Block 28 Sp 13

HMA Characterization Stiffness 1

Stiffness of HMA Mixtures

Senior/GraduateHMA Course


Important for PredictingPavement Performance

• Used to predict:– Critical stresses and strains

• ELSM5• WESLEA

– Fatigue cracking– Permanent deformation characteristics


General Terms

• Dynamic load– Load applied using a

sinusoidal wave form

• Repeated load– Load pulse applied then

removed– Rest period between loads

Load

Load

Time

Time


Elastic Viscous

TimeAA

B

C

Strain in-phase = 0o

Strain out-of-phase = 90o

Dynamic Loading

Strain

Stress

Strain

Stress


Resilient ModulusRepeated Load

Strain

TimeLoad Period

Rest Period

Instantaneous Recoverable

Strain

TotalRecoverable

Strain


Stiffness

• Fundamental to the analysis of pavement response to traffic loading

• Various methods– Axial resilient (ASTM D3497)– Diametral resilient (ASTM D4123)– Flexural dynamic– Shear dynamic


Resilient Modulus

• Axial (without pressure cell)

OEM, Inc., 2000


Axial Resilient Stiffness

10

100

1,000

10,000

0 10 20 30 40 50

Temperature, C

Stif

fnes

s, k

siLow Air VoidsHigh Air Voids

Affect of Temperature and Air Voids

(Tayebali, Tsai, and Monismith, 1994)


Axial Resilient Stiffness

10

100

1,000

10,000

0 10 20 30 40 50

Temperature, C

Stif

fnes

s, k

siOptimumHigh AC Content

Affect of Temperature and Asphalt Content



Diametral Resilient Modulus


Diametral Stiffness

10

100

1,000

10,000

0 10 20 30 40 50

Temperature, C

Dia

met

ral R

esili

ent

Mod

ulus

, ks

i

Opt. ACHigh AC


Increased AC content = decreased modulus


Diametral Stiffness

10

100

1,000

10,000

0 10 20 30 40 50

Temperature, C

Dia

met

ral R

esili

ent

Mod

ulus

, ks

i

4% Air Voids8% Air Voids


Increased air voids = decreased modulus


Diametral Stiffness

10

100

1,000

10,000

0 10 20 30 40 50

Temperature, C

Dia

met

ral R

esili

ent

Mod

ulus

, ks

i

0.5 Hz1.0 Hz


No significant influence on frequency (rest period)


Axial vs. Diametral

• Axial more sensitive to air voids than diametral• Diametral stiffness about 35 to 45% greater than axial

stiffness• Elastic modulus difficult to measure even at

moderately warm temperatures (40oC (104oF)) because of excessive sample deformation– Best to limit test temp to < 25oC (77oF)


Flexural Stiffness

Beam


Flexural Stiffness

• Typical beams– 38 x 38 x 381 mm (1.5 x 1.5 x 15 in)

• Testing parameters– 0.1 sec load, 1.67 Hz haversine– range of temperatures

• 0 to 25oC (32 to 77oC)• Used to estimate stiffness at critical strain

anticipated in pavement


Comparisons

0

300

600

900

1,200

Flexural Axial Diametral

8% Air Voids

4% Air Voids

20oC (68oF)

Resilient Modulus, psi


Comparisons

0

300

600

900

1,200

Flexural Axial Diametral

Opt AC

High AC

20oC (68oF)

Resilient Modulus, psi


Dynamic Shear Modulus

Can be conducted over a range of frequencies or at a fixed frequency


Stiffness

• General conclusions, independent of method used to estimate– Sensitive to:

• Asphalt binder type• Aggregate type• Air-void content• Temperature

Block 28 Sp 13

Technology

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