Mechanical and microstructural characteristics of different interfaces in cold recycled mixture containing cement and asphalt emulsion

Cold recycling technique is an effective strategy of cost reduction, resource conservation and energy consumption in pavement maintenance and rehabilitation. However, the mechanical mechanism of cold recycled mixture is not clear in the process of strength formation. The research was to put forward practical suggestions for mechanical property improvement of cold recycled mixture and generalize the application more efficiently to reach an ultimate purpose of energy and resource conservation. Various mixing orders were designed to construct different interfacial features and reflect on mechanical properties, and two-phase “aggregate-binder” interfaces were considered as the main interface forms to respectively reveal the mechanical and microstructural characteristics of cold recycled mixture. The mechanical properties of different mixing orders were compared and analyzed by the indirect tensile tests and dynamic modulus tests. Besides, the microstructure was observed by scanning electron microscope and the backscattered images were captured to extract microvoid parameters. Results indicated that different mixing orders and interface forms had a great influence on the mechanical properties of cold recycled mixture. Within the range of binder content applied in cold recycled mixture, the strength of the interface involving the recycled asphalt particles was larger than that of the interface involving the new aggregates. The indirect tensile strength of the interface involving asphalt emulsion was larger than that of the interface involving cement, while the microvoidage was just on the contrary. In conclusion, mixing cement before adding asphalt emulsion is not conducive to the improvement of mechanical properties. It is suggested that cement and asphalt emulsion should be fully mixed before the contact between aggregate and mortar in order to realize the optimal performance of cold recycled mixture; otherwise, mineral aggregates mixed with asphalt emulsion first can also be a better alternative.