Asphalt

BACKGROUND

OBJECTIVE

MATERIALS

METHODOLOGY

CONCLUSIONS

ACKNOWLEDGEMENTS

Study of the Thermal Stress Development of Asphalt Mixtures using the Asphalt Concrete Cracking Device (ACCD)Moses Akentuna1, Sang Soo Kim1, Munir Nazzal1, Ala R. Abbas2, Mir Shahnewaz Arefin2

1Departement of Civil Engineering, Ohio University, Athens, Ohio2Department of Civil Engineering, The University of Akron, Akron, Ohio

Asphalt pavements exposed to colder temperatures may crack when higherthermal stresses are induced in them. The thermal stress retrained specimentest (TSRST), the IDT creep and strength test, the Semi-Circular Bending (SCB)test and the Disk-Shaped Compact Tension Test (DC(t)) are some examples oftest methods developed over the years for the study of low temperaturecracking potential. The Asphalt Concrete Cracking Device (ACCD) is a testmethod developed as a simpler alternative to the thermal stress retrainedspecimen test (TSRST) for cracking temperature determination. ACCD testresults have been found to correlate with TSRST results for 5 SHRP asphaltmixtures at a correlation coefficient of 0.86. Most studies conducted on thethermal cracking potential of asphalt mixtures places much emphasis on thefailure stress and strain and cracking temperature. The magnitude of thermalstress development is as important as the cracking temperature and failurestrain in road pavements because it affects the durability of asphalt pavementssubjected to thermal loading. This may become critical when consideringstresses due to thermal loads together with stresses due to traffic loads. It hasbeen found that traffic loads applied during critical cooling events resulted in asubstantial increase in tensile stresses in pavements. The accumulation ofthermal stresses within asphalt mixtures is also important in analyzing thermalfatigue cracking of asphalt mixtures. For example, the magnitude of thermalstress increase in an asphalt mixture can influence the number of cyclesrequired to cause thermal fatigue cracking. In thermal fatigue cracking, anincrease in the magnitude of thermal stress development will reduce thenumber of cycles required to cause the asphalt mixture to crack.

Test Asphalt Mixture Samples in the ACCD device to determine the effect of the following asphalt mixture properties on thermal stress accumulation

• Asphalt binder PG grade• Addition of Recycled Materials (RAP and RAS)• Air Void Content• Binder Content• Aggregate and Asphalt Mixture CTE

Aggregates: Crushed Limestone (High CTE and Low CTE ); Round Gravel Five Asphalt Binder Grades (PGs 64-22, 70-22, 76-22, 88-22 and 64-28) Recycled Materials : Recycled Asphalt Shingles (RAS) and Recycled Asphalt

pavement (RAP)

1. Determine CTE of Aggregates 2. Determine Creep Stiffness and m-value of binder using the BBR

3. Prepare 150mm diameter Superpavegyratory mixture samples

4. Cut Mixtures into two halves for CTE testing

5. Measure CTE of Asphalt Mixtures the Ohio CTE Device

Relationship between strain and temperature expressed as:

𝜀 = 𝐶 + 𝛼𝑔 𝑇 − 𝑇𝑔 + 𝑅 𝛼𝑙 − 𝛼𝑔 𝑙𝑛 1 + exp [ 𝑇 − 𝑇𝑔 /𝑅

𝛼 =𝑑𝜀

𝑑𝑇= 𝛼𝑔 +

𝛼𝑙 − 𝛼𝑔 𝑒𝑥𝑝𝑇 − 𝑇𝑔𝑅

1 − 𝑒𝑥𝑝𝑇 − 𝑇𝑔𝑅

ε = thermal strain; C = a constant αl = slope of liquid state asymptote; αg = the slope of glassy state asymptote; T = average temperature of test specimen;Tg = glass transition temperature and R = width of the glass transition region

6. Core and Notch mixture Samples7. Test Cored Asphalt Mixture Samples in the ACCD Apparatus 150 mm

60.5 mm

22.6 mm

22.5 mm

Notch

Compacted Asphalt

Mixture

ACCD Ring

Strain Gauge Locator

Strain

Gauge

a

b

P

ACCD Ring

b

a

Cored Asphalt Mixture

a =11.3mmb = 30.25mm a = 30.25mm

b = 75mm

Thermal Stress generated in mixture determined from the expression:

