OVERLAY DESIGN OF FLEXIBLE PAVEMENTS: OAF PROGRAM

A fully computerized rational method for the design of flexible overlays of flexible pavements is presented. The overlay thickness is determined based on a fatigue distress function developed from analyzing AASHO Road Test data. It relates horizontal tensile strain in the asphalt layer to the number of equivalent 18-kip (80-kN) axle loads to failure. A rutting criterion is not included because (a) rutting in unbound layers has generally stabilized by the time an overlay is contemplated, (b) rutting in the asphalt layer can be controlled by proper mix design, (c) the addition of an overlay decreases the stresses in the unbound layers, and (d) fatigue requirements generally dictate the overlay thicknesses. The existing pavement is evaluated by using nondestructive dynamic deflection measurements and a visual survey that includes general observations of drainage, the existence of rutting, and the presence and type of cracking. The deflection data are analyzed by using linear elastic theory in which the existing pavement is represented by a four-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. The in situ asphalt modulus is modified for temperature effects, 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.