Relaxation behavior of basalt fiber reinforced polymer bars under different environmental conditions

Abstract Relaxation behavior of fiber-reinforced polymer (FRP) bars is one of the main issues considered in prestressing applications. Recently, basalt FRP (BFRP) bars have been developed as an alternative to other types of FRP bars. However, the available studies on their relaxation behavior are still limited, especially under severe environmental conditions. In this study, the long-term relaxation behavior of BFRP bars was studied, the initial stress level under different environmental conditions are the variables considered in this investigation. A total of twenty-four specimens of BFRP bars of nominal diameter 6 mm were used in this study. The relaxation behavior and the residual tensile strength of BFRP after relaxation were experimentally assessed. The expected relaxation loss after a million hours was estimated based on the extrapolated statistical analysis of the experimental results. The test results showed that the relaxation behavior of BFRP bars under different initial stress levels have linear relationship with logarithm of time, similar to other types of FRP bars, and are significantly affected by the initial stress level. Furthermore, the test results demonstrated that the relaxation behavior of BFRP bars is slightly affected by seawater (pH 8.1) and can be recommended for use in marine structures. On the other hand, the test results showed that the acidic (5% HCl) and alkaline (10% NaOH) environmental conditions greatly impacted the relaxation behavior of BFRP bars, such that although BFRP bars stressed to 50% of their ultimate strength survived 1000 h under normal conditions, exposure to different environmental conditions resulted in their rupture under the same initial stress level. Finally, the expected million-hour relaxation losses of BFRP bars range between 7.76% and 13.75% under normal conditions based on the initial tensile stress level, while introducing seawater can increase this ratio to an average of 16.13%.