TREATED BASE COURSEPERFORMANCE
IN ALASKA
Prepared byR. Scott Gartin, P.E. and David C. Esch, P.E.Alaska Department of Transportation & Public FacilitiesStatewide Research, Fairbanks, Alaska
June 1991
Prepared for:Alaska Department of TransportationStatewide Research2301 Peger RoadFairbanks, AK 99709-5399
FHWA-AK-RD-91-13
Alaska Departm
ent of Transportation & Public Facilities
Research &
Technology Transfer
10. Work Unit No. (TRAIS)
14. Sponsoring Agency Code
7. Author(s)
13. Type of Report and Period Covered
1. Report No. 2. Government Accession No.
4. Title and Subtitle
3. Recipient's Catalog No.
17. Key Words 18. Distribution Statement
16. Abstract
15. Supplementary Notes
19. Security Classif. (of this report) 20. Security Classif. (of this page) 21. No. of Pages 22. Price
8. Performing Organization Report No.
11. Contract or Grant No.
Technical Report Documentation Page
5. Report Date
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Form DOT F 1700.7 (8-72) Reproduction of completed page authorized
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This form was electronically produced by Elite Federal Forms, Inc. Modified to PDF 9/2000 by FHWA Alaska Division
METRIC (SI*) CONVERSION FACTORSAPPROXIMATE CONVERSIONS TO SI UNITS APPROXIMATE CONVERSIONS FROM SI UNITS
Symbol When You Know Multiply By To Find Symbol Symbol When You Know MultiplyBy
To Find Symbol
LENGTH LENGTH
in inches 2.54 cm mm millimeters 0.039 inches inft feet 0.3048 m m meters 3.26 feet ftyd yards 0.914 m m meters 1.09 yards ydmi miles 1.61 km km kilometers 0.621 miles mi
AREA AREA
in2 square inches 6.452 centimeters squared cm2 mm2 millimeters squared 0.0016 square inches in2
ft2 square feet 0.0929 meters squared m2 m2 meters squared 10.764 square feet ft2
yd2 square yards 0.836 meters squared m2 km2 kilometers squared 0.39 square miles mi2
mi2 square miles 2.59 kilometers squared km2 ha hectares (10,000 m2) 2.53 acres acac acres 0.395 hectares ha
MASS(weight)
MASS(weight)
oz Ounces 28.35 grams g g grams 0.0353 ounces ozlb Pounds 0.454 kilograms kg kg kilograms 2.205 pounds lbT Short tons (2000 lb) 0.907 megagrams mg mg megagrams (1000 kg) 1.103 short tons T
VOLUME VOLUME
fl oz fluid ounces 29.57 milliliters mL mL milliliters 0.034 fluid ounces fl ozgal gallons 3.785 liters liters liters liters 0.264 gallons galft3 cubic feet 0.0328 meters cubed m3 m3 meters cubed 35.315 cubic feet ft3
yd3 cubic yards 0.765 meters cubed m3 m3 meters cubed 1.308 cubic yards yd3
Note: Volumes greater than 1000 L shall be shown in m3
TEMPERATURE(exact)
TEMPERATURE(exact)
oF Fahrenheittemperature
5/9 (aftersubtracting 32)
Celsiustemperature
oC oC Celsius temperature 9/5 (then add 32) Fahrenheittemperature
oF
These factors conform to the requirement of FHWA Order 5190.1A *SI is thesymbol for the International System of Measurements
-40oF
-40oC
0
-200
3240
2037
40 60 80100oC
8098.6
120 160 200212oF
TREATED BASE COURSEPERFORMANCE
IN ALASKA
By
R. Scott Gartin, P.E. and David C. Esch, P.E. Alaska Department of Transportation & Public Facilities
Statewide Research. Fairbanks, AlaskaJune 1991
Prepared forState of Alaska
Department of Transportation and Public FacilitiesStatewide Research
2301 Peger Road, Fairbanks, AK 99709-5399
In cooperation withU.S. Department of Transportation
Federal Highway Administration
FHWA-AK-RD-91-13
The contents of this report reflect the views of the authors, who areresponsible for the facts and accuracy of the data presented herein. Thecontents do not necessarily reflect the official views or policies of the AlaskaDepartment of Transportation & Public Facilities or the Federal HighwayAdministration. This report does not constitute a standard, specification, orregulation.
TABLE OF CONTENTSPAGE
LIST OF TABLES AND FIGURES v-vi
ABSTRACT vii
IMPLEMENTATION STATEMENT viii
1.0 INTRODUCTION 1
2.0 BACKGROUND 2
2.10 Emulsified Asphalt Treated Bases (Cold Mix) 32.11 Heave Rate Test 5
2.20 Asphalt Cement Treated Base (Hot Mix) 62.30 Other Types of Treated Bases (not considered in this report) 7
3.0 DESCRIPTION OF SECTIONS 8
3.10 Richardson Highway near Tiekel and Thompson Pass 83.11 Thompson Pass – Emulsified Asphalt Treated Base Section 83.12 Tiekel – Asphalt Treated Base Section 93.13 Untreated Base Section 10
3.20 Alaska Highway near Northway 103.30 Elliot Highway near Fox 11
4.0 FIELD VERIFICATION AND EVALUATION 13
4.01 Modulus Backcalculation Procedures 134.10 Richardson Highway near Tiekel Pass 14
4.11 Core Sample Test Results 154.12 Backcalculation Procedures 15 4.121 Emulsified Asphalt Treated Base Section 16 4.122 Asphalt Treated Base Section 17 4.123 Untreated Base Section 19
4.20 Alaska Highway near Northway 204.21 1988 Backcalculation Results 214.22 1989 Backcalculation Results 23
iii
4.30 Elliot Highway near Fox 24
5.0 LABORATORY RESULTS 28
5.10 Richardson Highway near Tiekel Pass 285.20 Alaska Highway near Northway 295.30 Elliot Highway near Fox 30
6.0 PERFORMANCE ANALYSIS 31
7.0 CONCLUSIONS 37
8.0 BIBLIOGRAPHY 38
iv
LIST OF TABLES AND FIGURES
PAGETABLES
2.1 Laboratory Heave Rate Classification System 6
4.1 Richardson Highway Emulsified Asphalt Treated Base Moduli 16
4.2 Richardson Highway Emulsified Asphalt Treated Base Section Layer
Properties (Spring) 16
4.3 Richardson Highway Emulsified Asphalt Treated Base Section LayerProperties (Fall) 17
4.4 Richardson Highway Asphalt Treated Base Moduli 17
4.5 Richardson Highway Asphalt Treated Base Section Layer Properties(Spring) 18
4.6 Richardson Highway Asphalt Treated Base Section layer Properties(Fall) 18
4.7 Richardson Highway Untreated Base Moduli 19
4.8 Richardson Highway Untreated Base Section Layer Properties (Spring) 19
4.9 Richardson Highway Untreated Base Section Layer Properties (Fall) 20
4.10 Alaska Highway Base Course Moduli – 1988 21
4.11 Alaska Highway Layer Properties – 1988 22
4.12 Alaska Highway Base Course Moduli – 1989 23
4.13 Alaska Highway Layer Properties – 1989 24
4.14 Elliot Highway Base Moduli and Temperatures 25
4.15 Elliot Highway Measured Thaw Depth vs. Computed Depth toStiff Layer 26
v
4.16 Elliot Highway Layer Properties 27
5.1 Richardson Highway Laboratory Strain and Modulus Test Results 28
5.2 Alaska Highway Laboratory Strain and Modulus Test Results 29
5.3 Elliot Highway Laboratory Strain and Modulus Test Results 30
6.1 Critical Strains and Predicted Pavement Life 33
FIGURES
6.0 Comparison of Failure Criteria 36
vi
ABSTRACT
This report summarizes work by various researchers over the past several yearsregarding treated base courses used for highway construction in Alaska. Hot-mixasphalt and asphalt emulsion (cold-mix) treated bases are considered. Control sectionsof untreated base course are analyzed for comparison where data was available.
