PROJECT NO. 9-57 COPY NO. ___ EXPERIMENTAL DESIGN FOR FIELD VALIDATION OF LABORATORY TESTS TO ASSESS CRACKING RESISTANCE OF ASPHALT MIXTURES PROJECT FINAL REPORT Prepared for National Cooperative Highway Research Program Transportation Research Board of The National Academies Fujie Zhou, David Newcomb, Charles Gurganus, Seyedamin Banihashemrad, Eun Sug Park, Maryam Sakhaeifar, and Robert L. Lytton Texas A&M Transportation Institute The Texas A&M University System College Station, TX 77843-3135 April 2016 TRANSPORTATION RESEARCH BOARD OF THE NATIONAL ACADEMIES PRIVILEGED DOCUMENT This report, not released for publication, is furnished only for review to members of or participants in the work of the NCHRP. This report is to be regarded as fully privileged, and dissemination of the information included herein must be approved by the NCHRP.
Transcript
PROJECT NO. 9-57 COPY NO. ___
EXPERIMENTAL DESIGN FOR FIELD VALIDATION OF
LABORATORY TESTS TO ASSESS CRACKING RESISTANCE OF ASPHALT MIXTURES
PROJECT FINAL REPORT
Prepared for National Cooperative Highway Research Program
Transportation Research Board of
The National Academies
Fujie Zhou, David Newcomb, Charles Gurganus, Seyedamin Banihashemrad, Eun Sug Park, Maryam Sakhaeifar, and Robert L. Lytton
Texas A&M Transportation Institute The Texas A&M University System
College Station, TX 77843-3135 April 2016
TRANSPORTATION RESEARCH BOARD
OF THE NATIONAL ACADEMIES PRIVILEGED DOCUMENT
This report, not released for publication, is furnished only for review to members
of or participants in the work of the NCHRP. This report is to be regarded as fully privileged, and dissemination of the information included herein must be
approved by the NCHRP.
ACKNOWLEDGMENT OF SPONSORSHIP This work was sponsored by one or more of the following as noted:
x American Association of State Highway and Transportation Officials, in cooperation with the Federal Highway Administration, and was conducted in the National Cooperative Highway Research Program,
Federal Transit Administration and was conducted in the Transit Cooperative Research Program,
Federal Aviation Administration and was conducted in the Airport Cooperative Research Program,
Research and Innovative Technology Administration and was conducted in the National Cooperative Freight Research Program,
Pipeline and Hazardous Materials Safety Administration and was conducted in the Hazardous Materials Cooperative Research Program, which is administered by the Transportation Research Board of the National Academies.
DISCLAIMER
This is an uncorrected draft as submitted by the research agency. The opinions and conclusions expressed or implied in the report are those of the research agency. They are not necessarily those of the Transportation Research Board, the National Academies, or the program sponsors.
Project No. 9-57 Copy No.__
EXPERIMENTAL DESIGN FOR FIELD VALIDATION OF
LABORATORY TESTS TO ASSESS CRACKING RESISTANCE OF ASPHALT MIXTURES
DRAFT FINAL REPORT
Prepared for
National Cooperative Highway Research Program Transportation Research Board
of The National Academies
Fujie Zhou David Newcomb Charles Gurganus
Seyedamin Banihashemrad Eun Sug Park
Maryam Sakhaeifar and
Robert L. Lytton
Texas A&M Transportation Institute The Texas A&M University System
College Station, Texas
April 2016
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CONTENTS
List of Figures ................................................................................................................................ v
List of Tables ................................................................................................................................ vi Acknowledgments ...................................................................................................................... viii Executive Summary ...................................................................................................................... 1
Appendix A. Literature Review on Laboratory Cracking Test ........................................... A-1
Appendix B. Summary of Asphalt Cracking Test Workshop .............................................. B-1
Appendix C. Potential Field Test Sections .............................................................................. C-1
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LIST OF FIGURES
Figure 1. DCT specimen dimensions. ........................................................................................... 18 Figure 2. SCB-AASHTO TP 105 specimen geometry (AASHTO TP 105)................................. 19 Figure 3. SCB-LTRC specimen geometry. ................................................................................... 19 Figure 4. SCB-IL specimen geometry. ......................................................................................... 20 Figure 5. OT specimen geometry.................................................................................................. 21 Figure 6. BBF specimen geometry. .............................................................................................. 21 Figure 7. IDT specimen geometry. ............................................................................................... 22 Figure 8. LTPP climate zones with projected latitude and longitude. .......................................... 46 Figure 9. Diurnal temperature ranges for March (NWS 2015). .................................................... 47 Figure 10. Desired locations for field experimental test sections for thermal cracking. .............. 47 Figure 11. Information sheet for potential thermal cracking in cold climate section. .................. 51 Figure 12. Information sheet for potential thermal cracking in diurnal cycling climate test
section. .............................................................................................................................. 52 Figure 13. Temperature cycling region for reflection cracking. ................................................... 62 Figure 14. Potential reflection cracking section in a steady state climate. ................................... 64 Figure 15. Potential reflection cracking section in hard-freeze climate. ...................................... 65 Figure 16. Precipitation map created by PRISM Climate Group, Oregon State University. ....... 74 Figure 17. Possible climate zones for fatigue cracking test sections. ........................................... 75 Figure 18. Potential bottom-up fatigue cracking test section in high temperature/moisture
cycling region.................................................................................................................... 77 Figure 19. Potential bottom-up fatigue cracking test location in other climate location. ............. 78 Figure 20. Solar gain map. ............................................................................................................ 86 Figure 21. Expanded high and low solar gain regions. ................................................................. 86 Figure 22. Top-down cracking climate regions. ........................................................................... 87 Figure 23. Potential top-down cracking test section in a hard-freeze climate. ............................. 89 Figure 24. Potential top-down cracking test section in no-freeze climate. ................................... 90 Figure 25. Example of pavement forensic study flow chart. ........................................................ 97 Figure 26. Example information sheet of potential section from DOT letting. .......................... 100
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LIST OF TABLES Table 1. Laboratory cracking tests. ................................................................................................. 7 Table 2. Twelve cracking tests considered at the workshop. ........................................................ 11 Table 3. Example of weighted scores for reflection cracking tests. ............................................. 12 Table 4. Cracking tests selected at the workshop. ........................................................................ 12 Table 5. Identified candidate laboratories for ruggedness test (and later for ILS). ...................... 14 Table 6. Proposed ruggedness testing variables for selected cracking tests. ................................ 16 Table 7. Proposed experiment design for up to seven variables (ASTM E 1169-14). ................. 23 Table 8. Number of specimens for ruggedness testing. ................................................................ 23 Table 9. Needed raw materials for ruggedness test. ..................................................................... 24 Table 10. Eight run combinations for DCT test with six test variables. ....................................... 25 Table 11. Eight run combinations for SCB-TP105 test with five test variables. .......................... 25 Table 12. Eight run combinations for SCB-LTRC test with five test variables. .......................... 26 Table 13. Eight run combinations for SCB-IL test with five test variables. ................................. 26 Table 14. Eight run combinations for IDT-Florida test with four test variables. ......................... 26 Table 15. Eight run combinations for OT test with six test variables........................................... 27 Table 16. Eight run combinations for BBF test with six test variables. ....................................... 27 Table 17. Budget for ruggedness studies. ..................................................................................... 29 Table 18. Schedule for ruggedness testing. .................................................................................. 29 Table 19. Proposed laboratories for ILS of cracking tests. ........................................................... 31 Table 20. Number of specimens for ILS. ...................................................................................... 32 Table 21. Needed raw materials for ILS. ...................................................................................... 33 Table 22. Precision statistics sample (reference: ASTM E691). .................................................. 35 Table 23. Time and budget for ILS. .............................................................................................. 36 Table 24. Schedule for ILS testing. .............................................................................................. 37 Table 25. Candidate set of runs for factors X1, X2, X3. .............................................................. 41 Table 26. D-Optimal design with seven runs for main effects. .................................................... 41 Table 27. D-Optimal design with 18 runs for main effects and two-way interactions. ................ 42 Table 28. Field experimental design factors identified for thermal cracking. .............................. 44 Table 29. D-optimal experimental Design 1 for thermal cracking: 6 test sections. ...................... 45 Table 30. D-optimal experimental Design 2 for thermal cracking: 15 test sections. .................... 45 Table 31. Four types of field experimental test facilities in United States. .................................. 48 Table 32. D-optimal experimental Design 1 for thermal cracking with possible test
sections. ............................................................................................................................. 50 Table 33. Materials per section for validation of thermal cracking. ............................................. 55 Table 34. Time and budget* for thermal cracking study. ............................................................. 57 Table 35. Schedule for thermal cracking study for Design 1. ...................................................... 58 Table 36. Schedule for thermal cracking study for Design 2. ...................................................... 58 Table 37. Field experimental design factors identified for reflection cracking. ........................... 59 Table 38. D-optimal experimental Design 1 for reflection cracking: 7 test sections. .................. 60 Table 39. D-optimal experimental Design 2 for reflection cracking: 25 test sections. ................ 61 Table 40. D-optimal experimental Design 1 for reflection cracking with possible test
sections. ............................................................................................................................. 63 Table 41. Needed materials per section for validation of reflection cracking. ............................. 67 Table 42. Budget* for reflection cracking study. ......................................................................... 69
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Table 43. Schedule for reflection cracking study for Design 1. ................................................... 70 Table 44. Schedule for reflection cracking study for Design 2. ................................................... 70 Table 45. Field experimental design factors identified for bottom-up fatigue cracking. ............. 71 Table 46. D-optimal experimental Design 1 for fatigue cracking: 8 test sections. ....................... 72 Table 47. D-optimal experimental Design 2 for fatigue cracking: 26 test sections. ..................... 73 Table 48. Potential fatigue cracking test sections from ALF at FHWA Turner-Fairbank
Highway Research Center. ................................................................................................ 75 Table 49. D-optimal experimental Design 1 for bottom-up fatigue cracking with possible
test sections. ...................................................................................................................... 76 Table 50. Needed materials per section for validation of fatigue cracking. ................................. 80 Table 51. Budget* for bottom-up fatigue cracking study. ............................................................ 81 Table 52. Schedule for bottom-up fatigue cracking study for Design 1. ...................................... 82 Table 53. Schedule for bottom-up fatigue cracking study for Design 2. ...................................... 82 Table 54. Field experimental design factors identified for top-down cracking. ........................... 84 Table 55. D-optimal experimental Design 1 for top-down cracking: nine test sections. .............. 84 Table 56. D-optimal experimental Design 2 for top-down cracking: 26 test sections. ................ 85 Table 57. D-optimal experimental Design 1 for top-down cracking with possible test
sections. ............................................................................................................................. 88 Table 58. Needed materials per section for validation of top-down cracking. ............................. 92 Table 59. Budget* for top-down fatigue cracking study. ............................................................. 94 Table 60. Schedule for top-down fatigue cracking study for Design 1. ....................................... 94 Table 61. Schedule for top-down fatigue cracking ctudy for Design 2. ....................................... 95 Table 62. Modified LTPP forensic field activities........................................................................ 95 Table 63. Summary of Budgets and Timelines for Cracking Tests. ........................................... 101
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ACKNOWLEDGMENTS
The research reported herein was performed under NCHRP Project 9-57 by the Texas A&M Transportation Institute (TTI), a member of The Texas A&M University System. Dr. Fujie Zhou, Research Engineer at TTI served as the principal investigator, and Dr. David Newcomb, senior research engineer at TTI, served as the co-principal investigator. The other authors of this report are Mr. Charles Gurganus and Mr. Seyedamin Banihashemrad, both research assistants; Dr. Maryam Sakhaeifar, assistant professor of civil engineering; Dr. Eun Sug Park, research scientist; and Dr. Robert L. Lytton, professor of civil engineering.
NCHRP Project 9-57 produced nine cracking test videos and one crack test booklet, and included a cracking test webinar and a facilitated workshop. The research team gratefully appreciates the following individuals for their time and efforts:
• Dr. Soohyok Im and Mr. Tony Barbosa at TTI and Drs. Nathan Morian and Elie Hajj at the University of Nevada at Reno for the cracking test videos.
• Ms. Debbie Murillo at TTI for facilitating the cracking test webinars and designing the cracking test booklet and Ms. Tina Geiselbrecht at TTI for facilitating the Cracking Test Workshop.
• Dr. Bill Buttlar (University of Illinois at Urbana Champaign), Dr. Elie Hajj (University of Nevada at Reno), Dr. John Harvey (University of California at Davis), Dr. Richard Kim (North Carolina State University), Dr. Robert Lytton (Texas A&M University), Dr. Mihai Marasteanu (University of Minnesota), Dr. Louay Mohammad (Louisiana State University), Dr. Rey Roque (University of Florida), and Mr. Tom Scullion (TTI) for sharing selflessly the development histories of different cracking tests at the cracking test webinars.
• Mr. Chris Abadie (Louisiana Department of Transportation), Mr. Phil Blankenship (Asphalt Institute), Dr. Judy Corley-Lay (North Carolina Department of Transportation), Dr. Jo Daniel (University of New Hampshire), Mr. Jeff Dean (Oklahoma Department of Transportation), Dr. Stacey Diefenderfer (Virginia Department of Transportation), Dr. Adam Hand (Granite Construction), Mr. Gerry Huber (Heritage research group), Dr. David Jones (University of California at Davis), Dr. Mike Mamlouk (Arizona State University), Mr. Dan Oesch (Missouri Department of Transportation), Mr. David Powers (Ohio Department of Transportation), and Dr. Peter Sebaaly (University of Nevada at Reno) for the time and effort spent attending and constructively contributing to the Cracking Test Workshop.
The research team deeply appreciates the workshop coordination and logistical and technical guidance from NCHRP officers Drs. Ed Harrigan and Anthony Avery. Special thanks go out to the panel members for the time, effort, and valuable input into this project.
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EXECUTIVE SUMMARY
Asphalt mix designs are becoming more and more complex with the increasing uses of recycled materials, recycling agents, binder additives/modifiers (such as recycled engine oil bottom), and multiple warm-mix asphalt technologies. These changes have altered the performance of mixtures both positively and negatively so that volumetric mix design alone is not sufficient for evaluating the potential behavior of asphalt mixtures, especially on the cracking behavior of asphalt mixes. Thus, there is an urgent need to establish and implement reliable performance tests that can be used to eliminate brittle mixes or used to model asphalt pavements to predict cracking.
The overall objective of this research was to develop an experimental design for field validation of laboratory tests selected under this study in order to assess the cracking potential of asphalt mixtures. A three-step process was taken to achieve the objective: (1) identify and select cracking tests, (2) refine these selected cracking tests through a ruggedness test and inter-laboratory study (ILS), and (3) develop an experimental design and associated time and cost for validation of the selected cracking tests. This draft final report first summarizes the cracking test selection process and the final cracking tests selected for field validation in Chapter 2. The necessary steps for refining each cracking test method through ruggedness testing and ILS for each cracking test are detailed in Chapter 3. A D-optimal experimental design approach was employed to develop field experimental design for each of the four modes of cracking: thermal, reflection, bottom-up fatigue, and top-down. The detailed experimental designs for field test sections and associated laboratory tests for each type of cracking are provided in Chapter 4. In Chapters 3 and 4, estimates of time and costs are given for the ruggedness testing, ILS, and validation testing. Chapter 5 concludes the report with a summary and proposed research.
3
CHAPTER 1. INTRODUCTION
BACKGROUND
Cracking is a primary mode of distress that frequently drives the need for rehabilitation of asphalt pavements. There are four major modes of asphalt pavement cracking— thermal, reflection, fatigue, and top-down—that are affected by numerous factors and interactions. In the past, volumetric mixture design gave a reasonable level of comfort for performance since the materials were relatively consistent within a given jurisdiction. Meanwhile, asphalt mix designs are becoming more and more complex with the increasing uses of recycled materials, recycling agents, binder additives/modifiers (such as recycled engine oil bottom), and multiple warm-mix asphalt technologies. These changes have altered the performance of mixtures both positively and negatively so that volumetric mix design alone is not sufficient for evaluating the potential behavior of asphalt mixtures. Thus, there is an urgent need to establish and implement reliable performance tests that can be used to eliminate brittle mixes or used to model asphalt pavements to predict cracking.
RESEARCH OBJECTIVE
The overall objective of this research is to develop an experimental design for field validation of laboratory tests selected under this study in order to assess the cracking potential of asphalt mixtures. This final report documents the selected cracking tests, proposed refinements, the field validation plan for the selected tests, and associated time and cost estimations.
RESEARCH APPROACH
The overall research approach employed in National Cooperative Highway Research Program (NCHRP) 9-57 was to synthesize information gathered from both a critical review of relevant research and state mixture design practices and a workshop with invited experts in order to develop a comprehensive experimental plan and budget for a follow-up project to improve and validate the selected cracking tests for routine asphalt mixture designs. Seven specific steps briefly listed below were taken to achieve the research objective. Detailed information is presented in the following chapters of this report.
• Step 1: Review relevant research and mixture design practice of each state. • Step 2: Prepare the Cracking Test Workshop. • Step 3: Conduct the Cracking Test Workshop. • Step 4: Develop plans for laboratory refining the selected cracking tests. • Step 5: Develop field validation plans for the selected cracking tests. • Step 6: Develop a schedule and budget for laboratory refining and field validation
plans. • Step 7: Document results in the final report and meet with the NCHRP Project 9-57
panel.
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REPORT ORGANIZATION
This final report documents the overall research results of NCHRP Project 9-57. Chapter 1 provides an introduction. Chapter 2 describes the overall cracking test selection process and the final cracking tests selected for further laboratory refining and field validation. The overall selection process of the cracking tests included (1) identifying 10 cracking tests through critically reviewing relevant research, (2) developing videos to document these cracking tests, (3) conducting cracking test webinars prior to the workshop, and (4) conducting the Cracking Test Workshop to select cracking tests and identify critical factors for field validation. Chapter 3 defines the steps to refine each cracking test method through ruggedness tests and inter-laboratory study. The field validation plan for these cracking tests is described in Chapter 4. Detailed experimental design and associated laboratory tests for each mode of cracking (thermal, reflection, fatigue, and top-down) are provided in Chapter 4. In Chapters 3 and 4, estimates of time and costs are given for the ruggedness tests, the inter-laboratory studies, and the validation testing. Chapter 5 concludes the report with a summary and proposed research.
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CHAPTER 2. SELECTION OF ASPHALT CRACKING TESTS
INTRODUCTION
The selection of asphalt cracking tests is a three-step process. First, the research team critically reviewed relevant research and state best practices on mix designs and identified 10 cracking tests for the Cracking Test Workshop experts to consider. Next, the research team coordinated with the NCHRP Project 9-57 panel to prepare a cracking test selection workshop. The workshop preparation included (1) identifying workshop participants; (2) conducting two crack test webinars in which the original developers of the cracking tests presented each test method to the attendees of the Cracking Test Workshop; (3) making high-definition cracking test videos that were posted on the web and visually displayed the key steps of the cracking tests presented in the webinars; (4) developing a cracking test reference booklet tailored for the workshop participants; and (5) developing a workshop agenda. Finally, seven cracking tests were selected at the Cracking Test Workshop held at the National Academy of Sciences Beckman Center in Irvine, California, on February 11–12, 2015. A brief summary of the whole cracking test selection process and the results of the Cracking Test Workshop is provided in the following sections.
IDENTIFICATION OF LABORATORY CRACKING TESTS
A comprehensive literature review was performed to identify cracking tests suitable for routine asphalt mixture designs. Detailed literature review results are presented in Appendix A. Table 1 presents a summary of the identified 10 cracking tests; some were considered practical while others showed promise. Several of them are either currently used by state highway agencies or are being considered for implementation in the future, as noted from the cracking test survey. These 10 cracking tests are:
• Disk-shaped compact tension (DCT) test: ASTM D7313-13. • Semi-circular bend (SCB) test at low temperature: AASHTO TP105-13. • Indirect tension (IDT) for low-temperature cracking: AASHTO T322. • Uniaxial thermal stress and strain test (UTSST): University of Nevada at Reno. • Texas overlay test (OT): Tex 248-F. • Bending beam fatigue (BBF) test: AASHTO T321. • SCB at intermediate temperature: Louisiana Transportation Research Center. • IDT-UF for top-down fatigue cracking: University of Florida. • Simplified viscoelastic continuum damage (S-VECD) fatigue test: AASHTO TP107. • Repeated direct tension (RDT) test: Texas A&M University. When the research team recommended these 10 cracking tests for the workshop
participants to consider, the following aspects were considered: • Test variability: The monotonic loading types of cracking tests (e.g., IDT, SCB, DCT,
thermal stress-restrained specimen test [TSRST], or Uniaxial Thermal Stress and Strain Test) generally have much lower variability with coefficients of variation (COV), often less than 15 percent. In contrast, the repeated loading types of cracking tests, including OT, BBF, S-VECD, and direct tension (DT), may have much higher
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COVs (>30 percent) than the monotonic tests. Relatively higher COV may be the inherent nature of the repeated loading types of cracking tests.
• Interpretation of test results: Cracking test results can be classified into index- or mechanistic-oriented parameters. Index parameters, such as fracture energy or OT cycles to failure, are often directly correlated to field cracking distresses and mechanistic-oriented parameters, like creep compliance from IDT and those from S-VECD and DT, and need to be combined with cracking models in order to evaluate hot-mix asphalt’s cracking resistance. Either index or mechanistic-oriented tests may be adequate for routine use as the software associated with mechanistic-oriented analyses matures.
• Correlations to field performance: Some of the existing tests are of the index type, which is more suitable for mix design and QC/QA; some are more mechanistic oriented and suited for performance analysis; and some can be used for both. Regardless of whether the tests are index or mechanistic oriented, and monotonic or repeated load, any laboratory test adopted for routine use must be validated and have a good correlation with field performance.
• Test simplicity (or complexity): Technician training requirements, time for preparing and testing specimens, and difficulty in analyzing data are also factors for consideration. For example, IDT needs the least time for specimen preparation; in contrast, the beam fatigue test, UTSST, S-VECD, and DT need the most time for specimen preparation. Tests listed in Table 1 require shaping prior to testing. Additionally, the SCB requires notching, the OT needs gluing, and the DCT requires notching and drilling.
• Sensitivity to mix design parameters: Cracking tests should be able to distinguish performance related to the characteristics and volumetric properties of asphalt mixtures such as binder type and content, air voids, reclaimed asphalt pavement (RAP) or recycled asphalt shingle (RAS) content, aggregates, and so forth. In general, the repeated loading cracking tests are more sensitive to mix variables than monotonic loading tests.
• Other factors: Equipment availability and cost, the availability of test methods (AASHTO, ASTM, state standard, or draft), compaction methods, and direction of loading all need to be considered in choosing cracking tests.
It is likely that some compromises may have to be made when selecting cracking tests for routine use. Therefore, a dedicated workshop with a balanced representation of practitioners and topical experts is warranted in order to obtain consensus on candidate cracking tests for further evaluation in the follow-up project(s).
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Table 1. Laboratory cracking tests. Laboratory test Correlation to
field performance
Test variability Test simplicity (or complexity)
Test sensitivity to mix design
parametersa
Equipment cost and
availability
Adoption by states Test
name Cracking
type Test
standard Test configuration Specimen geometry
Cracking parameter
DCT
Low-temperature
cracking and
reflection cracking
ASTM D7313
(Monotonic test)
D1 = 6 in. T1 = 2 in.
2 holes D = 1 in.
ND1 = 2.46 in.
Fracture energy
Good correlation with low-temperature
cracking validated at MnRoad.
Low (COV=10-
15%)
Training: little time Specimen prep: 4 cuts and 2 holes Instrumentation: gluing 2 studs Testing2: 1–6 min. Analysis: area integration Interpretation: quick and easy (pass/fail criteria).
Asphalt binder,
aggregate, RAP/RAS, and aging;
insensitive to AV3 and Pb
3
Commercially available; Cost: $49,000.
Adopted by Minnesota;
being considered by
Colorado, South
Dakota, and Montana.
SCB
Low-temperature
cracking
AASHTO TP105
(Monotonic test)
D = 6 in. T = 1 in.
ND = 0.6 in.
Fracture energy
Good correlation with low-temperature
cracking validated at MnRoad.
Medium (COV=20
%)
Training: medium time Specimen prep: 3 cuts Instrumentation: gluing 3 studs Testing: 30 min. Analysis: area integration Interpretation: quick and easy (pass/fail criteria).
Asphalt binder,
aggregate, RAP/RAS, AV and Pa
Commercially available; Cost: $52,000.
Being considered by Utah, South
Dakota, Pennsylvania, and Montana.
Bottom-up and top-
down fatigue
cracking
LTRC (Monotonic
test)
D = 6 in. T = 2.25 in.
ND = 1, 1.25 and 1.5 in.
Energy release rate
Fair correlation to field cracking
from the Louisiana Pavement
Management System.
Medium (COV=20
%)
Training: very little time Specimen prep: 4 cuts Instrumentation: none Testing: 5–10 min. Analysis: area integration and regression Interpretation: quick and easy (pass/fail criteria).
Asphalt binder,
aggregate, RAP/RAS
Commercially available; Cost: $20,000;
Adopted by Louisiana;
being considered by
Oklahoma and New Mexico.
IDT Low-
temperature cracking
AASHTO T322: Dt and tensile
strength test (monotonic
tests)
D = 6 in. T = 1.5-2.0 in.
Creep compliance and tensile
strength
Creep compliance and tensile strength
inputs to TCMODEL.
Calibrated and validated
through original SHRP-I and
MEPDG.
Low (COV<11
%)
Training: medium time Specimen prep: 2 cuts Instrumentation: relatively easy Testing: 1–2 hours Analysis: short and easy with data analysis software Interpretation: longer time with cracking model to predict performance.
Asphalt binder,
aggregate, RAP/RAS,
aging
Hydraulic test machines can be used. With test machine, more than $100,000.
AASHTO T322 is
required by AASHTOWa
re.
8
Laboratory test Correlation to field
performance
Test variability Test simplicity (or complexity)
Test sensitivity to mix design
parametersa
Equipment cost and
availability
Adoption by states Test
name Cracking
type Test
standard Test configuration Specimen geometry
Cracking parameter
Top-down cracking
University of Florida:
Mr test, Dt test, and tensile strength test (cyclic and monotonic
tests)
D = 6 in. T = 1.5–2.0 in.
Energy ratio
Validated with field cores in Florida study and confirmed at National Center for Asphalt Technology (NCAT) test track.
Possibly low, similar to AASHTO T322.
Training: medium time Specimen prep: 2 cuts Instrumentation: relatively easy with gauge point template Testing: 1–2 hours Analysis: easy with data analysis software Interpretation: short and easy (pass/fail criteria).
Insensitive to change in binder viscosity (Roque et al. 2010)
Being adopted by Florida.
TSRST/UTSST
Low-temperature
cracking
(Monotonic test)
L = 10 in. W = 2 in. T = 2 in.
Fracture temperature
Validated with test sections
during SHRP program.
MnRoad test results showed
moderate correlation with
field performance.
Low (COV = around 10%)
Training: long time and intensive Specimen prep: difficult and long Instrumentation: easy and short Testing: 3–5 hours Analysis: easy and short Interpretation: quick and easy (pass/fail criteria).
Asphalt binder,
aggregate, AV, Pb, and
aging
Commercially available;
Cost: $98,000
Being considered by
Nevada.
Texas Overlay
Test
Reflection cracking
and bottom-up fatigue cracking
Tex-248-F (cyclic tests)
L= 6 in. W = 3 in. T = 1.5 in.
No. of cycles
(or fracture parameters:
A and n)
Good correlation with reflection cracking validated in Texas, California, and New Jersey; promising correlation with fatigue cracking validated with FHWA-ALF and NCAT test track.
Relatively high
(COV=30–50%)
Training: little time Specimen prep: 4 cuts Instrumentation: none Testing: 1 min–3 hr Analysis: easy and short Interpretation: quick and easy (pass/fail criteria).
Binder, aggregate, Pb, RAP/RAS, aging, etc.
Commercially available; Cost: $46,000
Adopted by Texas and
New Jersey; being
considered by Montana, Nevada,
Florida, and Ohio.
9
Laboratory test Correlation to field
performance
Test variability Test simplicity (or complexity)
Test sensitivity to mix design
parametersa
Equipment cost and
availability
Adoption by states Test
name Cracking
type Test
standard Test configuration Specimen geometry
Cracking parameter
Bend Beam
Fatigue Test
Bottom-up fatigue
cracking
AASHTO T321
(cyclic tests)
L = 15 in. W = 2.5 in.
T = 2 in.
No. of cycles
(or fatigue equation)
Correlation with bottom-up fatigue cracking historically validated.
Very high (COV>50
%)
Training: medium time Specimen prep: difficult and long Instrumentation: almost none Specimen testing: hours to days Data analysis: easy and quick Date Interpretation: quick and easy (or combine with pavement analysis program to predict pavement fatigue life.)
Binder, aggregate, Pb, RAP/RAS, aging, etc.
Frame (fixture) commercially available. Universal testing machine needed; could be > $100,000.
California—special
pavement design; being considered by Nevada and
Georgia.
S-VECD
Bottom-up and top-
down fatigue
cracking
AASHTO TP107 (cyclic tests)
(AASHTO TP79 E*
test for data analysis)
S-VECD: D = 4 in.
L = 5.1 in.
(E*: D = 4 in. L = 6 in.)
Fatigue equation
and damage parameters
(or predicted
no. of cycles)
S-VECD used with MEPDG or more advanced models (LVECD and VECD-FEP++) to simulate pavement performance. Validated with FHWA-ALF test lanes and verified in North Carolina.
Not defined
Training: very long time Specimen prep: 2 cuts and 1 coring Instrumentation: easy with a special glue jig Testing: hours to 1 day (3 more days if E* test is considered) Analysis: easy if using ALPHA-fatigue software Interpretation: quick and easy if only number of cycles is concerned (or combine with pavement analysis programs [LVECD and VECD-FEP++] to predict pavement fatigue life).
Not available Commercially available; Cost: $97,000
Being considered by
Oklahoma, Georgia, and Pennsylvania.
Direct tension
Bottom-up and top-
down fatigue
cracking
Texas A&M
University (cyclic tests)
D = 4 in. L = 6 in.
Paris’ law parameters (or No. of
cycles)
Correlations with bottom-up and top-down fatigue cracking being developed under several research projects. Model and methods being validated with LTPP data.
Not defined
Training: very long time Specimen prep: 2 cuts and 1 coring Instrumentation: medium time and difficulty Testing: 1–2 hours Analysis: need special software Interpretation: still under development.
Model coefficients functions of AV, Pb, gradation; modulus, aging, etc.
Universal test machine needed for direct tension test; >$100,000.
Unknown
Note: D = diameter; L = length; W = width; T = thickness; ND = notch depth; AV = Air voids; Pb = Percent asphalt binder. a Testing refers to the time for running the test only.
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WORKSHOP PREPARATION
Participants
Participants in the Cracking Test Workshop included the NCHRP 9-57 panel, members of the research team, and invited experts recommended by the research team and approved by the project panel. Thirty-one individuals were invited to attend, including 11 panel members, the NCHRP senior program officer and senior program assistant, four members of the research team, and 14 invited experts. There was a high degree of interest in the workshop, with 94 percent of the invitees participating.
Cracking Test Webinars
It was critical for each invited participant to have access to information concerning these cracking tests before the workshop. Two half-day webinars were conducted so each cracking test developer could present the development history, test features, lab-to-field correlation, and implementation status of each cracking test. Note that the IDT tests for low-temperature and top-down cracking are similar except for the test temperatures, and the three other tests were presented for low-temperature cracking. Thus, the focus of the IDT test at the webinar was on top-down fatigue cracking, and a total of nine cracking tests were discussed. Each presenter had 30 minutes for presentation and a 10-minute question/answer time for the participants to directly ask test developers questions on specific cracking tests.
Cracking Test Videos
A concerted effort was made to develop nine videos for the 10 cracking tests. Note that one video was developed for IDT tests for both low-temperature and top-down cracking. The purpose of developing cracking test videos was to further assist workshop participants in understanding each cracking test and to visualize the key steps for performing it, from specimen preparation to final data analysis. These cracking test videos are available to the public through the following links:
A 12-page cracking test summary booklet was designed specifically for the workshop. The key aspects of each of the 10 cracking tests were described in the booklet in order to provide a condensed version of the interim report.
WORKSHOP: SELECTION OF CRACKING TESTS
One of the main objectives of the Cracking Test Workshop was to select tests for further laboratory and field evaluation of thermal, reflection, bottom-up, and top-down fatigue cracking. In addition to the 10 cracking tests listed in Table 1, two more new cracking tests were recommended at the workshop by the participants: the SCB test in Illinois (SCB-IL) for thermal cracking and the modified Overlay test for top-down cracking. Thus, a total of 12 cracking tests, as categorized in Table 2, were discussed at the workshop. Appendix B documents the detailed workshop information.
Table 2. Twelve cracking tests considered at the workshop.
A four-step process was used to select cracking tests, as described below: • Step 1: Determine the weighting factors for seven attributes of each cracking test: (a)
availability of test method, (b) test simplicity, (c) test variability, (d) sensitivity to mix parameters, (e) complexity of data analysis, (f) availability/cost of test equipment, and (g) lab-to-field correlation. The weighting factor ranged from 1 (least important) to 5 (most important). The weighting factors from each workshop participant were then averaged for later use to select cracking tests for fatigue, reflection, thermal, and top-down cracking.
• Step 2: Determine the rating score for each cracking test under each cracking mode in terms of the seven attributes listed above. The participants were divided into four groups: fatigue, reflection, thermal, and top-down. Each group was responsible for rating the cracking tests under each cracking mode category.
• Step 3: Calculate the weighted score for each test under each cracking mode. A weighted score was determined by multiplying the scores by the attribute weighting factors. The weighted scores were then totaled for an overall ranking of the test methods. Table 3 shows an example of the weighted scores for the reflection cracking tests.
• Step 4: Select 2–3 cracking tests for each cracking mode. Rank each cracking test from the highest to the lowest total score and select the first two or three tests under
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each cracking mode. The final candidate cracking tests selected by the workshop participants are listed in Table 4.
Table 3. Example of weighted scores for reflection cracking tests.
Reflection Cracking
OT SCB-LTEC BBF DCT Avg. Rating WF Final
Score Avg. Rating WF Final
Score Avg. Rating WF Final
Score Avg. Rating WF Final
Score Availability of Test Method 4.7 2.9 13.5 4.0 2.9 11.5 4.5 2.9 13.0 4.7 2.9 13.5
Total Score 102.4 94.9 89.8 89.0 Note: Ave. rating-averaged rating from all participants of thermal cracking group; WF-weighting factor; Final score is calculated by ave. rating x WF. The total score is the sum of the final score in each characteristic of test method. Higher total score is preferred.
Table 4. Cracking tests selected at the workshop.
Thermal cracking tests
Reflection cracking tests
Fatigue cracking tests
Top-down cracking tests
1. DCT 2. SCB-IL 3. SCB (AASHTO
TP105)
1. OT 2. SCB-LTRC 3. BBF
1. Beam fatigue 2. SCB-LTRC 3. OT*
1. IDT-Florida 2. SCB-LTRC
*OT for fatigue cracking was added later by request of the panel.
SUMMARY
This chapter documents the three-step process of selecting cracking tests for laboratory routine mix design and performance prediction. The final seven cracking tests selected at the workshop were (a) DCT, (b) SCB-IL, (c) SCB-TP105, (d) SCB-LTRC, (e) OT, (f) BBF, and (g) IDT-Florida. Since most of these seven cracking tests did not go through ruggedness testing and inter-laboratory study, further laboratory evaluation is needed to refine test procedures and develop precision for each test procedure, which is discussed in Chapter 3.
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CHAPTER 3. REFINEMENT OF THE SELECTED CRACKING TESTS
INTRODUCTION
A total of seven cracking tests were suggested for routine use to characterize cracking resistance of asphalt mixes in terms of thermal, reflection, bottom-up fatigue, and top-down cracking. Some of them are new tests (like SCB-LTRC, SCB-IL, IDT-Florida) that are not standardized yet; some tests have been in use for decades, like BBF and OT. However, none of them is in a stage of routine use in the asphalt mix design process, although OT has been routinely used to design stone-matrix asphalt (SMA) and porous friction course (PFC) mixtures in Texas. Thus, it is critical to evaluate and refine the test procedures through ruggedness testing and inter-laboratory study (ILS). A detailed ruggedness test plan and inter-laboratory study for each cracking test are described in the following sections.
RUGGEDNESS TESTING
The main purpose of performing ruggedness testing is to identify those factors that significantly influence the cracking resistance measurements of each specific cracking test method and to estimate how closely these factors need to be controlled. Basically, the ruggedness test is a kind of sensitivity test on variables of a test method rather than materials. For a given test method, the variables may include test temperature, specimen dimensions, loading rate, and so forth. Through ruggedness testing, the sensitive test variables will be identified and the associated tolerance for each sensitive variable will be defined.
The plan described below was developed by following the ASTM E 1169-14: Standard Practice for Conducting Ruggedness Tests guidelines. ASTM E1169-14 recommends that ruggedness testing should be done by a single laboratory with uniform material and should precede an ILS. Specifically, the proposed plan includes the seven items listed below:
• Identified laboratories for ruggedness testing. • Asphalt mixture for ruggedness testing. • Test variables. • Ruggedness testing design. • Execution of ruggedness testing. • Statistical analysis of ruggedness testing results. • Possible revision of the test method as needed.
Identified Laboratories for Ruggedness Testing
The research team contacted a variety of research laboratories around the United States that are potentially capable of performing all or part of the seven selected cracking tests. The research team evaluated each laboratory based on its history, research experience, and, more importantly, the AASHTO Materials Reference Laboratory (AMRL) certification. Table 5 lists the identified laboratories for performing a ruggedness test. Capabilities of performing cracking tests in each laboratory are also listed in Table 5. Note that these laboratories are also the candidates for conducting ILS at a later time.
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Table 5. Identified candidate laboratories for ruggedness test (and later for ILS).
FHWA Turner-Fairbank Pavement Research Center DCT, SCB-IL, SCB-TP105, IDT-Florida
P
Florida DOT (FDOT) IDT-Florida P
Florida State University (FSU) OT O
Heritage Research DCT, SCB-IL P
Illinois DOT (IDOT) SCB-IL, DCT P
Louisiana Transportation Research Center (LTRC) SCB-IL, SCB-LTRC, BBF P
Mathy Technology & Engineering Services DCT, SCB-IL, SCB-LTRC P
Minnesota DOT (MnDOT) DCT, SCB-IL P
NCAT at Auburn University DCT, SCB-IL, SCB-LTRC,
SCB-TP105, OT, BBF, IDT-Florida
P
Rutgers University OT, BBF P
Texas A&M Transportation Institute (TTI) DCT, SCB-IL, SCB-LTRC,
SCB-TP105, OT, BBF, IDT-Florida
P
Texas DOT (TxDOT) OT P
University of Arkansas (UARK) SCB-IL, SCB-LTRC, SCB-TP105, IDT-Florida
P
University of California Pavement Research Center (UCPRC) BBF, OT P
University of Florida (UF) IDT-Florida O
University of Illinois at Urbana Champaign (ICT-UIUC) DCT, SCB-IL, SCB-LTRC,
SCB-TP105, OT, BBF, IDT-Florida
P
University of Massachusetts at Dartmouth (UMass) DCT, SCB-IL, SCB-LTRC,
OT, BBF P
University of Minnesota (UM) DCT, SCB-IL, SCB-LTRC, SCB-TP105, IDT-Florida
O
University of New Hampshire DCT, SCB-IL, SCB-LTRC,
SCB-TP105, BBF, IDT-Florida
O
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Asphalt Mixture for Ruggedness Testing
As noted previously, the purpose of the ruggedness test is to evaluate the test method rather than asphalt mixtures. A more uniform asphalt mixture will lead to a better identification of the test method sensitivity. Thus, a typical Superpave dense-graded asphalt (DGA) mixture with nominal maximum aggregate size (NMAS) of 9.5 mm is recommended. Additionally, to reduce impact of mixture components on the final results, a virgin mix with either PG64-22 or PG70-22 is desired for the ruggedness test.
Test Variables
The sensitivity of variables for each test method is the focus of the ruggedness test. These variables are features of the test method and subject to control by the test method. After reviewing each test method and associated literature, the research team identified some overall test method variables, namely loading rate, test temperature, crack opening rate, specimen dimension, notch depth, and air voids of test specimen. Table 6 lists the identified variables for each cracking test, followed by detailed descriptions.
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Table 6. Proposed ruggedness testing variables for selected cracking tests.
Test name Test standard Test
configuration
Specimen geometry (Note 1)
Test variability Ruggedness testing variables Standard factor High level Low level Tolerance
• DCT: Recently, the Minnesota Department of Transportation (MnDOT) performed limited ruggedness testing on the DCT test. Based on this research and information from the test method found in ASTM D7313, the test variables and associated upper and lower variation for NCHRP 9-57 are proposed below: o Specimen thickness: The target thickness for laboratory-compacted specimens
could be changed 50 ± 5 mm o Crack opening displacement: The crack-mouth opening displacement (CMOD)
opening rate is 1 mm/min. The crack opening displacement rate could be changed within 5 percent of the target value.
o Test temperature: The DCT is often conducted at 10°C warmer than the performance grade (PG) low temperature grade. The high and low level for the ruggedness test could be varied ±0.5°C.
o Location of holes: Figure 1 shows the location of holes in a DCT specimen. The dimension for specimen is mm. The location of holes could be changed ±5 mm horizontally and vertically.
o Notch depth: The notch depth for a standard specimen is 62 mm; the range could be ±3 mm.
o Air voids: The air voids could be changed 7 ± 0.5 percent.
Figure 1. DCT specimen dimensions.
• SCB-AASHTO TP105: The proposed test variables and associated upper and lower variation for SCB-AASHTO TP 105 are: o Specimen thickness: The target thickness for laboratory-compacted specimens
could be changed 25 ± 5 mm, as shown in Figure 2. o Crack opening displacement: The CMOD opening rate is 0.03 mm/min. The crack
opening displacement rate could be changed within 5 percent of the target value. o Test temperature: The SCB-AASHTO TP 105 is often conducted at 10°C warmer
than the PG low temperature grade. The high and low level for the ruggedness test could be varied ±0.5°C.
o Notch depth: The notch depth for a standard specimen is 15 mm, and the variation range could be ±3 mm.
o Air voids: The air voids could be changed within 7 ± 0.5 percent.
