THE FATIGUE OF BITUMEN AND BITUMINOUS MIXES

Abstract

We have carried out laboratory investigations into the phenomenon of fatigue both in bitumen (asphalt cement) itself and in mixes of mineral aggregate bound with bitumen as used in asphalt pavements. The objectives are to determine the role of the binder and to attempt to establish a method of predicting road performance from laboratory fatigue tests. Research into the fatigue of bitumen has been carried out on thin film specimens that have been subjected to sinusoidal shearing over a range of temperatures and frequencies. In tests with a constant stress amplitude, the initial strain for a given fatigue life has been found to depend strongly on the complex modulus of the bitumen, and the slope of the fatigue curve (log life against log initial strain) to be related to the phase angle between the stress and the strain. When plotting the slope of the fatigue curve against the phase angle there is a maximum which is more pronounced for bitumens of low temperature susceptibility. The fatigue life of bitumen can be interpreted in terms of energy: the energy dissipated per cycle of stress being related to the fatigue damage per cycle, and the fatigue resistance may be regarded as the capacity to dissipate this energy into heat. Films of filler/bitumen mixtures have also been tested in the same apparatus and it appears that a filler bitumen behaves like an unfilled one of harder grade. Tests have also been carried out on varous asphalt mixes, including a dense wearing course type and an economical base course type with a low binder content. The object has been to correlate the fatigue behavior of the mixes in binding with that of the bitumen films on the one hand, and with the development of cracking in a slab of mix loaded by a rolling wheel on the other. This last test simulates the tri-axial loading that occurs in practice in an asphalt pavement. Two types of loading have been used in the bending tests: sinusoidal to provide a link with the usual mode of laboratory testing, and intermittent to give strains whose wave-shapes are similar to those that have been recorded under moving traffic in asphalt layers of pavements. It has been found that the rest periods between successive groups of waves, corresponding to the spacing between vehicles, give fatigue lives that are longer than those with continuous sinusoidal loading. In this way, realistic data are being developed for incorporation into asphalt pavement design procedures. /AUTHOR/