Bond Stress-slip Relationship Research Articles

Bond performance and constitutive model of steel bar in G-UHPC under cyclic loading

The increasing embrace of the low-carbon and environmentally sustainable construction approach has opened exciting opportunities for using environmentally friendly geopolymer based ultra-high-performance concrete (G-UHPC) in engineering structures, drawing significant attention to its performance. Seismic performance is a crucial factor in structural design, and it is greatly affected by the bond behavior between steel bar and concrete. As a result, a thorough understanding of the bond behavior of steel bars to G-UHPC provides essentially theoretical support for the structural design and nonlinear finite element analysis of G-UHPC structures, which is vital for their widespread implementation in earthquake engineering. This study tested 54 specimens from 18 groups to examine the bond performance of G-UHPC and steel bars under cyclic loading. The influence of concrete and steel bar types, protective layer thickness, bond length, and steel fiber on critical bond performance indicators were evaluated, and the bond stress-slip hysteretic and skeleton curves were analyzed. Subsequently, based on the experimental data, a constitutive model was proposed to predict the bond stress-slip relationship of steel bars and G-UHPC under cyclic loading. The results indicated that the steel bars exhibited superior bond performance with G-UHPC as compared to NSC. The bond and residual strength between steel rebars and G-UHPC was approximately 1.73 times and 3 times greater, respectively, in comparison with that between NSC and steel rebars. Increasing the strength of steel bars only resulted in a limited 2 % improvement in bond strength. The steel bar diameter, protective layer thickness, bond length, and the dosage and length of steel fibers in G-UHPC significantly influenced the bond performance. The addition of irregular-shaped steel fibers to G-UHPC has demonstrated potential in improving energy consumption, with the most remarkable enhancement observed for the inclusion of twisted steel fibers. Specifically, the incorporation of twisted steel fibers signally enhanced the energy consumption to approximately 1.3 times the energy dissipation as compared to straight steel fibers with the same dosage and aspect ratio. In addition, the accuracy of the proposed constitutive model has been validated.

Experimental study of bond behavior of geopolymer concrete under different curing condition using a pull-out test

Researchers recently have focused on geopolymer concrete as it has several advantages compared to traditional Portland cement, such as lower carbon footprint, high early strength, chemical resistance and long-lasting structural performance. This paper designed 192 pull-out specimens to investigate the influence of factors on the bond-slip behavior between geopolymer and steel bar such as curing condition (ambient and 80 °C for 7 hours), longitudinal rib angle, the position of longitudinal bar, bar diameter (12 mm and 24 mm), stirrup usage (with and without stirrup) and bond length (2d and 4d). The maximum bond strength, bond failure modes, and bond stress-slip relations were discussed. The test results showed that the longitudinal rib angle, and the position of rebar play a vital role in determining the bond strength as well as the bar diameter. In comparison with the specimens without the stirrup, the bond performance of specimens with stirrup increased more significantly, and this phenomenon became even more pronounced as the bar diameter increased. The stirrup usage in samples under ambient and heat cure increased the bond strength by an average of 45 % and %40, respectively. Moreover, it has been determined that the curing conditions have a significant effect on the bond performance, as especially the compressive and tensile strengths of the samples under heat cure improve. In samples exposed to both ambient and heat cure, doubling the bond length increased the bond strength by 70 %. Furthermore, increasing the bar diameter increased the bond strength of the samples exposed to ambient and heat curing by a maximum of 84 % and 79 %, respectively. The findings lay the groundwork for understanding the bond behavior of geopolymer, which is critical for the successful application of geopolymer-bonded rebar.