• SCB-LTRC: The proposed test variables and associated upper and lower variation for SCB-LTRC are: o Specimen thickness: The target thickness for laboratory-compacted specimens
could be changed 57 ± 5 mm. o Loading rate: The SCB- LTRC is cross-head controlled and the deformation rate
is 0.5 mm/min. The crack opening displacement rate could be changed within 5 percent of the target value.
o Test temperature: The SCB- LTRC is often conducted at room temperature 25°C. The high and low level for the ruggedness test could be varied ±1°C.
o Notch depth (Figure 3): The three required notch depths for a standard specimen are 25.4, 31.8, and 38.1 mm; the range could be within ±3 mm.
o Air voids: The air voids could be changed 7 ± 0.5 percent.
Figure 3. SCB-LTRC specimen geometry.
150
1.5
15
25
20
• SCB-IL: The proposed test variables and associated upper and lower variation for SCB-IL are: o Specimen thickness: The target thickness for laboratory-compacted specimens
could be changed 50 mm ±5 mm (Figure 4). o Loading rate: The SCB-IL is a cross-head controlled test with a loading rate of
50 mm/min. The crack opening displacement rate could be changed within 5 percent of the target value.
o Test temperature: The SCB-IL is often conducted at room temperature 25°C. The high and low level for the ruggedness test could be varied ±0.5°C.
o Notch depth: The notch depth for a standard specimen is 15 mm, and the variation range could be ±3 mm.
o Air voids: The air voids could be changed 7 ± 0.5 percent.
Figure 4. SCB-IL specimen geometry.
• Texas OT: The proposed test variables and associated upper and lower variation for the Texas OT are: o Specimen height (Figure 5): The target height for laboratory-compacted
specimens could be changed 38 ± 1 mm. o Opening displacement: The maximum opening displacement is 0.635 mm. The
opening displacement rate could be changed within 2 percent of the target value. o Test temperature: The Texas OT is often conducted at room temperature 25°C.
The high and low level for the ruggedness test could be varied ±1°C. o Specimen width: The target width for laboratory-compacted specimens could be
changed 76 ± 3 mm. o Loading rate: The Texas OT is a cyclic displacement-controlled test with a
triangle-loading waveform operating at 10 seconds per cycle (0.1 Hz). It could be changed ± 1 second.
o Air voids: The air voids could be changed 7 ± 0.5 percent.
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Figure 5. OT specimen geometry.
• BBF: The proposed test variables and associated upper and lower variation for the bending beam fatigue test are: o Specimen height (Figure 6): The target height for laboratory-compacted
specimens could be changed 50 ± 6 mm. o Test temperature: The BBF is often conducted at room temperature 20°C. The
high and low level for the ruggedness test could be varied ±0.5°C. o Specimen length: The target length for laboratory-compacted specimens could be
changed 380 ± 6 mm. o Specimen width: The target width for laboratory-compacted specimens could be
changed 63 ± 6 mm. o Loading frequency: The load rate is 10 Hz. It could be changed by ±1 Hz. o Air voids: The air voids could be changed 7 ± 0.5 percent.
Figure 6. BBF specimen geometry.
• IDT-Florida: The proposed test variables and associated upper and lower variation for the IDT-Florida are: o Specimen thickness (Figure 7): The target thickness for laboratory-compacted
specimens could be changed 50 ± 5 mm. o Creep time: In the creep test, a load level that produces a horizontal deformation
is held constant for 1,000 seconds. The time could change within ±5 percent. o Horizontal deformation low range: In the creep test, a horizontal deformation for
a constant load in 1000 seconds could vary between 0.0025 mm and 0.019 mm. The low range of horizontal deformation could change within ±5 percent.
o Horizontal deformation high range: The high range of horizontal deformation in a creep test is 0.019 mm. This horizontal deformation could change within ±5 percent.
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o Loading rate: The strength test determines the tensile strength of a specimen by loading the specimen at a constant rate of 50 mm/minute until failure. All of these loading rates could be changed within ±5 percent.
o Test temperature: The test temperature for the top-down fatigue cracking is 10°C for all three tests. The high and low level for the ruggedness test could be varied ±1°C.
o Air void percentage: The air voids could be changed 7 ± 0.5 percent.
Figure 7. IDT specimen geometry.
Ruggedness Testing Experimental Design
Since many test variables are involved, the fractional factorial Plackett-Burnam (PB) designs are often used with ruggedness tests to determine the effects of the test method variables. The PB designs just consider two levels for each variable, and the levels chosen should be reasonably large relative to measurement error. Thus, the high and low levels should be set at the extreme limits that could be expected to exist between different qualifying laboratories.
Table 7 shows a recommended experiment design for up to seven variables (A through G) with each variable set at two levels: (-1) for low level and (1) for high level. For five variables, use Columns A, B, C, D, and F and, for six variables, use Column A, B, C, D, F, and G. As discussed previously, the maximum number of variables for the seven cracking tests is up to seven. Thus, the PB designs shown in Table 7 are suitable for the seven cracking tests evaluated under NCHRP 9-57. Note that the design shown in Table 7 is balanced by providing equal numbers of high and low level runs for every variable. The main effect refers to the difference between the average response of runs at the high level and the average response of runs at the low level. When the effect of a variable is the same regardless of the levels of other variables, then the main effect is the best estimate of the variable’s effect.
150 mm 50 mm
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Table 7. Recommended experiment design for up to seven variables (ASTM E 1169-14).
PB Order, Run # A B C D E F G 1 1 1 1 -1 1 -1 -1 2 -1 1 1 1 -1 1 -1 3 -1 -1 1 1 1 -1 1 4 1 -1 -1 1 1 1 -1 5 -1 1 -1 -1 1 1 1 6 1 -1 1 -1 -1 1 1 7 1 1 -1 1 -1 -1 1 8 -1 -1 -1 -1 -1 -1 -1 Ave + Ave - Main Effect
Specifically, when transferring the PB designs shown in Table 7 to each cracking test ruggedness test, the variability of each cracking test should also be considered. In general, monotonic tests have smaller variability than repeated loading tests. Considering the variability of each cracking test, the research team proposed the number of specimens required for the ruggedness test, as listed in Table 8. Three replicates of specimens are recommended for monotonic cracking tests (DCT, SCB-IL, SCB-TP105, SCB-LTRC, and IDT-Florida) and five for repeated loading cracking tests (BBF and OT). Additionally, a safety factor of 1.5 is considered when calculating the number of specimens.
Table 9 shows the raw materials needed for running the ruggedness test on the selected
seven cracking tests.
Table 8. Number of specimens for ruggedness testing. Test Test
Table 9. Needed raw materials for ruggedness test. Laboratory-Mixed, Laboratory-Compacted (LMLC) PER MIXTURE/SPECIMEN TYPE Mixture/Specimen Description
No. of Specimen Mixture (kg) Aggregate (kg) Binder (liter)
DCT 36 181 175 9
SCB-IL 36 71 68 3.5
SCB-TP105 36 45 43.5 2.3
SCB-LTRC 36 91 87.5 4.5
OT 60 301 292 15.2
BBF 60 511 495 26
IDT-Florida 36 181 175 9
Execution of Ruggedness Testing
Essentially, the laboratories chosen to perform ruggedness testing for different cracking tests should follow each specific test procedure meticulously. The test specimens for each cracking test should be prepared and tested in one AMRL-certified laboratory. Additionally, the equipment for conducting ruggedness tests should be calibrated, and the operator should be trained to avoid or reduce any potential systematic error (or bias).
The recommended design for up to seven variables (Table 7), based on fractional factorial PB designs with two levels of (-1) for low level and (+1) for high level, should be specified for each cracking test. As discussed previously, there should be eight runs for each cracking test with different test variables. The number of replicates for each run depends on the cracking test variability. Three replicates of specimens are recommended for monotonic cracking tests (DCT, SCB-IL, SCB-TP105, SCB-LTRC, and IDT-Florida), and five for repeated loading cracking tests (BBR and OT). Table 10 through Table 16 detail the specific test variables to run the ruggedness test for each selected cracking test.
For the preparation of test specimens in the lab to simulate plant production, the AASHTO R30 procedure as modified in NCHRP 9-52 is recommended. Essentially, this requires aging of the loose mix at 116oC for warm mix asphalt and 135oC for hot mix asphalt for 2 hours prior to compaction. Long-term aging should follow AASHTO R30 for 5 days at 85oC. The compaction should follow AASHTO R35 for volumetric design and the gyrations should be adjusted to achieve the desired air void content.
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Table 10. Eight run combinations for DCT test with six test variables. PB order, run #
Specimen thickness 50±5 mm (T)
Crack opening displacement 1±0.05 mm/min (COD)
Test temperature PG low+10±0.5°C (t)
Location of holes 25±5 mm (LH)
Notch depth 62±3 mm (ND)
Air void 7±0.5% (AV)
1 55 mm 1.05 mm/min PG low+10.5°C 20 mm 59 mm 6.50% 2 45 mm 1.05 mm/min PG low+10.5°C 30 mm 65 mm 6.50% 3 45 mm 0.95 mm/min PG low+10.5°C 30 mm 59 mm 7.50% 4 55 mm 0.95 mm/min PG low+9.5°C 30 mm 65 mm 6.50% 5 45 mm 1.05 mm/min PG low+9.5°C 20 mm 65 mm 7.50% 6 55 mm 0.95 mm/min PG low+10.5°C 20 mm 65 mm 7.50% 7 55 mm 1.05 mm/min PG low+9.5°C 30 mm 59 mm 7.50% 8 45 mm 0.95 mm/min PG low+9.5°C 20 mm 59 mm 6.50%
Table 11. Eight run combinations for SCB-TP105 test with five test variables.
1 30 mm 0.0315 mm/min PG low+10.5°C 12 mm 6.50% 2 20 mm 0.0315 mm/min PG low+10.5°C 18 mm 7.50% 3 20 mm 0.0285 mm/min PG low+10.5°C 18 mm 6.50% 4 30 mm 0.0285 mm/min PG low+9.5°C 18 mm 7.50% 5 20 mm 0.0315 mm/min PG low+9.5°C 12 mm 7.50% 6 30 mm 0.0285 mm/min PG low+10.5°C 12 mm 7.50% 7 30 mm 0.0315 mm/min PG low+9.5°C 18 mm 6.50% 8 20 mm 0.0285 mm/min PG low+9.5°C 12 mm 6.50%
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Table 12. Eight run combinations for SCB-LTRC test with five test variables. PB order, run #
Specimen thickness 57±5mm (T)
Loading rate 0.5±0.025 mm/min (LR)
Test temperature 25±1°C (t)
Notch depth 25.4, 31.8, 38.1 ±3 mm (ND)
Air void 7±0.5% (AV)
1 62 mm 0.525 mm/min 26°C 22.4, 28.8, 35.1 6.50% 2 52 mm 0.525 mm/min 26°C 28.4, 34.8, 41.1 7.50% 3 52 mm 0.475 mm/min 26°C 28.4, 34.8, 41.1 6.50% 4 62 mm 0.475 mm/min 24°C 28.4, 34.8, 41.1 7.50% 5 52 mm 0.525 mm/min 24°C 22.4, 28.8, 35.1 7.50% 6 62 mm 0.475 mm/min 26°C 22.4, 28.8, 35.1 7.50% 7 62 mm 0.525 mm/min 24°C 28.4, 34.8, 41.1 6.50% 8 42 mm 0.475 mm/min 24°C 22.4, 28.8, 35.1 6.50%
Table 13. Eight run combinations for SCB-IL test with five test variables.
PB order, run #
Specimen thickness 50± 5 mm (T)
Loading rate 50±2.5 mm/min (LR)
Test temperature 25±0.5°C (t)
Notch depth 15 ±3 mm (ND)
Air void 7±0.5% (AV)
1 55 mm 52.5 mm/min 25.5°C 12 mm 6.50% 2 45 mm 52.5 mm/min 25.5°C 18 mm 7.50% 3 45 mm 47.5 mm/min 25.5°C 18 mm 6.50% 4 55 mm 47.5 mm/min 24.5°C 18 mm 7.50% 5 45 mm 52.5 mm/min 24.5°C 12 mm 7.50% 6 55 mm 47.5 mm/min 25.5°C 12 mm 7.50% 7 55 mm 52.5 mm/min 24.5°C 18 mm 6.50% 8 45 mm 47.5 mm/min 24.5°C 12 mm 6.50%
Table 14. Eight run combinations for IDT-Florida test with four test variables.
PB order, run #
Specimen thickness 50±5 mm (T)
Loading rate 50±2.5 mm/min(Lr)
Test temperature 10±1°C (t)
Air void 7±0.5% (AV)
Creep Time 1000±5% (sec)
Horizontal deformation low range 0.0025±5%
Horizontal deformation High range 0.019±5%
1 55 mm 52.5 mm/min 11°C 7.50% 1050 0.002488 0.018 2 45 mm 52.5 mm/min 11°C 6.50% 950 0.002513 0.018 3 45 mm 47.5 mm/min 11°C 7.50% 1050 0.002488 0.020 4 55 mm 47.5 mm/min 9°C 7.50% 1050 0.002513 0.018 5 45 mm 52.5 mm/min 9°C 7.50% 1050 0.002513 0.020 6 55 mm 47.5 mm/min 11°C 6.50% 950 0.002513 0.020 7 55 mm 52.5 mm/min 9°C 6.50% 950 0.002488 0.020 8 45 mm 47.5 mm/min 9°C 6.50% 950 0.002488 0.018
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Table 15. Eight run combinations for OT test with six test variables. PB order, run #
Specimen height 38±1 mm (H)
Crack opening displacement 0.635±0.013 (COD)
Test temperature 25±1ºC (t)
Specimen width 76±3 mm (W)
Loading period 10 sec±1 sec (LP)
Air void 7± 0.5% (AV)
1 39 mm 0.648 mm 26ºC 73 mm 9 sec 6.50% 2 37 mm 0.648 mm 26ºC 79 mm 11 sec 6.50% 3 37 mm 0.622 mm 26ºC 79 mm 9 sec 7.50% 4 39 mm 0.622 mm 24ºC 79 mm 11 sec 6.50% 5 37 mm 0.648 mm 24ºC 73 mm 11 sec 7.50% 6 39 mm 0.622 mm 26ºC 73 mm 11 sec 7.50% 7 39 mm 0.648 mm 24ºC 79 mm 9 sec 7.50% 8 37 mm 0.622 mm 24ºC 73 mm 9 sec 6.50%
Table 16. Eight run combinations for BBF test with six test variables.
PB order, run #
Specimen height 50±6 mm (H)
Test temperature 20±0.5°C (t)
Specimen length 380±6 mm (L)
Specimen width 63±6 mm (W)
Loading frequency 10 Hz±1 Hz (LF)
Air void 7±0.5% (AV)
1 56 mm 20.5°C 386 mm 57 mm 9 Hz 6.50% 2 44 mm 20.5°C 386 mm 69 mm 11 Hz 6.50% 3 44 mm 19.5°C 386 mm 69 mm 9 Hz 7.50% 4 56 mm 19.5°C 374 mm 69 mm 11 Hz 6.50% 5 44 mm 20.5°C 374 mm 57 mm 11 Hz 7.50% 6 56 mm 19.5°C 386 mm 57 mm 11 Hz 7.50% 7 56 mm 20.5°C 374 mm 69 mm 9 Hz 7.50% 8 44 mm 19.5°C 374 mm 57 mm 9 Hz 6.50%
Statistical Analysis of Ruggedness Testing Results
The purpose of the statistical analysis of ruggedness test is to determine whether the variable effects are significant. For the seven cracking tests evaluated, this is a two-step analysis process.
• Step 1—Estimate variable effects: The main variable effects are the differences between average responses at the high (+1) and the low (-1) levels. The average of test results (five replicates for the cracking tests with higher variability and three replicates for those cracking tests with less variability) will be calculated in each PB order run. “Ave +” in Table 6 for Variable A is the average of the average test results that Variable A is considered as high level (+1) in running the test. Subsequently, “Ave -” is the average of the average test results when the variable is considered as low level (-1). The main effect is the difference between Ave + and Ave – for each variable.
• Step 2—Statistical tests of variable effects: The variable effects can be determined using the Student’s t-test. The t-test statistic for a variable is the main effect divided by the standard error of effects (seffect). It is worth mentioning that the standard error
28
of effects is the same for all variables in an orthogonal and balanced design (see Table 6). If the calculated t-value for a variable is greater than the t-value corresponding to a 0.05 significance level, the variable is statistically significant at a level of 0.05.
= × (3-1)
where: N = number of runs in the design (N = 8). Rep = number of replicates of design (Rep=5 for those with five replicates; 3 for those with three replicates). sr = standard error of the test results.
More detailed statistical analysis and examples can be found in ASTM E1169-14: Standard Practice for Conducting Ruggedness Tests.
Possible Revision of Cracking Test Methods
With the statistical variable effects calculated, whether or not the test method is rugged with the variables tested can be determined. Any cracking test can be considered as rugged with regard to the variables tested if no effects are identified as statistically or practically significant, provided the experiment was carried out in the proper way. On the other hand, if some effects are practically or statistically significant, then the test method should be modified or a new specification regarding the acceptable range for identified variables should be provided. Note that sometimes variable effects are statistically significant, but it is not practical to make changes. For such cases, no modification to the test method is necessary. If needed, another round of ruggedness tests may have to be conducted. After each cracking test is considered as rugged, the precision of each test can be determined through an ILS that is discussed in the next section.
Proposed Schedule and Budget
Ruggedness testing is to be done in one laboratory for each cracking test. The laboratory must be very well versed in the test procedure, so it will take multiple laboratories to establish parameters for all of the cracking tests, although one contractor needs to oversee the entire effort. This contractor must be responsible for specimen fabrications to distribute to the other participating laboratories. The budget and schedule are presented in Tables 17 and 18, respectively. Since the performing laboratories are to be proficient in the test method, there is no provision for buying equipment or for testing familiarization. The cost basis for the ruggedness testing budget is time and effort (person-months) at a rate of $140,000 per person/year. This rate is based on a review of recent projects and allows for fringe benefits and approximately 45 percent in indirect costs. The total cost for ruggedness testing is estimated to be $285,575.
Travel 2 Panel Meetings/4 Site Visits @ $1500 ea. 9000 Misc. Equipment Repair, Supplies, Etc. 7000 Total 285,575
Table 18. Schedule for ruggedness testing.
Month 1 2 3 4 5 6 7 8 9 10 11 12 Review/Revise
Experiment Design
Laboratory Preparation
Prepare and Ship Samples
Conduct Experiment
Analyze Results
Prepare, Review,
Revise Report
INTER-LABORATORY STUDY PLAN TO DEFINE PRECISION OF THE SELECTED CRACKING TESTS
Laboratory test results are impacted by many different factors, such as operator, test equipment, calibration of the equipment, and environment (temperature, humidity, etc.). These factors vary with changing laboratories. The variability of the test results obtained by different operators or with different equipment will usually be greater than the variability of between-test results from a single operator with the same equipment. To quantify the variability within a single laboratory and between different laboratories, an ILS is warranted.
The main purpose of performing an ILS is to determine the precision of a test method in terms of its repeatability and reproducibility. Repeatability focuses on the variability between independent test results obtained within a single laboratory in the shortest practical period of time by a single operator with a specific set of test apparatus using test specimens taken at random from a single quantity of homogeneous material prepared for the ILS. Reproducibility deals with the variability between single test results obtained in different laboratories, each of which has applied the test method to test specimens taken at random from a single quantity of homogeneous material prepared for the ILS.
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The ILS plan being described below was developed following ASTM E 691-14: Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method. Specifically, the proposed ILS plan includes six items listed below:
• Identification of laboratories for ILS. • Proposal of asphalt mixtures for ILS. • ILS experimental design for each cracking test. • Execution of ILS for each cracking test. • Statistical analysis of ILS results. • Development of a precision statement for each cracking test. For the preparation of test specimens in the lab to siumulate plant production, the
AASHTO R30 procedure as modified in NCHRP 9-52 is recommended. Essentially, this requires aging of the loose mix at 116oC for warm mix asphalt and 135oC for hot mix asphalt for 2 hours prior to compaction. Long-term aging should follow AASHTO R30 for 5 days at 85oC. The compaction should follow AASHTO R35 for volumetric design and the gyrations should be adjusted to achieve the desired air void content.
Identified Laboratories for ILS
ASTM E691-14 requires that the ILS begin with eight or more laboratories in order to allow for attrition because the final precision statement of a test method should be developed based on acceptable test results from a minimum of six laboratories. It is highly recommended that any laboratory qualified to run the test routinely be included in the ILS. ASTM E691-14 defines qualified as “proper laboratory facilities and testing equipment, competent operators, familiarity with the test method, a reputation for reliable testing work, and sufficient time and interest to do a good job.” In those laboratories that meet all the other requirements but have had insufficient experience with the test method, the operators should be given an opportunity to familiarize themselves with the test method and practice its application before the ILS starts. Note that many ILS efforts have turned out to be unsuccessful due to a lack of familiarization. The familiarization of the test method is extremely important to the success of the ILS for each cracking test. Based on the requirements of the minimum number and qualification of the ILS, the research team listed all potential ILS laboratories in Table 19, for ILS of each cracking test. These laboratories have been contacted to assess their capabilities.
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Table 19. Proposed laboratories for ILS of cracking tests. Test method Identified laboratories for ILS DCT 1. AAT 2. AI 3. FHWA-TF 4. Heritage Research 5. ICT-UIUC 6. MnDOT
The number and type of materials required for an ILS will depend on many factors (such as providing a variety of asphalt mix types, the difficulty and expense involved in preparing and distributing samples, the difficulty and length of the time required for performing the test, etc.). ASTM E691-14 suggests that an ILS of a test method should include at least three materials (asphalt mixtures), and for development of broadly applicable precision statements, six or more materials should be included in the study. Each material in an ILS study should be homogeneous prior to molding the specimens. Considering the types of asphalt mixtures often used, the research team recommends three typical Superpave fine DGA mixes for the ILS. Either PG58-28 or PG70-22 can be used for the ILS. It is worth noting that it is important to know and report the true binder grade. However, the temperature for running a ruggedness test or ILS for tests like DCT or SCB_TP105 would be selected by the PG standard grade rather than the true grade. Based on the pilot run results (to be discussed later), some materials may be changed.
• Virgin DGA mix with a NMAS of 19 mm aggregates. • Virgin DGA mix with a NMAS of 9.5 mm aggregates. • DGA mix with high binder replacement from either RAP or post-consumer RAS.
ILS Experimental Design for Each Cracking Test
In the design of an ILS, an acceptable number of specimen replicates for each material must be specified in order to obtain a good estimation of the repeatability (generally the repeatability standard deviation). It is generally sound to limit the number of replicates for each material in each laboratory to a small number. When considering the repeatability of asphalt mixture cracking tests, it is recommended that the cracking tests with a monotonic load (i.e., DCT, SCB, and IDT) use three replicates, and those cracking tests with a repeated load (i.e., OT and BBF) use five replicates. Table 20 shows the number of required samples for ILS for each cracking test. Note that all samples are prepared in a central location. The central location will mold and ship the samples to the selected laboratory to conduct the ILS. Additionally, the
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quantity of material prepared for the testing should be 50 percent more than is actually needed. This quantity means that if three test results (three types of asphalt mixtures) are needed for eight laboratories, and each test needs three specimens for low variability (except the SCB-LTRC, which requires nine), then 108 specimens should be provided. For high variability tests, five specimens for each material are needed. Table 21 shows the raw materials needed for ILS for each cracking test. The total for all the tests is 1116 samples that will need to be prepared, shipped, and tested.
Additionally, the ILS coordinator needs to communicate to each ILS laboratory the importance of following correct cracking test protocols and address any potential errors in the test procedure that may arise.
Table 20. Number of specimens for ILS.
Test method Test variability # Labs # Material # of
No. of Specimen Mixture (kg) Aggregate (kg) Binder (Liter)
DCT 108 542 525 27.3
SCB-IL 108 211 205 10.6
SCB-TP105 108 136 131 6.9
SCB-LTRC 108 271 263 13.7
OT 180 902 875 45
BBF 180 1531 1485 77
IDT-Florida 108 542 525 27.3
Execution of ILS for Each Cracking Test
As noted previously, the familiarization of the test method is extremely important to the success of the ILS for each cracking test. In order to get more reliable results, the ILS for each cracking test should be run in two phases:
• Pilot Run: ASTM E691-14 suggests conducting a pilot run with one or two materials in order to ascertain that the test protocol and ILS procedures are clear for each ILS laboratory. Basically, the pilot run, serving as a familiarization process, will help the laboratories get familiar with the ILS procedure. The pilot run results will be an indication of how well each laboratory will perform in terms of promptness and following the protocol. As a result, the laboratories with poor performance can be identified and assisted with corrective actions.
• Full-Scale Run: A full-scale run includes the preparation of the replicate samples. The total number of samples should be 50 percent more than needed in order to account for breakage or other issues associated with handling, shipping, and so forth. All these samples should be prepared at a central location, randomly mixed, and then shipped to the ILS laboratories. Each ILS laboratory should perform each cracking test according to the instructions of the ILS coordinator. The ILS coordinator should
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contact each ILS laboratory to check the specimen conditions and test progress. The completed test results should be evaluated by the ILS coordinator immediately upon receipt so that any unusual test results can be detected. Retesting with replacement specimens could be performed, if necessary.
Statistical Analysis of ILS Results
The statistical analysis of ILS test results has three purposes: (a) determining whether the collected data are consistent enough to form a basis for a test method precision statement, (b) investigating and acting on inconsistent data, and (c) establishing a precision statement. Essentially, the statistical analysis is simply a one-way analysis of variance (within and between laboratories) that is done separately for each material. First, it is necessary to examine the consistency of the data. Outliers are identified through a consistency analysis that considers two consistency statistics: h-value and k-value. The “h” consistency statistic is “an indicator of how one laboratory’s cell average, for a particular material, compares with the average of the other laboratories,” as stated in ASTM E691-14: : Standard Practice for Conducting an Interlaboratory Study to Determine the Precision of a Test Method. Similarly, the “k” consistency statistic is “an indicator of how one laboratory’s within-laboratory variability, under repeatability conditions on a particular material, compares with all of the laboratories combined. Values of ‘k’ larger than 1 indicate greater within-laboratory variability than the average for all laboratories.” Critical values are often chosen at a significance level of 0.5 percent. If outliers are identified, an investigation should be conducted to locate the source of errors—clerical, sampling, or procedural. If the investigation discovers no error, the unusual data should be kept, and the precision statistics should be published inclusive of those data. Otherwise, these outliers should be excluded from the statistics for a precision statement.
The fundamental precision statistics of the ILS on any test method are the repeatability standard deviation and the reproducibility standard deviation. A brief discussion on these two key statistics and related statistical concepts are described below:
• Repeatability standard deviation within laboratories, sr, is calculated by the following equation:
= ∑ ⁄ (3-2)
where: = the repeatability standard deviation. s = the cell standard deviation defined in Equation 3-3. p = the number of laboratories in the ILS.
= ∑ ( − ) ( − 1)⁄ (3-3)
where: xx = the average of the test results in one cell. x = the individual test results in one cell. n = the number of test results per cell.
• Reproducibility standard deviation between laboratories, sR, is calculated by the following equation:
= ( ) + ( ) ( − 1)/ (3-4)
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where: sx = the standard deviation of the cell average calculated in Equation 3-5. = the repeatability standard deviation.
n = the number of test results per cell.
= ∑ ( ) (3-5)
where: = the average of the cell averages for one material. = the individual cell averages. p = the number of laboratories in the ILS.
Development of Precision Statement for Cracking Tests
As discussed previously, the seven cracking tests for characterizing the cracking resistance of asphalt mixes are not in the stage of practical applications in routine mix design. It is critical to evaluate the test procedures by providing a precision statement through ILS. With a statistical analysis, the precision statement for each cracking test can be presented in the format of Table 22, as recommended by ASTM E691-14. An official precision statement should be prepared in accordance with ASTM E177: Use of the Terms Precision and Bias in ASTM Test Methods. The statement of precision and bias for each cracking test should at least include these specified elements:
• A brief description of the ILS procedure. • Materials (mixtures) used. • Number of laboratories. • Number of test results per laboratory per material. • Data analysis. • A description about any deviation from test method. • Result of precision statistics.
Table 22. Precision statistics sample (reference: ASTM E691). Material r R
A (DGA-19 mm) B (DGA-9.5 mm)
C (DGA-high binder replacement) Note: = average of cell averages. = Standard deviation of cell averages. = repeatability standard deviation. = reproducibility standard deviation. r = repeatability limit, r=2.8Sr. R = reproducibility limit, R=2.8SR.
Proposed Schedule and Budget
The ILS should be performed by one contractor with subcontracts or cooperative agreements with other participating laboratories. Frequently, ILSs are performed among laboratories on a volunteer basis, but it is assumed for this discussion that there will be some
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reimbursement for time and effort at a rate of $500 per test. Preparation of all samples will take place in one laboratory (assumed to be the prime contractor) and the ILS testing is to be done in eight laboratories (subcontractors) for each cracking test. There will be two rounds of testing: one for a pilot run and one for the full-scale run. It is assumed here that the pilot run will be conducted with one material, and the full-scale will be done with three materials. For the pilot run, it is assumed that six replicate tests will be done by each laboratory to account for potential problems in specimen handling and preparation or testing. This will mean a total of 496 specimens for pilot testing. For the full-scale testing, there will be three samples of three materials for low variability tests and five samples of three materials for high variability tests. The SCB-LTRC will require three notch depths on three samples for one result. This will make the total number of samples 1116. The time allotted for the ILS is 18 months for the tasks presented inTable 23 and Table 24.
This chapter discussed ruggedness testing in order to evaluate and refine seven selected tests for thermal, reflection, bottom-up fatigue, and top-down cracking. The main purpose of performing a ruggedness test is to identify those test variables that significantly influence the cracking resistance measurements of each specific cracking test method. Detailed experimental designs, schedule and budget for conducting ruggedness tests are proposed in order to estimate how closely these factors need to be controlled. If effects of some test variables are practically and statistically significant, the test method should be modified, and a new specification regarding the acceptable range for identified variables should be developed. Following that, an ILS needs to be performed to determine the precision of a test method in terms of its repeatability and reproducibility. A detailed ILS experiment design with specific steps and the schedule and budget were provided to conduct the ILS and develop the precision statement for each cracking test. After the ruggedness and ILS testing, the next step is to validate the cracking tests and develop cracking criteria with field test sections, which is described in the next chapter.
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CHAPTER 4. EXPERIMENTAL DESIGN FOR VALIDATING THE SELECTED CRACKING TESTS
INTRODUCTION
A total of seven cracking tests have been selected for use in asphalt mix design to address the cracking problems discussed in Chapter 2. Chapter 3 developed a plan to refine these laboratory tests through the ruggedness and the ILS testing. This chapter focuses on the development of an experimental design for validating these seven cracking tests and establishing pass/fail criteria. First, this chapter describes the D-optimal experiment design concept, followed by specific experimental designs for thermal, reflection, bottom-up fatigue, and top-down cracking, respectively. The purpose of the experimental designs is to establish and validate criterion for each cracking test. Additionally, a forensic study plan is proposed to address premature pavement failures.
It is important to understand that the objective of the experiment designs presented in this chapter is to validate the cracking tests associated with the cracking types, not to study the mechanisms of the different types of cracking. The experiment designs are configured to provide a wide range of cracking performance so that the validation may be done on sections likely to crack as well as sections less likely to crack. In such way, the cracking tests can be validated for materials and conditions intended to show poor cracking performance and against those that will perform well. This will help to guard against the occurrence of false positive results.
D-OPTIMAL EXPERIMENTAL DESIGN
The goal of statistical experimental design is to obtain valid data in order to achieve research objectives as efficiently as possible (i.e., by using as few test sections as possible for a given degree of precision). There are three options that can be considered for developing an experimental design: full factorial design, fractional factorial design, and D-optimal design. Using the full factorial design for field test sections is unrealistic for the experiments being considered in this project. The D-optimal designs and the fractional factorial designs are useful when the number of runs in the full factorial design (consisting of all possible factor-level combinations) is higher than the study can afford. They both choose a subset of runs from a full factorial design. Some fractional factorial designs can still be D-optimal designs although they are chosen through a different mechanism (e.g., the one-half fraction of the 2k design, where k is the number of factors) rather than optimizing a statistical criterion. There are, however, many cases for which the tabulated classical fractional factorial designs do not even exist (most of the tabulated classical fractional factorial designs are only available in designs where each factor has two levels). Also, the practical constraints in the experiment may prevent the use of classical tabulated designs. For example, the maximum number of runs or test sections available may be too small to be accommodated by classical designs; in addition, some factor-level combinations are impossible to run. It needs to be emphasized that in such cases, selecting a subset of the full factorial designs arbitrarily will not lead to a good design. The factor effects estimated by such an arbitrary design are likely to have poor statistical properties. There is also no guarantee that all important factor effects can be estimated by that design.
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A D-optimal design is a computer-generated statistical experimental design that chooses the best subset of all possible factor-level combinations (e.g., binder/aggregate combinations) within the constraints of the experiment (e.g., limited number of test sections) by maximizing the determinant of the information matrix X′X, where X is the design matrix. It can address the limitations of the traditional designs while enabling the estimation of all important effects (e.g., main effects and two-way interaction effects) with a considerably smaller number of test sections than required by traditional designs. For example, if there are five factors, each having four levels, the number of factor-level combinations for a full factorial design exceeds 1000, which is prohibitive. However, only 16 test sections for the D-optimal design are needed to estimate all of the main effects. The D-optimal designs minimize the standard errors of the parameter estimates for a pre-specified model so that the factor effects of interest can be estimated more precisely than by using any other designs with the same number of test sections. Although more test sections lead to a more precise estimation of the factor effects, the maximum number of test sections that can be afforded will depend on the resources.
An example is provided below to illustrate the D-optimal design. Suppose that there are three factors of interest, X1, X2, and X3, in the experiment, and researchers are interested in estimating all main effects under a linear model. The levels considered are:
• X1: 2 levels (L1, L2). • X2: 3 levels (L1, L2, L3). • X3: 4 levels (L1, L2, L3, L4). A full factorial design with the above three factors consists of 24 factor-level
combinations, given in Table 25, and it is the candidate set of runs for D-optimal designs. D-optimal designs select a subset of runs from Table 25Table , maximizing the D-efficiency (i.e., minimizing the generalized variance of main effects’ estimates) for a pre-specified number of runs.
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Table 25. Candidate set of runs for factors X1, X2, X3. Run X1 X2 X3
Assume that due to resource limitations, only up to 20 runs can be afforded, and a smaller
number of runs are preferred. The minimum number of runs needed to estimate all three main effects is seven in this case, as listed in Table 26. The minimum number of runs needed to estimate all three main effects and two-way interactions among them is 18, and those are listed in Table 27Table .
Table 26. D-Optimal design with seven runs for main effects.
Note: Seven is the minimum number of runs required to estimate all three main effects. Because this design is saturated, there are no degrees of freedom for error (i.e., there will be no error term for testing). If testing of parameters in addition to estimation is needed, at least one more run is needed to estimate an error term.
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Table 27. D-Optimal design with 18 runs for main effects and two-way interactions. Run X1 X2 X3
Note: Eighteen is the minimum number of runs required to estimate all three main effects and two-way interaction effects among them. Because this design is saturated, there are no degrees of freedom for error (i.e., there will be no error term for testing). If testing of parameters in addition to estimation is needed, at least one more run is needed to estimate an error term.
In summary, D-optimal designs can easily accommodate physical constraints in the
experiment (such as the maximum number of runs that can be afforded or infeasible factor-level combinations), even in cases where classical designs cannot, and are optimal (in the sense that standard errors of the resulting coefficient estimates of the fitted model are minimized) among all the designs that are subject to the same constraints. The research team developed a series of field experimental designs for validating cracking tests by utilizing D-optimal designs to accommodate the constraints in resources while ensuring that none of the important factor effects are confounded.
The tests selected for validation will be judged according to their sensitivities to the important effects. For instance, if the test can detect differences in mixture properties such as asphalt content, void content, VMA, binder type, etc. it will be considered a good candidate for adoption. On the other hand if the mixture test cannot distinguish between brittle (poor performing) mixtures and ductile (crack resistant) mixtures it will not be a good candidate. The D-optimal experiemental designs will help identify tests which can make these distinctions while accounting for the effects of other parameters that might affect the performance. These experimental designs are discussed below.
EXPERIMENTAL DESIGN FOR VALIDATING THERMAL CRACKING TESTS
This section briefly discusses thermal cracking mechanisms and associated influential factors before presenting the experimental design for validating thermal cracking tests, including both a field experimental design and a laboratory testing plan. The research team also reviewed and documented potential field test sections for validating thermal cracking tests in this section as well.
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Thermal Cracking Mechanism and Influential Factors
Thermal cracking is tied to the climatic conditions of either a slow temperature differential developed seasonally along with contraction and expansion cycles in very cold climates or due to large diurnal temperature differentials in arid climates with a fast temperature differential over a short period of time. Thermal cracking occurs when thermal stress in the material exceeds its tensile strength. Traffic volume often accelerates the number of cracks and the rate of cracking deterioration. Many factors influence thermal cracking development. The research team, with input from the experts at the Cracking Test Workshop, recommended the key factors and their variation levels for thermal cracking experimental design, as listed in Table 28.
In Table 28, the climate factors were narrowed to cold and dry/ hot. Cold climates are those that experience a seasonal drop to below freezing and maintain a cold weather pattern for several months. The dry/ hot regions represent those areas in the United States that experience significant temperature movement between hot and cold. These large diurnal temperature ranges correspond with thermal fatigue. For any given mix, the colder the pavement surface temperature, the greater the potential for thermal cracking. Pavement surface temperature is highly related to ambient temperature. Many low-temperature cracks occur when the temperature decreases to a level below the glass transition temperature and remains at that level for a period of time.
Another climate component that should be captured is the cooling rate that generates thermal cracking when daily temperature differentials are the highest. By including mix type as a key factor, the study will capture the effect of the temperature-stiffness relationship of the asphalt binder. Both regular DGA mixtures and SMA mixes are considered. It is worth noting that fine DGA mixtures are desirable for all experimental designs for thermal, reflection, fatigue and top-down cracking, but coarse DGA mixtures are acceptable as well. For DGA mixtures, both regular binder and a special binder with a one-grade lower performance grade on the low end are recommended to differentiate the mix performance. The structure variable was limited to asphalt concrete (AC) layer thickness. The substrate is not considered because thermal cracking is believed to originate at the top of the asphalt layer. By using AC layer thickness, the follow-up project should be able to compare the performance of thick vs. thin asphalt layers with regard to thermal cracking. In general, the consensus is that there is a lower occurrence of thermal cracking in thicker pavements. This was confirmed at the Ste. Anne Test Road, where an increase of asphalt layer thickness from 4 to 10 inches resulted in one-half the cracking frequency when all other variables were the same (Deme and Young 1987), probably because the greater cross section of the pavement reduced the thermal stresses. By including this as a key factor in the study, the effect of pavement thickness on thermal cracking can be better understood. Traffic is included as a key factor to assist in validating the assumption that thermal cracking is primarily environmentally driven.
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Table 28. Field experimental design factors identified for thermal cracking. Key factor Variation level
Climate 1) Cold regions—areas with few freeze-thaw (F-T) cycles that enter prolonged cold seasons.
2) Diurnal cycling regions—areas that can experience large daily temperature fluctuations in dry and hot regions. These large diurnal temperature ranges correspond with thermal fatigue.
Mix type 1) DGA mixture with regular PG binder (DGA-Regular PG). 2) DGA mixture with the binder having the same high end PG but one grade lower in terms of PG low end (DGA_PG-Lower). 3) SMA mixtures.
Pavement structure
1) Thick AC: > 150 mm (6 inches). 2) Thin AC: ≤ 150 mm (6 inches).
Field Experimental Design for Validating Thermal Cracking Tests
The objective of the experimental design is to assess the effects of study factors on thermal cracking development in the field and then to validate the capability of the selected thermal cracking tests for differentiating the different performance of these experimental test sections. More test sections lead to a more precise estimation of the factor effects, as described in Table 28. However, the maximum number of test sections that can be afforded depends on the resources available. The research team proposed two levels of experimental design:
• Design 1: the most affordable design that simply focuses on estimating all main effects.
• Design 2: desirable design that estimates both all main effects and two-way interaction effects.
The two levels of experimental design provide flexibility for the follow-up project to make decisions based upon the available funding.
Table 29 and Table 30 present the D-optimal Designs 1 and 2 for thermal cracking, respectively. Note that the number of test sections was kept as small as possible in developing each experimental design while still ensuring that all important factor effects can be estimated.
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Table 29. D-optimal experimental Design 1 for thermal cracking: 6 test sections. Test section Climate Mixture Structure Traffic
1 Cold DGA_Regular PG Thick AC High
2 Cold SMA Thin AC High
3 Cold DGA_PG-Lower Thin AC Low
4 Diurnal cycling regions DGA_PG-Lower Thick AC High
5 Diurnal cycling regions SMA Thick AC Low
6 Diurnal cycling regions DGA_Regular PG Thin AC Low Note: Six is the minimum number of test sections required to estimate all four main effects in this case.
Table 30. D-optimal experimental Design 2 for thermal cracking: 15 test sections. Test section Climate Mixture Structure Traffic
1 Cold DGA_PG-Lower Thick AC High
2 Cold DGA_Regular PG Thick AC Low
3 Cold DGA_PG-Lower Thick AC Low
4 Cold SMA Thick AC Low
5 Cold DGA_Regular PG Thin AC High
6 Cold DGA_PG-Lower Thin AC High
7 Cold SMA Thin AC High
8 Cold DGA_Regular PG Thin AC Low
9 Diurnal cycling regions DGA_Regular PG Thick AC High
10 Diurnal cycling regions DGA_PG-Lower Thick AC High
11 Diurnal cycling regions SMA Thick AC High
12 Diurnal cycling regions DGA_Regular PG Thick AC Low
13 Diurnal cycling regions DGA_Regular PG Thin AC High
14 Diurnal cycling regions DGA_PG-Lower Thin AC Low
15 Diurnal cycling regions SMA Thin AC Low
Identification of Field Test Sections for Validating Thermal Cracking Tests
Much of the identification of test sections revolves around the climate. For thermal cracking, the traditional climate regions within LTPP provide general discrimination points between hot and cold. In cold regions, low-temperature cracking will occur when the temperature becomes cold enough to induce a contraction in the pavement that overcomes the tensile strength. Figure 8 is a recreation of the LTPP climate regions over a Mapquest®
screenshot with projected latitude and longitude lines. The figure also consists of grid points at the latitude and longitude intersections. To avoid selecting locations near the boundaries of the
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climate lines, locations for thermal cracking in cold regions will be north of 40°N in either the wet or dry LTPP zones. This will lead to the selection of pavements that experience a more monotonic decline in temperature and limit any undesired fluctuations that might occur in sections between the freeze line used by LTPP and 40°N. Additionally, the cold climate will end at 120°W to avoid the insulated area along the Pacific Coast.
