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 structural performance, VDOT researchers are also looking at the positive environmental aspects of using pavement recycling. Recent work completed at the National Center 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 sections that are tested under natural environmental conditions with accelerated loading. Agencies and industry partners fund experiments on 200-ft-long test sections where controlled loading and embedded instrumentation allow researchers to quantify the pavement response during three-year test cycles. In 2012, VDOT sponsored the construction 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 elsewhere 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 designation 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 materials 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 constructed 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 existing 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 million 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 additional 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 quantify 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 suspected that Section N4 experienced some internal damage during the second research cycle, as evidenced by the increased maximum 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 Figure 2, the change in strain with respect to temperature was much less for Section S12. This was attributed to the cement-treated FDR layer, which was not expected to experience a change in stiffness with temperature changes. The temperature dependent behavior was expected for the CCPR layers and is known to happen in AC layers.
From the testing of the NCAT test sections, VDOT learned several key pieces of information regarding the structural performance of pavements built using CCPR and FDR. First, it was possible to build pavements using CCPR that could be expected to have long service lives under heavy truck traffic.
Virginia continues to be a leader in applying pavement recycling techniques on high-volume roadways, and the test sections’ measured responses supported this position. Second, the inclusion of the FDR base layer played a major role in the performance of Section S12 and was credited with keeping the strain values low, similar to other previously-built track sections determined to have perpetual performance. Third, since FDR equipment was used to produce the stabilized layer in Section S12, it suggested that a similar process could be followed elsewhere (either to stabilize existing pavement foundations or to create a stabilized foundation on newly built roads or added lanes using materials imported to the project site).
In addition to the structural response, the researchers also studied the economic and environmental impacts of including CCPR and FDR. Since Section S12 was considered a perpetual pavement structure, it was compared with two other pavement sections built in 2003 (that did not include CCPR and FDR) and were similarly regarded as perpetual. Table 1 shows unit costs for selected materials based on recently awarded VDOT pavement rehabilitation projects. Table 1 also shows the assumed density for each material required to convert from a weight-based unit cost ($/ton) to an area-based pavement section cost ($/SY). The upper part of Table 2 shows the as-constructed layer thicknesses of Section S12 and the two sections constructed in 2003 that did not include CCPR or FDR: 2003 N3 and 2003 N4 (coincidentally, the 2003 sections were also located on Sections N3 and N4 and the year of construction is used to differentiate them from the CCPR sections built in 2012). The lower part of Table 2 shows the pavement section cost ($/SY), the pavement structural number (SN) and the structure normalized pavement section cost ($/SY/SN). The pavement section cost was calculated by using the values shown in Table 1. The structural number was calculated by using typical VDOT pavement design values. Finally, the structure normalized pavement section cost was calculated by dividing the pavement section cost by the structural number to normalize the section costs with respect to structural capacity. By observing this normalized cost, it can be seen that Section S12, built using pavement recycling techniques, was approximately 30% less expensive per square yard.
In addition to cost, the environmental implications of the pavement structure are important to consider. The percent recycled content for each section shown in Table 2 was calculated as a weighted average per inch of material. Section S12 contained 76% recycled material; the two sections constructed in 2003 contained none. To characterize the environmental benefits of pavement recycling at NCAT completely, a life-cycle assessment (LCA) is needed. Although this has not yet been completed, the Federal Highway Administration released a case study in 2020 that did investigate VDOT’s I-81 project constructed in 2011 that included a similar combination of CCPR and FDR and were the same RAP source as Section S12. The authors calculated that materials and initial construction energy demand were reduced approximately 50 to 70%, and Global Warming Potential (GWP) was reduced by 40 to 70% compared to a “conventional” design (https://www.fhwa.dot.gov/pavement/sustainability/case_studies/hif19078.pdf).
One of the goals of VDOT’s pavements research program is to implement research findings into the business practices of VDOT. The findings of the research conducted at NCAT directly led to the use of pavement recycling techniques for the widening and reconstruction of Segments II and III on I-64 near Williamsburg. Both the widened and the reconstructed lanes used the combination of CCPR over an FDR foundation to produce a pavement structure having approximately 70% recycled materials. The work performed on Segments II and III included approximately 71 and 83 lane miles of recycled pavement, respectively. These quantities made them the second largest pavement recycling projects globally at the time of their respective construction.
VDOT continues to look for applications where pavement recycling can rehabilitate existing pavements and construct new lanes. By implementing the results of pavement research, VDOT is leading the nation in using pavement recycling techniques on high-volume roadways.