Bond properties of GFRP rebars in UHPC under different types of test

Bond properties of Glass Fiber-Reinforced Composite (GFRP) rebars in Ultra-High-Performance Concrete (UHPC) are the key to designing the FRP-reinforced UHPC structures. In this paper, bond properties between sand-coated/deformed GFRP rebars and UHPC within various embedment lengths and covers were estimated using three types of tests, namely the pullout test, modified pullout test and beam test. It was revealed that the dominant failure mode was the pulling-out of GFRP bars, evidenced by interlaminar shearing devastation, showing the shearing of the ribs and a significant reduction of the rebar diameter. Two classes of bond stress-slip relationships were summarized depending on the embedment length. The micro-slip, slip, decline and oscillation parts were captured, with the oscillation characteristic being more related to the ribs of GFRP rebars. Bond strengths of GFRP rebars under the beam test increased by 4.2 %− 10.0 % to the pullout test, and those obtained from the modified pullout test and pullout test remained little difference within 3.3 %. Both increasing the embedment length and lowering the cover could cause a linear decrease in the bond strength. Specifically, regarding beam tests, the bond strength demonstrated reductions of 11.4 % and 29.5 %, respectively, when the embedment length increased from 2.5d to 5d and 5d to 10d. Moreover, the bond strengths under three types of tests were compared with the ones predicted by existing methods, and the design recommendation was presented.

Bond behavior of deformed rebar in concrete containing recycled brick aggregate and waste rubber

The reuse of aggregates and crumb rubber (CR) in concrete is gaining interest globally for reducing the adverse effects on the environment and maintaining construction sustainability. The bond strength of reinforcing rebars in such concrete is one of the key issues to be addressed before their practical application, particularly in structural elements. Pull-out tests were performed on 80 specimens to assess the bond performance of deformed rebars in rubberized recycled brick aggregate concrete (RRBAC). After investigating the strength properties of RRBAC, pull-out tests were performed on two sizes of cylindrical specimens. RBA%, CR%, rebar diameter (d), embedment length (le), and concrete cover-to-rebar diameter ratio (c/d) were among the test variables in this bond behavior investigation. Bond stress-slip relationships were obtained, examined, and average bond strengths were compared with the available codes. Concrete split failure was observed in 79% of the tested specimens, whereas rebar fracture was not seen in any of the tests. The bond strength was found to be decreased with the inclusion of RBAs and CRs though relative improvement was observed for higher percentage RBAs. Additionally, regression analysis was performed on the experimental data to produce a closed-form equation for forecasting the bond strength for RRBAC. With a strong coefficient of determination (R2 = 0.97) and low values of error terms, the predicted bond strengths were seen to be a good agreement with the test results.

Experimental study on the bond-slip behavior of steel tube-coal gangue concrete

This study investigated the bond-slip behavior at the interface between steel tubes and coal gangue concrete, considering the coal gangue coarse aggregate replacement rate, the strength grade of coal gangue concrete, and the bond length as variable parameters. Eleven groups of push out tests were conducted on steel tube coal gangue concrete specimens. The bond slip curve and the longitudinal strain distribution curve of the steel tubes were obtained, and the effects of these parameters on bond strength were analyzed. The results showed that the load-slip curves at both the loading and free ends of the specimens were similar, with slip occurring earlier at the loading end. The regularity of the test curves in repeated experiments indicated that interfacial bond stress was primarily determined by friction in the later stages. The longitudinal strain of steel tubes was approximately exponentially distributed along their length. The interfacial bond strength increased with the increase of coal gangue aggregate replacement rate and was more pronounced in specimens with higher strength coal gangue concrete. Longer bond lengths resulted in greater bond failure loads, with a lower rate of increase in higher strength coal gangue concrete specimens compared to those with lower strength. Based on the three stage bond stress-slip relationship for steel tube-concrete, a bond-slip constitutive model for steel tube-coal gangue concrete was established. This provided a reference for mechanical analysis and interface performance design of steel tube-coal gangue concrete.

