Steady-state deformation behaviour of asphaltic mixtures

Permanent deformation of asphaltic materials has a major contribution to rutting. Under static loading, the steady-state deformation of asphaltic materials is the key component of the permanent deformation. The steady-state deformation behaviour of a standard 70/100 penetration grade pure bitumen and two types of asphaltic mixtures used in the UK pavements, namely a 10mm Dense Bitumen Macadam (DBM), which is a dense graded mixture, and a 30/10 Hot Rolled Asphalt (HRA), which is a gap graded mixture have been studied. Using Dynamic Shear Rheometer (DSR) Constant stress creep tests were conducted on the bitumen at10°C and20°Cover a range of stress levels. Uniaxial creep and constant strain rate tests have been conducted over a range of stress levels, strain rates and temperatures, and triaxial creep tests have been conducted over a range of stress levels and confining pressures on the mixtures. The steady-state deformation behaviour of the bitumen is found to be linear at stress levels less than 100 kPa and non-linear power law creep with a power exponent of 2.6 at stress levels higher than 500 kPa. The steady-state deformation behaviour of both mixtures is found to be well captured by the Modified Cross Model which predicts linear viscous behaviour at low stress levels and non-linear viscous behaviour at high stress levels.  The steady-state deformation behaviour of the DBM mixture is found to be more sensitive to the stress level. The temperature dependency of the steady-state deformation behaviour is found to be well captured by the Williams-Landel-Ferry (WLF) equation.  It is found that confinement has a stiffening effect on the steady-state deformation behaviour of the mixtures. The stiffening effect of the confining stress is found to be higher for the DBM mixture.