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Bennert wins K.B. Woods Award for third time
Dr. Thomas Bennert, director of CAIT's Pavement Resource Program, was named winner of the K.B. Woods Award for outstanding paper by the Transportation Research Board (TRB). The paper covers environmentally sustainable strategies to incorporate higher percentages of recycled asphalt pavement (RAP) in new paving projects. He and his co-authors Dr. Jo Sias Daniel (University of New Hampshire) and Dr. Walaa Mogawer (University of Massachusetts-Darmouth) will be recognized at the TRB Annual Meeting in January 2015.
The K.B. Woods Award was established by the TRB Executive Committee in 1971 and is given annually for an outstanding paper published in the field of transportation facility design and construction.
Only one other person has won the K.B. Woods Award more than Bennert; this is his third win in seven years. Bennert and Dr. Ali Maher were honored with the Woods award in 2008 for their paper “Field and Laboratory Evaluation of a Reflective Crack Interlayer in New Jersey,” and in 2011 for “Influence of Production Temperature and Aggregate Moisture Content on Performance of Warm-Mix Asphalt.”
State transportation agencies in the Northeast are promoting the use of sustainable materials in transportation infrastructure and investigating different strategies that will enable them to use higher percentages of recycled asphalt pavement (RAP) than they do now. The “trick” is not sacrificing pavement performance; fatigue and thermal cracking are particular concerns in the Northeast, and the general workability and handling of higher RAP content asphalt mixes is important in all regions and climates. [Photo of asphalt milling courtesy of Black Rock Enterprises, LLC, Old Bridge, NJ]
Bennert's winning paper this year, “Strategies for Incorporating Higher RAP Percentages: Review of Northeast States Implementation Trials,” recounts a study that looked at three main strategies: 1) Using a softer asphalt binder grade to offset the stiff RAP asphalt binder; 2) Limiting the amount of RAP binder credited to the total asphalt content of the asphalt mixture; and 3) Using a performance-based specification that the high RAP content mixture must meet for acceptance. The research involved actual field trials and evaluated commercially-produced RAP mixtures to provide insight as to how these different strategies performed.
Findings indicated the three strategies examined have different levels of complexity.
Using softer asphalt binders, requires minimal or no changes in the asphalt mixture design. However, when specifying a softer asphalt binder, there may be issues with local availability, which may increase general costs.
When limiting the percentage of RAP binder in the total asphalt content of the mix, generally, the asphalt mixture has to be redesigned since additional virgin binder would need to be added. Without a redesign, the mixture may fail established volumetric requirements, but after redesign is completed, the original performance grade (PG) mix typical in the region could still be used.
Employing performance-based specifications for high RAP mixtures, both a redesign and change in asphalt binder grade may be necessary, depending on performance requirements set by the state agency. However, unlike the other two strategies (using softer binders or limiting RAP content), DOTs can have a certain level of confidence that the high RAP mixture produced will perform to their expectations when utilizing performance-based specifications.
In this study, the softer binder grade strategy did result in slightly better low-temperature cracking properties in thermal stress tests, but was not as significant for critical cracking analysis. When evaluating the intermediate temperature cracking performance, the use of a softer binder grade did not improve the crack propagation resistance. This may indicate that production (mixing temperature, silo storage time, etc.) and mixture parameters (volumetrics, gradation, etc.) may negate or minimize the effectiveness of bumping to a softer PG graded binder.
For the RAP binder contribution project on Route 25 in New York, the 75 percent and 50 percent RAP mixtures both achieved better intermediate fatigue performance when compared to the baseline 100 percent RAP contribution mixture. Although the resultant increase in asphalt content due to limiting the RAP binder contribution clearly improved the intermediate fatigue performance, low temperature properties were not evaluated and definitive conclusions cannot be drawn. It should also be noted that this mixture only contained 20 percent RAP. However, the methodology does seem to be viable at higher RAP contents and a promising alternative.
A list of all K.B. Woods award winners is posted at trb.org/AboutTRB/WoodsAward.aspx.