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7 CHAPTER 2 State of Practice A review of the existing literature was conducted to answer pavement thickness, greater than that required to keep the following specific questions: strains below the endurance limit, would not provide addi- tional life. This concept has significant design and eco- 1. What is an endurance limit? nomic implications. 2. What field or laboratory studies support the existence of The concept of the endurance limit was originally devel- an endurance limit? oped for metals (4, 5). Barret et al. (5) describe the endurance 3. What HMA material factors affect fatigue life and conse- limit for metals as being a stress below which, for uncracked quently might affect the endurance limit for different materials, the plot of stress versus cycles to failure becomes materials? essentially horizontal and fatigue does not occur. Figure 2.1 4. What are the best methods of measuring fatigue life in the illustrates the theoretical concept of the endurance limit, as it laboratory? would be applied to HMA. 5. What analysis methods should be used to analyze fatigue The concept of the endurance limit was first implemented data in order to identify the endurance limit for a given for paving materials by the Portland Cement Association. mixture? An examination of fatigue tests conducted by various researchers on Portland cement concrete beams, discussed A tremendous volume of literature was identified related to in Baladi and Snyder (6) and Huang (7), indicated that if the the factors affecting fatigue life, methods of measuring fatigue stress ratio is kept below 0.45, the concrete will have an life, and analyzing fatigue data. Three very good summaries of essentially infinite fatigue life. The stress ratio was defined this literature were identified, one by Epps and Monismith (1) as the ratio of stress induced in the concrete pavement to the produced in the early 1970s and two additional summaries concrete's modulus of rupture. A maximum of 10 million produced as part of the Strategic Highway Research Program cycles to failure was used for the majority of this testing (one (SHRP) (2, 3). Therefore, an attempt was not made to sum- sample was tested to approximately 20 million cycles) (Fig- marize all of the references related to factors affecting fatigue ure 2.2). life and fatigue measurement. Monismith and McLean (8) first proposed an endurance limit of 70 micro-strain (ms) for asphalt pavements. It was observed that the log-log relationship between strain and Arguments for the Existence of bending cycles converged below 70 ms at approximately the Endurance Limit 5 million cycles. Maupin and Freeman (9) noted a similar Pavements have been designed primarily to resist rutting convergence. Using low-strain design principles, Monismith of the subgrade and bottom-up fatigue cracking. In classi- and McLean (8) designed a pavement structure that increased cal pavement design, as design load applications increase, the fatigue life of the pavement from 12 to approximately pavement thickness must also increase. There is a growing 19-plus years. belief that for thick pavements, bottom-up fatigue cracking In the field, Nunn (10) in the United Kingdom (UK) and does not occur. The concept of an endurance limit has been Nishizawa et al. (11) in Japan proposed concepts for long-life developed, representing a strain level resulting from a com- pavements for which classical bottom-up fatigue cracking bination of HMA stiffness and thickness, below which would not occur. Nunn (10) defines long-life pavements as bottom-up cracking will not initiate. Therefore, additional those that last at least 40 years without structural strengthen-

OCR for page 7
8 1000 Target Micro Strain 100 Endurance Limit 10 1 1 10 1, 1, 10 10 10 10 10 1, 10 00 00 00 0, 0 ,0 0, ,0 ,0 0 0, 00 0 00 00 0, 00 0 00 00 0, 0 ,0 ,0 0 00 00 0, 00 00 0 ,0 0 00 Number of Cycles to 50% Stiffness Figure 2.1. Idealized concept of the endurance limit. ing. The UK's pavement design system was based on experi- Surface initiated cracking was common in high-traffic mental roads that had carried up to 20 million standard axles. pavements, but there was little evidence of bottom-up When this study was conducted, these relationships were being fatigue. Surface initiated cracks tended to stop at a depth of extrapolated to more than 200 million standard axles. Nunn 4 in. (100 mm). (10) evaluated the most heavily traveled pavements in the UK, It was observed that the stiffness of thick pavements was most of which had carried in excess of 100 million standard increasing with time, most likely due to binder aging. This axles to evaluate the then current design system. Nunn (10) would not tend to occur if the pavement was weakening due concluded the following: to accumulated damage. A minimum thickness for a long-life pavement was recom- For pavements in excess of 7.1 in. (180 mm) thick, rutting mended as 7.9 in. with a maximum thickness of 15.4 in. This tended to occur in the HMA layers. range is based on a variety of factors such as binder stiffness. Source: Huang, Yang, H., Pavement Analysis and Design, 2nd ed., 2004, p. 317. Reprinted by permission of Pearson Education, Inc. Upper Saddle River, NJ. Figure 2.2. Illustration of endurance limit for concrete pavements (7).