Bond durability of basalt-fiber-reinforced-polymer (BFRP) bars embedded in concrete in aggressive environments

Recently, basalt-fiber-reinforced-polymer (BFRP) bars have emerged as a promising alternative to glass-fiber-reinforced-polymer (GFRP) bars. So far, however, BFRP bars have not been incorporated into design standards and specifications. This is due to limited studies and lack of knowledge on the performance of the bars in concrete, in particular, their bond durability when exposed to aggressive environments. This paper presents some results of an extensive research program investigating the bond durability behaviors of BFRP bars in concrete structures and the long-term bond-strength-retention predications of the BFRP bars on the basis of short-term tests results. This research included testing deformed BFRP bars measuring 12 mm in diameter. Pullout specimens were tested with direct tensile loading after being exposed to an alkaline solution (pH 12.9) for 1.5, 3, and 6 months at temperatures of 40 °C, 50 °C, and 60 °C. This paper investigated the effects of alkaline environment, exposure periods, and elevated temperatures on bond strength as well as the degradation mechanism and mode of failure of the BFRP-reinforced specimens. In addition, optical microscopy and scanning electronic microscopy were used to investigate the degradation of BFRP bars tested. The test results indicate an initial increase in the bond strength of the conditioned specimens as the temperature increased compared to their unconditioned specimens. After 1.5 months of exposure, the specimens conditioned at 50 °C and 60 °C, respectively, had bond-strength increases of 25% and 26%, while the specimens conditioned at 40 °C exhibited no noticeable changes (a minor decrease of 4.3%). Nevertheless, the bond strength of the conditioned specimens deteriorated during immersion. The highest bond-strength reductions occurred in the conditioned specimens after 6 months of exposure at 40 °C (a 16% loss), followed by specimens conditioned at 50 °C (7% loss) and 60 °C (5% loss) compared to their counterparts at 1.5 months. Lastly, the long-term bond-strength-retention predications of the BFRP bars after 50 years of service life in dry, moist, and moisture-saturated environments with mean annual temperatures between 5 °C and 35 °C ranged from 71% to 92%.