Construction and Maintenance Practices for Permeable Friction Courses (2009)

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24 Within the United States, OGFC has been used to describe HMA with an open aggregate grading that is used as a wear- ing layer to improve frictional properties. These mixes evolved through experimentation with plant mix seal coats. The ini- tial interest in these mix types came from problems associated with chip seals. Primarily, loose aggregates that either were not adequately seated during construction or dislodged by traffic were breaking windshields. Additionally, there was a time con- straint problem with setting the chip seal aggregates during a sudden rainstorm (3). During the 1930s Oregon began exper- imenting with the plant mix seal coats to improve frictional properties. During the 1940s, California also began using the plant mix seal coats as drainage interlayers and as an alterna- tive to chip seals and slurry seals. During the late 1940s, a num- ber of the western states began to use these mixes to improve frictional properties. An additional benefit when using these plant mix seal coats as a wearing layer was that hydroplaning and splash/spray was reduced (3). Even though plant mix seals provided excellent frictional properties and reduced potential for hydroplaning and splash/ spray, their use did not become widespread until the 1970s. The primary problems with these mixes were related to dura- bility and draindown. Because the plant mix seals had an almost uniform aggregate gradation with little fine aggregate, there was very little aggregate surface area. The term draindown describes the asphalt draining from the aggregates during stor- age and transportation. Asphalt binder that has drained from the aggregate structure results in pavement areas that have too little asphalt binder and areas that are very rich in asphalt binder. Areas without a sufficient amount of asphalt binder were prone to raveling, while areas rich in asphalt binder could become slick and did not provide the desired frictional properties. In the 1970s, the FHWA initiated a program to improve the skid resistance of the nation’s roadways (4). The plant mix seal coats were one of the tools an agency could use to improve frictional resistance and gained popularity. According to the 1978 NCHRP Synthesis Number 49, these plant mix seals became known as OGFCs (5). In 1980, the FHWA published a mix design procedure for these mix types (6). The procedure entailed an aggregate gradation requirement, a surface capac- ity of coarse aggregate, determination of fine aggregate con- tent, determination of optimum mixing temperature, and resistance of the designed mixture to water. OGFC mixtures designed in accordance to the FHWA procedure were suc- cessful at performing their intended function: removing water from the pavement surface and improving frictional resis- tance. However, a number of states noted that the OGFC pave- ments were susceptible to sudden and catastrophic failures (7). Failures observed during the 1970s and 1980s were caused by mix design, material specification, and construction prob- lems. These problems primarily involved mix temperature during construction. Gradations associated with the OGFCs of the 1970s and 1980s were much coarser than typically used dense-graded mixes (Marshall and Hveem designed mixes). Additionally, few states were using modified asphalt binders. Because of the open nature of the aggregate gradings and neat asphalt binders, there were problems of draindown during transportation to the project site. To combat the draindown problems, most owners would allow contractors to reduce the mixture’s temperature during production. The drain- down and mixture temperature problems led to catastrophic raveling and delamination, respectively. These problems were of such magnitude that a number of states put a moratorium on the use of OGFC mixtures during the 1980s. A survey of state highway agencies conducted by Kandhal and Mallick in 1998 (8) indicated that 19 states (38 percent) were currently using OGFCs. Over 70 percent of the states using OGFCs reported service lives of 8 years or more. The vast majority of the states reporting good performance indicated the use of coarser gradations than the FHWA mix design pro- cedure (8) required and the use of stiffer, polymer-modified binders. The question must be asked, “If OGFCs did not perform in the 1970s and 1980s, why did states continue to evolve them such that performance improved?” The answer is simple, C H A P T E R 3 Overview of Permeable Friction Courses

