Short- and Long-Term Prestress Losses in Basalt FRP Prestressed Concrete Beams under Sustained Loading

The favorable mechanical properties of basalt fiber–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 behavior of such structural members has not yet been investigated extensively. Therefore, this paper aims to give information on long-term behavior and prestress losses of pretensioned BFRP-reinforced concrete beams based on the collected experimental data. The testing program 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 unloaded monitoring was 7% of the initial strain. During the following 6 months of monitoring under sustained loading, an additional 0.3% reduction was recorded on average. The loss was dependent on the initial strain.