Influence of the construction method on the long-term behavior of timber-concrete composite beams

Timber-concrete composite beams (TCCs) are made from a concrete topping fastened to a timber beam. Different methods can be used to construct the TCCs. The concrete topping can be either poured on the timber beam with preinstalled connectors or prefabricated off-site and then connected to the timber beam. The timber beam can be shored during assembling with the concrete topping or left unshored. The use of the shored construction is usually recommended as a way to reduce long-term deflection; however, it is costly. This paper investigates the influence of the construction method on the long-term behavior. A previously developed rigorous uniaxial finite element (FE) software has been first validated on some long-term tests performed in Sweden on TCCs with prefabricated concrete slabs and different connection systems. The FE model has then been used to carry out some analyses where the deflection in the long term has been compared with the same TCC constructed in different ways. Parameters investigated include the mode of construction of the concrete slab (prefabricated off-site or cast in situ), the assembly of the composite beam (with shored or unshored timber beam), the storage time of the concrete slab, and the time between the end of construction and the application of the live load. The outcomes of this numerical study indicate that by prefabricating the concrete slab it is possible to reduce the long-term deflection, particularly when the slab is stored for at least 28–56 days before it is connected to the timber beam and when the timber beam is shored for at least 1 day. In these cases, the effect of concrete shrinkage is reduced, as well as the instantaneous and delayed deflection due to the self-weight of the beam. Conversely, systems with concrete poured on the timber beam should be shored for a longer time (at least 7 days). Shoring the timber beam does not markedly change the increase in long term deflection due to concrete shrinkage; however, it does raise the tensile stresses in the concrete slab during the period the shores are left in place, leading to the potential for cracking when stiff shear connectors are used and for long shoring times.