The results are intended for use by highway engineers interested in the effects ofincorporating treated base courses in their project. Hundreds of field tests wereperformed. The pavement layer moduli have been backcalculated and means ofanalysis for design purposes are presented.
Highway sections that have been considered and analyzed are:
1. Richardson Highway in the Tiekel River and Thompson Pass areas.2. Alaska Highway near Northway.3. The Elliot Highway near Fox.
Tasks performed in the analysis of the treated base sections include:
1. Cataloging sections: obtaining project as-built information.2. Field verification and evaluation: core samples were taken and Falling Weight
Deflectometer data was obtained and moduli backcalculated.3. Laboratory work: asphalt contents, gradations, densities, and laboratory resilient
modulus tests were performed on the core samples when intact cores wererecovered.
4. Performance analysis: The effects of using treated base in the pavementstructure are analyzed.
This report presents the results of each task for the various sections. The results areused to predict performance and cost/benefits of using treated bases, and areconsiderations for future designs.
vii
TREATED BASE COURSE PERFORMANCE IN ALASKA
1.0 INTRODUCTION
Engineers have long realized the structural and economic advantages inherent toaggregate stabilization. In many areas of Alaska, clean, durable aggregates normallyutilized for base course either require long hauls from outside of, or are difficult to obtainwithin the project limits. Stabilization of the available lower quality materials for use asbase course is sometimes considered. In pavement design work, stabilization is oftenused to improve the base course material properties. These properties can be strength,durability, stiffness, cohesion, permeability, frost susceptibility, workability and others.The difficulty in design lies in the assignment of specific properties to the differentpavement structure layers before these layers have been constructed. Data frompreviously constructed roadways is therefore essential in selecting properties for designanalysis. No such analysis of treated base layers has ever been attempted in Alaska.
It is obvious that installation of a stiffer layer beneath a pavement will decrease thehorizontal strain at the bottom of the pavement. Excessive horizontal strains in thepavement have been found to lead to fatigue cracking and failures. In fact, horizontalstrain at the bottom of the pavement is the criteria used to design and predict life interms of number of repetitions to failure. Treated bases therefore can increasepavement fatigue life and should be considered by the prudent designer. The problemlies in the availability of data to predict the moduli of treated bases for design. Thisreport presents data from existing sections for reference in design.
1
2.0 BACKGROUND
The following section gives background information and general designconsiderations regarding the two types of base treatments. References are providedso the reader requiring greater detail may find more information particular to theirneeds. Treated bases have only been considered in Alaska when the available basematerials were marginal in frost susceptibility due to excessive fines (-#200)contents. Emulsified asphalts may be used with aggregates ranging from gravels orcrushed rock to slightly silty sands. Asphalt cements are generally used only ongravels with at least some material retained on the 3/8” sieve.
The structural function of a pavement is to support a wheel load on the pavementsurface while transferring and spreading that load to the subgrade without exceedingeither the strength of the subgrade or the internal strength of each layer of thepavement structure itself. The subgrade ultimately carries all traffic loads.
The base course is considered the most critical structural element of the pavement.In conjunction with the overlying asphalt surface, its purpose is to distribute trafficwheel loads over the lower layers. To perform this function, the bases must be builtwith necessary internal strength and frost action resistance properties. In this respectasphalt treated bases have advantages over untreated base courses. Each wheelload on a pavement system slightly deflects the pavement structure causing both thetensile and compressive stresses. Asphalt treated pavement layers have both tensileand compressive strength to resist those internal stresses. Untreated granular baseshave no tensile strength except for that induced by soil moisture tension effects,which are lost upon saturation. Therefore, asphalt treated bases resist the effects ofsaturation and freezing, and spread the wheel load over broader areas thanuntreated granular bases. As a result, less total pavement structural thickness isrequired using an asphalt treated base.
2
2.10 EMULSIFIED ASPHALT TREATED BASES (COLD MIX)
An asphalt emulsion consists of three basic ingredients: asphalt, water and anemulsifying agent. Some emulsifying agents contain a stabilizer. In some cases, suchas when using a sandy aggregate, portland cement is added to aid in thedevelopment of early initial strength. Lime may be added to improve the strippingresistance and to speed curing.
Asphalt emulsions are divided into three categories: anionic, cationic, and nonionic.The anionic and cationic classes refer to the electrical charges surrounding theasphalt particles. Like charges repel one another, and unlike charges attract. Theletter “C” in front of an emulsion type denotes cationic. The absence of the letter "C”indicates anionic or nonionic emulsion.
Emulsions are further classified on the basis of how quickly they "set” on revert toasphalt cement. The relative terms RS, MS and SS have been adopted to indicaterapid setting, medium setting and slow setting emulsion.
The prefix "HF” on medium setting anionic emulsions indicates "high float.” Highfloat emulsions have a specific quality that permits a thicker film coating withoutdanger of runoff [11].
The suffixes 1 or 2 used on the emulsion type indicates the relative viscosity of theemulsion. The number 1 is used with lower viscosity grades. The "h" thatfollows certain grades simply means that a harder base asphalt was used. An "S"following the grade indicates that a required solvent content was included duringproduction.
The key to obtaining good results using emulsified asphalt is to select the rightemulsion for the aggregate or construction system Involved. Generally the rapid setemulsions are only used in spray seal applications or with very coarse aggregate.
Slow setting and medium setting emulsions are more often used for base treatment.Slow setting emulsions are used with dense-graded aggregates due to their greaterability to coat the finer particles. Medium setting emulsions are used with open-graded aggregates to prevent the emulsion from draining off prior to set.
3
The predominating charge and other properties of the aggregate surface determinewhether cationic or anionic emulsion will produce the best results. The only way tobe sure is to test in the laboratory for setting rate and strength properties.
Asphalt emulsion may be mixed with aggregate in any one of the following ways:
1) at a central mixing plant2) in place with a travelling mixer3) on a mixing area or in windrows on the roadway.
For estimating the approximate emulsified asphalt content needed to fully coat adense-graded mix the following empirical formula may be used [13]:
P = (0.05A + 0.1B + 0.5C) x 0.7 (1)
where: P = % of total emulsified asphalt by weight of dry aggregateA =% of aggregate retained on the No.8 sieveB =% of aggregate passing the No.8 and retained on the No.200 sieveC = % of aggregate passing the No.200 sieve
As an example using D-1 [15] graded aggregate with 35% passing the No.8 sieveand 6% passing the No.200 sieve yields an estimate of:
P =[0.05(100-35) +0.1(35-6) + 0.5(6)3 x0.7 = 6.4%
Mix designs may be performed by the Marshall test method (6). The Asphalt Instituterecommends emulsified asphalt treated base thicknesses of 6.5 inches to 11.5inches depending on the quality of the base aggregate - ranging from processed andwell graded to sands or silty sands [14].
The Alaska DOT&PF has more commonly utilized a second design philosophy foremulsion treated bases. This approach involves the use of lower emulsion contentswhich are sufficient only to coat and bind together the fine particles, thereby reducingfrost heaving in a laboratory test cabinet, to the "low or negligible" category.
4
Typically, this may indicate a minimum emulsion content as low as two to threepercent. Most Alaskan emulsion base stabilization projects have utilized thisapproach to treat base layers which have degraded or broken down to become frostsusceptible after placement or service.