Experimental study of post-fire bond behavior of concrete-filled stiffened steel tubes: A crucial aspect for composite structures

The slip between steel and concrete components and the bond strength between the two materials are important factors in load transfer under fire and cooling conditions. This study investigates the effect of the post-heating interaction of concrete and steel on the performance of composite members considering the bond between the two materials. Unidirectional push-out tests were conducted on circular concrete-filled stiffened steel tube (CFSST) columns after experiencing heating and subsequent cooling. The parameters under study were the diameter-to-thickness ratio of the tube (63.5, 31.75, and 21.17), the number of shear connectors (0, 4, and 6), and the target temperature (25, 250, 500, and 750 °C). The results showed that shear connectors effectively prevent concrete from sliding against the steel surface, facilitating load transfer from concrete to the steel tube. This increased interlocking between steel and concrete surfaces, leading to a substantial improvement in bond stiffness by 1–284% and bond strength by 2–590% for specimens with 4 and 6 shear studs. Moreover, the reduction in the diameter-to-thickness ratio enhanced interlocking between the steel tube and concrete core, leading to an increase in bond stiffness by 4–201% and bond strength by 10–216%. The application of thermal loading and the subsequent cooling induced macro-changes in the geometry of both the concrete core and steel tube, resulting in significant improvements in the bond properties of CFSST columns. In the specimens without shear connectors, slip between the concrete and steel occurred without any obstacle, while in the specimens with shear connectors, failure occurred in the form of the local crippling of the tube and the crushing of concrete around the shear stud together with the outward local buckling of the steel tube around the shear stud. Ultimately, a model to capture the bond stress-slip relationship of CFSST columns after thermal loading was proposed and then evaluated against the experimental data of the present study, which showed a good correlation between the experiment and prediction results.

Improved Bond Stress-Slip Relationships for Carbon Fibre-Reinforced Polymer-Strengthened Masonry Triplets

Carbon fibre-reinforced polymer (CFRP) emerges as a viable solution for reinforcing unreinforced masonry (URM) walls subjected to shear loads. While masonry structures are straightforward to construct, the complexity of the construction materials, especially in terms of their mechanical properties, poses challenges for numerical studies of their structural behaviour. Walls, being fundamental components in masonry construction, play a crucial role in transferring both horizontal and vertical lateral forces. This study investigates the enhancement of masonry wall behaviour through the reinforcement of CFRP. CFRP reinforcement increases ductility and strength, reducing the risk of failure under shear conditions. Additionally, CFRP composites present a practical solution to strengthening masonry structures compared to traditional reinforcement. However, brick, mortar, and CFRP have not been thoroughly investigated. Experimental tests on the bond behaviour of different configurations of CFRP-retrofitted masonry triplets have not been performed before and are therefore presented in this paper. Triplet specimens, comprising three bricks and two mortar joints, both with and without CFRP strengthening, were subjected to bond testing. The study affirms that masonry triplets strengthened with CFRP under shear loads exhibit strength levels at least four to six times greater than those without CFRP. The experimental work was carried out with eight different CFRP configurations on triplet masonry, and each test was repeated four times. Further, the bond stress-slip relationship in the case of masonry triplets with and without CFRP was predicted with new mathematical equations based on the conducted test results. These equations were included in the commercial finite element software ANSYS and used to conduct simulations of CFRP-reinforced masonry triplets. The numerical results indicate good agreement between the finite element model and the test results. The outcome of this research improves the current knowledge on the use of CFRP to reinforce masonry walls with brick and mortar, which will contribute to the understanding of the effect of CFRP on masonry structures.

Strain-rate effect on the bond strength between concrete and reinforcing bars in dynamic pull-out tests

Static and dynamic pull-out tests were conducted to investigate the strain-rate effect on the bond strength between concrete and rebar and figure out the required dynamic development length. The concrete strength and loading rate were considered variables. The test results indicated that the bond strength increased at high strain rates, and the strain-rate sensitivity became more significant with the decrease in concrete strength. Moreover, the bond stress–slip relationship varied with strain rates. Based on these results, dynamic increase factor (DIF) models of bond strength were suggested. The required dynamic development length was evaluated by comparing the suggested bond DIFs to DIF of rebar yield strength. The dynamic development length was similar to or less than the static development length when rebars with yield strength above 400 MPa and concrete with compressive strength below 40 MPa were used. However, the dynamic development length was expected to be longer with concrete above 40 MPa than the static development length.

