Long-term performance of timber-concrete composite flooring systems

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Timber concrete composites (referred to as TCC beams here onwards) consist of a concrete slab integrally connected to the timber joist by means of a shear connector. The coupling of a concrete layer on the compression side and timber on the tension side of cross-section results in efficient use of both materials. As the timber joist is mainly subjected to tension and bending while the concrete flange is mainly subjected to compression. The connection plays an important role for the composite action in determining the structural and serviceability performance of the system. Use of stiff and strong connection system contributes to a suitable bending strength and stiffness of the TCC together with other mechanical properties. Design of timber-concrete composite systems requires verification of serviceability and ultimate limit states. With the increasing trend in long span and light-weight construction, design of these floors may be governed by serviceability limit states and deflection under long-term load is one of the serviceability criteria that need to be addressed. The long term behaviour of timber-concrete structures depends on a number of phenomena taking place in its components. Phenomena such as creep and shrinkage effects in concrete, creep, shrinkage or swelling effects in timber and creep in connection affect long term strength, stiffness and deflection behaviour of timber-concrete composites. Creep due to variation in the moisture (mechano-sorptive creep) plays a major role in the long term behaviour of TCC floors. Few long-term experimental tests conducted so far have been reported in the literature. The objectives and scope of this study are to conduct long-term experimental test on timber-concrete composite beams, analyse the results to determine the creep coefficient of the composite system and compare the experimental results with the analytical solutions in accordance with Eurocode 5, in which the effective modulus method is used to account the effect of creep. To achieve the aforementioned objectives, a long-term laboratory investigation was started in August 2010 on four 5.8m span TCC beams with four different connector types. The specimens have been under sustained loads of 1.7kPa and subjected to a cyclic humidity conditions whilst the temperature remains quasi constant (22 °C). During the test, the mid-span deflection, moisture content of the timber beams and relative humidity of the air are continuously monitored. The long-term test is still continuing, two TCC beams were unloaded and tested to failure after 550 days, while the other two TCC beams are still being monitored and this report included experimental results up to the first 1400 days only. The long-term investigation on the two timber only composite floor beams commenced on March 2013 and the results are reported for the first 800 days from their commencement.
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