Fire performance of Fibre Reinforced Polymer (FRP) bars in reinforced concrete: an experimental approach

Abstract

During the past two decades, Fibre Reinforced Polymer (FRP) bars have been applied as viable alternatives to internal steel reinforcement of concrete, owing to their numerous benefits over steel reinforcement including comparatively high tensile strength and non-corrosive properties. However, there are limitations on the use of FRP as reinforcement, where fire resistance of structures is required, due to a lack of understanding of the behaviour of FRP materials at elevated temperature. This hinders application of FRP materials in many cases. To understand the complexities of FRP bars’ response at elevated temperature, this thesis examines current design guidance and literature to highlight gaps in understanding. The experimental work within the thesis focusses on three commercially available FRP bars; two Glass FRP (GFRP) bars and one Carbon FRP (CFRP) bar. Bench-scale characterisation tests using Dynamic Mechanical analysis (DMA) and Thermogravimetric analysis (TGA) have been performed to understand the deterioration of FRP bars at elevated temperature. The experimental work has defined a glass transition (Tg) and decomposition temperature (Td) range for each of the FRP bars. Using the results from the bench-scale characterisation tests and direct tensile tests, a novel predictive model for the reduction in tensile strength of FRP materials at high temperature has been proposed. A study on the bond capacity of fibre reinforced polymer (FRP) bars in concrete at elevated temperature demonstrated the requirement for cold anchorage of the reinforcement. To further determine the impact of cold anchorage on FRP reinforced concrete (RC) beams, tests were carried out with both continuous and lap spliced FRP at ambient temperature and under sustained load with transient localised heating. Cold anchorage of the reinforcement was maintained throughout testing and confirmed with local temperature measurements. The results demonstrate that cold anchorage (i.e. maintained below the onset of the glass transition range) of FRP bars is necessary to ensure their safe use as internal reinforcement in concrete, unless unrealistically deep concrete cover is provided. Cold anchorage may be provided in a number of ways; continuity of reinforcement across compartments, bent bars in the anchorage zone or increased concrete cover at anchorage zones. Where this is provided the performance of FRP bars is demonstrated – for the particular conditions of the current study – to be satisfactory under full service loads and at reinforcement temperatures exceeding the decomposition of the polymer matrix (>380°C for the bars in the current study). The research has identified a minimum suite of tests necessary to characterize thermo-mechanical behaviour of proprietary FRP bars. By understanding the effects of temperature on the polymer resin matrix and on the FRPs’ tensile and bond properties, and by rationally optimizing the placement and anchorage of the bars, this thesis has demonstrated FRP reinforcements may be designed as fire-safe alternatives to steel reinforcement for concrete.