This synthesis explores the state of knowledge and state of the practice regarding the use of fiber additives 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, among 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 use. The U.S. state response rate to the survey was 96.0% (48 of 50). Numerous international asphalt engineers and researchers were contacted by e-mail to gather information on the use of fibers and on current international research. About 28 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 (OGFCs/PFCs) to control drain-down of the binder from the mix. In the past, fibers were used in dense-graded mixes in some states, but that usage has decreased in the past 20 years or so. Use of SMAs and porous mixes is also on the decline in some states because of the high costs, but that situation is fluid and subject to change. A wide variety of fiber types has been used in asphalt mixtures, including cellulose, mineral, synthetic polymer, and glass fibers, as well as some less common fiber types. Recycled fiber materials–such as newsprint, carpet fibers, and recycled tire fibers–have also been used. These different types of fibers have benefis and disadvantages that make them better suited for some applications than others. For example, cellulose is not strong in tension, but it is absorbent and holds asphalt, therefore it is well-suited to reducing draindown in open-graded mixes but not for reinforcing dense-graded concrete. Also, because of the different applications, sources, and types of fibers, test methods used to characterize them vary. Typically, asphalt mixtures with fibers are designed using the same procedures as conventional mixtures. The only common addition is the use of a draindown test for SMA and OGFC mixes. Some types of fibers, particularly those that are highly absorptive or have a high surface area, require increased binder contents, which may improve mix durability but also may increase costs. Fiber quality is ensured through supplier certification in most states where fiber properties are specified. Mixture production and pavement construction are also typically similar to conventional construction practices. The only difference in mix production equipment is the presence of a mechanism to introduce fibers into the asphalt mix plant. This may be accomplished by blowing loose or pelletized fibers into a drum mix plant (typically through the reclaimed asphalt pavement collar) or by adding premeasured, meltable bags of fibers into the pug mill or weigh hopper on a batch plant. There are other, less common methods as well. Handling and storing fibers properly at the production facility are important to ensure the success of the fiber addition. A great deal of research has been conducted over nearly 50 years on the use of fibers in asphalt. Much of this research has focused on the laboratory and field behaviour of fiber-modified asphalt binders and mixtures. The results of the research have been mixed, especially regarding the use of fibers in dense-graded mixtures. In some cases, fibers have reportedly improved the rutting and cracking tendencies of binders and dense mixes; in other cases, there has been no significant improvement. Generally, fibers appear to be more effective at improving the performance of marginal or lower-quality mixtures. Fibers have rarely been detrimental to performance in dense mixes, but if they do not improve performance, they may not be cost-effective. In SMA and OGFC mixes, fibers have clearly been shown to reduce draindown and are commonly used, although alternative materials– such as polymer-modified binders and recycled asphalt shingles–can be used. The survey results in chapter three show that 30 of 50 states currently allow or require the use of fibers in some asphalt mixtures. By far the most common use is in SMA and OGFC mixes. A few other states indicated that they would use fibers but are currently not constructing SMAs or OGFCs. For SMAs and OGFCs, the use of cellulose or mineral fibers is typical. The few current applications of fibers in dense-graded mixes use various synthetic polymer fibers. Information on the cost-effectiveness of the use of fibers in different applications is almost nonexistent. The use of fibers internationally is quite similar to that in the United States; that is, the use of cellulose or mineral fibers in gap- and open-graded mixtures is routine in many countries. There appears to be growing interest in developing parts of the world in using locally available plant-based materials–such as coconut, jute, hemp, and sisal–as sources of fibers; this allows the benefis of fibers to be taken advantage of economically while creating a market for locally produced materials. Case examples of the use of fibers in asphalt by local and state agencies are provided in chapter four. The first case example outlines the common questions an agency might face when it considers the possibility of using fibers. The second describes the history of one agency that used large quantities of fibers in the past to reinforce dense-graded mixtures but has reduced usage drastically after making other changes in its mix design procedures and specifiations. A third case example describes another agency’s path toward implementing fibers in open- and gap-graded mixtures, and the contractors’ experiences as they began using the materials. The last two case examples highlight ongoing research efforts; one in a state with little to no prior experience with the use of fibers in asphalt and the other in a state that uses fibers extensively in SMA and OGFC, and is exploring using them in dense-graded mixtures as well. The information reported here shows a number of gaps in the state of knowledge. Information to address these gaps was found to be lacking or inconsistent. Although the use of fibers to reduce draindown in gap- and open-graded mixes is quite well established and clearly successful, the effects of using fibers for other reasons are less clear. Research is needed to determine or clarify the following: * Cost-effectiveness of fiber mixes; * Use of mechanistic-empirical pavement design with fiber-reinforced mixtures; * Standardized guidance on production and construction of fiber mixes; * Fiber quality and interactions; * Test methods to verify the presence and distribution of fibers; * Health, safety, and environmental issues; * Performance mechanisms and material characteristics with different types of fibers, perhaps through a comprehensive performance study; and * Potential impacts on recycled materials. Overall, the reported success of fiber-reinforced asphalt mixtures is quite promising. Their use in open- and gap-graded mixtures is well established. Opportunities exist to increase the use of fibers in other applications, provided their benefits can be clearly and consistently demonstrated. Additional guidance on mixture and pavement design, critical fiber properties, and the cost-effectiveness of fibers in different applications could make this sometimes overlooked tool a more widely used method to improve pavement performance. (Author/publisher)
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