2.11 HEAVE RATE TESTS
Heave test procedures used in this method have been developed by USA CRREL,and modified in the Materials Laboratory of the Alaska Department of Transportationand Public Facilities over the past ten years, for evaluating the performance of soilstabilizing agents and for comparing the frost heave susceptibilities of base andsubbase aggregates at differing gradations and fines contents. The testingprocedure involved the removal of the + ¾ inch particles and compaction of samplesin fine layers, using a vibratory hammer in 6 inch diameter by 5½ inch highsegmented ring molds. Compaction is followed by sample saturation by overnightsoaking. Heave measurements are recorded after draining down the water level to ½inch above the base of the sample. A fixed +15°F air temperature is applied abovethe samples for 72 hours, while maintaining a +40°F temperature beneath thesamples. Samples are classified on the basis of the heave occurring between 48 and72 hours after the start of freezing, during which time the freezing rate approximates½ inch per day.
A Heave Classification System which was originally developed for Corps ofEngineers heave tests performed at a freezing rate of ½ inch per day in taperedmolds, is applied to the heave test results shown in Table 2.1.
5
TABLE 2.1 LABORATORY HEAVE RATE CLASSIFICATION SYSTEM
Average Heave Rate48 - 72 Hrs (mm/day) Heave Rate Classification
0.0 – 0.5 Negligible0.5 – 1.0 Very Low1.0 – 2.0 Low2.0 – 4.0 Medium4.0 – 8.0 High
Greater than 8.0 Very High
2.20 ASPHALT CEMENT TREATED BASE (HOT MIX)
Asphalt cements are graded according to three different systems. They are:penetration, viscosity, and viscosity after aging. Penetration graded asphalts are notcommonly used in the United States anymore. Viscosity graded asphalt cements aremost commonly used in Alaska. Their names begin with “AC” and a numberindicating their viscosity-higher numbers mean higher viscosity. High viscosityasphalt cement tends to become brittle (or less ductile) at cooler temperatures, solow viscosity asphalt cements, i.e., AC-2.5, are more often used in Alaska. The thirdgrade of asphalt are viscosity graded after being aged in an oven. Their designationsbegin with “AR”, which stands for “Aged Residue". The idea is to identify what theviscosity characteristics will be after it is placed in the pavement.
The amount of asphalt required for a given mixture should be determined byappropriate laboratory testing, e.g. the Marshall Method. Generally the larger theaggregate the less asphalt is required to obtain stability and other criteria. Expectedminimum asphalt contents by weight of total mixture may range from 3-5 percent,depending on gradation, absorption and specific gravity of the aggregate. Marshalldesign criteria for light traffic may be used in asphalt treated bases. These criteria asrecommended by the Asphalt Institute [13] are:
6
No. of blows = 35 each end of specimen
Stability = 750 lbs.
Flow = 8 -18 (0.01 inch)% Air Voids = 3 - 5
% Voids in Mineral Aggregate (VMA) = 12 – 16 *
* Depending on the largest sieve size in the aggregate specification upon whichany material is retained.
2.30 OTHER TYPES OF TREATED BASES (not considered in this report)
Cut-back asphalts which contain petroleum solvents, may also be used for base coursetreatment, with applications similar to emulsions. However, more stringentenvironmental considerations are in order. Since cut-back asphalts are liquidhydrocarbons that cure by evaporation and are subject to runoff which couldcontaminate surface waters.
Portland cement has also been used with some success In other locations in Alaskasuch as Bethel and Barrow, for base course and soil stabilization [17]. These uses willnot be discussed further herein.
7
3.0 DESCRIPTION OF SECTIONS
The following are summarizations of site histories stabilized base work performed atseveral treated base sections that have bean constructed In Alaska. Generalinformation about the projects and sites as given here includes but is not limited to:
1. Project location2. Age3. Layer thicknesses4. Asphalt type5. Gradation of aggregate6. Purpose of treatment7. Method of installation8. Any construction related problems encountered
3.10 RICHARDSON HIGHWAY NEAR TIEKEL AND THOMPSON PASS
Two sections in the Thompson Pass vicinity with treated base courses wereconsidered here. Both have subgrades consisting of alluvial deposits or rock cuts.One was constructed using in-place emulsified asphalt treated base and the secondusing hot mix asphalt treatment with fixed plant mixing and conventional paverplacement. A third section nearby, constructed using untreated crushed aggregatebase, was used for comparison.
3.11 THOMPSON PASS - EMULSIFIED ASPHALT TREATED BASE SECTION
In 1985 a project (No. F-071-1 (55)) was constructed from milepost 25 to 35 on theRichardson Highway which required the emulsion base stabilization work, using atravelling mixer. The asphalt emulsion treated base thickness varies from 4 to 6inches in thickness here. The emulsified asphalt used was a cationic, low viscosity,grade CSS-1, which Chevron Oil Company produced from AC-5 (medium viscosityasphalt), at the site using a portable emulsion mill.
8
The treated base was surfaced with 1-1/2 inches of asphalt concrete pavementmade with AC-2.5 (low viscosity hot-mix asphalt). The base course was installedusing a paver. The embankment beneath the base had been constructed in 1980,using 24” of shot rock.
Gradations used in the base course were generally 1-1/2” minus material [10], madeusing reconditioned pavement and base from the previous project (1980). Theaggregate used in the project was found to be a highly degradable graywacke whichhad excessive thaw-weakening after the original placement contract was completed.The treatment was used to help/stabilize the fines in the aggregate and minimizefurther degradation.
The mix design for the treated base determined that 3.5% of CSS-1 was thedesirable emulsion content [10]. The top layer of the aggregate base tended to ravelin this project. This resulted in 5-6% overruns in the asphalt concrete pavementsince the loose material had to be swept away prior to paving. The primary reasonfor the raveling was that the base course was not placed and compacted in finalposition for several days after the treatment. The base took an initial set during thisperiod, and had to be aggressively re-worked to obtain the final cross-section.
3.12 TIEKEL - ASPHALT TREATED BASE SECTION
This section was constructed between mileposts 35 and 40 on the RichardsonHighway in 1984. The base course was treated with AC-2.5 after an Extra WorkOrder was written by the Resident Engineer. The base thickness is 4” and issurfaced with 2” of asphalt concrete, which also was made using AC-2.5.
The base course was a D-1 gradation [1], with excess fines (>6%) due todegradation, hence the Extra Work Order. Type I aggregate [1], that is 100% passingthe 1” sieve, was used in the final pavement.
9
3.13 UNTREATED BASE SECTION
A section adjacent to the two previous sections was constructed with untreatedcrushed aggregate base course. The section spans from milepost 40 to 43.Construction last took place here in 1976, when 2” of asphalt concrete pavementwas placed over the existing (1964) Bituminous Surface Treatment (BST), with 6” ofD-1 base, 6” of select material, and unclassified embankment beneath. The BSTlayer was constructed using two chip gradations, one ¾” minus and the other 3/8”minus. This usually results in a net surface thickness of ¾” since the 3/8” minusmaterial is designed to fit in the voids of the gap-graded ¾” minus material.
The current (1976) pavement was constructed using I" minus aggregate with AC-2.5asphalt. An asphalt emulsion tack coat (CRS-2) was used between the BST and theasphalt concrete pavement. CRS-2 is a cationic, rapid setting, high viscosityemulsion.
3.20 ALASKA HIGHWAY NEAR NORTHWAY
Emulsified asphalt treated base course was constructed on the Alaska Highwaybetween mile posts 1238 and 1253 in 1984 (Project No. IR-l-OA1 -1(2)). The projectspanned from milepost 1235 to 1253, so only the first 3 miles of base wereconstructed using no treatment.
The typical section includes 2" of asphalt concrete pavement, 4" of CrushedAggregate Base, 18" of Borrow A, and 36" of non-frost susceptible (NSF) sand.Subgrade material is generally sand or silty sand often of high moisture content [7,8].
The original project specifications called for untreated base with 0-6% by weightpassing the #200 sieve and a minimum degradation value of 45. The Contractor wasunable to meet these specifications with material available near the project site, sothey were changed by Extra Work Order (No. 10) to 0-10% passing the #200 sieveand a degradation value of 40. The excess fines due to degradation of the materialwere found to make it highly frost susceptible and subject to saturation because ofmoisture being trapped in it due to rains. In fact, the base was so weak that alligator(fatigue) cracking occurred immediately after paving. This led to the realization thatbase improvement by stabilization was mandatory.
