Half and Half
One year later, a mat with 50-percent RAP looks really good.
by P. Michael Harnsberger,
Western Research Institute
All photos courtesy of Western Research Institute.
A stretch of asphalt in Canada, laid down in September 2009, is beginning to reveal its secrets.
Initial tests on the road included examination of a section which used 50-percent Reclaimed Asphalt Pavement (RAP) provided by the old road it replaced. The first results are positive. And that’s good news to proponents of using higher percentages of RAP, a process which had environmental (fewer emissions) and budgeting (lower cost) plusses for road building.
In March 2009, the Asphalt Research Consortium (ARC), consisting of the Western Research Institute, Texas A&M University, the University of Nevada-Reno, the University of Wisconsin-Madison and Advanced Asphalt Technologies, met with Manitoba Infrastructure and Transportation (MIT) to discuss research being conducted by the ARC and the desire of MIT to incorporate “green” technologies into its asphalt paving construction. The ARC was working on a five-year research project funded by the Federal Highway Administration (FHWA) with significant emphasis on “green” technologies like higher levels of RAP, warm-mix technologies, and cold-mix technologies.
Both the ARC and MIT were interested in incorporating higher levels of reclaimed asphalt pavement (RAP) into pavements to reduce the cost of new pavement and to recycle the value and materials from the older pavements. Recycling of asphalt pavement into new pavement is not new, but using higher percentages of RAP in the surface lifts of pavement is not nearly as common. But constructing sections with high RAP content was not the only concern. MIT and the ARC wanted to monitor the performance of pavements to obtain valid data on the value of using d materials.
Designing the Road
In the early discussions, MIT was planning on using RAP on a 28.5-kilometer (about 17-mile section of Provincial Trunk Highway 8 from Gimli to Hnausa. The ARC was interested in using 50-percent RAP in the top two lifts of the pavement and providing asphalt binder and mix testing of the proposed materials prior to construction. MIT was also very interested and enthusiastic to try higher levels of RAP in surface courses and to have additional resources to help test and evaluate the materials. Since the old pavement on the project highway was being completely removed, it was advantageous to be able to use all of the available old pavement in reconstructing the highway. The final plan for the RAP pavement sections included a section with 50-percent RAP, a section with 50-percent RAP and a softer grade of asphalt, a section of 15-percent RAP, and a section of conventional hot-mix, all of which were to be constructed in sequential pavement sections in the same project using the same contractor and the same materials. The plan allows for a direct comparison of the effect of RAP on pavement performance.
The MIT project personnel conducted mix design testing of the four proposed mixes and provided materials to the ARC members at the University of Nevada-Reno (UNR), the University of Wisconsin-Madison (UWM), and Western Research Institute (WRI). The MIT mix designs adjusted the aggregate gradation to account for the gradation of the RAP at both the 15-percent and 50-percent levels. In some cases where high RAP content is planned, it is recommended that the RAP be fractionated into coarse and fine fractions using the ¼-inch sieve. Because high RAP content paving mixtures are not common in Manitoba, the contractor was not required to fractionate the RAP into coarse and fine fractions.
The mix designs used the Marshall mix design method with a target air void of 4-percent, 5.1-percent asphalt content, and 13-percent voids in the mineral aggregate (VMA). The geology of Manitoba, being mostly an ancient lake bed, makes it challenging to economically use substantial quantities of rock. Therefore, most of the aggregate gradations are on the finer side of the maximum density line and manufactured sand is used as a method to attain VMA. A plot of the Manitoba aggregate gradation is shown in Figure 1.
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