Structural Numbers for Reclaimed Asphalt Pavement Base and Subbase Course Mixes

Abstract

The 1993 AASHTO pavement design guide equation for flexible pavement structures requires the determination of a structural number, which is a function of the layer coefficient, thickness, and drainage coefficient of each layer of pavement. Empirical correlations for layer coefficients have been developed based on resilient moduli. The principal factor affecting the drainage coefficient is the hydraulic conductivity. The objective was to determine how the layer and drainage coefficients of Massachusetts Highway Department (MHD) base and subbase aggregates would be affected by the introduction of reclaimed asphalt pavement (RAP). MHD gray, dense, graded, crushed-stone base and gravel-borrow subbase were combined with 0 to 50 percent RAP (by weight); 9 mixtures were tested, and 27 resilient modulus and 12 hydraulic conductivity tests were conducted. Results from the resilient modulus tests indicate that the resilient moduli of the RAP/base and RAP/subbase mixtures generally increased with an increase in the percentage of RAP. Therefore, the layer coefficient and the structural number increased. The hydraulic conductivity of the RAP/crushed-stone base was not affected by the percentage of RAP in the mixture, but the hydraulic conductivity of the RAP/gravel-borrow subbase increased with an increase in the percentage of RAP. Nonlinear elastic analyses of typical MHD pavement structures indicated a small increase in the structural number with an increase in the percentage of RAP in the base and subbase. Therefore, the addition of RAP to the base and subbase materials tested is considered to be beneficial.