10
Therefore a Work Order ("K") was written directing the Contractor to use emulsifiedasphalt (CSS-1) treatment on the base [9]. The application rate of CSS-1 wasestablished to be 4% of the base course quantity.
The emulsion was placed in two applications from a tanker using a spray bar withintermediate grader blading of the base. It was found that compaction was bestaccomplished using a rubber tired rather than a steel wheeled roller. Once theemulsion material had started to break, the steel wheeled roller would tend to ride upon hard points, achieving little compaction. This method provided very poor resultsduo to inadequate mixing, even with up to seven grader passes. It is not to berecommended!
3.30 ELLIOT HIGHWAY NEAR FOX
In August of 1985 a 4” emulsified asphalt (CSS-1) treated base was constructed onthe first 7 miles of the Elliot Highway (Project No. F-065-1[5]) Most of the project wasreconditioning work including pulverizing the old pavement and base course, treatingit with emulsion and then constructing a 1-1/2” asphalt concrete pavement surfaceusing AC-2.5. Two “Subgrade Stabilization" sections, totaling approximately 1 mileinvolved excavation to grade, placement of subgrade stabilization fabric beneath andon top of Borrow Type A, installation of Subbase Grading E [1], and placement of thetreated base and pavement on top. Three test areas (Extra Work Order #1) in theSubgrade Stabilization sections were built with dynamic compaction, using a 10 tonweight dropped 20 feet from a crane. The Subgrade Stabilization sections hadshown severe embankment deformation prior to the onset of the project due to poordrainage and/or subgrade conditions (3). The poor foundation conditions in thisproject include ice-rich organic silt subject to annual, sometimes increasing, thawcycles [4]. Crushed aggregate base course was used, where required to fill voids, inthe project.
The purpose of the base treatment was to optimize the project money spent onpaving. The highway receives heavy truck traffic, since it is the route to the PrudhoeBay oil fields. The previous pavement, constructed in 1970, was becoming alligatorcracked even in areas of stable foundation. Inadequate snow removal from the sideslopes was also attributed to the cracking problems. Leaving snow on the side
11
slopes does not allow the pavement sublayers to drain as the spring thaw proceeds.
Specifications for the construction of the treated base were somewhat vague andconsidered to be of an experimental nature [3]. The Contractor used a C.M.I. RotoMill as a mixer and a tank truck mounted spray bar to distribute the asphalt emulsion.The correct percent moisture was determined by trial and error to be between 5.5and 6.5% in order to obtain material stability while obtaining acceptable asphaltcoating. Twenty-five days elapsed between the construction of the treated base andthe pavement surfacing. In this time rain and heavy traffic began to cause ruts andpotholes. The highway was finally paved in late September, when air temperatureswere in the 40 to 50 degree (Fahrenheit) range. The minimum air temperature forpaving (by specification) was 40 degrees.
Severe longitudinal and transverse cracking was noted after the first day of paving.This was first attributed to excessive compactive effort and inferior asphalt. TheContractor was ordered to decrease the number of roller passes on the pavementwhich decreased the cracking and compaction specifications were still met. Over$25,000 in patching repair work had to be done to the pavement surface in order tobring it to an acceptable level. Later the need for patching was attributed, by theEngineer [3], to:
1. High thermal gradients between the asphalt concrete and base.2. Thin mat thickness- 1-1/2”.3. Possible slippage between the base and surface courses, not influenced by
the application of CSS-1 Tack Coat, since cracking occurred in both areaswhere the Tack Coat was applied and not.
4.. The extended time between treated base and pavement construction.
The Engineer recommended more stringent specifications regarding the length oftime allowed for treated base to be left prior to paving.
12
4.0 FIELD VERIFICATION AND EVALUATION
Core samples were taken in each of the sections discussed in this report. This workand extractions performed to determine residual asphalt contents of the existingbases was done by Dean Baldassari.
4.01 MODULUS BACKCALCULATION PROCEDURES
The Falling Weight Deflectometer (FWD) data was collected using a Dynatest 8000machine. That is a trailer mounted test machine which drops a weight onto a rubbercovered plate which is hydraulically lowered to the pavement surface. Data collected(usually to a computer disk) is peak load and deflections at the load center plus 6radial locations.
The data is backcalculated by first converting the layers to equivalent thicknesses ofthe subgrade moduli. Then using a combination of Boussinesq and linear elastictheories to determine layer moduli stresses and strains [15]. This is done using theDynatest program: ELMOD version 3.1. For simplicity the ELMOD program assumesall layers to have a Poisson’s ratio of 0.35.
As a rule for surface layers that are thinner than ½ the FWD plate radius (5.91”),moduli may not be accurately backcalculated but are determined in terms of therecorded pavement temperature at the time of the tests. This is accomplished bygiving a reference modulus at 77 degrees F and using a conversion function intrinsicto the ELMOD program. The reference modulus assumed here was 350 ksi.
Moduli for the asphalt concrete surface layers were all computed based on themeasured surface temperatures at the time of the FWD testing. The equation used is[5]:
( )( )
���
�
−−−=
° 32325.21
77 10 CTLOG
FMTM
R
R (2)
13
where: T = The measured pavement temperatureC = 77°F, reference temperature
The results are for the analysis of the second FWD drop at drop height 2, that is, for anominal 9000 load. The first drop is used as a seating load. A maximum depth to stifflayer was assumed to be 240” when running the ELMOD program.
Subgrade thicknesses presented in this section are computed using equation (3)shown in Section 4.3. The thicknesses indicate the Mean subgrade thickness to astiff layer. e.g.. permafrost or rock, to obtain the best match of the measured FWDdeflection basin using linear elastic behavior. A few of the points tested requireddepths to stiff layer of greater than 240 inches for best fits to the deflection basin. TheELMOD program, in this case, calculates non-linear subgrade modulus coefficients.Since only from none to 2 or 3 test points had these results on each test date, thisdata is not included here. For computation of the Mean depth to stiff layer the depthswere assumed to be 240 inches for these few points.
The Mean, standard deviation, minimum and maximum values for the backcalculatedmoduli are given in the following tables. It is common practice in mechanistic designto deduct 2 standard deviations from the mean moduli to use as a design section.However, notice in the values given below, that if this is done blindly, one may obtainvalues lower than can be found in the field and the results may be over-conservativeregarding this factor. Alternative methods based on estimated remaining life andstatistical analysis to break a project into representative sections are available usingthe ELMOD program.
4.10 RICHARDSON HIGHWAY NEAR TIEKEL PASS
The following discussion is regarding core samples and Dynatest Falling WeightDeflectometer (FWD) data that was obtained. The core samples were taken andanalyzed by Dean Baldassari in 1988. The FWD date was obtained in the spring andfall of 1990. Tests were taken at 0.1 to 0.2 miles on center in the wheel path closestto the shoulder. Backcalculation results using the Dynatest program ELMOD,Version 3.1, are also presented herein.
14
The short section with untreated base adjacent to the treated base sections inthis vicinity was also FWD tested at the times mentioned earlier. This section isanalyzed and data presented for comparison with the treated base sections.
The site was visited by Dave Esch and Scott Gartin in early October of 1990.The entire stretch of highway here was performing well, without any noticeablefailures spots. Fill heights beneath the pavement layers were noted to be 36”minimum thickness.
4.11 CORE SAMPLE TEST RESULTS
A core sample taken in this section (near MP 37) had extractions performed inthe laboratory. The results are that the base sample contained 5.6% by weightof aggregate of asphalt (AC-2.5) with unit weight of 150.1 pcf. The mix issimilar to asphalt concrete. An extraction was also performed on the 2” asphaltconcrete pavement layer and the asphalt content was determined to be 5.9%.
