Microstructure of synthetic composite interfaces and verification of mixing order in cold-recycled asphalt emulsion mixture

Cold recycling technology is getting more and more attention due to economic and environmental benefits by reduced energy consumption and resource conservation. However, its application has been limited to the base and subbase layer because of complicated components and poor crack resistance for the last decades. The design method has shortcomings especially the traditional mixing order, which may be one of the reasons for poor crack resistance. To support the viewpoint above, synthetic composite interfaces were designed to simulate the worst situation during the traditional mixing process and observed in micro scale by scanning electron microscopy. In the microstructure of the traditional mixing order, it was apparent that cement paste had a number of microdefects, a signal of lower interfacial adhesive strength. Moreover, based on the microscopic observation, the adding sequence of cement ought to be changed and two optimized mixing orders were designed of which the difference was verified by the SEM observation of synthetic composite interfaces and the mechanical experiments for different curing time. It can be summarized that mechanical performance was consistent with the microscopic observation. The traditional mixing order was the worst one both in the strength and moisture sensitivity. Finally, the optimal mixing order is put forward to decrease the possibility of the adverse interface, that is, the graded aggregates are mixed with additional water first to reach the workability, while cement, asphalt emulsion and mineral powder are mixed to form cement-asphalt emulsion mortar, finally mixing them all up.