Determining the Endurance Limit of HMA Mixture

Fatigue cracking is one of the most prominent distresses in asphalt pavement. Considerable research has been done to develop theoretical models, experimental laboratory testing techniques, and data analysis methods to predict the fatigue performance of asphalt paving mixtures. The majority of fatigue studies were conducted in order to determine the curves and relationships that are called the transfer functions.Usually, the transfer functions are given based on the test results conducted in more than 250 micro strains. But in long life pavements, we need to examine the behavior of asphalt mixtures in lower strains. Long life or the perpetual is defined as more than 40 years without any major structural failures or repairs.These pavements have three asphalt layers. The surface layer and the middle layer should have the high rutting resistance and the lower layer should have enough fatigue resistance so that there would not be any failure during operating the asphalts. As per these characteristics, and high rate of passing traffic during operating the asphalt, these kinds of pavements need an asphalt thicker layer compared to common pavements. The requirement made the designers to find an optimized thickness which is either economical or has suitable serviceability during operating period without the need to be prepared. To prevent the failure caused by the fatigue of the lower layer, it is very important to find the strain level below which no fatigue damage occurs. This strain level is called "endurance limit".The endurance limit is level of strain below which fatigue damage does not occur for any number of load repetitions. The concept of the endurance limit was originally examined and developed for the metal materials but the asphalt mixtures (Viscoelastic materials) were studied less than the latter.That is a valid concept for hot mix asphalt and its quantifications could contribute in the efficient design of long life flexible pavements with a significantly reduced life cycle cost. In the recent years, some researchers have conducted various studies to determine the endurance limit of hot mix asphalt. In 1972, Monismith and Mclean first proposed an endurance limit of 70 micro-strain for asphalt pavement. It was observed that log-log relationship between strain and bending cycles converged at approximately 70 micron-strain at approximately 5 million cycles.In 2000, Carpenter et al, based on the fatigue tests results in different levels of strain, observed that an obvious difference happens in the asphalt mixtures behavior, more than 1.1E+7 in the fatigue life.In 2006 based on Brown et al study, by conducting the traffic studies, a mix which provided 50 million cycles or more, was considered to be below the endurance limit.In this paper, using four - point flexural bending beam test, the endurance limit of different types of asphalt mix based upon 50-percent stiffness reduction and the parameter "dissipated energy change ratio" were investigated. Comparing the result of two methods shows that the fatigue life based on the "Rate of dissipated energy change" criterion is more than the result of the 50-percent stiffness reduction (direct or extrapolation method). Using the criterion suggested by Carpenter (11 millions of loads repetition) and the criterion suggested by Brown (50 millions of load repetition), the endurance limit has been determined for all the tested samples. The endurance limit for all the samples based on the fatigue life resulted from "plateau value" is between 400 to 1000 micro-strain. And the fatigue life based on the stiffness reduction criterion is between 200 to 700 micro-strains. Based on the result of this study, for every type of asphalt mixture, the number of load repetition curve-strain was drawn and the behavior change section was determined for each type of asphalt mixture. The strain corresponding with section can be considered the mixture endurance limit. This is a method for determining the endurance limit corresponding with the concept suggested by Carpenter but separately, the special behavior change section is determined for each type of asphalt mixture. As per the suggested method, the endurance limit of the asphalt mixtures types is between 160 to 270 micro-strains. The results show that the endurance limit depends on the asphalt mixture characteristics.Also using the results of tests and the characteristics of tested specimens, a relationship has been suggested for predicting the endurance limit of hot mix asphalt. This relationship shows that increasing the bitumen percentage and reducing the asphalt mixtures air voids, the endurance limit will be increased. Also, increasing bitumen penetration grade for increasing the fatigue life of the asphalt mixture is a positive effect. Also, increasing the nominal size of materials and the percentage of fine materials and filler in the asphalt mixture cause increased value of the endurance limit.