Two core samples were taken in the emulsified asphalt treated section (nearMP 27). The average asphalt contents were determined to be 2.2 and 3.2% byweight of aggregate. Recall that the mix design called for 3.5% asphalt. Theaverage density was found to be 151.7 pcf.
4.12 BACKCALCULATION PROCEDURES
For the purpose of backcalculation the section layers were given according tothe pavement and base thicknesses. A 24” borrow embankment was alsoassumed under each section, since this was the case in the emulsion treatedsection and can be assumed for the others to obtain comparative results.
The asphalt cement treated base proves to be mechanistically superior in thedata below. However, the emulsified asphalt treated base has significantlyhigher moduli (about 72%) than untreated base.
15
4.121 EMULSIFIED ASPHALT TREATED BASE SECTION
The results (in ksi) for the 5” emulsified asphalt treated base section are as follows:
Table 4.1Richardson Highway Emulsified Asphalt Treated Base Moduli
DATE NO. OFTESTS
MEAN BASEMODULUS
STANDARDDEVIATION
MINIMUMMODULUS
SURFACETemp. F°
4/19/90 37 70 20.2 26 64
4/24/90 50 72 19.2 34 59
5/05/90 37 86 17.7 56 52
5/17/90 37 87 20.9 38 48
5/24/90 37 83 24.4 34 55
9/24/90 32 115 32.1 49 44
Summarization of all the backcalculation data for the spring emulsified asphalt treatedbase section is given in the following table. Recall that the pavement thickness is 1.5inches.
Table 4.2Richardson Highway Emulsified Asphalt Treated Base Section Layer Properties(Spring)
STATISTICPAVEMENTMODULUS
(ksi)
BASEMODULUS
(ksi)
BORROWMODULUS
(ksi)
SUBGRADEMODULUS
(ksi)
SUBGRADETHICKNESS
(in)
MEAN 607 79 50 22 44
STANDARDDEVIATION 112.1 21.8 13.7 8.7 --
MINIMUM 325 26 16 7 --
MAXIMUM 738 147 92 88 --
16
Table 4.3Richardson Highway Emulsified Asphalt Treated Base Section Layer Properties
(Fall 9/24/90)
STATISTICPAVEMENTMODULUS
(ksi)
BASEMODULUS
(ksi)
BORROWMODULUS
(ksi)
SUBGRADEMODULUS
(ksi)
SUBGRADETHICKNESS
(in)
MEAN 851 115 72 24 62
STANDARDDEVIATION 13.8 32.1 20.2 7.9 --
MINIMUM 834 49 31 11 --
MAXIMUM 862 211 133 46 --
Table 4.122 Asphalt Treated Base Section
The results for the 4 inch asphalt treated base section (in ksi) are as follows:
Table 4.4Richardson Highway Asphalt Treated Base Moduli
DATE NO. OFTESTS
MEAN BASEMODULUS
STANDARDDEVIATION
MINIMUMMODULUS
SURFACETemp. F°
4/19/90 28 124 39.5 37 64
4/24/90 48 97 33.4 19 71
5/05/90 27 151 39.5 62 54
5/17/90 27 111 30.7 54 48
5/24/90 27 112 26.8 58 55
9/24/90 27 187 63.3 83 45
The surface deflections measured here were lower than in the previous section,resulting in the higher moduli. Recall that the pavement thickness here is 2 inchesand the Borrow thickness assumed was 24 inches.
17
Summarizing the backcalculation date for the spring hot mix asphalt treated basesection gives:
Table 4.5Richardson Highway Asphalt Treated Base Section Layer Properties (Spring)
STATISTICPAVEMENTMODULUS
(ksi)
BASEMODULUS
(ksi)
BORROWMODULUS
(ksi)
SUBGRADEMODULUS
(ksi)
SUBGRADETHICKNESS
(in)
MEAN 532 116 74 20 55
STANDARDDEVIATION 147.8 39.1 24.8 7.4 --
MINIMUM 325 19 12 6 --
MAXIMUM 738 244 155 64 --
Since asphalt moduli are highIy temperature dependent and surface temperatures atthe test times varied from in the 40’s to 70’s, the surface layer moduli can beexpected to vary greatly The FWD crew did not take many temperature readingswhile testing, so the temperature data here may not be assumed to be highlyaccurate for all tests.
Table 4.6Richardson Highway Asphalt Treated Base Section Layer Properties (FalI-9/24/90)
STATISTICPAVEMENTMODULUS
(ksi)
BASEMODULUS
(ksi)
BORROWMODULUS
(ksi)
SUBGRADEMODULUS
(ksi)
SUBGRADETHICKNESS
(in)
MEAN 834 187 118 21 60
STANDARDDEVIATION 0 63.6 40.2 14.7 --
MINIMUM 834 83 52 10 --
MAXIMUM 834 382 242 91 --
18
4.123 UNTREATED BASE SECTION
The results for the 6 inch untreated base section are as follows:
Table 4.7Richardson Highway Untreated Base Moduli
DATENO. OFTESTS
MEAN BASEMODULUS
(ksi)
STANDARDDEVIATION
(ksi)
MINIMUMMODULUS
(ksi)
SURFACETemp. F°
4/19/90 10 43 9.1 31 64
4/24/90 10 44 6.1 35 78
5/05/90 10 50 6.9 35 72
5/17/90 10 46 10.1 17 48
5/24/90 10 49 5.6 38 55
9/24/90 10 56 11.1 28 45
Summarization of all the spring backcalculation data for the 6” untreated base section isin the following table. The pavement thickness assumed was 2 ¾ inches (2” of ACpavement + ¾” BST) and the Borrow thickness is 24 inches.
Table 4.8Richardson Highway Untreated Base Section Layer Properties (Spring)
STATISTICPAVEMENTMODULUS
(ksi)
BASEMODULUS
(ksi)
BORROWMODULUS
(ksi)
SUBGRADEMODULUS
(ksi)
SUBGRADETHICKNESS
(in)
MEAN 511 46 29 15 39
STANDARDDEVIATION 142.5 8.2 5.0 3.0 --
MINIMUM 341 17 11 6 --
MAXIMUM 738 60 37 21 --
19
Table 4.9Richardson Highway Untreated Base Section Layer Properties (FaIl- 9/24/90)
STATISTICPAVEMENTMODULUS
(ksi)
BASEMODULUS
(ksi)
BORROWMODULUS
(ksi)
SUBGRADEMODULUS
(ksi)
SUBGRADETHICKNESS
(in)
MEAN 834 56 35 14 33
STANDARDDEVIATION 0 11.1 6.9 3.9 --
MINIMUM 834 28 17 8 --
MAXIMUM 834 65 40 21 --
4.20 ALASKA HIGHWAY NEAR NORTHWAY
Several core samples were taken in the vicinity of this project. Table 5.7 includesthe asphalt content test result of samples that were tested in the laboratory by DeanBaldassari.
The target asphalt content for the project was 4% of the base, by weight. Accordingto these results it was never reached. Dean told me that it had been raining at thetime of application of the emulsion, and that the treated base section of the projectis located on a downgrade, in the direction of increasing mileposts. He theorizedthat the asphalt emulsion may have run down the grade in the rain, causing thehigher asphalt contents near the bottom. Review of project profiles show the gradesto be rolling.
Two years (1988 & 89) of falling weight deflectometer tests were performed near thelocations that the core samples were taken. The following two tables show theresults of backcalculatlon of moduli using ELMOD. Here the data was in hard copyform, so had to be converted to metric format and FWD data files were created byusing and editing existing computer disk files. The seventh (outer) sensor readingsappear to be low in the 1989 data, evidently due to electrical or mechanicalproblems. This likely explains some of the wide variation in the data for that year.
20
Moduli for the 4” emulsified base course were determined after fixing the asphaltconcrete layer modulus, dependent on the recorded test temperature, and alsobackcalculating the 18” Select A and subgrade moduli. The following two tablessummarize the results.
