The behavior of asphalt-bound layers, unbound aggregate bases, and foundation soils (subgrades) may be affected by such variables as gradation, asphalt and (or) moisture contents, type of aggregate, density, method of compaction, temperature, magnitude and frequency of loading, duration of each load cycle, and other less significant variables. The complex interaction of all these variables yields a composite behavior for a particular pavement structure that could become manifest in some form of distress or even complete "failure."

Flexible pavements are susceptible to rutting. But it is not well known where and to what extent rutting takes place within a pavement structure. Rutting is a result of the lateral distribution (generally approximated by a normal distribution) or scatter of wheel passes across the wheelpaths. A large percentage of wheel passes occur within relatively narrow paths on the pavement surface. It is this distribution of traffic that causes accumulated deformations to occur, producing ruts. If these ruts are to be estimated or predicted for design purposes, the behavior of the materials must be known or "parameterized."

To determine where in the pavement structure and to what extent rutting occurs and to determine the factors that control rutting, a comprehensive laboratory testing program was performed. Various traffic and environmental parameters were controlled in the study; and from the data, mathematical models that described the rutting behavior of an asphaltic concrete, a dense-graded aggregate, and a subgrade soil were formulated. A traffic and a temperature model were also formulated to provide necessary input into the rutting models. These models have been collected and programmed into a computer program entitled PAVRUT. Using this program, an estimated rut depth can be calculated for any flexible pavement, assuming the volume and characteristics of the traffic stream and the properties of the paving materials are known.

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The contents of this report reflect the views of the authors who are responsible for the facts and the accuracy of the data presented herein. The contents do not necessarily reflect the official views or policies of the University of Kentucky, of the Federal Highway Administration, nor the Kentucky Transportation Cabinet. This report does not constitute a standard, specification, or regulation.