steel-FRP Composite Bars Research Articles

Bond behavior between bundled steel-FRP composite bars and grouted corrugated duct

The utilization of steel-fiber reinforced polymer (FRP) composite bars (SFCBs) reinforcement in precast concrete structures offers numerous benefits, including controllable post-yield stiffness and reduced residual displacement. Employing bundled reinforcement can improve construction efficiency. This study presents direct pull-out tests conducted on SFCBs embedded in grouted corrugated ducts. The results show that bond strength decreases with both the number of bars in a bundle (nb) and the actual embedded length (la). The bond strength of single bar SF-1b-3d with 3d embedded length is 24.21 MPa, while that of 2-bar bundled SF-2b-3d and 3-bar bundled SF-3b-3d is 17.41 MPa and 12.61 MPa, respectively. Additionally, the error in predicting the bond strength between bundled SFCBs and grouted corrugated duct using the equivalent diameter method is found to be less than 8.4%. Utilizing the mBPE constitutive model, an analytical hardening-slip model with a linear slip field assumption accurately predicts the full-range behavior, bond stress distribution, and SFCB's axial stress distribution. Furthermore, the effects of different parameters on yield slip sy, ultimate slip su, and critical anchorage length Ltr are investigated by considering different local bond-slip constitutive models (varying peak bond strength τm and its corresponding slip sm and ascending section coefficient (α) as variables. The yield slip sy varies from 0.1 mm to 1.3 mm, while the ultimate slip su can reach a maximum of 35.0 mm. Furthermore, the validity of existing models for evaluating the anchorage length of single SFCB and bundled SFCBs grouted in corrugated duct connections is discussed, and a formula to estimate the critical anchorage length of bundled SFCBs and grout considering the number of bars within one bundle (nb) is proposed.

Experimental and numerical study on flexural performance of ultra-high performance concrete frame beams reinforced with steel-FRP composite bars

This paper presents the bending tests of four ultra-high performance concrete (UHPC) frame beams and one normal strength concrete (NSC) frame beam, all reinforced with steel-FRP composite bars (SFCBs). A comprehensive analysis was carried out, encompassing evaluation of the failure mode, crack propagation, bearing capacity, deformation, strain response, and plastic rotational capacity of the frame beams. Investigating the effects of concrete type, reinforcement type, and beam-end reinforcement ratio on the flexural performance of the frame beams was a key aspect of this study. A three-dimensional finite element (FE) model of the frame beam was established and rigorously verified. The developed model enabled a detailed parametric analysis involving the steel ratio, the yield strength of the inner core steel bar, the elastic modulus of the FRP, and the ultimate tensile strength of the SFCB. The results indicated a consistent failure mode of all frame beams: crushing of concrete at the beam-end, initiating a sequence of plastic hinge occurrence starting at the beam-end and then progressing to mid-span. The substitution of normal strength concrete with UHPC significantly enhanced various aspects of the frame beams, including the flexural capacity, deformation, ductility, ultimate energy dissipation, and plastic rotational capacity, while inhibiting the generation and expansion of cracks. Notably, the plastic rotation angle of SFCB-UHPC frame beams was 4.9 times greater than those of steel-UHPC frame beams, emphasizing the effectiveness of SFCB in enhancing the beam-end plastic rotational capacity. A decrease in the beam-end reinforcement ratio significantly reduced the flexural capacity, ultimate energy dissipation, and beam-end plastic rotational capacity, while improving ductility. Additionally, the study established a formula for calculating the equivalent plastic hinge length, utilizing the relative compressive zone height and effective section height of the beam-end controlling section as variables, which demonstrated good alignment between predicted and experimental results.

Durability of SFCB reinforced low-alkalinity seawater sea sand concrete beams in marine environment

Seawater sea-sand concrete (SWSSC) provides an alternative to normal concrete, while Steel-FRP Composite Bars (SFCB) offer corrosion resistance in offshore and marine structures. Despite their benefits, the long-term performance of SFCB-reinforced SWSSC structures remains under-explored. This study evaluated the durability of such beams, particularly focusing on the effects of using Portland cement versus low-alkalinity calcium sulphoaluminate (CSA) cement. The investigation involved tensile and pull-out tests conducted on SFCB, and flexural tests conducted on the beam components after accelerated marine environmental exposure over periods ranging from one to six months. Results highlighted that SFCB embedded in low-alkalinity CSA cement exhibited improved retention of tensile strength, elastic modulus, and bonding strength by 32.0 %, 39.1 %, and 56.7 % respectively, compared to that embedded in normal Portland cement concrete. Microstructural analyses showed much less hydrolysis of the resin matrix and shallower corrosion of the basalt FRP (BFRP) cover on the SFCB. Furthermore, flexural tests demonstrated superior cooperative performance between low-alkalinity CSA concrete with SFCB in beam components, showing only marginal degradation in crack density, crack strength, and yielding points. The ultimate failure load of beams made with CSA concrete was 1.89 times higher than those made with normal SWSSC after 6 months of exposure. These results underscore the potential of low-alkalinity concrete in improving the long-term durability and structural performance of SFCB-reinforced marine concrete infrastructures.