4.21 1988 BACKCALCULATION RESULTS
Base course moduli ksi for given test dates:
Table 4.10 Alaska Highway Base Course Moduli – 1988
MILEPOST BASE TYPE 4/28/88 5/12/88 5/19/88 5/25/88 ASPH. %
1236.8 UNTREATED 22 24 17 21 0
1237.2 UNTREATED 16 24 21 21 0
MEAN UNTREATED 19 24 19 21 0
1239.2 TREATED 32 33 34 31 1.8
1239.4 TREATED 34 33 42 33 1.8
1244.8 TREATED 33 32 34 29 1.7
1247.8 TREATED 47 48 53 51 2.45*
1250.2 TREATED 37 35 38 39 3.4
1250.4 TREATED 42 38 40 36 3.2
MEAN TREATED 38 37 40 37 2.4
TEST TEMP.(DEG. F) N/A 55 58 50 55 N/A
* This is the average of two core samples taken at Milepost 1247.7 and 1247.9 whereno FWD tests were performed.
21
Notice the rather sharp increase of moduli between the untreated base course andthe treated base in these tables. The core sample results indicate minimal asphaltcontent treated sections, yet the results here show an average increase in modulusof approximately 82% between the mean of the untreated base tests and the meanof the treated base tests. Also note that the base moduli are generally increasingwith increasing asphalt content. The data can be used to observe general trends,but since the asphalt content is highly variable in this area (due to rainfall at the timeof application and rolling grades causing runoff on the sprayed surface), noconclusions should be made regarding optimal content.
Summarizing the all the spring 1988 backcalculation data gives:
Table 4.11 Alaska Highway Layer Properties – 1988
BASE TYPE STATISTICPAVEMENTMODULUS
(ksi)
BASEMODULUS
(ksi)
BORROWMODULUS
(ksi)
SUBGRADEMODULUS
(ksi)S.G THICK
(in.)
UNTREATED MEAN 617 21 13 10 20
UNTREATED STD. DEV. 50.4 2.7 1.7 1.7 --
UNTREATED MIN. 560 16 10 6 --
UNTREATED MAX 698 24 15 12 --
TREATED MEAN 617 38 24 18 113
TREATED STD. DEV. 50.4 6.4 4 6.2 --
TREATED MIN. 560 29 18 11 --
TREATED MAX. 698 53 33 33 --
22
4.22 1989 BACKCALCULATION RESULTS
Base course moduli in ksi for 1989 given test dates are:
Table 4.12 Alaska Highway Base Course Moduli – 1989
MILEPOST BASE TYPE 4/12/89 4/25/89 5/08/89 5/17/89 ASPH. %
1236.8 UNTREATED NG* 20 29 15 0
1237.2 UNTREATED NG* 15 22 19 0
MEAN UNTREATED -- 18 26 17 0
1239.2 TREATED 27 41 36 36 1.8
1239.4 TREATED 35 49 32 NG* 1.8
1244.8 TREATED 28 33 39 35 1.7
1247.8 TREATED 44 78 190** 195** 2.45
1250.2 TREATED 47 51 64 54 3.4
1250.4 TREATED 38 52 NG* 124** 3.2
MEAN TREATED 37 51 72 89 2.4
TEST TEMP.(DEG. F) N/A 45 58 42 54 N/A
* Could not backcalculate modulus here due to errors in FWD data.
** Modulus here may be high due to non-fatal errors in the FWD data.
In the above table, the moduli are much more variable due to the sensor problemswith the FWD machine, but similar trends to the 1988 data may be seen.
Summarizing all the spring 1989 backcalculation data gives:
23
Table 4.13 Alaska Highway Layer Properties – 1989
BASE TYPE STATISTICPAVEMENTMODULUS
(ksi)
BASEMODULUS
(ksi)
BORROWMODULUS
(ksi)
SUBGRADEMODULUS
(ksi)S.G. THICK
(in.)
UNTREATED MEAN 745 18 11 15 25
UNTREATED STD.DEV 147.6 6.5 4.1 17.8 --
UNTREATED MIN. 560 8 5 5 --
UNTREATED MAX. 919 29 18 55 --
TREATED MEAN 728 60 38 13 62
TREATED STD.DEV 144.8 46.5 29.3 9.0 --
TREATED MIN. 560 27 17 4 --
TREATED MAX. 919 195 123 38 --
4.30 ELLIOT HIGHWAY NEAR FOX
Five core samples were taken in this section. Extractions and sieve analysis wereperformed in the laboratory. The average asphalt content determined from the fivecores was 1.82% of the aggregate weight. The gradations meet the specifications forD-1 [1,2] but are 1 to 2% high on the #200 sieve.
Intensive FWD testing was performed in the Hilltop Cafe area on this route in April,May and June of 1989, Hilltop Cafe is located about 5 miles up the Elliot Highwayfrom the Fox scale house. The testing spanned .725 mile on 0.025 mile centers (30tests/date). A thermal probe was installed within the test area to measure the thawdepth as spring progressed.
The thermal probe gave temperature readings at 3” depth intervals to a depth of over6 feet. Therefore a reading was also taken at 3” in depths which is indicative of thetemperature of the emulsified asphalt treated base (EATB). From one to threetemperature readings were taken each test day. The average temperature readingtaken in the base is also given in the following table.
24
The typical section here is 1.5” of asphalt concrete pavement, 4” of asphalt emulsiontreated base, 6” of subbase over old embankment and subgrade. Forbackcalculation purposes the pavement layer was estimated based on the recordedsurface temperatures and three sublayer moduli were determined.
The following table summarizes the backcalculation results for the 4” emulsiontreated base course.
Table 4.14 Elliot Highway Base Moduli and Temperatures
Temperature dependence of treated base modulus cannot be established from thegiven data since the test times were not recorded. The times of temperaturereadings were recorded. The variance between morning and afternoontemperatures (when recorded) was sometimes greater than 20 degrees. However,the moduli here are seen to decrease slightly relative to increasing temperatures.
25
MEANMODULUS
(ksi)
STANDARDDEVIATION
(ksi)
MINIMUMMODULUS
(ksi)
RECORDEDSURFACE
TEMP(DEG. F)
MEANRECORDEDEATB TEMP
(DEG. F)4/19/89 100 17.8 73 60 43.84/26/89 105 19.0 77 56 46.64/28/89 102 17.7 75 55 46.15/03/89 93 17.3 61 60 48.85/05/89 92 17.7 61 56 44.15/10/89 87 18.6 51 52 44.15/12/89 80 17.9 50 52 45.65/17/89 78 15.8 45 76 53.65/19/89 87 16.2 53 61 53.05/25/89 83 15.6 54 75 63.66/07/89 88 14.2 65 76 70.9
The ELMOD program also computes an equivalent depth to stiff layer (EDSL) when amaximum depth is given on the input screen. The EDSL is computed in terms of
the computed moduli and given layer thicknesses.
The equation used to determine the EDSL is [5]:
ss
is
ii H
EEHEDSL +
����
�=
−
=
3/11
1(3)
where: n is the number of layers above the stiff layer Hi is the thickness of layer iEi is the modulus of layer i Es and Hs are the modulus and thickness of the subgrade material
above the stiff layer.
Equation (3) can be solved for the thickness of subgrade to stiff layer (Hs), and thenthe computed depth to stiff layer (CDSL) by summing the known layer thicknesseswith the subgrade H. The following table gives the average measured thaw depthresults and the average CDSL (inches) for each date. The table of “THAW” valuesare MEAN interpolated depths of the 32 degree (F) isotherm in inches. Dates are in1989.
