Application of surface free energy techniques to evaluate bitumen-aggregate bonding strength and bituminous mixture moisture sensitivity

This paper presents a combination of surface free energy testing techniques for aggregates and bitumen, with mechanical moisture-sensitivity assessment techniques for identification of compatible bitumen–aggregate combinations. Results from the surface free energy measurements and calculations were compared with data from the rolling bottle test and the saturation ageing tensile stiffness test in order to establish relationships amongst the three techniques. The mineralogical composition and microstructure of the aggregates were characterised using a mineral liberation analyser. For the same bitumen type, moisture sensitivity of mixtures containing limestone aggregates were found to be lower than mixtures containing granite aggregates. The presence of high amounts of quartz and feldspar (up to 50%) in the granite compared with almost none in limestone, and the significant amount of calcite in limestone (96% or more) versus none in granite were cited as possible reasons for the differences in performance of the two aggregate types. The porosity of the worst performing aggregates was higher than the best performing aggregate. The study concludes that moisture damage in the bituminous mixture is a function of the physico-chemical surface free energy properties of bitumen and aggregates as well as the mineralogical composition of aggregates.