Characterization of mesoscale fracture damage of asphalt mixtures with basalt fiber by environmental scanning electron microscopy

The crack resistance of asphalt mixture determines the service life of asphalt pavement to a great extent. Pavement cracking is strongly influenced by the internal structures of asphalt mixtures, such as the aggregate skeleton and the interstitial components (i.e., fine aggregate matrix (FAM)). Basalt fiber is widely used for cracking resistance in asphalt mixtures because of its excellent mechanical properties. However, due to the lack of an effective real-time observation technique, the damage suppression mechanism of basalt fiber in the asphalt mixture is unclear. This study aims to develop a novel methodology that can effectively characterize the evolution of instantaneous damage of asphalt mixture with basalt fiber and assess its fracture properties. This methodology is based on the characterization of the FAM (i.e., the fine matrix portion that governs cracking performance) through a mesoscopic tension test termed the in-situ direct tension (ISDT) test. The mixtures with and without basalt fiber were designed for consideration. The semicircular bending (SCB) and the ISDT tests were conducted to compare the fracture properties between the asphalt mixture and FAM. The fracture property indices were calibrated using the feature parameters of force–displacement curves. The sigmoidal-shaped crack area density model was proposed with consideration of the full development of the cracking area density curve. The results show that the asphalt mixture and FAM have a comparable increasing trend in fracture energy and fracture toughness with the increase of basalt fiber content. The lagging effect of crack initiation and propagation was observed in fiber asphalt mixtures and FAM. The results indicate that the lagging effect of the asphalt mixture on the fine portion controls the crack propagation speed. It is valuable to investigate the fracture property of the fine portion rather than focusing on that of the whole asphalt mixtures. The proposed ISDT test can be recognized as an efficient implementation to evaluate the fracture performance of fiber asphalt mixtures.