Table 4.15 Elliot Highway Measured Thaw Depth vs. Computed Depth to StiffLayer
DATE 4/19 4/26 4/28 5/03 5/06 5/10 5/12 5/17 5/19 5/24 6/07THAW 27 34 37 46 47 50 51 53 55 60 >78CDSL 39 47 54 55 55 57 58 57 58 57 55
It is interesting to note the fair correlation between the computed depth to stiff layer(CDSL) and the measured depth of the 32 degree isotherms. Computed stiff layerdepths are generally slightly below the thaw depth until towards the end of the testing.
26
The thaw temperature of the sublayer soils has not been established. Soils usually thawat temperatures above 32 degrees or take time to complete the phase change fromfrozen to liquid due to the latent heat of the soil which is dependent on its density andmoisture content.
Summarizing the all the backcalculation data for the spring 1989 emulsified asphalttreated base section gives:
Table 4.16 Elliot Highway Layer Properties
STATISTICPAVEMENTMODULUS
(ksi)
BASEMODULUS
(ksi)
BORROWMODULUS
(ksi)SUBGRADEMODULUS
SUBGRADETHICKNESS
(in)
MEAN 536 90 52 13 42
STANDARDDEVIATION 112 19.1 11.1 3.3 --
MINIMUM 335 45 26 6 --
MAXIMUM 783 149 87 23 --
27
- 5.0 LABORATORY RESULTS
The following section presents the results of laboratory resilient modulus testing ofcore samples performed in 1989. The samples were placed in the DOT&PFResearch section’s computer analyzed apparatus. Here they are hit with 0.1 secondpulse loads at approximately 1 second intervals with load and deformation measuredon the diametrial samples. The loads and the deformation were converted to resilientmodulus and strain according to standard methods [16]. The resilient modulus isdefined as the ratio of the deviator stress to recoverable strain. The tests wereperformed at temperatures of approximately 50 degrees Fahrenheit.
5.10 RICHARDSON HIGHWAY NEAR TIEKEL PASS
Three cores obtained from the milepost 37 vicinity were tested. Recall that this is inthe hot mix asphalt treated base section. The only extraction performed on a corehere resulted in 5.6% asphalt content by weight of aggregate. These cores are otherthan the ones that had extraction tests. The cores were numbered 5, 6 and 7.Tensile strain and moduli were determined on two samples from each core. Thecoding used was “t” and “b.” Results are presented in the following table.
Table 5.1 Richardson Highway Laboratory Strain and Modulus Test Results
SAMPLE NUMBER TENSILE STRAINx 106
RESILIENT MODULUS(ksi)
MP-37-5t 132 149
MP-37-5b 121 285
MP-37-6t 143 156
MP-37-6b 117 302
MP-37-7t 121 107
MP-37-7b 100 291
MEAN 122 215
28
The ratio between Mean laboratory moduli and Mean field moduli for the asphalttreated base is 215/116 = 1.85. In other words the laboratory moduli are on theaverage 1.85 times greater than the field tested moduli for this section.
5.20 ALASKA HIGHWAY NEAR NORTHWAY
Tensile strain and moduli tests were also performed on core samples taken in thevicinity of FWD tests in this area. The Mean asphalt content from cores for thissection was 2.4% by weight. The results including milepost locations are presentedin the following table.
Table 5.2 Alaska Highway Laboratory Strain and Modulus Test Results
MILEPOSTPOSITION
SAMPLE NUMBER
TENSILESTRAIN
x 106
RESILIENTMODULUS
(ksi)1239.2 AK HWY 4 148 329
1239.2 AK HWY 4A 153 294
1239.4 AK HWY 5 115 432
1239.4 AK HWY 5A 113 541
1244.8 AK HWY 6 119 328
1244.8 AK HWY 6A 106 387
1244.8 AK HWY 6B 158 297
1247.7 AK HWY 8 101 417
1247.7 AK HWY 8A 121 380
1250.2 AK HWY 10 148 414
1250.2 AK HWY 10A 165 312
1250.4 AK HWY 11 101 388
1250.4 AK HWY 11A 145 310
1250.4 AK HWY 11B 133 241
ALL MEAN 130 362
29
The ratio here between laboratory and field tested moduli is 362/60 = 6.03. This isusing the 1989 field data (see Table 4.12).
5.30 ELLIOT HIGHWAY NEAR FOX
Laboratory resilient modulus tests were performed on 6 cores sampled on this highway.The test results are given in the following table:
Table 5.3 Elliot Highway Laboratory Strain and Modulus Test Results
SAMPLENAME TENSILE STRAIN
x 106
RESILIENT MODULUS
(ksi)
ASPHALTCONTENT
(%)EILOT 1 118 478 1.6 (SAMPLE 1B)
EILOT 1A 129 423 NO DATA
EILOT 2 144 568 NO DATA
EILOT 2A 132 436 1.2
EILOT 4 161 251 1.47 (SAMPLE 4B)
EILOT 6 132 257 .96 (SAMPLE 6B)
MEAN 136 402 1.31
The mean ratio between laboratory and field resilient moduli for the Elliot Highway is:402/90 = 4.47
30
6.0 PERFORMANCE ANALYSIS
The question arises as to how to analyze or design an asphalt treated base toestimate how long it will last. Fatigue related surface cracking of the pavement likelyinitiates at the bottom of the treated base layer. Therefore, the horizontal strain at thebottom of the treated base for the design load should be computed for design [8].Two typical equations are generally used to estimate the fatigue life of a pavement.The NCHRP 1-1OB (Eq. 4) and the Asphalt Institute Equation (Eq. 5) [12].
1fN = 0.0579 hε 854.0291.3 −−RM (4)
where: Nf = number of 18-kip, dual tire single axle loads to fatigue failurehε = maximum horizontal strain at the bottom of the asphalt treated layer,
in/inMR = resilient Modulus of the asphalt treated layer (psi)
fAN = 0.07958 C hε 854.0291.3 −−RM (5)
where: Nf, hε and MR is as aboveC = 10M
M = 4.84 [Va/(Vv + Va)- 0.69]Va = Volume of asphalt, %Vv = Volume of air voids, %
The Asphalt Institute equation is less conservative than the NCHRP 1-10B equation forasphalt contents greater than about 3% by weight. It could be used to estimate fatiguelives of pavements with bases of less asphalt content than 3%. However it becomesvery conservative, relative to Equation 4 when the asphalt contents are less than 2%, asthey were in some of the extracted core samples here. The equation most typically usedin Alaska for fatigue is a modified version of the Asphalt lnstitute Equation. The Alaskafatigue criteria takes Equation 5 and sets C equal to 1.0.
31
This version has been found to acceptably model fatigue in Alaskan pavements.Since volume of asphalt and air voids (%) data was not found in this study, Equation5 is not considered further.
The following table gives computed strains, due to a typical 18kip dual tire truck axleload, at the base of the asphalt concrete (AC) and treated base courses for themean section at each location. The number of loads to fatigue failure is computed foreach layer using the Alaska fatigue equation as described in the previous paragraph.The strains were computed using the ELSYM5 program, which is a public domainpavement analysis program utilizing linear elastic theory. The critical strains werenormally located directly under the tire centers in the longitudinal direction. “StiffLayer Moduli” were assumed to be 500 ksi. All Poisson’s ratios were assumed at0.35.
Notice that in most cases the predicted number of loads to failure is moreconservative in the AC layer. However, assuming a tight bond between the AC layerand treated base, cracking is not likely to initiate in the AC. The strains and numberof loads to fatigue failure in AC layers underlain with treated bases are shown InTable 6.1 only for Informational and comparative purposes.
Some of the AC layers were only 1.5 inches, which being so thin, often results inlower computed strain due to neutral-axis problems in the application of linear elastictheory. This will be discussed later. Also notice that the computed strains at the baseof the pavement are always higher in the untreated base sections - even whencompared to the strains at the bottom of the base course. It can be concluded thattreating the base course improves pavement fatigue life.
