HMA Characterization Thermal Cracking 1

Thermal Cracking

Senior/GraduateHMA Course

HMA Characterization Thermal Cracking 2

Thermal Cracking

HMA Characterization Thermal Cracking 3

Background

• Associated with volumetric contraction that occurs when temperature decreases• Cyclic• One time

• Quick, large drop• Thermal stresses develop

• Crack occurs when tensile stresses exceed mix strength

• Temp at which this occurs is called fracture temperature

HMA Characterization Thermal Cracking 4

Background

• Low pavement surface temperature• Rate of cooling• Pavement age

Colder

Temperature Gradient

T Max

Gradient of thermal stresses

HMA Characterization Thermal Cracking 5

Background

0.1

1

10

100

-50 -40 -30 -20 -10 0Temperature, C

Str

ess

, kg

/cm

2

Thermal Stress in Pavement Tensile Strength of Mix

Fracture Temperature, C

Thermal Fatigue Consideration

Low Temp Consideration

HMA Characterization Thermal Cracking 6

Test Methods

• Recently Recommended Tests• Thermal Stress Restrained Specimen Test

(TSRST)• Indirect Tension

• Advanced Topics• Direct Tension• Flexural Bending

• C*-Line Integral

HMA Characterization Thermal Cracking 7

TSRST

• Thermal Stress Restrained Specimen Test (TSRST)• 5o or 10oC/hr temperature drop

Stress

Temperature

TF Tt

Stress Relaxation

Fracture Strength Slope = dT/dS

HMA Characterization Thermal Cracking 8

TSRST

• Advantages• Direct measure of development of thermal stress

build up• Eliminates the need to mathematically estimate

• Disadvantages• Beams have to made and cut (time consuming)• Difficult to obtain good alignment in load frame

HMA Characterization Thermal Cracking 9

TSRST

-35

-30

-25

-20

-15

-10

-5

0

AAG-1 AAG-2 AAK-1 AAK-2

Fra

ctu

re T

em

p,

C

8%, RB4%, RB8%, RL4%, RL

Influence of Air Voids and Agg. Type

AAG-1 approx. AC 20 AAG-2 approx. AC 10AAK-1 = AC 30, AAK-2 = AC 10

RB = Watsonville GraniteRL = Chert

HMA Characterization Thermal Cracking 10

TSRST

0

0.5

1

1.5

2

2.5

3

3.5

4

4.5

AAG-1 AAG-2 AAK-1 AAK-2

Fra

ctu

re S

tre

ng

th,

MP

a8%, RB4%, RB8%, RL4%, RL

Influence of Air Voids and Agg. Type

AAG-1 approx. AC 20 AAG-2 approx. AC 10AAK-1 = AC 30, AAK-2 = AC 10

RB = Watsonville GraniteRL = Chert

HMA Characterization Thermal Cracking 11

TSRST

• Slope is greater for samples with lower voids• Specimens with lower voids fracture at higher stresses• Some influence of aggregate on fracture temperature• Major influence of asphalt source rather than viscosity

26

Influence of HMA Properties

HMA Characterization Thermal Cracking 12

Desirable Material Properties

• Asphalt binder• Low temperature viscosity• Temperature susceptibility

• Aggregate characteristics• High abrasion resistance• Low freeze-thaw loss• Low absorption

HMA Characterization Thermal Cracking 13

Material Properties ThatHave Minimal Effect

• Asphalt binder content• Increasing % AC increases

coefficient of thermal contraction

• Offset by decreasing stiffness• Air voids

• Minimal affect

HMA Characterization Thermal Cracking 14

Pavement Structure

• Narrow width pavements have closer crack spacing• Examples

• 7.3 m (24 ft) width may have crack spacing of 30 m (100 ft)

• 50 to 30 m (50 to 100 ft; airports) may have crack spacing > 45 m (150 ft)

• Thicker pavements = less cracking• Friction coefficient between HMA and base

• Prime coat reduces thermal cracking

HMA Characterization Thermal Cracking 15

Pavement Structure (continued)

• Subgrade• More cracks on sand than cohesive subgrades

• Construction flaws• Steel roller compaction of HMA layers at high

temps and low mix stiffness creates transverse flaws

• Cracks may be initiated at these points

HMA Characterization Thermal Cracking 16

QUESTIONS ?