Dynamic bond stress-slip relationship of steel reinforcing bars in concrete based on XGBoost algorithm

The bond stress-slip relationship between a deformed steel bar and concrete under various loading rates is required to appropriately simulate the ultimate conditions of concrete structures under dynamic loading. Furthermore, the corresponding key bond parameters are crucial to ensure the accuracy of the bond-slip model. This study analyzed the effect of concrete strength, geometric profile of steel bar, concrete cover, and stress state around the bond region on the dynamic bond parameters and bond-slip curves. For this purpose, a database containing 1056 pullout specimens was established, which was used to train three ensemble learning models, including Extreme Gradient Boosting Decision Tree (XGBoost), Gradient Boosting Regression Tree, and Random Forest, thereby predicting the bond parameters. The ensemble learning models exhibited higher accuracy than existing empirical models, with the XGBoost algorithm attaining the highest accuracy. Furthermore, the predicted bond parameters were found to express the bond response of the steel bar under different loading rates and lateral confinements with good accuracy. The model proposed in this study provides a new approach for determining the bond parameters and improves the accuracy of bond stress-slip model.

Experimental study on bond performance between carbon/glass-hybrid-fiber-reinforced polymer bars and concrete

Coating GFRP bars with a layer of carbon fibers containing carbon/glass-hybrid-fiber-reinforced polymer (HFRP) bars has been proven to improved interlaminar shear durability. Thus, the bond performance of the HFRP bar-concrete interface depends on the behavior of the HFRP-reinforced concrete structure. In this context, the study investigates the effects of the compressive strength of concrete and the embedded length of HFRP bars on the bond behavior of the HFRP bar-concrete interface and compares the results with the data on the interfacial bond of GFRP bar to concrete. The results demonstrate that all the specimens fail at the FRP bar–concrete interface under the pullout. The bond strength and bond stiffness of the HFRP bar–concrete composites decrease with an increase in the embedded length of the HFRP bar from 4 to 10 times its diameter. Raising the compressive strength of the concrete enlarges the bond strength and bond stiffness of the HFRP bar–concrete composites. Furthermore, the outer layer of carbon-fiber on the GFRP bar decreases the impact of the embedded length of the HFRP bar on the bond strength and bond stiffness of the HFRP bar–concrete composites. Finally, the improved modified mBPE model can accurately predict the bond stress–slip relationship of GFRP/BFRP/HFRP bar–concrete composites.

Bond responses and anchorage length of GFRP bar in precast recycled aggregate concrete

Precast recycled aggregate concrete (PRAC) employs precast reject-derived recycled aggregates (PRAs) and exhibits mechanical properties on par with or superior to those of natural aggregate concrete (NAC). Nevertheless, the durability of PRAC is hampered by aged mortar. To address this issue, integrating high durability glass fiber reinforced polymer (GFRP) bars into PRAC emerges as a potential solution for constructing enduring structures. Building upon this context, the study focuses on the bond responses between PRAC and GFRP bars, an area with limited existing research. Three main tasks were conducted: Firstly, a comprehensive test program involving 108 pullout samples was conducted to investigate the key variables' effects on bond failure mode and stress-slip relationship. Subsequently, an equation was developed to determine the peak bond strength of recycled aggregate concrete (RAC) based on available test data. Additionally, reliability analyses were performed to discuss the anchorage length of GFRP bar embedded in RAC. The findings are as follows: (a) The PRAC's bond capacity initially increased then decreased with rising PRA content. At full replacement, PRAC's bond strength was 28.3% lower than NAC. However, a two stage mixing approach restored PRAC's bond capacity to NAC levels. (b) Helical wrapping of the GFRP bar consistently exhibited the highest bond capacity, with an average bond strength 51% greater than a sand coated GFRP bar and 21% higher than a GFRP bar treated with both helical wrapping and sand coating. (c) Current codes specifying the critical anchorage length for FRP bar in NAC were often unsafe for RAC. Hence, a new expression, factoring in RA content, type and RAC mixing method, was proposed to determine the suitable anchorage length.

