Fatigue of bitumen and bituminous mixes

Abstract

This paper gives details of an investigation into the fundamental fatigue properties of bitumen and bituminous mixes. Fatigue tests carried out under constant bending stress, at varying temperatures between −13·5°C and +25°C, show that the material exhibits fatigue properties over wide ranges of stress and that for a particular temperature and speed of loading the log stress-log number of cycles to failure relationship is linear between 104 and 108 cycles. The life under constant stress is highly dependent on the temperature of the test, a low temperature giving a longer life at a particular stress; it is also dependent to some extent on the speed of loading, but taking into account the stiffness of the material which depends on temperature, speed of loading, rheological characteristics and composition of the mix, it has been found that when the logarithm of the strain is plotted against the logarithm of the number of cycles to failure all experimental results at different speeds and temperatures for one mix lie with a certain amount of scatter about a straight line. It appears therefore that the factor affecting the fatigue life is one of strain rather than stress, and the effects of temperature and speed can be accounted for by their effect on the stiffness of the specimen. This has been confirmed by tests under constant torsional strain at different temperatures between −20°C and +40°C, but at the higher temperatures under this type of loading the fatigue life includes a considerable crack-propagation time. Similar results have been obtained from mixes containing different amounts of aggregate, but as the quantity of aggregate in the mix is reduced so the life for a given strain increases, suggesting that the criterion of failure may be one of tensile strain in the bitumen present in the mix. Some tests have also been carried out on pure bitumen specimens at different temperatures.Careful examination of the fatigue cracks and failure surfaces shows that in nearly all cases failure originates on the principal tensile plane. The effects of such factors as surface conditions, void content and rest periods have also been studied.