IMPROVED DESIGN OF UNBONDED CONCRETE OVERLAYS

Abstract

An unbonded concrete overlay is an excellent choice for long-lasting rehabilitation of a deteriorated concrete pavement. However, concerns over thickness and joint design of unbonded concrete overlays hinder their more widespread use. This paper reviews the existing approaches to unbonded overlay thickness design, examines actual field behavior and long-term performance of these overlays, and presents a framework for an improved design approach which relates design to performance. Four approaches to unbonded overlay design are considered: the traditional Corps of Engineers procedure, design of the overlay as a new pavement, and multilayer pavement design using either elastic layer theory or plate theory. These approaches are evaluated with respect to their theoretical bases, inherent assumptions, sensitivity to various design inputs, and compatibility with actual field performance of unbonded overlays. The major distresses in jointed unbonded overlays are linear cracking and comer breaks due to combined loading and curling; reflection cracking and other distresses are much less significant. Continuously reinforced unbonded overlays exhibit little distress of any kind except reflection cracking over transverse joints. However, the objectives of the existing approaches to unbonded overlay thickness design have little connection to the observed performance of either overlay type. Finite element analysis holds the most promise for an improved approach to unbonded overlay design. Two­ dimensional, plate theory-based multilayer finite element programs (or equations developed from them) are capable of realistically modelling most, but not all, of the complex factors which must be considered. Recommendations are given for further research needed to fully develop improved methods for unbonded overlay analysis and design.