Shear Mechanism of a Novel SFCBs-Reinforced Composite Shear Connector: Experimental, Theoretical Investigations and Numerical Model.

Traditional stud and perfobond leiste (PBL) shear connectors are commonly used as load-transferring components in steel-concrete composite structures. Composite shear connectors fully utilize the advantages of traditional stud and PBL shear connectors. In order to maximize the advantages of composite shear connectors, a novel shear connector for complex environments was proposed. The steel-FRP composite bars (SFCBs) with excellent fatigue resistance and corrosion resistance were introduced to replace the steel bars. This study discussed the failure modes, load-slip curves, and load-strain curves of the composite shear connector. In addition, a finite element analysis (FEA) model was developed to analyze the influence of various factors on its shear behavior. Results showed that compared with traditional composite shear connectors, the introduction of SFCB resulted in a promotion of 7.85% in shear stiffness, and it also led to a significant increase of 63.61% in ductility, further enhancing the mechanical performance. Meanwhile, FEA models were well fitted to the test results, and parametric analysis showed variate effects on shear bearing capacity. In the end, an equation was established to calculate the shear capacity of composite shear connectors, which could provide a reference for further research and engineering applications.

Axial compressive behavior of low-alkalinity seawater sea-sand concrete column reinforced with hydrophobic longitudinal SFCBs and FRP hoops

To address corrosion problem of steel bars and shortage of freshwater and river sand in marine and offshore structures, a novel hydrophobic-modified longitudinal steel-FRP composite bars (SFCB) and FRP hoops reinforced low-alkalinity seawater sea-sand concrete (SWSSC) is developed. However, axial compressive behavior of hybrid SFCB and FRP reinforced columns is not clear and the applicability of existing design methods is low. Here, nine hybrid reinforced columns have been designed and tested. Typical failure modes of columns and axial compressive behavior of constituent materials (i.e., hydrophobic reinforcements and low-alkalinity SWSSC) are investigated. It is found that the synergistic confining effect formed by hybrid reinforcement are significant. The mesh confinement formed by densely distributed FRP hoops and longitudinal SFCBs improves the local buckling behavior, and effectively constrains the lateral expansion of concrete core, resulting in improved compressive strength and ductility of column. The compressive contribution of single longitudinal SFCB to ultimate load capacity of column can be enhanced effectively, up to 36.3 %, by increasing steel content of SFCB. Based on this, a prediction model of hybrid reinforced columns is proposed, which can accurately evaluate the ultimate load capacity. This work provides valuable experimental observations and load capacity model for hybrid reinforced column, which promote its design and application in marine engineering.

Bond behavior and modeling of steel-FRP composite bars in engineered cementitious composites

The combination of the steel-FRP composite bar (SFCB) and engineered cementitious composite (ECC) holds favorable applications in terms of durable resilient structures. However, a clear understanding of the interfacial bond performance of this novel combination is currently lacking. Therefore, the bond behavior and mechanism of the SFCB-ECC interface were investigated through direct pullout tests. A total of 48 cube specimens were fabricated, with the test variables including bar type, bar diameter, embedment length, and matrix type. The experimental results indicated that the bond failure mode of SFCBs and FRP bars differed from that of deformed steel bars, attributed to the damage of resin-rich ribs in relatively high-strength ECC. The bond-slip curves revealed both the fundamental differences and similarities among steel bars, SFCBs, and FRP bars. The Poisson effect and shear lag effect increased with a decrease in bar stiffness and an increase in diameter. The combined impact of the interaction between the bar stiffness and diameter resulted in varying bond strengths for steel bars, SFCBs, and GFRP bars, despite their similar surface geometries. A theoretical model for the bond strength at the SFCB-ECC interface was derived using the thick-walled cylinder principle and subsequently validated with test data. Finally, a unified empirical model framework for predicting the bond strength of various bar-matrix combinations was proposed based on the theoretical model. The accuracy of this method was evaluated using a collected database.

