Nebraska LTAP


Date of this Version


Document Type



Morcous, G., and Abo-Elkhier, M., (2021). Design and Detailing of Cast-in-Place and Precast Concrete Approach Slabs. NDOT Research Report M108.


Approach slab is a structural concrete slab that spans from the backwall 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 traffic loads over the backfill behind the abutments to avoid differential settlement that causes bumps at the bridge ends. Cast- in-place concrete approach slab is the current practice in US with various spans, reinforcement, thicknesses, joints, and concrete covers. NDOT has observed premature cracking in a significant number of approach slabs, which could result in a shorter service life and costly repairs/replacements as well as traffic closures and detours. The objective of this project is to investigate the extent and causes of approach slab cracking and propose necessary design, detailing and construction changes that could mitigate this deterioration. The literature on current approach slab practices by other state DOTs is reviewed and an analytical investigation is conducted using finite element analysis to evaluate the performance of different approach slabs under live load, volume changes due to shrinkage and temperature, and soil friction. Several parameters are considered in this investigation, skew angle, bridge width, joint location, and connection type. Analysis results indicate that volume changes cause high tensile stresses along abutment line, which result in the observed cracking. Several design changes are proposed and precast concrete approach slab alternatives are considered as promising solutions that could result in longer service life and accelerated construction.