Mix Proportion Optimisation Design and Performance Evaluation of a Cold‐Recycled Mixture Based on Uniform Design and Bailey Verification

To eliminate the gradation variability of reclaimed asphalt pavement (RAP) materials and improve the road performance of cold‐recycled mixtures, this study analyses the gradation variability of recycled materials and conducts gradation design using the actual particle sizes of extracted aged materials. An optimisation method for the mix proportion of cold‐recycled mixtures is proposed by combining the uniform method for design and the Bailey method for verification. A comparative study on the road performance of cold‐recycled mixtures with two different gradations was then carried out. Based on low‐temperature beam bending and semi‐circular bending (SCB) tests, the low‐temperature performance of the recycled mixtures was comprehensively evaluated from the perspectives of conventional analysis and energy analysis. Moreover, the correlation between fracture energy, strain energy density and the low‐temperature indices specified in the current technical code was analysed to determine the low‐temperature performance evaluation index for cold‐recycled mixtures in severely cold regions. The results show that gradation design using only the sieving data of milled RAP materials will lead to an overall finer gradation of the recycled mixtures. For the optimised gradation verified by the uniform design and Bailey verification, the dynamic stability of the mixtures is increased by 9%~15% and the water stability is improved by 3%~6% under different cement contents. In addition, the flexural tensile strength and flexural tensile strain of the mixtures are increased by 3% and approximately 6%, respectively after gradation optimisation, indicating an improvement in low‐temperature performance, with the optimal effect achieved at a cement content of 1.5%. Through the correlation analysis of fracture energy, strain energy density and flexural tensile strain, fracture energy is proposed as the low‐temperature performance evaluation index for cold‐recycled mixtures, and the recommended value of this index is determined to be 1000 J/m 2 for severely cold regions in winter. The research results provide a theoretical basis for the gradation design and performance evaluation of recycled mixtures applied in severely cold regions.