Keywords: fibres, fabric, reinforced concrete, ground floor slabs, plastic analysis.

This paper presents a new quasi-plastic method for predicting the structural behaviour of slabs on grade under point and line loading. Slabs on grade are widely used in warehouse and industrial buildings where their working surface must be maintained to a high tolerance. As a result, the performance of such floors under service loads is critical.

The analysis of slabs on grade is complex and current design guidance uses a mixture of either elastic and, or plastic analysis techniques. The amount of reinforcement present in slabs on grade is typically much less than for a suspended slab and therefore the application of classical plastic analysis techniques is less easy to justify.

The reinforcement present in slabs on grade usually comprises either fabric reinforcement or steel fibres. The amount of reinforcement in a typical slab on grade is such that the elastic moment capacity is higher than the plastic moment capacity. This is in sharp contrast to the behaviour of reinforced concrete sections.

A new quasi-plastic analysis method for slabs on grade under the action of line loads is presented in this paper. The method is largely analytical and is does not require the use of finite element software capable of modelling material non-linearity. The method comprises two basic steps. The initial step involves a non-linear analysis of the slab section, which yields a simple model for the moment-curvature relationship for the slab. The second phase of the method uses this moment-curvature relationship in combination with classic elastic analyses to identify the location of primary and secondary cracks. The method can be used to determine the allowable loads for an existing slab, or can be used for design.

The paper considers the performance of concrete slabs on grade that are reinforced with either steel fabric or steel fibres. The paper demonstrates the extent to which the load capacity of the slab can be increased by placing the fabric reinforcement close to the bottom of the slab. In the case of fibre reinforced slabs on grade, the paper concludes that fibres may significantly increase the allowable load capacity of a ground floor slab under the action of line loading even when the plastic moment capacity of the fibre reinforced slab is very low. This result is dependent of the fibre reinforcement being capable of restricting the propagation of the initial tensile crack under the line load to the slab's top surface.

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