Evaluating the efficacy of Ground Tire Rubber technologies and developing guidelines, tests and specification limits pertaining to GTR use in asphalt binders and mixtures.
In the United States alone, about 300 million scrap tires are generated each year (1). If these tires are not either properly disposed of or recycled into a safe application, there are significant environmental impacts as well as public health effects (1). One of the most prevalent civil engineering applications for recycled rubber tires is to grind them into crumb rubber, or ground tire rubber (GTR) and use them in asphalt mixtures. In the 1980s and 1990s, there was interest from the federal government in implementing this technology, but it has not been explored in depth by the federal government in the United States since the publication of NCHRP Synthesis 198 (2) in 1994.
Today, there is a higher-level of interest in sustainability in the highway construction industry, as well as a larger volume of scrap tire generation, with possibly even larger traffic volumes creating more scrap tires as the use of autonomous vehicles becomes prevalent. Additionally , other sustainable technologies such as recycled asphalt pavements (RAP) and warm mix asphalt (WMA) have become more common in the asphalt industry, and there is more and more interest to use these techniques in combination with GTR. While many agencies are allowing and encouraging the use of GTR in asphalt pavements, there are still many open questions to be answered regarding the most effective ways to use GTR and its effect on binder, mixture, and pavement performance. Including the resulted in increasing of mixtures price and required asphalt binder content. A comprehensive study is proposed that will evaluate the current state of practice in creating and using GTR, the performance of binders and mixtures modified with GTR, and the development of guidelines for GTR use.
The research study is intended to evaluate the effects of using GTR in asphalt binders and mixtures and provide a framework for future usage of these materials. The framework presented will include a synthesis of different methods used for producing GTR, mixing it with asphalt materials, and testing these binders and mixtures, and will evaluate a wide variety of these materials.
There is a high demand for new guidelines for sustainable practices in asphalt pavement construction. Given that a long time has passed since the last NCHRP project related to GTR use in asphalt, it is necessary to urgently focus on this area. There is a growing availability of scrap tire rubber, as well as an increasing demand for pavement construction and maintenance. In addition to incorporating a widely-available recyclable material, this technology also has the added benefit of typically improving binder, mixture, and pavement performance. However, to gain the full benefits of using this technology, it is imperative to do a comprehensive study of GTR and GTR-modified asphalt materials to ensure that the use of GTR is resulting in high quality projects across the country. The findings from this study will help the pavement industry to move in a more sustainable direction, but also improve the quality and cost-effectiveness of projects constructed using GTR, and help guide state and federal agencies in writing more comprehensive and forward-thinking specifications for this material.
The existing literature reveals two primary processes for incorporating GTR into asphalt materials (1,2). These are primarily referred to as a wet process and a dry process. The major difference between these two processes is that the dry process involves mixing coarser GTR fractions in with the aggregate (often referred to as “rubber-filler”), while the wet process is performed by mixing finer fractions of GTR with the asphalt binder before mixing it with aggregates (which is often called “asphalt-rubber”). The wet process has created some environmental concerns due to an elevated temperature required for mixing and obtaining a complete blending between the asphalt binder and the GTR.
Aside from methods for use of GTR in asphalt binders and mixtures, the existing literature focuses extensively on the applications for which GTR use can improve the properties of binder and mixtures. In the existing literature, the addition of GTR to asphalt binders has been shown to stiffen asphalt binders and has been compared favorably to polymer modification (1). In addition, it has been shown to improve fatigue cracking resistance of asphalt mixtures (3). Besides using GTR in standard hot mix asphalt (HMA) mixtures, other uses revealed in the literature include using asphalt-rubber as a chip seal binder, an underseal to absorb stresses between an existing layer and overlay layer, and a binder for Porous Friction Course (PFC) mixes (1).
Finally, the existing literature provides insight into the drawbacks of using GTR in asphalt applications. One of the most significant issues in using GTR is the problem of storage stability. GTR has a higher density than typical asphalt binders, and therefore can create the problem of separation when stored. Previous studies indicate that these asphalt-rubber storage stability becomes worse when the storage temperature is higher, as well as when larger GTR particles are used (4). Another major concern regarding using GTR is the elevated cost associated with it (3). However, the high cost of using this technology could possibly be compensated for by a longer service life of the pavement based on Life Cycle Cost Analysis (LCCA) (1). Finally, there are still environmental implications, including the temperatures associated with asphalt-rubber production, as well as potentially dangerous contaminants coming from GTR-modified asphalt binders and mixtures (5).
Specifically, this study must consider, but not be limited to, the following aspects:
Review existing literature for (i) available methods used for the production of GTR that can be used for modification of asphalt binders from NCHRP Synthesis 198 and other studies (ii) methods used to modify asphalt binders and mixtures with GTR, and (iii) test methods used to evaluate GTR modified binder including existing binder specifications as well as novel tests that can be used to characterize binders and mixtures that use GTR.
Evaluate the properties of binders and mixtures which have been modified with GTR. This evaluation should evaluate the effect of modification on stiffness, ductility, temperature sensitivity, strength, and other properties of these materials. This evaluation should be comprehensive including a wide variety of mixtures and binders, as well as other technologies such as recycled materials or WMA additives in combination with GTR. Finally, storage stability should be examined for different types of materials.
Develop guidelines for the use of GTR in asphalt mixtures. These guidelines should include a range of tests for the binder and mixture, as well as proposed limits for these tests. These guidelines should also include best practices for implementing this technology safely.
Provide draft AASHTO specifications related to the materials and tests described above.
State DOTs and local governmental agencies
|Sponsoring Committee:||AFK20, Asphalt Binders
|Research Period:||24 - 36 months|
|RNS Developer:||Darren Hazlett (AFK 20 Chair), Amit Bhasin (AFK 20 CRC), Elie Hajj (AFK 50 Chair), Shane Underwood (AFK 50 CRC), Ramez Hajj (AFK 20 Friend), and Members of AASHTO COMP|
|Source Info:||1. Shu, Xiang, and Baoshan Huang. "Recycling of waste tire rubber in asphalt and portland cement concrete: An overview." Construction and Building Materials 67 (2014): 217-224.|
2. Epps, Jon A. Uses of recycled rubber tires in highways. NCHRP Synthesis Vol. 198. Transportation Research Board, 1994.
3. Huang, Baoshan, Louay N. Mohammad, Philip S. Graves, and Chris Abadie. "Louisiana experience with crumb rubber-modified hot-mix asphalt pavement." Transportation Research Record 1789, no. 1 (2002): 1-13.
4. Navarro, F. J., Partal, P., Martınez-Boza, F., & Gallegos, C. (2004). Thermo-rheological behaviour and storage stability of ground tire rubber-modified bitumens. Fuel, 83(14-15), 2041-2049.
5. Azizian, Mohammad F., Peter O. Nelson, Pugazhendhi Thayumanavan, and Kenneth J. Williamson. "Environmental impact of highway construction and repair materials on surface and ground waters: Case study: crumb rubber asphalt concrete." Waste management 23, no. 8 (2003): 719-728.
|Index Terms:||Tires, Asphalt rubber, Bituminous binders, Asphalt mixtures, Mix design, Recycled materials, |
|Cosponsoring Committees:||AFK50, Structural Requirements of Asphalt Mixtures|