Seismic behavior of novel resilient and corrosion-resistant composite columns using strain-hardening engineered cementitious composites and steel-FRP composite bars

This paper proposed a novel composite column with exceptional resilience and corrosion resistance achieved through the casting of engineered cementitious composites (ECCs) in the plastic hinge region and the utilization of steel-FRP composite bars (SFCBs). A total of six specimens were tested under constant axial load and cyclic lateral load to examine their seismic performance and reparability. The influence of the type of longitudinal reinforcement (steel bar, SFCB, and GFRP bar) and the matrix type (ECC and concrete) in the plastic hinge region was discussed. Test results indicated that configuring SFCB in the columns facilitated drift-hardening characteristics, which in turn enhanced resilience. The adoption of ECC in the plastic hinge region further reduced residual deformation of the composite columns while significantly improving both lateral strength and ductility. The maximum repairable limit of the specimens increases by a factor of 1.36, 1.86, and 2.5, respectively, as the FRP content increases from 0% to 31.3%, 56.4%, and 100%. The specimens reinforced with SFCBs can be quickly restored to their intended use without repair, before reaching a 2% drift ratio. The slip effect of SFCBs has a substantial impact on increasing the lateral displacement of test columns, and this effect tends to increase with an increase in FRP content. A fiber model was developed for predicting the skeleton curves, and its accuracy was validated through comparisons between the predicted and test results.

Bond durability between steel-FRP composite bars and concrete under seawater corrosion environments

Steel fiber-reinforced polymer (FRP) composite bars (SFCBs) can enhance the damage-controllable features of reinforced concrete (RC) structures, offering a potential solution to corrosion issues in conventional RC structures. However, the durability of SFCBs bonded to concrete remains insufficiently studied. Seawater exposure experiments were conducted on SFCBs-concrete specimens with two diameters (12 and 16 mm) at different exposure times (30, 180, and 360 days) and temperatures (23, 40, and 60 °C) in this study. Subsequent pull-out tests were performed to assess long-term bonding behaviors. The results indicate that the hydrolysis of the glass FRP (GFRP) matrix is the primary cause for the reduction in shear capacity of SFCB ribs and the degradation of bond properties between SFCB and concrete. With increasing aging time and temperature, the bond strength of SFCB-concrete gradually decreases. Thicker GFRP layer SFCB-concrete specimens exhibit better bond strength retention rates. Moreover, a predictive model for the bond strength of SFCB-concrete was proposed. As per the model predictions, the bond strength retention after 10 years of seawater immersion is 50% for 12 mm diameter SFCB, while the 16 mm diameter SFCB achieves a retention of 70%. These findings will provide valuable references for the implementation of SFCB-reinforced concrete structures.

Bond behavior between steel-FRP composite bars and engineered cementitious composites in pullout conditions

Steel-FRP composite bar (SFCB) reinforced engineered cementitious composites (ECC) members are promising to apply in structural engineering due to their excellent mechanical properties and durability. The bond behavior between SFCB and ECC was investigated by direct pullout tests. The experimental variables included the diameter of SFCB, bond length, type of matrix, compressive and tensile strengths of ECC, and type of reinforcement. All ECC specimens failed in a ductile manner with ECC keys being crushed and sheared off. The bond strength and bond toughness of ECC specimens were higher than those of concrete specimens possessing similar compressive strengths. An interfacial bond-slip model was proposed to predict the bond behavior of SFCB embedded in ECC. An iteration procedure was performed utilizing the developed bond-slip model to analyze the distributions of bond stresses and slips. Notably, an increase in bond length and the yielding of SFCB led to heightened non-uniformity in bond stress and slip distributions. Furthermore, equations were derived to predict the embedment lengths of SFCB applicable to various scenarios.

Compressive Performance of Longitudinal Steel-FRP Composite Bars in Concrete Cylinders Confined by Different Type of FRP Composites.

