Critical Evaluation of Use of the Procedure of Superpave Volumetric Mixture Design for Modified Binders

Abstract

This study examined the possible interference of modified binders with the standard procedure of Superpave^® volumetric mixture design. Sensitivity of volumetric properties and moisture damage performance were evaluated with four binders modified with and without polymeric additives. The study focused on effects of compaction temperatures and vertical pressure on densification of mixtures and on effects of compaction temperatures on moisture damage. The zero shear viscosity concept was used to estimate acceptable mixing and compaction temperatures. The compaction energy index was used to study changes in compaction effort, and cohesion and adhesion of binders were measured to study moisture damage potential. Results indicated that temperature effects were somewhat marginal and that unless temperatures were reduced to below 80°C, effects on volumetric properties were rather small. It was also found that using a target zero shear viscosity of 1.5 Pa·s for mixing and 3.0 Pa·s for compaction could provide reasonable compaction temperatures for binders used in this study. Varying vertical stress used in the gyratory compactor showed significant effects on volumetric properties. Changing pressure from 600 kPa to 300 kPa resulted in 3.0% to 4.0% increase in air voids, more significant than changing viscosity by more than 10 times. It is postulated that the focus on keeping temperature high during compaction in the lab and in the field might be unfounded. Moisture damage does not appear to be affected significantly by the variation in binder viscosity profiles for the binders and aggregates used in this study. Because of the limited sample size, more work is needed in this area to study the effect of mixing and compaction temperatures on moisture damage.