Evaluation of hot-mix asphalt susceptibility to temperature-induced top-down fatigue cracking by means of Uniaxial Cyclic Tensile Stress Test

A dominant failure mode in hot-mix asphalt (HMA) layers is material fatigue, occurring when the asphalt layer is repeatedly loaded by tensile stresses. The maximum stress arises at the bottom of the asphalt layer and, in consequence, crack development is initiated at the bottom. Various techniques are known that simulate bottom-up fatigue cracking in the laboratory. Fatigue failure is also observed on top of the pavement. Especially in cold climates, temperature-induced top-down cracking initiated at the pavement surface is a well-known failure mode. Usually, horizontal tensile stresses in the surface layer are smaller than stresses in the bottom layer. At cold temperatures and when temperature falls within a short period of time, the traffic-induced stress is superposed by temperature-induced stress, and the total stress may come up to the tensile strength of the material. In order to test the material's susceptibility to temperature-induced top-down fatigue cracking in the laboratory, the Uniaxial Cyclic Tensile Stress Test (UCTST) was established and recently introduced in European Standards (prEN 12697-46). In this test, a prismatic shaped HMA beam is subjected to a constant tensile load (representing the temperature-induced stress) which is superposed by a sinusoidal tensile load (representing the traffic load). This loading situation results in a visco-elastic strain response on the one hand, and in an accumulation of visco-plastic strain on the other. Different failure modes are observed in UCTST. In this paper, the failure modes occurring in UCTST, i.e. cracking of the specimen, stiffness decrease, and creep, are discussed based on the results from laboratory testing of nine different HMA and on variation of loading conditions.