Fiber Additives in Asphalt Mixtures (2015)

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CONTENTS 1 SUMMARY 5 CHAPTER ONE INTRODUCTION Background, 5 Synthesis Approach, 6 7 CHAPTER TWO LITERATURE REVIEW: USE OF FIBER ADDITIVES IN ASPHALT MIXTURES Fiber Materials and Mixtures, 7 Mix Design with Fibers, 11 Production, Construction, and Acceptance of Fiber Mixtures, 12 Performance of Fiber Mixtures, 13 Costs and Benefits of Fiber Additives in Asphalt Mixtures, 22 24 CHAPTER THREE SURVEY RESULTS: CURRENT U.S. AND INTERNATIONAL EXPERIENCE U.S. Survey Results, 24 State Specifications and Test Methods, 26 International Experience, 27 29 CHAPTER FOUR CASE EXAMPLES Case 1. Agency Considering Use of Fibers, 29 Case 2. Agency with Varying Fiber Usage, 29 Case 3. Contractors’ Experiences with Fibers in Asphalt Mixtures, 30 Case 4. Ongoing Research on Fibers in Dense-Graded Asphalt, 31 Case 5. State with High Fiber Usage Researching Other Applications, 32 33 CHAPTER FIVE CONCLUSIONS 35 REFERENCES 39 BIBLIOGRAPHY 42 APPENDIX A SURVEY QUESTIONNAIRE 46 APPENDIX B SURVEY RESPONDENTS 48 APPENDIX C TABULATED SURVEY RESPONSES Note: Many of the photographs, figures, and tables in this report have been converted from color to grayscale for printing. The electronic version of the report (posted on the web at www.trb.org) retains the color versions.

SUMMARY FIBER ADDITIVES IN ASPHALT MIXTURES 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 draindown 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 benefits 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

2 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 behavior 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 benefits 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 specifications. 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;

3 • 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.

5 The use of fibers in specialty mixes (such as cold mix or curb mixes) and in spray-applied pavement preservation treatments is not considered; however, if an agency mentioned such applications in its comments, the comments have been included in the summary response tables. BACKGROUND The use of fibers in asphalt mixes dates back many decades. Or longer: Button and Epps (1981) maintain that the earliest use of fibers in asphalt was the use of straw in ancient Egyptian building specifications. In the United States, asbestos fibers were used as early as the 1920s (Serfass and Samanos 1996), and this usage continued until the 1960s, when health and environmental concerns put an end to it (Busching et al. 1970). Cotton fibers were used in the 1930s (Busching et al. 1970), but they tended to degrade over time (Freeman et al. 1989). Since then many types of fibers have been used in various applications and different parts of the world. Fibers were reportedly used to provide the following benefits (Busching et al. 1970; Peltonen 1991): • Increased tensile strength resulting in increased resistance to cracking, • Reduced severity of cracking when it did occur, • Increased fatigue resistance, • Increased rutting resistance as a result of lateral restraint within the mixture, • Increased abrasion resistance, • Higher asphalt contents leading to increased durability, and • Potential lower life cycle costs arising from longer service life. Early applications were in dense-graded mixtures. Beginning in 1991, the first SMA mixtures were placed in the United States after more than 30 years of successful use in Europe (Cooley and Brown 2001). These mixes were designed in Europe mainly to resist studded tire wear but were found to be highly resistant to permanent deformation as well. Usage in the United States increased rapidly: by 1997 more than 140 SMA projects across the country were evaluated by the National Center for Asphalt Technology (NCAT) (Cooley and Brown 2001). These mixes generally used cellulose or mineral fibers to help hold the asphalt CHAPTER ONE INTRODUCTION Fibers have been used to reinforce paving materials for many decades in various parts of the world. Their use in stone matrix asphalt and porous or open-graded mixtures to prevent draindown of the binder from the aggregate particles is very common. Less common is the use of fibers in dense-graded mixtures to increase stability (reduce rutting) and improve resistance to cracking. Cracking of asphalt pavements appears to be an increasing concern in many states, so identification of a potential tool to reduce cracking could be very beneficial. This synthesis is intended to explore past and current use of fibers in asphalt mixtures. Many types of fibers are available for incorporation into asphalt paving mixtures. Cellulose and mineral fibers are commonly used in gap-graded stone matrix asphalt (SMA) and open-graded or porous mixtures. Polypropylene and polyester fibers were previously used in dense-graded mixtures and are still used to some extent. Various polymers, steel wool, and other fibers are also sometimes added to asphalt mixtures. The relative benefits and issues with these various types of fibers are not well documented. The appropriate specifications and material characteristics to ensure the best performance in different climates, under different traffic loadings, and in different applications are also not widely recognized. This synthesis assembles and summarizes the available literature on asphalt mixtures with fiber additives. Agencies were surveyed to determine their current and past use of fibers in asphalt, their testing and mix design procedures, performance history, and other information. In particular, the synthesis panel examined the following: • Types of fibers (e.g., materials, dimensions, applications, sources); • Specifications, test methods, and acceptance criteria; • Fiber quality, interactions, and supply issues; • Health, safety, and environmental issues; • Use of fibers in both experimental and routine construction; • Mix design, production, placement, and acceptance issues/resolutions; • Factors that affect performance (e.g., climate, traffic, application, fiber type); • Performance mechanisms/material characteristics; • Costs/benefits; and • Impact on recycled materials.

6 the topics they addressed (e.g., performance, testing, types of fibers). More than 100 references were identified and reviewed. Information on past research and practices from this literature review is presented in chapter two. The second approach was aimed at getting a picture of the current state of the practice. A Qualtrics electronic survey was developed and distributed to all 50 states in the United States. Responses were received from 48 states for a response rate of 96.0%. The responses are summarized in chapter three. A copy of the survey, list of agencies responding to the survey, and tabulations of survey responses are provided in the appendices. Chapter three also includes a summary of some international experience with the use of fibers in asphalt mixes gleaned from interviews and e-mail with personal contacts. Following up on the initial survey, selected organizations were interviewed by phone or questioned by e-mail to elicit additional information. These follow-up interviews provided the basis for the case examples in chapter four. Finally, on the basis of the literature review, survey responses, and interviews, topics that need additional research were identified. This list and the conclusions drawn from the synthesis are presented in chapter five. binder in the gap-graded aggregate structure; that is, to prevent draindown of the binder. Fibers are also used in open-graded friction courses (OGFCs) or porous asphalt mixes to prevent draindown. These mixes have open-graded aggregate structures and high air voids to create stone-on-stone contact to resist rutting, reduce noise (McGhee et al. 2013), reduce splash and spray, and improve friction (Watson et al. 1998). In summary, there are two main uses for fibers: (1) to prevent draindown in gap- and open-graded mixes, and (2) to strengthen dense-graded asphalt mixes to resist rutting and cracking. These uses plus other potential benefits and applications of the use of fibers are explored in this synthesis. SYNTHESIS APPROACH A variety of approaches was used to collect the information presented in this synthesis. The first was a comprehensive literature search performed using the TRID database, Google, and Compendex. Pertinent references were also provided by some of the survey respondents, the panel members, and others. The references were reviewed and categorized as to