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ASPHALT RESEARCH CONSORTIUM |
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An FHWA Research Program Comprising Western Research Institute, Texas A&M University, University of Wisconsin-Madison, University of Nevada-Reno and Advanced Asphalt Technologies. |
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Asphalt Research Correspondent |
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This and future issues of the Asphalt Research Correspondent will highlight a selection of research activities being conducted by different Asphalt Research Consortium (ARC) partners. The ARC partners are all working on the following research elements: · Moisture Damage · Fatigue Cracking · Engineered Materials · Vehicle-Pavement Interaction · Technology Development · Validation · Technology Transfer For this issue, the University of Nevada, Reno and the University of Wisconsin-Madison researchers were asked to present a couple of research topics from their ARC programs. The next issue will highlight activities being conducted by Western Research Institute and Texas A&M University. The overall objectives, strategic plan, and work plans of the Asphalt Research Consortium can be accessed through our website at: www.arc.unr.edu |
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The Asphalt Research Consortium Completes its 1st Year of Research |
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Vol 2 Issue 1 May 2008 |
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Designing Hot Mix Asphalt Pavements for a Variety of Loading Conditions: Three Case Studies |
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As part of the Asphalt Research Consortium, the University of Nevada research team is working on two simultaneous projects: Critically Designed HMA Mixtures, and Pavement Response Model to Dynamic Loads (3D-MOVE). The objective of the research on critically designed HMA mixtures is to develop a practical test method to identify the critical combination of temperature and loading rate of HMA mixtures. This research will deliver a laboratory test that enables state highway agencies to identify the critical conditions of a given HMA mix. As hot-mix asphalt (HMA) mix is placed at a given project location, it is subjected simultaneously to local environmental conditions and traffic loading. Because of the viscoselastic nature of the HMA mix, its behavior is highly dependent on temperature, rate of loading, and the stress distribution at the tire-pavement interface. The relationship between pavement temperature and air temperature has been established and verified based on the data from the Long Term Pavement Performance (LTPP) studies. This relationship has been accepted in the Superpave system. The loading rate of the HMA mix depends on traffic speed. The loading rate varies from short under freeway traffic to long under urban traffic. The stress distribution at the tire–pavement interface depends on the complex interaction of factors that include tire type, tire structure, tire inflation pressure, and tire load. The objective of the research on pavement response is to develop a model to predict dynamic loads from moving vehicles and their effect on the response of flexible pavements. This research will deliver a tool that will enable state highway agencies to assess whether a given HMA mix will be subjected to its critical condition under actual field loading. The following case studies present situations in which the results of these two research efforts would be highly applicable. Case Study 1 HMA for Intersections & Off-ramps. Case Study 2 Accelerated Rutting at High Temps. Case Study 3 Impact of Heavy Agricultural Vehicles |
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Above Image: This asphalt pavement has clearly been subjected to loads that are outside its critical condition. |
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Question: We have several pavement analysis models already. Why do we need another? Answer: The pavement analysis models currently available to the pavement engineering community are based on the linear elastic solution of a multi-layer pavement system subjected to static load, with the stress distribution at the tire–pavement interface described as circular and uniform. These limiting conditions work well when a truck equipped with the typical dual radial tires is traveling at 50-60 mph, and the ambient weather conditions are moderate. Any deviations from these loading and environmental conditions, however, pose a significant challenge for the current analytical models. |
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Asphalt mixtures designed for high performance require binders that are resistant to damage, including rutting, fatigue cracking, and cold weather cracking. The ARC team at University of Wisconsin-Madison is leading efforts to help address and mitigate different types of damage for engineered, high-performance asphalts. As a starting point, UW turned to existing practices in rheology—the study of deformation and flow—and laboratory tests and models that describe and predict damage to asphalt binders. While useful to a point, these tools also have significant limitations: some of the parameters involved (such as G* and sin δ) help describe elasticity of binders but are limited in explaining how damage evolves over time. Further, the mathematical basis for these tools may be insufficient—linear models for viscoelasitcity may be too simple and fail to take into account all the necessary variables. Evidence suggesting that the best way to differentiate between good and poor asphalt modifiers is at extreme conditions (that is, in high stress and strain testing domains) underscores the need for research focusing on new testing and analysis techniques for high-performance modified asphalts. In late 2007 and early 2008, the UW team achieved important progress in critical research on binder damage, including binder rutting resistance and binder fatigue resistance. Binder Rutting Resistance In research on binder rutting resistance, the creep and recovery behaviors of binders were compared with those of mixtures. In particular, the behavior of tertiary—or unstable—flow of binders was studied in great detail. Initial results clearly showed that binders play a role in the tertiary flow of mixtures. The flow number of mixtures under repeated creep loading, as measured in the simple performance test, appears to be highly affected by binder type, as is creep behavior under high stress conditions. Binder Fatigue In research on binder fatigue resistance, a new binder test was found to effectively predict pavement fatigue. The new test, which can be run easily in the dynamic shear rheometer, is called the binder yield energy test—the BYET—and follows a simple constant strain loading test to measure binder load capacity before yielding. This test is based on a similar test of asphalt mixture failure used previously by numerous researchers as a surrogate test for mixture fatigue. The test results help derive mathematical functions to describe fatigue damage evolution. These in turn are used to estimate the damage effect of such factors as traffic volume and pavement structure. Binder Damage Resistance Characterization: Rutting Applied stresses and loading times affect the creep behavior of asphalt binders. However, new research results from the UW research team show that, under certain combinations of stress and loading time, some binders show non-linear behavior that resembles tertiary flow in mixtures. However, even under identical testing conditions, this non-linear effect is not seen for all binders. Instead, the creep behavior and the conditions under which the tertiary-like behavior begins are found to vary significantly among binders and depend on testing geometry, stress, loading time and temperature. Beyond investigating the behavior of different binders, mixtures produced using identical binder were also studied to further investigate binder creep behavior and tertiary flow of mixtures. For further description of results and accomplishments for these binder projects, click here. |
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University of Wisconsin-Madison Takes New Approaches to the Study of High-Performance Asphalts |
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