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Update on Balanced Mix Design Randy West, Ph.D., P.E. Director, National Center for Asphalt Technology
Update on Balanced Mix Design
Randy West, Ph.D., P.E.Director, National Center for Asphalt Technology
Balanced Mix Design – a definition
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An asphalt mix design that uses practical performance tests on appropriately conditioned specimens to ensure resistance to common distresses and considers mix aging, traffic, climate and location within the pavement structure.
Why change?
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Most asphalt technologists are not satisfied with the current long term performance of our pavements. There is a desire to significantly improve the life of asphalt pavements.
Why change?
4
• Volumetric properties do not tell us anything about the quality of the binder, or about the interactions of different binder components and additives.
• Vbe = the volume of effective asphalt = VMA – Va
• Vbe is dependent on Gsb which is not a reliable property• Gsb of source materials are subject to change over time, but not often verified.
• Gsb has a low level of precision
• Gsb of RAP aggregate is questionable
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With the current volumetric mix design system…
WMA additives
Recycled Shingles
Fractionated RAP
Recycled Tire RubberRecycled Plastic
Recycling agents
BMD Optimum Asphalt Content
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Cra
ckin
g
Ru
ttin
g
Binder Content
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Cra
ckin
g
Ru
ttin
g
Binder Content
Min Max
BMD Optimum Asphalt Content
BMD Performance Diagram
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Numerous options to adjust mixes
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Gradation Asphalt Content
Modifiers
RAP Content
RAS Content
Rejuvenator
BMD Implementation Status
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California
⚫ Bending Beam Fatigue
⚫ Superpave Shear Test
Illinois
⚫ Illinois Flexibility Index
⚫ Hamburg Wheel Tracker New Jersey
⚫ Overlay Test
⚫ Hamburg Wheel Tracker
Texas
⚫ Overlay Tester
⚫ Hamburg Wheel Tracker
Louisiana
⚫ Semi-Circular Bend Test
⚫ Hamburg Wheel Tracker
BMD Pilot Projects
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2019 BMD Workshops
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Sacramento, CA
⚫ October 1, 2019
Pewaukee, WI
⚫ December 5-6, 2019 King of Prussia, PA
⚫ July 16-17, 2019
Taylorsville, UT
⚫ Sept. 26, 2019
Atlanta, GA
⚫ June 26, 2019
Springfield, MO
⚫ August 21, 2019
Culpepper, VA
⚫ Oct. 29, 2019
Completed BMD Research Studies
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Completed & OngoingBMD Research Studies
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The BIG questions
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1. What performance tests will be used in BMD for each state?
2. How will the performance tests be used? Where will they fit in the mix design process (i.e. the Framework)?
3. What criteria should be used in specifications?
4. What aging/conditioning protocols should be used for mixtures?
5. How will the performance tests be used in Quality Assurance?
6. What should you do to get ready for BMD?
Cracking Group Experiments
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NCAT Test Track
Top-down cracking
MnROAD
Low-temperature cracking
NCAT Test Track
America’s asphalt pavement proving ground
NCAT Cracking Group Sponsors
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Selected Top Down Cracking Tests
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SCB-LAEnergy Ratio OT-NCATIFIT OT-TX
All tests have been conducted on:1. lab prepared mix after short-term aging2. lab prepared mix after short-term and critical aging3. plant mix samples that were reheated 4. plan mix samples that were reheated and critically aged
IDEAL-CT
critical aging for Auburn, AL = loose mix oven aging at 135C for 8 hours
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Section N8May 21, 2019
NCAT CG Field Performance
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Section Description
Cracking(% of lane area)
Crit. AgedCT Index*
Start of this Cycle 12/6/19
N1 20% RAP (Control) 10.3 10.6 8.1
N2 Control w/ High Density 6.9 7.5 10.8
N5 Low AC, Low Density 3.5 9.3 7.6
N8 20% RAP 5% RAS 16.6 70 2.4
S5 35% RAP PG 58-28 0 0 16.3
S6 Control w HiMA 0 0 18.7
S13 AZ Rubber Mix 0 0 68.4
* Specimens compacted to 7.0±0.5% air voids
Selected Top Down Cracking Tests
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SCB-LA IFIT
Unable to identify worst and best performing mixes
IDEAL-CTEnergy Ratio OT-NCATOT-TX
Selected Top Down Cracking Tests
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SCB-LAEnergy Ratio OT-NCATIFIT OT-TX
Not practical enough for routine mix design
IDEAL-CT
Selected Top Down Cracking Tests
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IFIT IDEAL-CT
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17
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MnROAD Cracking Group Test Sections
Test sections constructed August 2016
MnROAD Cracking Group Sponsors
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MnROAD Cracking Group
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Low temperatures January 30, 2019
MnROAD Cracking Group Field Performance through April 2019
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Cell Key Mix Factors
Transverse Cracking
(ft.)
