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The favourable mechanical properties of basalt fibre-reinforced polymer (BFRP) bars, such as their excellent strength-to-weight ratio, resistance to corrosion, lower environmental impact and electromagnetic neutrality, make them attractive for internal reinforcement of concrete elements with specific service requirements. Prestressing has emerged as a possible method for limiting the deflections and cracking of BFRP reinforced flexural RC elements. However, long-term behaviour of such structural members has not yet been investigated extensively. Therefore, this paper aims to give information on long-term behaviour and prestress losses of pretensioned BFRP reinforced concrete beams based on the collected experimental data. The testing programme includes long-term analysis of six BFRP concrete beams pretensioned to different prestress levels; namely, 20%, 30% and 40% of the ultimate tensile capacity of the bars. The monitored testing stages included initial tensioning of the bars, casting and curing of concrete, transfer of prestressing force to the concrete, long-term unloaded phase after transfer, sustained loading application, long-term sustained loading phase, unloading and final destructive testing, conducted in a controlled indoor environment. During all phases of the testing strain levels in the BFRP bars and deflections were continuously monitored. The results of continuous strain monitoring show that the average loss of strain over the period of initial 90 days of Journal of Composites for Construction unloaded monitoring was 7% of the initial strain. During the following 6 months of monitoring under sustained loading recorded an additional 0.3% reduction on average. The loss was dependent on the initial strain.

Fibre Reinforced Polymers (FRP) are becoming a popular reinforcement option for RC elements mainly due to good strength to weight ratio and resistance to corrosion. The main limitation for their wider application is their relatively low Young’s modulus, which results in unfavourable serviceability performance, in terms of early development of deflections and cracks. Among others, prestressing has been suggested as one of the possible approaches to addressing this issue, with encouraging results from research conducted so far. This experimental study aimed to explore prestress losses of basalt fibre reinforced polymer (BFRP) reinforced pretensioned concrete beams. Five beams were produced, three of them internally reinforced with 6mm diameter BFRP bars, pretensioned to 20%, 30% and 40% of the ultimate load level of prestress. Additionally, two beams, acting as control samples, were reinforced with unprestressed BFRP and steel bars of same cross-sectional area, respectively. The dimensions of all samples were 125x200x1900 mm. Prestress losses were monitored with the aid of strain gauges attached to the reinforcing bars, as well as load cells. The strain readings were continuously taken during the pretensioning process, from initial application of the prestressing force, during casting and curing of concrete, until releasing of the beams from prestressing devices after curing. Ultimately, all samples were subjected to a quasi-static, load-controlled, four-point bending test until destruction. The results provide the information about the flexural behaviour of pretensioned BFRP reinforced beams, along with insight into some of the initial prestress losses of these elements.

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