Behaviour of pre-Tensioned hollow-core slabs with in-situ concrete topping

Nowadays, hollow-core slabs are probably the most popular kind of precast slabs used in many kinds of projects. A long tradition in many countries in Europe arid North America is application of cast in-situ topping, most often 50 mm thick. At present, medium-strength concrete is used as a topping, e.g.C25/30 or C30/37, protected against shrinkage with fabrics of relatively thin wires. The long discussions at modifications of the Standards and finally very conservative recommendations in EN 1992- l-l:2004 caused the new situation. To treat such slabs as composite structures the transversal interface reinforcement or special shaping of precast members' surface is necessary. This is because live load is usually imposed not as uniformly distributed but as local (concentrated or linear). Therefore, the possible design shear resistance at the interface is very small, and transverse reinforcement must be introduced. Designers and contractors are against such rules, particularly in situations where topping is cooperating with transversal and longitudinal parts surrounding prefabricated parts. An experimental program was developed in order to clarify the actual behaviour of pre-Tensioned hollow-core slabs with typical topping placed in-situ, without interface reinforcement or any social preparation of the interface. The full scale members were tested under linear transversal loading. In the first step the elements over one year old were tested under instantaneous loading up to failure, while in the second step quite new elements - firstly subjected to long-Term loading - were finally also tested up to failure. In both cases the cooperation of topping was very good, up to the level of over 95% of failure load. The tests confirmed the observations from practice and opened the way to omit in particular cases the conservative rules given for all kinds of composite structures "with concrete cast at different times" .