Shear strengthening of headstocks using prestressed fibre strips

There is an urgent need to repair concrete structures that are failing short of their expected design life. There are many concrete bridges in Australia and around the world that are inadequate in terms of strength and serviceability. As engineers we need to find the best possible solution to repair these structures instead of just tearing them down and replacing them. Sustainable engineering practice is a requirement, and repair rather than replace is the best option for sustainability. External post tensioning offers a method of repairing these bridge structures that requires minimal work to put in place and significant improvements in strength and serviceability. This repair method gives engineers a viable and economical alternative to replacing bridge structures. The downside of external post tensioning work to date is the use of mainly steel tendons in the post tensioning components. These components are inherently vulnerable to corrosion under the conditions in which these structures exist. A viable alternative needs to be found. This research aims to show that fibre reinforced plastic (FRP) laminates can be this viable alternative. The main focus of this research was the development and construction of an FRP post tensioning system and testing it on a model bridge headstock, analyzing the resulting effects on strength and serviceability. The main focus of the testing was on enhancing the shear capacity of the headstocks. Some testing was also conducted on the effects of epoxy on shear cracking. A comparison was also made between the predicted capacity of headstocks using AS3600 and the actual observed capacity. The system has two main components - a jacking block and an FRP laminate wrap. The jacking block is constructed from high strength polymer concrete. It is designed to last as long as standard concrete but with increased strength and durability. The laminate wrap is made from an economical fibre continuously woven around the bridge headstock. The jacking blocks are used to exert a compressive force into the laminate strip which is then transferred to the concrete structure. This results in the same compressive force on the concrete that would occur through conventional steel post tensioning. When the system was tested on the model headstocks, increases in the shear strength were observed with the addition of the prestressing system. The increase in capacity was dependent on the extent of the shear cracking prior to strengthening. When the headstock was repaired using just the prestressing system a strength increase of 16% was observed. When the existing cracks were repaired with epoxy and then prestressed the strength increase was observed to be 46%. This shows a beneficial capacity increase by the use of epoxy and prestressing. The comparison of the test results with the predicted capacity from AS3600 showed that AS3600 adequately predicted the capacity of the new specimens. However, testing revealed that AS3600 could not adequately predict the capacity of the specimens with existing cracking. The author concluded that AS3600 was unable to model the failure of specimens strengthened with prestressing with the presence of existing cracks. AS3600 also does not readily assess diagonal compression failure as this was observed to be occurring in the test specimens. Overall further research into the design of FRP prestressing systems is required. There are obvious benefits of FRP over steel products as there are no corrosion problems. However further research into the design of the jacking system would be warranted and also the cost of such systems. Further analysis is also required into the effects of initial cracking on the predictions of AS3600. The standard does not address the effects of this initial cracking when predicting strengthened capacities. Further research also needs to be conducted in the assessment of diagonal compression failure loads as this is not easily computed with existing methods.