High performance concrete railway sleepers

© 2016 Dr. Javad TaherinezhadRail transport is the safest and one of the most efficient transportation modes to move both passengers and goods. It has a range of advantages such as low travel time, high capacity, energy efficiency and low impact on the environment. The demand for rail transport is expected to keep its continuously increasing trend over the next decade. Conventional track or ballasted track is still widely used in a large number of newly established lines, even those constructed for high-speed or heavy-haul trains. Sleepers are an essential component of ballasted tracks and play a significant role in track safety and performance. While timber, steel and concrete have been used to make sleepers since the establishment of railways, nowadays, prestressed concrete sleepers (PCSs) are most commonly used as a solution to the increasing rail demand around the world. Although PCSs are a small structural element, their behaviour is very complicated due to different loading conditions, small dimensional tolerances, prestressing, steam curing, subjected to cyclic and high-magnitude dynamic loads and exposure to various environmental conditions. In addition, several reports show premature damage and early failure of PCSs that require unplanned replacement. Therefore, over the last few decades, many research projects have been aimed at identifying key behaviours, responses and interactions with other components, as well as main issues of PCSs in the track. These projects have shed some light on several aspects such as loading conditions, interactions with other components, dynamic responses and load carrying mechanisms of PCSs. The most critical problems of PCSs also have been identified globally, namely, cracking, especially from dynamic loads. However, the literature on performance of materials, particularly under dynamic loading, is still very limited. High performance concrete (HPC) is believed to be an appropriate solution to the most critical problems and unresolved issues of PCSs and is a subject of interest to the railway industry. In this regard, recently, a few research projects have been conducted to apply specific kinds of HPC as a material of PCSs in order to address their cracking or durability issues. Nevertheless, the cracking behaviour of PCSs made of HPC under dynamic loading is still lacking. Furthermore, some issues of PCSs are attributed to the inadequacy of current analysis (quasi-static) and design (permissible stress) methodology of the current standard codes of PCSs. This has motivated few researchers and organisations to shift the current design concept to a rational limit states format code. Nonetheless, establishment of a new code needs further investigation to recognise the behaviour of material under railway loads; for example, to identify the influence of dynamic loading on strength enhancement of PCS material due to strain rate effects. This study was mainly aimed at proposing the most appropriate types of HPC in order to make durable PCSs that possess sufficient resistance to cracking from high-magnitude dynamic loads. To achieve this main aim, a few specific objectives were proposed: (a) identify the level of strain rates in PCSs and their effects on material enhancement; (b) recognise the effectiveness of concrete grade on controlling PCS cracking; (c) identify the properties of concrete that effectively influence PCS cracking; and (d) identify the most appropriate types of HPC based on the findings in the previous objective. The current study was divided into five parts: (a) a comprehensive review of behaviour of PCSs; (b) an overview of specific behaviour and properties of HPC types that potentially can be suitable to make PCSs; (c) development and validation of an elaborated finite element model to simulate the behaviour of a PCS in track conditions; (d) a parametric study to investigate the influence of various factors on structural behaviour, especially cracking behaviour, of PCSs; and (e) propose the most appropriate types of HPC to produce durable PCSs. An elaborated finite element model was developed using the LS-DYNA package. The model consists of a full-scale PCS and its surrounding components. The model was calibrated using results of an experimental study, an analytical computer program, and a numerical track model. The finite element model is capable of simulating loading conditions of the track as well as all key behaviours of concrete materials under high-magnitude dynamic loads. Structural behaviour and severity of sleeper cracking were investigated using the model for a number of parameters, such as strain rate effects, concrete grade, key mechanical properties of concrete material, different levels of prestressing force, and specific combinations of these parameters. The results of this study suggest that the model developed may be used to analyse the structural behaviour of PCSs made of different materials. The effects of strain rates are recommended to be taken into consideration during the analysis and design process of PCSs. It was found that using higher grades of concrete is not an effective way to control PCS cracking from dynamic loads. The results proved that fracture energy of concrete is the most effective property of concrete to enhance the crack resistance of PCSs. It is revealed that including fibres in the concrete material is the most practical way to increase concrete fracture energy. Finally, it is recommended that using appropriate combinations of fibres and silica fume in concrete material may be the best option to make durable sleepers with adequate resistance to cracking from dynamic loads. This thesis also provides a framework to investigate the performance of different materials to make PCSs, which can be used in future studies. The thesis is expected to pave the design path to select the most appropriate types of HPC to make PCSs in various conditions and to contribute in the establishment of a new design approach