This paper presents an experimental study on the compressive performance of longitudinal steel-fiber-reinforced polymer composite bars (SFCBs) in concrete cylinders confined by different type of fiber-reinforced polymer (FRP) composites. Three types of concrete cylinders reinforced with (or without) longitudinal SFCBs and different transverse FRP confinements were tested under monotonic compression. The results showed that the post-yield stiffness of SFCBs is higher when confined with high elastic modulus carbon fiber-reinforced polymer (CFRP) composite than with low elastic modulus basalt fiber-reinforced polymer (BFRP) composite. Decreasing confinement spacing did not significantly improve the compressive strength of SFCBs in concrete cylinders. The compressive failure strain of SFCBs could possibly reach 88% of its tensile peak strain in concrete cylinders confined by CFRP sheets, which is significantly higher than the value (around 50%) in previous studies. Existing design equations, which applied a strength reduction factor or a maximum compressive strain of concrete to consider the compressive contributions of SFCBs in concrete members, underestimate the load-carrying capacity of SFCB-reinforced concrete cylinders. The design equation that considers the actual compressive stress of SFCBs gives the most accurate prediction; however, its applicability and accuracy need to be verified with more experimental data.

Investigation on bond behavior of steel-FRP composite bars in engineered cementitious composites by spliced beam tests

The collaborative utilization of steel-FRP composite bar (SFCB) and engineered cementitious composites (ECC) represents a favorable choice for engineering structures exposed to aggressive environments, offering benefits involving both mechanical behavior and durability. This paper primarily studies the bond performance of SFCB embedded in ECC through spliced beam tests coupled with finite element analysis. The findings demonstrate that with an appropriate cover thickness or transverse confinement, spliced ECC beams exhibit failure modes characterized by splitting and pull-out. Under such conditions, the bond-slip response demonstrates limited sensitivity to changes in cover thickness and transverse confinement. Conversely, increasing the cover thickness or introducing transverse confinement in spliced concrete beams, which exhibit splitting failure, significantly enhances the bond strength. Furthermore, in contrast to spliced concrete beams, spliced ECC beams are observed to demonstrate a bond toughness that is 3.85–12.10 times greater. The discrepancy in bond toughness between ECC and concrete diminishes as the cover thickness and transverse confinement increase. Notably, the uniform distribution of bond stress in the splice region of ECC beams remains unaffected by variations in splice length. To incorporate the impact of bond stress distribution when calculating the splice length of SFCB in ECC beams, a coefficient of 0.85 is introduced.

Axial-flexural performance of columns reinforced by steel-FRP composite bars and FRP ties

The ductile and corrosion-resistant steel-FRP composite bar (SFCB) is a promising longitudinal reinforcement for marine engineering in harsh environments. However, the study about the axial-flexural performance of longitudinal SFCBs reinforced concrete columns, especially combined with corrosion-resistant FRP ties, is very limited. The lack of valuable experimental observations and design methods seriously limits its design and application. Here, 15 square concrete member specimens with different reinforcement types are tested under concentric compression, eccentric compression, and pure flexural loading, to investigate the effect of longitudinal SFCB and FRP tie on the axial-flexural behavior of columns. Results show that the yielding characteristic of longitudinal SFCB leads to two representative failure modes (tension-controlled and compression-controlled failure). Using equal-stiffness SFCB to replace steel bar contributes to a better axial-flexural behavior of columns, including the same failure mode, similar deformation, and higher load capacity. Furthermore, a theoretical model that considers the unique constitutive relationship of SFCB and confinement conditions of different ties is developed to predict the failure modes and axial-flexural capacity of columns, showing great accuracy. These findings provide a valuable experimental basis and safe design tactics for this corrosion-resistant and ductile hybrid reinforcement scheme of marine infrastructure.

Seawater sea-sand concrete columns using durable and ductile steel-FRP composite bars and FRP ties: A promising hybrid scheme for marine engineering

To overcome corrosion of steel bar and scarcity of freshwater and river sand, durable and ductile steel-FRP composite bar (SFCB) provides a promising longitudinal reinforcement alternative in seawater sea-sand concrete (SWSSC) columns for marine infrastructures. The unique mechanical properties and lower compressive strength of SFCB inevitably influence axial compressive performance of column. However, very few studies on axial compressive performance of hybrid longitudinal SFCBs and FRP ties reinforced concrete (hybrid-RC) columns lead to a lack of valuable experimental observations and failure mechanisms. Here, an experimental investigation of axial compressive performance for hybrid-RC columns is conducted. Results indicate that SFCB has a bilinear stress–strain characteristic with high elastic modulus under compression and can maintain its integrity until concrete crushing. Using SFCB as longitudinal reinforcement leads to satisfactory axial compressive behavior including high strength and great ductility for columns. Nevertheless, FRP ties show low confinement efficiency since the worse mechanical properties of bent portion and the slipping of lap splice. With the experimental and theoretical analysis for hybrid-RC columns, the prediction model of axial load capacity is developed and validated preliminarily, showing great accuracy. These findings provide a meaningful experimental basis and safe design tactics for this hybrid reinforcement scheme of SWSSC.

