Optimising water content in cold recycled foamed asphalt mixtures

During cold recycling, water is added to facilitate the dispersion of foamed asphalt in the mixture and to achieve uniform mixing and help compaction by providing sufficient lubrication. Too little water may cause difficulty in workability and compaction of the mixture, but too much water may extend the curing time and reduce density and strength. Therefore, the optimum water content (OWC) was considered as one of the most important factors in mix design procedures for cold recycling. Currently, mix design procedures for cold recycled foamed asphalt mixtures suggest adding water to the mixture at an optimum content to facilitate mixing and compaction. However, there is no standard method for determining the optimum total water content (OTWC) for cold recycling mixtures. Several empirical relationships were developed to determine the OTWC based on modified Proctor test results for Reclaimed Asphalt Pavement (RAP)/aggregate. However, the compaction effort in the modified Proctor test for RAP/aggregate may not match that for mixtures, which is compacted using the Superpave Gyratory Compactor (SGC) or Marshall hammer. A study is underway to improve the design method for cold recycled foamed asphalt mixtures with 100% RAP. The purpose of this paper is to optimise the design procedure by developing a new method to determine OTWC. SGC was used to compact RAP instead of the modified Proctor test to match the compaction effort recommended for foamed asphalt mixtures. A regression model was developed to calculate the OTWC for a mixture based on the determined OWC of the RAP, foamed asphalt content, and binder type as factors. The method for determining OTWC for a mixture was validated using six different mixtures and was found to correlate well with the measured OTWC, even though two of six mixtures had underestimated OTWC due to different binder source. Further comparisons with other two OTWC determining methods showed the mixtures at the proposed OTWC had improvement in indirect tensile strength.