Page 52 - Mechanics of Asphalt Microstructure and Micromechanics

Mechanical Proper ties of Constituents 45

elastic range. It might be possible that zones at aggregate-aggregate contacts may ex-
perience some plastic deformation. For computing efficiency, aggregates may be con-
sidered as linear elastic and brittle with a stress-strain relationship as linear elastic-
perfect plastic. While generally aggregates are anisotropic, the random orientation
effects and the material anisotropy may cancel out. For the sake of simpler material
characterization and computational efficiency, the anisotropy behavior may be ne-
glected in current literature. Strengths of rocks are typically considered to follow
Mohr-Coulomb criteria.
More complicated models such as the continuum damage model, the viscoplasticity
model, and the elastoplasticity model with back stresses, while more accurate, may not
be necessary for modeling the behavior of aggregate. During the compacting, aggregate
breakage may be modeled as brittle, linear elastic until fracture. For more advanced
modeling, rate independent elastic-plasticity may be considered.
2.2.1.1 Concepts of Mechanical Properties
Aggregate strength, usually crushing strength, represents maximum compressive or
tensile stress that an aggregate particle can carry before breaking. Unconfined compres-
sive strength (UCS), aggregate crushing value (ACV), and aggregate impact value (AIV)
provide strength data.
Aggregate stiffness is an aggregate particle’s resistance to deformation and is based
on its modulus of elasticity. The modulus of elasticity is calculated from the applied
compressive or tensile stress divided by the corresponding recoverable strain.
Toughness is the resistance of aggregates to crushing and impact fracture during
transport, placing, compaction, and under traffic. British tests such as aggregate crush-
ing value (ACV) and aggregate impact value (AIV) are usually employed to determine
the aggregates’ toughness.
Durability or resistance to weathering is the ability of an aggregate particle to re-
sist disintegration due to wetting and drying, heating and cooling, and freezing and
thawing. Common tests such as the soundness test or the unconfined freeze-thaw test
are used to assess the aggregates’ susceptibility to weathering.
Hardness or wear resistance is the aggregate surface’s ability to resist polishing or
wearing due to rubbing and/or friction produced by externally applied forces, such as
vehicles or foot traffic. Thus, the aggregate surface should wear non-uniformly to main-
tain a high level of surface friction. Hence, a high percentage of hard, well-bonded
mineral grains is desired in the aggregate to resist the abrasive smoothing action of
tires. Tests used to evaluate aggregate’s hardness are Los Angeles abrasion, micro-deval
abrasion, and polished stone value tests.
Degradation resistance is an aggregate’s ability to resist fractionation when sub-
jected to applied forces such as those from mixer blades, compaction, heavy wheel
loads, and grinding action.

2.2.1.2 Typical Mechanical Properties from Literature
Table 2.1 presents the typical mechanical properties of mineral rocks. It should be not-
ed that the rock properties represent those of intact rocks. When intact rocks (with
distributed discontinuity) are crushed into aggregates of different sizes, the porosity of
aggregates becomes smaller and therefore the modulus of aggregates are typically
larger for aggregates of smaller size. Some relatively simple micromechanics ap-
proaches, such as Mori-Tanaka, self-consistent method, and Hsih-Strikman method,
may be used to estimate these properties.