Experimental testing of residual strain and displacement development in pretensioned BFRP reinforced concrete beams

The utilisation of fibre reinforced polymers (FRPs) as reinforcement for concrete elements has attracted attention mainly due to their high tensile strength, light weight and corrosion resistance. Lately, there has been an interest in basalt FRPs as a more economically competitive and environmentally friendly option. Basalt FRP is manufactured from a widely available volcanic rock in a process which does not require any additives, giving it an edge over the currently more popular glass-fibre reinforced plastic (GFRP) reinforcement. Pretensioning of FRP reinforcement with low longitudinal modulus of elasticity, such as GFRP and BFRP has been proposed as an effective solution to the concerns regarding the serviceability performance of flexural elements reinforced with these composite materials. In previous research it has been demonstrated that prestressing even at low levels can significantly reduce deflections and postpone cracking of BFRP reinforced concrete elements. This research presents an experimental investigation of pretensioned BFRP reinforced concrete tested under quasi-static loading and unloading cycles at 5kN load increments until failure. A comparison with an unprestressed sample is also provided to examine the effectiveness of prestressing at improving the structural performance of the beams. The samples were equipped with internal strain gauges and linear displacement transducers to monitor the development of strains in the reinforcement, deflections and concrete surface strains during testing. Close monitoring of the anchorage zone and the development of cracks was also conducted. Based on the experimental results it can be concluded that the prestressing of BFRP reinforced beams delays the development of residual strains and residual displacements upon unloading. Furthermore, the increase in the prestress level further reduced the residual strain and displacements.