Experimental study on the influence of concrete cracking on timber concrete composite beams

Concrete cracking in timber-concrete composite beams causes a stiffness reduction of the concrete and increases deformations and stresses. The consideration of this effect in design is indispensable, because otherwise an unsafe design is obtained. However, existing design recommendations range from ignoring the effect to suggesting a 60% stiffness reduction of the concrete. A comparison of FEM simulations and reported bending tests reveal that the concrete cracking alone, as proposed in state-of-art literature, is not sufficient to explain measured stiffness reductions. The connector's slip modulus must be decreased as well when cracking occurs. In novel laboratory shear tests, small gaps simulating the concrete cracking show that the slip modulus can decrease by more than 70%. A comparison of laboratory beam tests with FEM analysis show that initial stiffness and crack initiation is predicted correctly by the standard's recommendations. However, the FEM results are only conservative if the connectors' slip modulus is additionally decreased by 60%, when concrete cracking is observed.