Figure 8. LTPP climate zones with projected latitude and longitude.
The experimental design requires field test sections in the diurnal cycling regions. When identifying locations throughout the United States that experience significant temperature swings within a single day, the research team turned to the National Weather Service (NWS) historical climatic data. Figure 9 shows diurnal temperature ranges in the whole United States in March. The research team chose March for this illustration because it historically consists of the most days with large temperature differentials. Figure 9 also indicates that the southern portion of the High Plains, which is dry and hot, has the largest daily temperature differential. This differential is often characterized by temperatures falling 22 to 28°C (40 to 50°F) within a single hour. Cracking might occur in the fall or spring in a climate that experiences large daily temperature swings, thereby creating thermal fatigue. In summary, Figure 10 shows the desired locations for diurnal cycling/dry/hot sections and cold sections.
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Figure 9. Diurnal temperature ranges for March (NWS 2015).
Figure 10. Desired locations for field experimental test sections for thermal cracking.
Structure and mixture should be considered when selecting test locations for thermal cracking. Ideally, pavement sections used for the study will be new and uncracked. Evaluating overlays over cracked subsurfaces creates the potential for mistakenly identifying reflection cracking for thermal cracking. Additionally, the study should evaluate the difference between thick and thin HMA sections. It is thought that thermal stresses are reduced with thicker pavement sections. Presently, the point of division between thick and thin is 150 mm (6 inches). For thermal cracking, three different types of mixes are recommended: (a) DGA mixture with regular PG binder (DGA-Regular PG), (b) DGA mixture with the binder having the same high end PG but one grade lower in terms of PG low end (DGA_PG-Lower), and (c) SMA mixtures. Traffic is also considered here. It is important to define high traffic and low traffic in order to distinguish between field selections. This delineation is applicable to all cracking types and
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should be noted for all selected sections. The definitions for high and low traffic are based on ESALs and are as follows:
Available Test Sections for Validating Thermal Cracking Tests
In the last two decades, a variety of field test sections have been constructed, tested, and monitored for different purposes. Field sections, depending on traffic loading, can be categorized into APT sections, full-scale experiment test tracks, test roads, and in-service pavements. Table 31 summarizes the features and strengths/weaknesses of each type of test section. While results may be rapidly obtained and conditions are well defined in APT test sections, they are subject to slow-moving traffic loading and are problematic with respect to aging and moisture effects. Full-scale test tracks allow for realistic loading on full-sized pavement structures, but their service lives only span one to four years, making the aging limited. In contrast, in-service pavements can well consider natural aging and moisture effects but take considerably longer for results and may present difficulties in determining traffic loading accurately. However, some in-service pavements are equipped with weigh-in-motion (WIM) stations so that well-documented traffic loading is available. All four types of test sections, depending on cracking types, should be considered for validating cracking tests.
Table 31. Four types of field experimental test facilities in United States.
MnRoad LTPP-GPS/SPS sections and state DOT sections
Traffic load Known traffic; well controlled traffic; often overloaded
Known traffic; WesTrack: 4 units of tractor/ trailer—triple combinations NCAT track: four fully loaded trucks
Known traffic; Real traffic
Unknown traffic (most of time); real traffic; many SPS sections equipped with WIMs
Traffic speed Slow; around 5–12 mph
Around 40–45 mph Real traffic and real speed (around 60 mph)
Real traffic and real speed (around 60 mph)
Test period Several months 1–3 years 4 years Several years to more than 15 years
Environment Temperature is often controlled
Natural weather Natural weather Natural weather
Aging effect Artificial aging can be considered, but not natural aging
Impact of short-term aging on performance is considered.
Impact of short/medium-term aging is considered
Impact of long-term aging is addressed
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APT facilities generally are not appropriate for validating thermal cracking due to the required pavement section length and the secondary effect of traffic loading. Meanwhile, the NCAT test track is located in neither a cold region nor a diurnal cycling/dry/hot region. Thus, the only two potentials are MnRoad and LTPP/SPS sections. With LTPP sections, a data search can be focused on climate regions meeting the cracking requirements. This may be done on both active and out-of-study SPS sections where the Materials Reference Library (MRL) inventory indicates if a binder is available. Expanding the search to sections that are no longer active relies on the distress data available from LTPP and material storage over the previous two decades. Nonetheless, if the binder is listed as available in the MRL inventory, it may be possible to have exact construction materials on which to perform tests. A limiting factor with using older LTPP sections is that matching the mixture parameters in the experimental design becomes more difficult. Meeting mixture specifications is difficult when relying on LTPP sections even when the proposed test sections meet other parameters in the experiment design. All potential test sections should be further verified, and necessary testing should be performed prior to inclusion in the follow-up project.
Figure 10 provides a starting point for locations of potential thermal cracking section. This figure indicates that New Mexico is likely the best state to find diurnal cycling regions for thermal cracking. The LTPP program recently completed construction on five SPS10 sections along IH 40. These sections, with a thick asphalt layer and high traffic, are prime candidates for thermal cracking in a diurnal cycling region. While New Mexico is well suited for the diurnal cycling region, the facility at MnRoad is ideal for thermal cracking in a cold climate. As of this report, MnRoad has raw materials available from sections constructed in 2008. In addition to material and distress data from MnRoad 2008 sections, MnDOT is preparing for reconstruction of several new sections. These sections will offer the follow-up project access to additional test sections for top-down, thermal, and possibly bottom-up fatigue cracking. Table 32 is an extension of Table 29 with possible sections included. The potential sections meet only the climate and structure parameters. For example, MnRoads Cells 16–21 are not SMA, yet they are thin AC sections in a cold climate. The follow-up project will have to evaluate these types of trade-offs to help determine parameter interaction. Information sheets have been developed for possible test sections. Figure 11 represents an information sheet for a potential thermal cracking section in a cold climate, while Figure 12 represents one for a diurnal cycling climate. Additional information sheets can be found in Appendix C.
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Table 32. D-optimal experimental Design 1 for thermal cracking with possible test sections. Test section Climate Mixture Structure Traffic Sections
1 Cold DGA_Regular PG Thick AC High MnRoad Cell 15, LTPP 18-A901
2 Cold SMA Thin AC High MnRoad Cell 16, MnRoad Cell 17, MnRoad Cell 18, MnRoad Cell 20, MnRoad Cell 21
Potential Cracking Test Location: Thermal Cracking—Cold Climate—Thick HMA—High Traffic Top-down Cracking—Hard Freeze/Low Solar Gain—High Volume/High Speed Material Notes: Binder is PG58-34 with 20% RAP
Figure 11. Information sheet for potential thermal cracking in cold climate section.
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LTPP Section: 35-0501 State: New Mexico (35) Roadway and Direction: IH 10 EB Experiment No.: SPS-5
Date of Surface Construction: Sept. 1996 Status: Out-of-study
Pavement Cross Section:
Layer No. Type Thickness(inche
s) Description Date 1 SG – Coarse-grained, clayey sand w/ gravel - 2 Base 12 Soil-Aggregate Mix (mainly coarse grained) Sept. 1995 3 AC Layer 3.7 Dense-Graded HMA Sept. 1995 4 AC Layer 2.2 Recycled AC, cold lay, mixed in place Sept. 1995 5 AC Layer 0.3 Dense-Graded HMA Sept. 1995 6 AC Layer – Chip Seal Sept. 1995 7 AC Layer 2 Dense-Graded HMA Sept. 1996 8 AC Layer 1 Dense-Graded HMA Sept. 1996 9 AC Layer 1.3 Open Graded HMA Sept. 1996
Tot. AC Thickness = 10.5 SPS-5, out of study in March 2003 - Traffic Data: Number of Years with AADTT Data: 19 Latest AADTT: 9593 Year: 1989 Latest KESAL: 318 Year: 2000 Performance from last surface date:
1 7.1 33.2 132.7 8 April 2002 Patch Pot Holes 5/6/2002 0.6 11.8 154.3 16
Sept. 2002 Crack Seal 11/3/2002 1.6 101 134.4 31 May 2003 Patch Pot Holes 5/19/2003 6.9 1.5 236.4 24 Potential Cracking Test Location: Thermal Cracking—Dry, Hot Climate—Thick AC—High Traffic Top-Down Cracking—High Solar Gain—No Freeze—High Volume, High Speed Material Notes: Binder sampled 9/7/1996, AC Cement Virgin. AC Cement for RAP also sampled on 9/7/1996. Figure 12. Information sheet for potential thermal cracking in diurnal cycling climate test
section.
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From the information sheets, it is clear that some sections can pertain to multiple cracking types. For example, the potential sections illustrated in Figure 11 and Figure 12 are also potential sections for top-down cracking. It is worth mentioning that SPS-10 is a project that can offer a number of potential test sections. The purpose of SPS-10 is to capture short- and long-term performance of warm-mix asphalt (WMA). SPS-10 will add sections to long-term pavement performance (LTPP) since the work is to be performed on locations not previously included. Work will consist of WMA overlays over flexible pavements. In addition to the test sections in New Mexico, construction was recently completed on sections near Wichita Falls, Texas. Wichita Falls is near the temperature cycling region and these sections might be possibilities for either thermal cracking or top-down cracking. Information on these sections has not yet been populated into LTPP InfoPave but should be available for the follow-up project. It is anticipated that sections will be completed in the near future in Oklahoma. Much like Wichita Falls, the sites in Oklahoma present a possibility for thermal cracking in a diurnal region. Other anticipated SPS-10 locations are listed below, along with how they correspond with thermal cracking.
• Arizona—possible diurnal temperature cycling region for thermal cracking. One location is along IH 40 in the northern part of Arizona and another location is on IH 10 in the southern part of Arizona.
• Southern California—possible diurnal temperature cycling region for thermal cracking.
• Western Nevada—depending on the location (i.e., high altitude), could serve as a cold temperature thermal cracking. Also, if this location sits in a valley, it could experience diurnal characteristics. Either way, this is a potential thermal cracking section.
• North Central Oregon—more than likely will fall too close to the Pacific Coast to be considered a low-temperature possibility, but should be verified within the follow-up project.
• Eastern Washington—candidate for low-temperature cracking. • Manitoba, Canada—candidate for low-temperature cracking. • Ontario, Canada—candidate for low-temperature cracking. • Southern Georgia—not applicable for thermal cracking. • Northern Florida—not applicable for thermal cracking. Last, construction sections from NCHRP 9-47A, Field Performance of Warm Mix
Technologies, offer the potential for thermal cracking sections. Raw materials from NCHRP 9-47A are currently available and are being stored by the project investigators at NCAT. There is no guarantee that these raw materials will remain available for the follow-up project. The locations within NCHRP 9-47A and their relation to thermal cracking are listed below:
• US 12 in Walla Walla, Washington: Located in southeastern Washington near the Oregon border, this site is an option for low-temperature thermal cracking.
• IH 66 in near Centreville, Virginia: Located near the Maryland border, this site does not provide ideal geography to be included in a thermal cracking study.
• County Road 513 near Rapid River, Michigan: Located on the Upper Peninsula, this site is a prime possibility for cold-weather thermal cracking.
• Montana County Route 322 in Fallon County, Montana: Located in eastern Montana, this site is especially suited for cold weather thermal cracking.
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• Calumet Ave in Munster, Indiana: Located near the shores of Lake Michigan, this site meets the cold-weather thermal cracking parameters.
• US 98 in Jefferson County, Florida: The climate in Florida does not lend itself to thermal cracking, and this section should not be considered for this type of cracking.
• Little Neck Parkway in New York, New York: Located toward the southern border of the cold climate thermal cracking, this site nonetheless has raw materials available; this location should be given proper consideration.
• State Road 84 in Casa Grande Arizona: Located south of Phoenix, this site presents the only possibility within NCHRP 9-47A for a diurnal thermal cracking location.
The list above focusses only on meeting the climate parameters for thermal cracking. No regard has been given to the other variables because the fact that raw materials remain available for these sections is more than enough for their consideration in a follow-up project. These locations provide constructed projects that have been in service for approximately five years at the time of this writing. At the time of this report, approximately 5 gal of binder is available for each location with various amounts of aggregate, including RAP material.
Laboratory Experimental Design for Validating Thermal Cracking Tests
A successful laboratory evaluation of asphalt mixtures starts with sampling and ends with data interpretation, but each stage of the whole process is crucial. Thus, the experimental design will address the plans for each stage separately, as detailed below.
Plan for Sampling and Inventory The sampling includes acquiring raw materials and plant mixtures during production and
construction and field cores after construction. NCHRP’s Research Results Digest (RRD) 370: Guidelines for Project Selection and Materials Sampling, Conditioning, and Testing in WMA Research Studies is available for guidance in sampling materials.
• Raw Materials: Asphalt binders, aggregates, RAS, RAP, and any additives (anti-stripping agent, WMA additives, recycling agents, fibers, and others if used) should be sampled at the plant location during construction of the test sections and utilized later for LMLC specimens. In the case of existing test sections such as LTPP, efforts will be made to obtain the raw materials from storage facilities. For instance, significant quantities of aggregate, subgrade, binder, and HMA are being stored in the FHWA-supported MRL in Reno for future testing by other researchers. Table 33 lists the raw materials being collected for each test section for validating three thermal cracking tests.
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Table 33. Materials per section for validation of thermal cracking. RAW MATERIAL PER INDIVIDUAL SPECIMEN
Specimen description No. of specimen Mixture (kg) Aggregate (kg)
Binder (l)
DCT 1 5 4.9 0.25
SCB-IL 1 7.8 7.6 0.390
SCB-TP105 1 5 4.9 0..5
RAW MATERIAL PER EACH TEST SECTION
Mixture/Specimen Description
No. of specimen Safety factor
No. of LMLC specimen
Mixture (kg)
Aggregate (kg)
Binder (l)
DCT 3 2 6 29.94 29.03 1.51
SCB-IL 3 2 6 15.42 14.97 0.76
SCB-TP105 3 2 6 9.98 9.53 0.39
Total Raw Material for each Test Section 55.34 53.53 2.65
PLANT MIX PER EACH TEST SECTION
Mixture/Specimen Description
No. of specimen Safety factor
No. of PMLC specimen
Mixture (kg)
DCT 3 2 6 29.94
SCB-IL 3 2 6 15.42
SCB-TP105 3 2 6 9.98
Total Plant Mixture for each Test Section 55.34
• Plant Mixtures: Plant-mixed materials should be sampled at the plant or construction
site. The plant mixtures should be used either for compacting specimens at the plant site or for later reheating at a central laboratory as plant-mixed, laboratory-compacted (PMLC) samples. The size of the sampling container should be consistent for all plant mixtures sampling because it may impact the properties of asphalt mixtures when reheating and molding specimens for further laboratory evaluation. Plant production information (such as plant type and characteristics, baghouse fines handling, production rate, plant temperatures, silo storage times, and haul times) should be recorded. Additionally, field laydown and compaction processes should be recorded, including the type and number of items of equipment, paving speed, mat thickness, mat temperature, and on-site climate conditions. The amount of plant mix material required is displayed above in Table 33.
All the sampled raw materials and plant mixtures should be inventoried before transporting to a central facility for testing and storing.
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Plan for Transportation and Storage The sampled and inventoried materials will need to be transported from the construction
site to a temporary storage site before finally arriving at the testing facility. The samples should be marked and labeled before sending to testing laboratories in order to easily identify the samples. The LTPP program suggests the following information on tags and labels:
• Section identification number, which is a unique six-digit number. • Core/sample location (as marked on sample layout plans). • Sample code (four-digit code that identifies sample type [e.g., core or bulk sample],
material type [e.g., PCC or AC], and sample number). • Date of sampling. • Field set (one-digit number, which will be 1 for the first round of sampling and 2 in
instances where a second round of sampling is performed at the same general location in the future).
Materials must be handled with care to avoid damage during transit and storage. Storage temperature does have an impact on the properties of both plant mixtures and field samples. Even if the specimens are stored at room temperature, there is continuous asphalt binder absorption and aging that affect cracking resistance (Walubita et al. 2012). Ideally, the asphalt binders, plant mixtures, and other necessary materials should be stored at a lower temperature (below 40°F).
Plan for Laboratory Thermal Cracking Tests Specimen preparation for LMLC and PMLC, including short- or long-term aging
condition, should be determined based on existing AASHTO T 312, R 30 standards and the results of NCHRP 9-54. The thermal cracking tests, including DCT, SCB-IL, and SCB-TP105, should be conducted on the specimens prepared with materials from each experimental section following the test methods refined through the ruggedness test and ILS.
The laboratory test results can be compared with the field-measured thermal cracking to establish the correlation for each thermal cracking test. If needed, a detailed thermal cracking modeling approach can be developed as well. Based on the correlation and modeling (if needed), thermal cracking criteria for each cracking test are then developed.
Proposed Schedule and Budget
The thermal cracking experiment for Design 1 will require six test sites; three will be located in a cold climate, and three will be in a diurnal cycling climate. Given the climate requirements, it is conceivable that all three cold climate sections can be located at one facility (MnRoad), and that the two high-traffic locations of the three diurnal cycling sites might be found in a LTPP SPS section; the other diurnal cycling site could conceivably be an LTPP site close by. Design 2 requires 15 sections, including eight in cold climates and seven in diurnal cycling climates. The estimated budget for Design 1 and Design 2 are shown in Table 34 and the schedules are shown inTable 35 and Table 36. Since construction and subsequent monitoring of any test sites will add at least $500,000 per test site in construction costs and add years to the study while waiting for cracks to form, the budget and schedule below reflect the case where adequate sites exist along with the materials to perform the cracking tests. It is probable that adequate sites exist for Design 1, while it is unlikely that Design 2 can be accomplished without additional construction.
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Table 34. Time and budget* for thermal cracking study. Experiment Design 1
Travel 2 panel meetings/3 state meetings and site visits
@$1500 ea.
7500 2 panel meetings/8 state meetings and site visits
@$1500 ea.
15,000
Misc. Equipment Repair,
Supplies, Etc.
7000 14,000
Total 254,600 356,505 *Costs are exclusive of overhead charges or profit margin.
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Table 35. Schedule for thermal cracking study for Design 1. Month 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15
Identify Sites, Verify
Performance, Materials
Availability Obtain
Material Samples Prepare LMLC,
Compact PMLC
Samples Conduct Testing
Analyze Test Results Prepare, Review,
Revise Report
Table 36. Schedule for thermal cracking study for Design 2. Month 1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18
Identify Sites, Verify
Performance, Materials
Availability Obtain
Material Samples Prepare LMLC,
Compact PMLC
Samples Conduct Testing
Analyze Test Results Prepare, Review,
Revise Report
EXPERIMENTAL DESIGN FOR VALIDATING REFLECTION CRACKING TESTS
This section briefly discusses reflection cracking mechanisms and associated influential factors before presenting the experimental design for validating reflection cracking tests, including both a field experimental design and a laboratory testing plan. The research team also reviewed and documented all potential field test sections for validating reflection cracking tests in this section as well.
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Reflection Cracking Mechanism and Influential Factors
Reflection cracking is due to the movement of the segmented substrate below the surface layer, which causes cracks to be transmitted through the surface. The movements may be horizontal due to the expansion and contraction of lower substrate segments, or they may be vertical as traffic loads pass over the crack and load is transferred from segment to segment, or they may be a combination of both. The major factors influencing reflection cracking and their variation levels identified at the Cracking Test Workshop (see Chapter 2) are listed in Table 37.
Climate is included as a key factor in reflection cracking because it is known to play a primary role, particularly in cold climates (Rigo 1993). Kim and Buttlar (2002) stated that daily temperature variations and the resulting thermal contractions of pavement layers are driving forces of reflection cracking. Mukhtar and Dempsey (1996) described how both seasonal and daily temperature changes cause the existing pavement to contract and produce tensile stresses in the overlay immediately above the joint or crack. Zhou et al. (2009) detailed the effect of underlying conditions on reflection cracking. These conditions play a critical role on the speed at which reflection cracks propagate to the surface. Underlying conditions combined with material properties in the way of mix type and overlay thickness greatly influence reflection cracking and are accounted for as key factors. From a mix type standpoint, softer binder or higher binder content typically resists crack propagation much more effectively. Aging and recycled components adversely affect reflection life. Three types of mixtures are identified for this experiment design: DGA, a performance mixture, and crack attenuating mixture. The DGA mixture should provide the least resistance to cracking, followed by the performance mixture, and the most crack resistance should be found in the crack attenuating mixture. Overlay thickness not only reduces stress at the bottom of the layer, but also slows propagation. A small increase in thickness can have a profound effect on cracking life. The final key factor included is traffic. Traffic creates stresses over the joints and/or cracks in the old pavement, eventually leading to reflection cracking. Traffic is needed to drive reflection cracking; therefore, it has only one variation level, which is high traffic.
Table 37. Field experimental design factors identified for reflection cracking.
Key factor Variation level Climate 1) Steady state regions—typically moderate to hot weather to avoid thermally
induced low-temperature cracking. 2) High temperature cycling regions—areas that can experience large daily
temperature fluctuations. These regions can include areas that experience fluctuations around freezing or those in regions that are dry and hot.
Existing pavement types
1) Cracked AC/Granular base. 2) Cracked AC/cement treated base (CTB). 3) Joint plain concrete pavement (JPCP) with poor load transfer efficiency
(LTE) at joints. 4) JPCP with good LTE at joints.
Mix type 1) Superpave DGA mixture. 2) Performance mixture (such as SMA, rubber mixes, etc.). 3) Special crack resistant mixes (such as Strata, Texas CAM, etc.).
Asphalt overlay thickness
1) Thin AC: ≤ 50 mm (2 inches). 2) Medium AC: 50–150 mm (2–6 inches).
Traffic High: > 300,000 ESAL/year.
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Field Experimental Design for Validating Reflection Cracking Tests
The objective of this field experimental design is to assess the effects of study factors on reflection cracking development in the field and then to validate the capability of the selected reflection cracking tests for differentiating the different performances of these experimental test sections. More test sections lead to a more precise estimation of the factor effects described in Table 37. However, the maximum number of test sections that can be afforded depends on the resources available. Similar to thermal cracking, two separate D-optimal experimental designs were developed for reflection cracking:
• Design 1: the most affordable design that simply focuses on estimating all main effects.
• Design 2: desirable design that estimates both all main effects and two-way interaction effects.
Table 38 and Table 39 present D-optimal Designs 1 and 2 for reflection cracking, respectively. These two levels of experimental designs provide flexibility to make decisions based upon available funding. Note that the number of test sections was kept as small as possible in developing each experimental design while still ensuring that all important factor effects can be estimated.
Table 38. D-optimal experimental Design 1 for reflection cracking: 7 test sections.
Test section
Climate Existing pavement type Mixture Overlay thickness
Traffic
1 Steady state Cracked AC/Granular base
DGA ≤ 50 mm (2 inches)
> 300,000 ESAL/year
2 Steady state Cracked AC/CTB base Special crack resistant mix
≤ 50 mm (2 inches)
3 Steady state JPCP with low LTE Performance mix
≤ 50 mm (2 inches)
4 Steady state JPCP with high LTE Special crack resistant mix
50–150 mm (2–6 inches)
5 Temperature cycling
Cracked AC/Granular base
Special crack resistant mix
≤ 50 mm (2 inches)
6 Temperature cycling
Cracked AC/CTB base Performance mix
50–150 mm (2–6 inches)
7 Temperature cycling
JPCP with low LTE DGA 50–150 mm (2–6 inches)
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Table 39. D-optimal experimental Design 2 for reflection cracking: 25 test sections. Test section
Climate Existing pavement type Mixture Overlay thickness
Traffic
1 Steady state Cracked AC/Granular base Special crack resistant mix
≤ 50 mm (2 inches)
> 300,000 ESAL/year
2 Steady state Cracked AC/Granular base DGA 50–150 mm (2–6 inches) 3 Steady state Cracked AC/Granular base Performance mix
4 Steady state Cracked AC/CTB base Special crack resistant mix
≤ 50 mm (2 inches)
5 Steady state Cracked AC/CTB base DGA 6 Steady state Cracked AC/CTB base Performance mix 50–150 mm
(2–6 inches) 7 Steady state JPCP with low LTE Special crack resistant mix
8 Steady state JPCP with low LTE DGA ≤ 50 mm (2 inches) 9 Steady state JPCP with low LTE Performance mix
10 Steady state JPCP with high LTE Special crack resistant mix
50–150 mm (2–6 inches)
11 Steady state JPCP with high LTE DGA 12 Steady state JPCP with high LTE Performance mix ≤ 50 mm
(2 inches) 13 Temperature
cycling Cracked AC/Granular base Special crack
resistant mix 50–150 mm (2–6 inches)
14 Temperature cycling
Cracked AC/Granular base DGA ≤ 50 mm (2 inches)
15 Temperature cycling
Cracked AC/Granular base Performance mix
16 Temperature cycling
Cracked AC/CTB base Special crack resistant mix
17 Temperature cycling
Cracked AC/CTB base Special crack resistant mix
50–150 mm (2–6 inches)
18 Temperature cycling
Cracked AC/CTB base DGA
19 Temperature cycling
Cracked AC/CTB base Performance mix ≤ 50 mm (2 inches)
20 Temperature cycling
JPCP with low LTE Special crack resistant mix
21 Temperature cycling
JPCP with low LTE DGA 50–150 mm (2–6 inches)
22 Temperature cycling
JPCP with low LTE Performance mix ≤ 50 mm (2 inches)
23 Temperature cycling
JPCP with low LTE Performance 50–150 mm (2–6 inches)
24 Temperature cycling
JPCP with high LTE DGA ≤ 50 mm (2 inches)
25 Temperature cycling
JPCP with high LTE Performance mix 50–150 mm (2–6 inches)
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Identification of Field Test Sections for Validating Reflection Cracking Tests
Similar to thermal cracking, when identifying test locations, climate regions must be established. As described earlier, temperature cycling is an area that experiences high diurnal temperature ranges. Steady state areas are those that might experience seasonal temperature variations but do not experience large daily temperature differences between the high and low. Using the information from the National Weather Service and the other sources discussed above, Figure 13 was created to indicate areas within the United States that experience the highest diurnal temperature ranges. This zone is overlaid on a map that indicates the traditional LTPP climate zones.
Figure 13. Temperature cycling region for reflection cracking.
To avoid cold thermal issues, an attempt should be made to select steady state locations below 40°N. This will not always be possible, but it should be noted when reflection cracking locations are selected that could also face thermal stresses.
It is also important to note that some areas within the temperature cycling region can experience more movement around freezing than others. This can roughly be established as north of 35°N. During testing and evaluation of sections in the temperature cycling zone, effort should be made to capture how often the cycling includes temperatures below freezing. Attention to this detail might provide additional insight into how cold temperature cycling affects reflection cracking compared with warmer temperature cycling.
A total of three overlay mixes and two different overlay thicknesses are considered in the experimental matrix. Additional parameters to be considered are existing pavement types and conditions. For reflection cracking, the existing pavement types include both JPCP and cracked asphalt pavements with either granular or CTB base. Within the JPCP, the project should evaluate areas that have both high and low LTE. To identify the LTE at joints, initial forensic studies might be required. Deflections from the FWD can greatly improve the understanding of LTE within JPCP. In fact, forensics prior to section selection for reflection cracking might be imperative in order to gain a needed understanding of existing pavements. This includes
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knowledge of the existing asphalt layer thicknesses, if they are not to be milled off. These data can be obtained through ground penetrating radar (GPR). Traffic is not directly considered in the experimental design for reflection cracking, but high-volume traffic is required for all reflection cracking sections.
Available Test Sections for Validating Reflection Cracking Tests
Table 40 is an extension of Table 38 and gives the experimental design for the reflection cracking test with possible sections for testing. The selection of these sections focused on meeting the climate, pavement type, and overlay thickness, and acknowledging that these were as-built sections and the materials were determined long before this study. Additionally, the load transfer across JPCP will need to be determined through non-destructive means for confirmation.
Figure 14 and Figure 15 are examples of potential reflection cracking sections from LTPP in either a steady state region or a temperature cycling region. As with all potential sections, these do not meet every parameter within the design. It will be imperative for the follow-up group to determine where compromises can and should be made regarding sections to test. Additionally, all sections indicated within this report are subject to material availability and field investigation.
Table 40. D-optimal experimental Design 1 for reflection cracking with possible test
sections. Test section
Climate Existing pavement type
Mixture Overlay thickness
Sections
1 Steady state Cracked AC/Granular base
DGA ≤ 50 mm (2 inches)
2 Steady state Cracked AC/CTB base
Special crack resistant mix
≤ 50 mm (2 inches)
LTPP 01-0103, 01-0105
3 Steady state JPCP with low LTE Performance mix ≤ 50 mm (2 inches)
4 Steady state JPCP with high LTE Special crack resistant mix
50-150 mm (2–6 inches)
LTPP18-A901
5 Temperature cycling
Cracked AC/Granular base
Special crack resistant mix
≤ 50 mm (2 inches)
6 Temperature cycling
Cracked AC/CTB base
Performance mix 50-150 mm (2–6 inches)
7 Temperature cycling
JPCP with low LTE DGA 50-150 mm (2–6 inches)
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LTPP Section: 01-0103 State: Alabama (1)
Roadway and Direction: US 280 WB Experiment No.: SPS-1
Date of Surface Construction: Apr-91 Status: Out-of-Study 6/15/05 Pavement Cross Section:
Layer No. Type Thickness(inches) Description Date 1 SG – Fine-Grained soils: sandy silt - 2 Base 7.4 HMAC treated base April 1991
3 AC
Layer 2.8 Dense-Graded HMA April 1991
4 AC
Layer 1.4 Dense-Graded HMA April 1991 Tot. AC Thickness = 4.2
Traffic Data: Number of Years with AADTT Data: 0 Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
Potential Cracking Test Location: Top-down Cracking—Low Solar Gain—No Freeze Bottom-up Cracking Reflection Cracking—Steady State Climate—Cracked Asphalt Material Notes: Binder sampled 2/4/1993, AC20 Aggregate sample on MRL inventory
Figure 14. Potential reflection cracking section in a steady state climate.
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LTPP Section: 18-A901 State: Indiana (18) Roadway and Direction: IH 70 EB Experiment No.: SPS-9J Date of Surface Construction: Jul-97 Status: Out-of-study (09/05/2004) Pavement Cross Section:
Layer No. Type Thickness(inches) Description Date 1 SG – Coarse-grained, poorly graded sand with clay - 2 Base 8 Soil-Aggregate Mix (mainly coarse grained) Jan. 1997 3 Conc. 10 JRCP Jan. 1997
4 AC
Layer 4.5 Dense-Graded HMA Jan. 1997
5 AC
Layer 1.8 Dense-Graded HMA July 1997 Tot. AC Thickness = 6.3 HMA overlay over JRCP July 1997 Traffic Data: Number of Years with AADTT Data: 2 Latest AADTT: 33785 Year: 2003 Latest KESAL: 3197 Year: 2001 Performance from last surface date:
Potential Cracking Test Location: Thermal Cracking—Thick AC—High Traffic—Cold Climate Reflection Cracking—Jointed Concrete—Steady State Climate Top-down Cracking—Low Solar Gain—Hard Freeze Material Notes: 18-0900 exists on the binder list. Does this include A901?
Figure 15. Potential reflection cracking section in hard-freeze climate.
Raw materials associated with reflection cracking can be difficult to find. The fact that this experimental design builds a project that is focused on cracking tests, not field performance, makes the procurement of raw materials, particularly binder, critical. While many cracked pavements or jointed concrete pavements have been overlain, it has not been common to do this in a research environment where the raw materials from construction are maintained for future research. With that in mind, it is worth noting that many original SPS locations recently entered GPS-7 classification within LTPP. Moving from SPS to GPS-7 indicates that JPCPs have been overlain with HMA. This overlay work is often done by local DOTs through traditional construction and contracting. Unfortunately, this often means that raw materials are not maintained beyond the life of the contract or required DOT retention policies. Most of these
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sections come from either GPS-3, JPCP or SPS-9J, Superpave Asphalt Binder Study, AC Overlay on joint concrete pavement. Many of these sections are now requiring mill and overlays. A few examples are listed below:
• Section 31-3024: Originally a GPS-3 section of JPCP constructed in 1987. Overlain with 4 inch HMA and classified as a GPS-7B section in June 2012.
• Wisconsin A900 Sections: Originally in SPS-9J. Original overlay was placed in 1992, with a mill and overlay performed in 2011 that moved the section into GPS-7S. Another mill and overlay was performed in 2012.
Not having the raw materials eliminates the use of these sections for cracking tests; however, the activity on these sections provides potential to locate future sections. Using the assumption that other projects that began as GPS-3 or SPS-9J projects will require overlay work in the near future, researchers within the follow-up project can locate these sections within LTPP and contact state DOTs to determine if overlay work is expected. Examples are listed below:
• Section 19-3055 in Iowa was originally part of GPS-3. It was moved to GPS-7B in 2003 and currently has 5.8 inches of AC over 10 inches of JPCP. The AC overlay was placed in 2003. The LTPP performance history indicates transverse cracking appeared in 2007 and has grown during each subsequent inspection. Additionally, longitudinal cracking appeared in 2007 and grew substantially between 2007 and 2009. It is possible that the managing agency will schedule a mill and fill in the coming years. If that is the case, this becomes a potential reflection cracking location.
• Section 17-5151 in Illinois was moved into GPS-7B in 1990 and was overlain with 1.5 inches of AC pavement in 2001. Performance data indicates longitudinal and transverse cracking. The AC surface on this section is approximately 14 years old at the time of this report. It is possible it will have additional years of wear prior to the follow-up project. Whether it requires a mill and inlay during the follow-up project is not known, but it presents a lead and somewhere to start when contacting managing agencies.
The same logic can be applied to other sections within LTPP. Obviously, the methodology above does not guarantee finding sections, but it presents options, particularly when trying to find sections that meet the variables within the experimental design. A somewhat similar methodology is presented at the end of this chapter, specifically as it relates to identifying future projects to be bid upon within a state DOT. This methodology further illustrates the difficulty in finding field sections to validate cracking tests. When it becomes necessary to have the exact construction materials, the obvious course of action is to work with DOTs on projects that are currently under construction. The limitation with this course of action is the failure to capture all necessary variables and the fact that if the DOT receives the desired performance from the project, it should not crack prior to completion of the follow-up project. This reality must be faced by all future researchers.
Laboratory Experimental Design for Validating Reflection Cracking Tests
A successful laboratory evaluation of asphalt mixtures starts with sampling and ends with data interpretation, but each stage of the process is crucial. Thus, the experimental design will address the plans for each stage separately, as detailed below.
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Plan for Sampling and Inventory The same plan of sampling and inventory developed for thermal cracking is applicable to
reflection cracking. The main difference is in the amount of materials needed for reflection cracking due to the different cracking tests to be conducted. Table 41 shows the required raw materials and plant mixes from each test section for reflection cracking tests. Again, all the sampled raw materials and plant mixtures should be inventoried before transporting and storing to a central facility for testing.
Table 41. Needed materials per section for validation of reflection cracking.
RAW MATERIAL PER INDIVIDUAL SPECIMEN
Specimen description No. of specimen Mixture (kg) Aggregate (kg)
Binder (l)
OT 1 5 4.85 0.25
SCB-LTRC 1 5 4.85 0.25
BBF 1 8.53 8.26 0.43
RAW MATERIAL PER EACH TEST SECTION
Mixture/Specimen Description
No. of specimen Safety factor
No. of LMLC specimen
Mixture (kg)
Aggregate (kg)
Binder (l)
OT 5 2 10 49.90 48.53 2.65
SCB-LTRC 3 2 6 14.97 14.52 0.76
BBF 5 2 10 85.28 82.55 4.16
Total Raw Material for each Test Section 150.15 145.60 7.57
PLANT MIX PER EACH TEST SECTION
Mixture/ Specimen Description
No. of specimen Safety factor
No. of PMLC specimen
Mixture (kg)
OT 3 2 6 49.90
SCB-LTRC 3 2 6 14.97
BBF 3 2 6 85.28
Total Plant Mixture for each Test Section 150.15
Plan for Transportation and Storage The same transportation and storage plan used for the thermal cracking experiment can
be used for reflection cracking tests. Basically, materials must be handled with care to avoid damage during transit and storage. Ideally, the asphalt binders, plant mixtures, and other necessary materials should be stored at a lower temperature (below 40°F).
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Plan for Laboratory Reflection Cracking Tests Specimen preparation for LMLC and PMLC, including short- or long-term aging
condition, should be determined based on existing AASHTO T 312, R 30 standards and the results of NCHRP 9-54. The reflection cracking tests, including OT, SCB-LTRC, and BBF, should be conducted on the specimens prepared with materials from each experimental section following the test methods refined through the ruggedness test and ILS.
The laboratory test results can be compared with the field-measured reflection cracking to establish the correlation for each reflection cracking test. If needed, a reflection cracking modeling approach can be developed as well. Based on the correlation and modeling (if needed), reflection cracking criteria for each cracking test are then developed.
A report should document the whole testing process, findings, analysis, conclusions, and recommendations.
Proposed Schedule and Budget
The reflection cracking experiment will require the construction of test sections on existing roadways. For Design 1, if the available LTPP sections are suitable and the raw materials are available, five to seven new test sections will need to be constructed. These sections will most probably be in separate locations. If suitable sites can be found, then the construction costs will probably be approximately $500,000 each for a 500-ft (152 m) section with 100-ft sampling areas at each end. Thus for Design 1, a construction budget of $2.5 million to $3.5 million will be needed. For Design 2, it will be likely that 15 to 25 sections will need to be built at an estimated cost of $7.5 million to $13 million. The performance of these sections will need to be tracked over a period of two to ten years, probably at a cost of about $5000 per section per year. These costs are excluded from Table 42 because it may be necessary to separate the construction and performance monitoring from the testing portion of the work. The estimated budget for Design 1 and Design 2 are shown in Table 42, and the schedules are shown in Table 43 and Table 44.
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Table 42. Budget* for reflection cracking study. Experiment Design 1
EXPERIMENTAL DESIGN FOR VALIDATING BOTTOM-UP FATIGUE CRACKING TESTS
This section briefly discusses bottom-up fatigue cracking mechanisms and associated influential factors before presenting the experimental design for validating fatigue cracking tests, including both a field experimental design and a laboratory testing plan. The research team also reviewed and documented all potential existing field test sections for validating fatigue cracking tests in this section as well.
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Bottom-up Fatigue Cracking Mechanism and Influential Factors
Fatigue cracking is the result of repetitive traffic loads being applied to the pavement surface, resulting in a tensile strain at the bottom of the asphalt layer below that which would cause fracture due to many load applications. It is caused by a combination of an inadequate structural cross-section and a weak or brittle asphalt mixture. The major factors involved in fatigue cracking and their variation levels identified at the Cracking Test Workshop (see Chapter 2) are listed in Table 45.
Climate can impact fatigue cracking in asphalt pavements in a number of ways. One is the change in binder stiffness combined with environmental effects that reduces the pavement’s ability to resist cracking. High precipitation can cause subgrade and base saturation, weakening the support and facilitating fatigue cracking. The climate factor used in the experiment design will seek to capture warm, wet areas in one group and other climate zones outside of that group. Traffic is an obvious factor in fatigue cracking. Typically, heavier loads and greater repetitions lead to more fatigue damage, and this may be captured in the project by including two levels of traffic within the study. Generally speaking, improved fatigue cracking resistance can be achieved by increasing the effective volume of binder, reducing the nominal maximum aggregate size, increasing the in-place density of the mixture, and the addition of polymer modifiers to the asphalt binder. Decreased fatigue cracking resistance can be achieved by doing the opposite of these in addition to using increased amounts of recycled materials, especially post-consumer RAS. Considering different levels within the mix type factor will help differentiate the degree of cracking. Fatigue cracking generally occurs in AC layers less than 150 mm (6 inches) thick. Thicker AC layers typically crack from the top-down rather than the bottom-up. In thin layers, the bending experienced in an AC layer depends upon the underlying support. Weaker underlying layers cause more bending in the surface. By including mix type, structure, and subgrade as key factors, the behaviors described above will be captured in the study.
Table 45. Field experimental design factors identified for bottom-up fatigue cracking.
Key factor Variation level Climate 1) High temperature and moisture cycling regions—areas that
experience fluctuations around freezing. 2) All other regions—areas except high temperature/moisture cycling
regions. Traffic 1) High: > 300,000 ESAL/year
2) Low: ≤ 300,000 ESAL/year Mix type 1) Very good cracking resistant mix
2) Good cracking resistant mix 3) Medium cracking resistance mix 4) Poor cracking resistance mix
Pavement structure (AC thickness of all sections must be less than 6 inches)
1) AC with granular base 2) AC with CTB base
Subgrade 1) Poor 2) Good
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Field Experimental Design for Validating Fatigue Cracking Tests
The objective of this field experimental design is to assess the effects of study factors on fatigue cracking development in the field and then to validate the capability of the selected fatigue cracking tests for differentiating the performance of these experimental test sections. More test sections lead to a more precise estimation of the factor effects as described in Table 35. However, the maximum number of test sections that can be afforded depends on the resources available. Similar to thermal cracking, two separate D-optimal experimental designs are developed for fatigue cracking:
• Design 1: the most affordable design that simply focuses on estimating all main effects.
• Design 2: a desirable design that estimates both all main effects and two-way interaction effects.
Table 46 and Table 47 present D-optimal Designs 1 and 2 for fatigue cracking, respectively. These two levels of experimental designs provide flexibility to make decisions based upon available funding. Note that the number of test sections was kept as small as possible in developing each experimental design while still ensuring all important factor effects can be estimated.
Table 46. D-optimal experimental Design 1 for fatigue cracking: 8 test sections.
