Use of Recycled and Waste Fibers in Asphalt Concrete

A significant problem associated with asphalt concrete mixtures is the lack of durability resulting from an inadequate asphalt cement content. Additional asphalt cement added to increase durability results in flushing, bleeding and a significant loss In stability. Fibers have been used in stone matrix asphalt and open-graded friction course pavements successfully throughout Europe to allow higher asphalt cement contents for many years. The presence of recycled and waste fibers in mixtures offer the potential of durable, longer lasting roads. This paper summarizes the findings of two independent studies. One investigation studied four fiber types in an asphalt concrete with a polymer modified asphalt cement to measure the effectiveness of commercial and recycled/waste fibers. All four fibers; commercial cellulose and polyester and waste nylon and ground carpet, were added to an asphalt concrete mixture at the same proportions. Stabilities, air voids and "voids in the mineral aggregate" (VMA) were measured to evaluate mixture properties as affected by the different fiber types. Stripping resistance tests, Resilient Modulus and indirect tensile stress tests were conducted according to the Asphalt-Aggregate Mixture Analysis System (AAMAS) to measure performance of the fiber mixtures. A comparison study was conducted on mixtures compacted to densities simulating the densities achieved at construction. Indirect Tensile Strengths and Resilient Modulus were measured. Evaluation of overall performances of the different fiber types suggests all fibers can be used in asphalt concrete mixtures. The waste nylon fibers and the ground carpet showed a higher VMA and an increase in stability versus the commercial fibers. All of the fibers were effective in increasing the asphalt cement content by 0.3 to 0.4 percent. BACKGROUND Inadequate asphalt cement contents in asphalt concrete mixes have been labelled as a significant problem for asphalt concrete mixtures, due to the resulting lack of durability in today's pavement systems. An initial solution often involves changing the aggregate gradation to allow for an increase of the asphalt cement content or simply increasing the asphalt content. Often this results in flushing and bleeding pavements and significant increases in permanent deformation of the pavement. Most of this problem can be alleviated with proper aggregate gradation to provide adequate space for the asphalt cement. However, the optimal amount of asphalt cement resulting from these changes in gradation may not be enough to ensure performance of dense graded mixtures. A different approach to reducing this problem is to add fibers to the mixture to stabilize the asphalt cement, allowing for higher asphalt cement percentages. Fiber stabilization of stone matrix asphalt and open graded friction courses has been successfully used throughout Europe. The use of fibers in dense graded 2 mixtures is relatively new technology and has not been widely used. Fiber enhancement of dense graded mixes may offer the potential of durable, longer lasting roads without an increased risk of permanent deformation. Earlier research suggests that the addition of fibers will improve binder-mixture properties by increasing the amount of asphalt cement, which in turn improved durability characteristics of the mixture. Several man-made, commercially available fibers exist that can be purchased for use in asphalt concrete mixtures. The asphalt industry has recently received a mandate to use waste and recycled material in asphalt concrete pavement construction. This investigation addresses the issue of performance of fiber asphalt concrete and how waste and recycled fibers compare to commercially available fibers. The waste/recycled fibers are by-products that must be used in an environmentally safe and effective manner or they will be disposed in landfills. This paper combines the findings of two studies of recycled/waste fibers in dense graded asphalt concrete. One program was conducted in the laboratories of CTLVThompson, Inc. in Denver, Colorado and the second comparative study was conducted at the University of Texas at Austin under the supervision of Dr. Thomas W. Kennedy. Laboratory investigations were conducted to measure the properties, performance, and effectiveness of the recycled fibers and commercially available fibers. CTL/THOMPSON STUDY PROGRAM The CTL/Thompson, Inc. laboratory investigation measured the mixture properties of a single asphalt cement-aggregate blend stabilized with four different fiber additives, two commercial fibers and two waste/recycled fibers. Questions addressed by this investigation about the use of fibers are: * Is there a difference in performance between commercially available fibers and waste/recycled fibers? * In a dense graded type mixture, is the void structure large enough to allow the fibers to be inert within the mix or do the fibers act as an aggregate? * Does increased asphalt cement content imply improved durability? Results of this research are intended to provide preliminary comparison between commercially available fibers and waste/recycled fibers, quantifying their laboratory mixture properties. To provide true comparisons between the fibers, the aggregate source, aggregate gradation and base asphalt cement remained constant throughout the study. 3 FIBERS This investigation selected four fibers from locally available sources for comparison testing. Two commercially man-made fibers were selected from fibers presently available to the asphalt paving industry. Two waste/recycled fibers were selected from potential market sources. All of the fibers were added to the mixtures at a concentration of 0.3 percent by total weight of mix. The two commercial fibers selected were cellulose and a polyester fiber. The cellulose was a cylindrical pellet composed of 50% cellulose fiber and 50% asphalt cement, by total weight. The fiber has an average length of 0.043 inches and a thickness of 0.002 inches. The polyester was a loose polyester fiber with an average length of 0.25 inches. The cellulose and polyester fibers are currently available to the asphalt paving industry. The two waste/recycled fibers selected were nylon, a processed waste fiber, and ground carpet fibers from processed carpet. The nylon fibers are a processed waste fiber from mills in Georgia and/or South Carolina. The Nylon 66 fiber stock has a mix denier of 2 to 6 and was cut into 0.25 inch lengths. The nylon waste fibers result from interruptions of spooling operations, with a yearly supply of 40 million pounds. The ground carpet fibers were cut from waste carpets. Carpet fiber lengths varied from a maximum length of 0.5 inches. The carpet backing was included in the cutting process. The carpet fibers are a recycled post-consumer fiber, ground to average fiber length. The yearly supply of the carpet fiber is up to 200 million pounds. The waste nylon fibers are made as pure nylon strands, collected from interruption in the spooling operations, cleaned and cut to required length. The length and diameter of the waste nylon can be specified within range of commercial denier and desired length. The recycled carpet fibers are taken from homes and offices as waste carpet, ground with dust separation to average fiber length control.