Effects of shrinkage on the long-term stresses and deformations of composite concrete slabs

Composite one-way concrete floor slabs with profiled steel decking used as permanent formwork are commonly used in the construction of floors in buildings. The steel decking supports the wet concrete of a cast in situ slab and, after the concrete sets, acts as external reinforcement. Relatively little research has been undertaken on the time-dependent in-service behaviour of such slabs and little guidance is available to structural engineers for predicting the long-term deflection. The drying shrinkage profile through the thickness of a slab is known to be greatly affected by the impermeable steel deck at the slab soffit, but this has not been quantified to date. This paper presents the results of laboratory measurements of the non-uniform distribution of shrinkage through the thickness of composite slabs, and the resulting effects on the long-term deflection of such slabs are considered. An analytical procedure for the determination of time-dependent stresses and deformations in composite floor slabs is described, and the results of analyses are presented for several common decking profiles using the measured shrinkage profiles.