Load Related Cracking
(% of lane area)16 Moderate RAP + RAS 58 1.5
17 Low RAP + RAS 70 6.3
18 Moderate RAP 35 3.8
19 Moderate RAP, extra AC 61 0.4
20 High RAP, softer binder 0 0.2
21 Moderate RAP, softer binder 28 1.1
22 Limestone agg. and 9.5 mm NMAS 50 4.4
23 Moderate RAP, highly mod. binder 43 14.9
MnROAD Cracking Group Tests
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IFIT Low Temp. SCBOT-NCAT IDT Creep Compliance & Strength
Cantabro IDEAL-CT DCT
Intermediate Temperature Tests Low Temperature Tests
other tests are being performed by other research organizations
Ongoing Decisions & Work Ahead
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• Selection of Tests
• Ruggedness and Precision studies
• Benchmarking current mixes
• Setting criteria
• Training
• Pilot Projects
Questions
UnderstandingBond Strength
Randy West, Ph.D., P.E.Director, National Center for Asphalt Technology
1. What is the function of tack coat?2. What can happen when tack coat is not properly applied?3. How can you tell if your tack coat is sufficient?
The purpose of a tack coat is…
• to bond layers of a pavement together.
Slippage Failures
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Forensic Investigations
Orlando Airport
US 45 Michigan
City of Auburn
I-65 Alabama
I-10 Florida
I-94 Minnesota
I-70 Colorado
Forensic Investigations - Cores
Orlando Airport
US 45 Michigan
City of Auburn
I-65 Alabama
I-10 Florida
I-94 Minnesota
I-70 Colorado
NCAT Test Track Section S5, 2007
Evolution of Cracking in Section S5
1. Weak bond between binder and base layers
2. Debonding between binder and base layers
3. “Middle-up” crack initiated at bottom of binder
4. “Middle-up” crack reaches surface of pavement
5. Full depth crack extends to bottom of base
1 2 3 4 5
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COMPRESSION
TENSION
Bonded
COMPRESSION
TENSION
Unbonded
• 5 plywood strips (layers)
• 48” x 4” x 11/32”
• 60 and 160 pound loadings
• Bonded (glued) and Unbonded
Bonding Demonstration(courtesy of FHWA/AI Tack Workshop)
½” Deflection,
60lb Load
¼” Deflection,
160lb Load
Fully BondedUnbonded
Bonding Demonstration(courtesy of FHWA/AI Tack Workshop)
NCAT First Tack Coat Study
• What is the best type of tack coat material?
• What is the best application rate?
US-17
Literature on Bond Strength Testing
Juris-diction
Switzer-land
Germany AustriaUnited
KingdomQuébec Florida
Standard SN 671 961 DIN 1999ÖNORM 3639-1
SG3/05/234
unknown FM-599
Type Shear Shear Tensile TorqueTensile (in-situ)
Shear
Criteria 12 to 15 kN unknown1.0 to 1.5 N/mm²
N.A. N.A. N.A.
Photo
Bond Strength Experiment Design
Tack Coat Type:
CRS-2, CSS-1, PG 64-22
Application Rate:
0.02, 0.05, & 0.08 gal/yd2
Gradation:
19.0 mm Coarse & 4.75 mm Fine
Test Temperature:
50, 77, & 140°F
Normal Load:
0, 10, and 20 psi
Sample Preparation and Testing
• SGC specimens cut in half for specimen base
• Tack coat was applied to uncut surface of base
• Tacked specimen base was put back in SGC mold, loose mix compacted on top
• Bond strength test determined after four hours at the test temperature
• A servo-hydraulic testing machine loaded at 2”/min, maintained the temperature, and captured load and strain data.
