FUNCTIONAL AND STRUCTURAL FLEXIBLE PAVEMENT OVERLAY DESIGN FOR INDIANA TO OVERCOME A DEFICIENCY IN THE 1986 AASHTO GUIDE

A procedure for designing the thickness of asphaltic concrete overlays of flexible pavements in Indiana was developed. In analyzing the research data collected, a deficiency in the 1986 AASHTO Guide became apparent: the overlay design procedure addresses only structural overlays and ignores functional overlays. Many miles of asphalt pavement in Indiana are overlaid for functional rather than for structural reasons. The research included testing on 30 flexible pavement test sections statistically selected to be representative of flexible pavements in Indiana. Two approaches were taken: an empirical approach that calculates the overlay thickness required to provide functional performance (ride quality) over the life of the pavement and a structural overlay method. Flexible overlay design Method 2 of the 1986 AASHTO Guide for the Design of Pavement Structures was selected for structural capacity design. Method 2 uses nondestructive test (NDT) deflection data to calculate overlay thickness. If found, a negative value for overlay thickness indicates that sufficient structural capacity is present without adding an overlay. The functional performance approach used Indiana flexible pavement historical data to produce a regression equation relating overlay thickness to anticipated future traffic, overlay design life, pavement condition at design life end, and CBR. Simultaneous use of the two design methods was recommended to IDOH.