ABSTRACT
In the United Kingdom, the long term performance predictions for High Modulus Bases (HMB’s) containing hard low penetration grade binders have been based on the projection of the behaviour of “conventional” flexible base materials manufactured with 50 or 100 penetration grade binder. Whether failure is assumed to occur by crack initiation at the top (as more recent evidence suggests) or by fatigue crack initiation at the bottom of a bituminous pavement layer, it is of concern that HMB might be fundamentally more “brittle” than “conventional” bituminous material. This characteristic may lead to more rapid crack propagation especially at lower temperatures and after a degree of ageing. Hard grade bitumen samples from different sources were subjected to accelerated ageing procedures: the Rolling Thin Film Oven Test (RTFOT); and the High Pressure Ageing Test (HiPAT). Rheological and cohesion testing were carried out using a Dynamic Shear Rheometer and a Vialit Pendulum Cohesion tester. A cellular method was adopted for analysing the overall binder properties. The rheological and cohesive characteristics of the low penetration grade binders tested changed in a different manner and at different rates with ageing, for each different source, with the two straight-run binders tested showing the least susceptibility to ageing.
