An investigation was carried out into the bond development length characteristics of 9.3mm indented prestressing strands in concrete sleepers. The study consisted of both experimental work and theoretical analysis study of results obtained. The main objective was to determine whether the 9.3mm strand would be suitable for use in pretensioned concrete railway sleepers.
In pretensioned concrete railway sleepers, maximum loading acts on the railway sleeper through rail seats, however there is limited space for the bond development length of the strands. Therefore, the bond requirements in this area are particularly significant. Past research has revealed that current Australian Standard provisions for the required development length are too conservative. Reducing the quantity of materials required by refinement of the code would lead to more economic sleeper design.
The aim of experimental study was to determine the development length required for the 9.3mm strands, by using the modified pullout test in which the fully stressed strand was encased in concrete. The test specimens was prestressed and then steam cured, follow with the ambient cure in the usual manner for pretensioned concrete railway sleepers. From these results, conclusions about the development length of the strand were drawn. An analysis was then carried out to attempt to predict the results obtained using a bond stress versus slip model of the type given in the European code for traditional concrete reinforcement. This analysis was duplicated for the pullout test results obtained from a previous experimental study into the bond characteristics of a 5mm indented wires. A reasonable prediction of the bond stress verses slip relationship was achieved by altering the parameters given in the European code.
It was concluded from the experimental work and analysis that a development length of 400mm is adequate for 9.3mm prestressing strand. Also it was revealed that the current Australian Standard code for calculating development length was over conservative. Furthermore, a bond stress verses slip model was developed which could reasonably predict the experimental results.
From the experimental work and analysis, it is recommended a pretensioned concrete railway sleeper test be conducted to confirm the development length required. Also, further study in the region on decreasing and residual stress of local bond stress-slip curve is recommended.