ABSTRACT: The principal objective of this research was to study the influence of the environment on the thickness of flexible pavements. Environmental variables considered include general soil conditions and temperature effects. As identified in previous studies, six climatic zones were recognized. Weather information and soil properties were collected for 175 typical stations covering the continental United States, excluding Alaska. Based on the criteria of rutting of 1.25 cm (0.5 in.) and thermal cracking of 115 m/1000 ms (35 ft/l,000 ft²), appropriate asphalt-cement grade were selected for each station. To consider the interaction of temperature and modulus with fatigue damage, the concept of effective modulus was introduced. The effective modulus calculated by using the appropriate asphalt grade was found to be nearly constant within a zone but varied considerably from one zone to another. A sensitivity analysis was performed on the AASH0 flexible pavement design equation, the purpose of which was to determine the effect of the regional factor and the soil support value on the structural number. After these two items had been combined with the change in the layer coefficient due to modulus change, their overall effect on pavement thickness was evaluated. The ratio of the thickness required at a given station to that required at reference conditions [namely, asphalt effective modulus of 34.5 kPa (5 x 105 psi), regional factor of 1.0, and soil support value of 5.0] is defined as the depth factor. The depth factor ranged from as low as 0.45 in Florida, parts of Mississippi, Alabama, Georgia, and the Carolinas to as high as 1.60 in regions of Montana, North Dakota, and South Dakota. The higher the depth factor, the more severe the influence of environment on pavement performance. Examples to illustrate how the depth factor may be incorporated into the AASHTO flexible design are given.

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