THE ROLE OF PESSIMUM VOIDS CONCEPT IN UNDERSTANDING MOISTURE DAMAGE TO ASPHALT CONCRETE MIXTURES

On the basis of a hypothesis that voids in asphalt pavements are a major source of water damage, a test system was developed by Oregon State University as part of SHRP Project A-003A to evaluate the major factors that influence water sensitivity. The Environmental Conditioning System (ECS) was used to develop a test procedure that includes measurement of permeability, vacuum wetting (partial saturation), cycling at various temperatures, and continuous repeated loading while monitoring the resilient modulus after each conditioning cycle. The development aspects of the ECS conditioning procedure will not be discussed in this paper, as they have been documented elsewhere by Terrel and Al-Swailmi. This paper gives a brief overview of the theoretical aspects of water sensitivity, followed by a more detailed description of the role of air voids and water accessibility of asphalt mixtures in the mechanism of the water sensitivity. If asphalt concrete specimens are water conditioned, the retained strength is typically somewhat lower than that for the original dry mixture. This effect tends to be tempered by the voids in the mixture. Mixtures with minimal voids that are not interconnected are essentially impermeable. When air voids increase beyond some critical value, they become larger and interconnected. The test results show that the worst behavior in the presence of water occurs in the range where most conventional mixtures are compacted. Thus, the term "pessimum voids" can be used to describe a void system (i.e., the opposite of optimum).