Long-term draindown is the term adopted in this study to describe the downward flow of binderin a porous asphalt mixture due to the force of gravity at high temperatures experienced duringservice, especially in warmer climates. The concept of long-term draindown has beenqualitatively noted in field observations of porous asphalt mixtures including open gradedfriction courses (OGFC). This research quantified the long-term draindown of three differentOGFC mixtures by measuring the permeability of the mixes throughout an 84-day conditioningperiod at 140oF (60oC). The results showed that the permeability of all three mixtures decreasedover the first 56-days and then leveled off, or slightly increased for the remaining 28-days. Thepermeability data was used to calculate the rate of permeability loss over the initial 56-dayperiod for each mix. To verify that the permeability reduction was due to binder draindown, thedistribution of binder content of randomly selected specimens was measured. The results fromthis study indicated that cellulose fibers did not have any influence on the long-term draindownof the mixtures evaluated. Also, the mixture made with a ground tire rubber (GTR) modifiedbinder exhibited a lower rate of permeability loss compared to the other two mixtures made withan SBS modified binder. Finally, one of the mixes evaluated was produced as a warm mixOGFC using Evotherm™ 3G and its long-term draindown potential was similar to the control mixeven though this mix did not contain fibers and it had a similar binder content as the other mixes.
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