Effects of mixing procedures and rap sizes on stiffness distribution of hot recycled asphalt mixtures

The quality of hot recycled asphalt mixture prepared by conventional laboratory procedures tends to be overestimated. This is because the long RAP preheating time, for instance 2 h at 110 °c might entirely soften RAP and coincidentally enhance the mixing between RAP and virgin materials thus the quality of hot recycled mixture. In order to manufacture the laboratory specimens that better present the quality as produced in the industrial mixing plant, this research employed the mixing method that simulates the mixing mechanism actually occurs in the asphalt mixing plant in which RAP at ambient temperature is mixed with superheated virgin aggregate at 215 °c for 2, 4, 6 and 8 min before this blend is mixed with virgin binder. The proportion of RAP in the recycled mixture is 40% and bitumen 160/220 penetration grade is used as virgin binder. Two sizes of RAP, 20 and 40 mm are used. The stiffness of recycled asphalt specimens composed of different sizes of RAP and prepared by different mixing procedure including the conventional approach is recorded and analysed. The result indicates with short mixing time, RAP tends to work as inert black rock and the complete blending situation only approached if the mixing duration significantly increased. This finding is against the premise that RAP and virgin binder are completely blending in the recycled mixture design literature. However, longer RAP/virgin aggregate mixing times could result in recycled hot asphalt mixtures with higher stiffness modulus and better homogeneity. In addition, RAP size seriously affects the level of blending hence the stiffness variation of recycled mixtures.