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5 CHAPTER 1 Introduction and Research Approach Introduction mix properties that will improve the resistance to bottom-up cracking will also improve the resistance to top-down crack- Fatigue cracking has been an issue in the design and per- ing. Whether it is top-down cracking or bottom-up cracking, formance of hot mix asphalt (HMA) pavements ever since the likely cause of fatigue distress is tensile strains in the HMA hot mix asphalt pavements began to be used. It has long been at critical points in the pavement structure. understood that pavement structures that are too thin fail in There is now a much wider use of polymer modified asphalts fatigue under repeated loads. This type of structural failure than in past years. There have been very few fatigue studies on results from fatigue of the HMA mixtures and results in the mixtures containing modified asphalts, and it is not clear how formation of alligator cracking. much improvement, if any, can be obtained from modified There are a number of properties in the mixture that can mixtures. be adjusted to improve the resistance to fatigue but the most Typically, fatigue data are analyzed by plotting the number important property by far is the thickness of the HMA and its of cycles to failure versus the maximum tensile strain or stress resulting effect on the tensile strain at the bottom of the HMA. in the HMA specimens. It has been shown that plotting these It is this strain at the bottom of the asphalt that is the primary data on a log-log (log of cycles to failure versus log of applied cause of the bottom-up fatigue cracking. Ultimately, a method stress or strain) plot will provide approximately a straight line. is needed to determine the expected strain at the bottom of the It is very time consuming to conduct fatigue tests at very high asphalt layer more accurately and to determine the effect of numbers of cycles so, generally, researchers have applied fairly this expected strain on the fatigue resistance of the mixture. high stress or strain values so that the number of cycles to fail- Determination of fatigue life based on laboratory tests gener- ure will not be so high as to provide excessive test time. It is ally does not provide an accurate predictor of what is observed generally believed that approximately 10 specimens must be in the field and a shift factor must be applied. There are many tested for each mixture being evaluated to provide a suitable reasons that probably lead to the need for a shift factor, includ- relationship between applied stress or strain and number of ing: aging, rest periods, healing, densification under traffic, cycles to failure. temperature fluctuations, concept of constant stress versus Since most of the testing has been done at higher stress constant strain, simulation of field compaction versus labora- or strain levels, there has not been much work to look at the tory compaction, and property changes due to other environ- expected performance at lower stress or strain levels even mental conditions such as aging of the binder and moisture though these lower stress levels are typical in the field. Gener- damage. The shift factor can be significant (e.g.,15 to 20 times ally, a best-fit line is determined for the data at these relatively or more of the laboratory estimate of the fatigue life). high strain or stress levels and the performance of mixtures Much research was performed in the 1960s and 1970s to at lower stress or strain levels is extrapolated from the data. study fatigue of HMA. Until recent years it was believed that Hence, there is very little understanding of the actual per- fatigue cracking always started at the bottom of the HMA formance that would be expected at these lower levels. layers and propagated up through the HMA layers, eventually Some work has indicated that there might be a level of stress reaching the surface. However, recently there is evidence that or strain below which no damage occurs to the test specimen. some projects have exhibited fatigue failure due to top-down The stress or strain level below which no fatigue damage orig- cracking. Generally, it is believed that any modification to the inating from the bottom of the pavement structure occurs has