Behavior of high-strength concrete (HSC) box girders reinforced with GFRP bars, ties, and spirals under torsion

To date, the torsional behavior of high-strength concrete (HSC) box girders reinforced with glass fiber-reinforced polymer (GFRP) reinforcement has not been investigated. This study reports on the experimental results of HSC box girders reinforced with GFRP bars, ties, and spirals under pure torsion. Seven full-scale reinforced concrete (RC) box girders with a total length of 4,000 mm, width of 380 mm, height of 380 mm, and wall thickness of 100 mm were examined over a clear span of 2,000 mm. The test parameters included the type, configuration, and amount of the web reinforcement and the concrete type. All specimens have the same longitudinal reinforcement ratio. The test specimens comprised three box girders reinforced with GFRP bars and individual ties, three specimens reinforced with GFRP bars and continuous spirals, and one box girder entirely reinforced with steel reinforcement as a reference specimen. Two girders were fabricated with normal-strength concrete (NSC)-one reinforced transversally with GFRP spirals; the other reinforced with ties-to investigate the influence of concrete type on torsional behavior. The test findings indicate that, at the same web reinforcement ratio and reinforcement configuration, the HSC specimens achieved higher pre- and post-cracking torsional strength and stiffness than their counterpart NSC specimens. The continuous spiral configuration instead of individual ties improved the overall torsional performance of the tested box girders. Moreover, the stirrup's capacity strongly influenced the torsional strength of the GFRP- and steel-reinforced concrete box girders, but the stirrup stiffness did not affect the ultimate torque.