Test section
Climate Traffic Mixture Pavement structure
Subgrade
1 All others High Very good cracking resistance mix
AC/CTB base Poor
2 High temperature/ moisture cycling regions
High Good cracking resistance mix
AC/granular base
Poor
3 All others High Medium cracking resistance mix
AC/granular base
Good
4 High temperature/ moisture cycling regions L1
High Poor cracking resistance mix
AC/CTB base Good
5 Low Very good cracking resistance mix
AC/granular base
Good
6 All others Low Good cracking resistant mix
AC/CTB base Good
7 High temperature/ moisture cycling regions
Low Medium cracking resistance mix
AC/CTB base Poor
8 All others Low Poor cracking resistance mix
AC/granular base
Poor
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Table 47. D-optimal experimental Design 2 for fatigue cracking: 26 test sections. Test
High Very good cracking resistance mix AC/granular base Poor 2 High Very good cracking resistance mix AC/CTB base Good
3 All others High Very good cracking resistance mix AC/CTB base Poor 4 High temperature/
moisture cycling regions
High Good cracking resistance mix AC/granular base Poor 5 High Good cracking resistance mix AC/CTB base Good
6 All others High Good cracking resistance mix AC/granular base Good 7 High Good cracking resistance mix AC/CTB base Poor 8 High temperature/
moisture cycling regions
High Medium cracking resistance mix AC/granular base Good 9 High Medium cracking resistance mix AC/CTB base Poor
10 All others High Medium cracking resistance mix AC/granular base Poor 11 High Medium cracking resistance mix AC/CTB base Good 12 High temperature/
moisture cycling regions
High Poor cracking resistance mix AC/granular base Good
13 All others High Poor cracking resistance mix AC/granular base Poor 14 High Poor cracking resistance mix AC/CTB base Good 15 High temperature/
moisture cycling regions
Low Very good cracking resistance mix AC/CTB base Poor
16 All others Low Very good cracking resistance mix AC/granular base Good 17 High temperature/
moisture cycling regions
Low Good cracking resistance mix AC/granular base Good 18 Low Good cracking resistance mix AC/CTB base Poor
19 All others Low Good cracking resistance mix AC/granular base Poor 20 Low Good cracking resistance t mix AC/CTB base Good 21 High temperature/
moisture cycling regions
Low Medium cracking resistance mix AC/granular base Poor 22 Low Medium cracking resistance mix AC/CTB base Good
23 All others Low Medium cracking resistance mix AC/granular base Good 24 Low Medium cracking resistance mix AC/CTB base Poor 25 High temperature/
moisture cycling regions
Low Poor cracking resistance mix AC/CTB base Poor
26 All others Low Poor cracking resistance mix AC/granular base Good
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Identification of Field Test Sections for Validating Fatigue Cracking Tests
As described earlier, temperature cycling occurs in areas that experience high diurnal temperature ranges. Other areas do not experience large daily temperature differences and moisture cycling. Figure 16 is from the PRISM Climate Group at Oregon State University and offers a glimpse into annual precipitation around the United States. Using the information from the National Weather Service and the other sources discussed above, Figure 17 was created to indicate areas within the United States that experience high temperature and moisture cycling regions. This zone is overlaid on a map that indicates the traditional LTPP climate zones.
High temperatures and significant precipitation overlap in the climate zones shown in Figure 17. In the follow-up study, if the team desires to capture the effects of significant rainfall and how it facilitates bottom-up fatigue cracking in warm climate, the Gulf Coast offers the most promising location. The advantage to this region is the existence of the NCAT facility in Alabama and the Louisiana-LAF. Additionally, the follow-up project might wish to consider the effects of freeze-thaw on bottom-up fatigue cracking. If this is the case, sections should be chosen north of 40°N, and if high precipitation is desired, using Figure 16 as a guide, sections should be chosen from eastern Minnesota, eastward.
Figure 16. Precipitation map created by PRISM Climate Group, Oregon State University.
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Figure 17. Possible climate zones for fatigue cracking test sections.
Available Test Sections for Validating Fatigue Cracking Tests
Possibly the best candidate for potential test sections to validate bottom-up fatigue cracking comes at the FHWA Turner-Fairbank Highway Research Center. A study was recently completed on 10 test lanes with varying amounts of recycled material. Most likely, the primary benefit to using these sections is the different binder types with varying amounts of recycled material that will assist in meeting the mixture parameters listed in Table 46 and Table 47. The 10 potential sections from the ALF experiment at FHWA’s facility are listed in Table 48.
Table 48. Potential fatigue cracking test sections from ALF at FHWA Turner-Fairbank
Table 49 is a re-creation of Table 46 with a column for potential test sections. These potential sections were chosen based on climate with traffic, structure, and subgrade considered when feasible. Mixture type was not considered in the initial evaluation of potential sections.
Figure 18 and Figure 19 represent potential test sections for bottom-up fatigue cracking in a high temperature/moisture cycling region and other climate regions respectively. This does not include the sections listed above that are available at Turner-Fairbank Highway Research Center.
Table 49. D-optimal experimental Design 1 for bottom-up fatigue cracking with possible
test sections.
Test section Climate Traffic Mixture Pavement
structure Subgrade Sections
1 All others High Very good cracking resistant mix
Potential Cracking Test Location: Top-down Cracking—Low solar gain—No Freeze Bottom-up Cracking Material Notes: Binder sampled 10/03/1996, AC-30
Figure 18. Potential bottom-up fatigue cracking test section in high temperature/moisture cycling region.
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MnRoad Cell No: 21 State: Minnesota
Road Section and Direction: IH 94 WB Experiment No.: MnRoad
Date of Surface Construction: Sept. 2008 Status: Complete
Pavement Cross Section:
Layer No. Type Thickness(inches) Description Date 1 SG – Clay Subgrade June 2008 2 Base 7 Sand—select blend of reclaimings June 2008 3 Base 12 MnRoad Gravel Class 3 June 2008 4 Base 12 Class 5 Gravel Base June 2008
Tot. AC Thickness = 5 Traffic Data: Number of Years with AADTT Data: 20 Latest ESAL: 683751 Year: 2013 Latest KESAL: 684 Year: 2013 Performance from last surface date:
Potential Cracking Test Location: Thermal Cracking—Cold Climate—Thin HMA Bottom-up Fatigue Cracking—High Traffic—Granular Base—Poor Subgrade Material Notes: Binder is PG58-28 with RAP Figure 19. Potential bottom-up fatigue cracking test location in other climate location.
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Most pavements are designed in a way that bottom-up cracking should not occur. However, prior to the fall in oil prices in December 2014, the United States was experiencing rapid oil sector development that significantly damaged roadways. This was particularly evident in North Dakota, Pennsylvania, and South Texas. While the loading produced by this type of development has lessened over the past year, similar roadway impacts could occur in the future. These locations offer potential for bottom-up cracking sections. Researchers of this topic in the future should be mindful of economic development and seek out locations for additional study. For example, the Permian Basin near Odessa, TX, remains highly active with oil and gas sector traffic even when the price of oil is fairly low.
Laboratory Experimental Design for Validating Fatigue Cracking Tests
A successful laboratory evaluation of asphalt mixtures starts with sampling and ends with data interpretation, but each stage of the whole process is crucial. Thus, the experimental design will address the plans for each stage separately, as detailed below.
Plan for Sampling and Inventory The same plan of sampling and inventory developed for thermal cracking is applicable to
fatigue cracking. The main difference is the amount of materials needed for fatigue cracking due to the different cracking tests to be conducted. Table 50 shows the required raw materials and plant mixes from each test section for fatigue cracking tests. Again, all the sampled raw materials and plant mixtures should be inventoried before transporting and to a central facility for storing and testing.
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Table 50. Needed materials per section for validation of fatigue cracking. RAW MATERIAL PER INDIVIDUAL SPECIMEN
Specimen description No. of specimen Mixture (kg) Aggregate (kg)
Binder (l)
OT 1 5 4.85 0.25
SCB-LTRC 1 5 4.85 0.25
BBF 1 8.53 8.26 0.43
RAW MATERIAL PER EACH TEST SECTION
Mixture/Specimen Description
No. of specimen Safety factor
No. of LMLC Specimen
Mixture (kg)
Aggregate (kg)
Binder (l)
OT 5 2 10 49.90 48.53 2.65
SCB-LTRC 3 2 6 14.97 14.52 0.76
BBF 5 2 10 85.28 82.55 4.16
Total Raw Material for each Test Section 150.45 145.60 7.57
PLANT MIX PER EACH TEST SECTION
Mixture/ Specimen Description
No. of specimen Safety factor
No. of PMLC Specimen
Mixture (kg)
OT 3 2 6 49.90
SCB-LTRC 3 2 6 14.97
BBF 3 2 6 85.28
Total Plant Mixture for each Test Section 150.45
Plan for Transportation and Storage The same transportation and storage plan as the thermal cracking plan can be used for
fatigue cracking. Basically, materials must be handled with care to avoid damage during transit and storage. Ideally, the asphalt binders, plant mixtures, and other necessary materials should be stored at a lower temperature (below 4°C [40°F]).
Plan for Laboratory Fatigue Cracking Tests Specimen preparation for LMLC and PMLC samples, including short- or long-term
aging, should be determined based on existing AASHTO T 312, R 30 standards and the outcomes of NCHRP Project 9-54. The fatigue cracking tests, including BBF, SCB-LTRC, and OT, should be conducted on the specimens prepared with materials from each experimental section following the test methods refined through the ruggedness test and ILS. The laboratory test results can be compared with the field-measured fatigue cracking to establish the correlation for each fatigue cracking test. If needed, a detailed fatigue cracking modeling approach can be developed as well. Based on the correlation and modeling (if needed), fatigue cracking criteria for each cracking test are then developed.
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Proposed Schedule and Budget The bottom-up fatigue cracking experiment may not require the construction of test
sections due to the number of LTPP and MnROAD sections available for the study. It appears that the only sections for Design 1 that were not found included roads with CTB in warm, moisture-cycling climates and other climates. It is likely that such pavements could be found through a survey of states. Design 2 could require the construction of some sections. It is likely that these sections will most probably be in separate locations. The construction costs will probably be approximately $500,000 each for a 500-ft (152 m) section with 100-ft sampling areas at each end. If construction of sections is needed, the performance of these sections will need to be tracked over a period of five to 15 years in order to allow fatigue cracking to appear. These costs are excluded from Table 51 because it may be necessary to separate the construction and performance monitoring from the testing portion of the work. The estimated budget for Design 1 and Design 2 are shown in Table 51, and the schedules are shown in Table 52 and Table 53.
Table 51. Budget* for bottom-up fatigue cracking study.
EXPERIMENTAL DESIGN FOR VALIDATING TOP-DOWN CRACKING TESTS
This section briefly discusses top-down cracking mechanisms and factors before presenting the experimental design for validating top-down cracking tests, including both a field experimental design and a laboratory testing plan. The research team also reviewed and documented all potential existing field test sections for validating top-down cracking tests in this section as well.
Top-Down Cracking Mechanism and Influential Factors
The mechanisms for top-down cracking are not as well defined as the other types of cracking discussed. This is because it has often been mistaken for bottom-up fatigue cracking in relatively thick asphalt pavements. Like bottom-up cracking, it is considered primarily load
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related, but unlike bottom-up cracking, top-down cracking tends to propagate slowly after initiation. Many factors influence top-down cracking development. The major factors and their variation levels identified at the Cracking Test Workshop (see Chapter 2) are listed in Table 54.
Mixture type factors include gradation and air void levels. In-place air voids should be considered high above 9 percent. Coarse mixtures with higher air void contents tend to crack faster than those with finer gradations and lower void contents do. Finally, traffic levels were selected based upon the speed of the vehicles and the number of heavy vehicles. Slower vehicle speeds are expected to correlate with a greater tendency to crack due to the slower viscoelastic response of the material to the loading.
Climate factors included in this experiment design include temperature and solar radiation. This is because top-down cracking is believed to be strongly related to the stiffness of the surface mix. Higher solar radiation in areas without freezing temperatures ages asphalt on the surface faster than it will age asphalt in areas with lower solar radiation and lower temperatures. There is a decreasing gradient to aging with layer depth, which consequently reduces crack resistance and makes it easier for top-down cracking (Leech and Nunn 1997; Svasdisant et al. 2002; Worel 2003; Wamburga et al. 1999; Koohi et al. 2012). Material factors include material composition, modulus gradient, fracture, and thermal properties of asphalt concrete mixtures. Komoriya et al. (2001) and De Freitas et al. (2005) found that the initiation of top-down cracking is affected by binder content, aggregate gradation, and binder-aggregate adhesion. High air voids at the pavement surface lead to greater aging and moisture damage, which increase the likelihood of top-down cracking due to a higher surface tensile stress (Svasdisant et al. 2002). Higher air voids in DGA mixtures can be achieved through using an asphalt content of about 0.5 percent lower than the mix design would suggest. Conversely, lower air voids are possible be increasing the asphalt content between 0.3 to 0.5 percent higher than the design asphalt content. Research studies indicate that top-down cracking is caused mainly by non-uniform stress distribution beneath rolling tires, including vertical, longitudinal, and transverse stresses (De Beer et al. 1997; Fernando et al. 2006; Myers et al. 1999; Weissman 1999). It turns out that some load positions can have a much greater effect than others can, thus the need to split the key factor of traffic into three different variation levels. The final key factor is pavement structure, and it has only one level. The pavement thickness associated with top-down cracking for the follow-up project only considers thick pavements since these are the structures that are most likely to exhibit top-down cracking. By only including thick pavements, the possibility of encountering fatigue cracking while evaluating top-down cracking will be minimized.
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Table 54. Field experimental design factors identified for top-down cracking. Key factor Variation level Climate 1) Hard freeze, low solar radiation
2) Hard freeze, high solar radiation 3) No freeze, low solar radiation 4) No freeze, high solar radiation
Mix type 1) DGA coarse mixture, high AV 2) DGA coarse mixture, low AV 3) DGA fine mixture, high AV 4) DGA fine mixture, low AV
Traffic 1) High volume (>300,000 ESAL/year), high speed 2) Low volume (≤ 300,000 ESAL/year), local (low) speed 3) High volume (>300,000 ESAL/year), low speed
Pavement structure Thick AC: ≥ 150 mm (6 inch)
Field Experimental Design for Validating Top-Down Cracking Tests
The objective of this field experimental design is to assess the effects of factors on top-down cracking development in the field and then to validate the capability of the selected top-down cracking tests for differentiating the performance of these experimental test sections. More test sections lead to a more precise estimation of the factor effects as described in Table 54. However, the maximum number of test sections that can be afforded depends on the resources available. Similar to thermal cracking, two separate D-optimal experimental designs are developed for top-down cracking:
• Design 1: the most affordable design that simply focuses on estimating all main effects.
• Design 2: a desirable design that estimates both all main effects and two-way interaction effects.
Table 55 and Table 56 present D-optimal Designs 1 and 2 for top-down cracking, respectively. These two levels of experimental designs provide flexibility to make decisions based on available funding. Note that the number of test sections was kept as small as possible in developing each experimental design while still ensuring that all important factor effects can be estimated.
Table 55. D-optimal experimental Design 1 for top-down cracking: nine test sections.
Test section Climate Traffic Mixture 1 Hard Freeze, High Solar Low volume, low speed DGA fine, high AV 2 Hard Freeze, High Solar High volume, low speed DGA coarse, high AV 3 Hard Freeze, Low Solar High volume, high speed DGA fine, low AV 4 Hard Freeze, Low Solar High volume, low speed DGA fine, high AV 5 No Freeze, High Solar High volume, high speed DGA coarse, low AV 6 No Freeze, High Solar Low volume, low speed DGA coarse, high AV 7 No Freeze, High Solar High volume, low speed DGA fine, low AV 8 No Freeze, Low Solar High volume, high speed DGA fine, high AV 9 No Freeze, Low Solar Low volume, low speed DGA coarse, low AV
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Table 56. D-optimal experimental Design 2 for top-down cracking: 26 test sections. Test section Climate Traffic Mixture
1 Hard Freeze, High Solar High volume, high speed DGA coarse, high AV 2 Hard Freeze, High Solar High volume, high speed DGA coarse, low AV 3 Hard Freeze, High Solar High volume, high speed DGA fine, high AV 4 Hard Freeze, High Solar Low volume, low speed DGA coarse, high AV 5 Hard Freeze, High Solar Low volume, low speed DGA coarse, low AV 6 Hard Freeze, High Solar Low volume, low speed DGA fine, low AV 7 Hard Freeze, High Solar High volume, low speed DGA fine, high AV 8 Hard Freeze, High Solar High volume, low speed DGA fine, low AV 9 Hard Freeze, Low Solar High volume, high speed DGA fine, high AV
10 Hard Freeze, Low Solar High volume, high speed DGA fine, low AV 11 Hard Freeze, Low Solar Low volume, low speed DGA coarse, low AV 12 Hard Freeze, Low Solar Low volume, low speed DGA fine, low AV 13 Hard Freeze, Low Solar High volume, low speed DGA coarse, high AV 14 Hard Freeze, Low Solar High volume, low speed DGA coarse, low AV 15 Hard Freeze, Low Solar High volume, low speed DGA fine, high AV 16 No Freeze, High Solar High volume, high speed DGA coarse, high AV 17 No Freeze, High Solar High volume, high speed DGA fine, high AV 18 No Freeze, High Solar Low volume, low speed DGA coarse, low AV 19 No Freeze, High Solar Low volume, low speed DGA fine, high AV 20 No Freeze, High Solar Low volume, low speed DGA fine, low AV 21 No Freeze, High Solar High volume, low speed DGA coarse, high AV 22 No Freeze, High Solar High volume, low speed DGA fine, low AV 23 No Freeze, Low Solar High volume, high speed DGA coarse, low AV 24 No Freeze, Low Solar High volume, high speed DGA fine, high AV 25 No Freeze, Low Solar High volume, high speed DGA fine, low AV 26 No Freeze, Low Solar Low volume, low speed DGA coarse, high AV 27 No Freeze, Low Solar Low volume, low speed DGA fine, high AV 28 No Freeze, Low Solar High volume, low speed DGA coarse, high AV 29 No Freeze, Low Solar High volume, low speed DGA coarse, low AV 30 No Freeze, Low Solar High volume, low speed DGA fine, low AV
Identification of Field Test Sections for Validating Top-Down Cracking Tests
Much of the identification challenge with top-down cracking revolves around the climate. An in-depth study of desired climate parameters with top-down cracking in order to assist in test section identification was undertaken. In reviewing solar gain references, Figure 20 was created. This figure was created primarily based on information from the National Renewable Energy Laboratory (NREL) and indicates the areas of high solar gain and low solar gain. High solar gain is most prevalent in what is considered far west Texas and the desert southwest. The lowest solar gain in the continental United States takes place along the eastern seaboard into the Great Lakes region and stretches just into the far north reaches of the Great Plains.
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Figure 20. Solar gain map.
From Figure 20, it is easy to deduce the no freeze/high solar gain region and the hard freeze/low solar gain region. It is slightly more difficult to find consistent regions with the other two combinations. Figure 21 is an expanded version of a solar gain map, followed by Figure 22, which delineates the regions that should be used for top-down cracking (NREL 2015).
Figure 21. Expanded high and low solar gain regions.
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Figure 22. Top-down cracking climate regions.
An effort was made during the delineation of the desired regions for top-down cracking to keep away from the dividing lines established in the traditional LTPP climate zones. More specifically, there was an attempt made to establish the no-freeze line at the 35°N parallel and the hard-freeze line at the 40°N parallel. For the most part, this was achievable, except with the hard freeze/high solar gain region. Additionally, the hard-freeze line was also set at 120°W to avoid regions insulated by weather patterns along the Pacific Coast.
Structure is omitted from the experimental matrix only because of the assumption that top-down cracking will likely take place on sections with high overall stiffness. With this in mind, it is assumed that top-down sections will consist of thick asphalt sections (likely 150 mm [6 inches] or thicker) over good subgrades.
Available Test Sections for Validating Top-Down Cracking Tests
Table 57 is a recreation of Table 55 with potential test sections listed corresponding with climate parameters, while traffic components are included when distinguishable within available data. Figure 23 and Figure 24 are examples of information sheets detailing potential top-down cracking tests. Additional information sheets are included in Appendix C. As indicated in Figure 23, MnRoad can provide many test sections for top-down cracking. NCAT also provides an excellent source for potential top-down cracking sections in a no-freeze climate.
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Table 57. D-optimal experimental Design 1 for top-down cracking with possible test sections.
= 14 Traffic Data: Number of Years with AADTT Data: 20 Latest ESAL: 683751 Year: 2013 Latest KESAL: 684 Year: 2013 Performance from last surface date:
Year Fatigue Cracking (m2) WP Long. Crack
(m) NWP Long. Crack
(m)
Trans. Cracking (count)
Potential Cracking Test Location: Thermal Cracking—Cold Climate—Thick HMA—High Traffic Top-down Cracking—Hard Freeze/Low Solar Gain—High volume/High speed Material Notes: Binder is PG58-34 with 20% RAP
Figure 23. Potential top-down cracking test section in a hard-freeze climate.
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LTPP Section: 01-0101 State: Alabama (1) Roadway and Direction: US 280 WB Experiment No.: SPS-1 Date of Surface Construction: Apr-91 Status: Out-of-Study 6/15/05 Pavement Cross Section:
Layer No. Type Thickness (inches) Description Date
1 SG – Fine-Grained soils: sandy silt - 2 Base 7.9 Crushed Stone April 1991 3 AC Layer 6.2 Dense-Graded HMA April 1991 4 AC Layer 1.2 Dense-Graded HMA April 1991
Tot. AC Thickness = 7.4 Traffic Data: Number of Years with AADTT Data: 0 Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
Potential Cracking Test Location: Top-down Cracking—Low Solar Gain—No Freeze Material Notes: Binder sampled 2/4/1993, AC20 Aggregate sample on MRL inventory
Figure 24. Potential top-down cracking test section in no-freeze climate.
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Furthermore, many of the locations planned for SPS-10 have potential for top-down cracking. These are listed below:
• Arizona—top-down cracking, no freeze, high solar gain. Specifically, sites are planned along IH 40 in northern Arizona and IH 10 in southern Arizona.
• Southern California—top-down cracking, no freeze, high solar gain. • Western Nevada—top-down cracking, hard freeze, high solar gain. • North Central Oregon—borderline location for top-down cracking, hard freeze, low
solar gain. • Eastern Washington—top-down cracking, hard freeze, low solar gain. • Manitoba, Canada—top-down cracking, hard freeze, low solar gain. • Ontario, Canada—top-down cracking, hard freeze, low solar gain. • Southern Georgia—borderline location for top-down cracking, no freeze, low solar
gain. • Northern Florida—borderline location for top-down cracking, no freeze, low solar
gain. • Central New Mexico—top-down cracking, no freeze, high solar gain. • Central Oklahoma—between climate regions, particularly solar gain regions. • North Central Texas—between climate regions, particularly solar gain regions. Locations that offer thermal cracking potential also provide possibilities for top-down
cracking locations. Therefore, the sections from NCHRP 9-47A are listed below with the climate parameters associated with top-down cracking. As discussed in the thermal cracking section of this chapter, raw materials (binder and aggregate) are available for these sections, making them attractive options for the follow-up project.
• US 12 in Walla Walla, Washington: borderline top-down cracking, hard freeze, low solar gain.
• IH 66 near Centreville, Virginia: somewhat between climate zones and not particularly suited for top-down cracking within the desired climate parameters.
• County Road 513 near Rapid River, Michigan: top-down cracking, hard freeze, low solar gain.
• Montana County Route 322 in Fallon County, Montana: top-down cracking, hard freeze, low solar gain.
• Calumet Ave. in Munster, Indiana: top-down cracking, hard freeze, low solar gain. • US 98 in Jefferson County, Florida: somewhat between climate zones and not
particularly suited for top-down cracking within the desired climate parameters. • Little Neck Parkway in New York, New York: borderline top-down cracking, hard
freeze, low solar gain. • State Road 84 in Casa Grande, Arizona: top-down cracking, no freeze, high solar
gain. The list above focusses only on meeting the climate parameters for thermal cracking. No
regard has been given to the other variables because the fact that raw materials remain available for these sections is more than enough for their consideration in a follow-up project. These locations provide constructed projects that have been in service for approximately five years at the time of this writing. At the time of this report, approximately 5 gal of binder is available for each location with various amounts of aggregate, including RAP material.
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Laboratory Experimental Design for Validating Top-Down Cracking Tests
The experimental design will address the plans for each stage of the validation separately, as detailed below.
Plan for Sampling and Inventory The same plan of sampling and inventory developed for thermal cracking is applicable to
top-down cracking. The main difference is the amount of materials needed for top-down cracking due to different cracking tests to be conducted. Table 58 shows the required raw materials and plant mixes for top-down cracking tests. Again, all the sampled raw materials and plant mixtures should be inventoried before transporting and storing to a central facility for testing.
Table 58. Needed materials per section for validation of top-down cracking.
RAW MATERIAL PER INDIVIDUAL SPECIMEN
Specimen description
No. of specimen
Mixture (kg) Aggregate (kg) Binder (l)
IDT-Florida 1 5 4.85 0.25
SCB-LTRC 1 5 4.85 0.25
RAW MATERIAL PER EACH TEST SECTION
Mixture/Specimen Description
No. of specimen
Safety factor
No. of LMLC specimen
Mixture (kg)
Aggregate (kg)
Binder (l)
IDT-Florida 3 2 6 29.94 29.03 1.51
SCB-LTRC 3 2 6 14.97 14.52 0.76
Total Raw Material for each Test Section 44.91 43.55 2.27
PLANT MIX PER EACH TEST SECTION
Mixture/ Specimen Description
No. of specimen
Safety factor
No. of PMLC specimen
Mixture (kg)
OT 3 2 6 29.94
SCB-LTRC 3 2 6 14.97
Total Plant Mixture for each Test Section 44.91
Plan for Transportation and Storage The same transportation and storage plan used for thermal cracking can be used for top-
down cracking. Basically, materials must be handled with care to avoid damage during transit and storage. Ideally, the asphalt binders, plant mixtures, and other necessary materials should be stored at a lower temperature (below 40°F).
Plan for Laboratory Top-Down Cracking Tests Specimen preparation for LMLC and PMLC, including short- or long-term aging
condition, should be determined based on existing AASHTO T 312, R 30 standards and the
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outcomes of NCHRP 9-54. The top-down cracking tests, including IDT-Florida and SCB-LTRC, should be conducted on the specimens prepared with materials from each experimental section following the test methods refined through the ruggedness testing and ILS.
The laboratory test results can be compared with the field-measured top-down cracking to establish the correlation for each top-down cracking test. If needed, a detailed top-down cracking modeling approach can be developed as well. Based on the correlation and modeling (if needed), top-down cracking criteria for each cracking test are then developed.
Proposed Schedule and Budget
The top-down fatigue cracking experiment may not require the construction of test sections due to the number of LTPP and MnROAD sections available for the study. It appears that the only sections for Design 1 that were not found were high-volume, low-speed sections. It is likely that such pavements can be found through a survey of states. Design 2 might require the construction of some sections since coarse-graded surface mixtures may be difficult to find in the required locations. It is likely that these sections will most probably be in separate locations. The construction costs will probably be approximately $500,000 each for a 500-ft (152-m) section with 100-ft sampling areas at each end. If construction of sections is needed, the performance of these sections will need to be tracked over a period of five to 15 years in order to allow top-down cracking to appear. These costs are excluded from Table 59 because it may be necessary to separate the construction and performance monitoring from the testing portion of the work. The estimated budget for Design 1 and Design 2 are shown in Table 59, and the schedules are shown in Table 60 and Table 61.
FORENSIC STUDY PLAN FOR POORLY PERFORMING TEST SECTIONS
Forensic study plans discussed here are applicable to all four types of cracking. In a forensic study, poorly performing test sections will not only provide additional information regarding cracking mechanisms and field performance, they can also assist in lab-to-field validation. When raw construction materials are available for testing, cracking tests will be performed on LMLC mixes. In this case, forensics will be used to further understand the field performance of the mixture and how effectively the cracking test captures that performance. Table 62 presents LTPP forensic activities taken from the LTPP forensic report (MACTEC 2004).
Table 62. Modified LTPP forensic field activities.
Field activity Purpose Core at crack Visual determination of crack origin Core at saw and seal locations Locate joint relative to saw cut Core at reflection cracks Determine joint versus crack Trench Detailed layering study; determine location of any
permanent deformation GPR Layer thickness, material condition Drainage evaluation Excavate and assess condition of drainage elements Dipstick Cross slope
What is not shown in Table 62 is the measurement of deflections. These tests can be
useful in determining layer stiffness and should be considered during the forensic investigation. The LTPP guidelines for forensic investigations emphasize that no two pavement sections are exactly alike, and a forensic plan should be established for each section depending on the desired outcome. For pavement sections identified for inclusion in laboratory cracking testing, most factors will be known. These factors include raw material availability and knowledge of construction information. The process becomes more complicated when performing forensics on
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poorly performing sections that were not initially part of the study. This activity needs to be considered in the follow-up project, but care must be taken to remember that the study is focused on the validity of laboratory cracking tests to predict field performance. Forensic testing can exclusively focus on field performance if the desired information and outcomes are not clearly identified. Regarding forensics, it is important to note that when cores are taken, regardless of the location, effort should be made to identify where the crack begins and the direction in which it is propagating.
With the above in mind, the follow-up project will likely need to identify poorly performing sections by reaching out to owner agencies. The goal is to identify poorly performing sections with regard to cracking performance. These sections should not have received any maintenance work prior to the investigation. This fact makes identifying and evaluating these sections difficult from a timing perspective. Agencies are not likely to leave distressed pavement unattended for long periods of time. Therefore, it is recommended that once a poorly performing section is identified, the investigating team schedule forensic activities within a month of section identification.
The information that should be considered in the forensic study is listed below: • As-built construction data:
o Provides information on layer thickness. • Construction material information:
o Provides material source information. o Includes designs used by asphalt mix producers.
• Construction QA/QC data: o Provides information on quality of construction.
§ Data should include lab-molded densities, core results, longitudinal joint density profiles (if available), segregation profiles (if available), thermal profiles (if available), gradations, and other appropriate QC/QA data for flexible pavements.
§ For new construction, data should be obtained for aggregate base and subgrade layers. This should include compaction information, gradations, and other tests. This type of information may provide valuable insight into pavement surface behavior, especially fatigue cracking where the lower layers are not supporting the surface.
o Provides information on the quality of the material as it was produced by the supplier.
o Provides insight into the status of the pavement immediately upon completion and when initially placed in (or back in) service.
The LTPP forensic report emphasizes that no two pavement sections are exactly alike, and forensic plans should be established to reflect an individual section. When studying existing poorly performing sections, it will be difficult to acquire the materials used during construction. Specifically, the binder used on the day of construction may be difficult to obtain unless it is stored in the LTPP MRL or at a site associated with an existing experiment such as MnRoad or NCAT. Nonetheless, forensic studies can provide valuable insight into pavement behavior, and if the other raw materials can be obtained, additional lab testing is warranted. While the binder will not be the same, a similar grade binder should be available that will allow the team to investigate how the material would behave if it were produced again. This can greatly help managing agencies make decisions regarding future projects and possibly anticipate issues with current
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sections of like material that are in service. The research team suggests a flowchart similar to Figure 25 in order to develop a detailed forensic plan that can be used for all four types of cracking.
Figure 25. Example of pavement forensic study flow chart.
SUMMARY
Validation of the selected cracking tests is a critical step for incorporating the cracking tests into the routine mix design process and, consequently, good cracking performance of asphalt pavements. This chapter focused on developing an experimental design for validating the selected seven cracking tests and establishing associated pass/fail criteria. The D-optimal
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experiment design concept was described, followed by specific experimental designs for thermal, reflection, bottom-up fatigue, and top-down cracking, respectively. The experimental design for each cracking type includes both a field experimental design and a laboratory testing plan. The purpose of the experimental designs is to establish and validate criterion for each cracking test, not to investigate the mechanisms of cracking. Additionally, a forensic study plan is also proposed to address the potential premature pavement failure issue.
Selecting potential test sections is one of the most challenging features of the validation process. Fortunately, studies such as LTPP SPS 10, NCAT, and MnRoad provide excellent possibilities for section locations. Even within these studies, it is likely that not every parameter within the experimental matrix will be met. Using other LTPP sections from SPS studies offers a breadth of climate locations with a variety of pavement structures. Many of the potential sections within Appendix C are LTPP sections. Using these sections relies upon historical distress data collected through routine monitoring and the use of materials that have been stored for several years. Nonetheless, using these sections presents the opportunity to use exact construction materials, an important element of the lab-to-field validation.
Sections within NCHRP 9-47A were discussed as possible locations for top-down and thermal cracking where raw materials are currently available. Other recent NCHRP projects were completed that have limited potential for future sections. The reason that the potential is limited is the lack of raw materials—an issue mentioned a number of times. For example, no raw materials are available from NCHRP 9-49A. From NCHRP 9-49, Evaluation of the Moisture Susceptibility of WMA Technologies, a small amount of binder remains from the Texas project. The Texas location consists of nine test sections along FM 973 near the Austin-Bergstrom International Airport. Aggregate is not available outside of the damaged IDT specimens that remain. IDT specimens are also available from the locations in Iowa, Montana, and New Mexico, but this offers little in the way of useable materials for the follow-up project. From NCHRP 9-52, Short-Term Laboratory Conditioning of Asphalt Mixtures, plant mix materials are available. This material is not ideal for the follow-up project, but it provides an option if raw materials cannot be procured for other locations. What the summary above does illustrate is the need for researchers within the follow-up project to review active and recently completed NCHRP projects that might offer possible sections for crack test validation.
The potential exists to use current projects being let by state DOTs as test sections. The advantages to this method include isolating states within desired climate regions and acquiring materials during construction. The most significant disadvantage to this method is the fact that the sections will not likely begin to crack during the period of the follow-up study. Regardless, an example of information that can be easily acquired through DOT websites regarding project information is shown in the information sheet below (see Figure 26).
Lab-to-field validation is extremely important, and while 40 LTPP sections, nine MnRoad sections, and 10 APT sections at Turner-Fairbank have been identified (with additional options at NCAT and other APT facilities), finding sections that meet all parameters within the experimental matrix will prove difficult. It is likely that compromise will have to be made on what parameters are imperative within the lab-to-field validation. The sections identified here offer a starting point for the follow-up project, but sections were not found to completely fulfill the experiment designs.
Additionally, it is worth noting that the cracking tests discussed here may be too time-consuming to be of practical use in quality control, but a research effort needs to be undertaken to move performance testing into project quality assurance for plant-mixed, laboratory
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compacted specimens. Testing of core samples may be problematic due to issues with sample size and geometry as well as potential anisotropy in field compacted mixtures. Further work is still needed to refine/simplify/develop new cracking tests for practical use in quality control.
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13438
SH 13, Main St., Grangeville
9/22/2015 Status:
Layer No. TypeThickness
(in)1 SG -2 Subbase -3 Mill -4.84 AC Layer 4.8
4.8
256152 Year: 2019
750 Year: 2019
Typical Sections and Construction Info.:
Top-Down Cracking - Hard Freeze - Low Solar Gain - Low Volume/Local Traffic
Material Notes:Planned binder is PG70-28 at 5.5% by weightPlanned aggregate is 1/2" aggregate for SuperPave HMA Class SP-3
Bottom-up Cracking - Low traffic
Mill off existing between C&GCL SP-3 HMA Superpave
Tot. AC Thickness = Multiple width sections, but same depth
Potential Cracking Test Location:Thermal Cracking - Cold Climate - Thin AC - Low TrafficReflection Cracking - More info would be needed from DOT regarding underlying layers
Unknown
DOT Project No: State: Idaho
Roadway:
County:Let Date: State DOT Const. Project
Pavement Cross Seciton:
DescriptionUnknown
Figure 26. Example information sheet of potential section from DOT letting.
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CHAPTER 5. SUMMARY AND PROPOSED RESEARCH
SUMMARY
Asphalt mix designs are becoming more complex due to the increased use of recycled materials, recycling agents, binder additives/modifiers (such as recycled engine oil bottom), and multiple warm-mix asphalt technologies. These changes have altered the performance of mixtures both positively and negatively so that volumetric mix design alone is not sufficient for evaluating the potential behavior of asphalt mixtures, especially cracking behavior. Thus, there is an urgent need to establish and implement reliable performance tests that can be used to eliminate brittle mixes or model asphalt pavements to predict cracking.
The overall objective of this research is to develop an experimental design for field validation of laboratory tests selected under this study to assess the cracking potential of asphalt mixtures. A three-step process was taken to achieve the objective: (1) identify and select cracking tests, (2) refine these selected cracking tests through the ruggedness test and ILS, and (3) develop an experimental design for validation of the selected cracking tests. This interim report first summarizes the cracking test selection process and the final cracking tests selected for field validation. The necessary steps for refining each cracking test method through the ruggedness test and ILS for each cracking test are detailed in Chapter 3. A D-optimal experimental design approach was employed to develop field experimental designs for each of four cracking behaviors: thermal, reflection, bottom-up fatigue, and top-down. The detailed experimental designs for field test sections and associated laboratory tests for each type of cracking are provided in Chapter 4. In Chapters 3 and 4, the schedule and budget for executing these proposed plans are proposed as well. A summary of the budgets and timelines for the validation studies for the cracking tests are given in Table 63. Chapter 5 concluded the report with a summary and proposed research.
Table 63. Summary of Budgets and Timelines for Cracking Tests.
Cracking Type Experiment Design 1 Experiment Design 2 Time, mos. Cost*, $ Time, mos. Cost*, $
*Costs are exclusive of overhead charges or profit margin. As with any dynamic topic, progress is currently being made on all the tests presented in
this report. The contractors for the follow-up projects need to keep apprised of these developments, especially as they relate to current efforts on ruggedness and ILS testing. They should pay particular attention to on-going research at the MnROAD and the NCAT Test Track with respect to validation studies.
PROPOSED RESEARCH
NCHRP Project 9-57 developed detailed plans for refining the selected cracking tests in the laboratory and validating these cracking tests in the field. It is highly proposed that these plans are executed in a follow-up project.
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REFERENCES
De Beer, M., Fisher, C., and Jooste, F. J. (1997). “Determination of Pneumatic Tire/Pavement Interface Contact Stresses under Moving Loads and Some Effects on Pavement with Thin Asphalt Surfacing Layers,” Proceedings of the 8th International Conference on Asphalt Pavements, Seattle.
De Freitas, E. F., Pereira, P., Picado-Santos, L., and Papagiannakis, A. T. (2005). Effect of construction quality, temperature, and rutting on initiation of top-down cracking. Transportation Research Record: Journal of the Transportation Research Board, 1929(1), 174–182.
Deme, I. J. and F. Young (1987) Ste. Anne Test Road Revisited 20 Years later, Proceeding of Canadian Technical Asphalt Association, Vol. 42, pp. 254-283.
Fernando, E., Musani, D., Park, D., and W. Liu. (2006). Evaluation of Effects of Tire Size and Inflation Pressure on Tire Contact Stresses and Pavement Response, FHWA/TX-06/0-4361-1, Texas Transportation Institute, College Station, Texas.
Kim, J., and W. Buttlar (2002), Analysis of Reflective Cracking Control System Involving Reinforcing Grid over Base-isolating Interlayer Mixture, Journal of Transportation Engineering, Vol. 128, No. 4, pp. 375-384.
Komoriya, K., T. Yoshida, and H. Nitta. (2001). WA-DA-CHI-WA-RE Surface Longitudinal Cracks on Asphalt Concrete Pavement. Presented at 80th Annual Meeting of the Transportation Research Board, Washington, D.C.
Koohi, Y., J. J. Lawrence, R. Luo, and R. L. Lytton. (2012). “Complex Stiffness Gradient Estimation of Field-Aged Asphalt Concrete Layers Using the Direct Tension Test.” Journal of Materials in Civil Engineering, ASCE, Vol. 24, No. 7, pp. 832–841.
Leech, D. and M. E. Nunn. (1997). “Deterioration mechanisms in flexible roads,” Proceedings of the 2nd European Symposium on the Performance and Durability of Bituminous Materials, Leeds.
Mukhtar, M. T. and B. J. Dempsey. (1996). Interlayer Stress Absorbing Composite (ISAC)for Mitigating Reflection Cracking in Asphalt Concrete Overlays. Final Report, Transportation Engineering Series No. 94, Cooperative Highway and Transportation Series No. 260, University of Illinois, Urbana, IL.
Myers, L., R. Roque, B. Ruth and C. Drakos. (1999). “Measurement of Contact Stresses for Different Truck Tire Types to Evaluate Their Influence on Near-Surface Cracking and Rutting.” Transportation Research Record No. 1655, Transportation Research Board, pp. 175–184.
Svasdisant, T., M. Schorsch, G. Y. Baladi and S. Pinyosunun. (2002). “Mechanistic Analysis of Top-Down Cracks in Asphalt Pavements,” Transportation Research Board 81st Annual Meeting, January, Washington, D.C.
Rigo, J., M. (1993). “General introduction, main conclusion of the 1989 conference on reflection cracking in pavements, and future prospects,” Proceedings of the 2nd RILEM Conference on Reflection Cracking in Pavement, Liege, Belgium.
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Wamburga, J. H. G., J. N. Maina and H. R. Smith. (1999). “Kenya Asphaltic Materials Study,” Transportation Research Board 78th Annual Meeting, January, Washington, D.C.
Weissman S. L. (1999). “Influence of Tire-Pavement Contact Stress Distribution on Development of Distress Mechanisms in Pavements,” In Transportation Research Record: Journal of the Transportation Research Board, No.1655, pp. 161–167.
Worel, B. (2003). “MnRoad HMA Performance,” Presented at the MnRoad Workshop, Mn/Road Office of Materials, http://mnroad.dot.state.mn.us, Maplewood, MN, February.
Zhou, F., S. Hu, and T. Scullion. (2009). Mechanistic-Empirical Asphalt Overlay Thickness Design And Analysis System, Report FHWA/ TX-09/0-5123-3. FHWA, Texas A&M Transportation Institute, College Station, Texas.
APPENDIX A. LITERATURE REVIEW ON LABORATORY CRACKING TEST
INTRODUCTION
Cracking is a primary mode of distress that frequently drives the need for rehabilitation of asphalt pavements. There are several modes of asphalt pavement cracking, including low-temperature, reflection, fatigue, and top-down which are affected by numerous factors and interactions. In the past, volumetric mixture design gave a reasonable level of comfort for performance as the materials were relatively consistent within a given jurisdiction. Asphalt mixtures today are becoming more complex with different binder modifiers and the increasing use of both recycled materials and warm mix asphalt technologies. These changes have altered the performance of mixtures both positively and negatively so that volumetric mix design alone is not sufficient for evaluating the potential behavior of asphalt mixtures. Thus, there is an urgent need to establish and implement reliable performance tests that can be used to eliminate brittle mixes or model asphalt pavements to predict cracking.
Research Objective
The overall objective of this research is to develop an experimental design for field validation of selected laboratory tests to assess the cracking potential of asphalt mixtures. This appendix reviews existing laboratory cracking tests and a survey of highway agencies to assess the current state of implementing cracking tests.
Appendix Organization
This appendix documents the findings from the literature review and agency survey. The first section provides an introduction. The next few sections describe the literature review findings on thermal (or low-temperature), reflection, bottom-up fatigue, and top-down fatigue cracking, respectively. The final section concludes the appendix with a summary of discussion on cracking test evaluation and selection.
THERMAL CRACKING AND LABORATORY TESTS
Introduction
Cracking of asphalt pavements due to cold temperatures or temperature cycling can occur in many regions of the United States. Cracking that results from cold temperatures generally is referred to as low temperature cracking; cracking that is induced by thermal cycling is often referred to as thermal fatigue cracking. Low temperature cracking is often associated with the northern states and Canada in North America. Meanwhile, thermal fatigue cracking generally is associated with areas that experience large extremes in daily temperatures, such as the southwestern states. Both low-temperature cracking and thermal fatigue cracking are commonly referred to as thermal cracking.
