Fiber Additives in Asphalt Mixtures (2015)

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33 CHAPTER FIVE CONCLUSIONS laboratory and field performance of fiber-reinforced dense- graded mixtures, however, have yielded mixed results. In some cases, the fibers improved performance, especially in terms of rutting and cracking resistance; in other cases, the fibers have not resulted in significant performance improvements. It appears that fibers might be more effective in marginal or lower-quality mixtures that are prone to rutting and cracking. The benefits of fibers for reducing draindown in gap- and open-graded mixes are more clearly defined in the literature, which may explain their common use in those mixes. The use of fibers internationally is quite similar to that in the United States; that is, cellulose or mineral fibers are routinely used in gap- and open-graded mixtures. Fibers are less commonly used in dense-graded mixes, but when they are used, synthetic polymer fibers are most prevalent. There appears to be growing interest in using locally available plant-based materials—such as coconut, jute, hemp, and sisal—as sources of fibers in developing parts of the world. Case examples of the use of fibers in asphalt by local and state agencies are provided in chapter four. These case examples include the following: • Common questions an agency might have when it is considering the possibility of using fibers; • An agency that saw a great decrease in the use of fibers as they implemented new mix design procedures and specifications; • An agency’s and its contractors’ experiences with the implementation of fibers in open- and gap-graded mixes; • An ongoing research project being conducted by a state with little previous experience with the use of fibers in asphalt; and • Another ongoing research effort in a state that uses fibers extensively in SMA and OGFC and is exploring their use in dense-graded mixtures as well. On the basis of the information reported here, a number of gaps in the state of knowledge have been identified. Research is needed to clarify or document the cost-effectiveness of using different types of fibers in different applications; how to characterize fiber mixes for mechanistic-empirical pavement design; best practices for production and construction of This synthesis compiles available information on the use of fibers in asphalt mixtures. It outlines the many types of fibers that have been used, their properties and how they are tested, mix design tests for fiber mixes, the types of applications in which fibers have been used, and lab and field performance of fiber mixes, and other topics. The information in this synthesis was gathered through a thorough review of the available U.S. and international literature. In addition, a survey of U.S. and Canadian state/ provincial agencies was conducted to determine the current status of fiber asphalt usage. The U.S. state response rate to the survey was 96.0% (48 of 50). About 30 states report using fiber in asphalt mixes. By far the majority of the use is in stone matrix asphalt (SMA) and open- graded or porous friction courses (OGFC or PFC) to control draindown of the binder from the mix. In the past, fibers were frequently used in dense-graded mixes in some states, but that usage has decreased in the past 20 years or so, though interest appears to be increasing again. The use of SMAs and porous mixes is also on the decline in some states because of the high cost, but that situation is fluid and subject to change. Cellulose and mineral fibers are most commonly used in SMA and porous mixes in the United States. The use of synthetic polymer fibers is less common but perhaps increasing. There is also interest in using recycled or waste fibers, provided they can perform as well as virgin fibers. Of the states that use fibers, most do not routinely require any additional tests or changes to the mix design procedures, with the exception of adding a draindown test for open- or gap-graded mixes. Fiber quality is ensured through supplier certifications in most states in which fiber properties are specified. Because some fibers are absorbent or have large surface areas to coat with binder, the binder contents in fiber mixes may need to be increased, which can have beneficial effects on durability but might increase mix costs. Mix production and placement are largely unchanged, except that some means of introducing the fibers into the mix plant is required. If the fibers are properly stored and handled, construction issues can usually be minimized. Research into the use of fibers in asphalt has been extensive over the past 4 to 5 decades. Studies of the

34 fiber mixes; critical fiber characteristics to ensure quality; test methods to verify the presence and distribution of fibers; health, safety, and environmental issues with the use of different types of fibers; performance mechanisms of different types of fibers (perhaps through a comprehensive performance study); and the future recyclability of fiber mixes and effects on use with recycled materials. In summary, some uses of fibers in asphalt mixtures are well established and successful. Opportunities exist to use fibers in other applications to extend pavement service lives and improve the level of service, but more research is needed to ensure that those potential benefits are consistently realized.