ASPHALT BINDERS AND ASPHALT MIXTURES - Materials for Civil and Construction Engineers - page 366

of the viscous properties of the material being evaluated. Mathematically,
this is expressed as

E* = ƒ E* ƒ cos f + i ƒ E* ƒ sin f (9.22)

where

t i
t p

f = * 360

t i = time lag between of stress and strain, s

for a stress cycle, s
number

t p = time

i = imaginary

For a pure elastic material, and the complex modulus is equal
to the absolute value, or the dynamic modulus. For a pure viscous material,

f = 0, E*

f = 90°.

The dynamic modulus obtained from this test is indicative of the stiff-
ness of the asphalt mixture at the selected temperature and load frequen-
cy. The dynamic modulus is correlated to both rutting and fatigue cracking
of HMA.

Triaxial Static Creep Test In the static compressive creep test, a total
strain-time relationship for a mixture is measured in the laboratory under
unconfined or confined conditions. The static creep test, using either one
load-unload cycle or incremental load-unload cycles, provides sufficient
information to determine the instantaneous elastic (recoverable) and plastic
(irrecoverable) components (time independent), and the viscoelastic and vis-
coplastic components (time dependent) of the material's response. In this test,
the compliance, D(t) is determined by dividing the strain as a function of time
by the applied constant stress e 1 t 2 , s o :

2
s o

1

e t

D 1 t 2 = (9.23)

Figure 9.30 shows typical test results between the calculated compli-
ance and loading time. As shown, the compliance can be divided into three

D(t)

Secondary

Te r tiary

Flow Time When
Shear Deformation Begins

Primary

FIGURE 9.30 Compliance ver-
sus loading time for the static triaxial
creep test. time