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16 Research conducted during the SHRP recommended a shift Materials factor of 10 between laboratory beam fatigue results and field performance, equating to 10% cracking in the wheel-path (46). The literature indicated that the primary material proper- Considering this shift factor, laboratory testing to 50 million ties affecting fatigue life are binder content, binder stiffness, cycles would equate to approximately 500 million loading and air void content. The literature indicated that aggregate cycles in the field or approximately the maximum possible gradation, type, shape, and angularity have more limited loading in a 40-year period. Based on these analyses, a mix effects. Different nominal maximum aggregate sizes, with that provided 50 million cycles or more of fatigue life in the their corresponding differing minimum VMA requirements, laboratory was considered to be indicative of a long-life will tend to produce differing volumes of asphalt binder. pavement. If pavements were designed to have a strain level at the en- Phase I durance limit, then all pavements, regardless of traffic, would be designed with approximately the same thickness of HMA A full-factorial experiment was conducted to evaluate the (assuming the same underlying support). This approach is existence of an endurance limit and to identify factors affect- contrary to the way pavements have been designed in the past ing the endurance limit. Two main factors were included in and is unlikely to be cost effective for future designs. Hence, the experiment. Two additional factors were fixed. These fac- it is more reasonable, especially for highways with low to tors along with their levels are as follows: medium traffic, to design a pavement for the expected traffic during an extended period of years (e.g., 40 to 50) than to Nominal maximum aggregate size (NMAS)--19.0 mm simply design at the endurance limit. Aggregate type--granite Practically speaking, highways with lower traffic levels can Asphalt content--optimum and optimum + 0.7% be designed with less pavement structure and still have long Binder stiffness--PG 67-22 and PG 76-22 lives. Hence, the amount of traffic has to be a critical element in the design process. In the past there has been very little The lower lifts of part of the structural experiment from fatigue testing at low strains (very high cycles to failure) and the 2003 NCAT Test Track were replicated to provide two this study was designed to identify the relationship between of the mixes for the experiment. The mixes were a 19.0 mm strain and cycles to failure at these very low strains (high cycles NMAS granite mixture at optimum asphalt content with to failure). In the process of evaluating low strains, it was felt both neat PG 67-22 and SBS modified PG 76-22 binder. The that the "endurance limit" would be better identified. average field gradations, shown in Table 3.1, were used as the Two test procedures were utilized to evaluate the existence target gradation for the laboratory study. Previous research of the endurance limit: beam fatigue tests and uniaxial ten- on fatigue has indicated that fatigue results are relatively insen- sion tests. Beam fatigue tests have been the most widely used sitive to gradation. Therefore, it was felt that the use of the method for testing fatigue in the United States. Uniaxial ten- average gradation would be appropriate. sion tests have provided an alternative that allowed a more The base mix was placed in four lifts for sections N3 and N4 fundamental analysis by modeling the viscoelastic material and two lifts in the other sections. The asphalt content of the behavior using Schapery's correspondence principle, contin- base layer for section N8 was intentionally set at 0.5% above uum damage mechanics, and work potential theory. optimum. Therefore, it was not included in the average. Table 3.1. Production gradations for base layers of 2003 NCAT Test Track structural experiment. N3 - N3 - N4 - N4 - Sieve Size N1 N2 Upper Lower Upper Lower N5 N6 N7 N8 Average 1" 100 100 100 100 100 100 100 100 100 100 100 3/4" 92 93 100 90 92 88 92 90 90 92 92 1/2" 80 84 84 79 79 77 79 78 78 83 80 3/8" 71 74 75 68 66 66 66 71 71 73 70 No. 4 49 53 57 50 49 49 49 53 53 54 52 No. 8 40 43 48 44 43 42 43 44 44 45 44 No. 16 33 35 42 39 36 36 36 36 36 37 37 No. 30 24 24 33 30 26 28 26 27 27 26 27 No. 50 13 14 20 16 14 16 14 15 15 14 15 No. 100 8 9 11 9 8 9 8 9 9 9 9 No. 200 5.5 5.5 6.7 5.6 5.5 5.5 5.5 5.7 5.7 5.5 6 Asphalt Content 4.3 4.5 4.3 4.6 4.7 4.4 4.7 5.0 5.0 5.2 4.7