Bond Strengths @ 140°F
0
10
20
30
40
50
60
70
80
90
Low Medium High Low Medium High Low Medium High
0 psi 10 psi 20 psi
Application Rates and Normal Pressure
Bo
nd
sh
ear
str
en
gth
(p
si)
Fine-graded,
CRS-2
Fine-graded,
CSS-1
Fine-graded,
PG 64-22
CRS-2
CSS-1
PG64-22
0
10
20
30
40
50
60
70
Low Medium High Low Medium High Low Medium High
0 psi 10 psi 20 psi
Application Rates and Normal Pressure
Bo
nd
sh
ear
str
en
gth
(p
si)
Coarse-graded,
CRS-2
Coarse-graded,
CSS-1
Coarse-graded,
PG 64-22
Coarse-Graded Mixes Fine-Graded Mixes
CRS-2
CSS-1
PG64-22
0
50
100
150
200
250
300
350
400
450
500
Low Medium High Low Medium High Low Medium High
0 psi 10 psi 20 psi
Application Rates and Normal Pressure
Bo
nd
sh
ear
str
en
gth
(p
si)
Fine-graded,
CRS-2
Fine-graded,
CSS-1
Fine-graded,
PG 64-22
0
50
100
150
200
250
300
350
400
450
Low Medium High Low Medium High Low Medium High
0 psi 10 psi 20 psi
Application Rates and Normal Pressure
Bo
nd
sh
ea
r s
tre
ng
th (
ps
i)
Coarse-graded,
CRS-2
Coarse-graded,
CSS-1
Coarse-graded,
PG 64-22
Bond Strengths @ 77°F
CRS-2
CSS-1
PG64-22
Coarse-Graded Mixes Fine-Graded Mixes
CRS-2
CSS-1
PG64-22
Effects of Temperature & Normal Pressure
0
100
200
300
400
500
600
700
0 psi 10 psi 20 psi 0 psi 10 psi 20 psi 0 psi 10 psi 20 psi
50°F 77°F 140°F
Test Condition
Bo
nd
Sh
ear
Str
en
gth
(p
si)
NCAT (ALDOT) Bond Strength Test
77°F, no normal pressure Sensitive to key variables including tack coat
type, application rates, surface textures
Ability to evaluate bond strength of pavement cores
Low cost, quick simple
For most conditions, the PG 64-22 provided better bond strengths than the CSS-1 and CRS-2
Lower application rates yielded higher bond strengths for all tack coat types
Field Validation Work
• Test sections set up on seven field projects
• Each section had a different tack coat application rate
• Cores taken after HMA compacted and cooled.
Measurement of Application Rate, ASTM D 2995
~100 m Geotextile pads for checking tack rate
Low Rate Medium Rate High Rate
Test Section Layout
Application rates measured using ASTM D 2995
~100 m Geotextile pads for checking tack rate
Low Rate Medium Rate High Rate
Test Section Layout
Application rates measured using ASTM D 2995
Low Rate Medium Rate High Rate
Test Section Layout
HMA overlay
3 random cores + 2 at pads per section
Low Rate Medium Rate High Rate
Test Section Layout
Recommendations
• The bond strength test is capable of measuring effects of tack coat type, application rate and surface texture.
• 100 psi proposed as a preliminary criterion
• Tack coat application rates should be specified in terms of residual asphalt.
• Milled/micro-milled surfaces have better bond than unmilledsurfaces
• Bond develops quicker for PG 67-22 and NTSS-1HM
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The GoodThe Bad
The Ugly
This is what happens to tack picked up on truck tires.
