Overview of the Test

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CHAPTER 2 LITERATURE REVIEW

2.1 Background

The current flexible pavement design methodology used by Bina Marga Directorate General of Highway, DGH is derived from the results of the American Association of State Highway Officials AASHO Road Test, conducted in the late 1950’s. A basic nomograph and design equation were developed from the road test results including inputs of soil modulus, traffic, ride quality serviceability, and the capacity of the pavement structure structural number. Using the flexible pavement design equation or nomograph in conjunction with these inputs, the designer arrives at a design thickness for the pavement layers.

2.2 AASHO Road Test

2.2.1 Overview of the Test

The AASHO Road Test was conducted from 1958 to 1960 near Ottowa, Illinois. The primary purpose of the road test was to determine the effect of various axle loadings on pavement behavior. Both flexible and rigid pavements were tested in the study, along with several short span bridges. Six two-lane test loops were created for trafficking, including four large loops and two small loops. Hot mix asphalt HMA and base thicknesses were varied within each test loop to determine the effect of axle loadings on different pavement cross sections. Individual lanes were subjected to repeated loadings by a specific type and weight of vehicle. Single and tandem axle vehicles were used for trafficking. Bias-ply tires were used with pressures of approximately 70 psi. Only 2 million equivalent single axle loads ESALs were applied over the course of the test. One of the key products of the road test was the concept of load equivalency, which accounts for the effects of the axle loads on pavements in terms of an equivalent single axle load ESAL. Under this concept, the damage imposed by any vehicle is based on its axle weights compared with a standard 18,000 lb axle load. The 7 ESAL values for other axles express their relative effect on pavement structure. If the number and types of vehicles using the pavement can be predicted, then engineers can design the pavement for anticipated a number of 18 kips equivalent single axle loads 18 kips ESAL. Virtually, all heavy-duty pavements built in the United States since the mid-1960s have been designed using the principles and formulas developed from the Road Test Davis, 2009. The test vehicles ranging in gross weight from 2,000 lb to 48,000 lb. The improved paving materials that are used today such as Superpave mixes, stone mastic asphalts, and open graded friction courses were not available during the road test. Within the pavement cross section, only one type of HMA, granular base material, and sub grade soil were used. The thickest HMA pavement was 6 inches. All results from the road test are a product of the climate of northern Illinois within a two-year period HRB, 1962.

2.2.2 Results of AASHO Road Test