32
Table 6.1 Critical Strains and Predicted Pavement Life
PROJECTLOCATIONBase Type
AC STRAINAC (x 106)
Nf, AC(millions)
BASESTRAIN(x 106)
Nf, BASE(millions) NR (millions)
Richardson Hwy,EATB (Spring)
134 5.1 201 7.6 7.5
Richardson Hwy,EATB (Fall)
90 14.0 139 18.6 31.9
Richardson Hwy,ATB (Spring)
106 12.2 143 16.8 15.2
Richardson Hwy,ATB (Fall)
64 43.8 89 53.1 82.8
Richardson Hwy,Untreated (Spring)
234 0.98 -- -- 1.6
Richardson Hwy,Untreated (Fall)
173 1.7 -- -- 6.0
Alaska Hwy,Untreated, 1988
425 0.1 -- -- 0.2
Alaska Hwy, EATB1988
284 0.4 361 2.10 1.7
Alaska Hwy,Untreated, 1989
428 0.09 -- -- 0.2
Alaska Hwy, EATB,1989
192 1.3 245 5.0 4.8
Elliot Hwy,EATB
106 12.1 251 3.3 8.2
33
Using linear elastic theory, e.g., the ELSYM5 program, for design of pavements iscomplicated when pavement thicknesses of less than about two inches are considered.For certain combinations of layer thicknesses and moduli, the horizontal strainscomputed under the wheel load will decrease with decreasing pavement thickness. Thisis contrary to reality, but is a phenomenon of neutral axis, similar to as found in beams.The maximum computed horizontal strain may be below the bottom of the surface layerwhen thin pavements are analyzed. With the computed strains being lower, theestimated pavement life will appear to increase with decreasing pavement thickness.
This problem can be avoided by adopting other failure criteria such as limiting stress inthe unbound layers, that is at the top of the untreated base or subbase. The author hasused a modified version of the Danish stress criteria [15] to control from the point wherepavement strains are computed as decreasing using elastic theory programs. Verticalstresses computed in this situation are found to be reasonable. This is a roughnesscriterion, above which pavement roughness is estimated to become unacceptable. Theequation is:
(6)
Where: NR = Number of 18 kip, dual tire single axle loads to roughness failureMr = Modulus of the unbound layer (ksi)
vσ = computed stress 1 inch into the top unbound layer (psi)F(R) = function of Regional factor yielding a critical stress (psi)
= 18.6 –1.2RR = Regional factor, 1 to 5 with 1 being best to 5 being the worst case.
These may vary on a seasonal basis.b = 1.16 when rM < 23 ksi = 1.0 when rM ≥ 23 ksi
34
26.30772.
23)(856.52
−− �
��
�
���
��
����
=b
rvrR
MRF
MN σ
The equation is best utilized by computers. Regional factors may be varieddepending on the season and location being considered, based on engineeringjudgment. Figure 6.0 shows the relationship between the fatigue and roughnesscriteria for a typical pavement section. Computed values for NR are included in Table6.1, using R=4 in Spring and R=2 in Fall.
The roughness criteria here is shown only to control in the weaker emulsified asphalttreated base sections. For estimating the structural equivalent thickness of treatedbase as compared to untreated base the following equation may be used:
(7)
where: hTB = the equivalent thickness of treated base coursehUTB = the thickness of untreated baseEUTB = the modulus of the untreated base courseETB = the modulus of the treated base course
The equation is based on Odemark's method (14) and is used assuming thePoisson's ratios of both layers are the same. The 0.8 value is a correlation factor formulti-layered system equivalence to linear elastic behavior.
Using Equation 7 for an example with (EUTB/ETB) equal to 0.5 and comparing it with a6 inch untreated base gives:
0.8(6 in) (.5)1/3 = 3.8 inches
That is, 3.8 inches of treated base at the given modular ratio is equivalent inperformance to 6 inches of untreated base course. The AASHTO road test (1958-60)data was used by the Asphalt Institute who have concluded that asphalt treated baseis equivalent to about twice its thickness of untreated base. For this result, usingEquation 7 you would need a treated base modulus equivalent to about 4 times thatof untreated base.
35
31
8.0�
���
�=
TB
UTBUTBTB E
Ehh
7.0 CONCLUSIONS
After considering the results of the previous study, the following conclusions can bemade regarding the sections tested:
1. Stiffness (modulus) of treated bases are improved over untreated bases. Thetreated bases were constructed of marginal aggregates (high P 200) and themoduli are generally much higher than similar untreated bases.
2. The emulsified asphalt treated bases analyzed in this study generally hadasphalt contents below the design amounts. This including, rainy weatherduring construction, rolling grades, application techniques not well specifiedand emulsion type may not be appropriate for the aggregate.
3. Pavement life, related to traffic, is improved by installation of treated bases.Treated bases have tensile strength properties which help distribute loads,decreasing strains and stresses within and under them, and thereby lastinglonger than untreated base.
4. Pavement designs for asphalt treated bases should consider horizontal strainat the base of the pavement and vertical stress in the underlying layers. Strainin the AC layer may be analyzed if there is doubt in the bonding of the tackcoat between the AC and the treated base.
5. Laboratory determined moduli for the treated bases tested in this study areabout 2 to 6 times greater than field tested moduli. The emulsified asphalttreated bases gave the highest laboratory moduli relative to thebackcalculation results.
6. Equivalent thicknesses of asphalt treated bases relative to untreated bases toyield equivalent bearing capacities may be estimated using Equation 7 for costanalysis. Modulus estimates may be obtained using an FWD andbackcalculating typical sections or referring to the data presented here.
36
8.0 BIBLIOGRAPHY
1) Alaska DOT&PF, "Standard Specifications for Highway Construction," 1981.
2) Alaska DOT&PF, “Standard Specifications for Highway Construction,” 1988.
3) Bennett, J., “Elliot Highway, Fox to Mile 7, Project History,” Project F-065-1(5)/A20022/60205, 1986.
4) Brazo, G., “Elliot Highway, 0-7 Mile Paving, A-86O52~ Memorandum to RalphSwarthout, May 18, 1984.
5) Dynatest Engineering A/S. “ELMOD/ELCON, Evaluation of Layer Moduli and OverlayDesign,” User’s Manual, March, 1990.
6) Materials Section, Alaska DOT&PF, "Alaska Test Methods,” Test T-1 7, January1980.
7) Materials Section, Alaska DOT&PF, “Engineering Geology and Hydrology Report,Alaska Highway Mile 1253 to 1235,” August 1979.
8) Materials Section, Alaska DOT&PF, “Engineering Geology and Hydrology Report,Alaska Highway Mile 1253 to 1235 - Supplement 2,” June 1981.
9) Project IR-l-OA1-1(2), Alaska Highway, Mile 1253-1235 “Emulsified Asphalt TreatedBase History”, 1984.
10) Simpson. J., “Emulsified Asphalt Treated Base Study Area,” Project F-071-1(55)Report, 1986.
11) The Asphalt Institute, “A Basic Asphalt Emulsion Manual,” Volume 1, FHWA-IP-79-1,January 1980.
12) The Asphalt Institute, “Research and Development of the Asphalt Institute’sThickness Design Manual (MS-1) Ninth Edition,” RR-82-2-2, 1981.
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13) The Asphalt Institute, "The Asphalt Handbook," Manual Series No. 4 (MS-4), 1989.
14) The Asphalt Institute, “Thickness Design - Asphalt Pavements for Highways andStreets," Manual Series No.1 (MS-1). September 1981.
15) Ullidtz, P., "Pavement Analysis,” Pg. 191 Elsevier, Amsterdam, 1987.
16) Yoder, E.K. & Witczak, M.W., "Principles of Pavement Design," 2nd Edition, Pg. 268-269, John Wiley & Sons, Inc., New York, 1975.
17) R&M Consultants Inc., C. Vita et al., “Bethel Airport CTB-AC Pavement PerformanceAnalysis," Alaska DOT&PF Report No. AK-RD-86-31, 1986.
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