VAA Spring/Summer 2020

30 SPRING/SUMMER 2020 Figure 4 shows the temperature normalized strain from those gauges located at the bottom of the CCPR layer. The data shown were normalized with respect to tempera- ture because the mid-depth pavement temperature over the time period varied from approximately 92°F (June) to 51°F (December). Without this normalization, other performance trends would be masked by the change in stiffness with respect to temperature. The trend in Figure 4 shows that the strain decreases with respect to time. The researchers suggest that this decrease in strain could be due to continued curing of the recycled layers. Additional curing will increase the stiffness of the pave- ment section and thus reduce the measured strain. Previous work by VTRC showed that stiffening of a recycled pavement section by curing in the field could be observed at least one year from construction. As noted previously, the maximum strain is expected at the bottom of the OGDL layer, below where the strain gauges were placed on Segment II. To calculate the strain on the bottom of the OGDL layer, the research- ers modeled the pavement system using a layered elastic software program (WESLEA for Windows). The primary model inputs included the pavement layer thicknesses and the stiffness of each layer. The thick- ness values were obtained from cores that were collected after construction. Initial stiffness values were taken from previous research efforts by VTRC and adjusted until the modeled strain at the bottom of the CCPR layer matched the values measured from the field instrumentation. Figure 5 shows the strain distribution within the pavement structure based on the modeling results. From Figure 5, it can be seen that the modeled strain at the bottom of the OGDL layer is less than approximately 50 microstrains, well within limits shown to result in a very long-lasting pavement. This compares favorably with the temperature normalized strain values from Section S12 at NCAT shown in Figure 6. Given that the pavement structure for Segment II is thicker than Section S12, a lower strain value is expected. Using these initial measurements, the researchers can begin to monitor the I-64 recycled pavement and determine the relative health and expected life of the pavement section. △ continued from page 29 Tensile Microstrain at 68°F Jun-2019 Jul-2019 Aug-2019 Sep-2019 Oct-2019 Nov-2019 Dec-2019 Jan-2020 45 40 35 30 25 20 15 10 5 0 Figure 4. Tensile Strain Data from I-64 Segment II (approximately 0.1 to 0.4 million ESALs). Depth, in Horizontal Microstain 0 5 10 15 20 25 40 30 20 10 -10 -20 -30 -40 -50 -60 0 AC CCPR OG FDR Figure 5. Modeled Tensile Strain for I-64 Segment II. AC = asphalt concrete, CCPR = cold central plant recycling, OG = open graded drainage layer, FDR = full depth reclamation. Tensile Microstrain at 68°F Nov-2018 Dec-2018 Jan-2019 Feb-2019 Mar-2019 Apr-2019 May-2019 Jun-2019 Jul-2019 Aug-2019 Sep-2019 Oct-2019 Nov-2019 Dec-2019 Dec-2019 Jan-2020 Feb-2020 Mar-2020 250.0 200.0 150.0 100.0 50.0 0.0 Figure 6. Tensile Strain Data from NCAT Section S12 (approximately 20 to 26 million ESALs). THE SCIENCE OF RECYCLING