25 safety. OGFCs most likely provide the safest wearing sur- face for the nation’s roadways. OGFCs have been shown to have excellent frictional resistance, reduce splash and spray, reduce the potential for hydroplaning, improve night visi- bility and improve visibility of pavement markings. Addi- tional benefits of using OGFCs include reduced pavement noise, smoother pavements, increased fuel economy, and use of relatively thin layers. The property of OGFC that leads to these safety benefits is the relatively high permeability of OGFC compared to dense- graded HMAs. Because of the very coarse gradation and lack of fines, OGFCs have very high air void contents in the range of 15 to 22 percent. These high air void contents result in inter- connected voids that allow water to infiltrate into the OGFC layer. Without water on the pavement surface, the frictional properties of the pavement improve, splash and spray is re- duced, and the potential for hydroplaning is greatly reduced. OGFCs that are designed to have at least 18 percent air voids are termed PFCs, a special type of OGFC specifically designed to have high air void contents, typically 18 to 22 percent, for removing water from the pavement surface. Other types of OGFCs also are used within the United States. In some states, friction courses having an open-grading are used; however, the purpose of these friction courses is to provide a safe riding surface by improving frictional properties and/or to reduce tire/pavement noise. Air void contents for these type OGFCs are generally 12 to 15 percent. U.S. Experiences with PFCs OGFCs have been used in the United States for many years; however, PFCs only recently have been utilized as a pavement surface layer option. The Oregon Department of Transporta- tion (DOT) has been using OGFC since the 1970s (4). This mix would not explicitly meet the definition of a PFC, but was placed at a thickness (2 in.) that led to rapid removal of water from the pavement surface. Likely, the first specified PFC in the United States was by the Georgia DOT (10). In 1992, the Georgia DOT built some test sections on I-75 that were specif- ically designed to be coarser and have higher air void contents than GDOT’s current version of OGFC (10). After these field experiments, GDOT developed specifications for what they termed Porous European Mixes (PEM) (11). These PEMs are considered the first generation of PFCs used in the United States. Characteristics of these Georgia PFCs were that mod- ified asphalt binders and stabilizing additives were included. A 1998 survey on the use of OGFC in the United States (8) indicated that most DOTs reporting good performance with OGFCs had adopted coarser gradations and modified asphalt binders similar to the Georgia DOT PEM mixes. After the 1998 survey (8) was published, the National Cen- ter of Asphalt Technology undertook a research project to develop a mix design procedure for new-generation OGFCs. These mixes are considered PFCs. This research study led to a number of DOTs adopting specifications for PFCs. A number of states in the southern part of the United States now utilize PFCs as the wearing layer on all interstates. Based upon the results of the survey conducted during the course of NCHRP Project 9-41, nine DOTs are currently specifying PFCs. Seven of these DOTs are located in the southeast, ranging from Texas to North Carolina. Though they did not respond to the ques- tionnaire, the researchers also are aware that the New Jersey (12), Indiana (13), and Louisiana (14) DOTs have placed PFCs. In most instances, PFCs are used on high-traffic, high-speed pavements within the United States. Huber (7) states that PFC mixes are more desirable on high speed roadways. High speeds are needed to generate enough hydraulic action under vehicle tires to allow the PFC layer to be self-cleaning and, therefore, maintain permeability longer. European Experiences with PFCs In 1990, a significant portion of Transportation Research Record No. 1265 was devoted to the use of OGFCs in Europe. Within most of the articles of this publication, the OGFCs were called porous asphalt. These porous asphalts would clas- sify as PFCs as most are designed to have more than 20 per- cent air voids. Isenring et al. (15) state that PFCs first were used in Switzerland during 1972 on an airport runway and first placed on highway pavements in the late 1970s and early 1980s. As of 1990, Spain had placed more than 3.6 million yd2 (3 million m2) of porous asphalt (16) with the first applica- tion placed in 1980. Within the Netherlands, the first PFC was placed in 1972 (17). During the 1980s, placement of PFC became more widespread because of the noise-reduction characteristics provided by these mix types. The literature also showed a number of other European countries that utilize PFCs, including Italy, France, Belgium, Austria, and the United Kingdom (4, 18). Alderson (19) provided a comprehensive report on the use of PFCs in Australia. PFC are utilized in Australia because they provide better wet weather skid resistance, reduced noise, reduced splash and spray, improved visibility of pavement markings and smoother riding surfaces. Based upon the survey described in Chapter 2, both Canada and Japan also are utilizing PFCs. Iwata et al. (20) indicate that PFCs are widely used on the expressways of Japan. Iwata et al. (20) described an initiative in which the Japan Highway Public Corporation planned to construct almost 7,200 miles (11,520 km) of expressway with PFC being the wearing surface.