Study on the flexural behaviours of precracked hollow core beams strengthened with core filling and unbonded prestressing steel strands

Extensive investigations show that the core-filling method can markedly improve the shear behaviour of hollow core beams (HCBs). However, few investigations were carried out to study the flexural behaviours of the HCBs strengthened with this method. In this study, the flexural behaviours were first studied, where the HCBs were strengthened with a hybrid strengthening method of the core-filling and external unbonded prestressing steel strands. Six types of HCB specimens were fabricated and investigated. The initial damage and prestress levels were selected as the main parameters. Before strengthening, the HCB specimens were loaded to 40.6 %, 66.3 % and 100 % of their ultimate load to pre-set some cracks in the concrete beams, which corresponded to the maximum crack widths of 0.2 mm, 0.4 mm and wider than 1.5 mm, respectively. Compared with the control beam, the strengthening with the core-filling improved the flexural load carrying capacity by 3.93 %, whereas the hybrid strengthening method improved by 33.67 %; the cracking loads for the two strengthening methods increased by 10.34 % and 128.28 %, respectively. Moreover, the experimental results showed that the initial flexural stiffness can be markedly improved with the hybrid strengthening method. However, the initial damage level had little influence on the final crack width. In addition, an analytical model was proposed to evaluate the flexural behaviors of the strengthened HCBs. The simulation results of the preload, ultimate load and failure modes agreed well with the experimental ones.