Page 97 - MODELING OF ASPHALT CONCRETE

Overview of the Stif fness Characterization of Asphalt Concr ete 75

D ∞
Creep compliance Iog D(t) D o 1 n

D , D , t , n Log (time)
∞
o
o
FIGURE 3-7 Creep compliance.
approaches the asymptote of D . The slope n can never be greater than 1.0. The creep
∞
compliance is related to the relaxation modulus. This is determined in a uniaxial test in
which a strain is imposed on the material and held constant. The stress in the material
then relaxes as the time after the strain was first applied increases. The relaxation
modulus is the time-dependent stress divided by the constant strain. Chapter 6 deals
exclusively with the relationship between this property and the creep compliance. At
this point it simply needs to be known that the theory of linear viscoelasticity may be
used to convert between any of the linear viscoelastic response functions as long as the
measurements actually represent linear viscoelastic behavior.
Surface Energies
The surface energies of the asphalt can be measured in a number of different test devices.
The instrument which was used in measuring the surface energies that are tabulated in
Table 3-1 were measured with the Wilhelmy plate apparatus. The apparatus is pictured
in Fig. 12-3, and a detailed explanation of the testing procedure is given there. There are
three components of surface energy: the nonpolar or Lifshitz-van der Waals, and the
Lewis acid and Lewis base polar components. There is a hysteresis effect between the
wetting and the dewetting surface energies. The wetting surface energies are associated
with the healing of the microfractures in the asphalt while the dewetting surface
energies are associated with the fracture of the asphalt. The total surface energy is made

Surface New Asphalt Aged Asphalt
Energy
Components,
ergs/cm 2 Water Wetting Dewetting Wetting Dewetting
Γ LW 21.6 8.80 13.62 14.91 6.76
Γ – 25.5 1.50 18.87 1.74 15.28
Γ + 25.5 2.81 10.52 1.07 15.03
Γ AB 51.0 4.13 28.18 2.64 30.30
Γ TOTAL 72.6 12.93 41.80 17.55 37.06

TABLE 3-1 Wetting and Dewetting Surface Energies of Asphalt and Water