Laboratory Evaluation of Long-Term Draindown of Porous Asphalt Mixtures

Abstract

Long-term draindown is the term adopted in this study to describe the downward flow of binder in a porous asphalt mixture due to the force of gravity at high temperatures experienced during service, especially in warmer climates. The concept of long-term draindown has been qualitatively noted in field observations of porous asphalt mixtures, including open graded friction courses (OGFC). This research quantified the long-term draindown of three different OGFC mixtures by measuring the permeability of the mixes throughout an 84-day conditioning period at 60°C. The results showed that the permeability of all three mixtures decreased over the first 56 days and then leveled off or increased for the remaining 28 days. To verify that the permeability reduction was due to binder draindown, the distribution of binder content of randomly selected specimens was measured. The results from this study indicated that cellulose fibers did not have any influence on the long-term draindown of the mixtures evaluated as the other mixtures [one made with ground tire rubber modified binder and the other with Evotherm 3G (MWV Specialty Chemicals, North Charleston, South Carolina)] exhibited slightly lower rates of permeability loss despite not containing fibers.