Development of prefabricated timber-concrete composite floors

Sammanfattning: Timber-concrete composite structures were originally developed for bridges and upgrading of existing timber floors, but finds today new applications in multi-storey buildings. Most of the research performed to date has been focused on systems where "wet" concrete is cast on top of timber beams with mounted connectors. A novel composite system where the concrete slab is prefabricated off-site with the connectors embedded and then connected on-site to the timber joists is presented in this thesis. A notable benefit of prefabricating the concrete slab is that most of the concrete shrinkage will develop when the slab is not yet connected to the timber beam, markedly reducing the correlated increase in deflection and flexural stresses in the composite beam. Another advantage of the prefabrication is the reduction in construction cost by moving work from the building site to the workshop. In this research special attention has been paid to develop and use connection systems easy to produce and mount in order to speed up the construction process. Different types of shear connectors suitable for prefabrication were investigated; coach screws (alone or combined with a timber joist notch), metal plates (glued or nailed), dowels, and toothed metal plates embedded in the concrete slab. The experimental test programme includes: (i) direct shear tests to failure of 7 different connection systems, (ii) full-scale bending tests to failure followed by a numerical analysis, (iii) long-term tests under sustained load of full-scale composite beams and a following numerical analysis. The outcomes of the experimental tests show that it is possible to achieve good structural performance with a prefabricated system, particularly using the connection with coach screws and timber notches. The mechanical properties of the prefabricated connections can be used in the design method in Eurocode 5. The experimental-numerical investigations indicate that the new shear connections are suitable for prefabricated timber-concrete composite structures and perform equally well as "wet" systems. It is possible to achieve high load-carrying capacity and stiffness (up to 98%) using the new shear connectors. It is feasible to manufacture timber-concrete composite structures as prefabricated elements. An additional benefit is that some of the systems are fully demountable, allowing the owner to dissemble the timber beams and concrete panels at the end of their service life. From the numerical analysis it was found that at the end of service life (50 years) the prefabricated timber-concrete composite floors behave adequately. The mid-span deflection is within the acceptable limit of L/250. Numerical studies show that prefabrication off-site of the concrete slab results in a reduction of the long-term deflection if the slab is cured for at least 56 days before it is connected to the timber beam. Furthermore, the construction method (propped or unpropped) affects traditional timber-concrete systems with cast-in-situ concrete more. The props should be left inplace for at least seven days using cast-in-situ concrete, while for prefabricated concrete slabs the props need to be left in place only for one day, leading to a significant economic advantage. Lastly, the long-term deflection is hardly affected by the time between the end of construction and the live load application.