𝜎𝜃𝜃 =𝑃𝑏2

𝑏2 −𝑎2+

𝑎2𝑏2𝑃

𝑟2 𝑏2 −𝑎2

𝑃 =𝐸𝜀𝜃𝜃

𝑏2

𝑏2 −𝑎2+

𝑎2𝑏2

𝑟2 𝑏2 −𝑎2

E is modulus of elasticity of ACCD Ring𝜀𝜃𝜃 is hoop strain measured from ACCD test

RESULTS AND DISCUSSION

Effect of Binder Grade

0

5

10

15

-30 -20 -10 0

Ther

mal

Str

ess

(MPa

)

Temperature (°C)

PG 64-22PG 64-28PG 70-22PG 76-22PG 88-22

Effect of Recycled Materials (RAP and RAS

0

2

4

6

8

10

12

-30 -20 -10 0

Ther

mal

Str

ess

(MPa

)

Temperature (°C)

PG 64-22

PG 64-22+RAS

PG 64-22+RAP

0

5

10

15

20

-30 -20 -10 0

Ther

mal

Str

ess

(MPa

)

Temperature (°C)

PG 70-22

PG 70-22+RAP

PG 70-22+RAS

Effect of Compaction Effort (Air Void Content)

02468

1012141618

-30 -20 -10 0

Ther

mal

Str

ess

(MPa

)

Temperature (°C)

PT 1 (High)

PT1 (Low)

PT2 (High)

PT2 (Low)

Effect of Binder Content

02468

1012141618

-30 -20 -10 0

AC

CD

Th

erm

al S

tres

s (M

Pa)

Temperature (°C)

PT1(5.3)

PT1(5.8)

PT1(6.3)

0

2

4

6

8

10

12

14

16

-30 -20 -10 0

Ther

mal

Str

ess

(MPa

)

Temperature (oC)

PT2(5.3)

PT2(5.8)

PT2(6.3)

Effect of Aggregate/Mixture CTE

10

12

14

16

18

20

22

24

-30 -20 -10 0

CTE

, µ

ε/°C

Temperature, (°C)

PG 64-22 (Low CTE Agg.)PG 76-22 (Low CTE Agg.)PG 64-22(High CTE Agg.)PG 76-22 (High CTE Agg.)

0

2

4

6

8

10

12

14

-30 -20 -10 0AC

CD

Th

erm

al S

tres

s (M

Pa)

Temperature (⁰C)

PG 64-22 (Low CTE)

PG 64-22 (High CTE)

PG 76-22 (Low CTE)

PG 76-22 (High CTE)

• PG 88-22 mixture recorded the lowest thermal stress development

• PG 88-22 mixture had the lowest creep stiffness and highest m-value at -18⁰C

• Stiffness and the method of polymer modification of asphalt has significant effect on thermal stress development.

• RAP and RAS contain stiff binder and hence resulted in higher thermal stress development

• RAP resulted in higher thermal stress development due compositional difference in RAS

• Low air void mixtures recorded higher thermal stress development

• Mixture stiffness have bee found to decrease with increasing air void resulting in decreased thermal cracking potential (Bazin and Saunier, 1967; Fromm and Phang; 1972)

• Increased binder content resulted in increased thermal stress development• This may be due to the increased mixture stiffness due to the increased binder

content.

• Similar thermal stress development for both low and high CTE mixtures at relative warmer temperatures

• High CTE mixtures recorded higher thermal stress development at relatively colder temperatures

• Binder grade and the type of modification showed significant effects on the thermal stress development during cooling.

• Mixtures with low binder stiffness resulted in smaller thermal stress development except mixtures with PPA modified binder which had relative low stiffness and showed relatively large thermal stress.

• The addition of RAP and RAS to asphalt mixtures resulted in an increase in thermal stress development.

• RAP mixtures, however, exhibited a higher increase in thermal stress development as compared to RAS mixtures.

• An increase in the air void content above the design value (less compaction) resulted in a reduction in thermal stress development during cooling.

• Increasing the binder content tended to increase the magnitude of thermal stress development probably due to increased stiffness.

• Mixtures prepared with high CTE aggregates were found to exhibit a higher thermal stress development compared to mixtures prepared with low CTE aggregate.

The authors would like to thank the Ohio Department of Transportation (ODOT) and the Federal Highway Administration (FHWA) for their financial support. The authors would also like to extend their gratitude to many ODOT engineers and contractors who helped in this study.