MODIFICATION OF PAVING ASPHALTS BY DIGESTION WITH SCRAP RUBBER

Abstract

The service performance of sprayed surface treatments laid where traffic-induced stress is severe or where the existing pavement is cracked can be significantly improved if comminuted scrap rubber is digested in the asphaltic cement before spraying. Work performed to identify the factors that are of importance in optimizing asphalt performance is reported. Measurement of the deformation response of the rubber digestions to sinusoidal loading indicated that the modified binder had improved response under the loading produced by traffic at high pavement temperatures and that produced by thermal contraction at low pavement temperatures. A simple and rapid test procedure was developed to measure the response under loading conditions similar to these. The procedure is to subject a prism of the binder at 60 deg C to a shear strain of 1.0-in creep and then determine the elastic recovery when the stress is removed. The most important factor affecting elastic recovery was found to be the morphology of the rubber particles as determined by the comminution process used in their manufacture. A simple bulk-density test was used to characterize this morphology. Digestions of natural (truck-tire) rubber are generally superior to those that incorporate synthetic (car-tire) rubber but are more affected by changes in the time or temperature of digestion. Where a cryogenic comminution process had been used, only digestions of natural-rubber particles produced a significant improvement in asphalt properties. Elastic recovery of strain is linearly related to rubber concentration. (Author)