Investigation of Warm Mix Asphalt Additives Using the Science of Tribology to Explain Improvements in Mixture Compaction

Abstract

Part of asphalt pavement production is the compaction process of loose mix. The quality of compaction process has a profound impact on the quality of the final product. With the advent of Warm Mix Asphalt (WMA) additive use, the compaction process became more efficient. In 2010 estimated 4 million tons of WMA asphalt were produced using the WMA additive technology (Hansen and Newcomb 2011). The additives allowed for improved compaction (requiring less compactive effort) and compaction at lower-than-typical temperatures. This thesis is focused on quantifying the improved mixture compaction through novel binder testing. The science of tribology has been used to understand and quantify the way certain WMA additives improve compaction process. It has been found that chemical WMA additives are capable of creating boundary lubrication films protecting aggregate particles from direct solid-solid contact, hence reducing friction between the particles. The new Asphalt Boundary Lubrication test was developed to quantify the boundary lubrication of binders in terms of boundary coefficient of friction. The test quantified the influence of aggregate mineralogy on the lubricating properties of certain WMA additives. It was also discovered that WMA modified binders improve boundary lubrication as compared with control binders. The binder asphalt boundary lubrication test results and mixture volumetric density data show good ranking. Additionally, at high testing temperature (145�C), boundary coefficient of friction provided explanation for changes in mastic viscosity, which can be well correlated with mixture volumetric data.