Shrinkage Deformations of Composite Slabs with Open Trapezoidal Sheeting

Composite steel-concrete slabs that have integral steel shuttering at the soffit of a reinforced concrete slab are commonplace in building infrastructure. Inevitable shrinkage of the concrete represents an indirect (or non-mechanical) strain that results in deformations of the composite slab, resulting in long-term effects in which creep of the concrete also plays a role. Because the impervious steel sheeting prevents moisture egress at the slab soffit whereas at the top of the slab such moisture egress can occur, there is a variation of the shrinkage strain through the depth of the slab, resulting in warping-type deformations. In addition, the composite action between the steel and concrete results in partial shear interaction between these elements, and this interacts with the shrinkage response. Surprisingly little appears in the published literature on rational modelling of the behaviour of composite slabs incorporating partial interaction that are subjected to shrinkage straining, despite its practical importance and significance. Based on fundamental principles of mechanics, a theoretical model of this shrinkage behaviour that includes partial interaction is developed, and prescriptive equations are derived. These may be used to assess the deflections and stresses in the concrete due to restrained shrinkage which may lead to cracking of the slab. It is also shown that the two effects of partial interaction and shrinkage straining counteract each other when slab deflections are considered.