Better Roads Digital Magazine
Issue link: http://read.dmtmag.com/i/276457
Better Roads March 2014 5 Evaluation of WMA performance in Florida Use of warm-mix asphalt is indicated for paving in cold am- bient conditions or long hauls from the plant, during which the temperature of hot-mix asphalt can swiftly decline to the point that it loses workability. Before WMA, plants would produce mixes at higher-than- optimum temps in the hopes that the mix would retain work- ability on arrival at the jobsite. But this excessive heat had the effect of "prematurely aging" the liquid binder in the mix by burning off the lighter hydrocarbons that give the pavement resistance to thermal cracking and other ills. Typically, those lighter fractions would be driven off by slow weathering and aging during the years. But WMA is useful for hot climates as well, as their volatile organic compound (VOC) emissions are substantially below that of HMA mixes, enhancing working conditions and the urban environment. One by one, states are evaluating the benefits of WMA – which comes at a higher price than con- ventional HMA – and Florida is that latest warm-climate state to review its use. In Florida, WMA performance in terms of cracking, rutting, and pavement smoothness is comparable to that of HMA, say Bouzid Choubane, Sanghyun Chun, Hyung Suk Lee, Patrick Upshaw, James Greene and Abdenour Nazef, of the Florida DOT Materials Research Park in Gainesville, in their paper, Evaluation of Warm-Mix Asphalt (WMA) Performance in Florida. They say with WMA, asphalt fumes and 26 aerosols have been reported to be reduced by 30 to 50 percent, while reductions reported for carbon dioxide (CO 2 ) and sulfur dioxide (SO 2 ) have been in the range of 30 to 40 percent. Reductions of 10 to 30 percent of carbon monoxide (CO) and 20 to 25 percent of dust also have been reported. Florida DOT started using the WMA technology in 2006 based on reported benefits due to lower asphalt mixing and compac- tion temperatures. "Limited field and laboratory studies have indicated that WMA may have similar performance as traditional hot-mix asphalt," they write. "However, some studies have also sug- gested that the use of WMA may increase long-term rutting and stripping potential, and hence the long-term perfor- mance evaluation of the WMA mixtures is still warranted." With the primary objective of obtaining long-term field performance data of Florida's WMA, the respective field performance of six representative WMA projects using five different technologies (one chemical additive and four water foaming technologies) was monitored and documented. The performance of the WMA projects was then compared to the historical performance data of Florida's asphalt pavements. "The results to date indicate that WMA performance in terms of cracking, rutting and pavement smoothness is comparable to that of HMA," the authors conclude. "These projects will continue to be monitored to further assess their long-term field performance." WMA modifier eliminates drain-down fibers from OGFCs In addition to its other benefits, at least one warm-mix asphalt modifier can eliminate the expense of adding fibers to liquid binder used for open-graded frictions courses, say Bradley J. Putman and Kimberly R. Lyons, Glenn Department of Civil Engineering, Clemson University, South Carolina, in their paper, Evaluation of Warm Mix and Rubber Modified OGFC Test Sec- tions Made Without Fibers in South Carolina. Open-graded friction courses (OGFCs) enhance the tire-pavement interface while keeping surface water off the pavement, making them safer than conventional pavements. But because they lack fines, drain-down of liquid binder in early OGFCs quickly led to raveling of their surfaces, so much that they were banned by some departments of transportation (DOTs). Since then, the advent of polymer modified asphalt binder has led to more durable OGFCs. While modifiers will help keep liquid asphalt binder from moving within an open- graded mix, it still may puddle at the bottom of a haul truck or settle lower in the lift of mix just placed. This drain-down of asphalt seriously compromises the du- rability of OGFCs. The problem with the drain-down of liquid asphalt is solved by use of fibers – typically cellulose, but also mineral filler – which are used to hold the binder in place. But research presented at TRB indicates a WMA modifier can halt drain-down in OGFCs as well. Pulman and Lyons' paper summarizes the performance of OGFC mixtures used to construct two separate experimental pavement test sections in South Carolina, with the main objective to evaluate OGFC mixtures made without fibers. One test section consisted of three OGFC mixtures (control with cellulose fibers ground tire rubber modified without fibers, and WMA without fibers), and the second consisted of two OGFC mixtures (control with cellulose fibers and WMA without fibers). "Material was sampled from the asphalt plant during pro-