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A precast concrete structure reinforced by steel-fiber-reinforced polymer (FRP) composite bars (SFCBs) shows good durability and controllable post-yield stiffness, which makes this kind of structure suitable for marine infrastructure. The connection technology is one of the critical issues of a precast concrete structure with hybrid reinforcement. This paper presents an experimental study on the bond-slip testing (27 pullout specimens) of composite bars connected by a grouted deformed pipe splice (GDPS) connector with different bond lengths. The reinforcement included SFCBs and pure FRP bars. The test results showed that the failure modes could be classified into three categories: rebar pullout before or after the inner steel bar yielded, rupture of the FRP wrapped on the SFCB, and mixed failure of bar pullout with a partial fiber fracture. The average bond strength of the ordinary steel bar was approximately 146.8% that of the SFCB connector with the same anchored length. When the anchored length of the SFCB specimen was 15 d (d : bar diameter), the specimen could be fully anchored to fracture. An explicit hardening bond-slip model considering the post-yield stiffness of the SFCB was used to predict the bond-slip behavior of the GDPS connector, and the experimental and analytical results agreed well with each other, which demonstrates that the proposed model could provided a reference for the analysis and design of connectors for SFCB-reinforced precast concrete structures. [ABSTRACT FROM AUTHOR]

• We tested 5 concrete beams with bundled hybrid longitudinal reinforcements. • The reinforcements including steel bars, hybrid steel/FRP bars. • The failure modes and static performance of the corresponding beams were analyzed. Fiber-reinforced polymers (FRPs) are widely used in the strengthening of concrete structures due to their light weight, high strength, and good durability. For precast concrete structures, bundled FRP/steel bars can substantially ease the construction process. In this paper, experimental studies were conducted on five concrete beams with different types of bundled reinforcements. The test results showed that all the beams exhibited concrete crushing failure modes after the steel bar yielded, and the plastic development of the steel bar was restrained by the elastic FRP bar. As the reinforcement concentration increased, the bond behavior between the longitudinal reinforcement and the surrounding concrete decreased; the postcracking stiffness and crack quantity of the corresponding concrete beam decreased, whereas the crack width increased. Both the initial stiffness and postyield stiffness of the concrete beam with 3-bar bundles were approximately 50% of that of the beam with double-bar bundles. The displacement ductilities of all the concrete beams were greater than six. Due to the differences in the bond behavior, the ultimate displacements of the beams with 3-bar bundles and 6-bar bundles were approximately 1.6 and 1.9 times the ultimate displacement of the beam with single-bar reinforcement, respectively. [ABSTRACT FROM AUTHOR]

Bundled reinforcement can improve construction efficiency by reducing the number of connectors required. The steel-fiber reinforced polymer (FRP) composite bar (SFCB) is a novel reinforcement with high strength, moderate modulus of elasticity and good durability. Four groups of pullout tests were carried out, with parameters including the number of bars within each bundle, bonded length, and rebar type (steel bar, BFRP bar, SFCB). The failure modes of bar pullout without or after yielding, rebar fracture, and concrete splitting were observed. Compared to the single-bar specimen, the two-bar and three-bar bundled specimens resulted in approximately 30% and 40% decrease in average bond strength. The yielding slip s y of the three-bar bundled SFCB specimen is 3.8 times that of the single-bar specimen, indicating that bundled reinforcement can effectively control s y. A simplified bond strength prediction method and a bond stress-slip constitutive model based on the equivalent effective area approach are proposed. Finite element (FE) models were established and verified, and the failure modes of bar fracture are obtained by varying the bonded lengths for different bundled reinforcements, the ultimate slip of four-bar bundled BFRP bars increased by 127%. Recommendations for the development length of bundled SFCBs are proposed based on the FE results, which provide enough safety redundancy. • The bond behavior between bundled bars and concrete by beam-end pullout tests was investigated. • The average bond strength decreases by approximately 30% and 40% for two-bar and three-bar bundled specimens. • A simplified bond strength prediction method and a bond stress-slip constitutive model are proposed. • Recommendations for the development length of bundled SFCBs are introduced based on the FE results. [ABSTRACT FROM AUTHOR]

