No abstract

Durability is one of the most important properties of an asphalt mixture. A key factor affecting the durability of asphalt pavements is moisture damage. Moisture damage is generally considered to be the result of two main mechanisms; the loss of adhesion between bitumen and aggregate and the loss of cohesion within the mixture.

a b s t r a c tThe effects of aggregate mineralogical composition on moisture sensitivity of aggregate-bitumen bonds were investigated using four aggregate types (two limestone and two granite) and two bitumen grades (40/60 pen and 70/100 pen). Moisture sensitivity (or water resistance) of the aggregate-bitumen bonds were characterized using retained strength obtained from three different tensile tests (peel, PATTI and pull-off). The results showed significant differences in the amount of moisture absorbed by a given aggregate which suggested strong correlations between aggregate mineral composition and moisture absorption. For most of the aggregate-bitumen bonds, failure surfaces transformed from cohesive to adhesive with conditioning time thereby confirming the strong influence of moisture on aggregate bonds. The three tensile tests used in this study showed similar rankings in terms of moisture sensitivity but the pull-off test was found to be the most sensitive. The effect of bitumen on moisture sensitivity was found to be lower than the effect of aggregates, with the moisture absorption properties of the aggregates depending strongly on certain key minerals including clay, anorthite and calcite. Strong correlations were also found between mineral compositions and moisture sensitivity with clay and anorthite having strong negative influence while calcite showed positive effect on moisture sensitivity. Previous studies have identified various mineral phases like albite, quartz, and k-feldspar, as detrimental in terms of moisture sensitivity. The results appear to support the extension of the existing list of detrimental aggregate minerals to include anorthite and clay while supporting the case of calcite as a moisture resistant mineral.

A common manifestation of moisture-induced damage in asphalt mixtures is the loss of adhesion at the aggregate-asphalt mastic interface and/or cohesion within the bulk mastic. This paper investigates the effects of moisture on the aggregate-mastic interfacial adhesive strength as well as the bulk mastic cohesive strength. Physical adsorption concepts were used to characterise the thermodynamic work of adhesion and debonding of the aggregate-mastic bonds using dynamic vapour sorption and contact angle measurements. Moisture diffusion in the aggregate substrates and in the bulk mastics was determined using gravimetric techniques. Mineral composition of the aggregates was characterised by a technique based on the combination of a scanning electron microscope and multiple energy dispersive X-ray detectors. Aggregate-mastic bond strength was determined using moisture-conditioned butt-jointed tensile test specimens, while mastic cohesive strength was determined using dog bone-shaped tensile specimens. Aggregate-mastic bonds comprising granite mastics performed worse in terms of moisture resistance than limestone mastic bonds. The effect of moisture on the aggregate-mastic interfacial bond appears to be more detrimental than the effect of moisture on the bulk mastic.

In this study, the moisture sensitivity of different kinds of aggregates and bituminous binders is examined by comparing the performance between five empirical test methods for loose mixtures -static immersion test, rolling bottle test (RBT), boiling water test (BWT), total water immersion test and the ultrasonic method -with more fundamental surface energybased test data. The RBT and BWT results showed that limestone aggregates perform better than granite aggregates and that, for unmodified binders, stiffer binders provide better moisture resistance compared with softer binder. Both tests were sensitive to aggregate type, binder type and anti-stripping agent type. Ranking of the mixtures by RBT and BWT was in general agreement with the surface energy-based tests, especially for mixtures that performed worst or best in RBT and BWT. The magnitude of the work of debonding in the presence of water was found to be aggregate type dependent which suggests the physico-chemical properties of aggregates may play a fundamental and more significant role in the generation of moisture damage, than bitumen properties.