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15 CHAPTER 3 Research Plan Introduction and a single steering axle (three axle groups). Strain traces indicate that the tandem axle results in two distinct load Based upon the review of the literature, a controlled labo- repetitions (49). Assuming that one is designing a perpetual ratory experimental plan was developed. The experimental pavement for the tandem axle load, the steering axles would plan was developed with the primary objective of testing the have a lower loading so, theoretically, in a perpetual pavement hypothesis that there is an endurance limit for HMA mixtures. design they would do no damage to the pavement. Thus, each As a secondary objective, some of the HMA material proper- Class 9 vehicle would provide four load repetitions to the pave- ties that affect the endurance limit were investigated. ment for a maximum total of 1,317,504,000 axle load repeti- A working definition of the endurance limit was devel- tions in a 40-year period. This represents a theoretical maxi- oped as a framework for testing within the experimental plan. mum loading where every truck is fully loaded and the design Although the endurance limit is defined as an essentially in- lane is at maximum capacity for 24 hours a day, 7 days a week, finite fatigue life for metal alloys, testing for an infinite life is for a 40-year period. This loading condition would be expected impractical. The endurance limit must be defined in practi- to be even more severe than a dedicated truck lane. A similar cal, usable terms if it is to have meaning. For example, the methodology was used by Mahoney to calculate the maxi- literature has defined 40 to 50 years as a reasonable lifetime to mum number of ESALs expected in a 40-year period (unpub- be considered as a long lasting, or perpetual, pavement. Hence, lished data). determining a strain level that results in 40 to 50 years (or In actual mixed traffic streams, the highest percentage of even more) of pavement life is a very practical way to identify trucks tends to be about 50%, which would reduce the maxi- the endurance limit. mum number of load repetitions to 940,473,600 or a maxi- The Highway Capacity Manual states that the maximum mum number of load repetitions of 592,876,800 for 25% number of passenger cars per hour per lane for a freeway at a trucks. Even the most heavily traveled highways do not free flow speed of 65 mph is 2,350 (48). In rolling terrain, a maintain traffic streams at capacity 24 hours a day and not single truck or bus would replace 2.5 passenger cars (48). all trucks are loaded. The fact that all trucks are not fully Thus, one would expect a maximum of 940 trucks per hour, loaded is illustrated by a Washington DOT study of 10 weigh- 22,560 trucks per day, or a maximum of 329,376,000 trucks in-motion sites over a one-year period, which indicated in a 40-year period. Such a case might represent a dedicated that the typical number of ESALs for a Class 9 vehicle was truck lane running at capacity 24 hours a day, 7 days a week, 1.2 (50). If 1.2 "design load" axles were applied per truck for 365 days a year, an unlikely occurrence. By comparison, the the maximum number of trucks per lane in a 40-year period very heavily traveled section of Interstate 710 in California (329,376,000), a total of 395,251,200 load repetitions would carried a maximum of 9,650 trucks per day in the design be applied. Also, in winter months in many parts of the lane (20). By calculating the appropriate heavy-vehicle adjust- country, the pavement stiffness is very high, and this results ment factor and determining its impact on traffic flows (48), in significantly lower strains. Therefore, it is a reasonable mixed traffic streams with 25% and 50% trucks would pro- assumption that the maximum possible number of load duce a maximum of 148,219,200 (10,152 trucks per day) and repetitions expected in a 40-year period is approximately 235,118,400 trucks in a 40-year period, respectively. 500 million. This could be considered as a practical target Consider, for example, an FHWA Class 9 vehicle or five-axle for evaluating parameters (strain or energy) indicating an single trailer, which typically consists of two tandem axles endurance limit.