Seismic degradation behavior of steel-FRP composite bar reinforced concrete beams in marine environment based on experimental and numerical investigation

This study investigated the seismic behavior of seawater sea sand concrete (SWSSC, which is made of seawater, sea sand, ordinary aggregate and cement) beams reinforced with steel bars in a marine environment. To improve the durability of the specimens, different types of reinforcements, namely fiber reinforced polymer (FRP), steel-FRP composite bars (SFCBs), and macro basalt fibers, were introduced to replace steel bars owing to their superior durability and high strength. The failure mode, hysteretic response, energy dissipation capacity, and curvature of the pedestal were analyzed. Furthermore, a long-term performance prediction method based on finite element (FE) modeling was proposed. Results showed that the bond between the longitudinal reinforcements and SWSSC in steel and SFCB reinforced concrete (RC) beams degraded significantly after corrosion. Additionally, slips were observed during reciprocating loading. The total cumulative dissipated energy of the SFCB RC beam was approximately the same as that of the steel RC beam. Moreover, the corresponding value of SFCB and macro basalt fiber hybrid reinforced concrete specimen was almost twice that of the steel RC beam owing to a stable confinement of fibers on concrete. After corrosion, steel and SFCB RC beams exhibited a decrease in the lateral displacement at the top and a larger curvature at the pedestal, indicating that the rotation of the corroded specimens mainly occurred at the beam-column joint. The accuracy of the FE model was verified by comparing the obtained results with the experimental results. The prediction results revealed that the seismic performance of SFCB RC beams in the marine environment declined in the first three years and then stabilized gradually.which is made of seawater, sea sand, ordinary stone and cement.

Flexural behaviour of concrete beams reinforced with steel-FRP composite bars

Corrosion of steel reinforcement affects the durability of concrete structures. This led researchers to look forward to alternatives for steel reinforcement. One of these alternatives is Fiber Reinforced Polymer (FRP) reinforcement, which has high tensile strength as well as excellent corrosion resistance. However, the use of FRP reinforcement always comes at the expense of ductility. Thus, Steel Fiber Composite Bars (SFCBs), which consist of a steel core and FRP protective coating, could be a suitable alternative to conventional steel/FRP reinforcement. In the current work, fourteen concrete beams were tested to investigate the flexural performance of concrete beams reinforced with manually produced SFCBs. All beams had similar dimensions of 200 mm width, 300 mm height, and a clear span of 1700 mm. The main parameters were the reinforcement type (steel/SFCB) and the concrete compressive strength (30, 40, and 50 MPa). The experimental results were presented in terms of; crack pattern, failure modes, cracking load, ultimate load, load-crack width response, load-deflection curves, and the reinforcing bars' strain curves. The test results showed that, at the same load level, using SFCBs as tensile reinforcement increased the beams' mid-span deflection and crack width compared with those reinforced with steel bars. Finally, an analytical study was performed to predict the ultimate loads, maximum crack width and maximum deflection at service load of SFCBs reinforced beams. The results from the analytical study were very close to the test results, validating the accuracy of the suggested formulas for usage in practice.

Deformation ability of precast concrete columns reinforced with steel-FRP composite bars (SFCBs) based on the DIC method

A steel-fiber reinforced polymer (FRP) composite bar (SFCB) is a novel reinforcement composed of an inner steel bar and outer continuous FRP. Moreover, SFCBs have excellent corrosion resistance and stable positive post-yield stiffness and can be used to control structural damage. However, the relatively small fracture strain of the outer FRP may limit the deformation ability of the concrete members reinforced with SFCBs. Bundled reinforcement can weaken the interfacial adhesion to improve the structural deformation capacity and reduce the connections of the reinforced precast structures to enhance the construction efficiency and quality. In this paper, a quasistatic test of six concrete columns was conducted, and the deformation of the column foot was observed by the digital image correlation (DIC) method to investigate the effects of the reinforcement type (steel bar and SFCB), the number of bars in each bundle (1, 2, and 3), and the construction method (cast-in-place, general precast, and precast with full-length grouting) on the deformation capacity. The crack spacing of the precast concrete column reinforced with single-bar SFCBs increased by approximately 10% compared to the precast reinforced concrete (RC) column, and the crack spacing decreased with an increasing number of bars within each bundle. The proportion of the lateral displacement caused by rigid body rotation due to strain penetration of the longitudinal reinforcement at the precast column foot in total lateral drift was approximately 35%; the smaller growth rate of residual rotation in concrete columns reinforced with SFCBs produced a superior post-disaster resetting ability. The curvature distribution range of columns reinforced with SFCBs was wider and more uniform than that of RC columns. Overall, the utilization of SFCBs as longitudinal reinforcement improved the equivalent plastic hinge length by approximately 120% compared with RC columns, thus greatly reducing the curvature requirement of the column bottom. Moreover, the bundled reinforcement reduced the growth rate of plasticity. The precast specimen with full-length grouting altered the failure mode and achieved a considerable deformation capacity with the longest equivalent plastic hinge length of approximately 435 mm, which is 2.77 times that of the conventional RC column.

