REVISION OF NAASRA INTERIM GUIDE TO PAVEMENT THICKNESS DESIGN.

The design procedures in the NAASRA Interim Guide to Pavement Thickness Design (1979) may be classified as empirical or partly mechanistic. The mechanistic elements of these procedures consist of predicting and limiting the surface deflection as a means of controlling the horizontal tensile strain in the asphalt surfacing or other bound layers. While this approach is simple to apply, and has a link with the common field practice of measuring deflections, it is only a rough control on strain levels. A more rigorous and consistent approach is proposed for the revised NAASRA Guide, based on strain prediction by structural analysis. The particular strains of interest are those associated with the primary modes of distress to which pavements are susceptible: cracking and rutting. The paper discusses structural analysis models, elastic properties of materials, flexible pavement design, and other aspects of the subject. Refs.