This section first discusses thermal cracking mechanisms and factors that affect thermal crack development, and then the focus is placed on practical laboratory tests for characterizing
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thermal cracking resistance of asphalt mixtures. Finally, a summary is presented at the end of this section.
Thermal Cracking Mechanisms
The overall mechanism for thermal cracking is tensile stress in the asphalt layer, as shown in Figure A-1 (Hass et al. 1987). Contraction induced by cooling result in thermal tensile stress development in the restrained surface layer. Thermal stress is usually greatest in the longitudinal direction of the pavement, since there is more restraint in that direction. This restraint can be affected by the friction between the asphalt layer and the underlying material, with lower friction resulting in larger crack spacing. Additionally, the thermal stress is greatest on the pavement surface because the pavement surface temperature is the lowest during the cooling process. Depending on the magnitude of these stresses and the asphalt mix fracture resistance, transverse cracks may develop at different points along the length of the pavement.
Figure A-1. Cross Section of Cold Pavement Showing Temperature and Thermal Stress
Gradients (Haas et al. 1987).
For the low temperature cracking, as the temperature continuously drops, the tensile stress increases. When the stress at some point becomes equal to or exceeds the tensile strength of the asphalt mixture (Figure A-2), a crack develops at the surface of the pavement. The crack then propagates through the depth of the layer with additional thermal cycles. Note that the additional thermal cycles could be one or many, depending on many factors (such as temperature and cooling rate, tensile strength and relaxation properties of asphalt mixture, etc.). This type of thermal cracking has been well studied and simulated in the laboratory. For a new pavement, cracks generally have been observed to occur at 100–200 ft spacing. As the pavement ages and/or more extreme temperature drops happen, the crack spacing often decreases to 10–20 ft.
For the thermal fatigue cracking in southwestern states, the temperature does not drop low enough for the tensile stress to equal the tensile strength of the asphalt mixture. Under extreme daily temperature cycles, the temperature stress is greatest at night and decreases during the warmer daytime temperature. Failure does not occur immediately, but develops at the surface of asphalt pavements over a period of time, which is similar to the traditional bottom-up fatigue cracking. This type of thermal cracking is difficult to duplicate in the laboratory due to the long test time, although limited study (Sugawara and Moriyoshi 1984, Epps 1997) is available in the literature.
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Figure A-2. Prediction of Fracture Temperature for a Restrained Strip of Asphalt
Concrete (Hills and Brien 1966).
Carpenter (1983) proposed temperature ranges where one or the other type of thermal cracking is predominant. As shown in Figure A-3, 20ºF (−7ºC) was suggested as the threshold temperature for the low temperature or thermal fatigue cracking. Below 20ºF, thermal cracking is more likely to be due to low temperature cracking; above that point, the thermal fatigue cracking may be the case as in the southwestern states.
Figure A-3. Temperature Ranges Associated with Different Types of Thermal Cracking
(Carpenter 1983).
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Factors Influencing Thermal Cracking Development
Major factors that influence thermal cracking are listed below: • Temperature: For any given mix, the colder the pavement surface temperature, the
greater the potential for thermal cracking. The pavement surface temperature is highly related to the ambient air temperature and wind speed. The majority of low temperature cracks are initiated when the temperature decreases to a level below the glass transition temperature and is maintained at this level for a period of time.
• Cooling rate: The greater the cooling rate, the greater the tendency for thermal cracking. Pavements may crack in the fall or spring when the pavement structure is subjected to the greatest temperature differential between day and night.
• Asphalt binder type: It has been often suggested that the single most important factor affecting the degree of low temperature cracking is the temperature-stiffness relationship of the asphalt binder. The stiffness at a cold temperature and the temperature susceptibility are the most important considerations. The lower the performance grade (PG) low end, the better the resistance of the asphalt binder to thermal cracking. Anderson et al. (1989), the Committee on Characteristics of Bituminous Materials (1988), and Carpenter and Van Dam (1985) have conducted comprehensive studies on the relationship of asphalt binder to low temperature cracking.
• Coefficient of thermal contraction (and/or expansion): The larger the coefficient of thermal contraction, the greater the frequency of thermal cracking. The aggregate type, binder content, and volumetric properties of asphalt mix have an impact on the coefficient of thermal contraction.
• Pavement thickness: In general, the thicker the asphalt layer, the lower the occurrence of thermal cracking. At the Ste. Anne Test Road, an increase of asphalt layer thickness from 4 to 10 inches resulted in one-half the cracking frequency when all other variables were the same (Deme and Young 1987), probably because the greater cross section of the pavement reduced the thermal stresses.
• Aging: The older the pavement, the greater the incidence of thermal cracking. This is associated with aging resulting in increased stiffness of asphalt mixes and the decreased stress relaxation.
• Subgrade type: The frequency of low temperature shrinkage cracking usually is greater on pavements on sand subgrade compared with cohesive soils. This is due to the greater friction developed over granular materials reducing the sliding of the asphalt mat.
Laboratory Test Methods for Thermal Cracking
Over the years, a number of laboratory test methods have been used to study the thermal cracking of asphalt pavements. Overall, there are two types of laboratory tests in the literature: index and mechanistic tests. The result of an index-oriented test and associated pass/fail criterion are often directly used to evaluate thermal cracking resistance of asphalt mixtures. This is very suitable for asphalt mix design and QC/QA. The results of mechanistic-oriented tests are often used as the input to a thermal cracking model for predicting field performance of asphalt mixtures during a given service time. All available test methods for thermal cracking in the
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literature were reviewed, but only those often used or the most promising tests are included here. The format for reviewing each test is:
• Description of the test. • Availability of test standard (AASHTO, ASTM, or state test procedure). • Test condition parameters. • Material properties measured. • Test variability. • Test simplicity (or complexity) in terms of technician training requirements,
specimen preparation and instrumentation, performing the test and analyzing data, and interpretation of test results.
• Test sensitivity to asphalt mix components and mix design properties (such as air void).
• Test equipment availability and associated cost. • Lab-to-field correlation and associated criteria.
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Disk-Shaped Compact Tension (DCT) Test
Description
Buttlar and his co-workers (Wagoner et al. 2005) developed the DCT test for characterizing cracking resistance of asphalt mixtures at low temperatures. Currently, the DCT test is an ASTM standard test method: D7313-13, and Minnesota Department of Transportation (MnDOT) is developing an alternative version of DCT test. A disk-shape specimen with a 2.46 inch notch (Figure A-4) is pulled apart until the post peak level has reduced to 0.1 kN. The DCT is often conducted at 10ºC warmer than the PG low temperature grade in a crack-mouth opening displacement (CMOD) controlled mode with an opening rate of 1 mm/min. Figure A-4 shows DCT test setup, CMOD gauge, and a typical test curve. The fracture energy (Gf) is calculated by determining the area under the Load-CMOD curve normalized by the initial ligament length and thickness. The larger the Gf, the better the cracking resistance of asphalt mixtures.
Figure A-4. DCT Test Setup, CMOD Gauge, and Typical Test Curve (Marasteanu et al.
2012).
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Availability of Test Methods (AASHTO, ASTM, Agency, Institutional References)
ASTM D7313-13: Standard Test Method for Determining Fracture Energy of Asphalt-Aggregate Mixtures Using the Disk-Shaped Compact Tension Geometry
Test Condition Parameters
• Geometry of the specimen: 6 inch diameter and 2 inch thick; two 1 inch holes; one 2.46 inch notch.
• CMOD opening rate: 1 mm/min • Test temperature: low temperature PG + 10°C
Properties Measured
Fracture energy (Gf)
Test Variability
The typical COV of the DCT test is around 10 percent, which is pretty low.
Test Simplicity (or Complexity)
• Specimen preparation: The major concern on the DCT test is the specimen preparation, although the specimen can be easily made from Superpave gyratory compactor or field cores. The DCT sample preparation involves four cuts (two cuts to make 2 inch thick sample, one cut to create a flat surface for CMOD gauge, and one cut for the 2.46 inch notch) and two coring operations for the tension holes.
• Researchers at the University of Illinois have determined the average fabrication time per specimen to be in the 10 to 15 minute range for DCT testing, which includes the four saw cuts and two cored holes. This is based upon mass production of at least a dozen test specimens. The fabrication of fewer test specimens will obviously lead to a longer per‐specimen preparation time (Buttlar 2012).
• Specimen instrumentation: The CMOD gauge needs to be mounted to two sides of cracking mouth, which is easy and fast.
• Running the test and technician training: the running time for DCT is short and easy. Although the DCT test itself takes only 1 to 6 minutes to perform, the actual amount of testing time per specimen is probably more akin to 15 minutes, accounting for stabilization of test temperature, loading samples into the test apparatus (Buttlar 2012). It is envisioned that performing the DCT test requires little technician training if the commercially available DCT tester with integrated operating software is employed.
• Data analysis: It seems easy to calculate the DCT fracture energy (Gf). However, a data analysis program or Excel Macro is needed, since the integration is involved.
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• Interpretation of test results: It is very easy to interpret the DCT test results (Gf) which involves comparing the results with the established pass/fail criterion for thermal cracking.
Test Sensitivity to Asphalt Mix Composition and Design Parameters
Braham et al. (2007) measured 28 asphalt mixtures designed for cold climates, and four parameters were investigated: 1) aggregate type (limestone and granite); 2) test temperature (three temperatures −2ºC below the low temperature grade [low temperature], 10ºC above the low temperature grade [mid-temperature], and 22ºC above the low temperature grade [high temperature]); 3) asphalt content (design asphalt content and design content plus 0.5 percent); and 4) air voids (4 percent and 7 percent). It was found that the DCT fracture energy is sensitive to binder content at higher temperatures, aggregate type, and temperature, but not sensitive to asphalt content at low and mid-temperatures and the air voids, as shown in Figure A-5. This finding was later confirmed by Dave et al. (2011). Dave et al. (2011) also found that aging had limited effect on fracture energy when aging is induced using the AASHTO R30 protocol. Recently, Hill et al. (2013) found that the inclusion of RAP led to reduced DCT fracture energy and consequently potentially increased thermal cracking irrespective of the WMA additive employed. Arnold et al. (2014) concluded that the mixtures containing RAS had lower DCT fracture energies. Thus, the DCT test is sensitive to the presence of recycled materials (RAP and RAS).
(a) Fracture energy vs. test temperature, asphalt content, and air voids (Braham et al. 2007)
(b) Fracture energy vs. RAP (Hill et al. 2013) (c) Fracture energy vs. RAS (Arnold et al. 2014)
Figure A-5. Sensitivity of DCT Test to Asphalt Mix Composition and Design Parameters.
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Availability and Cost of the Test Equipment
Currently, Testquip LLC manufactures the DCT test equipment specifically for running ASTM D7313-13. The price for a single test unit with a DCT validator is $49,000.00. Alternatively, a universal servo-hydraulic testing system equipped with an environmental chamber can be used to perform the DCT test.
Laboratory-to-Field Correlations and Cracking Criteria
Under the national pooled fund study: Investigation of Low Temperature Cracking in Asphalt Pavements – Phase II, field thermal cracking data was correlated to DCT fracture energy (Marasteanu et al. 2012). Figure A-6 shows such correlation. From these results (Figure A-6), a minimum of fracture energy of 400 J/m2
is suggested for protection against thermal cracking. Fracture energy in the range of 350–400 J/m2
is considered borderline, and may be permissible on less critical projects, where a low to moderate degree of thermal cracking can be tolerated. For critical projects, a factor of safety can be achieved by specifying a minimum of fracture energy of 600 J/m2.
Furthermore, a thermal cracking specification is proposed by Buttlar et al. for asphalt mix design (Marasteanu et al. 2012). Since the DCT test results presented in Figure A-6 were based on cores taken out of older pavements, a 15 percent increase in fracture energy is proposed in the pooled fund study to take into account the fact that these requirements are specified for laboratory-mixed and laboratory-compacted mixtures with short-term aging. Specification limits for three levels of project criticality are provided in Table A-1. Note that the specification applies for surface mixes only.
Additionally, the DCT fracture energy (Gf) combining with other viscoelastic properties of asphalt mixtures (such as creep compliance) also can be used as inputs to a mechanistic model (such as ILLI-TC) to predict thermal cracking development of asphalt pavements.
Figure A-6. Correlation between DCT and Thermal Cracking (Marasteanu et al. 2012).
0
200
400
600
800
1000
1200
1400
0 200 400 600 800 1000
Tran
sver
se C
rack
ing
(m/5
00m
)
Fracture Energy (J/m2) - CMOD Basis
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Table A-1. Recommended Low Temperature Cracking Specification for Loose Mixes (Marasteanu et al. 2012).
Contents Project Criticality/Traffic Level
Low <10M ESALs
Moderate 10-30M ESALs
High >30M ESALs
Minimum Fracture Energy (J/m2)@low-temperature
PG+10ºC 400 460 690
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Semi-Circular Bend (SCB) Test
Description
The SCB test for low temperature cracking was developed by Marasteanu and his co-workers (Li and Marasteanu 2004, Marasteanu et al. 2012). Currently, the SCB test is an AASHTO provisional standard test: AASHTO TP105-13. Similar to the DCT test, the SCB test is to obtain the fracture energy of an asphalt mixture specimen. The testing is conducted at 10ºC warmer than the PG low temperature grade. Also similar to the DCT test, the SCB test is run in a CMOD controlled mode. Figure A-7 shows the SCB test setup and typical test results.
Meanwhile, the SCB test has three aspects which are different from the DCT test, as listed below:
• Specimen thickness: The SCB uses 1 inch thick specimen, while the DCT specimen is 2 inch thick;
• CMOD opening rate: The opening rate for SCB test is 0.03 mm/min which is 33 times slower than the DCT loading rate. This increases the duration of the test to as much as 30 minutes.
• Fracture energy calculation: The SCB fracture energy (Gf) is calculated by determining the area under the Load-load line displacement (LLD) curve normalized by the initial ligament length and thickness. Note that LLD is measured using a vertically mounted Epsilon extensometer. The CMOD measurement is used for maintaining the test stability in the post peak region of the test rather than calculating fracture energy.
Figure A-7. SCB Test Setup, LLD Extensometer, and Typical Plot of SCB Test at Different Temperatures: TL-12ºC below TM, TM-binder PG low limit+10C, and TH-12ºC above TM
(Li et al. 2011).
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Availability of Test Methods (AASHTO, ASTM, Agency, Institutional References)
AASHTO TP 105-13 Standard Method of Test for Determining the Fracture Energy of Asphalt Mixtures Using the Semicircular Bend Geometry (SCB)
Test Condition Parameters
Geometry of the specimen; CMOD opening rate (0.03 mm/min); test temperature
Properties Measured
Fracture energy (Gf)
Test Variability
The typical COV associated with SCB testing is around 20 percent (Marasteanu et al. 2012).
Test Simplicity
• Specimen preparation: The SCB specimen can be made from laboratory compacted specimens or field cores (Figure A-8). Basically, it requires four cuts and two notches to obtain two SCB specimens, but no holes are required in the specimen.
• Specimen instrumentation: The installation of CMOD gauge is the same (or similar) as the DCT test. Additionally, an Epsilon extensometer is mounted on the SCB specimen to measure the LLD for calculating fracture energy.
• Running the test and technician training: Running the SCB test includes four steps: 1) contact loading step, 2) seating load step, 3) three small amplitude loading cycles, and 4) fracture test step. It is not difficult to run the SCB test with commercially available test equipment with the designated software that integrates all these test steps. Otherwise, a certain amount of training time is needed for technicians to become familiar with the test.
• Data analysis: Similar to the DCT test, the SCB data analysis also needs a specific fracture energy calculation program or an Excel Macro due to the area of integration.
• Interpretation of test results: Similar to the DCT test, the SCB fracture energy can be directly compared with the established pass/fail criterion for thermal cracking.
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Test Sensitivity to Asphalt Mix Composition and Design Parameters
Li and Marasteanu (2004) investigated the influence of asphalt binders (PG58-40, PG58-34, and PG58-28) used in MnRoad on SCB fracture energy, and found that the SCB fracture energy is sensitive to asphalt binder grade. At −30ºC the mixture with PG58-40 binder has the highest fracture energy and the one with PG58-28 has the lowest. Li et al. (2008a) later studied the impact of aggregate type (granite vs. limestone), air voids (4 percent vs. 7 percent), and asphalt content (optimum asphalt content vs. optimum asphalt content+0.5 percent) on fracture energy. They found that the SCB fracture energy is sensitive to the aggregated type and the air voids, but not to asphalt content. The mixtures with granite aggregate had higher fracture energy than those with limestone, all other factors being constant; higher air voids most likely result in mixtures with lower fracture energy. Meanwhile, richer mixtures (higher asphalt content) do not necessarily result in higher fracture energy. Additionally, the impact of RAP contents on SCB fracture energy was evaluated by Li et al. (2008a) and West et al. (2013). Li et al. (2008a) found that the control mixtures (0 percent RAP) had the higher fracture energy and while 20 percent RAP mixtures exhibited similar fracture resistance to the control mixtures; the 40 percent RAP mixtures had significantly lower fracture resistance at the low temperature (Figure A-9). However, recent results from NCHRP Project 9-46 (West et al. 2013) show that the SCB fracture energy does not provide a consistent effect for mixtures with high RAP contents. They found that the SCB fracture energy was not significantly affected by RAP content except in one particular case.
Figure A-9. Sensitivity SCB Fracture Energy to Mix Compositions and Design Parameters
(Li et al. 2008a).
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Availability and Cost of the Test Equipment
The same DCT test equipment manufactured by Testquip LLC can be used to run the SCB test with an addition of the SCB fixture, which costs $3400. Thus, the total cost for an SCB test is around $52,400. Alternatively, a universal servo-hydraulic testing system equipped with an environmental chamber can be used to perform an SCB test.
Laboratory-to-Field Correlations and Cracking Criteria
Similar to the DCT, the SCB fracture energy is correlated with the total length of transverse cracking observed in the field test sections in Illinois, Minnesota, and Wisconsin (Marasteanu et al. 2012). Figure A-10 shows the relationship.
Based on the results plotted in Figure A-10, a limiting value of 350 J/m2 was proposed.
This value may be adjusted to a limit of 400 J/m2 to account for aging effects.
Figure A-10. Field Data Suggesting a Minimum SCB Fracture Energy of 350 J/m2 to
Prevent Thermal Cracking (Marasteanu et al. 2012).
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Indirect Tensile (IDT) Test
Description
The IDT test for low-temperature cracking was developed in the Strategic Highway Research Program (SHRP) by Roque and his co-workers at Pennsylvania State University (Roque and Buttlar 1992, Buttlar and Roque 1994). Currently, it is the AASHTO Standard Method of Test: T322-07. AATHTO T322 measures both creep compliance and tensile strength of asphalt mixtures at low temperatures, which are the required inputs to the AASHTOWare Pavement ME design for the prediction of low-temperature cracking of flexible pavements.
Mixtures are often tested at three different temperatures determined by the PG grade of the asphalt binder. A cylindrical specimen: 6 inch diameter by 1.5-2.0 inch thickness is loaded in compression across a diametral plane. In the creep test, a load level that produces a horizontal deformation between 0.00125 mm and 0.019 mm is held constant for 1,000 seconds. The horizontal and vertical deformations are recorded during the loading process and are used to calculate creep compliance and stiffness as a function of time. The strength test determines the tensile strength of a specimen by loading the specimen at a constant rate of 12.5 mm/minute until failure. The specimen dimensions and peak load are then used to calculate the failure strength. Figure A-11 shows examples of IDT test setup and the creep compliance curves (Roque and Buttlar 1992).
Figure A-11. IDT Test Setup and Example of Creep Compliance Curves (Roque and
Buttlar 1992).
Availability of Test Methods (AASHTO, ASTM, Agency, Institutional References)
AASHTO T322-07: Determining the Creep Compliance and Strength of Hot Mix Asphalt (HMA) Using the Indirect Tensile Test Device.
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Test Condition Parameters
Geometry of the specimen (6 inch diameter and 2 inch thick); Loading time for creep test; Loading rate (12.5 mm/min) for tensile strength test; Test temperatures.
Properties Measured
Creep compliance; Tensile strength.
Test Variability
Christensen and Bonaquist (2009) assembled historical tensile strength and creep compliance data from a national IDT ruggedness study, and statistically analyzed the data. As reported in NCHRP Report 530, the COVs of tensile strength and creep compliance are 7 and 11 %, respectively.
Test Simplicity (or Complexity)
• Specimen preparation: The test specimen can be made easily from laboratory compacted mixtures or field cores.
• Specimen instrumentation: The specimen instrumentation is not difficult with a glue jig to locate the studs for the LVDTs.
• Running the test and technician training: The most difficult part of AASHTO T322 is the creep compliance test in which a proper load level is needed to produce a horizontal deformation of 0.00125 mm to 0.0190 mm for 6 inch diameter specimen. A trial and error approach may be needed to find the proper load level. There is a great deal of technician training for running AASHTO T322.
• Data analysis: IDT data analysis is not difficult with the software developed by University of Florida.
• Interpretation of test results: Different from the DCT and the SCB tests, IDT test data (creep compliance and strength) need to be combined with TCMODEL to predict low-temperature cracking performance of asphalt mixtures, although they can be used for a simple comparison purpose.
Test Sensitivity to Asphalt Mix Composition and Design Parameters
Since the early 1990s, the IDT has been widely used to evaluate thermal cracking resistance of asphalt mixtures. Historical data show that both creep compliance and tensile strength are sensitive to asphalt binder grade, binder content, aggregate type, air voids, RAP, and aging.
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Availability and Cost of the Test Equipment
Commercially IDT test equipment is not widely available at this time, but the IDT test can be performed with most universal hydraulic test machines with an IDT test fixture and a low-temperature chamber.
Laboratory-to-Field Correlations and Cracking Criteria
The measured creep compliance and tensile strength are generally mechanistic-oriented parameters and often used as the inputs to TCMODEL for low-temperature cracking prediction. Since the completion of the SHRP program in the early 1990s, the IDT test and associated creep compliance and tensile strength for predicting low-temperature cracking have been well documented and further refined in the process of developing the MEPDG.
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Thermal Stress Restrained Specimen Test (TSRST)
Description
The TSRST was originally developed as a part of the SHRP at Oregon State University (Jung and Vinson 1994). It was initially published as AASHTO Standard TP10, but it was dropped from the current standards. A long beam (2 inch × 2 inch × 10 inch) is used for the test. As the temperature drops, usually at a rate of 10ºC/hr., the specimen is restrained from contracting thus inducing tensile stresses. Figure A-12 shows a typical test results from TSRST. The fracture strength and the fracture temperature are measured as part of the TSRST test.
Recently a modified version of TSRST (see Figure A-13) was developed by Sebaaly and Hajj at University of Nevada at Reno (Hajj et al. 2010), called the Uniaxial Thermal Stress and Strain Test (UTSST). Instead of a long beam test specimen, the UTSST constrained specimen is made with a cylindrical laboratory compacted mixture in a typical size of 2.25 inch diameter and 5.5 inch height (Figure A-13). Additionally, a new module is added to allow the direct measurement of the resulting thermal strain developed within an unrestrained specimen concurrently with the stress development in the strained specimen (Morian et al. 2014).
Figure A-12. Typical Results from TSRST (Jung and Vinson 1994).
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Figure A-13. UTSST Setup and Strained Specimen Preparation (Morian et al. 2014).
Availability of Test Methods (AASHTO, ASTM, Agency, Institutional References)
The TSRST test method was initially published as AASHTO Standard TP10, but it was dropped from the current standards. However, it has been included in the European Standard EN12697-46 to characterize asphalt mixtures for thermal cracking resistance (European Standard 2012).
Test Condition Parameters
Specimen geometry (2 inch X 2 inch X 10 inch beam); Cooling rate (10ºC/hr).
Properties Measured
Fracture temperature; Fracture strength.
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Test Variability
The COV for fracture temperature was close to or below 10 percent, and for fracture strength was close to or below 20 percent, as reported by Jung and Vinson (1994).
Test Simplicity (or Complexity)
• Specimen preparation: In general, it is difficult to make TSRST specimens, because the long beam has to be cut from a large slab which requires a rolling compactor to fabricate. The specimen for the new version of the TSRST, the UTSST, can be made from cylindrical lab compacted samples, but the second unrestrained specimen is constructed of two 57-mm diameter specimens with a thin layer of epoxy between them (Figure A-13).
• Another difficult part of preparing the TSRST specimen is gluing the long beam to the pulling platen, although a special glue jig can help.
• Specimen instrumentation: No specimen instrumentation is needed. • Running the test and technician training: Performing the TSRST is not difficult, but it
may take several hours to fracture a beam. Running the test may not require much technician training, but specimen preparation and gluing the specimen to the tension platen must be done very meticulously.
• Data analysis: Fracture temperature is directly measured so that data analysis is very easy. Fracture strength is computed by knowing the maximum force measured by the load cell and the cross-sectional area of the specimen.
• Interpretation of test results: Interpretation is very easy in that the measured fracture temperature is compared directly against the required fracture temperature.
Test Sensitivity to Asphalt Mix Composition and Design Parameters
The TSRST results (fracture temperature, transition temperature, fracture strength, and the slope) are sensitive to key asphalt mix characteristics including asphalt binder grade, degree of aging, air void content, and aggregate type (Jung and Vinson 1994). Similar observation is recently reported by Marasteanu et al. (2007).
Availability and Cost of the Test Equipment
Commercial TSRST equipment is available. The cost is around $100,000 for a test system with the capability of testing three TSRST specimens at the same time.
Laboratory-to-Field Correlations and Cracking Criteria
Research performed by Monismith et al. (1969), Fabb (1974), Carpenter (1983), Arand (1987), Sugawara et al. (1982), Janoo (1989), and Jung and Vision (1994), showed that TSRST can be used to evaluate the susceptibility of asphalt mixtures to low temperature
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cracking. The TSRST was the test recommended by the SHRP A-003A research group for characterizing low temperature cracking, but it was not selected by the SHRP A-001 group for the Superpave mix design. Thus, the TSRST has not been widely used.
Limited data are available in the literature to show the laboratory TSRST-to-field correlation. It was originally validated by six field test sections, as documented by Zubeck et al. (1996). Recently, Marasteanu et al. (2007) employed the TSRST with specimens recovered from MnRoad. The best performance was obtained for MnRoad 34 with a PG58-34, and specimens from MnRoad 19 with a PG64-22 binder were the most susceptible to thermal cracking. It was concluded that the TSRST results provide a good quantitative indication of low temperature resistance for asphalt mixtures. However, up to now a TSRST based low temperature cracking criterion has not been developed.
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Fenix Test
Description
The Fénix test was developed in Spain by Perez-Jimenez et al. (2010). Figure A-14 shows the Fenix test setup. One-half of a 4 inch diameter and 2.5 inch thick cylindrical specimen prepared by laboratory compaction or field core is glued to the steel plates, and each plate is attached to a loading platen. A 6-mm deep notch is made in the middle of its flat side where two steel plates are fixed. The test is performed in tension with a constant displacement velocity (1 mm/min) at a specified temperature.
Two parameters: dissipated energy and tensile stiffness index are calculated based on the measured load-displacement curve. The dissipated energy is the area under the load-displacement curve divided by the specimen thickness and initial ligament length. The tensile stiffness index (Figure A-14) is calculated as half the peak load divided by the displacement before peak load.
Figure A-14. Fenix Test Set-up, Load-Displacement Curve, and Failed Specimen (Perez-
Jimenez et al. 2013).
Availability of Test Methods (AASHTO, ASTM, Agency, Institutional References)
No standard test method is available for the Fenix test.
Test Condition Parameters
Test specimen geometry; Loading rate (1 mm/min); Test temperature.
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Properties Measured
Dissipated energy; Tensile stiffness index.
Test Variability
The COV of Fenix test ranges from 8.5 percent to 15 percent (Perez-Jimenez et al. 2010).
Test Simplicity (or Complexity)
The specimen preparation is similar to the SCB test. Additional work is required to glue the specimen to two steel plates. Running the Fenix test is simple and the dissipated energy calculation is similar to both the SCB and DCT tests. There is no clear information available regarding interpreting the Fenix test results to field performance.
Test Sensitivity to Asphalt Mix Composition and Design Parameters
Perez-Jimenez et al. (2010) found that the dissipated energy calculated from the Fenix test is sensitive to test temperature, binder grade, and content, as shown in Figure A-15.
Figure A-15. Fenix Test-Dissipated Energy at Different Test Temperatures, Binder Type
and Contents (Perez-Jimenez et al. 2010).
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Availability and Cost of the Test Equipment
There is no commercially available Fenix test equipment, but it does not need any specific test equipment as any universal hydraulic loading system should work. Depending on test temperature, a temperature chamber is needed to perform the Fenix test.
Laboratory-to-Field Correlations and Cracking Criteria
The Fenix test has been used by researchers at the Technical University of Catalonia, Spain, to evaluate fatigue, low-temperature, and reflection cracking resistance of asphalt mixtures. However, up to now there is no laboratory-to-field correlation available in the literature.
Summary
Thermal cracking occurs in both northern and southwestern parts of the United States. Thermal cracking mechanisms are well defined in the literature, and its development is impacted by many factors, such as environment, asphalt binder grade, aggregate type, and air voids. Identifying a cracking test is vital to the design of asphalt mixtures. This section reviewed the most practical and promising cracking tests for thermal cracking in terms of variability, simplicity (or complexity), sensitivity to asphalt mix components and mix design properties, lab-to-field correlation and associated criteria, availability of test standards, availability of test equipment and associated costs. Additional discussion on test method evaluation and selection is presented in the appendix’s final summary.
REFLECTION CRACKING AND LABORATORY TESTS
Introduction
Reflection cracking is one of the primary distresses for asphalt overlays of existing flexible and rigid pavements. In addition to affecting ride quality, the penetration of water and debris into these cracks accelerates the deterioration of the overlay and the underlying pavement, consequently reducing service life. This section first discusses reflection cracking mechanisms and factors that affect reflection crack development, and then the description of the practical laboratory tests for characterizing reflection cracking resistance of asphalt mixtures. Finally, a summary is presented at the end of this section.
Reflection Cracking Mechanisms
Reflection cracking can result, as shown in Figure A-16, both from traffic and environmentally induced causes. The existing joints or cracks affect reflection cracking in two potential ways (Monismith and Cotzee, 1980). The existing joints or cracks result in stress concentrations at the bottom of the asphalt overlays which lead to crack growth into the asphalt overlay. If the stress-concentrating effect of the existing joints or cracks has been nullified by some means, a secondary effect is that the maximum deflection (under wheel load) of the pavement will occur at the crack. Thus, the greatest stresses in the overlay will occur at the joint
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or crack making it the most likely place for crack growth to initiate as an indirect effect of the existing crack.
Stress concentration plays the dominant role in reflection cracking. Thus, the basic mechanism causing reflection cracking is stress concentration in the overlay due to the movement in the underlying pavement in the vicinity of joints or cracks. This movement may be induced by bending or shearing action resulting from traffic loads or daily and seasonal temperature changes. In fact, any reflection cracking is caused by the combination of these three mechanisms. Every traffic load will induce two shearing plus one bending effect on the HMA overlay (see Figure A-16). Moreover, these bending and shear stresses are affected by the daily temperature. Thus, the combination of all three mechanisms is needed to successfully model reflection cracking.
Fracture mechanics failure modes associated with slab movements at the crack interface correspond to the three modes shown in Figure A-17 (Ponniah and Palph, 1989). The horizontal movement of slab (Mode I loading) is normally associated with traffic (bending) loading and temperature variations. This is the most common mode (opening mode) of reflection cracking in pavements. The vertical movement (Mode II loading) of the underlying slabs is traffic induced in which shear stress in the overlay is induced when traffic passes over the joint or crack. Lateral movement (Mode III loading) of concrete slabs is a less common cause of reflection cracking and occurs when uneven slab support is present (Mukhtar and Dempsey 1996, Chou 2004).
(a) Movements caused by thermal and traffic loading (Nunn 1989).
(b) Bending and Shear Stress Variation vs. Wheel Loading Position (Lytton et al. 2009)
Figure A-16. Mechanisms of Reflection Cracking.
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Figure A-17. Three Common Failure Modes Associated with Slab Movements (Ponniah
and Palph, 1989).
Factors Influencing Reflection Cracking Development
Major factors that influence reflection cracking are listed below: • Traffic: Traffic loading is an obvious cause of reflection cracking in asphalt
pavements due to the stress concentration that occurs immediately above the joints or cracks in the old pavements. Moving wheel loads cause both downward overall deflection (bending effect) and differential vertical movements (shear effect) between the underlying slabs (or segmented substrates). These are exacerbated when (1) a void is present under the joint or crack, (2) the load transfer is poor between slabs, or (3) the pavement is overloaded. Both shear and bending stresses in overlays are created under traffic loads. (Rigo, 1993; Yoder and Witczak, 1975; De Bondt, 1999; Mukhtar and Dempsey, 1996; Chou, 2004).
• Environment: Cyclic temperature variations are one of the primary causes of reflection cracking (Rigo, 1993) especially in very cold climates. De Bondt (1999) stated that if an overlaid jointed or cracked pavement structure is subjected to a temperature drop, the opening at joints caused by shrinkage of the old slabs induces a significant opening stress in the overlay. Kim and Buttlar (2002) stated that daily temperature variations and the resulting thermal contractions of pavement layers are driving forces of reflection cracking. Mukhtar and Dempsey (1996) described that both seasonal and daily temperature changes cause the existing pavement to contract and produce tensile stresses in the overlay immediately above the joint or crack.
• Existing pavement conditions: The condition of the existing pavement prior to asphalt overlay is critical to reflection cracking development (Zhou et al. 2009). The poorer load transfer at joints or cracks, the earlier the reflection cracking occurs. Joints or cracks where the load transfer efficiency is below 70 percent should be treated in order to extend overlay life (Zhou et al. 2009); it is always beneficial to treat the joints or cracks before placing an asphalt overlay. Milling before placing an overlay is
Mode I: Normal Tension
Mode II: Normal Shear
Mode III: Parallel Shear
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often the best solution to eliminate shallow surface cracking in an existing asphalt pavement. Cracking and seating or rubblizing a concrete pavement prior to overlay are effective in reducing or eliminating slab movement. Additionally, the existing layer thickness and layer modulus have a significant influence on the propagation of reflection cracking.
• Overlay thickness: A thicker overlay not only reduces the stress or crack potential in the bottom of the layer, but also slows the crack propagation. Thus, it will take a longer time for reflection cracks to propagate to the surface of a thicker overlay. It is worth noting that asphalt overlay life in terms of reflection cracking is not linearly proportional to overlay thickness. A 4 inch asphalt overlay can have twice the life of a 3 inch overlay (Zhou et al. 2009).
• Overlay material properties: Overlay material properties have a fundamental influence on the reflection cracking propagation. The asphalt mixture with softer binder or higher asphalt binder content often have much better capability to resist cracking propagation and consequently have longer reflection cracking life. Generally, the asphalt mixtures with RAP/RAS tend to have relatively poor reflection cracking resistance. Additionally, aged asphalt mixtures often perform poorly in terms of reflection cracking.
• Reflection cracking counter-measures: A variety of counter-measures have been used to retard reflection cracking. These counter-measures include interlayers (such as stress-absorbing membrane interlayers [SAMI], crack-attenuating mixtures [CAM]), fabric, and reinforcing materials (such as steel net, glass grid, etc.). The effectiveness of these counter-measures varies, depending on application conditions but they tend to delay the occurrence of reflection cracks rather than preventing them.
Laboratory Test Methods for Reflection Cracking
Many laboratory test methods have been developed and used to study the reflection cracking of asphalt pavements. However, only limited tests have been verified by field performance data. The researchers reviewed all available test methods for reflection cracking in the literature, but only those validated or promising tests are discussed here. For each test method, it is discussed in the following format:
• Description of the test. • Availability of test standard (AASHTO, ASTM, or state test procedure). • Test condition parameters. • Material properties measured. • Test variability. • Test simplicity (or complexity) in terms of technician training requirements,
specimen preparation and instrumentation, performing the test and analyzing data, and interpretation of test results.
• Test sensitivity to asphalt mix components and mix design properties (such as air void).
• Test equipment availability and associated cost. • Lab-to-field correlation and associated criteria.
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Texas Overlay Test (OT)
Description
The Texas OT was originally developed by Lytton and his co-worker in late 1970s (Germann and Lytton 1979). Zhou and Scullion (2004) completely updated and standardized the OT test procedure which has been adopted by TxDOT (Tex-248-F) and NJDOT (B-10). The key parts of the Texas OT consist of two steel plates underlying an asphalt mix sample, one plate is fixed and the other is movable horizontally to simulate the opening and closing of joints or cracks in the old pavements beneath an overlay (see Figure A-18).
The typical OT specimen size is 6-inch long by 3 inch wide and 1.5 inch high which can be easily prepared with a laboratory compactor or field cores. The Texas OT is a cyclic displacement-controlled test with a triangle loading wave form in 10 second per cycle. The OT is often run at room temperature 77°F with a maximum opening displacement of 0.025 inch, although both test temperature and opening displacement can vary. The test failure is defined as 93% load reduction from the maximum load measured at the first cycle. At the end of the test, the number of load cycles to failure is reported. Additionally, fracture properties (A and n) can be deduced from the measured load vs. displacement curve if needed (Zhou et al. 2007).
Figure A-18. Texas OT and Typical Results.
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Availability of Test Methods (AASHTO, ASTM, Agency, Institutional References)
TxDOT Standard Tex-248-F: Test Procedure for Overlay Test.
NJDOT Standard B-10: Overlay Test for Determining Cracking Resistance of HMA.
Test Condition Parameters
Test specimen geometry (6-inch long by 3 inch wide and 1.5 inch high); Loading frequency (0.1 Hz); Test temperature (77 ºF); Maximum opening displacement (0.025 inch).
Properties Measured
Number of cycles to failure; Fracture properties (A and n).
Test Variability
A recent study by Walubita et al. (2012) noted that the COV for OT results is approximately 30 percent for most dense- graded mixes, which is higher than most monotonic crack tests (such as DCT, SCB, IDT). In general, the study indicated that the sample drying method, glue quantity, number of sample replicates, air voids, sample age at the time of testing, and temperature variations are some of the key impacts on the OT repeatability and variability.
Walubita et al. (2012) commented that a high variability would be experienced with any repeated load cracking tests, and they concluded later that the repeated load cracking tests (such Texas OT, bending beam fatigue test, S-VECD) should not be compared with monotonic crack tests (such as DCT, SCB, IDT). Relatively higher COV may be the inherent nature of the repeated loading type of cracking tests.
Test Simplicity (or Complexity)
• Specimen preparation: Texas OT specimens can be prepared either from laboratory compacted specimens or field cores. Figure A-19 shows the process of preparing an OT specimen with a laboratory sample. With a single blade saw, four cuts are needed; with a double blade saw, only two cuts are needed. No internal cuts or drills (notches or holes) are required. The only additional work is to glue the cut and dried the OT specimen to the testing platens with a glue jig.
Figure A-19. Texas OT Specimen Preparation (Tex-248-F).
• Specimen instrumentation: No instrumentation is required. • Running the test and technician training: Performing the OT is very easy with the
commercial OT machines so that a minimum of technician training is needed. The test time can range from minutes to a maximum of 3 hours. OT machine can automatically detect the failure and then stop by itself. If the AMPT with OT kit is used for OT testing, some training may be required.
• Data analysis: At end of the OT, the operator can read the number of cycles to failure so no secondary data analysis is involved. If the users are interested in deducing fracture properties (A and n) of asphalt mixtures, special software is needed.
• Interpretation of test results: Data interpretation is very easy since the measured OT cycles can be directly compared against the established OT requirement to see if the mixture passes. Alternatively, the measured fracture properties (A and n) can be used as inputs to an asphalt overlay design and analysis system (such as TxACOL) to predict reflection cracking development.
Test Sensitivity to Asphalt Mix Composition and Design Parameters
The Texas OT is sensitive to asphalt binder content, binder grade, aggregate gradation, RAP/RAS, air void, aging, and other mix design properties (Zhou and Scullion 2005; Zhou et al. 2006; Zhou et al. 2011, Zhou et al. 2013, Walubita et al. 2012).
Availability and Cost of the Test Equipment
The Texas OT is commercially available at a cost of approximately $50,000. Additionally, IPC Global of Australia provides Texas OT kits for the AMPT machine which allows AMPT users to run the Texas OT. The cost for the Texas OT kits is around $3500.
Laboratory-to-Field Correlations and Cracking Criteria
The Texas OT has good correlation with observed field reflection cracking in Texas, New Jersey and Nevada, as reported by Zhou and Scullion (2005), Zhou et al. (2006), Bennert
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and Ali (2008), Hajj et al. (2010), and Walubita et al. (2012). Bennert et al. (2009) successfully applied the OT results to identify reasons for premature reflection cracking on I-495 in New Jersey in a forensic study. The good correlation between laboratory and field was further confirmed with California HVS asphalt overlay tests with rubber- and polymer- modified asphalt binders (Zhou et al. 2010). After reviewing many reflection cracking tests, Loria-Salazar (2008) concluded that the Texas OT is very few laboratory test method to have undergone field validation that exhibited consistency between laboratory test results and their corresponding field performance. Furthermore, the Texas OT device has been widely used to evaluate the effectiveness of different geosynthetic materials (Germann and Lytton 1979, Pickett and Lytton 1983, Button and Lytton 1987, Cleveland et al. 2003).
TxDOT has the following cracking requirements for different mixes:
The DTC test as discussed in Section 2 can also be used for reflection cracking.
Availability of Test Methods (AASHTO, ASTM, Agency, Institutional References)
ASTM D7313-13.
Test Condition Parameters
Geometry of the specimen; CMOD opening rate (1 mm/min); test temperature.
Properties Measured
Fracture energy (Gf).
Test Variability
The typical COV associated with DCT testing is around 10 percent.
Test Simplicity (or Complexity)
The same as that discussed in Chapter 2.
Test Sensitivity to Asphalt Mix Composition and Design Parameters
The same as that discussed in Chapter 2.
Availability and Cost of the Test Equipment
The same as that discussed in Chapter 2.
Laboratory-to-Field Correlation and Cracking Criteria
As early as 2005 (Wagoner et al 2005), Buttlar and his co-workers developed the DCT test for reflection cracking, and the fracture energy (Gf) from the DCT has been used for evaluating reflection cracking (Wagoner et al. 2006, Kim et al 2009, Dave et al. 2010). The laboratory measured Gf values have been compared with the field reflection cracking
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observed on field test sections in New York, Iowa, and Illinois as well as the University of Illinois-Urbana-Champaign (UIUC)-ATLAS APT test facility. It was concluded that the DCT-based Gf values of various asphalt mixtures were generally in good agreement with the field performance data. Due to the complexity of reflection cracking mechanisms and multiple influential factors, DCT- Gf based reflection cracking criteria have not been established yet. Instead, an integrated laboratory testing (both IDT-strength and creep compliance and DCT- Gf) and reflection crack modeling approach was emphasized (Wagoner et al. 2006).
