Aggregate Absorption in HMA Mixtures

Designing hot mix asphalt (HMA) that will perform for many years is a complex balancing act of selecting an appropriate design asphalt binder content that is sufficiently high to provide durability but not so high as to lead to rutting problems. One of the factors that has to be considered during the design of HMA is the absorption of asphalt by aggregates. Most all aggregates used in the production of HMA have some absorptive characteristics. The objective of this project was to evaluate asphalt absorption through the production and construction process. In order to accomplish this objective, six on-going HMA construction projects were visited and HMA sampled. The HMA was sampled from four locations through the production and construction process, including: the slat conveyor prior to the mix being placed in the silo, trucks prior to transportation, the paver, and the compacted HMA layer (cores). Laboratory testing was conducted on these samples to evaluate the amount of asphalt that was absorbed by the aggregates throughout the production and construction process. Based upon the results of testing, it was concluded that asphalt absorption continues from the production process through the construction process. Samples obtained from the slat conveyor and the truck had similar average asphalt absorption values, while the paver and core samples also had similar average asphalt absorption values. However, the asphalt absorption at the paver and core locations were significantly higher than the other two. This suggested that most of the absorption took place during mix transportation to the job site. It was attempted to compare the amount of asphalt absorption that took place during mix design (based upon the job mix formula) to the amount of asphalt absorption that took place during the production and construction process; however, this attempt was not successful. Generally, the amount of asphalt absorption in the field was much higher than that shown on the job mix formulas.