VAA Virginia Asphalt Fall/Winter 2021

VAASPHALT.ORG 21 PERMEABLE PAVEMENT known at the time about the associated maintenance requirements and costs. This prevented the direct comparison of PPSs to other BMP options based on a life-cycle cost analysis. In response, VDOT undertook a study to better understand the long-term infiltration performance and the associated maintenance requirements of a porous asphalt PPS based on operating andmain- taining a parking facility constructed using this material. The study’s objectives were to determine the most effective maintenance practices for keeping the infiltration rate of porous asphalt at an acceptable level, to calculate the costs of these practices and to estimate how long the porous asphalt parking lot would serve a functional stormwater BMP. Having this performance, maintenance and cost information would help VDOT decide which BMPs to use in the future. The parking facility monitored was constructed in the spring of 2013. It is approximately 1 acre in size and serves as the overflow lot for a larger park and ride lot adjacent to the site. The area under the entire lot was constructed to serve as a reservoir and subbase, composed of 18 to 24 inches of No. 2 stone placed on top of a woven subgrade stabilization geotextile fabric. A 3-inch base of a perme- able asphalt mixture with a nominal maximum aggregate size of 19.0 mm was placed directly on top of the subbase. This was over- laid with 1.5 inches of a permeable asphalt surface mixture with a nominal maximum aggregate size of 9.5 mm. Binders used for the base and surface mixtures were PG 70-22 and PG 76-22, respectively. The lot was divided into four sections to evaluate the effectiveness of different maintenance practices, each receiving a different type of maintenance treatment or maintenance interval. Field infiltration measurements were taken in each section using multiple embedded single-ring, falling head infiltrometers. Values were captured after construction and before and after each maintenance treatment for approximately four years. The study found that regardless of the maintenance practices used, the average permeability of the porous asphalt lot dropped signifi- cantly over the 4-year study period, but at 123 in/hr, it remained well above the 10 in/hr threshold required for the site to be effective as a BMP (2). Based on the infiltration rates measured, the site was projected to infiltrate at a rate greater than the threshold for 12 to 15 years at a cost that was significantly less than the costs projected for other BMP options originally considered for the lot. Performance findings have been similar for a pervious concrete VDOT parking facility constructed near Salem. After several years of use, the site is still performing well, with monthly vacuuming (see figure 1). Still Room for Improvement in Performance Similar to VDOT’s projects, there are numerous studies docu- menting the performance of PPSs. Still, these systems have room for improvement. Work continues on methods to improve binder performance by incorporating additives such as nanosilica and fibers (3, 4). In addition, advances are being made with respect to maintenance options, optimal timing of maintenance based on site-specific parameters, and even complete rehabilitation options such as partial depth milling (5, 6). It is anticipated that all of these efforts will result in PPSs, and porous asphalt specifically, becoming a viable option for even more locations. continues on page 23 △ Completed Porous Asphalt Park & Ride Facility.