Phillip B. BlankenshipConsulting Engineer

Blankenship Asphalt Tech & Training

Cracking Tests for Asphalt Mixtures and How to Mixture Condition

Region 4 Topic

A Fun Time to Be in Asphalt

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• Industry is demanding a cracking test

• SHRP started on cracking tests in ~1990 but never fully developed

• Binder spec (PAV DSR) was meant for unmodified binders and was a catch all

o Not stout enough for the cracking component that we need today

• Cracking tests are popping up seemingly everywhere

• Users are now in the game and notice the cracking

What Should Have Happened…

• Superpave required Level 1, 2, and 3 testing based on traffic (ESAL) load

o Level 1 (Volumetrics + TSR) was intended only for 1 million ESALS or less

o Level 2 and 3 were to be used for higher traffic loads and included rutting and cracking performance test

• Since we saw such good initial Superpave pavement performance in 1993-2000, Levels 2 and 3 “complication” were soon forgotten

The Need for Performance Testing

It all started in 1919

• Asphalt Association (later Asphalt Institute) was formed and hired Prevost Hubbard and Frederick Field as researchers

• Research led to the Hubbard-Field design method using rammers (like a Marshall hammer but with 2 size hammers) in mid 1920’s

AI Magazine article by Gerry Huber 2/15/2013

Hubbard-Field Stability

• Hubbard-Field Stability is the first known asphalt performance test.

• Sample was loaded by turning the wheel

• Dial gage recorded the maximum load

Hubbard-Field Stability test at

AI headquarters 8-2013

Testing Then and Now

• By the 1940’s:

o Hubbard-Field stability test

o Hveem stability test

o Marshall stability and flow

o Recorded data by hand or charts

• Today

o TSR, Hamburg wheel tracker, APA, Texas overlay tester, 4-point flexural fatigue, fracture energy (3-4 tests), resilient modulus, shear modulus, dynamic modulus, AMPT dynamic modulus/flow number, and more

Technology Today

• We can control test from 0.01 Hertz to 25 Hertz (25 cycles a second)

• Technology allows us to record data at fast rates like 100+ points a second

• Temperature control to the nearest 0.5C (mix) and 0.1C (binder)

o Need of strict temperature control is something we learned during the SHRP research 1987-1992.

• The problem still remains…

Traffic & Load Growth on RuralInterstate System

0%

100%

200%

300%

400%

500%

600%

700%

1970 1975 1980 1985 1990 1995 2000 2005

Change Since 1970

Rural Average Daily Load

Rural Average Daily Traffic

FHWA Highway Statistics 2003

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Why do we need a mixture cracking test if the binder (PG 64-xx) is

certified to meet a low temperature?

The Basics of Performance Testing

Allow us to verify our estimates

Design and check for potential distresses

Custom design for specific loading

Think out-of-the-box with new materials and modifiers

Fundamental Performance Tests

• Flexural Beam Fatigue

o Brittleness

• Asphalt Mixture Performance Test

o Dynamic modulus (used in MEPDG for design)

o Flow number (rutting)

• Indirect Tension Test

o Low temperature cracking b creep compliance (resting load on cold samples) to see how the move

Photos from Asphalt Institute

Performance Tests

• Other tests

o Hamburg Wheel Tester

o Asphalt Pavement Analyzer

o Several energy based tests

o Overlay (crack) tester

Cracking Test Evaluation Project

We Need a Test(s) that Is…

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• Sensitive properties of mix components

• Sensitive to mixture aging

• Repeatable and reproducible

• Easy to implement

o Less trimming/coring

o Small footprint

o Easy to run

• Practical, low cost

Review of Tests

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1. AMPT Push/Pull Fatigue (S-VECD)

2. Indirect Tensile Strength (IDT)

3. 4-Point Bending Beam Fatigue

4. Dissipated Creep Strain Energy (DCSE)

5. Disk-Shaped Compact Tension [DC(T)]

6. Texas Overlay Test

7. Semi-Circular Bending (SCB)-ASTM

8. Semi-Circular Bending (SCB)-AASHTO (later version called iFit)

9. Indirect Tensile Asphalt Cracking Test (IDEAL-CT)at Intermediate Temperature

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AMPT Push/Pull Fatigue (S-VECD)

AMPT Push/Pull Fatigue (S-VECD)

• Draft AASHTO standard by Richard Kim

• 18C / 23C

o Not recommended to run over 21C

• Various Strains

• Software builds curve based on three tests

AMPT Push/Pull Fatigue (S-VECD)

• Good test for design

• Not intended for 24-hr aged mixtures

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Indirect Tensile Strength (IDT)

Indirect Tensile Strength (IDT)

• ASTM D 6931

• Related AASHTO T322

• 25.0C and 4.0C

• Rate of Movement: 12.5 and 50 mm/min

Indirect Tensile Strength (IDT)

Simplest test, but just says that mix

gets stiffer

Indirect Tensile Strength (IDT)

Indirect Tensile Strength (IDT)

So what can we learn? Confirms that we need correct

temperature/loading rate for cracking sensitivity. Peak load alone is not

the answer…but combine with time/distance FRACTURE ENERGY

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4-Point Bending Beam Fatigue

4-Point Bending Beam Fatigue

• 4-point bending beam fatigue (1950’s / SHRP)

• AASHTO T321 & ASTM 7460

• Examined• 20.0C & 15.0C• Sine & haversine

waves

• Rate of Movement: 10Hz, various strains (strain rates)

• Ex: 300 ms = 0.16mm/0.1sec or 98mm/min

• 2 beams for average (per strain)

Beam Fatigue – 20C & sine

Beam Fatigue - 15C & sine

EXAMPLE: KY Density Study Findings with 24-hr Loose Mix Conditioning – M. Anderson

Beam fatigue device has been used to better understand

pavement cracking potential.

