OVERLAY DESIGN OF FLEXIBLE PAVEMENTS USING DYNAFLECT

This paper presents a fully computerized rational method for the design of flexible overlays of flexible pavements using dynaflect. The overlay thickness is determined based on a fatigue distress function which was developed from analyzing the AASHO test road data, and relates horizontal tensile strain in the asphalt layer to the number of equivalent 18 kip (80 kn) axle loads to failure. The existing pavement is evaluated using non-destructive dynamic deflection measurements and a visual survey which includes general observations regarding drainage, the existence of rutting, and the presence and type of cracking. The deflection data are analyzed using linear elastic theory in which the existing pavement is represented by a 4-layer model consisting of a pavement layer, base and subbase layers, and a subgrade layer. The in-situ layer stiffnesses are determined by matching measured deflections with those computed from layer theory. The design procedure recognizes that asphalt modulus is temperature dependent and the in-situ asphalt modulus is compensated for temperature and this adjusted modulus is used in design computation. The base and subgrade materials are corrected for stress effects when the state of stress is changed as a result of adding an overlay. The procedure also incorporates an environmental factor. (Author/TRRL)