Performance-Based Mix Design for Porous Friction Courses (2018)

Below is the uncorrected machine-read text of this chapter, intended to provide our own search engines and external engines with highly rich, chapter-representative searchable text of each book. Because it is UNCORRECTED material, please consider the following text as a useful but insufficient proxy for the authoritative book pages.

1 Porous Friction Course (PFC) asphalt mixes are primarily used to improve safety by increasing the frictional properties of the pavement surface during wet weather, and reduc- ing the potential for hydroplaning by allowing surface water to drain through the pave- ment. Some countries have reported a 90%–95% reduction in backsplash and spray for PFC mixtures when compared to dense-graded mixes. This is especially beneficial at night during wet weather. When a film of water is on the pavement surface, it can reflect a vehicle’s headlights, and the glare can prevent the driver from following the pavement markings. The film of water also mitigates the reflective beads in the pavement markings and keeps the driver from being able to distinguish lane stripes. Despite these benefits, the use of PFC has diminished over the years due to durability and service life issues. The durability problems are generally evidenced by raveling, and once the distress begins, it progresses rapidly. Out of the 21 agencies that responded to the National Center for Asphalt Technology (NCAT) 2014 survey that are currently using PFC designs, the average service life for a PFC pavement is between 8 and 10 years. Agencies that were achieving greater than 12 years of service life for PFC were asked what, if any, special consideration was given when designing and maintaining PFC pavement. Their responses are as follows: • Eliminated the use of gravel, lightweight aggregate, and slag due to past performance issues. • Required fiber stabilizer in all mixes. • Replaced liquid anti-strip with hydrated lime. • Increased mix production temperature to 320°F, which resulted in improved smoothness and more uniform texture. Porous pavements in cold regions pose two major concerns, the first of which is the use of ice-control materials. These materials can cause the pavement to clog and subsequently lose permeability and increase the noise level between the tire and pavement. The 2014 NCAT survey showed that many state agencies do not use PFC mixtures because of prob- lems encountered with snow and ice removal. For this study six mix designs were selected based on their field performance and min- eralogy. Three aggregate types (granite, limestone, and traprock) were used in this study. Three poorly performing mix designs were selected from Florida (limestone), South Carolina (granite), and Virginia (traprock), while three well performing mix designs were selected from Florida (limestone), Georgia (granite), and New Jersey (traprock). Based on agency com- ments, the well performing mixes have had service lives up to 19 years before being replaced, while the poorly performing mixes were replaced within 10 years. To address PFC performance issues, several laboratory tests and performance parameters were used to evaluate the potential for durability problems when selecting the optimum S U M M A R Y Performance-Based Mix Design of Porous Friction Courses

2 Performance-Based Mix Design of Porous Friction Courses binder content of PFC mixtures. In Part 1, a performance-based mix design procedure was to be developed based on procedures described in ASTM D7064 and AASHTO PP 77. These procedures are similar to each other and include requirements for selecting materials, design gradation, optimum binder content, and performance testing for draindown, raveling, per- meability, and moisture susceptibility. The experiment in Part 2 was designed to evaluate the contribution of filler, binder modification, fiber, and thickness to nominal maximum aggregate size (NMAS) ratio to the resistance of PFC mixtures to raveling and cracking. Based on this research, the following guidelines for design of PFC mixtures are proposed. • Design gradation(s) can be selected for available aggregate materials based on the gradation bands shown in Table 1 of the revised procedure (Appendix A). These gradations can be selected depending on the primary objective of the mix. A PFC mixture can be designed with a coarse gradation for permeability and rutting resistance, or a fine gradation for noise reduction, and with high P-200 for improved durability. • The evaluation of the increased P-200 specimens showed marked improvement in terms of durability and cohesiveness of the designs. It is suggested that agencies revise PFC specifica- tions to allow 2%–6% P-200 in the mix. This revision will be helpful where raveling is the primary form of distress so long as drainage capability based on air voids and permeability can be attained. • Optimum binder content is determined for the selected gradation to meet the air void, Cantabro loss, and permeability criteria that are required to achieve the functional capability of a PFC mixture, as shown in Table 2 of the revised procedure (Appendix A). If test results show the mix design meets the air void, permeability, and Cantabro requirements, perfor- mance testing can be conducted for the design. Otherwise, the mix is redesigned with a new trial aggregate gradation. • Performance testing is then conducted on the mix design that meets the functional require- ments. Draindown and moisture susceptibility may be conducted. Optional test procedures that may be conducted to address performance concerns are cohesive shear strength, rutting, and cracking. If test results show the mix design meets all the performance requirements shown in Table 2 of the revised procedure (Appendix A), the mix design is finalized for report- ing. Otherwise, the mix design needs to be adjusted.