Page 392 - Mechanics of Asphalt Microstructure and Micromechanics

384 Ch a p t e r E l ev e n

shape, angularity, and texture. Through calibrating the model constants of a limited
number of selected mixes (designed to evaluate the binder stiffness and temperature
effects, aggregate gradation effects, and aggregate shape, angularity, and texture ef-
fects), the model predicted compaction process will yield differences in slope and the
initial void content; these will present the calibrations of both laboratory and field com-
paction slopes. The ratio between the field slope and the lab slope presents a measure
of the shift factor.

11.3.2 Macro-Constitutive Models-Critical State Soil Mechanics
(CSSM) Model
Huerne (2004) made an analogy between soil and AC and applied the CSSM (Wood,
1990) model to simulate asphalt compaction. Huerne’s theory is summarized as fol-
lows:
1. p-q space where:

σ + σ + σ ⎡ (σ xx −σ yy ) 2 + (σ yy −σ zz ) 2 + (σ zz −σ xx ) 2 ⎤ 1 2/
τ
2
2
3
p xx yy zz , q = ⎢ + (τ 2 xy + τ + τ ) ⎥ (11-14)
zx
yz
3 ⎢ ⎣ 2 ⎥ ⎦
2. Mohr-Coulomb yield surface and critical state
3. Associated flow
4. VMA and void ratio equivalency:
Recoverable behavior: VMA = VMA k − klog(p)
Irrecoverable behavior: VMA = VMA l − llog(p)
Where VMA k , k, VMA l , and l are material constants related to aggregate gradation
characteristics, and binder contents.
Huerne also introduced the modeling of cooling through the use of Fourier’s law
and the empirical relationship to temperature drop. The FEM simulations of experi-
mental sections indicate that the model and the FEM procedure simulate the compac-
tion process nicely.

11.3.3 DEM Simulation of Field Compaction
In investigating how important factors influence the field compaction of pavement ma-
terials, the 3D particle flow code (PFC3D, Itasca, 2003), which is based on DEM, is used
as the simulation tool (Figure 11.18). The investigation includes:

• Particle shape effect;
• Particle contact property effect; and
• Temperature effect.

11.3.3.1 Particle Shape Effect
The DEM view can look at how aggregates move during the compaction process. The
particle shape effect is evaluated through clustering small balls together. Case 1 (Figure
11.19) is the simplest case for balls only. Case 2 clusters two balls into a rectangular