A Study of Air Voids and Effective Air Voids in Hot Mix Asphalt

The concept of air voids (AV) continues to be used as the primary control parameter in the mix design of hot mix asphalt (HMA). AV are estimated from the bulk specific gravity and the maximum specific gravity. Their accurate estimation is very important, since samples for various tests are compacted to target AV, which are calculated on their basis. This study explored the impact of gradation and asphalt content on the AV, effective air voids (EAV), and apparent and maximum specific gravity of a fine- and a coarse-graded HMA. Samples were tested for bulk specific gravities, apparent and maximum specific gravities, rutting tolerance index, and cracking tolerance index. The results indicate that significant differences exist between AV measured by the different procedures and EAV, and that the differences are greater for a coarse-graded mix than for a fine-graded mix. Major sample-to-sample variations exist for the apparent specific gravity values, which could be significantly different from the generic maximum specific gravity values that are used for each mix. This has a significant impact on the estimation of AV and variability of test results. A strong correlation between the ratio of EAV and AV is presented. Based on the results of this study, and because EAV are strongly correlated to permeability and aging, it is recommended that the concept of EAV be considered in the regular mix design of coarse-graded HMA.