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Wheel Reflection Cracking (WRC) Device
Description
Gallego and Prieto (2006) presented the WRC device, which simulates both thermal and traffic movements, for evaluating reflection cracking resistance of asphalt overlays. A schematic representation is shown in Figure A-20. During the test, one of the plates remains immobile while the other is displaced horizontally by the action of the tensile force applied to one end at a speed ranging from 0.001 to 50 μm/hr. Deflection is simulated by placing a prism-shaped rubber block under the rocker support; see Figure A-21. As a result, a vertical displacement occurs caused by the strain in the rubber block. This equipment allows different magnitudes of deflection to be simulated by varying the hardness or thickness of the rubber blocks used, to which end different heights of rocker support are available.
The 12×12×2.4 inch (305×305×60 mm) specimens used in the test were cut from slabs. The test temperature of 5±1°C was chosen to simulate the low-temperature distress mechanism and for running the test. However, the test can be performed over a range of temperatures between 0°C and 20°C. The crack or joint opening speed is 600 μm/h. The researchers suggested the failure criteria when a relative movement of 0.2 mm occurs between the crack edges. Thus the result of the test developed in this research project is the time required for a relative vertical movement of 0.2 mm to occur between crack borders.
Figure A-20. Schematic Representation of WRC Test (Gallego and Prieto 2006).
Figure A-21. Photos of WRC Test (Gallego and Prieto 2006).
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Availability of Test Methods (AASHTO, ASTM, Agency, Institutional References)
Not available.
Test Condition Parameters
Specimen geometry (12×12×2.4 inch [305×305×60 mm]); Test temperature (5 °C); Crack or joint opening speed (600 μm/h).
Properties Measured
The time required for a relative vertical movement of 0.2 mm to occur between crack borders.
Test Variability
No variability information was found for this test.
Test Simplicity
Load control and setting up is complex. Test duration is long.
Test Sensitivity to Asphalt Mix Composition and Design Parameters
No sensitivity analysis was performed in this study.
Availability and Cost of the Test Equipment
Not available.
Laboratory-to-Field Correlations and Cracking Criteria
The researchers concluded that the results of the WRC test applied to different overlays have shown adequate consistency with the current Spanish experience in full-scale test sections.
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Summary
Reflection cracking is the major distress for asphalt overlays, and it occurs in the overlay right above the joints or cracks. The reflection cracking mechanism is well defined in the literature, and its development is affected by many factors, such as traffic, environment, existing pavement conditions, asphalt binder grade, aggregate type, and air voids. Identifying a cracking test for reflection cracking and establishing associated criteria are vital to design proper asphalt mixtures for different asphalt overlay applications. This section summarized and reviewed cracking tests for reflection cracking in terms of the test variability, test simplicity (or complexity), test sensitivity to asphalt mix components and mix design properties, lab-to-field correlation and associated criteria, availability of test standard, availability of test equipment, and associated cost. Additional discussion on test method evaluation and selection is presented in the final summary.
BOTTOM-UP FATIGUE CRACKING AND LABORATORY TESTS
Introduction
Bottom-up fatigue cracking is a major distress mode considered in pavement structural design, and it has been studied for decades so substantial information about bottom-up fatigue cracking is available in the literature. This section first discusses bottom-up fatigue cracking mechanisms and factors that affect its development, and then the focus is placed on practical laboratory tests for characterizing bottom-up fatigue cracking resistance of asphalt mixtures. Finally, a summary is presented at the end of this section.
Bottom-up Fatigue Cracking Mechanisms
Asphalt pavement bottom-up fatigue cracking is the result of repetitive traffic loads being applied to the pavement surface resulting in a tensile strain level at the bottom of the asphalt layer below that which would cause fracture due to a single load application. It is usually caused by a combination of an inadequate structural cross-section and a weak or brittle asphalt mixture. It has been proposed that the bottom-up fatigue cracking consists of the two phases of the degradation process: crack initiation and crack propagation. During the process of the crack initiation, microcracks grow from microscopic size until, as some research indicates, a critical length of about 7.5 mm is reached (Lytton et al. 1993). In the crack propagation process, a single crack or a few cracks grow until the crack(s) reaches the pavement surface. Researchers noted that both microcracks and macrocracks can be propagated by tensile or shear stresses or combinations of both. Thus, in a pavement structure, microcracks can form and grow in any location where tensile or shear stresses generated by traffic or environmental variations are sufficiently large. Any tensile or shear stress applied to a field where microcracks exist may cause them to grow, to reach critical size, and then to propagate as macrocracks.
The number of traffic load repetitions (Nf), to cause a crack to penetrate through the full depth of the pavement surface layer is the sum of the number of load repetitions for crack initiation (Ni), and the number of load repetitions required for the macrocracks to propagate to the pavement surface (Np). Both Ni and Np are affected by many factors, and many laboratory tests have been developed to evaluate bottom-up fatigue cracking resistance of asphalt mixtures, as discussed in the following sections.
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Factors Influencing Bottom-up Fatigue Cracking Development
Major factors that influence bottom-up fatigue cracking are listed below: • Traffic: Traffic loading is an obvious cause of fatigue cracking in asphalt pavements
due to the stress or strain that occurs in the HMA pavement. Heavier tire loads and higher traffic volume than anticipated cause more or earlier fatigue cracking in the pavement. In laboratory tests, the fatigue response has been shown to be a function of mode of loading which is the method by which stress and strain are permitted to vary during repetitive loading. Limits to the loading conditions range from the controlled-stress mode to the controlled-strain mode, the loading shape, and the duration of the load pulse. The results may be quite different depending upon the conditions (Tangella et al. 1990). Attempts have been made to determine what mode of loading best simulates actual pavement conditions (Monismith and Deacon 1969; Monismith et al. 1977).
• Environmental condition: Environmental condition influences the HMA material properties such as mixture stiffness and in turn influences the stress or strain in the pavement. The stiffening of the asphalt due to aging would likely reduce its ability to resist cracking since the brittleness of the mixture increases in cold temperatures. However, this might not be always true for other environmental conditions. Epps (1969) compared the fatigue performance of specimens obtained from pavements subjected to actual traffic loading to that of laboratory specimens of similar composition. He concluded that aging-induced stiffening of the field mix increases its fatigue life to the extent that it offsets the effect of higher in-situ air void contents and damage due to traffic.
• Pavement structural combination: Pavement structure (e.g., the thicknesses of HMA layer and base layer, the modulus of HMA layer and base layer) has a significant influence on the magnitude of the tensile strains and the potential location of fatigue crack initiation (Hu et al. 2008). For example, by comparing the tensile strains at different locations in the pavement structure, Hu et al. (2008) found that when the HMA layer modulus is high, the potential location of fatigue crack initiation depends on the HMA layer thickness. However, when the HMA modulus is low both top-down and bottom-up crack initiations are likely to occur simultaneously, irrespective of the HMA layer thickness.
• HMA material property: Asphalt mixture properties have fundamental influences on the fatigue cracking propagation. The asphalt mixture type, binder gradation, asphalt content, and air void content all have direct influence on the material properties in terms of stiffness, viscoelastic properties, and fracture properties.
• Specimen fabrication method: The primary objective of specimen fabrication or compaction is to produce “realistic” test specimens which can reasonably duplicate the corresponding in-situ asphalt paving in all major respects including composition, density, and engineering properties. Compaction methods utilized to fabricate test specimens include: (1) static compaction, (2) impact compaction, (3) kneading compaction, (4) gyratory compaction, and (5) rolling-wheel compaction. Tangella et al. (1990) pointed out that rolling-wheel, kneading, and gyratory methods produce test specimens more like the in-situ pavement than either static or impact compaction.
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Laboratory Test Methods for Fatigue Cracking
In the literature the research team found that many fatigue tests have been developed to evaluate and characterize the fatigue response of asphalt mixtures. During the SHRP study, Tangella et al. (1990) provided an excellent review of laboratory bottom-up fatigue cracking. Monismith and his SHRP A-003 team (Tayebali et al. 1994, Tayebali et al. 1995) evaluated the 4-point bending beam fatigue test and other tests, and finally recommended the 4-point bending beam fatigue test for characterizing fatigue resistance of asphalt mixtures. Later AASHTO adopted it as a Standard Test Method T-321. Another type of bending beam fatigue is the trapezoidal cantilever beam fatigue test which has been widely used in Europe. Most recently, there are some new fatigue tests being developed, such as SCB at intermediate temperature, simplified viscoelastic continuum damage (S-VECD) test (Lee and Kim 1998, Daniel and Kim 2002, Chehab et al. 2002, Bodin et al. 2004, Christensen and Bonaquist 2005, and Underwood et al. 2006), Texas OT, and semicircular direct tension test. For each of these fatigue cracking test methods, the following format is used for the discussion:
• Description of the test. • Availability of test standard (AASHTO, ASTM, or state test procedure). • Test condition parameters. • Material properties measured. • Test variability. • Test simplicity (or complexity) in terms of technician training requirements,
specimen preparation and instrumentation, performing the test and analyzing data, and interpretation of test results.
• Test sensitivity to asphalt mix components and mix design properties (such as air voids).
• Test equipment availability and associated costs. • Lab-to-field correlation and associated criteria.
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Flexural Bending Beam Test
Description
The flexural beam fatigue test has been in use for many years, but it was not standardized until the end of the SHPR program. The current AASHTO T321 is a refined version of the flexural beam fatigue test originally proposed by Monismith and his co-workers at the University of California Berkeley under the SHRP A-003A (Tayebali et al. 1994). The beam specimen is 15×2×2.5 in. (380×50×63 mm). The load rate is variable but is normally set at 10 Hz. Although the test can be run in stress- or strain-controlled mode, the strain-controlled mode is much more widely used because it appears to provide results that are more comparative to field observations. Generally, the testing is performed at intermediate temperatures, usually 68°F.
Figure A-22 shows a four-point flexural beam fatigue test apparatus (www.pavementinteractive.org). During the test, the beam is held in place by four clamps and a repeated haversine (or sinusoidal) loading is applied to the two inner clamps with the outer clamps providing a reaction load. This produces a constant bending moment over the center portion of the beam (between the two inside clamps).
Figure A-22. Four-Point Flexural Beam Fatigue Test Apparatus
(www.pavementinteractive.org).
Availability of Test Methods (AASHTO, ASTM, Agency, Institutional References)
AASHTO T321: Standard Method of Test for Determining the Fatigue Life of Compacted Hot-Mix Asphalt (HMA) Subjected to Repeated Flexural Bending
ASTM D7460 Standard Test Method for Determining Fatigue Failure of Compacted Asphalt Concrete Subjected to Repeated Flexural Bending
Beam geometry (15×2×2.5 inch); Specified strain level; Loading frequency (10Hz); Test temperature (68 °F)
Properties Measured
Number of cycles to failure; Dissipated energy; Fatigue equation
Test Variability
The COV of fatigue life measured from the flexural beam test is larger than 50%, as reported by Monismith and his co-workers under the SHRP study (Tayebali et al. 1994).
Test Simplicity (or Complexity)
• Specimen preparation: In general, it is difficult to make beam specimens, because the long beam has to be cut from a large slab which needs a rolling compactor to make.
• Specimen instrumentation: It requires very little instrumentation, just aligning the LVDT to the stud glued to the center of the beam specimen.
• Running the test and technician training: Running the flexural beam fatigue test is not difficult, and it does not require much technician training time. However, it may take a long time to finish one test, depending on the asphalt mixture and strain levels. High strain (400–800 microstrain) may be completed in a few hours. Low strain tests (200–400 microstrain) can take days. Even lower strain levels (50–100 microstrain) can take more than a month.
• Data analysis: Number of cycles to failure can be directly determined. • Interpretation of test results: If only number of cycles to failure is reported, then it is
easy to compare with the pass/fail criterion to judge the acceptance of the asphalt mixture. If the test result is a fatigue equation, then it often combines with pavement design system to predict pavement fatigue life.
Test Sensitivity to Asphalt Mix Composition and Design Parameters
It is well known that the beam fatigue test is sensitive to asphalt binder grade, binder content, air voids, aggregate type, strain level, and temperature (Tayebali et al. 1994).
Availability and Cost of the Test Equipment
Generally a universal test machine is needed to run the flexural beam fatigue test; and the cost of this type of machine is more than $150,000.
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Laboratory-to-Field Correlations and Cracking Criteria
The correlation between flexural beam fatigue test results and field fatigue cracking has been well studied. Many pavement design methods (such as AI, AASHTOWare Pavement ME design) use the flexural beam fatigue test to develop fatigue equations for asphalt mixtures.
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Trapezoidal Beam Fatigue Test
Description
The trapezoidal beam fatigue test is also called the 2-point bending beam test and is widely used in Europe. Tests have been conducted on trapezoidal specimens by the Shell researchers (van Dijk, 1975), Belgium researchers (Verstraeten, 1972), and LCPC researchers (Bonnot, 1986). The test is detailed in the EN12697 standards. Figure A-23 shows a 2-point bending team test machine made by Cooper Technology (www.cooper.co.uk). The tested trapezoidal beam size is about 10 inch (250 mm) high and 1 inch (25 mm) thick, the small and large base of the trapeze are 1 inch (25 mm) and 2.2 inch (55 mm) respectively. The test specimens are cemented onto a metallic part, which is firmly fixed to the bottom plate in an upright position. A thin metal plate 0.02 inch (0.5 mm) is cemented on top of the specimen and connected to a load cell. Two trapezoidal specimens are tested simultaneously.
Fatigue tests are usually run in a controlled strain mode at 15 Hz and 20°C. Normally strain levels between 120–190 με are chosen. Fatigue failure is defined as the point when the stiffness modulus decreases to a user-selected target level, normally 50 percent of its initial value.
Figure A-23. Two-Point Trapezoidal Beam Fatigue Test Apparatus (www.cooper.co.uk)
and Specimen.
Availability of Standard Test Methods (AASHTO, ASTM, Agency, Institutional References)
EN 12697-24 Annex A, EN 12697-26 Annex A, and NF-P-98-260-2.
Test Condition Parameters
Specimen geometry (Figure A-23); Strain level (120-190 microstrain); Temperature (20°C)
Stiffness at each load cycle; Fatigue life; Dissipated energy.
Test Variability
Not well defined.
Test Simplicity (or Complexity)
• Specimen preparation: In general, it is very difficult to make the trapezoidal beam specimen with current compaction devices. It is cut from large slabs, but it requires more effort to achieve a trapezoidal beam specimen of the correct dimensions.
• Specimen instrumentation: Simple; It consists of mounting the load cell platen. • Running the test and technician training: Running the 2-point beam fatigue test is not
difficult. There would be test development time to produce standard test methods and criteria for the US. As with the 4-point bending beam, it may require a long time for running the test, depending on the asphalt mixture itself and strain levels. However, results from two beams can be obtained in one test. Additionally, significant technician training time may be required to become familiar with the test.
• Data analysis: The number of cycles to failure can be easily determined. • Interpretation of test results: Similar to the 4-point flexural beam fatigue test.
Test Sensitivity to Asphalt Mix Composition and Design Parameters
Not much information is available from US studies. However, its widespread use in Europe suggests that the results are useful in designing asphalt mixtures.
Availability and Cost of the Test Equipment
Commercial test equipment is available from Cooper Technology of the UK, but the exact cost is not clear at this stage.
Laboratory-to-Field Correlations and Cracking Criteria
The 2-point beam fatigue test has been used in France pavement design method for years. Thus, the laboratory-to-field correlation should be acceptable.
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SCB at Intermediate Temperature
Description
The SCB test at intermediate temperature for fatigue cracking was recently proposed by Mohammad and co-workers at the Louisiana Transportation Research Center (LTRC) (Wu et al. 2005, Elseifi et al. 2012). The LTRC-SCB test is similar to the SCB for low-temperature cracking (AASHTO TP105-13), but there are five differences: (1) test temperature: room temperature (77°F); (2) SCB specimen thickness: 2.25 inch; (3) three notch depths required: 1.0, 1.25, and 1.5 inch; (4) loading rate: cross-head controlled and deformation rate of 0.5 mm/min; and (5) the calculated fracture property: critical energy release rate (Jc).
Figure A-24 shows the LTRC-SCB test setup and typical test result. The critical strain energy release rate (Jc) is the absolute value of the ratio of the slope of the fracture energies vs. the notch depths (Figure A-24) to specimen thickness. Higher Jc values are desirable for better fracture-resistant mixtures. A threshold of a minimum Jc of 0.65 kJ/m2 has been suggested as a failure criterion (Wu et al., 2005).
Figure A-24. SCB Test Setup and a Typical Test Result (Wu et al. 2005).
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Availability of Test Methods (AASHTO, ASTM, Agency, Institutional References)
LTRC-SCB method
Test Condition Parameters
Specimen geometry; Loading rate (0.5 mm/min.); Test temperature (77°F)
Properties Measured
Critical strain energy release rate Jc
Test Variability
COV of Jc is around 20%, as reported by Kim et al. (2012).
Test Simplicity (or Complexity)
• Specimen preparation: LTRC-SCB test specimens are very easy to prepare. Generally, three 6 inch diameter samples are cut into six specimens and two replicates for each notch depth. For each specimen, a cut is needed to generate a notch.
• Specimen instrumentation: No instrumentation is needed. • Running the test and technician training: Running the LTRC-SCB is very simple and
very little technician training is needed. • Data analysis: The calculation of the critical strain energy release rate involves the
integration of the area of load vs. displacement curve (strain energy) and calculation of the slope of the strain energy vs. notch depth. So a simple Excel Macro or a special data analysis program is needed.
• Interpretation of test results: Straight-forward; the measured Jc is compared with the established pass/fail criteria.
Test Sensitivity to Asphalt Mix Composition and Design Parameters
The LTRC-SCB test is sensitive to asphalt binder grade, aging, and RAP/RAS, as reported by Wu et al. (2005), Kim et al. (2012).
Availability and Cost of the Test Equipment
LTRC-SCB test is a very simple test, and most universal hydraulic testing machines can be used to perform the LTRC-SCB test.
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Laboratory-to-Field Correlations and Cracking Criteria
Kim et al. (2012) reported that the LTRC-SCB Jc has a fair correlation with field cracking data, as seen in Figure A-25.
Figure A-25. Correlation between Jc and Field Cracking Data (Kim et al. 2012).
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S-VECD Fatigue Test
Description
Kim and his co-workers (Underwood et al. 2010; Hou et al. 2010) recently simplified the original viscoelastic continuum damage fatigue model and associated tests, and renamed it as S-VECD fatigue test. Currently, the S-VECD fatigue test is an AASHTO provisional standard test: AASHTO TP107-14. The S-VECD fatigue test itself does not include dynamic modulus (E*) test, but E* test results are needed for performing the S-VECD fatigue damage analysis. The AMPT equipment can perform both E* and S-VECD tests (see Figure A-26). The S-VECD test includes a dynamic modulus finger print test and two cyclic fatigue damage tests at different strain levels. The VECD damage parameters are determined from the test data using the ALPHA-Fatigue software developed by Kim and his co-workers.
Figure A-26. AMPT Equipment and S-VECD Test Specimen.
Availability of Test Methods (AASHTO, ASTM, Agency, Institutional References)
AASHTO TP107-14: Standard Method of Test for Determining the Damage Characteristic Curve of Asphalt Mixtures from Direct Tension Cyclic Fatigue Tests
Test Condition Parameters
Specimen geometry (4 inch diameter and 5.1 inch high); Strain levels; Loading frequency (10 Hz); Test temperature
Properties Measured
Damage characteristic curve and S-VECD fatigue model
Test Variability
Not well defined.
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Test Simplicity (or Complexity)
• Specimen preparation: Preparing the S-VECD specimen is equivalent to that required for an E* specimen. A tall Superpave gyratory specimen is cored to a diameter of 4 inch and the ends are trimmed to achieve a height of 5.1 inch. The most critical and often difficult part is gluing the specimen to the end platens. Since this is a cyclic tension test, the specimen is glued perpendicular to the top and bottom platens, and even with a double blade saw and a special glue jig, avoiding eccentricity in the alignment is difficult.
• Specimen instrumentation: The same instrumentation for the E* test is needed for the S-VECD test.
• Running the test and technician training: AASHTO TP107-14 recommends a total of three tests be done at three different strain levels. The first cyclic fatigue test starts with the peak-to-peak on-specimen strain level of 300 microstrain. Then, the peak-to-peak on-specimen strain levels of the second and third specimens are determined based on the resultant number of cycles to failure of the first specimen. AASHTO TP107-14 guidelines on-specimen strain levels for the second and third specimens. The running time for the S-VECD test depends on the asphalt mixture itself and strain levels, and it may take hours or one day to finish all 3 tests. Running the S-VECD test is not difficult but extensive technician training is needed for the proper sample preparation and instrumentation.
• Data analysis: Data analysis is easy when using the ALPHA-Fatigue software. • Interpretation of test results: It is very simple if only the number of cycles to failure is
wanted. However, it can also be somewhat complex if LVECD or VECD-FE++ pavement performance analysis is employed to predict field performance.
Test Sensitivity to Asphalt Mix Composition and Design Parameters
No sensitivity analysis was available.
Availability and Cost of the Test Equipment
The AMPT is used to perform the S-VECD test, and it costs around $100,000.
Laboratory-to-Field Correlations and Cracking Criteria
Kim and his co-workers have validated the S-VECD fatigue models with the FHWA-ALF test lanes (Underwood et al. 2009), and it has been verified with in-service pavements in North Carolina (Park and Kim 2014).
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Texas Overlay Test (OT)
Description
The same description as presented in Section 3.
Availability of Test Methods (AASHTO, ASTM, Agency, Institutional References)
TxDOT Standard: Tex-248-F Test Procedure for Overlay Test.
Test Condition Parameters
Test specimen geometry; Loading frequency; Test temperature; Maximum opening displacement (smaller than OT for reflection cracking, say 0.017-0.020 inch).
Properties Measured
Number of cycles to failure; Fracture properties (A and n).
Test Variability
Same variability as discussed in Chapter 3.
Test Simplicity (or Complexity)
Same test simplicity as described in Chapter 3.
Test Sensitivity to Asphalt Mix Composition and Design Parameters
Same sensitivity as described in Chapter 3.
Availability and Cost of the Test Equipment
Same as described in Chapter 3.
Laboratory-to-Field Correlations and Cracking Criteria
The good correlation between the Texas OT and field fatigue cracking has been validated by FHWA-ALF fatigue test on polymer modified binders and the structural test sections at NCAT 2006 test track (Zhou et al. 2007, Hu et al. 2012). Most recently study by Tran and
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his co-workers at NCAT (Ma et al. 2015) employed the Texas OT to evaluate five asphalt mixtures placed at the structural sections of the 2009 NCAT test track. The OT was performed at much smaller openings, and the OT results were interpreted in the form of fatigue model, which is different from the traditional OT cycles. They found that the ranking based on the number of ESALs until one percent cracking observed at the test track was exactly the same as the ranking by the strain-corrected OT results. Clearly, further validation is needed, as concluded by Ma et al. (2015).
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Monotonic IDT Test
Description
The IDT has been used to characterize fatigue properties of asphalt mixtures for a number of years, but it has not been widely accepted. Kim and Wen (2002) performed indirect tensile creep and strength tests on WesTrack field cores using the MTS servo-hydraulic closed-loop testing machine. A creep test was conducted first, followed by a constant-crosshead rate monotonic test at 2 inch per minute until failure. They found that neither tensile strength nor the horizontal strain at peak stress provided any relationship to resistance to fatigue cracking. However, the fracture energy (the sum of the strain energy and the dissipated energy due to structural changes such as micro-cracking), defined by the area under the stress-strain curve in the loading portion, seems to have a very good correlation with the fatigue cracking percentage in the test track.
The IDT specimen diameter can be 4 inch or 6 inch (Figure A-27). For laboratory-molded specimens, height is 2.4 inch; for core specimens, the height must be a minimum of 1.5 inch. Loading strips consists of 0.75×0.75 inch square steel bars, which are machined on the surface to the curvature of the test specimen. The test is conducted at room temperature 70°F to 77°F.
Figure A-27. IDT Test Configuration.
Availability of Test Methods (AASHTO, ASTM, Agency, Institutional References)
Any IDT strength test procedure is applicable, although no standard exists for an IDT fatigue test.
Test Condition Parameters
Specimen geometry; Loading rate (2 inch/min.); Test temperature
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Properties Measured
Fracture energy
Test Variability
Monotonic IDT test variability is low, although detailed information is not available on this specific test.
Test Simplicity (or Complexity)
• Specimen preparation: Simple. • Specimen instrumentation: It is not necessary. • Running the test and technician training: Running the IDT strength test is easy and
technician training is minimal. • Data analysis: Similar to calculation of DCT fracture energy, it needs an Excel macro
or specific program to integrate the area of the stress/strain (or load/displacement) curve.
• Interpretation of test results: It is very simple if a pass/fail criterion is used.
Test Sensitivity to Asphalt Mix Composition and Design Parameters
Not available.
Availability and Cost of the Test Equipment
A hydraulic universal testing machine is adequate to perform this test.
Laboratory-to-Field Correlations and Cracking Criteria
Fracture energy measured from IDT is reported to have a good correlation with WesTrack fatigue data, as reported by Kim and Wen (2002).
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Summary
Bottom-up fatigue cracking is a major distress considered in the structural design of pavements. The cracking mechanism is generally defined in the literature, and its development is affected by many factors, such as traffic, environment, pavement structural combinations, asphalt binder grade, aggregate type, and air voids. Identifying a cracking test for the bottom-up fatigue cracking and establishing associated criteria are vital to the design of pavement structures and asphalt mixtures. This section summarized and reviewed cracking tests for bottom-up fatigue cracking in terms of test variability, test simplicity (or complexity), test sensitivity to asphalt mix components and mix design properties, lab-to-field correlation and associated criteria, availability of test standard, availability of test equipment and associated cost. More discussion on test method evaluation and selection is presented in the final summary.
TOP-DOWN FATIGUE CRACKING AND LABORATORY TESTS
Introduction
In recent years top-down fatigue cracking has attracted a great deal of attention from pavement engineers and researchers. This cracking initiates at the surface of the asphalt pavement and propagates downward through the asphalt layer. It usually develops in the longitudinal direction both within the wheel path and outside of the wheel path (Harmelink and Aschenbrener 2003). Top-down cracking has been identified in the US (Anderson et al. 2001; Harmelink and Aschenbrener 2003; Harmelink et al. 2008; Myers and Roque 2002; Uhlmeyer et al. 2000) and many other countries (Hugo and Kennedy 1985, Dauzats and Rampal 1987, Gerritsen et al. 1987, Matsuno and Nishizawa 1992, Nunn 1997, and El Halim et al. 2004).
According to Svasdisant et al. (2002), top-down fatigue cracking has three stages. As shown in Figure A-28, the first stage consists of a single short longitudinal cracking appearing outside of the wheel path. In the second stage, companion cracks develop 12 to 40 inch parallel to the original crack. The parallel cracks are connected through short transverse cracks in the third stage. The basic characteristics of top-down cracking include: 1) it is oriented in the longitudinal direction; 2) it can occur in a wide range of layer thicknesses, but thin (< 4 inch) and thick (> 8 inch) asphalt layers have more potential for top-down cracking (Lytton et al. 2013); and 3) it happens from 1 to 11 years after paving with a typical age for the first top-down crack being 3 to 8 years (Uhlmeyer et al. 2000)
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Figure A-28. Three Stages of Top-down Cracking (A: Stage 1, B: Stage 2, C: Stage 3) and
Field Cores Showing Cracks (Svasdisant et al. 2002).
Top-Down Cracking Mechanisms
The mechanisms of top-down cracking have not been clearly defined and well modeled yet. Through studies in a series of NCHRP projects such as NCHRP 1-42 (Christensen et al. 2004), 1-42A (Roque et al. 2010), and 1-52 (Lytton et al. 2013, ongoing), it is generally believed that top-down cracks that develop at different locations on the pavement surface may have different mechanisms. According to its mechanism, top-down cracking can be grouped into two categories: (1) construction-related top-down cracking; and (2) load-related top-down cracking. Construction-related top-down cracking results mainly from the segregation of aggregate in the asphalt mixture during construction. This type of cracking is a construction related distress that cannot be predicted from laboratory testing, and will not be considered in the NCHRP 9-57 project. The following discussion will focus on the mechanisms for load-related top-down fatigue cracking.
Load-related top-down cracking usually develops in the wheel paths, produced by (1) bending-induced surface tension away from the tire (Soon et al. 2003; Zou 2009; Roque et al. 2010); and (2) shear-induced near-surface tension at the tire edge due to the non-uniform 3-D tire-pavement contact stresses (Myers et al. 1998; Christensen et al. 2004) and accelerated by thermal stresses and binder aging effects (Myers 2000; Lytton et al. 2013).
• Bending-induced surface tension away from the tire: The mechanism for the top-down cracking due to this tension from the tire is illustrated in Figure A-29. Soon et al (2004) found that the traffic load induced horizontal tensile stresses ( xxσ ) occur at a location of five times the width of the loaded area in a very thin layer (i.e., 50 mm),
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and that the maximum tensile stress location occurs further away with increasing layer thickness. Roque et al. (2004, 2010), Birgisson et al. (2006) and Zou (2009) also recognized that radial truck tires induced transverse tensile stresses at the surface of the pavement that could be sufficient to initiate surface cracks. This bending-induced top-down cracking often occurs in pavements with thin to medium (50 to 200 mm, Myers et al. 1998) thick asphalt layers and is accelerated by an aged surface mixture, a thermal stress caused by cooling, and the stiffness gradient of the asphalt layer.
Figure A-29. Mechanism of Bending-Induced Top-down Cracking (Soon et al. 2004, Zou
2009).
• Shear-induced near-surface tension at the tire edge: This type of cracking is mainly caused by non-uniform stress distribution in the tire contact area. Accordingly, a number of researchers investigated the contact pressure distributions beneath rolling tires, including the non-uniform vertical stress, longitudinal stress, and transverse stress as shown in Figure A-30 (De Beer et al. 1997, Fernando et al. 2006; Myers et al. 1999; Weissman 1999) and they obtained the following characteristics: o The contact pressure distribution is three-dimensional and highly non-uniform; o Maximum vertical contact pressure can be up to twice the tire inflation pressure; o The transverse contact stress (perpendicular to the direction of travel) under a
smooth tire indicated inward shear (toward the center of the tire) with zero stresses at the tire center (zero resultant force);
o The longitudinal contact stresses (in the direction of travel) for free-rolling tires indicated that these stresses are lowest in magnitude with the peak stress typically occurring near to the front and rear positions of the tire;
o The contact stresses under grooved tires were more variable compared to a smooth tire, but the patterns were overall similar to those measured from the smooth tire.
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Figure A-30. Typical Contact Stress Distributions Measured for a Slow-Moving Load
Using Vehicle-Road Pressure Transducer Array (VRSPTA) (De Beer 1997).
• Tangential surface stress: Significant tangential forces can be produced in the pavement surface, which are demonstrated by numerical calculations (Bensalem et al. 2000; Collop and Cebon 1995; Gerritsen et al. 1987; Jacobs 1995; Matsuno and Nishizawa 1992; Luo and Prozzi 2007; Perdomo and Nokes 1993) and experimental testing (Myers et al. 1998; Pottinger 1992) of the tire-pavement contact stresses. These forces were estimated for their magnitude which suggests that they may be sufficient to cause large tensile/shear stresses (see Figure A-31) and localized failure near the pavement surface resulting in top-down cracking (Bensalem et al. 2000; Collop and Cebon 1995; Jacobs et al. 1992; Perret 2002, Myers et al. 1999, Mun 2003, Wang et al. 2003).
Figure A-31. Surface Tensile and Shear Stresses near Tire Edge (Myer et al. 1999).
(a) Tensile Stress (b) Shear Stress
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Factors Influencing Top-down fatigue Cracking Development
Load-related top-down fatigue cracking is significantly affected by numerous factors influenced by the material, the pavement structure, the traffic, and the climate, as summarized below:
• Material factors: Material factors include material composition, modulus gradient, fracture and thermal properties of asphalt concrete mixtures. Komoriya et al. (2001) and De Freitas et al. (2005) found that the initiation of top-down cracking is affected by binder content, aggregate gradation, and binder-aggregate adhesion. High air voids at the pavement surface lead to material aging and moisture damage which increase the likelihood of top-down cracking due to a higher surface tensile stress (Svasdisant et al. 2002). Some fracture properties such as creep compliance, dynamic modulus, and tensile strength were determined to evaluate the resistance performance of asphalt concrete to top-down cracking (Birgisson et al. 2006; Myers et al. 1998; Myers et al. 1999; Witczak and Fonseca 1996). Thermal stress contributes to top-down cracking, thus thermal conductivity was used in pavement thermal prediction for top-down cracking modeling (Highter and Wall 1984; Tan et al. 1997).
• Pavement structural factors: Pavement structural factors include pavement layer thickness and stabilized base courses. The bending-induced surface top-down cracking was found to occur in thin to medium thick asphalt layers (Myers et al. 1998; Zou 2009). Top-down cracking associated with the shear-induced near-surface tension at the tire edge was believed to be initiated in thicker asphalt layers according to viscoelastic continuum damage (VECD) model predictions (Roque et al. 2010). Lytton (2013) concluded that the top-down cracking potentially occurs in pavements with an asphalt layer less than 100 mm or greater than 200 mm. Svasdisant et al. (2002) concluded that all of the following conditions contribute to high surface tensile stresses resulting in top-down cracking: o An increasing ratio of the asphalt concrete (AC) surface layer modulus to the AC
base layer modulus. o A higher base layer modulus as in stabilized or rubblized concrete bases. o An increasing thickness of the AC layer with a high-modulus base layer like a
stabilized base layer. o A decreasing thickness of the AC layer with a low-modulus base layer such as
unbound aggregate base. • Traffic factors: Traffic factors include traffic load magnitude, distribution, and
spectrum. Research studies indicate that the top-down cracking is caused mainly by non-uniform stress distribution beneath rolling tires, including vertical, longitudinal, and transverse stresses, as shown in Figure A-32 (De Beer et al. 1997; Fernando et al. 2006; Myers et al. 1999; Weissman 1999). Significant tangential forces can be induced by these stresses and may be sufficient to cause high tensile/shear stresses leading to localized failure near the pavement surface (Bensalem et al. 2000; Collop and Cebon 1995; Jacobs et al. 1992; Perret 2002). The load spectra had significant effects on the development of the surface cracks (Myers and Roque 2002). They concluded that some load positions had significantly more effect than others, depending on the pavement structure and crack length. Figure A-32 shows that for a short crack length, the most critical position is located at under the tire rib (with
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highest stress intensity factor), while at a position away from load center for a longer crack length.
Figure A-32. Effect of Loading Position on Opening at the Top-Down Crack Tip for 200
mm Surface Layer with a High Stiffness Ratio (Myers and Roque 2002).
• Climate factors: Climate factors including solar radiation, wind speed, rain fall, etc. lead to the non-uniform distributions of pavement temperatures, aging, and thermal stresses. Pavement temperature profiles are predicted using climate factors as inputs (Diefenderfer et al. 2006; Van Bijsterveld et al. 2001) and pavement surface cracking was found to occur at higher pavement temperatures (De Freitas et al. 2005; Merrill 2000). Pavement surface tends to become brittle due to material aging and the mix stiffness has a decreasing gradient along the layer depth, which consequently reduces crack resistance and makes it easier for top-down cracking (Leech and Nunn 1997; Svasdisant et al. 2002; Worel 2003; Wamburga et al. 1999; Koohi et al. 2012).
Laboratory Test Methods for Top-Down Cracking
A few laboratory tests have been used to characterize top-down cracking, including IDT (Zhang 2000; Roque et al. 2002; Jajliardo 2003; Roque et al. 2004; Zou 2009; Roque et al. 2010; Baladi et al. 2002), DT test (Jacobs et al. 1995; Luo et al. 2013; Lytton et al. 2013), wheel tracking test (De Freitas et al. 2005; Groenendijk 1998; Wang et al. 2003), and S-VECD. Since S-VECD has been well discussed in a previous section, no further description is added here. Each test method is presented in this section in the following format:
• Description of the test. • Availability of test standard (AASHTO, ASTM, or state test procedure). • Test condition parameters. • Material properties measured. • Test variability.
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• Test simplicity (or complexity) in terms of technician training requirements, specimen preparation and instrumentation, performing the test and analyzing data, and interpretation of test results.
• Test sensitivity to asphalt mix components and mix design properties (such as air void).
• Test equipment availability and associated cost. • Lab-to-field correlation and associated criteria.
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University of Florida IDT Test for Top-Down Fatigue Cracking
Description
The IDT test for the top-down fatigue cracking was developed by Roque and his co-workers at the University of Florida (Roque et al. 2004, Kim et al. 2009, Roque et al. 2010). The University of Florida IDT test (Figure A-33) is the most often used method to obtain the fracture properties of asphalt mixtures for top-down cracking characterization. Basically, it is very similar to the IDT for the low-temperature cracking except the following two exceptions:
• Resilient modulus (Mr) test: The Mr test is an addition to the IDT for low-temperature cracking.
• Test temperature for the top-down fatigue cracking: 50ºF for all three tests (Mr, Creep compliance, and tensile strength tests).
Compliance Test, and (c) Tensile Strength Test (Roque et al. 2004, Kim et al. 2009)
Availability of Test Methods (AASHTO, ASTM, Agency, Institutional References)
• AASHTO T322-07: Determining the Creep Compliance and Strength of Hot Mix Asphalt (HMA) Using the Indirect Tensile Test Device.
• ASTM D7369: Standard Test Method for Determining the Resilient Modulus of Bituminous Mixtures by Indirect Tension Test.
Test Condition Parameters
Geometry of the specimen (6 inch diameter and 2 inch thick); Loading frequency for Mr test (0.1 s loading and 0.9 s resting); Loading time for creep test; Loading rate for tensile strength test; Test temperatures
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Properties Measured
Resilient modulus (Mr); Tensile strength (St); Creep compliance (intercept [D1] and slope (m) on a log-log plot of creep compliance versus loading time); Dissipated creep strain energy at failure (DCSEf)
Test Variability
Similar to the IDT for the low-temperature cracking as discussed in an earlier section. The repeated loading used in Mr testing may result in higher variability for Mr.
Test Simplicity (or Complexity)
• Specimen preparation: The test specimen can be easily made from laboratory compacted mixtures or field cores.
• Specimen instrumentation: The specimen instrumentation is not difficult with a glue jig to locate the studs for the LVDTs.
• Running the test and technician training: Normally, each of the IDT resilient modulus test, the IDT creep compliance test and the IDT strength test can be finished within 15 minutes. The same concern on the creep compliance test as discussed in an earlier section applies here. A trial and error approach may be needed to find such proper load level. A great deal of technician training may be required.
• Data analysis: IDT data analysis is not difficult with the software developed by University of Florida.
• Interpretation of test results: Both the dissipated creep strain energy threshold and the minimum energy ratios have been proposed by Roque et al. (2004). The calculated values from the IDT test can be directly compared with the proposed criteria to see if the mixture is OK. Thus, the interpretation of the IDT tests for the top-down cracking is simple.
Test Sensitivity to Asphalt Mix Composition and Design Parameters
IDT test results are sensitive to the asphalt mixture composition and design parameters such as binder content, air void content, gradation, etc. But no single mixture parameter has a strong correlation to top-down cracking (Roque et al. 2004). For example, the top-down cracking is not sensitive to the change of binder viscosity represented by PG level (Roque et al. 2010).
Availability and Cost of the Test Equipment
Commercially IDT test equipment is not widely available at this time, but the IDT test can be performed with most universal hydraulic test machines with an IDT test fixture and a temperature chamber.
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Laboratory-to-Field Correlations and Cracking Criteria
The fracture energy model developed at the University of Florida for top-down cracking used IDT tests and found the energy ratio (ER; i.e., the ratio of DCSEf to DCSEmin) is a good indicator to rank the cracking resistance of asphalt mixtures based on 27 field test sections in Florida (Roque et al. 2002; 2004; and 2010) and the NCAT test track (Timm et al. 2009). They verified that the DCSEf is a fundamental property that can be obtained from IDT tests and that a higher ER indicates a better cracking resistance. It must be noted that cracking evaluation parameter, ER, is dependent upon traffic level and does not directly consider the effects of thermally induced cracking.
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Direct Tension (DT) Test
Description
The DT test for the top-down fatigue cracking was recently developed by Lytton and his co-workers at the Texas A&M University (Luo et al. 2013; Lytton et al. 2013). This test method is being evaluated under NCHRP 1-52. The DT test is conducted on a cylindrical asphalt mixture specimen (as shown in Figure A-34), which is glued to end plates and subjected to uniaxial tensile loading. The load can be monotonic, repeated, or cyclic in strain or stress control depending upon the models used for characterizing the cracks. Load and axial deformation are recorded to determine viscoelastic and fracture properties of the material.
Lytton’s DT test suite for top-down fatigue cracking includes three components:
• Low strain repeated direct tension tests at 1 Hz and 50 ~ 70 microstrains for 200 cycles.
• High strain repeated direct tension tests at 1 Hz and 200 microstrains for 1000 cycles. • Creep and recovery tests at three temperatures.
Both viscoelastic properties and energy-based fracture properties can be determined from these three tests, and then these properties are used as input to a fracture mechanics model to predict top-down cracking resistance of asphalt mixtures.
Figure A-34. Configuration of Direct Tension Test (Luo et al. 2013).
Availability of Test Methods (AASHTO, ASTM, Agency, Institutional References)
Texas A&M University method.
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Test Condition Parameters
Specimen geometry; Low strain repeated DT test (60 microstrain and 200 cycles); High strain repeated DT test (200 microstrain and 1000 cycles); Creep and recovery tests at three temperatures
Properties Measured
Viscoelastic properties of asphalt mixtures (creep compliance); Modified Paris’ law fracture parameters (A′ and n′)
Test Variability
Not defined yet
Test Simplicity (or Complexity)
A tensile test is much more complicated than a compressive test due to the difficulties listed below:
• The testing specimen must be glued to the end plates to transfer the tensile loads, which may induce non-uniform stress distributions and stress concentrations at the ends of the specimen. Reducing the gauge length by using LVDTs fixed to the lateral surface of the specimen can reduce this influence, but a sample may still fail or fracture in the area outside of the gauge length of the specimen (i.e., the area near the end plates).
• Special fixtures or devices are needed for the preparation and installment of the testing specimen to ensure the alignment between the specimen and the tensile load in order to avoid eccentricity from being potentially introduced into the specimen.
