Fracture Degradation of Asphalt Concrete under Repeated Loading

Abstract

For an accurate estimation of pavement life, it is essential to consider the effect of repeated traffic and thermal loadings on the fracture resistance of asphalt concrete (AC) materials. Repeated traffic and environmental loadings induce degradation of the AC overlay’s strength as microcracks’ stress levels grow lower than that of the layer’s designed strength. This degradation makes AC materials less resistant to fracture. Hence, fracture failure initiates and propagates to failure at a repeated load relatively lower than the material’s strength. The current approach to determine the critical properties of AC materials is by conducting laboratory testing under monotonic loading and cyclic loading separately. Fatigue tests under cyclic loading provide bulk material properties, but they overlook possible discontinuity, such as cracks or joints in existing pavements. Fracture tests, on the other hand, are conducted under monotonic loading, which does not capture the loss of material strength during repeated traffic loading. This study investigated the degradation of the AC fracture resistance due to repeated progressive damage during loading. An experimental program applied monotonic and cyclic loading to the same specimen’s geometry to determine the fracture resistance degradation as damage grows at the crack tip. Fracture and fatigue tests were performed on notched semicircular bending specimens at various notch lengths, temperatures, loading frequencies, and loading amplitudes. The study found that the use of highly modified asphalt binders and higher binder contents increases the number of cycles for initiation and propagation. The flexibility index at 10°C differentiated between AC mixes with respect to the cracking potential. The fracture energy, however, related more so to the fatigue life. Similarly, the J-integral had good correlation with the number of failed cycles but not with the fatigue life. The impact of the degradation is pronounced at the prepeak load-displacement area and at a relatively higher temperature. The prepeak fracture energy correlated with the AC fatigue life.