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CA
AZ
CO
NM
TX
OKAR
LA
MO KY
AL GA
FL
VA
OH
MI
VTAK
MT
NV
ME
WA
OR
UT
KS
IDWY
ND
SD
MN
NE
WI
IA
IL IN
MS
TN
SC
NC
WV
PA
NY
CT
NJDEMD
DC
MANH
RI
CA
HI
Key:
PR
Do not perform bond strength testing
Do perform bond strength testing
States that Perform Interface Bond Strength Testing
2018 NCHRP Synthesis 916
WVDOH Bond Strength Summaries
Year Lots No Bond <100 psiAvg. BS
(psi)
Std. Dev. of BS (psi)
2013 146 40 (27%) 81 (55%) 113.1 37.6
2014 216 75 (35%) 97 (45%) 152.6 48.0
2015 439 86 (20%) 201 (46%) 117.0 37.7
2016 346 59 (17%) 116 (34%) 150.0 63.3
2017 667 130 (19%) 238 (36%) 103.6 70.6
2018 401 122 (30%) 184 (46%) 115.2 98.8
Factors that Affect Bond Strength
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• Milled vs Unmilled: milled typically much higher
• Cleanliness of Surface: cleanliness is next to godliness
• Tack Coat Type: emulsions, trackless, PG binder
• Tack Coat Quality: how & where is it checked?
• Tack Coat Appl. Rate: check residual rate, how much stays?
• Overlay gradation: coarse vs. fine?, literature is inconsistent
• Time and Traffic: bond strength typically improves
Key Takeaways
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• A good bond between asphalt layers is critical to long-term performance of flexible pavements.
• Measuring bond strength is the only way to ensure a good bond exists.
• WV bond strength data clearly show problems exist.
• Numerous factors can impact bond strength.
Further Analysis Recommended
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• How are those pavements with failing bond strengths performing?
• Are bond strengths of wheelpath cores ≤ non wheelpath cores?
• Evaluate tack coat materials and application practices.
• Examine practices for coring.
NCAT Reports
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Available Reference Materials
Gierhart, D., and Johnson, D. NCHRP Synthesis 516:
Tack Coat Specifications, Materials and Construction Practices. National Cooperative Highway Research Program,
2018.
Available Reference Materials
Federal Highway Administration. Tack Coat Best Practices – TechBrief.
Office of Asset Management, Pavements, and Construction, 2016.
Available Reference Materials
Decker, Dale S. Best Practices for Emulsion Tack Coats. Quality Improvement Publication 128,
National Asphalt Pavement Association, 2013
Available Reference Materials
Mohammad, L. N., Elseifi, M. A., Bae, A., Patel, N., Button, J., and Scherocman, J. A.
NCHRP Report 712 Optimization of Tack Coat for HMA Placement.
National Cooperative Highway Research Program, 2012.
Recommended Application Rates
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Storing Asphalt Emulsions
• Do not heat the emulsion above 185°F to avoid damaging the product.
• Do not let the emulsion freeze to avoid separation of the asphalt and water.
• Do not use forced air to agitate the emulsion, as it will break the emulsion.
• Do not allow excessive agitation by mixing or pumping.
Handling Asphalt Emulsions
• Agitate gently when heating emulsion to minimize skin formation in the tank
• Clean out lines and leave drain plugs open when not in service.
• Ensure that tanks and equipment contain accurate thermometers. Gradually warm up the pump to about 150°F to facilitate start up.
• Dilution of the asphalt emulsion should only occur at the emulsion manufacturing facility. Dilution should not be done by the contractor in the distributor tank
• Polymer-modified and non-tracking emulsions should not be diluted for use as tack coat
Effect of water (rain or damp surface)?
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“For the effect of water on the tacked surface, the majority of the cases showed no statistically significant difference between dry and wet conditions. This data indicates that a small amount of water can be flashed away by the hot HMA mat and, thus, have inconsequential effects on the quality of the tack coat. This study used only hot mix as the overlay material; the use of warm mix may change this finding. In addition, these results are based on using a small quantity of water to simulate rainy conditions. Therefore, a dry and clean surface is recommended to avoid the negative effects of water on the bonding at the interface.”
NCHRP Project 712
Effect of water (rain or damp surface)?
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NAPA QIP-128
“A small amount of moisture on the pavement surface should not be detrimental to long-term tack coat performance, although a damp pavement will slow the cure and break time of the tack coat emulsion. If the pavement surface layer is saturated with water and the existing pavement surface is damp or has standing water, the ability of the tack coat emulsion to provide adequate bond between the existing and the new pavement layers will be significantly compromised”.
