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S U M M A R Y The objective of this project was to recommend design, construction, maintenance, and rehabilitation guidelines that will maximize the advantages and minimize the disadvantages associated with the use of permeable friction courses (PFCs). The research approach entailed two primary tasks: an annotated literature review and a survey of state departments of trans- portation. As no laboratory or field work was conducted as part of this project, a significant amount of time and effort was spent on the literature review and survey of agencies. Infor- mation gathered from these two activities was categorized according to the following sub- jects: general use of PFCs, benefits of the use of PFCs, materials and mix design, inclusion of PFCs in structural design, construction of PFCs, maintenance of PFCs, rehabilitation of PFCs, performance of PFCs, and limitations on the use of PFCs. For each of the subjects listed above, the information gathered from the literature review and survey of agencies was used to develop a state-of-practice on the use of PFCs. This state of practice is considered to be representative of practices used around the world, as a signif- icant amount of literature was obtained and reviewed from other countries. The informa- tion gathered also was used to develop guidelines on the use of PFCs in order to accomplish the project objectives. PFCs have been used since the 1970s. The initial use of PFCs was in Europe. Europeans took the U.S. version of open-graded friction courses developed in the 1930s through the 1970s and, through research, improved the performance of these mixes. Improvements pri- marily included the use of modified asphalt binders and fibers. The modified binders and fibers alleviated some of the problems that were encountered with open-graded friction courses in the United States. Benefits realized from the use of PFCs are primarily associated with improved safety. PFCs have been shown to improve wet weather frictional properties, reduce the potential for hydroplaning, reduce the amount of splash and spray, and improve visibility. Other ben- efits identified in the literature included resistance to permanent deformation, smoother pavements (and, hence, improved fuel economy), reduced tire/pavement noise levels, and other environmental benefits. Materials and mix design properties specified for PFCs were obtained from around the world. Materials used to comprise PFCs are coarse aggregates, fine aggregates, asphalt binders, and stabilizing additives. Stabilizing additives are used in PFCs to minimize the potential for draindown because draindown was identified as a major problem with open-graded friction courses during the 1970s and 1980s. Numerous methods of designing PFC mixes were identified; based on the information, the design of PFC mixes includes four primary steps: selection of appropriate materials, selection of a design aggregate gradation, selection of optimum asphalt binder content, and performance testing. Construction and Maintenance Practices for Permeable Friction Courses 1
2For the most part, PFCs are not given structural value within pavement structures. The lit- erature did provide evidence, however, that PFCs do lead to cooler temperatures in under- lying pavement layers. Cooler temperatures within the underlying layers provide a net in- crease in stiffness within these layers. This alone indicates that PFCs do add some structural value. The literature also suggested that there are two properties needed in order to establish a minimum lift thickness for PFCs: rain intensity and the permeability characteristics of the PFC layer. This information was used to develop a simple method for determining the min- imum lift thickness for a PFC layer. Construction of PFC layers is similar to most hot-mix asphalt (HMA) mixes with some slight differences. The primary difference in production of PFC is incorporation of stabiliz- ing additives, namely fibers, because special equipment is needed for introduction of fibers. An important step in the construction process is transportation. Precautions should be taken to minimize the amount of cooling that occurs during transportation. Compaction of PFCs also is slightly different than for typical HMA; compaction is not conducted to meet some specified density, but rather, compaction is conducted to seat the aggregates. Only steel wheel rollers are used on PFCs, as vibratory rollers tend to fracture aggregates during com- paction and pneumatic tire rollers tend to pick up the PFC. The survey of agencies suggested that none of the agencies in the United States currently conduct operations to clean clogged PFC layers. Other general activities include preventative surface maintenance and corrective surface maintenance. Winter maintenance on PFCs is a perceived problem worldwide. Definitive methods for addressing PFC winter maintenance were not identified during this project. The literature suggested that experience was the only method for developing a winter maintenance program. However, the literature was explicit that PFC layers require a different winter maintenance program than typical dense-graded layers. PFCs reach freezing temperatures before dense-graded layers and stay at a freezing temperature longer. Therefore, more winter maintenance materials are required. Rehabilitation of PFC layers is reasonably uniform around the world. In most instances, rehabilitation involves milling the existing PFC layer and replacing it with another PFC layer or another type of HMA. The literature did suggest that PFCs should not be overlaid unless they are sufficiently sealed. The performance of PFC layers was divided into two separate categories: service life and performance life. Service life was defined as the length of time a PFC maintains its frictional properties and smoothness. Performance life was defined as the length of time the PFC maintains its beneficial properties. No specific literature was found that tracked the service or performance life of PFC layers. However, the literature and survey of agencies did sug- gest that most agencies expect 8 to 10 years of service life. Limitations identified within the literature were essentially either related to winter events or clogging of PFC layers. Areas prone to heavy snowfalls are not recommended for placement of PFC layers. Areas that contain a lot of dirt or debris (e.g., near farms) also are not recom- mended for PFC placement. Other situations where PFCs are not recommended include proj- ects that require long haul times, inlays, projects that require a lot of hand work, and critical pavement locations, including intersections or locations with heavy turning movements.