Shear behavior of concrete beams reinforced with corrosion-resistant and ductile longitudinal steel-FRP composite bars and FRP stirrups

Inheriting excellent corrosion resistance of FRP bars, steel-FRP composite bar (SFCB) has better elastic modulus and ductility, promising to be longitudinal reinforcement for concrete structures in harsh environments. While previous research primarily focused on the flexural behavior of hybrid longitudinal SFCB and FRP stirrups reinforced concrete (hybrid-RC) beams, the study about shear behavior has rarely been reported and valuable experimental observations are limited. The unique mechanical properties of SFCB causes the inapplicability of existing shear design method and the risk of brittle shear failure. Here, a simplified shear design method is proposed for hybrid-RC beams by analytical investigation and the shear behavior of hybrid-RC beams is investigated experimentally. Results show that SFCB exhibits similar transverse shear behavior to steel bar. Compared with steel bars reinforced concrete beams, hybrid-RC beams based on the proposed design method have better shear performance including same failure modes, similar deflection, narrower shear crack, and higher shear capacity, due to the post-yielding stiffness of SFCB. Furthermore, a modified calculation method of the shear capacity for hybrid-RC beams is proposed, exhibiting great feasibility and accuracy. These results can provide valuable experimental observations of shear behavior and promote the development of shear design tactics for hybrid-RC beams.

Flexural behavior of concrete beams reinforced by partially unbonded steel-FRP composite bars

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 lub is between 0.5D and 1.0D (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 lub 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.

Compressive Behavior of Steel-FRP Composite Bars Confined with Low Elastic Modulus FRP Spirals in Concrete Columns

Compressive Behavior of Steel-FRP Composite Bars Confined with Low Elastic Modulus FRP Spirals in Concrete Columns

Investigation on the progressive collapse resistance of three-dimensional concrete frame structures reinforced by steel-FRP composite bar

Compared with conventional steel bars, steel-FRP (fiber reinforced polymer) composite bars (SFCBs) have distinctive post-yield stiffness, and the high efficiency of SFCBs in enhancing the progressive collapse resistance of concrete frames has been confirmed by recent quasi-static experimental studies on SFCB beam–column subassemblages. However, the realistic progressive collapse of structures is a dynamic process, and the three-dimensional (3D) effects are essentially important for determining the collapse response. In this study, numerical models are built to explore the progressive collapse behavior of 3D frame structures reinforced by SFCBs. The static and dynamic capacities of five SFCB frames and one reinforced concrete (RC) frame are compared under different column–removal cases. Results show that by virtue of the post-yield stiffness, the static load factors of SFCB frames are much larger than those of RC frames at the same deformation, proving that SFCB frames have better robustness against collapse failure than RC frames. After the dynamic removal of columns, all RC frames are subjected to progressive collapse failure, while SFCB frames can successfully complete the dynamic load redistributions and quickly reach a steady state. Further, the dynamic increase factor (DIF) of SFCB frames is proposed based on the ratio of maximum dynamic flexural responses to static flexural responses of beam joint sections under the same gravity loads (1.2 times dead load plus 0.5 times live load), and the conservative DIFs of 1.55 and 1.3 are recommended for a corner column–removal case and the other single column–removal cases, respectively. Finally, parametric analyses are conducted to reveal the effect of beam longitudinal reinforcement ratio, beam span-to-depth ratio, and dynamic duration for the column removal on the progressive collapse resistance of SFCB frames. The results lay a foundation for the progressive collapse design of 3D SFCB frame structures.