Laboratory Characterization of Crumb Rubber Modified Stone Mastic Asphalt (SMA) Using Dynamic Modulus

Abstract

Increased and uncontrolled axle loads, and high temperatures stimulates distresses and premature failures in flexible pavements that incur additional costs in the form of pavement rehabilitation. Stone mastic asphalt (SMA) can be utilized to address this issue due to its stone-to-stone aggregate skeleton that resists load and reduces deformation. Polymer modification of asphalt is gaining recognition and mixtures are being designed with modified binder to improve the performance of the pavement over the design life. Crumb rubber is a polymer modifier obtained from scrap tires that helps increase stiffness of the binder and improve performance by reducing the intrinsic vulnerability of binder to temperature. This study utilizes crumb rubber modified SMA for dynamic modulus testing at four temperatures and six frequency sweeps. The experimental design included preparation of SMA 19 specimens with 6 different percentages of crumb rubber (0%, 4%, 6%, 8%, 10%, and 12%) mixed in bitumen. Addition of crumb rubber to the mix translated in an increase in the stiffness of the mix, as 64% increase in dynamic modulus (at average) was reported at 10% CR as compared to neat mixture. Master curves were produced using |E*| test results which revealed that 10% modified SMA was relatively stiffer as compared to the other mixtures. Two level factorial design of experiment was conducted for dynamic modulus and phase angle. Sensitivity analysis concluded that an increase in temp from 21.1 to 37.8° C translated in a 65% decrease in |E*| (at average) and an increase in frequency from 0.1 to 25 Hz dictated a 72% increase in |E*| (at average). Regression model (R 2 =75.3%; MAPE=13.82%) was generated to predict dynamic modulus as a function of temperature, frequency, and Crumb rubber. These research findings can be utilized for the deployment of crumb rubber modified SMA mixtures for heavily trafficked areas and implementation of mechanistic empirical pavement design for better performance and increased service life of pavements.