Cold recycling mix design approach targeting permanent deformation resistance

Cold recycling technology offers a sustainable construction pathway for pavements with a potential of recycling up to 100% of the existing in-situ materials. This study introduces a new approach to design emulsion-based cold recycling mixtures. The proposed mix design approach channels the design efforts to optimize the mixture's components and evaluate its long-term performance. The elements of the proposed design include altering aggregate gradation to achieve optimum packing; a method to selecting an optimum moisture content that is derived from both liquid components in the mixture; and selecting the residual asphalt content based on the mixture's resistance to permanent deformation. To evaluate the reliability of the proposed mix design method, an optimum mixing water content was derived for an optimum RAP gradation. The effect of varying the emulsion content while keeping the mixing water content constant was evaluated on the mixture's volumetrics, strength, and permanent deformation behavior. A conventional triaxial compression stress path was applied on samples compacted with several residual asphalt contents. The triaxial test results were compared with results obtained from Hamburg Wheel Tracking Test and Asphalt Pavement Analyzer rutting device. The increase in residual asphalt content at optimum mixing water content led to a systematic increase in strength and reduction in voids that stabilizes as the content increases. The permanent deformation experiments showed the importance of imposing a deformation criterion to select the residual asphalt content in the mix design stage. Additionally, the triaxial repeated load tests demonstrated the importance of using anticipated stress states to characterize permanent deformation resistance which cannot be captured by simple performance tests. Representative stress states in the CR layer should be taken into account during the mix design stage where higher stresses are anticipated in the recycled layer.