Page 452 - Mechanics of Asphalt Microstructure and Micromechanics

444 C hapter T h ir te en

between the mastic and aggregates (Stuart, 1990; Maupin, 1999). It has also been shown
through micro-mechanics that mixtures with thin asphalt films are prone to adhesive
failure, while those having thick asphalt films tend to fail cohesively (Lytton et al., 2005).
The adhesive failure, mostly known as stripping, takes place when water gets between
the aggregate surface and asphalt film and breaks the bonding, while the cohesive fail-
ure occurs within the asphalt film due to its combination with water. Moisture usually
infiltrates the asphalt pavement from the rise of underground water, rainfall, and/or
water vapor absorption/adsorption (Arambula et al., 2007a).
Some other factors influencing the loss of adhesion are poor mechanical interlock-
ing of aggregate and chemical interaction between the minerals on the aggregate sur-
face and the asphalt cement. Even though cohesive failure has been regarded as a less
important factor of moisture damage of HMA, work done by Kanitpong and Bahia
(2005) showed that the failure surfaces in asphalt mixtures obtained from the tensile
strength ratio (TSR) test were visually observed within the binder film without evi-
dence of apparent loss of adhesion to the aggregate particles. Fromm (1974) proved that
water could penetrate the asphalt film and form a water-in-asphalt emulsion (emulsifi-
cation) that causes asphalt particles to separate from the asphalt film (cohesive failure).
Ultimately, this leads to an adhesive failure when this emulsification boundary pro-
gresses to the aggregate surface.
Over the years, numerous laboratory tests have been developed to identify the
moisture susceptibility of asphalt mixtures (Epps et al., 2000; Caro et al., 2008a). They
can be classified into two main categories: (1) tests performed on loose mixes, such as
the static immersion and boil tests, and (2) tests performed on compacted mixes, such
as the indirect tensile strength, immersion compression, and modulus tests (Roberts et
al., 1996; Solaimanian et al., 2007). It has been agreed, as a general consensus in the in-
dustry, that laboratory tests performed on compacted mixtures are capable of convey-
ing better information on moisture sensitivity than tests on loose mixtures or their com-
ponents. As numerous types of unmodified and modified asphalt binders and a great
number of different aggregate minerals are used, combined with various construction
practices, traffic levels, and environmental conditions, testing to accurately predict
HMA moisture susceptibility has become quite a difficult task. Also, several mecha-
nisms have been recognized as sources of moisture damage: displacement, detachment
(debonding), spontaneous emulsification, film rupture, pore pressure, hydraulic scour-
ing (Hammons et al., 2006; Solaimanian et al., 2007; Caro et al., 2008a), degradation or
fracture of aggregate (Cheng et al., 2003), and mastic dispersion and desorption (Krin-
gos and Scarpas, 2005, 2008; Caro et al., 2008b). Even though these mechanisms are held
responsible for moisture damage, moisture damage is much more complex and an
identification of the damage mechanisms of various asphalt-aggregate systems in the
presence of water is still difficult to achieve. Usually, a synergistic interaction of the
identified mechanisms remains the best justification of the moisture damage process.
Table 13.3 presents these mechanisms in more detail.
13.3.2 Mechanics Method
The mechanisms in the previous section are mainly empirically identified. A new ap-
proach for evaluating moisture-related damage in asphalt mixes is based on the funda-
mental properties of component materials. Possible moisture-related damage predic-
tion is sought from these properties by comparing mechanical behaviors of dry and
conditioned (water-immersed) asphalt mixtures. In this respect, Cheng et al. (2002a,b;