Considering the Lubrication Role of Asphalt Binders for Mix Design of Low-Carbon HMA

Abstract: The asphalt research community has done a significant amount of work on selecting proper mixing and compaction temperatures and proper binder content in asphalt mixtures. Most of the work done to date is focused on mixture volumetric properties and the basic measure of achieving 4% voids with just enough binder at selected temperatures and number of gyration (or compaction effort). We have seen continuous changes in how to determine processing temperatures, how to decide gyrations, and/or how to control the range of binder content. With the new requirements for “Low Carbon Materials”, this debate is back in focus. Binders contribute a major portion of CO2 emissions while heating and construction contribute the other major portion. It is estimated that up to 80 % of the emissions for delivery of Hot Mix Asphalt are related to these two factors (binder amount and production/construction of mix).
What could be missed in this emerging debate is the reality that asphalt binders are not only “binders”, but they are also “lubricants”. In other words, asphalt binders can reduce friction between aggregates during mixture mixing and compaction and allow easier and better packing of aggregates. If this realization is accepted, then the science of Tribology would help us address these new challenges for producing more effective and low-carbon asphalt mixes. In the 1990’s we in the asphalt industry had to learn about “Rheology”; perhaps we now need to learn about “Tribology.”
This presentation will share our recent work from the ongoing NCHRP IDEA project which is designed to develop a new tribology test to measure asphalt lubricating properties and propose a method to determine more optimum mixing and compaction temperatures. To date, a simple testing and analysis procedure, using the existing ball-on-plate geometry, in the DSR has been developed to measure the asphalt lubrication in the form of Stribeck curve, which is well-known in the Tribology field. Using the test, the effects of specialty polymers, Warm Mix Additives (WMAs) as well as softening agents on the lubricating properties of base asphalts are evaluated. The results show the additives’ effects vary significantly, and more than viscosity measurements are needed to define potential lubrication in the mix. To determine the relationship between the coefficient of friction of binders and asphalt mixture compaction in the lab, corresponding asphalt mixtures were designed and compacted. The analysis shows a good correlation between the asphalt mixture workability (evaluated from the gyration compaction curve) and asphalt lubricating properties at certain Sommerfeld numbers (SN). Such correlation confirms the critical role of asphalt lubricity in determining the workability mechanism of asphalt mixtures.
The study’s findings show the strong potential benefit of using asphalt lubrication rather than viscosity to optimize compaction temperatures, with or without mix modifications. It is evident that Tribology concepts could allow to significantly lower the asphalt mixture production and placement temperatures to reduce emission; improve the compaction density to achieve a long-lasting asphalt pavement, and; possibly further optimize the required binder (lubricant) content to reach the design air void.