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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]

A steel-fiber-reinforced polymer (FRP) composite bar (SFCB) is a kind of rebar with inner steel bar wrapped by FRP, which can achieve a better anticorrosion performance than that of ordinary steel bar. The high ultimate strength of FRP can also provide a significant increase in load bearing capacity. Based on the adequate simulation of the load-displacement behaviors of concrete beams reinforced by SFCBs, a parametric analysis of the moment-curvature behaviors of concrete beams that are singly reinforced by SFCB was conducted. The critical reinforcement ratio for differentiating the beam’s failure mode was presented, and the concept of the maximum possible peak curvature (MPPC) was proposed. After the ultimate curvature reached MPPC, it decreased with an increase in the postyield stiffness ratio (rsf), and the theoretical calculation method about the curvatures before and after the MPPC was derived. The influence of the reinforcement ratio, effective depth, and FRP ultimate strain on the ultimate point was studied by the dimensionless moment and curvature. By calculating the envelope area under the moment-curvature curve, the energy ductility index can obtain a balance between the bearing capacity and the deformation ability. This paper can provide a reference for the design of concrete beams that are reinforced by SFCB or hybrid steel bar/FRP bar. [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]

This paper presents experimental and numerical results of concrete columns reinforced by longitudinal steel basalt fiber reinforced polymer (FRP) composite bars (SBFCB) and lateral stirrups steel-wire basalt FRP (BFRP) composite stirrups (SWBFCS). The main parameter is the un-bonded length of the SBFCB and concrete in the column base. Both SBFCBs and SWBFCSs consisting of inner steel bars or wires hybrid with outer BFRP. The axial compressive loading test of concrete cylinders confined by SWBFCS demonstrated the effective confinement of this new kind of stirrup. Moreover, in this paper, the bond relationship between the concrete and the SBFCBs was measured in three bridge columns. One column was utilized as a reference, and the other two with un-bonded lengths of 150 mm and 300 mm, covering the plastic hinge region above the column base. The hysteretic behavior, energy dissipation capacity, residual displacements, and curvatures of the three columns were analyzed. The results show that the maximum load of the proposed specimen with a 300 mm un-bonded length is increased by 15.8%, and the residual displacement was 9% smaller than that of the of the control specimen because of the un-bond length between concrete and SBFCB. [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]