Introducing a New Limit States Design Concept to Railway Concrete Sleepers: An Australian Experience

Over 50 years, a large number of research and development projects with respect to the use of cementitious and concrete materials for manufacturing railway sleepers have been significantly progressed in Australia, Europe, and Japan (Wang, 1996; Murray and Cai, 1998; Wakui and Okuda, 1999; Esveld, 2001; Freudenstein and Haban, 2006; Remennikov and Kaewunruen, 2008). Traditional sleeper materials are timber, steel, and concrete. Cost-efficiency, superior durability, and improved track stability are the main factors toward significant adoption of concrete materials for railway sleepers. The sleepers in a track system, as shown in Figure 1, are subjected to harsh and aggressive external forces and natural environments across a distance. Many systemic problems and technical issues associated with concrete sleepers have been tackled over decades. These include pre-mature failures of sleepers, concrete cancer or ettringite, abrasion of railseats and soffits, impact damages by rail machinery, bond-slip damage, longitudinal and lateral instability of track system, dimensional instability of sleepers, nuisance noise and vibration, and so on (Pfeil, 1997; Gustavson, 2002; Kaewunruen and Remennikov, 2008a,b, 2013). These issues are, however, becoming an emerging risk for many countries (in North and South Americas, Asia, and the Middle East) that have recently installed large volumes of concrete sleepers in their railway networks (Federal Railroad Administration, 2013). As a result, it is vital to researchers and practitioners to critically review and learn from previous experience and lessons around the world. Although those problems have been resolved through a systemic approach, there has been a significant demand to optimize the use of materials and to reduce wastes in concrete sleeper production. In doing so, there have been two research trends: materials and design improvement. The outcomes from both research directions must enhance and comply with the systemic performance and specific criteria as well as the operational environments of such railway networks. Often engineering specifications by rail authorities are in place to mitigate and monitor imminent risks that could potentially interconnect with other elements. Because of the systemic complexities, the potential of many material-driven researches becomes limited and relates to only traditional materials. For example, composite materials were developed purposely to equate just timber characteristics. Also, a recycled polymer material was tested as a timber-replacement alternative (Manalo et al., 2010). Breaking through the systemic complexities, a research outcome has led to an introduction of limit states design concept to concrete sleepers in Australia (Remennikov et al., 2012). The change in design concept (which is about 5–6 years behind the European counterpart) empowers the leaner and greener potential for manufacturing sleepers: either by reducing material wastes or by embracing new material innovation. The contemporary design philosophy for railway concrete sleepers is based on the “allowable stress principle” taking into account only the quasi-static wheel loads, which results in overly conservative, deficient design for concrete sleepers. The permissible stress design concept has fundamentally dominated in current Australian and some international design standards for concrete sleepers (i.e. in North America and Asia). Field data have also raised concerns about the permissible stresses design technique for concrete sleepers, which considerably relies on material strength reductions and then leads to over-designing concrete sleepers. It is well known that the permissible stress design method does not consider the ultimate strength of materials, probabilities of actual loads, and risks associated with failures and other operational and maintenance factors. Empirical data collected by railway organizations report that railway tracks, especially railway concrete sleepers, might have untapped capacity that could bring potential economic advantage to infrastructure owners (Kaewunruen and Remennikov, 2009a,b). The research project to study the actual load carrying capacity of concrete sleepers was developed as a collaborative project between several Australian universities and the industry partners within the framework of the Australian Cooperative Research Center for Railway Engineering and Technologies (RailCRC). The research tasks were required to perform fundamental studies of the loading conditions, the static