Introducing a New Test to Examine Moisture Susceptibility at the Interface of Bitumen and Stones

This paper introduces a novel test method, the moisture-induced shear thinning index (MISTI), to evaluate the susceptibility to moisture damage at the interface of bitumen and siliceous stone aggregates. In the MISTI test, glass beads are used as surrogates for stones. The test quantifies shear thinning as a fundamental material property to examine the extent of desorption of bitumen from stone aggregates when exposed to water. Because desorption of bitumen from stones by water molecules is implicated in moisture damage, the MISTI can be used to examine susceptibility to moisture damage in bituminous composites. The theory behind the test method is based on the target bitumen’s profile of adsorption–desorption to siliceous stones, which are highly susceptible to moisture damage. As the surface of silica becomes coated by water displacing formerly adsorbed bitumen molecules, the surface chemistry of the silica changes; this in turn alters the extent of shear thinning of the bituminous composite. Shear thinning is measured by applying a shear rate sweep ($0.1-100{\mathrm{s}}^{-1}$) to the bitumen–stone blend. The MISTI is defined as the ratio of the shear-thinning value measured in wet condition to the value measured in the dry condition. In an entirely moisture-resistant sample, the MISTI is 1, indicating no changes due to water conditioning. The value of MISTI indicates the degree of susceptibility to moisture damage; any deviation from 1 indicates change at the interface and susceptibility to moisture damage. The test was developed to detect bitumens with high amounts of acidic, water-soluble compounds. The outcomes of this study facilitate detecting nondurable combinations of bitumens and siliceous stones by providing a tool to characterize them based on their susceptibility to moisture damage.