• The controlled strain condition is very complicated and hard to achieve in a tensile test, especially when the strain is inferred from LVDT measurements. The machine crosshead displacement control is an alternative to performing the strain-controlled test, but corrections must be performed to eliminate the effects of the machine compliance on the results.
Test Sensitivity to Asphalt Mix Composition and Design Parameters
Luo et al. (2013) indicated that the DT test results are sensitive to the asphalt mix composition and design parameters, such as air void content, binder content, gradation, modulus, and aging.
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Availability and Cost of the Test Equipment
The AMPT is capable of performing the DT test, and the cost of the AMPT machine is around $100,000.
Laboratory-to-Field Correlations and Cracking Criteria
The correlation between the DT test and the top-down fatigue cracking is the energy-based fracture mechanics model that is currently being developed under NCHRP 1-52.
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Hamburg Wheel Tracking Test (HWTT)
Description
The HWTT is originally designed for evaluating the rutting resistance and/or stripping potential of asphalt mixtures. Currently it is AASHTO Standard test method T324. Wheel tracking tests are employed to characterize top-down cracking because, in some instances, surface cracks are observed to accompany the rutting in the field. The literature shows that HWTT and other accelerated loading tests can produce surface cracking in specimens during the rutting development (De Freitas et al. 2005, Groenendijk 1998, and Wang et al. 2003). Figure A-35 shows a HWTT machine.
Figure A-35. Hamburg Wheel Tracking Test Device.
Availability of Test Methods (AASHTO, ASTM, Agency, Institutional References)
AASHTO T 324: Standard Method of Test for Hamburg Wheel-Track Testing of Compacted Hot-Mix Asphalt (HMA).
Test Condition Parameters
HWTT is typically conducted on two side by side gyratory compacted specimens or field cores. For the HWTT, a 47-mm wide steel or rubber wheel subjected to a 705 N load is tracked across a wet (submerged in a temperature controlled water bath) or dry sample for 20,000 cycles or until 20 mm of vertical deformation occurs.
Properties Measured
For the purpose of evaluating top-down cracks, cracks accompanying the rutting in the wheel tracking tests are observed as shown in Figure A-36. De Freitas et al. (2005) employed the number of wheel passes to produce the first crack (Nf) and the total number
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of wheel passes to reach 15 mm (Nt). They found that Nf decreased with increasing temperature.
Figure A-36. Observed Surface Cracks in Wheel Tracking Tests.
Test Variability
Not well defined.
Test Simplicity (or Complexity)
Many DOTs have adopted the HWTT as their standard test for evaluating rutting and stripping problems of asphalt mixtures. This confirms that the test is simple and practical.
Test Sensitivity to Asphalt Mix Composition and Design Parameters
According to De Freitas et al. (2005), the top-down crack initiation in a wheel tracking test was reported to be sensitive to high air voids and coarse gradation. Wang et al. (2003) mentioned that surface cracks may be initiated at high temperatures (e.g., 60°C), however, no distinct cracks or permanent strains could be observed at a temperature of 25°C using the same wheel tracking test procedure.
Availability and Cost of the Test Equipment
The HWTT is commercially available and has been well accepted by DOTs; the cost for a standard unit is around $52,000.
Laboratory-to-Field Correlations and Cracking Criteria
Not available.
(a) De Freitas et al. (2005) (b) Wang et al. (2003)
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Summary
Top-down fatigue cracking has become a wide-spread problem in asphalt pavements. However, top-down cracking has not been clearly defined in terms of its mechanisms nor is it well modeled for crack initiation and propagation. The initiation and propagation of top-down fatigue cracking are affected by numerous factors, including asphalt mixtures, pavement structure, traffic loading and environmental variables. All of these considerations produce significant difficulties in proposing a single laboratory test to characterize the top-down cracking performance of a mixture. The IDT test proposed by Roque and his co-workers is the only test that has been validated with field performance data. The DT test being developed under NCHRP 1-52 is promising, but it is not near implementation. The HWTT results showed the surface cracks at high temperatures, but deep research is needed to analyze the phenomenon and whether there is any relationship to top-down cracking. More discussion is provided in the final summary.
SUMMARY
There are four major modes of pavement cracking: low-temperature (or thermal), reflection, fatigue, and top-down, which are affected by numerous factors and interactions. A rational laboratory test method for evaluating and characterizing asphalt mixture cracking resistance must be based upon a clearly defined cracking mechanism. Asphalt pavement cracking can be caused by either traffic loading, environment, or both. Table A-2 summarizes the mechanisms associated with each of the four types of cracking. Table A-2 also lists the factors affecting crack development. All four types of cracking are influenced by asphalt mix composition and volumetric properties, climate, traffic, layer thickness, and aging conditions. Specifically, for reflection cracking, the existing pavement conditions (such as load transfer efficiency at joints/cracks) are critical factors that must be considered.
Table A-2. Cracking: Types, Mechanisms, and Factors Affecting Its Development.
Cracking Type
Low-Temperature Cracking
Reflection Cracking Bottom-Up Fatigue Cracking
Top-Down Fatigue Cracking
Cracking mechanisms
Environment, aggravated by traffic. Either single event or after multiple temperature cycles.
Combination of load and environment due to thermal movement and bending and shearing stress under traffic.
Traffic loading. Tensile strain induces crack initiation at the bottom of asphalt layer; bending and shearing loads responsible for crack propagation.
A variety of laboratory tests have been developed for characterizing these four types of cracking and associated mechanisms. Table A-3 lists some practical cracking tests and some promising cracking tests, some of which are either currently used by departments of transportation or considered for implementation in the future as noted from the cracking test survey. When selecting a cracking test, the following aspects should be considered.
• Test variability: The monotonic loading type of cracking tests (e.g., IDT, SCB, DCT, TSRST or UTSST) generally have much lower variability with coefficients of variation (COV) often less than 15%. In contrast, repeated loading type of cracking tests, including Texas OT, beam fatigue test, and S-VECD test, may have much higher COV (>30%) than the monotonic tests. Relatively higher COV may be the inherent nature of the repeated loading type of cracking tests.
• Interpretation of test results: Cracking test results can be classified into index or mechanistic-oriented parameters. Index parameters, such as fracture energy or OT cycles to failure, are often directly correlated to field cracking distresses and mechanistic-oriented parameters, like creep compliance from IDT and those from S-VECD and DT, and need to be combined with cracking models to evaluate HMA cracking resistance. Either index or mechanistic-oriented tests may be adequate for routine use as the software associated with mechanistic-oriented analyses matures.
• Correlations to field performance: Some of the existing tests are of the index type which is more suitable for mix design and QC/QA; some are more mechanistic oriented and suited for performance analysis; and some can be used for both. Regardless of whether they are index or mechanistic-oriented, monotonic or repeated load tests, any laboratory test being adopted for routine use must be validated and have a good correlation with field performance.
• Test simplicity (or complexity): Technician training requirements, time for preparing and testing specimens and difficulty in analyzing data are also factors for consideration. For example, IDT needs the least time for specimen preparation; in contrast, the beam fatigue test, TSRST, S-VECD and DT need the most time for specimen preparation. Tests listed in Table A-3 require shaping prior to testing. Additionally, the SCB requires notching; the OT needs gluing; and the DCT requires notching and drilling.
• Sensitivity to mix design parameters: Cracking tests should be able to distinguish performance related to the characteristics and volumetric properties of asphalt mixtures such as binder type and content, air voids, RAP/RAS content, aggregates, etc. In general, the repeated loading cracking tests are more sensitive to mix variables than monotonic loading tests.
• Other factors: Equipment availability and cost, the availability of test methods [AASHTO, ASTM, State standard, or draft], compaction methods, and direction of loading all need to be considered in choosing cracking tests.
It is likely that some compromises may have to be made when selecting cracking tests for routine use. Therefore, a dedicated workshop with a balanced representation of practitioners and topical experts is warranted to obtain consensus on candidate cracking tests for evaluation in the follow-on project(s).
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Table A-3. Laboratory Reflection Cracking Tests. Laboratory test
Correlation to field
performance
Test variability Test simplicity (or complexity)
Test sensitivity
to mix design
parameters (Note 2)
Equipment cost and
availability
Adoption by States Test
name Cracking
type Test
standard Test configuration Specimen geometry (Note 1)
Cracking parameter
DCT
Low temperature
cracking and
reflection cracking
ASTM D7313
(Monotonic test)
D = 6 in. T = 2 in.
2 holes D = 1 in.
ND = 2.46 in.
Fracture energy
Good correlation with low-temperature
cracking validated at MnRoad.
Low (COV=10-
15%)
Training: little time Specimen prep: 4 cuts and 2 holes Instrumentation: gluing 2 studs Testing: 1-6 min. Analysis: area integration Interpretation: quick and easy (pass/fail criteria).
Asphalt binder,
aggregate, RAP/RAS, and aging; insensitive to AV and
Pb
Commercially available; Cost: $49,000.
Adopted by Minnesota;
Being considered by
Colorado, South
Dakota, and Montana.
SCB
Low temperature
cracking
AASHTO TP105
(Monotonic test)
D = 6 in. T = 1 in.
ND = 0.6 in.
Fracture energy
Good correlation with low-temperature
cracking validated at MnRoad.
Medium (COV=20
%)
Training: medium time Specimen prep: 3 cuts Instrumentation: gluing 3 studs Testing: 30 min. Analysis: area integration Interpretation: quick and easy (pass/fail criteria).
Asphalt binder,
aggregate, RAP/RAS, AV and Pa
Commercially available; Cost: $52,000.
Being considered by Utah, South
Dakota, Pennsylvania, and Montana.
Bottom-up and top-
down fatigue
cracking
LTRC (Monotonic
test)
D = 6 in. T = 2.25 in.
ND = 1, 1.25 and 1.5 in.
Energy release rate
Fair correlation to field cracking
from the Louisiana Pavement
Management System.
Medium (COV=20
%)
Training: very little time Specimen prep: 4 cuts Instrumentation: none Testing: 5-10 min. Analysis: area integration and regression Interpretation: quick and easy (pass/fail criteria).
Asphalt binder,
aggregate, RAP/RAS
Not commercially available; Any hydraulic test system can be used; Cost: unknown.
Adopted by Louisiana;
being considered by
Oklahoma, and New Mexico.
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Laboratory test Correlation to
field performance
Test variability Test simplicity (or complexity)
Test sensitivity
to mix design
parameters (Note 2)
Equipment cost and
availability
Adoption by States Test
name Cracking
type Test
standard Test configuration Specimen geometry (Note 1)
Cracking parameter
IDT
Low-temperature
cracking
AASTHO T322: Dt and tensile
strength test (monotonic
tests)
D = 6 in. T = 1.5-2.0 in.
Creep compliance and tensile
strength
Creep compliance and tensile strength
inputs to TCMODEL.
Calibrated and validated
through original SHRP-I and
MEPDG.
Low (COV<11
%)
Training: medium time Specimen prep: 2 cuts Instrumentation: relatively easy Testing: 1-2 hours Analysis: short and easy with data analysis software Interpretation: longer time with cracking model to predict performance.
Asphalt binder,
aggregate, RAP/RAS,
aging Hydraulic test machines can be used. With test machine, more than $100,000.
AASHTO T322 is
required by AASHTOWa
re.
Top-down cracking
University of Florida:
Mr test, Dt test, and tensile strength test (cyclic and monotonic
tests)
D = 6 in. T = 1.5-2.0 in.
Energy ratio
Validated with field cores in Florida study and confirmed at NCAT test track.
Possibly low, similar to AASTHO T322.
Training: medium time Specimen prep: 2 cuts Instrumentation: relatively easy with gauge point template Testing: 1-2 hours Analysis: easy with data analysis software Interpretation: short and easy (pass/fail criteria).
Insensitive to change in binder viscosity (Roque et al. 2010)
Being adopted by Florida.
TSRST/UTSST
Low temperature
cracking
(Monotonic test)
L = 10 in. W = 2 in. T = 2 in.
Fracture temperature
Validated with test sections
during SHRP program.
MnRoad test results showed
moderate correlation with
field performance.
Low (COV= around 10%)
Training: long time and intensive Specimen prep: difficult and long Instrumentation: easy and short Testing: 3-5 hours Analysis: easy and short Interpretation: quick and easy (pass/fail criteria).
Asphalt binder,
aggregate, AV, Pb., and aging
Commercially available;
Cost: $98,000
Being considered by
Nevada.
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Laboratory test Correlation to
field performance
Test variability Test simplicity (or complexity)
Test sensitivity
to mix design
parameters (Note 2)
Equipment cost and
availability
Adoption by States Test
name Cracking
type Test
standard Test configuration Specimen geometry (Note 1)
Cracking parameter
Texas Overlay
Test
Reflection cracking
and bottom-up fatigue cracking
Tex-248-F (cyclic tests)
L= 6 in. W = 3 in. T = 1.5 in.
No. of cycles
(or fracture parameters:
A and n)
Good correlation with reflection cracking validated in Texas, California, and New Jersey; Promising correlation with fatigue cracking validated with FHWA-ALF and NCAT test track.
Relatively high
(COV=30-50%)
Training: little time Specimen prep: 4 cuts Instrumentation: none Testing: 1 min. - 3 hr. Analysis: easy and short Interpretation: quick and easy (pass/fail criteria).
Binder, aggregate, Pb, RAP/RAS, aging, etc.
Commercially available; Cost: $46,000
Adopted by Texas and
New Jersey;
Being considered by
Montana, Nevada,
Florida, and Ohio.
Bend beam
fatigue test
Bottom-up fatigue
cracking
AASHTO T321
(cyclic tests)
L = 15 in. W = 2.5 in.
T = 2 in.
No. of cycles
(or fatigue equation)
Correlation with bottom-up fatigue cracking historically validated.
Very high (COV>50
%)
Training: medium time Specimen prep: difficult and long Instrumentation: almost none Specimen testing: hours to days Data analysis: easy and quick Date Interpretation: quick and easy (or combine with pavement analysis program to predict pavement fatigue life.)
Binder, aggregate, Pb, RAP/RAS, aging, etc.
Frame (fixture) commercially available. Universal testing machine needed; Could be > $100,000.
California - special
pavement design;
Being
considered by Nevada and
Georgia.
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Laboratory test Correlation to
field performance
Test variability Test simplicity (or complexity)
Test sensitivity
to mix design
parameters (Note 2)
Equipment cost and
availability
Adoption by States Test
name Cracking
type Test
standard Test configuration Specimen geometry (Note 1)
Cracking parameter
S-VECD
Bottom-up and top-
down fatigue
cracking
AASHTO TP107 (cyclic tests)
(AASHTO TP79 E*
test for data analysis)
S-VECD: D = 4 in.
L = 5.1 in.
(E*: D = 4 in. L = 6 in.)
Fatigue equation
and damage parameters
(or predicted
no. of cycles)
S-VECD used with MEPDG or more advanced models (LVECD and VECD-FEP++) to simulate pavement performance. Validated with FHWA-ALF test lanes and verified in North Carolina.
Not defined
Training: very long time Specimen prep: 2 cuts and 1 coring Instrumentation: easy with a special glue jig Testing: hours to 1 day (3 more days if E* test is considered) Analysis: easy if using ALPHA-fatigue software Interpretation: quick and easy if only number of cycles is concerned. (or combine with pavement analysis programs [LVECD and VECD-FEP++] to predict pavement fatigue life.)
Not available
Commercially available; Cost: $97,000
Being considered by
Oklahoma, Georgia, and Pennsylvania.
Direct tension
Bottom-up and top-
down fatigue
cracking
Texas A&M
University (cyclic tests)
D = 4 in. L = 6 in.
Paris’ law parameters (or No. of
cycles)
Correlations with bottom-up and top-down fatigue cracking being developed under several research projects. Model and methods being validated with LTPP data
Not defined
Training: very long time Specimen prep: 2 cuts and 1 coring Instrumentation: medium time and difficulty Testing: 1-2 hours Analysis: need special software Interpretation: still under development.
Model coefficients functions of AV, Pb, gradation; modulus, aging, etc.
Universal test machine needed for direct tension test; >$100,000.
Unknown
Note 1: D = diameter; L = length; W = width; T = thickness; ND = notch depth. Note 2: AV = Air Voids; Pb = Percent Binder
A-75
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APPENDIX B. SUMMARY OF ASPHALT CRACKING TEST WORKSHOP
INTRODUCTION
This appendix documents the Asphalt Cracking Tests Workshop held at the National Academy of Sciences Beckman Center in Irvine, CA on February 11 and 12, 2015. The workshop was sponsored by the National Cooperative Highway Research Program (NCHRP) as part of NCHRP Project 9-57, Experimental Design for Field Validation of Laboratory Tests to Assess Cracking Resistance of Asphalt Mixtures. Participants included members of the NCHRP Project 9-57 panel, key members of the research team, and invited experts with extensive experience on cracking tests from highway agencies, industry, and academia. The main objectives of the workshop were to select a suite of tests for thermal, reflective, bottom-up and top-down fatigue cracking and to identify potential field test sections for validating the selected cracking tests.
PARTICIPANTS
Participants in the Asphalt Cracking Tests Workshop included the NCHRP 9-57 panel, members of the research team, and invited experts recommended by the research team and approved by the project panel. The list of participants shown in Table B-1 included 20 percent academia, 30 percent industry personnel, and 50 percent agency, as suggested by the panel. Thirty-one individuals were invited to attend, including 11 panel members, the NCHRP senior program officer and senior program assistant, four members of the research team, and 14 invited experts. There was a high degree of interest in the workshop, with 94 percent of the invitees participating. Only two invited experts, one panel member, and one invited expert were not able to participate due to a schedule conflict and a personal matter, respectively.
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Table B-1. Workshop Invitees and Attendees List. No. Name Affiliation Role Attend 1 Mr. Chris Abadie Louisiana Department of Transportation Invited expert Yes 2 Mr. Anthony Avery National Cooperative Highway Research
Program Senior program assistant
Yes
3 Dr. Tom Bennert University of Rutgers Panel member No 4 Mr. Phil Blankenship Asphalt Institute Invited expert Yes 5 Dr. Pete Capon Rieth-Riley Construction Company, Inc. Panel member Yes 6 Ms. Gisel Carrasco Texas Department of Transportation Panel member Yes 7 Mr. Tim Clyne Minnesota Department of Transportation Panel member Yes 8 Dr. Audrey Copeland National Asphalt Pavement Association Panel member Yes 9 Dr. Judy Corley-Lay North Carolina Department of Transportation Invited expert Yes 10 Dr. Jo Daniel University of New Hampshire Invited expert Yes 11 Mr. Jeff Dean Oklahoma Department of Transportation Invited expert Yes 12 Dr. Stacey
Diefenderfer Virginia Department of Transportation Invited expert Yes
13 Dr. Ervin Dukatz Mathy Construction Company Panel member Yes 14 Ms. Tina Geiselbrecht Texas A&M Transportation Institute Research team Yes 15 Dr. Nelson Gibson Federal Highway Administration Panel member Yes 16 Dr. Adam Hand Granite Construction Invited expert Yes 17 Dr. Ed Harrigan National Cooperative Highway Research
Program Senior program officer
Yes
18 Mr. Gerry Huber Heritage Research Group Invited expert Yes 19 Dr. David Jones University of California at Davis Invited expert Yes 20 Dr. Hyung Lee Applied Research Associates, Inc. Panel member Yes 21 Dr. Robert Lytton Texas A&M University Research team Yes 22 Dr. Mike Mamlouk Arizona State University Invited expert Yes 23 Dr. David Newcomb Texas A&M Transportation Institute Research team Yes 24 Mr. Dan Oesch Missouri Department of Transportation Invited expert Yes 25 Mr. David Powers Ohio Department of Transportation Invited expert Yes 26 Dr. Murari Pradhan Arizona Department of Transportation Panel member Yes 27 Geoff Rowe Abatech Inc. Invited expert No 28 Dr. Peter Sebaaly University of Nevada at Reno Invited expert Yes 29 Dr. Nam Tran National Center for Asphalt Technology Panel member Yes 30 Mr. Jeff Uhlmeyer Washington State Department of Transportation Panel member Yes 31 Dr. Fujie Zhou Texas A&M Transportation Institute Research team Yes
WORKSHOP PREPARATION
In addition to inviting the participants, the research team prepared materials for the workshop, including: (1) an interim report providing a critical review of available cracking tests documented in the literature, (2) two cracking test webinars with nine cracking test developers,
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(3) high-definition videos that were posted on the web visually displaying the key steps of the nine test procedures presented on the webinars, (4) a cracking test reference booklet tailored for workshop participants, and (5) the workshop agenda. More detailed information is described below.
Interim Report
The research team critically reviewed laboratory tests for evaluating thermal, reflection, bottom-up, and top-down fatigue cracking. Each cracking test was evaluated in terms of five criteria: (1) test simplicity (or complexity), including technician training requirement, time for preparing, instrumenting, and testing specimens and difficulty in analyzing data, (2) test variability, (3) correlations to field performance, (4) sensitivity to mix design parameters, (5) availability of test method and equipment availability and cost. Based on the evaluation, the research team recommended 10 cracking tests for the panel and invited experts to consider when making the final selection at the workshop, namely:
• Disk-shape compact tension (DCT) test: ASTM D7313-13. • Semi-circular bend (SCB) test at low temperature: AASHTO TP105-13. • Indirect tension (IDT) for low-temperature cracking: AASHTO T322. • Uniaxial thermal stress and strain test (UTSST): University of Nevada at Reno. • Texas overlay test (OT): Tex 248-F. • Bending beam fatigue (BBF) test: AASHTO T321. • SCB at intermediate temperature: Louisiana Transportation Research Center. • IDT-UF for top-down fatigue cracking: University of Florida. • Simplified viscoelastic continuum damage (S-VECD) fatigue test: AASHTO TP107. • Repeated direct tension (RDT) test: Texas A&M University.
Cracking Test Webinars
It was critical for each invited participant to have access to information concerning these cracking tests before the workshop. Two half-day webinars were conducted so each cracking test developer could present the development history, test features, lab-to-field correlation, and implementation status of each test. Table B-2 lists the webinar schedule and the invited presenters. Note that the IDT tests for low-temperature and top-down cracking are similar except the test temperatures, and that three other tests were presented for low-temperature cracking. Thus, the focus of the IDT test at the webinar was on top-down fatigue cracking, and a total of nine cracking tests were discussed. Each presenter had 30 minutes for presentation and a 10-minute Q/A time for the participants to directly ask test developers the questions they may have had on specific cracking tests. Additionally, the two webinars were recorded, and the links were sent to each participant for replay as needed.
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Table B-2. Cracking Test Webinar Schedule.
Date Time (CST) Presenter Test
January 28, 2015 (morning)
8:30–9:10 Robert Lytton Texas A&M University
RDT
9:10–9:50 Mihai Marasteanu University of Minnesota
SCB at low temperature
9:50–10:00 Break
10:00–10:40 Bill Buttlar University of Illinois
DCT
10:40–11:20 Rey Roque University of Florida
IDT
February 3, 2015 (afternoon)
1:00–1:40 Tom Scullion Texas A&M Transportation Institute
Texas OT
1:40–2:20 Richard Kim North Carolina State University
S-VECD
2:20–3:00 John Harvey University of California at Davis
BBF
3:00–3:10 Break
3:10–3:50 Elie Hajj University of Nevada at Reno
UTSST
3:50–4:30 Louay Mohammad Louisiana State University
SCB at intermediate temperature
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Cracking Test Videos
A concerted effort was made to develop nine videos for the 10 cracking tests. Note that one video was developed for IDT tests for both low- temperature and top-down cracking. The purpose of developing cracking test videos was to further assist workshop participants in understanding each cracking test and to visualize the key steps for performing it from specimen preparation to final data analysis. These cracking test videos were available before and at the workshop, and they are still available to the public through the following links:
A 12-page cracking test summary booklet was designed specifically for the workshop. The key aspects of each of the 10 cracking tests were described in the booklet to provide a condensed version of the interim report. Compared to the interim report, the booklet was very convenient for participants to carry around and reference during the workshop breakout and plenary sessions.
Workshop Agenda
Based on the availability of the conference rooms at the Beckman Center, the workshop agenda was developed jointly by Ms. Tina Geiselbrecht, Workshop Facilitator, Dr. David Newcomb, Co-Principal Investigator, and Dr. Fujie Zhou, Principal Investigator for NCHRP 9-57. The agenda was approved by the panel and is shown in Table B-3.
Table B-3. NCHRP 9-57 Asphalt Cracking Tests Workshop Agenda. Date Time (PST) Agenda Item
Wednesday, February 11,
2015
8:00am–8:15am Welcome and Self-Introductions 8:15am–8:30am Review of Project and Workshop Goals 8:30am–9:00am Brief Summary of Test Procedures
• Assignments to Breakout Groups o Cold Temperature o Reflection Cracking o Top-Down Cracking o Bottom-Up Cracking
• Weighting of Factors 9:00–10:30am Breakout Groups Session No. 1: Viable Tests for Evaluation
• Cracking Mechanisms • Discussion of Associated Tests
10:30–11:30am Plenary Session: Results of Breakout Session No. 1 11:30am–12:15pm Lunch 12:15pm–1:45pm Breakout Groups Session No. 2: Needed Laboratory Evaluation
• Considerations: o Complexity (or simplicity) o Variability o Data Analysis o Lab-to-Field Correlation o Cost and availability of test equipment o Others
• Select Tests to Evaluate • Proposed Approaches to Laboratory Study • Test Method Ratings
1:45pm–2:45pm Plenary Session: Results of Breakout Session No. 2 2:45pm–4:15pm Breakout Groups Session No. 3: Desirable Features of Test Sections
• Experimental Considerations o Climate o Structural Sections
Table B-3. NCHRP 9-57 Cracking Tests Workshop Agenda (continued). Date Time (PST) Agenda Item
Thursday, February 12,
2015
8:00am–8:30am Review of Previous Day’s Discussions/Need for Further Discussion
8:30am–11:00am Session No. 4: Possible Test Sections • List of Existing Resources (e.g., NCAT Test Track, MnROAD,
SPS10, ALF, etc.) and Features (e.g., WIM, temperature/moisture probes, strain gauges)
• Need for Additional Test Sites o Possible Locations o Needed Features
11:00am–11:30am Summary of Workshop and Next Steps
11:30am: Adjourn
WORKSHOP OVERVIEW
The Asphalt Cracking Tests Workshop was facilitated by Ms. Tina Geiselbrecht and Dr. David Newcomb. Although the on-site workshop occurred in Irvine, CA, on February 11-12, 2015, it actually started in January 2015 when the invited participants reviewed the interim report, attended the two webinars, and watched the videos. Thus, most invited participants were familiar with the cracking tests being selected ahead of the on-site workshop.
Workshop Presentations
The workshop started with an introduction session followed by a presentation made by Drs. Zhou and Newcomb. The presentation discussed the goals of the workshop and the workshop material preparation. As a part of the presentation, the nine cracking test videos were shown to refresh the memory of the participants on the key issues of each test. At the end of the presentation, Facilitator Ms. Geiselbrecht emphasized the ground rules for the workshop, including both the plenary and breakout sessions.
Weighting Factors for Cracking Tests
Before the breakout sessions, each participant was requested to assign a weighting factor for seven aspects of a cracking test: (1) availability of test method, (2) test simplicity, (3) test variability, (4) sensitivity to mix parameters, (5) complexity of data analysis, (6) availability/cost of test equipment, and (7) lab-to-field correlation. The weighting factor ranged from 1 (least important) to 5 (most important). The weight factors from each workshop participant were then averaged for a later use to select cracking tests for thermal, reflection, bottom-up, and top-down fatigue cracking. Table B-4 shows the averaged weighting factors for the seven aspects of cracking tests.
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Table B-4. Averaged Weighting Factors.
Cracking Test Aspect Score
Availability of test method 2.9
Test simplicity 3.9
Test variability 3.8
Sensitivity to mix 4.2
Complexity—data analysis 3.0
Availability/cost of test equipment 3.4
Lab-to-field correlation 4.4
Breakout Groups and Associated Participants
Each participant was assigned to a specific cracking discussion group based on the specific interests expressed by the participants. Table B-5 shows the breakout groups. In each group, a coordinator and a note taker were selected. Either the group coordinator or the note taker reported the discussion results to the whole group at the plenary session.
Jo Daniel (note taker) Jeff Dean Pete Capon Audrey Copeland (note taker)
Adam Hand Stacey Diefenderfer Erv Dukatz Judy Corley-Lay
Gerry Huber David Jones Nelson Gibson (note taker)
Hyung Lee
Dan Oesch David Powers Mike Mamlouk Bob Lytton
Murari Pradhan Fujie Zhou (coordinator)
Peter Sebaaly Nam Tran
Jeff Uhlmeyer (coordinator)
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Selection of Cracking Tests
One of the main objectives of the Asphalt Cracking Tests Workshop was to select tests for further laboratory and field evaluation for thermal, reflection, bottom-up, and top-down fatigue cracking. Two breakout sessions and two plenary sessions were designated for achieving the objective. The first breakout session focused on cracking mechanisms for each type of cracking, and each breakout group reported the discussion summary to the whole group at the following plenary session.
The second breakout session discussed the 10 cracking tests presented by the research team and two new cracking tests proposed by the participants. These two new cracking tests were (a) an SCB test for low-temperature cracking, which was originally proposed by Dr. Imad Al-Qadi for Illinois DOT (referred as to SCB-IL); and (b) a Modified OT for top-down cracking originally proposed by Dr. Robert Lytton. During the second breakout session, a three-step process for rating each cracking test was followed within each cracking group:
• Step 1—First individual rating: Each participant rated each cracking test, without discussion, based on the information obtained at the workshop. Each cracking test was rated on a scale of 1 (least effective) to 5 (most effective) in terms of seven aspects of each cracking test: (1) availability of test method, (2) test simplicity, (3) test variability, (4) sensitivity to mix parameters, (5) complexity of data analysis, (6) availability/cost of test equipment, and (7) lab-to-field correlation.
• Step 2—Group discussion: Each participant explained their ratings. • Step 3—Final individual rating: After the group discussion, each participant rated
each cracking test again and the results of this final rating are listed in Table B-6 through Table B-9.
Based on the final ratings, each group recommended up to three cracking tests for each type of distress, as listed below:
• Thermal cracking: (1) DCT, (2) SCB-IL, and (3) SCB at low temperature. • Reflection cracking: (1) OT, (2) SCB at intermediate temperature, and (3) BBF. • Bottom-up fatigue cracking: (1) BBF, and (2) SCB at intermediate temperature. • Top-down fatigue cracking: (1) SCB at low temperature, and (2) IDT-UF. Note that the SCB-IL was not available in the literature before the Asphalt Cracking
Tests Workshop. The thermal cracking group recommended further study of the SCB-IL and the needed comparison with DCT and SCB at low temperature. During and after the workshop, the research team received more information about the test.
Additionally, workshop participants recommended regional or national ruggedness testing for these selected cracking tests ahead of the inter-laboratory study to define the accuracy and precision of each test. Also when sampling materials, sufficient materials should be collected to validate not just those tests selected for specific cracking type; other types of cracking tests listed above should be considered as well to avoid the scenario that the selected specific cracking test does not have the expected correlation with field performance.
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Table B-6. Final Rating Scores of Thermal Cracking Tests.
Thermal Cracking
DCT SCB-IL SCB at low temp UTSST IDT
Avg. Rating WF Final
Score Avg.
Rating WF Final Score
Avg. Rating WF Final
Score Avg.
Rating WF Final Score
Avg. Rating WF Final
Score
Availability of Test Method 4.6 2.9 13.3 3.6 2.9 10.4 4.2 2.9 12.1 3.4 2.9 9.8 4.6 2.9 13.3
Note: Avg. rating—averaged rating from all participants of thermal cracking group; WF—weighting factor; final score is calculated by avg. rating x WF. The total score is the sum of the final score in each characteristic of test method. Higher total score is preferred.
Table B-7. Final Rating Scores of Reflection Cracking Tests.
Reflection Cracking
OT SCB at Intermediate Temp BBF DCT
Avg. Rating WF Final
Score Avg.
Rating WF Final Score
Avg. Rating WF Final
Score Avg.
Rating WF Final Score
Availability of Test Method 4.7 2.9 13.5 4.0 2.9 11.5 4.5 2.9 13.0 4.7 2.9 13.5
Identification of Desirable Variables for Experimental Design
After the identification of cracking tests, the workshop participants spent a significant amount of time discussing and debating the desirable variables needed to be included in the experimental design to validate these cracking tests. The final consensus on desirable variables from each cracking group is tabulated in Table B-10.
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Table B-10. Desirable Variables for Experimental Design.
Climate 1. Cold region (single event low-temperature cracking )
2. Warm region (thermal fatigue cracking)
1. Temperature: moderate/hot
2. High temperature cycling
All climates, preferring high temperature and moisture cycling
1. Temperature: hard freeze/ frequent thermal cycling/no freeze
2. Solar radiation: low/medium/ high
Traffic Any level High and many trucks Lots of trucks 1. Truck: high/low
2. Speed: high/low
Structure New construction/no overlay
Overlay thickness as designed
Same pavement structure for any specific test location but varying mix types through gradation, binder, and RAP/RAS so that the mixes having a full spectrum of SCB index
1. AC thickness: thin/not thin
2. Subgrade: soft/stiff
Asphalt mixture
1. Two mix types: conventional mix and special mixes (SMA, Gap-graded, Novachip, cracking resistance mix)
Use existing test sections Combine both APT and field test sections
1. Combine with top-down cracking sections;
2. Min. 2000 tons of mix
3. Prefer test track rather than APT
4. Do not limit to LTPP
Combine with existing NCHRP studies, SPS10, test tracks
Identification of Potential Test Sections for Field Validation of Cracking Tests
Different types of test sections were discussed and some consensuses on the field test sections are listed below:
• Use existing state test sections, LTPP, and test tracks (MnRoad, NCAT test track). • Coordinate with other NCHRP projects (9-55, 9-58, 9-59). • Coordinate with FHWA and NCHRP for SPS10 test sections.
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• Combine both bottom-up and top-down cracking test sections.
Closing Session
Final thoughts about the workshop from the participants were solicited in a round-robin discussion at the end of the workshop. The comments included:
• Learned more than what I really expected; very good group of people to work with. • Learned a lot from the experts around here. • A really good and worthwhile meeting, very well balanced group and very open
discussion. • Very well organized and nice process. • Enjoyed the time and group discussion. • Enjoyed the discussion and got a lot out of the workshop; great combination of
DOTs, contractors, consultants, and academia. • Very useful to see what other people are doing. • Enjoyed the open discussion. • Enjoyed the process and glad to see the community moving forward on cracking tests. • Very productive meeting. • Enjoyed meeting different groups of people and getting their opinions. • Met and exceeded my expectations. • Enjoyed the process, great group discussion, and learned a lot. • Fantastic process and great discussion, and enjoyed it. • Time well spent; we should do this more often; better discussion than ETG. • A great group of energized but very collaborative people; enjoyed it. • Very dynamic, diversified group of people; very helpful discussion. • Very helpful discussion, and appreciate having inputs from contractors. • Thank everyone for participation; overwhelmed by the knowledge of the group. • Very positive discussion and nobody established walls to defend particular cracking
tests; great accomplishment.
WORKSHOP OUTCOMES
There are three major outcomes from the Asphalt Cracking Tests Workshop, as described in Table B-11.
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Table B-11. Cracking tests selected for the four types of cracking.
Items Thermal Cracking
Reflection Cracking
Bottom-up Fatigue Cracking
Top-down Fatigue Cracking
Selected cracking tests
1. DCT 2. SCB-IL 3. SCB at low
temp.
1. OT 2. SCB at
intermediate temp.
3. BBF
1. BBF 2. SCB at
intermediate temp.
1. SCB at intermediate temp.
2. IDT-UF
Key factors for designing field experimental test sections
1. Climate (temperature, moisture, solar radiation); 2. Traffic; 3. Pavement structure and subgrade; 4. Asphalt mixtures; 5. Existing pavement conditions for reflection cracking.
Potential field test sections
1. LTPP; 2. SPS10; 3. MnRoad; 4. NCAT Test Track; 5. Test sections under NCHRP 9-55, 9-58, and 9-59.
SUMMARY AND CONCLUSION
This report documents the Asphalt Cracking Tests Workshop held in Irvine, CA, at the National Academy of Sciences Beckman Center on February 11 and 12, 2015. The workshop was sponsored by NCHRP as part of NCHRP Project 9-57, Experimental Design for Field Validation of Laboratory Tests to Assess Cracking Resistance of Asphalt Mixtures. Participants included members of the NCHRP Project 9-57 panel, key researchers of the research team, and invited experts from highway agencies, industry, and academia. The main objectives of the workshop were to:
• Select a suite of crack tests for thermal, reflective, bottom-up, and top-down fatigue cracking,
• Identify desirable variables for field experimental design, and • Identify potential field test sections for validating those selected cracking tests. At end of the workshop, these three objectives were all accomplished. Additionally,
numerous recommendations were made by the workshop participants, and will be considered by the research team in preparing the experiment design.
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APPENDIX C. POTENTIAL FIELD TEST SECTIONS
LTPP Section: 35-0501 State: New Mexico (35) Roadway and Direction: IH 10 EB Experiment No.: SPS-5 Date of Surface Construction: Sept. 1996 Status: Out-of-study Pavement Cross Section:
Layer No. Type Thickness(inches) Description Date
1 SG – Coarse-grained, clayey sand w/ gravel – 2 Base 12 Soil-Aggregate Mix (mainly coarse grained) Sept. 1995 3 AC Layer 3.7 Dense-Graded HMA Sept. 1995 4 AC Layer 2.2 Recycled AC, cold lay, mixed in place Sept. 1995 5 AC Layer 0.3 Dense-Graded HMA Sept. 1995 6 AC Layer – Chip Seal Sept. 1995 7 AC Layer 2 Dense-Graded HMA Sept. 1996 8 AC Layer 1 Dense-Graded HMA Sept. 1996 9 AC Layer 1.3 Open Graded HMA Sept. 1996
Tot. AC Thickness = 10.5 SPS-5, out of study in March 2003 – Traffic Data: Number of Years with AADTT Data: 19 Latest AADTT: 9593 Year: 1989 Latest KESAL: 318 Year: 2000 Performance from last surface date:
11/13/2001 7.1 33.2 132.7 8 April 2002 Patch Pot Holes 5/6/2002 0.6 11.8 154.3 16
Sept. 2002 Crack Seal 11/3/2002 1.6 101 134.4 31 May 2003 Patch Pot Holes 5/19/2003 6.9 1.5 236.4 24
Potential Cracking Test Location: Thermal Cracking—Dry, Hot Climate—Thick AC—High Traffic Top-Down Cracking—High Solar Gain—No Freeze—High Volume, High Speed Material Notes: Binder sampled 9/7/1996, AC Cement Virgin. AC Cement for RAP also sampled on 9/7/1996.
Figure C-1. Potential Field Test Section: LTPP 35-0501.
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LTPP Section: 35-0502 State: New Mexico (35) Roadway and Direction: IH 10 EB Experiment No.: SPS-5 Date of Surface Construction: Sept. 1996 Status: Out-of-study Pavement Cross Section:
Layer No. Type Thickness (inches) Description Date
1 SG – Coarse-grained, clayey sand w/ gravel – 2 Base 12.5 Soil-Aggregate Mix (mainly coarse grained) Sept. 1995 3 AC Layer 3.3 Dense-Graded HMA Sept. 1995 4 AC Layer 3.2 Recycled AC, cold lay, mixed in place Sept. 1995 5 AC Layer 1.4 Dense-Graded HMA Sept. 1995 6 AC Layer – Chip Seal Sept. 1995 7 AC Layer 2.1 Recycled AC, hot laid, plant mix Sept. 1996 8 AC Layer 0.8 Open Graded HMA Sept. 1996
Tot. AC Thickness = 10.8 SPS-5, out of study in March 2007 – Traffic Data: Number of Years with AADTT Data: 19 Latest AADTT: 9593 Year: 1989 Latest KESAL: 743 Year: 2006 Performance from last surface date:
Potential Cracking Test Location: Thermal Cracking—Dry, Hot Climate—Thick—High Traffic Top-Down Cracking—High Solar Gain—No Freeze—High Volume, High Speed
Material Notes: Binder sampled 9/7/1996, AC Cement Virgin. AC Cement for RAP also sampled on 9/7/1996.
Figure C-2. Potential Field Test Section: LTPP 35-0502.
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LTPP Section: 35-0503 State: New Mexico (35) Roadway and Direction: IH 10 EB Experiment No.: SPS-5 Date of Surface Construction: Sept. 1996 Status: Out-of-study Pavement Cross Section:
Layer No. Type Thickness (inches) Description Date
1 SG – Coarse-grained, clayey sand w/ gravel – 2 Base 15 Soil-Aggregate Mix (mainly coarse grained) Sept. 1995 3 AC Layer 3.6 Dense-Graded HMA Sept. 1995 4 AC Layer 3.2 Recycled AC, cold lay, mixed in place Sept. 1995 5 AC Layer 1.4 Dense-Graded HMA Sept. 1995 6 AC Layer – Chip Seal Sept. 1995 7 AC Layer 4.4 Recycled AC, hot laid, plant mix Sept. 1996 8 AC Layer 1 Open Graded HMA Sept. 1996
Tot. AC Thickness = 13.6 SPS-5, out of study in March 2007 – Traffic Data: Number of Years with AADTT Data: 19 Latest AADTT: 9593 Year: 1989 Latest KESAL: 716 Year: 2006 Performance from last surface date:
Potential Cracking Test Location: Thermal Cracking—Dry, Hot Climate—Thick—High Traffic Top-Down Cracking—High Solar Gain—No Freeze—High Volume, High Speed Material Notes: Binder sampled 9/7/1996, AC Cement Virgin. AC Cement for RAP also sampled on 9/7/1996.
Figure C-3. Potential Field Test Section: LTPP 35-0503.
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LTPP Section: 35-0504 State: New Mexico (35) Roadway and Direction: IH 10 EB Experiment No.: SPS-5 Date of Surface Construction: Sept. 1996 Status: Out-of-study Pavement Cross Section:
Layer No. Type Thickness (inches) Description Date
1 SG – Coarse-grained, clayey sand w/ gravel – 2 Base 12.7 Soil-Aggregate Mix (mainly coarse grained) Sept. 1995 3 AC Layer 3.5 Dense-Graded HMA Sept. 1995 4 AC Layer 2.7 Recycled AC, cold lay, mixed in place Sept. 1995 5 AC Layer 1.7 Dense-Graded HMA Sept. 1995 6 AC Layer – Chip Seal Sept. 1995 7 AC Layer 4.6 Dense-Graded HMA Sept. 1996 8 AC Layer 0.6 Open Graded HMA Sept. 1996
Tot. AC Thickness = 13.1 SPS-5, out of study in March 2007 – Traffic Data: Number of Years with AADTT Data: 19 Latest AADTT: 9593 Year: 1989 Latest KESAL: 713 Year: 2006 Performance from last surface date:
Potential Cracking Test Location: Thermal Cracking—Dry, Hot Climate—Thick—High Traffic Top-Down Cracking—High Solar Gain—No Freeze—High Volume, High Speed Material Notes: Binder sampled 9/7/1996, AC Cement Virgin. AC Cement for RAP also sampled on 9/7/1996.
