Design And Evaluation Of Conductive Asphalt Concrete For Self-Healing

The research of self-healing asphaltic materials mainly focouses on mechanisms, modeling, characterizing methods and enhancement techniques of self-healing. Due to the low healing capacity of asphalt mixture at ambient temperature, self-healing enhancement techniques become urgent and important. Increasing the temperature to accelerate healing in asphalt and to delay the crack propagation is an effective way to extend the service life of asphalt pavement. This study aims to design the conductive asphalt concrete (CAC) which can be used for microwave heating/healing, and to investigate the electrical, pavement and healing performance. The main contents of this study include material composition design and preparation, electrical and pavement performance evaluation of CAC, and healing properties after microwave heating.Firstly, the modified continuous dense gradation (AC-13C) was selected to prepapre CAC by comparing the structural features of different types of asphalt mixtures and considering the design requirements of CAC. Steel fiber and graphite were chosen as conductive phase materials. The rheological properties of graphite modified asphalt binder and steel fiber distribution in asphalt mixture were also investigated. Results show that graphite can improve the high-temperature performance and viscosity of neat asphalt binder through stiffening the binder. According to fiber distribution coefficients difined by resistivity method and CT (Computed Tomography) scanned image analysis method, prolonging the total mixing time to 270 s can improve the fiber distribution in steel fiber reinforced asphalt mixture.Secondly, electrical and pavement performance of conductive asphalt mixture and plain asphalt mixture were comparatively evaluated. The conductivity of insulated asphalt concrete can be achieved by adding either graphite or steel fiber. Whereas steel fiber has greater conductive effectiveness. Through compositing steel fiber and graphite, the resistivity of CAC decreases gradually without compromising its mechanical properties. The bonding forces between asphalt and aggregates decrease with the addition of graphite due to its oil-absorbing and lubricating effect, which decreases the Marshall Stability and indirect tensile strength of asphalt mixture. By contrast, well-distributed steel fibers in the mixture can form dimensional reticular structure, which improves the Marshall Stability and fatigue resistance of asphalt mixture due to fibers' reinforcing and toughening effects. Both graphite and steel fiber can improve the high-temperature anti-rutting performance of asphalt mixture, while degrading the moisture susceptibility of asphalt mixture. Both plain asphalt mixture and graphite modified asphalt mixture exhibit brittle fracture in low-temperature indirect tensile test. The addition of steel fiber increases the fracture energy of asphalt mixture, which makes steel fiber reinforced asphalt mixture exhibit combined failure mode. The developed dynamic modulus master curves of conductive asphalt mixtures can be used as parameters of pavement material properties to be applied in the design of pavement material and structure.Finally, the microwave heating properties and mechanism of CAC were investigated. The optimized heating scheme for healing was determined after proposing reasonable healing test scheme and evaluation index. The thermal conductivity coefficient of plain asphalt mixture and conductive asphalt mixture tested through heat flow meter method are 1.13 W/(m·K) and 1.46 W/(m·K), respectively. The addition of conductive materials increases the microwave heating rate of asphalt mixture significantly. Asphalt mixture samples were tested through "fatigue-healing-refatigue" procedure, and the healing levels were characterized by defining healing index. Results show that CAC samples have higher healing levels than plain asphalt mixture samples under the same microwave heating time. This is because CAC samples absorb more microwave energy to accelerate healing. The critical temperatures for plain asphalt mixture and conductive asphalt mixture to start flow healing are 38? and 50?, respectively. The optimum heating time for above mixtures are 150 s and 120 s, which correspond to the optimum heating temperatures of 76? and 90?, respectively. Under the same total heating time, intermittent heating mode is more effective than continuous heating mode to improve the healing levels of aphalt mixture samples.In summary, CAC designed in this study can be heated through microwave to accelerate healing capacity. Further research can lay the foundation for the application of microwave heating to repair asphalt pavement.