Flexible pavement overlay design procedures. Volume 1: Evaluation and modification of the design methods

A review and evaluation of existing flexible pavement overlay design procedures, including the ARE procedure (FHWA Contract DOT-FH-11-8544) resulted in a fully computerized rational method for the design of flexible overlay of flexible pavements. The overlay method presented is a combination and modification of several existing methods and incorporates the latest concepts in pavement evaluation and overlay determination. The overlay thickness is determined based on a fatigue distress function developed from the AASHO Road Test data and relates horizontal tensile strain in the asphalt layer to the number of equivalent 18 Kip (80kN) axle loads to failure. The existing pavement is evaluated using NDT 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 is analyzed using 3-layer linear elastic theory; the in-situ layer stiffnesses are determined by matching measured deflections with those computed from layer theory. The in-situ asphalt modulus is compensated for temperature and this adjusted modulus is used in design computation. Base and subgrade moduli are corrected for stress effects when test loads differ from design loads, or when the state of stress is changed as a result of adding an overlay. Also incorporated is an environmental factor which enables NDT to be conducted during most of the year. This design method has been compared against overlay design methods used by 3 different states in the U.S. Limited comparisons show that the proposed procedure has universal applicability and results in overlay thicknesses consistent with local methods. This volume is the first in a series. The other is: Flexible Pavement Overlay Design Procedures. User Manual, Volume 2 (FHWA-RD-81/033). (FHWA)