Experimental study on flexural behavior and balanced reinforcement ratio of CFRP-steel composite bars reinforced coral sea-sand aggregate seawater concrete beams

To study the flexural behavior of CFRP-steel composite bars (C-FSCBs) reinforced coral sea-sand aggregate seawater concrete (CSSC) beams and to verify the accuracy of the established calculation method for balanced reinforcement ratio, 22 beams were designed and fabricated for four-point bending tests. The main variables included reinforcement ratio, concrete strength, shear-span ratio, types of reinforcing bar and concrete. The tests provided data on the failure modes, moment-deflection curves, crack widths, and strain development. The results indicated that under different reinforcement ratios, the failure modes of C-FSCBs reinforced CSSC beams include C-FSCB rupture failure, concrete crushing failure after C-FSCB yielding, and concrete crushing failure. The formula established for calculating the balanced reinforcement ratio can predict the failure modes of C-FSCBs reinforced CSSC beams. Within the range of the balanced reinforcement ratio, as the reinforcement ratio increased, the peak moment and flexural stiffness of the beams gradually improved, while the ductility initially increased and then decreased. Compared to reinforced CSSC beam, the peak moment of C-FSCBs reinforced CSSC beam increased by 14 %, while the ductility decreased by 27 %. Moreover, the pre-yield stiffness only decreased by 7 %, but post-yield stiffness increased by 52 %. The maximum crack width of reinforced CSSC beam was significantly smaller than that of C-FSCBs reinforced CSSC beam. Considering the strength utilization ratio of the beam, it is recommended that the maximum crack width limit for C-FSCBs reinforced CSSC beams be set at 0.5 mm. Based on the stress and strain analysis of the balanced-reinforced beams, a formula for calculating the flexural load-bearing capacity of C-FSCBs reinforced CSSC beams was established, demonstrating good calculation accuracy.