Page 62 - Mechanics of Asphalt Microstructure and Micromechanics

Mechanical Proper ties of Constituents 55

2.4.15 VEPCD Model Calibration in Compression
In this case, compression data are used under same calibration procedure for tension
modeling regarding the LVE and VECD models (Kim, 2009). The tension-compression
tests were performed at a strain amplitude below 70 micro-strains. Also, for viscoelastic
damage characterization, constant crosshead rate monotonic test results at 5°C were
used. Pseudo-secant modulus (C) and damage parameter (S) were calculated based on
stresses and pseudostrains. At peak stresses acquired from different tension rate tests,
C values were between 0.3 and 0.35.
Repetitive creep and recovery tests with fixed time and stress are performed for the
VP model calibration. As in the tension calibration, the permanent strains at the end of
rest periods are used to determine the p, q, and Y (VP) model coefficients. Creep compli-
ance obtained from the complex modulus, damage characteristic relationship (C-S), and
VP model coefficients (p, q, and Y) are used to verify the VEPCD model in compression.
2.4.16 Permanent Deformation Continuum Damage
The constant rate of strain tests to failure is used to calibrate the uniaxial continuum
damage model parameters, C(S) (damage function), a (material constant), and a(T)
(shift factor). Compression tests are conducted at multiple strain rates and at low tem-
peratures to minimize the viscoplastic effects (Kim, 2009). A nonlinear optimization
procedure is used to calibrate the damage function C(S) and the material constant, a.
Also, the uniaxial form of the damage model can be generalized to confined triaxial
conditions using a dual pseudostrain energy density function of axial pseudostrain and
confining pressure (Ha and Schapery, 1998).
Confined strain rate to failure tests, conducted at different conditions from those
used for the model calibration, were used to validate the multi-dimensional damage
model (viscoelastic, viscoplastic, and damage).

2.5 Nanoscale Characterization
In recent years, nanoscale modeling and simulation offer significant insights into the
behavior of asphalt binder, mastics, and asphalt binder-aggregate interfaces. Research-
ers in the AC field have also started applying these advanced tools. The following pres-
ents just a few of these methods.
2.5.1 Atomic Force Microscope (AFM)
The atomic force microscope (AFM) or scanning force microscope (SFM) is a very high-
resolution type of scanning probe microscopy, with resolution of fractions of a nanome-
ter (Bonnell, 2001). It provides 3D, real-space images of various materials based on de-
tecting the local interaction between a small probe tip and a surface. Depending on the
separation between probe tip and sample, a variety of tip-surface interactions can be
measured with the force microscope. In the AFM, the tip is attached to a spring in the
form of a cantilever and as the tip moves over the surface, the cantilever bends back and
forth in the z direction. A laser beam is directed onto the cantilever and as the cantilever
bends, the movement of the reflected beam is detected by a photo diode. The movement
of the tip over the surface is controlled by a piezoelectric ceramic, which can move in the
x, y, and z directions in response to applied voltages. Sample scanning is carried out by
the movement of the piezo ceramic in the x and y directions. A feedback circuit inte-
grates the photo diode signal and applies a feedback voltage to the z piezo to exactly
balance the cantilever bending. Since the probe force is proportional to the cantilever