VAA Virginia Asphalt Fall/Winter 2021

14 FALL/WINTER 2021 Brian Diefenderfer, Ph.D., PE, Principal Research Scientist, Virginia Transportation Research Council PAVEMENT RECYCLING Virginia Department of Transportation’s Research Program Over the last 10 to 12 years, the Virginia Department of Transportation has invested in a research program to investigate the ways pavement recycling can be used to reduce costs and improve the performance of VDOT’s pavement network. In addition to cost reductions and struc- tural performance, VDOT researchers are also looking at the positive environmental aspects of using pavement recycling. Recent work completed at the National Cen- ter for Asphalt Technology (NCAT) Test Track, located near Auburn University, has become a cornerstone of this effort. The NCAT Test Track is a 1.7-mile oval with more than 40 experimental pavement sec- tions that are tested under natural environ- mental conditions with accelerated loading. Agencies and industry partners fund exper- iments on 200-ft-long test sections where controlled loading and embedded instru- mentation allow researchers to quantify the pavement response during three-year test cycles. In 2012, VDOT sponsored the con- struction of three test sections to explore the potential structural benefits of using Cold-Central Plant Recycling (CCPR) and Full Depth Reclamation (FDR). CCPR and FDR are two pavement recycling techniques that have been used in Virginia and else- where in the U.S. Still, data regarding their in-place structural properties under heavy traffic was limited. Figure 1 shows the as-designed pavement structure for Sections N3, N4 and S12. The section name’s N or S letter designa- tion refers to the north and south tangent, respectively. The number portion of the section name is the sequential section number going around the track. All three sections included CCPR that consisted of 97% reclaimed asphalt pavement (RAP), which came from the I-81 recycling project built in 2011, 2% foamed asphalt, and 1% cement. That is, 97% of the CCPR mate- rials used came from a recycled material (RAP) known to be plentiful in urban areas of Virginia. Sections N3 and N4 included the same pavement design except that the asphalt concrete layer was two inches thicker in section N3. Likewise, N4 and S12 have the same design, except the aggregate base used in Section N4 was replaced by a cement stabilized layer in Section S12 con- structed using FDR equipment to mimic an FDR layer. In practice, FDR usually consists of RAP, aggregate base and a portion of the underlying subgrade; the FDR in Section S12 was completed by stabilizing the exist- ing aggregate base and a part of the existing subgrade but did not include RAP. All three test sections were trafficked between 2012 and 2017 for a total of 20 mil- lion equivalent single axle loads (ESALs). This is an equivalent traffic level to about ten years on I-81, where Virginia sees some of the highest truck traffic in the state. However, starting in 2018, only Sections N4 and S12 were trafficked for an addi- tional research cycle. At this time, the instrumentation responses began to show that Sections N3 and S12 were the better performers, with Section S12 being the best of the three. Therefore, between 2018 and 2020, an additional 10 million ESALS was applied to Sections N4 and S12, totaling 30 million ESALs (approximately 15 years of I-81 traffic). Instrumentation to measure pressure, strain and temperature within each test section were included during construction to quan- tify the performance of each pavement. Lower and more consistent pressures and strains generally indicate a well-performing pavement that can carry loading with little internal damage. Figure 2 shows the strain response for the VDOT test sections. From Figure 2, it can be seen that Section N4 had the highest and most variable strain responses while Section S12 had the lowest and most consistent. Therefore, it was sus- pected that Section N4 experienced some internal damage during the second research cycle, as evidenced by the increased maxi- mum strain with respect to time. In contrast, Section S12 was considered to respond as a perpetual pavement structure given low strain values observed over the 30 million ESALs applied. As seen in Fig- ure 2, the change in strain with respect to temperature was much less for Section S12. 4-inch AC 4-inch AC 5-inch CCPR 8-inch FDR Subgrade 5-inch CCPR 6-in Agg Base Subgrade N3 N4 S12 6-inch AC 5-inch CCPR 6-inch Agg Base Subgrade Figure 1 VDOT Test Sections Constructed in 2012.