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5Introduction Fatigue cracking has been an issue in the design and per- formance of hot mix asphalt (HMA) pavements ever since hot mix asphalt pavements began to be used. It has long been understood that pavement structures that are too thin fail in fatigue under repeated loads. This type of structural failure results from fatigue of the HMA mixtures and results in the formation of alligator cracking. There are a number of properties in the mixture that can be adjusted to improve the resistance to fatigue but the most important property by far is the thickness of the HMA and its resulting effect on the tensile strain at the bottom of the HMA. It is this strain at the bottom of the asphalt that is the primary cause of the bottom-up fatigue cracking. Ultimately, a method is needed to determine the expected strain at the bottom of the asphalt layer more accurately and to determine the effect of this expected strain on the fatigue resistance of the mixture. Determination of fatigue life based on laboratory tests gener- ally does not provide an accurate predictor of what is observed in the field and a shift factor must be applied. There are many reasons that probably lead to the need for a shift factor, includ- ing: aging, rest periods, healing, densification under traffic, temperature fluctuations, concept of constant stress versus constant strain, simulation of field compaction versus labora- tory compaction, and property changes due to other environ- mental conditions such as aging of the binder and moisture damage. The shift factor can be significant (e.g.,15 to 20 times or more of the laboratory estimate of the fatigue life). Much research was performed in the 1960s and 1970s to study fatigue of HMA. Until recent years it was believed that fatigue cracking always started at the bottom of the HMA layers and propagated up through the HMA layers, eventually reaching the surface. However, recently there is evidence that some projects have exhibited fatigue failure due to top-down cracking. Generally, it is believed that any modification to the mix properties that will improve the resistance to bottom-up cracking will also improve the resistance to top-down crack- ing. Whether it is top-down cracking or bottom-up cracking, the likely cause of fatigue distress is tensile strains in the HMA at critical points in the pavement structure. There is now a much wider use of polymer modified asphalts than in past years. There have been very few fatigue studies on mixtures containing modified asphalts, and it is not clear how much improvement, if any, can be obtained from modified mixtures. Typically, fatigue data are analyzed by plotting the number of cycles to failure versus the maximum tensile strain or stress in the HMA specimens. It has been shown that plotting these data on a log-log (log of cycles to failure versus log of applied stress or strain) plot will provide approximately a straight line. It is very time consuming to conduct fatigue tests at very high numbers of cycles so, generally, researchers have applied fairly high stress or strain values so that the number of cycles to fail- ure will not be so high as to provide excessive test time. It is generally believed that approximately 10 specimens must be tested for each mixture being evaluated to provide a suitable relationship between applied stress or strain and number of cycles to failure. Since most of the testing has been done at higher stress or strain levels, there has not been much work to look at the expected performance at lower stress or strain levels even though these lower stress levels are typical in the field. Gener- ally, a best-fit line is determined for the data at these relatively high strain or stress levels and the performance of mixtures at lower stress or strain levels is extrapolated from the data. Hence, there is very little understanding of the actual per- formance that would be expected at these lower levels. Some work has indicated that there might be a level of stress or strain below which no damage occurs to the test specimen. The stress or strain level below which no fatigue damage orig- inating from the bottom of the pavement structure occurs has C H A P T E R 1 Introduction and Research Approach
been termed fatigue endurance limit. If a pavement is designed and constructed so that under repeated traffic loads no dam- age occurs, then that pavement should last indefinitely without a structural failure. This pavement will still require overlays on some regular basis to maintain the surface in good operating condition, but the pavement structure should provide a very long life. Research Problem Statement Work is needed to determine if there is an endurance limit for HMA and, if so, how this information should be used in pavement design. One would not want to design all pave- ments, regardless of traffic level, so that the endurance limit is not exceeded since this would result in the same design thickness regardless of traffic level. For very high traffic lev- els it might be desirable to design the pavement so that the endurance limit is not exceeded, but for low traffic levels it is likely that this approach would be too expensive. How- ever, a similar concept could be employed for pavements subject to very low traffic levels to ensure a long life. As always, the pavement should be designed to provide the lowest life- cycle costs. Hence, there is a need first of all to determine if there is an endurance limit for HMA. Once the endurance limit is determined it is important to determine how this endurance limit would fit into the new mechanistic pavement design procedures. Objectives The objectives of this study are to 1. Test the hypothesis that there is an endurance limit in the fatigue behavior of HMA mixtures and measure its value for a representative range of HMA mixtures. 2. Recommend a procedure to incorporate the effects of the endurance limit into mechanistic pavement design methods. Scope This study included a literature review, laboratory testing, and analysis of field data. A mix design, used to construct struc- tural sections at the 2003 National Center for Asphalt Technol- ogy (NCAT) Test Track, was replicated. Four versions of the mix design were evaluated encompassing the effect of increased asphalt binder content and polymer modification. The mix- tures with higher asphalt binder contents were prepared at higher densities to simulate the improvement in compaction expected in the field. Beam fatigue and uniaxial tension test- ing were conducted to determine fatigue life. Beam fatigue testing was conducted at progressively lower strain levels until a fatigue life in excess of 50 million cycles was achieved. Data from the Long-Term Pavement Performance (LTPP) study were analyzed to determine if they supported the endurance limit concept. 6
