Structural Behaviour of Concrete Railway Sleepers

The railway concrete sleeper is an important part of the railway track structure. The sleeper member requires substantial engineering skills during the design and manufacturing process in order to fulfil the requirements set by modern train traffic. The irregularities of the train or the track generate dynamic loads that have to be accounted for during the design of sleepers. A static design system with a dynamic amplification factor is often used. However, the effects of rapid shifts in the motion of the sleeper are thereby difficult to capture. Coupling of a linear track model and a non-linear finite element sleeper model is presented in this work in order to examine the behaviour of a sleeper when subjected to dynamic loads. It was found that the inertial forces induced in the sleeper and the underlying ballast during a train passage result in an increased curvature of the sleeper, which may lead to increased crack propagation. An interactive use of both the track model and the sleeper model showed that a reduced sleeper flexural stiffness due to cracking had only a small influence on the global track response. Prestressed strands are used in sleepers to prevent cracking of the concrete. The prestress load is anchored through bond and fully transferred to the concrete prior to the rail seat position of the sleeper. In-depth knowledge concerning the bond mechanisms and the influencing parameters of strands is thus needed. Pull-through and push-in tests together with a developed numerical bond model were used in this work to determine the bond behaviour of different strands. It was found that three mechanisms - adhesion, friction and mechanical action - determines the bond at different stages of loading. The bond capacity of the three mechanisms can be improved by changing the properties of the strand surface or the surrounding concrete. Full-scale tests of sleepers in combination with finite element analysis showed that the anchorage capacity is to a large extent influenced by the initial bond behaviour of the strand, which is given by the adhesion between the strand and the concrete. The adhesion can be improved by increasing the micro roughness of the strand surface or by using additives, for instance silica, to the concrete that increase the density and the strength of the interfacial transition zone created between the strand and the surrounding concrete