Design and Detailing of Bridge Approach Slabs: Cast-in-Place and Precast Concrete Options

Abstract

Approach slab is a structural concrete slab that spans from the back wall of the abutment (i.e. end of the bridge floor) to the beginning of the paving section. The purpose of the approach slab is to carry the dead and live loads over the backfill behind the abutments to avoid differential settlement that causes bumps at the bridge ends. Cast-in-place (CIP) concrete approach slab is the current practice in most of the states in US with various spans, reinforcement, thicknesses, and concrete covers. However, it has been reported that most approach slabs experience cracking and settlement, which result in premature deterioration and shorter service life. The replacement of deteriorated approach slabs causes costly and long traffic closure and detouring. Precast concrete (PC) approach slabs is a promising solution that could provide longer service life and accelerated construction/replacement. This paper presents a literature on current approach slab practices and innovative precast concrete solutions. Also, an analytical investigation is conducted using finite elements to evaluate the performance of the current approach slab practices in the state of Nebraska. Several parameters are considered in this investigation, such as volume changes due to shrinkage and temperature changes as well as skew angle and bridge width. Analysis results indicate that volume changes cause high tensile stresses along abutment line, which results in longitudinal cracks. Also, high skew angles result in stress concentrations at the slab corners and the increase in slab width increases the stresses in transverse direction.