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CHAPTER 11 Design of Open-Graded Mixtures Open-graded friction course (OGFC) is a specialty HMA that uses an extremely open aggregate gradation to improve frictional resistance, reduce splash and spray, improve nighttime visibility, reduce hydroplaning, or reduce pavement noise levels. OGFC is specifically designed to have a large percentage of a single size coarse aggregate with a low percentage of fine aggregates and a very low percentage of materials finer than 0.075 mm (dust or mineral filler). A relatively single size coarse aggregate combined with a low amount of fine aggregate and dust provides for a much more open aggregate gradation compared to other HMA mix types. OGFCs were first developed during the 1940s through experimentation with plant-mix seal coats. Even though these plant-mix seal coats provided excellent frictional properties, their use spread slowly because they required a different mix design method and special construction considerations than typically used HMA. It was not until the 1970s, when the FHWA published a formalized mix design procedure that the use of OGFCs began to increase. This procedure entailed an aggregate gradation requirement, a surface capacity of coarse aggregate, determination of fine aggregate content, determination of optimum mixing temperature, and determination of the resistance of the designed mixture to moisture. During the 1970s and 1980s, some agencies observed performance problems when using OGFCs. Primarily, the problems were raveling and delamination. These distresses were caused by problems associated with mix design, material specification, and construction. The primary issue involved mix temperature during construction. The gradations associated with OGFCs were much coarser than typical dense-graded mixes. Additionally, unmodified asphalt binders were used with these OGFCs at that time. Because of the open nature of the aggregate gradation, there were problems with the asphalt binder draining from the coarse aggregate during transport. To combat these draindown problems, the mix temperature was reduced during production. This reduction in temperature resulted in two problems. First, the internal moisture within the aggregates was not removed and, second, the OGFC did not bond with the existing pavement when placed. These two issues resulted in the raveling and delamination problems frequently encountered with OGFCs during the 1970s and 1980s. During the 1990s, major improvements were made in HMA specifications and design methods. Additionally, new technologies were adopted from Europe for stone matrix asphalt (SMA), gap-graded hot mix asphalt (GGHMA), and OGFC. These improvements and ideas have resulted in new methods for designing and constructing OGFC, notably the inclusion of polymer-modified asphalt binders and stabilizing materials, which have significantly improved the resulting mixtures and the overall performance of OGFCs. Stability additives, such as mineral filler now help prevent draindown and allow higher mixing temperatures. This allows better bonding between the OGFC and the underlying layer through the use of a proper tack coat, reducing the potential for delamination. Additionally, the higher mixing temperatures help to dry the aggregate more 194