Time-Dependent Behaviour of Composite Concrete Slabs

Composite one-way concrete floor slabs with profiled steel decking as permanent formwork are commonly used in the construction industry. Despite their common usage, relatively little research has been undertaken on the in-service behaviour of such slabs and little guidance is available to practising engineers. Most previous experimental studies of composite slabs incorporating steel decking have involved short-term load tests that have concentrated on the strength, ductility and failure mode of the slabs. Long-term testing to study the time-dependent service load behaviour and in particular, the effects of creep and shrinkage of the concrete in this increasingly popular form of construction have been largely neglected.There exists an urgent need for research on the serviceability of composite slabs, as excessive deflection is often the critical design consideration, especially in long-span slabs. The situation has been exacerbated in recent years with the advent of high strength materials (both concrete and steel) without significantly higher stiffness, and the development of a wide range of new and innovative decking profiles. Serviceability rather than strength is often the governing limit state in the design of concrete elements, particularly in shallow deflection-sensitive elements such as floor slabs.The research in this thesis is the first detailed study of the effects of creep and shrinkage on the in-service behaviour of composite slabs. Laboratory experiments have been conducted on composite slabs to measure: the distribution of shrinkage strain through the thickness of the slabs; shrinkage-induced deflections in the slabs and time-dependent deflections and cracking under sustained loads. Analytical models have been developed and calibrated to predict these effects and reliable design guidance has been presented for the construction industry and practising design engineers. The change in the bond-slip relationship between profiled steel decking and concrete slab with time due to creep and shrinkage was not identified previously. The time-dependent effects of creep and shrinkage on the bond-slip characteristics for slabs with profiled decking during extended periods under sustained service loads have also been investigated