Verification of voids in coarse aggregate testing: Determining stone-on-stone contact of hot-mix asphalt mixtures

During NCHRP Project 9-8, Designing Stone Matrix Asphalt Mixtures, the voids in coarse aggregate (VCA) concept was selected as the preferred method to determine whether stone-on-stone contact existed in stone matrix mixtures. It is believed that stone-on-stone contact is important for open-graded friction course (OGFC) mixes to minimize the potential for rutting. Therefore, the same VCA requirements were adopted for OGFC mixes as had been developed for stone matrix asphalt mixtures. The objective of this study was to verify the VCA concept for defining stone-on-stone contact within OGFC mixtures by using digital imaging techniques such as analyzing particle contacts and the air voids size distribution. The effect of aggregate breakdown on stone-on-stone contact by the VCA method was also evaluated. The following conclusions are based on the information presented in this study. (a) A general guideline for determining the critical breakpoint sieve would be to select the finest sieve for which there is at least 10% of the total aggregate retained. This sieve size also should differentiate between the aggregate skeleton and filler particles. (b) Digital imaging provides a scientific method for identifying when stone-on-stone contact exists and supports the VCA. An advantage of digital imaging is that it can quantify the number of contacts between aggregate particles.