Figure C-4. Potential Field Test Section: LTPP 35-0504.
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LTPP Section: 35-0505 State: New Mexico (35) Roadway and Direction: IH 10 EB Experiment No.: SPS-5 Date of Surface Construction: Sept. 1996 Status: Out-of-study Pavement Cross Section:
Layer No. Type Thickness (inches) Description Date
1 SG – Coarse-grained, clayey sand w/ gravel – 2 Base 8 Soil-Aggregate Mix (mainly coarse grained) Sept. 1995 3 AC Layer 3.2 Dense-Graded HMA Sept. 1995 4 AC Layer 2.4 Recycled AC, cold lay, mixed in place Sept. 1995 5 AC Layer 1.6 Dense-Graded HMA Sept. 1995 6 AC Layer – Chip Seal Sept. 1995 7 AC Layer 2.5 Dense-Graded HMA Sept. 1996 8 AC Layer 0.8 Open Graded HMA Sept. 1996
Tot. AC Thickness = 10.5 SPS-5, out of study in March 2007 – Traffic Data: Number of Years with AADTT Data: 19 Latest AADTT: 9593 Year: 1989 Latest KESAL: 771 Year: 2006 Performance from last surface date:
Potential Cracking Test Location: Thermal Cracking—Dry, Hot Climate—Thick—High Traffic Top-Down Cracking—High Solar Gain—No Freeze—High Volume, High Speed Material Notes: Binder sampled 9/7/1996, AC Cement Virgin. AC Cement for RAP also sampled on 9/7/1996.
Figure C-5. Potential Field Test Section: LTPP 35-0505.
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LTPP Section: 35-0801 State: New Mexico (35) Roadway and Direction: IH 10 FR EB Experiment No.: SPS-8 Date of Surface Construction: Sept. 1995 Status: Active Pavement Cross Section:
Layer No. Type Thickness (inches) Description Date
1 SG – Coarse-grained, clayey sand Sept. 1995
2 Base 9.7 Soil-Aggregate Mix (mainly coarse
grained) Sept. 1995
3 AC
Layer 4.2 Dense-Graded HMA Sept. 1995 Tot. AC Thickness = 4.2 Latest surface info from 1995
Traffic Data: Number of Years with AADTT Data: 0 Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
Potential Cracking Test Location: Thermal Cracking—Thin AC—Low Traffic—Dry, Hot Climate Top-Down Cracking—High Solar Gain—No Freeze—Low Volume Bottom-Up Cracking Material Notes: Binder sampled 9/23/1996 and 09/25/2006, AC-10. Aggregate sampled on 09/07/1996, 09/11/1996 and 09/23/2006. Aggregate—20% RAP—90% Virgin sampled on 09/10/1996.
Figure C-6. Potential Field Test Section: LTPP 35-0801.
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LTPP Section: 35-0802 State: New Mexico (35) Roadway and Direction: IH 10 FR EB Experiment No.: SPS-8 Date of Surface Construction: Sept. 1995 Status: Active Pavement Cross Section:
Tot. AC Thickness = 7 Latest surface info from 1995 Traffic Data: Number of Years with AADTT Data: 0 Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
Potential Cracking Test Location: Thermal Cracking—Thick AC—Low Traffic—Dry, Hot Climate Top-Down Cracking—High Solar Gain—No Freeze—Low Volume Bottom-Up Cracking Material Notes: Binder sampled 9/23/1996 and 09/25/2006, AC-10. Aggregate sampled on 09/07/1996, 09/11/1996 and 09/23/2006.
Figure C-7. Potential Field Test Section: LTPP 35-0802.
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LTPP Section: 35-A001 State: New Mexico (35) Roadway and Direction: IH 40 WB Experiment No.: SPS-10 Date of Surface Construction: Oct. 2014 Status: Active Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Fine-grained soils: Silty Clay – 2 Base 8 Soil-Aggregate Mix (mainly fine grained) Aug. 2014 3 AC Layer 5.8 Dense-Graded HMA Aug. 2014 4 AC Layer 3.9 Dense-Graded HMA Aug. 2014 5 AC Layer 3.3 Recycled AC, hot laid, plant mix Oct. 2014
Tot. AC Thickness = 13 Mill and overlay in October 2014 Traffic Data: Number of Years with AADTT Data: 0 Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
Year Fatigue Cracking (m2) WP Long. Crack
(m) NWP Long. Crack
(m) Trans. Cracking
(count) 9/3/14 2.4 0 292.3 13
Oct 2014 Mill and overlay with hot-mix with recycled AC 11/14/14 0 0 0 0
Potential Cracking Test Location: Thermal Cracking—Thick AC—Dry, Hot Climate—High Traffic Top-Down Cracking—High Solar Gain—No Freeze—High Volume, High Speed Bottom-Up Cracking Material Notes:
Figure C-8. Potential Field Test Section: LTPP 35-A001.
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LTPP Section: 35-A002 State: New Mexico (35) Roadway and Direction: IH 40 WB Experiment No.: SPS-10 Date of Surface Construction: Oct. 2014 Status: Active Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Fine-grained soils: Silty Clay – 2 Base 8 Soil-Aggregate Mix (mainly fine grained) Aug. 2014 3 AC Layer 4.3 Dense-Graded HMA Aug. 2014 4 AC Layer 4 Dense-Graded HMA Aug. 2014 5 AC Layer 3.7 Dense-Graded WMA Oct. 2014
Tot. AC Thickness = 12 Mill and warm-mix overlay in October 2014 Traffic Data: Number of Years with AADTT Data: 0 Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
Year Fatigue Cracking (m2) WP Long. Crack
(m) NWP Long. Crack
(m) Trans. Cracking
(count) 9/3/14 2 0 299 3
Oct 2014 Mill and overlay with warm-mix with recycled AC 11/14/14 0 0 0 0
Potential Cracking Test Location: Thermal Cracking—Thick AC—Dry, hot Climate—High Traffic Top-Down Cracking—High Solar Gain—No Freeze—High Volume, High Speed Bottom-Up Cracking Material Notes:
Figure C-9. Potential Field Test Section: LTPP 35-A002.
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LTPP Section: 35-A003 State: New Mexico (35) Roadway and Direction: IH 40 WB Experiment No.: SPS-10 Date of Surface Construction: Oct. 2014 Status: Active Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Fine-grained soils: Silty Clay – 2 Base 8 Soil-Aggregate Mix (mainly fine grained) Aug. 2014 3 AC Layer 4.9 Dense-Graded HMA Aug. 2014 4 AC Layer 4.3 Dense-Graded HMA Aug. 2014 5 AC Layer 3.6 Dense-Graded WMA Oct. 2014
Tot. AC Thickness = 12.8 Mill and warm-mix overlay in October 2014 Traffic Data: Number of Years with AADTT Data: 0 Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
Year Fatigue Cracking (m2) WP Long. Crack
(m) NWP Long. Crack
(m) Trans. Cracking
(count) 9/3/14 4.5 0 165 15
Oct 2014 Mill and overlay with warm-mix with recycled AC 11/14/14 0 0 0 0
Potential Cracking Test Location: Thermal Cracking—Thick AC—Dry, Hot Climate—High Traffic Top-Down Cracking—High Solar Gain—No Freeze—High Volume, High Speed Bottom-Up Cracking Material Notes:
Figure C-10. Potential Field Test Section: LTPP 35-A003.
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LTPP Section: 35-A061 State: New Mexico (35) Roadway and Direction: IH 40 WB Experiment No.: SPS-10 Date of Surface Construction: Oct. 2014 Status: Active Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Fine-grained soils: Silty Clay – 2 Base 8 Soil-Aggregate Mix (mainly fine grained) Aug. 2014 3 AC Layer 4.3 Dense-Graded HMA Aug. 2014 4 AC Layer 3.8 Dense-Graded HMA Aug. 2014 5 AC Layer 3.4 Dense-Graded WMA Oct. 2014
Tot. AC Thickness = 11.5 Mill and warm-mix overlay in October 2014 Traffic Data: Number of Years with AADTT Data: 0 Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
Year Fatigue Cracking (m2) WP Long. Crack
(m) NWP Long. Crack
(m) Trans. Cracking
(count) 9/4/14 0 0 213.8 9
Oct 2014 Mill and overlay with warm-mix with recycled AC 11/14/14 0 0 0 0
Potential Cracking Test Location: Thermal Cracking—Thick AC—Dry, Hot Climate—High Traffic Top-Down Cracking—High Solar Gain—No Freeze—High Volume, High Speed Bottom-Up Cracking Material Notes:
Figure C-11. Potential Field Test Section: LTPP 35-A061.
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LTPP Section: 35-A062 State: New Mexico (35) Roadway and Direction: IH 40 WB Experiment No.: SPS-10 Date of Surface Construction: Oct. 2014 Status: Active Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Fine-grained soils: Silty Clay – 2 Base 8 Soil-Aggregate Mix (mainly fine grained) Aug. 2014 3 AC Layer 4.6 Dense-Graded HMA Aug. 2014 4 AC Layer 3.4 Dense-Graded HMA Aug. 2014 5 AC Layer 4.1 Dense-Graded WMA Oct. 2014
Tot. AC Thickness = 12.1 Mill and warm-mix overlay in October 2014 Traffic Data: Number of Years with AADTT Data: 0 Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
Year Fatigue Cracking (m2) WP Long. Crack
(m) NWP Long. Crack
(m) Trans. Cracking
(count) 9/4/14 0 0 288.9 9
Oct 2014 Mill and overlay with warm-mix with recycled AC 11/17/14 0 0 0 0
Potential Cracking Test Location: Thermal Cracking—Thick AC—Dry, Hot Climate—High Traffic Top-Down Cracking—High Solar Gain—No Freeze—High Volume, High Speed Bottom-Up Cracking Material Notes:
Figure C-12. Potential Field Test Section: LTPP 35-A062.
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LTPP Section: 35-0901 State: New Mexico (35) Roadway and Direction: IH 10 EB Experiment No.: SPS-9O Date of Surface Construction: Sept. 1996 Status: Out-of-study Pavement Cross Section:
Layer No. Type Thickness (inches) Description Date
1 SG – Fine-Grained Soils: Sandy Silt – 2 Base 9 Soil-Aggregate Mix (mainly coarse grained) Sept. 1995 3 AC Layer 2.6 Dense-Graded HMA Sept. 1995 4 AC Layer – – Sept. 1995 5 AC Layer 3.2 Recycled AC, cold lay, mixed in place April 1996 6 AC Layer 4.4 Dense-Graded HMA Sept. 1996 7 AC Layer 1 Open Graded HMA Sept. 1996
Tot. AC Thickness = 11.2 Layer 6 and 7 overlay in Sept. 1996 – Traffic Data: Number of Years with AADTT Data: 19 Latest AADTT: 9593 Year: 1989 Latest KESAL: Year: Performance from last surface date:
Potential Cracking Test Location: Thermal Cracking—Dry, Hot Climate—Thick AC—High Traffic Top-Down Cracking—High Solar Gain—No Freeze—High Volume, High Speed Material Notes: Binder sampled 9/9/1996 and 9/11/1996. AC 10
Figure C-13. Potential Field Test Section: LTPP 35-0901.
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LTPP Section: 35-0902 State: New Mexico (35) Roadway and Direction: IH 10 EB Experiment No.: SPS-9O Date of Surface Construction: Sept. 1996 Status: Out-of-study Pavement Cross Section:
Layer No. Type Thickness (inches) Description Date
1 SG – Fine-Grained Soils: Sandy Silt – 2 Base 12 Soil-Aggregate Mix (mainly coarse grained) Sept. 1995 3 AC Layer 2.1 Dense-Graded HMA Sept. 1995 4 AC Layer – – Sept. 1995 5 AC Layer 3.2 Recycled AC, cold lay, mixed in place April 1996 6 AC Layer 4.5 Dense-Graded HMA Sept. 1996 7 AC Layer 1 Open Graded HMA Sept. 1996
Tot. AC Thickness = 10.8 Layer 6 and 7 overlay in Sept. 1996 – Traffic Data: Number of Years with AADTT Data: 19 Latest AADTT: 9593 Year: 1989 Latest KESAL: Year: Performance from last surface date:
10/7/1999 0 1.1 0.6 0 10/17/2000 0 0 0 0 11/13/2001 0.5 13.3 31.1 1 May 2002 Patch pot holes 11/3/2002 6.5 142 102.3 1 5/20/2003 19 13 110.2 0 Aug. 2003 Strip seal 11/9/2003 3.7 114.7 30.6 8
Potential Cracking Test Location: Thermal Cracking—Dry, Hot Climate—Thick AC—High Traffic Top-Down Cracking—High Solar Gain—No Freeze—High Volume, High Speed Material Notes: Binder sampled 9/11/1996. PG64-22
Figure C-14. Potential Field Test Section: LTPP 35-0902.
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LTPP Section: 35-0903 State: New Mexico (35) Roadway and Direction: IH 10 EB Experiment No.: SPS-9O Date of Surface Construction: Sept. 1996 Status: Out-of-study Pavement Cross Section:
Layer No. Type Thickness (inches) Description Date
1 SG – Fine-Grained Soils: Sandy Silt – 2 Base 10.5 Soil-Aggregate Mix (mainly coarse grained) Sept. 1995 3 AC Layer 1.9 Dense-Graded HMA Sept. 1995 4 AC Layer – – Sept. 1995 5 AC Layer 3.2 Recycled AC, cold lay, mixed in place April 1996 6 AC Layer 4.8 Dense-Graded HMA Sept. 1996 7 AC Layer 1 Open Graded HMA Sept. 1996
Tot. AC Thickness = 10.9 Layer 6 and 7 overlay in Sept. 1996 – Traffic Data: Number of Years with AADTT Data: 19 Latest AADTT: 9593 Year: 1989 Latest KESAL: Year: Performance from last surface date:
Potential Cracking Test Location: Thermal Cracking—Dry, Hot Climate—Thick AC—High Traffic Top-Down Cracking—High Solar Gain—No Freeze—High Volume, High Speed Material Notes: Binder sampled 9/11/1996. PG58-22
Figure C-15. Potential Field Test Section: LTPP 35-0903.
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LTPP Section: 18-A901 State: Indiana (18) Roadway and Direction: IH 70 EB Experiment No.: SPS-9J Date of Surface Construction: Jul-97 Status: Out-of-study (09/05/2004) Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Coarse-grained, poorly graded sand with clay – 2 Base 8 Soil-Aggregate Mix (mainly coarse grained) Jan. 1997 3 Conc. 10 JRCP Jan. 1997 4 AC Layer 4.5 Dense-Graded HMA Jan. 1997 5 AC Layer 1.8 Dense-Graded HMA July 1997
Tot. AC Thickness = 6.3 HMA overlay over JRCP July 1997 Traffic Data: Number of Years with AADTT Data: 2 Latest AADTT: 33785 Year: 2003 Latest KESAL: 3197 Year: 2001 Performance from last surface date:
Potential Cracking Test Location: Thermal Cracking—Thick AC—High Traffic—Cold Climate Reflection Cracking—Jointed Concrete—Steady State Climate Top-Down Cracking—Low Solar Gain—Hard Freeze Material Notes: 18-0900 exists on the binder list. Does this include A901?
Figure C-16. Potential Field Test Section: LTPP 18-A901.
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LTPP Section: 04-A901 State: Arizona (4) Roadway and Direction: US 93 NB Experiment No.: SPS-9N Date of Surface Construction: Jan. 1993 Status: Out-of-study (06/01/2006) Pavement Cross Section:
Tot. AC Thickness = 6.9 Traffic Data: Number of Years with AADTT Data: 3 Latest AADTT: 5950 Year: 1995 Latest KESAL: 54 Year: 2005 Performance from last surface date:
Potential Cracking Test Location: Thermal Cracking—Thick AC—Dry, Hot Climate Top-Down Cracking—High Solar Gain—No Freeze Bottom-Up Cracking Material Notes: 04-A900 is available in MRL-PG76-10 Composite
Figure C-17. Potential Field Test Section: LTPP 04-A901.
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LTPP Section: 04-A903 State: Arizona (4) Roadway and Direction: US 93 NB Experiment No.: SPS-9N Date of Surface Construction: Nov. 1992 Status: Out-of-study (06/01/2006) Pavement Cross Section:
Tot. AC Thickness = 6.7 Traffic Data: Number of Years with AADTT Data: 3 Latest AADTT: 5950 Year: 1995 Latest KESAL: 54 Year: 2005 Performance from last surface date:
Potential Cracking Test Location: Thermal Cracking—Thick AC—Dry, Hot Climate Top-Down Cracking—High Solar Gain—No Freeze Bottom-Up Cracking Material Notes: 04-A900 is available in MRL-PG76-10 Composite
Figure C-18. Potential Field Test Section: LTPP 04-A903.
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LTPP Section: 04-B901 State: Arizona (4) Roadway and Direction: IH 10 WB Experiment No.: SPS-9O Date of Surface Construction: Mar-95 Status: Out-of-Study 02/01/2005 Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Coarse-grained, Silty sand w/ gravel Feb 1995 2 Base 10.8 Crushed gravel Feb 1995 3 AC Layer 2.8 Dense-Graded HMA Feb 1995 4 AC Layer 4.4 Dense-Graded HMA Feb 1995 5 AC Layer 3.5 Dense-Graded HMA March 1995
Tot. AC Thickness = 10.7 Traffic Data: Number of Years with AADTT Data: 24 Latest AADTT: 14699 Year: 2007 Latest KESAL: 1112 Year: 2005 Performance from last surface date:
Potential Cracking Test Location: Thermal Cracking—Thick AC—Dry, Hot Climate Top-Down Cracking—High Solar Gain—No Freeze Material Notes: PG76-10 Binder sampled on 03/25/1995
Figure C-19. Potential Field Test Section: LTPP 04-B901.
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LTPP Section: 04-B903 State: Arizona (4) Roadway and Direction: IH 10 WB Experiment No.: SPS-9O Date of Surface Construction: Mar-95 Status: Out-of-Study 2005 Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Coarse-grained, Silty sand w/ gravel Feb 1995 2 Base 10.8 Crushed gravel Feb 1995 3 AC Layer 3 Dense-Graded HMA Feb 1995 4 AC Layer 4.4 Dense-Graded HMA Feb 1995 5 AC Layer 4 Dense-Graded HMA March 1995
Tot. AC Thickness = 11.4 1995 Surface info. (Out-of-Study 2005) Traffic Data: Number of Years with AADTT Data: 24 Latest AADTT: 14699 Year: 2007 Latest KESAL: 1110 Year: 2005 Performance from last surface date:
Potential Cracking Test Location: Thermal Cracking—Thick AC—Dry, Hot Climate Top-Down Cracking—High Solar Gain—No Freeze Material Notes: AC 40 Superpave Binder sampled on 03/17/1995
Figure C-20. Potential Field Test Section: LTPP 04-A903.
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LTPP Section: 05-0803 State: Arkansas (5) Roadway and Direction: US 65 SB Experiment No.: SPS-8 Date of Surface Construction: Jan. 1996 Status: Active Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Fine-grained, Lean clay w/ sand – 2 Base 7.3 Crushed stone Jan. 1996 3 AC Layer 2.5 Dense-Graded HMA Jan. 1996 4 AC Layer 1.2 Dense-Graded HMA Jan. 1996
Tot. AC Thickness = 3.7 1996 Surface info. Traffic Data: Number of Years with AADTT Data: 0 Latest AADTT: Year: Latest KESAL: 205 Year: 2006 Performance from last surface date:
Potential Cracking Test Location: Top-Down Cracking—Low Solar Gain—No Freeze Bottom-Up Cracking Material Notes: Binder sampled for 05-0800 on 10/09/1997 ACHM binder sampled for 05-0800 on 10/08/1997
Figure C-21. Potential Field Test Section: LTPP 05-0803.
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LTPP Section: 05-0804 State: Arkansas (5) Roadway and Direction: US 65 SB Experiment No.: SPS-8 Date of Surface Construction: Jan. 1996 Status: Active Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Fine-grained, Lean clay w/ sand – 2 Base 12.7 Crushed stone Jan. 1996 3 AC Layer 5.7 Dense-Graded HMA Jan. 1996 4 AC Layer 1.6 Dense-Graded HMA Jan. 1996
Tot. AC Thickness = 7.3 1996 Surface info. Traffic Data: Number of Years with AADTT Data: 0 Latest AADTT: Year: Latest KESAL: 198 Year: 2006 Performance from last surface date:
Potential Cracking Test Location: Top-Down Cracking—Low Solar Gain—No Freeze Material Notes: Binder sampled for 05-0800 on 10/09/1997 ACHM binder sampled for 05-0800 on 10/08/1997
Figure C-22. Potential Field Test Section: LTPP 05-0804.
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LTPP Section: 28-0805 State: Mississippi (28) Roadway and Direction: SH 315 NB Experiment No.: SPS-8 Date of Surface Construction: Jan. 1996 Status: Active Pavement Cross Section:
Layer 2 Dense-Graded HMA Aug. 1995 Tot. AC Thickness
= 14 1995 Surface info. (Out-of-Study 2005) Traffic Data: Number of Years with AADTT Data: 0 Latest AADTT: Year: Latest KESAL: 436 Year: 2000 Performance from last surface date:
Year Fatigue Cracking (m2) WP Long. Crack
(m) NWP Long. Crack
(m) Trans. Cracking
(count) 9/18/96 0 0 0 0 8/6/97 0 0 0 0
3/31/98 0 0 0 0 11/15/99 0 0 0 2 1/13/00 0 0 0 0
10/12/00 0 0 0 0 Potential Cracking Test Location: Top-Down Cracking—Low Solar Gain—No Freeze Material Notes: Binder sampled 11/11/1992, AC-30
Figure C-24. Potential Field Test Section: LTPP 28-0902.
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LTPP Section: 01-0101 State: Alabama (1) Roadway and Direction: US 280 WB Experiment No.: SPS-1 Date of Surface Construction: Apr-91 Status: Out-of-Study 6/15/05 Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Fine-Grained soils: sandy silt – 2 Base 7.9 Crushed Stone April 1991 3 AC Layer 6.2 Dense-Graded HMA April 1991 4 AC Layer 1.2 Dense-Graded HMA April 1991
Tot. AC Thickness = 7.4 Traffic Data: Number of Years with AADTT Data: 0 Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
Potential Cracking Test Location: Top-Down Cracking—Low Solar Gain—No Freeze Material Notes: Binder sampled 2/4/1993, AC20 Aggregate sample on MRL inventory
Figure C-25. Potential Field Test Section: LTPP 01-0101.
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LTPP Section: 01-0102 State: Alabama (1) Roadway and Direction: US 280 WB Experiment No.: SPS-1 Date of Surface Construction: Apr-91 Status: Out-of-Study 6/15/05 Pavement Cross Section:
Layer No. Type Thickness (inches) Description Date
1 SG – Fine-Grained soils: sandy silt – 2 Base 12 Crushed Stone April 1991 3 AC Layer 2.8 Dense-Graded HMA April 1991 4 AC Layer 1.4 Dense-Graded HMA April 1991
Tot. AC Thickness = 4.2 Traffic Data: Number of Years with AADTT Data: 0 Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
April 2003 Full depth patching, strip seal 1/20/04 0 0 0 80 2/24/04 145.1 0 10.1 32 4/29/05 166.9 0 12.2 43
Potential Cracking Test Location: Top-Down Cracking—Low Solar Gain—No Freeze Bottom-Up Cracking Material Notes: Binder sampled 2/4/1993, AC20 Aggregate sample on MRL inventory
Figure C-26. Potential Field Test Section: LTPP 01-0102.
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LTPP Section: 01-0103 State: Alabama (1) Roadway and Direction: US 280 WB Experiment No.: SPS-1 Date of Surface Construction: Apr-91 Status: Out-of-Study 6/15/05 Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Fine-Grained soils: sandy silt – 2 Base 7.4 HMAC treated base April 1991 3 AC Layer 2.8 Dense-Graded HMA April 1991 4 AC Layer 1.4 Dense-Graded HMA April 1991
Tot. AC Thickness = 4.2 Traffic Data: Number of Years with AADTT Data: 0 Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
2/23/04 40.1 0 10.1 32 4/28/05 41.4 0 12.2 43 Potential Cracking Test Location: Top-Down Cracking—Low Solar Gain—No Freeze Bottom-Up Cracking Reflection Cracking—Steady State Climate—Cracked Asphalt Material Notes: Binder sampled 2/4/1993, AC20 Aggregate sample on MRL inventory
Figure C-27. Potential Field Test Section: LTPP 01-0103.
C-28
LTPP Section: 01-0104 State: Alabama (1) Roadway and Direction: US 280 WB Experiment No.: SPS-1 Date of Surface Construction: Apr-91 Status: Out-of-Study 6/15/05 Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Fine-Grained soils: sandy silt – 2 Base 12.2 HMAC treated base April 1991 3 AC Layer 4.9 Dense-Graded HMA April 1991 4 AC Layer 1.4 Dense-Graded HMA April 1991
Tot. AC Thickness = 6.3 Traffic Data: Number of Years with AADTT Data: 0 Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
Potential Cracking Test Location: Top-Down Cracking—Low Solar Gain—No Freeze Bottom-Up Cracking Material Notes: Binder sampled 2/4/1993, AC20 Aggregate sample on MRL inventory
Figure C-28. Potential Field Test Section: LTPP 01-0104.
C-29
LTPP Section: 01-0105 State: Alabama (1) Roadway and Direction: US 280 WB Experiment No.: SPS-1 Date of Surface Construction: Apr-91 Status: Out-of-Study 6/15/05 Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Fine-Grained soils: sandy silt – 2 Base 4 Crushed Stone April 1991 3 Base 4.1 HMAC Treated Base April 1991 4 AC Layer 2.8 Dense-Graded HMA April 1991 5 AC Layer 1.3 Dense-Graded HMA April 1991
Tot. AC Thickness = 4.1 Traffic Data: Number of Years with AADTT Data: 0 Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
1/20/04 2 243.5 0 113 2/23/04 105.7 0 8.6 104 4/28/05 131.3 0 8 63 Potential Cracking Test Location: Top-Down Cracking—Low Solar Gain—No Freeze Bottom-Up Cracking Reflection Cracking—Steady State Climate—Cracked Asphalt Material Notes: Binder sampled 2/4/1993, AC20 Aggregate sample on MRL inventory
Figure C-29. Potential Field Test Section: LTPP 01-0105.
C-30
LTPP Section: 01-0106 State: Alabama (1) Roadway and Direction: US 280 WB Experiment No.: SPS-1 Date of Surface Construction: Apr-91 Status: Out-of-Study 6/15/05 Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Fine-Grained soils: sandy silt – 2 Base 8 Crushed Stone April 1991 3 Base 8.4 HMAC Treated Base April 1991 4 AC Layer 5.5 Dense-Graded HMA April 1991 5 AC Layer 1.4 Dense-Graded HMA April 1991
Tot. AC Thickness = 6.9 Traffic Data: Number of Years with AADTT Data: 0 Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
Potential Cracking Test Location: Top-Down Cracking—Low Solar Gain—No Freeze Material Notes: Binder sampled 2/4/1993, AC20 Aggregate sample on MRL inventory
Figure C-30. Potential Field Test Section: LTPP 01-0106.
C-31
MnRoad Cell No: 15 State: Minnesota Road Section and Direction: IH 94 WB Experiment No.: MnRoad Date of Surface Construction: Sept. 2008 Status: Complete Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Clay Subgrade July 1992 2 AC Layer 11 PG64-22 HMA July 1992 3 Int. Layer 0.5 Microsurface Aug. 2003 4 Int. Layer 0.5 Microsurface Aug. 2004 5 AC Layer 1.5 PG58-34 WMA w/ 20% RAP Sept. 2008 6 AC Layer 1.5 PG58-34 WMA w/ 20% RAP Sept. 2008
Tot. AC Thickness = 14 Traffic Data: Number of Years with AADTT Data: 20 Latest ESAL: 683751 Year: 2013 Latest KESAL: 684 Year: 2013 Performance from last surface date:
Year Fatigue Cracking (m2) WP Long. Crack
(m) NWP Long. Crack
(m) Trans. Cracking
(count)
Potential Cracking Test Location: Thermal Cracking—Cold Climate—Thick HMA—High Traffic Top-Down Cracking—Hard Freeze/Low Solar Gain—High Volume/High Speed Material Notes: Binder is PG58-34 with 20% RAP
Figure C-31. Potential Field Test Section: MnRoad Cell No. 15.
C-32
MnRoad Cell No: 16 State: Minnesota Road Section and Direction: IH 94 WB Experiment No.: MnRoad Date of Surface Construction: Sept. 2008 Status: Complete Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Clay Subgrade June 2008 2 Base 7 Sand—select blend of reclaimings June 2008 3 Base 12 MnRoad Gravel Class 3 Aug. 2008 4 Base 12 Recycled PCC Aug. 2008
5 AC Layer 2 PG58-34 WMA w/ 20% non-wear RAP and Evotherm 3G Sept. 2008
6 AC Layer 3 PG58-34 WMA w/ 20% wear RAP and Evotherm 3G Sept. 2008
Tot. AC Thickness = 5 Traffic Data: Number of Years with AADTT Data: 20 Latest ESAL: 683751 Year: 2013 Latest KESAL: 684 Year: 2013 Performance from last surface date:
Potential Cracking Test Location: Thermal Cracking—Cold Climate—Thin HMA—High Traffic Material Notes: Binder is PG58-34 with RAP
Figure C-32. Potential Field Test Section: MnRoad Cell No. 16.
C-33
MnRoad Cell No: 17 State: Minnesota Road Section and Direction: IH 94 WB Experiment No.: MnRoad Date of Surface Construction: Sept. 2008 Status: Complete Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Clay Subgrade June 2008 2 Base 7 Sand—select blend of reclaimings Aug. 2008 3 Base 12 MnRoad Gravel Class 3 Aug. 2008 4 Base 12 Crushed conc. & aggregate blend Sept. 2008
5 AC Layer 2 PG58-34 WMA w/ 20% non-wear RAP and Evotherm 3G Sept. 2008
6 AC Layer 3 PG58-34 WMA w/ 20% wear RAP and Evotherm 3G Sept. 2008
Tot. AC Thickness = 5 Traffic Data: Number of Years with AADTT Data: 20 Latest ESAL: 683751 Year: 2013 Latest KESAL: 684 Year: 2013 Performance from last surface date:
Potential Cracking Test Location: Thermal Cracking—Cold Climate—Thin HMA Material Notes: Binder is PG58-34 with RAP
Figure C-33. Potential Field Test Section: MnRoad Cell No. 17.
C-34
MnRoad Cell No: 18 State: Minnesota Road Section and Direction: IH 94 WB Experiment No.: MnRoad Date of Surface Construction: Sept. 2008 Status: Complete Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Clay Subgrade June 2008 2 Base 7 Sand—select blend of reclaimings Aug. 2008 3 Base 12 MnRoad Gravel Class 3 Aug. 2008 4 Base 12 RAP Sept. 2008
5 AC Layer 2 PG58-34 WMA w/ 20% non-wear RAP and Evotherm 3G Sept. 2008
6 AC Layer 3 PG58-34 WMA w/ 20% wear RAP and Evotherm 3G Sept. 2008
7 AC Layer – Chip Seal July 2014 Tot. AC Thickness = 5 Traffic Data: Number of Years with AADTT Data: 20 Latest ESAL: 683751 Year: 2013 Latest KESAL: 684 Year: 2013 Performance from last surface date:
Potential Cracking Test Location: Thermal Cracking—Cold Climate—Thin HMA Material Notes: Binder is PG58-34 with RAP
Figure C-34. Potential Field Test Section: MnRoad Cell No. 18.
C-35
MnRoad Cell No: 20 State: Minnesota
Road Section and Direction: IH 94 WB Experiment No.: MnRoad
Date of Surface Construction: Sept. 2008 Status: Complete
Pavement Cross Section:
Layer No. Type Thickness(inches) Description Date 1 SG – Clay Subgrade June 2008 2 Base 7 Sand—select blend of reclaimings June 2008 3 Base 12 MnRoad Gravel Class 3 June 2008 4 Base 12 Class 5 Gravel Base June 2008
Traffic Data: Number of Years with AADTT Data: 20 Latest ESAL: 683751 Year: 2013 Latest KESAL: 684 Year: 2013 Performance from last surface date:
Year Fatigue Cracking (m2) WP Long. Crack
(m) NWP Long. Crack
(m)
Trans. Cracking (count)
Potential Cracking Test Location: Thermal Cracking—Cold Climate—Thin HMA Bottom-Up Fatigue Cracking—High Traffic—Granular Base—Poor Subgrade Material Notes: Binder is PG58-28 with RAP
Figure C-35. Potential Field Test Section: MnRoad Cell No. 20.
C-36
MnRoad Cell No: 21 State: Minnesota
Road Section and Direction: IH 94 WB Experiment No.: MnRoad
Date of Surface Construction: Sept. 2008 Status: Complete
Pavement Cross Section:
Layer No. Type Thickness(inches) Description Date 1 SG – Clay Subgrade June 2008 2 Base 7 Sand—select blend of reclaimings June 2008 3 Base 12 MnRoad Gravel Class 3 June 2008 4 Base 12 Class 5 Gravel Base June 2008
Traffic Data: Number of Years with AADTT Data: 20 Latest ESAL: 683751 Year: 2013 Latest KESAL: 684 Year: 2013 Performance from last surface date:
Year Fatigue Cracking (m2) WP Long. Crack
(m) NWP Long. Crack
(m)
Trans. Cracking (count)
Potential Cracking Test Location: Thermal Cracking—Cold Climate—Thin HMA Bottom-Up Fatigue Cracking—High Traffic—Granular Base—Poor Subgrade Material Notes: Binder is PG58-28 with RAP
Figure C-36. Potential Field Test Section: MnRoad Cell No. 21.
C-37
MnRoad Cell No: 24 State: Minnesota Road Section and Direction: LVR S. Side Experiment No.: MnRoad Date of Surface Construction: Oct. 2008 Status: Complete Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Sand Subgrade May 1992 2 Base 4 MnRoad Base Class 6 Sept. 1992 3 AC Layer 3 PG58-28 HMA Aug. 1993 4 Int. Layer 0.5 Slury Seal Sept. 2005 5 Mill -3 Mill existing AC layers June 2008 6 AC Layer 3 PG58-34 WMA w/ 20% wear RAP Oct. 2008
Tot. AC Thickness = 3 Traffic Data: Number of Years with AADTT Data: 20 Latest ESAL: 9969 Year: 2013 Latest KESAL: 10 Year: 2013 Performance from last surface date:
Year Fatigue Cracking (m2) WP Long. Crack
(m) NWP Long. Crack
(m) Trans. Cracking
(count)
Potential Cracking Test Location: Thermal Cracking—Cold Climate—Thin HMA—Low Traffic Bottom-Up Fatigue Cracking—Low Traffic—Granular Base Material Notes: Binder is PG58-34 with RAP
Figure C-37. Potential Field Test Section: MnRoad Cell No. 24.
C-38
MnRoad Cell No: 83 State: Minnesota Road Section and Direction: Farm Road Experiment No.: MnRoad Date of Surface Construction: Oct. 2007 Status: Complete Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Clay Subgrade Aug. 2007 2 Subbase 24 Clay Aug. 2007 3 Base 8 Gravel—Class 5 Sept. 2007
4 AC
Layer 3.5 PG58-34 HMA Oct. 2007 Tot. AC Thickness = 3.5 Traffic Data: Number of Years with AADTT Data: Latest ESAL: Year: Latest KESAL: Year: Performance from last surface date:
Year Fatigue Cracking
(m2) WP Long. Crack (m) NWP Long. Crack
(m)
Trans. Cracking (count)
Potential Cracking Test Location: Thermal Cracking—Cold Climate—Thin HMA Bottom-Up Fatigue Cracking—Low Traffic—Poor Subgrade Material Notes: Binder is PG58-34
Figure C-38. Potential Field Test Section: MnRoad Cell No. 83.
C-39
MnRoad Cell No: 84 State: Minnesota Road Section and Direction: Farm Road Experiment No.: MnRoad Date of Surface Construction: Oct. 2007 Status: Complete Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Clay Subgrade Aug. 2007 2 Subbase 24 Clay Aug. 2007 3 Base 9 Gravel—Class 5 Sept. 2007
4 AC
Layer 5.5 PG58-34 HMA Oct. 2007 Tot. AC Thickness = 5.5 Traffic Data: Number of Years with AADTT Data: Latest ESAL: Year: Latest KESAL: Year: Performance from last surface date:
Year Fatigue Cracking
(m2) WP Long. Crack (m) NWP Long. Crack
(m)
Trans. Cracking (count)
Potential Cracking Test Location: Thermal Cracking—Cold Climate—Thin HMA Bottom-Up Fatigue Cracking—Low Traffic—Poor Subgrade Material Notes: Binder is PG58-34
Figure C-39. Potential Field Test Section: MnRoad Cell No. 84.
C-40
LTPP Section: 49-0803 State: Utah (49) Roadway and Direction: SH 35 EB Experiment No.: SPS-8 Date of Surface Construction: Jan. 1996 Status: Active Pavement Cross Section:
Layer No. Type Thickness (inches) Description Date
1 SG – Coarse-Grained Soil: Clayey sand with gravel – 2 Base 41.2 Coarse-Grained Soil: Clayey sand with gravel Jan. 1996 3 Base 7.8 Crushed Gravel Jan. 1996
4 AC
Layer 4.9 Dense-Graded HMA Jan. 1996
5 AC
Layer – Dense-Graded HMA Aug. 1998 Tot. AC Thickness = 4.9
Traffic Data: Number of Years with AADTT Data: 9 Latest AADTT: 160 Year: 2009 Latest KESAL: Year: Performance from last surface date:
Potential Cracking Test Location: Bottom-Up Fatigue Cracking Material Notes: Binder sampled 10/17/1997, PG 58-34
Figure C-40. Potential Field Test Section: LTPP Section 49-0803.
C-41
LTPP Section: 49-0804 State: Utah (49)
Roadway and Direction: SH 35 EB Experiment No.: SPS-8
Date of Surface Construction: Jan. 1996 Status: Active
Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Coarse-Grained Soil: Clayey sand with gravel – 2 Base 41.2 Coarse-Grained Soil: Clayey sand with gravel Jan. 1996 3 Base 12 Crushed Gravel Jan. 1996 4 AC Layer 7.1 Dense-Graded HMA Jan. 1996 5 AC Layer – Dense-Graded HMA Aug. 1998
Tot. AC Thickness = 7.1 Traffic Data: Number of Years with AADTT Data: 9 Latest AADTT: 160 Year: 2009 Latest KESAL: Year: Performance from last surface date:
Potential Cracking Test Location: Top-Down Cracking—Hard Freeze, High Solar Gain Bottom-Up Fatigue Cracking Material Notes: Binder sampled 10/17/1997, PG 58-34
Figure C-41. Potential Field Test Section: LTPP Section 49-0804.
C-42
LTPP Section: 30-0901 State: Montana (30) Roadway and Direction: IH 15 NB Experiment No.: SPS-9N Date of Surface Construction: Oct. 1998 Status: Active Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Coarse-Grained: Well graded sand w/ gravel – 2 Base 1 Crushed Gravel Oct. 1998
3 AC
Layer 4.9 Dense-Graded HMA Oct. 1998
4 AC
Layer – Chip Seal July 2004 Tot. AC Thickness = 4.9 Traffic Data: Number of Years with AADTT Data: Latest AADTT: Year: Latest KESAL: 221 Year: 2009 Performance from last surface date:
Potential Cracking Test Location: Top-Down Cracking—Hard Freeze, Low Solar Gain Bottom-Up Fatigue Cracking Material Notes: Binder sampled 10/26/1998, PMAC Binder
Figure C-42. Potential Field Test Section: LTPP Section 30-0901.
C-43
LTPP Section: 30-0902 State: Montana (30) Roadway and Direction: IH 15 NB Experiment No.: SPS-9N Date of Surface Construction: Oct. 1998 Status: Active Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Coarse-Grained: Well graded sand w/ gravel – 2 Base 1 Crushed Gravel Oct. 1998
3 AC
Layer 4.7 Dense-Graded HMA Oct. 1998
4 AC
Layer – Chip Seal July 2004 Tot. AC Thickness = 4.7 Traffic Data: Number of Years with AADTT Data: Latest AADTT: Year: Latest KESAL: 221 Year: 2009 Performance from last surface date:
Potential Cracking Test Location: Top-Down Cracking—Hard Freeze, Low Solar Gain Bottom-Up Fatigue Cracking Material Notes: Binder sampled 10/26/1998, PMAC Binder
Figure C-43. Potential Field Test Section: LTPP Section 30-0902.
C-44
LTPP Section: 30-0903 State: Montana (30) Roadway and Direction: IH 15 NB Experiment No.: SPS-9N Date of Surface Construction: Oct. 1998 Status: Active Pavement Cross Section:
Layer No. Type
Thickness (inches) Description Date
1 SG – Coarse-Grained: Well graded sand w/ gravel – 2 Base 1 Crushed Gravel Oct. 1998
3 AC
Layer 4.8 Dense-Graded HMA Oct. 1998
4 AC
Layer – Chip Seal July 2004 Tot. AC Thickness = 4.8 Traffic Data: Number of Years with AADTT Data: Latest AADTT: Year: Latest KESAL: 221 Year: 2009 Performance from last surface date:
Potential Cracking Test Location: Top-Down Cracking—Hard Freeze, Low Solar Gain Bottom-Up Fatigue Cracking Material Notes: Binder sampled 10/26/1998, PMAC Binder
Figure C-44. Potential Field Test Section: LTPP Section 30-0903.
C-45
LTPP Section: 55-0805 State: Wisconsin (55)
Roadway and Direction: SH 29 EB Experiment No.: SPS-8
Date of Surface Construction: Nov. 1997 Status: Active
Layer 2.1 Dense-Graded HMA Nov. 1997 Tot. AC Thickness = 4.5 Traffic Data: Number of Years with AADTT Data: Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
Layer 2.1 Dense-Graded HMA Nov. 1997 Tot. AC Thickness = 7.2 Traffic Data: Number of Years with AADTT Data: Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
Layer 1.6 Dense-Graded HMA Dec. 1997 Tot. AC Thickness = 3.8 Traffic Data: Number of Years with AADTT Data: Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
Layer 1.4 Dense-Graded HMA Dec. 1997 Tot. AC Thickness = 1.4 Traffic Data: Number of Years with AADTT Data: Latest AADTT: Year: Latest KESAL: Year: Performance from last surface date:
