Efeito dos períodos de repouso no ensaio de fadiga em misturas asfálticas e em ligantes

Abstract

Traffic loading repetition can lead to fatigue damage in the asphalt mixtures used in pavement surface courses. Fatigue is evaluated in the laboratory through continuous cyclic tests. Meanwhile, in the field, loading is intermittent. In the laboratory test, the loss of stiffness throughout the test and the required amount of cycles to cause fatigue failure is observed. Different phenomena can cause loss of stiffness throughout loading and recovery over rest. Due to intermittent field loading, further study of the effect of rest periods (RPs) on stiffness is important. In addition, due to the expectation that rest may lead to self-healing of microcracks (a phenomenon designated as healing in the literature), it also makes relevant an investigation into the effect of RPs on fatigue life. The aim of this work is to contribute to the study of the fatigue cracking phenomenon in asphalt mixtures by evaluating the effect of RPs on the scales of the binder and the respective complete asphalt mixture. Materials at both scales will be submitted to fatigue tests with the same PRs. The materials used in the research are two granitic aggregates, fly ash, two asphalt cements (AC) 50/70 from Petrobras/LUBNOR and an AC 50/70 from Petrobras/REFAP. The characterization of the rigidity was performed using the axial compression dynamic modulus for the mixtures (E*) and the twisted complex shear modulus test for the binders (G*). Fatigue tests were performed at 19ºC and with repetitions at different strain ranges. In the same materials and under the same conditions of temperature and strain, the fatigue tests with 5 RPs of 4h were introduced. For the linear viscoelastic modeling (LVE) we used the 2S2P1D model (2 springs, 2 parabolic elements and 1 linear dashpot), with 7 constants, and the WLF (Williams-Landel-Ferry) equation, with 2 independent constants, for the representation. of the Time-Temperature Superposition Principle (TTSP). The 2S2P1D model, associated with the WLF equation, was satisfactory for the modeling of mixtures and binders. Out of the 9 fitted model constants values for binders, 6 could be kept for their respective mixtures, all linked to time-dependent properties, demonstrating that the viscoelastic behavior of the mixture is inherited from the asphalt binder and can be described by method known in the literature as SHStS transformation. Through the use of those methodologies, it was possible to relate the LVE behavior of the mixture with the rheological behavior of the its binder. Fatigue damage in the mixtures was directly related to damage in the corresponding binders. The relationship between the effects of the RPs in both investigated scales was investigated. There are differences in the material behavior regarding fatigue with and without RPs and although there is a recovery of the modulus during rest periods, this recovery is not observed in terms of an increase in the number of cycles at failure (Nf) for the tests with rest, which did not change significantly with rest. It is concluded that the modulus recovery must be related to other effects (reversible phenomena), not to healing.