Alireza Zeinali, Phillip B. Blankenship, Kamyar C. Mahboub

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Dissipated Creep Strain Energy (DCSE)

Dissipated Creep Strain Energy (DCSE)

• Draft standard by Rey Roque

• Uses IDT configuration

• Creep based on load & time

• 10C

• 3 samples for average

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Disk-Shaped Compact Tension

[DC(T)]

Disk-Shaped Compact Tension [DC(T)]

• ASTM D 7313

• Run at +10C from critical low temp PG

• -12.0C

• Rate of Movement: 1 mm/min

• 3 samples for average

EXAMPLE: Pavement Preservation with Chip Seal

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Texas Overlay Test

Texas Overlay Test

• Tx DOT Standard

• Tex-248-F

• 25C

• Rate of Movement: 0.6 mm/5 sec and returns (fatigue) or 7.2mm/min

• 0.1 Hz

• 6 samples for

Texas Overlay Test

Note: High error. Data is usually trimmed average.

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Semi-Circular Bending (SCB)-

ASTM

Semi-Circular Bending (SCB)-ASTM

• ASTM standard by Louay Mohammad

• 25C

• Rate of Movement: 0.5 mm/min

Semi Circular Bend (SCB) Test Fracture mechanics

Temperature: 25 C

Half-circular Specimen

– Laboratory prepared

– Field core

– 150mm diameter X 57mm thickness

– simply-supported and loaded at mid-point

Notch controls path of crack propagation

– 25.4-, 31.8-, and 38.0-mm

Loading type

– Monotonic

– 0.5 mm/min

– To failure

Record Load and Vertical Deformation

Compute Critical Strain Energy: Jc

0.0

0.2

0.4

0.6

0.8

1.0

1.2

1.4

1.6

0.0 0.5 1.0 1.5 2.0 2.5

Lo

ad

(kN

)

Deflection (mm)

Peak Load

notch a1

U1

Semi-Circular Bend Test Results, 25°C

• Can have high error. Usually based on 6 samples

• Higher temps or lower PG yields lower energy

o This is opposite of what should happen

0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

4-hr 24-hr

Jc, K

j/m2

Mixture Type

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Semi-Circular Bending (SCB)-AASHTO (later

version called iFit)

What About iFit?Semi-Circular Bending (SCB)-AASHTO

• AASHTO TP-124 by Imad Al-Qadi

• 25C

• Rate of Movement: 50 mm/min

• Focus on latest standard on Flexibility Index (FI)

What about iFit?

From Research Report No. FHWA-ICT-15-017, “Testing Protocols to Ensure Performance of High Asphalt

Binder Replacement Mixes Using RAP and RAS” by Al-Qadi, et.al.

What about iFit?

• Showing much promise

• Current work on field mixes

• More work to come on longer aged mixes

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Following slides from Texas A&M

Indirect Tensile Asphalt Cracking Test

(IDEAL-CT)at Intermediate

Temperature

Development of IDEAL-CT

IDEAL-CT concept

Development of IDEAL-CT

CTindex Equation

IDEAL-CT sensitivity

Sensitivity to RAP/RAS

IDEAL-CT sensitivity

Sensitivity to binder type

IDEAL-CT sensitivity

Sensitivity to binder content

Refer to NCHRP 9-57 for Further Info

How to Condition Mixtures

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• Short-term (absorption)

o 2 to 4 hours on loose mix at 135°C

• Stir hourly

• Longer time needed for > 2% water absorption

How to Condition Mixtures

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• Long-term (aging to simulate time in field)

o Currently three (3) options

• AASHTO R-30: 120 ± 0.5 hours at 85 ± 3°C on compacted sample

• UIUC/AI: 24 hours at 135°C on loose mix

• Proposed that aging at 135°C causes changes in the chemistry of the binder that are not realistic

• National Cooperative Highway Research Program (NCHRP) 09-54: Suggests 95°C as optimal temperature for aging loose mix. Time is adjusted based on climate conditions and pavement depth.

• One study cited 5 - 12 days corresponding to preliminary recommendations of NCHRP 09-54 study

Why 24 Hour Loose Mix Aging• Focus on aging of the top ~1-2 inches

• University of Illinois – study on in-place mixtures

o Andrew F. Braham, William G. Buttlar, Timothy R. Clyne

• AAPTP non-load associated cracking study

o Also found that 18hr loose mix 20hr PAV

• KY density study

Conclusions

• We need to condition mixtures to simulate proper field conditions at 7 to 10 years

o 95C aging or 24-hr loose mix aging @ 135

• All tests seem to recognize the conditioned mixtures except for the IDT strength

o Strength alone is not enough

• Need to accept tests for what they are and designed to do

• Adjust tests for climates

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Thank You

Phillip B. Blankenship, PEConsultant

Blankenship Asphalt Tech and Training, PLLC