• The experiments study the bending performance of concrete beams reinforced by partially unbonded steel-FRP composite bars (SFCBs). • The normal postyield stiffness ratio r b ′ keeps a constant approximately equal to 0.21, while the actual postyield stiffness ratio r b increases with the unbonded length. • An unbonded length l ub of 0.5 D , 1.0 D and 1.5 D can increase the energy ductility coefficient μ E of the SFCB reinforced concrete beam by 101%, 23% and 42%, respectively. • The differences between the results of the numerical models and the tests on the bearing capacity and deformation of the partially unbonded SFCB concrete beam were 5.8% and 2.3%. Fiber-reinforced polymers (FRPs) are linear elastic materials that are widely used to reinforce newly built bridges and strengthen existing structures due to its high strength and good durability. To avoid brittle failure, the strength utilization of FRPs in practical applications is relatively low. A steel-FRP composite bar (SFCB) is a composite bar compound of an inner steel bar and outer longitudinal FRPs; its advantages include a good initial modulus, high durability and controllable postyield stiffness. When a concrete structure is reinforced by SFCBs, due to the limited rupture strain of the SFCB's outer FRP, the deformability of the corresponding member is often controlled by the rupture of the FRP. To improve the deformability of an SFCB beam, an experimental study on concrete beams with partially unbonded SFCBs was conducted, and the failure modes, load–deflection curves and strain distribution were investigated. The test results showed that with partial unbonding treatment of the SFCB, the failure mode involving rupture of the SFCB's outer FRP during SFCB reinforced concrete failure can be delayed or transferred into failure due to concrete crushing after the SFCB's inner steel bar yielded, and the corresponding deformability and energy ductility can be improved. Then, a numerical 3D model was built and validated with the test results, and a parametric study of partially unbonded SFCB beams with different parameters was carried out. The simulation results show that the partial unbonding treatment can increase the deformability of the SFCB beam (with rupture of FRP) by two times while maintaining the stability of the load carrying capacity. When the unbonded length l ub is between 0.5 D and 1.0 D (D is the height of the concrete beam), balanced failure of the SFCB beam is more likely to occur (the postyield stiffness is between 0.20 and 0.25). An excessive unbonded length l ub will prematurely lower the depth of the compression zone, and the corresponding bearing capacity will decrease. As a result, when designing an SFCB reinforced concrete beam, the postyield stiffness ratio, reinforcement ratio and unbonded length should be comprehensively considered. [ABSTRACT FROM AUTHOR]

This paper presents an experimental and analytical investigation on the compressive performance of concrete cylinders internally reinforced with steel-fiber-reinforced composite bars (SFCBs) and externally confined with fiber-reinforced polymer (FRP) sheets, termed herein as S-F-RC cylinders. Eleven concrete cylinders, with different confinement conditions and with (or without) longitudinal SFCBs, were tested under monotonic compression. Two distinguishable failure mechanisms, global buckling and local buckling, were discovered for SFCBs in S-F-RC cylinders, under weak and strong confinement conditions, respectively. The S-F-RC cylinders exhibited a marked increase in failure strain of the confinement, directly translating to an enhanced compressive failure strain for the longitudinal SFCBs under strong confinement conditions. Existing models for FRP-reinforced concrete cylinders were found to underestimate the load-carrying capacity of S-F-RC cylinders. An existing compressive stress-strain model for SFCB was extended to the case of S-F-RC cylinders. The proposed method offers accurate predictions, effectively mitigating the discrepancies caused by varying confinement and material properties across different SFCBs. • Compressive performance of FRP-confined SFCB-reinforced concrete is presented. • Different buckling failure mode observed for SFCB under weak or strong confinement. • Model is developed for axial compressive strength of SFCB in FRP-confined concrete. [ABSTRACT FROM AUTHOR]