ABSTRACT
Bridge Approach concrete Slabs (BAS) are widely used as a gradual transition between concrete bridge slabs and adjacent concrete/asphat pavements or embankments. Their main function is to provide a smooth and structurally sound transition to the adjacent roadway pavement. A concrete approach slab is generally supposed to be cast on a uniform, well-compacted soil, and its end may be simply supported at the abutment or resting on a sleeper slab/beam or just on the underlying grade, i.e. slab on grade (SOG).
Several numerical idealizations have been adopted. These include linear Finite Element Method (FEM) and FE with partial soil support. Idealizations with limited nonlinearities were used to model BAS. FEM with full material nonlinearities for SOG considering the concrete slab and reinforcement properties are few. Analyses which consider both the reinforced concrete slab and underlying soil material nonlinearities are rare. Idealizations with material nonlinearities can help assess the actual factors of safety involved and help towards better design, as resulting displacements and loads provide bases for comparison with serviceability criteria derived from tests and pertinent standards. Some design guidelines take into account the potential loss of soil support and provide for partial degree of support to the BAS, by using an effective span factor. The most conservative central deflection situation is for the case of a slab on grade.
BAS seldom receive appropriate design considerations. Design practices are usually based on oversimplified assumptions, generally following the “absence of crack” criterion which will lead to a relatively thicker slab. Such designs mainly do not account for the actual inherent material nonlinearities of the concrete slab, the reinforcement and the interaction with the underlying soil, leading to varying relative stiffnesses which affect the straining actions especially under heavy bridge loads. The behavior of a concrete BAS is governed by soil-slab interaction, where the moments are reduced with the reduction in rigidity. Concrete cracking and the decrease in flexural rigidity can significantly affect the stress distribution, reduce the maximum moments and increase deflections. Neglecting this may lead to over conservative and uneconomical thicknesses and irrational designs. BAS can be considered as structurally active reinforced concrete slab on grade and designed assuming the slab to be cracked under mechanical loads and the reinforcement is provided and positioned according to the concrete design requirements, not only to control crack widths, but to contribute to the load carrying capacity, and applying the principles of reinforced concrete. The slab is designed to crack, and reinforcement is provided to resist the tensile stresses caused by the peak moment, while the slab thickness is designed so that concrete in tension can resist the relatively low negative moment. Thus the design results in thinner slabs. The effects of the reinforced concrete slabs and soil conditions under prevailing loads can be considered through finite element analyses with material nonlinearitis. However this may be a little tedious, and can be resorted to for special cases, boundary conditions, loadings and once needed. The bases for a simpler approximate approach for concrete BAS design accounting for concrete, reinforcement and underlying soil properties, as done for structurally active concrete SOG, are recommended, noting that the deflections for SOG are generally more conservative. Serviceability conditions including rideability quality often control the BAS design. An approach for estimating the deflections under loads greater than the concrete slab cracking loads is referred to. Nonlinear concrete slab analysis can lead to better understanding of the structural behavior of such complex elements, and improved economical designs can be attained.
