EVALUATING BMD CONVENTIONAL AND HIGH RAP SURFACE MIXTURES continues on page 13 △ Inside the environmental chamber, a dual tire carriage was mounted. A laser profiler, installed on the HVS carriage (Figure 1b) was used to conduct scans of the pavement surface and quantify the rut depth present at the surface. The loading was applied using a dual tire assembly, utilizing 11.00R22.5 tires inflated to a pressure of 105 psi. A data acquisition system (DAS) was also used to capture signals from various instruments. Experimental Lanes and Test Cells Six APT lanes were constructed with one mixture placed in each lane. Each lane had a length of 300 ft and a width of 10 ft. Within each lane, five test cells were labeled A through E. Each test cell had a length of 24 ft and the five test cells were positioned within the center 200 ft of each lane. Test cells A, C, and E were instrumented with strain gauges, pressure cells, moisture sensors, and thermocouples to collect pavement response data. Test cells B and D served as backup test cells to address unforeseen issues or unexpected damage during the experiment. Figure 2 provides a visual representation of the site layout for the APT testing. The pavement structure was consistent across all six constructed lanes. Each lane was built with a research mixture consisting of two 1.5-inch lifts, with a total thickness of three inches. Each research mixture was placed on top of a 12-inch VDOT 21B base layer, followed by an additional 26-inch layer of VDOT 21B as a subgrade layer. Loading Conditions For each rutting test, there were three phases applied to each cell: an initial wheel load of 9,000 pound force (lbf) for the first two weeks, followed by 12,000 lbf for approximately one week, and then finished with 15,000 lbf for approximately one week. This progressive loading protocol was designed to expedite the development of pavement distresses. The internal pavement temperature was maintained at 40°C (104°F). For the cracking test, the loading protocol implemented for each cell included a wheel load of 15,000 lbf until three million equivalent single axle loads (ESALs) was applied. The internal pavement temperature was maintained at 20°C (68°F). Rutting Experiment Measurements Figure 3a shows the rut depth measurements collected within the 12 rutting test cells, with 2 rutting cells (R1 and R2) for each lane. Mixture 45_HR showed the poorest rutting resistance compared to the other five mixtures, as indicated by its rapid accumulation of rut depth. This performance can be attributed to the significantly higher flexibility resulting from the high OBC used in the design and production of mixture 45_HR. The rutting performance of the remaining five mixtures appeared comparable. A ranking among the corresponding test cells R1 and R2 also proved difficult. Further investigation into the effect of environmental aging was conducted to explain the observed differences in performance between test cells R1 and R2. The progression of rut depth for each mixture, normalized with respect to aging and averaged with respect to cell R1 and R2, is shown in Figure 3b. All BMD HRAP mixtures showed higher rut depths compared to the control mixture 30_C. This can be attributed to the inclusion of an increased binder content, along with the use of an RA, and softer binder grade in BMD HRAP mixtures to balance the mixture and enhance the cracking resistance. Figure 1. (a) Dynatest Mark VI Heavy Vehicle Simulator; (b) Laser Profile Mounted on HVS Carriage (a) (b) 0 50 100 150 200 250 300 ft 10ft N 45_HR_L 30_O 60_HR_L_RA 45_HR 45_HR_RA 30_C L6 L5 L4 L3 L2 L1 A B B B B B B C C C C C C D D D D D D E E E E E E A A A A A Rutting Study Cell Cracking Study Cell Figure 2. Site Layout for the Accelerated Pavement Testing Experiment L1 through L6 = lane numbering; N = North; C = control; O = optimized; HR = high reclaimed asphalt pavement (RAP); RA = recycling agent; L = softer virgin binder grade 0 100,000 200,000 300,000 400,000 500,000 600,000 Measured Rut Depth, mm ESALs 20 18 16 14 12 10 8 6 4 2 0 30_C_R1 45_HR_R1 45_HR_L_R1 30_C_R2 45_HR_R2 45_HR_L_R2 30_O_R1 45_HR_RA_R1 60_HR_RA_R1 30_O_R2 45_HR_RA_R2 60_HR_RA_R2 (a) Figure 3. (a) Rutting Depth for 12 Rutting Test Cells; (b) Normalized Average Rut Depth Values / Curves. ESALs = equivalent single axle loads; C = control; O = optimized; HR = high reclaimed asphalt pavement (RAP); RA = recycling agent; L = softer virgin binder; R1 and R2 = rutting cells 00.511.522.533.544.55 Normalized Rut Depth, mm ESALs x105 25 20 15 10 5 0 30_C 30_O 45_HR 45_HR_RA 45_HR_L 60_HR_L_RA (b) VAASPHALT.ORG 11
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