Experimental and theoretical study of the behaviour of composite bridges during construction

The behaviour of steel-concrete composite bridges is influenced by time dependent effects in the concrete deck slab that can affect the durability of such structures. A detailed study of the causes of transverse cracking in concrete slabs of composite bridges has been carried out in order to understand better the most important parameters with respect to the origin of tensile stresses in the slab. In-situ measurements and laboratory tests have enabled the behaviour of concrete slabs to be observed from the moment they were placed. The results of measurements have demonstrated the important influence of concrete hydration and the pouring sequence on the tensile stresses in the slab. It was demonstrated that for typical bridges the most critical tensile stresses in the slab are induced during construction. Methods for limiting the effects of hydration, such as cooling the concrete or the use of low-heat cement, have been tested in the laboratory and compared to the results of numerical simulations. A criterion for evaluating the probability of crack formation in young concrete has been established on the basis of these simulations. Other tensile stress limiting methods, such as prestressing and the pouring sequence, have been proposed and analysed. Practical indications have been given as a function of the span length and of their short- and medium-term efficiency.