Flexural Strength of Post-tensioned concrete-filled fiber-reinforced-polymer rectangular tube beams

This study extends an extensive research program carried out at the University of Sherbrooke to design and assess the potential use of rectangular, concrete-tilled fiber-reinforced-polymer tube (CFFT) beams post-tensioned with steel tendons in bridge applications. This paper describes research to enhance the flexural performance of post-tensioned CFFT beams. Five rectangular post-tensioned CFFT beams were tested up to failure, and the effects of attaching a thin carbon-fiber-reinforced polymer (CFRP) laminate embedded in tension flange and its total reinforcement ratio as well as tube structure fiber laminate were investigated. Last, a simplified design approach is proposed based on strain combinability and force equilibrium to estimate the flexural moment capacity of the tested beams. The specimens with two inclined fiber patterns in the hoop direction or added CFRP laminate strips embedded in the bottom flange of the tubes exhibited substantially greater flexural strength, absorbed energy, and serviceability performance than the control specimens. The ductility index and energy ratio ranged from 8.3 to 10.6 and from 82% to 87%, respectively, which indicates ductile behavior. Also, adding CFRP laminate strips embedded in the bottom flange of the tubes enhanced the flexural strength by 17% on average compared with post-tensioned CFFT without CFRP laminate. The specimen with the CFRP laminates in the bottom flange of the tube achieved flexural strength and energy absorption that was comparable to the flexural strength and energy absorption of the specimen with two layers of inclined fiber patterns. The findings suggest that the design can be optimized to achieve more efficient post-tensioned CFFT structural members. The proposed design approach successfully predicts the flexural strength of the tested beams with an average of 1.05 +/- 0.05 for the partially confined concrete model and an average of 1.11 +/- 0.07 for the unconfined concrete model.