Characterisation of Fatigue Damage of Asphaltic Materials

Abstract

Four-point bending fatigue tests were conducted on asphalt specimens under controlled strain and stress to study damage evolution using the stiffness modulus. A common feature in these tests, irrespective of mode, was that modulus eterioration could be split into three phases; initial (rapid deterioration), intermediate and final in which failure occurred. Damage was modelled based on behaviour at the intermediate phase during which the modulus appeared to follow a straight-line v number of cycles. The effect of test mode on fatigue behaviour was facilitated by the inclusion in the model of a dissipated energy parameter, measured over the intermediate modulus evolution phase. A simple iterative approach was adopted in the model to produce a unique damage parameter that describes the material’s fatigue performance independent of test mode. The damage concept was applied to two asphalt mixtures; a stone mastic asphalt (SMA) used as surfacing on major roads and a dense bitumen macadam (DBM) used as binder course on lightly trafficked roads. The effect of binder modification on resistance to fatigue damage was also included in the study. The SMA binder was modified by the addition of different quantities of crumb rubber modifier (CRM) obtained from truck tyres to study the effect of CRM content. The DBM binder was modified by the addition of CRM from truck and car-tyres to study the effect of CRM origin. When plotted v initial strain, the deduced damage parameter showed a clear distinction in ranking the mixtures in accordance with their fatigue performance.