Bond stress-slip behavior of ultrahigh-strength steel bars with spiral grooves embedded in plain and steel fiber high-strength recycled aggregate concrete

This study investigated the bond behavior of ultrahigh-strength steel bars with spiral grooves (UHSSBs) embedded in high-strength recycled aggregate concrete (RAC). Forty-five pullout specimens under monotonic loadings were designed to evaluate the effect of several parameters on the bond strength. The variables included concrete strength (50 MPa–60 MPa); recycled concrete aggregate (RCA) replacement ratio (0 %, 50 %, and 100 %); embedment length (5db, 10db, and 20db); concrete cover (25 mm, 45 mm, and 68.7 mm); surface treatment; volume fraction of steel fibers in RAC and stirrup ratio. The failure modes of these tests mainly concentrated on two types, pullout-type bond failure and splitting-type bond failure. According to the typical bond stress-slip curves and trendlines of plotted values, the bond strength increased with increasing of concrete compressive strength. For specimens with splitting-type bond failure, the higher normalized bond strength was observed in higher RCA replacement ratio and concrete cover thickness. The factors of embedment length and surface treatment were also proven to be effective for bond strength. For specimens with pullout-type bond failure, the presence of steel fibers and stirrups substantially restricted the splitting crack growth and improved the bond failure modes. Combined with previous experimental results, simple empirical equations for the average bond strength and bond stress-slip relationship between UHSSBs and RAC under different types of bond failure were developed by regression analysis, which can capture the experimental data with high accuracy.

Bond strength analysis of post-installed rebar with chemical adhesive in concrete mixed with marble powder and glass fiber

This study investigates the impact of marble powder as a part of fine aggregate and the addition of glass fiber on the bond strength of concrete and deformed high-strength post-installed steel rebars using the epoxy chemical as an adhesive. Five marble dust powder (MDP) substitution percentages (i.e. 0%, 10%, 20%, 30%, and 35%) and the addition of glass fiber (i.e. 0.5%, 1%, and 1.5%) were used. The performance of deformed high-strength post-installed rebars in conventional concrete and concrete modified with 35% marble powder and 1% glass fiber was examined here utilizing pull-out tests on a total of 96 specimens. To examine the bond behaviors of deformed high-strength post-installed rebars at the epoxy–conventional concrete interface and epoxy-modified concrete with MDP and glass fiber interface, test criteria included concrete compressive strength, anchorage depth of rebar into concrete, rebar diameter, and concrete cover to rebar diameter ratio. For post-installed high-strength deformed rebar, bond stress–slip relationships were determined, examined, and subsequently compared with prior work and readily available codes. It was found that none of the specimens exhibited pull-out failure and that the majority of them displayed concrete splitting, concrete rapture, or rebar rapture failure. This suggests that epoxy resins are particularly successful as bonding agents to retrofit concrete structures at the steel–concrete contact. A closed-form equation for predicting the bond strength for post-installed high-strength deformed rebar was also created using regression analyses on the experimental data. With a significant coefficient of determination (R2 = 0.85) the observed predicted bond strengths and the test data were quite similar.

Early-Age Bond Behavior of Deformed Bars in Concrete Subjected to Bilateral Pressures

In RC structures during construction, the bond behavior of deformed bars in concrete is governed by the age-related concrete strength, the local confining pressure, the bar diameter, and the concrete cover thickness. In this paper, an experimental investigation was performed to quantify the influences of the curing age and the bilateral pressure on the bond behavior of deformed bars in early-age concrete with 204 pull-out specimens for different concrete strengths, bar diameters, and relative concrete cover thicknesses. It was shown that the failure mode was associated greatly with the age-related concrete strength, the concrete cover thickness, and the pressure level, and that the bond strength increased with the pressure level and curing age, whereas the corresponding slip decreased with the curing age. Based on the regression analysis, calculation models were developed to predict the ultimate bond stress and the corresponding slip. A bond–slip model is proposed to describe the bond stress–slip relationship of deformed bars in early-age concrete under bilateral pressures, and the model was verified with the test results.

Numerical study on the effects of non-uniform corrosion and confinement conditions on the bond performance of RC beams

A 3D model is established to simulate the bond performance of corroded RC beams, in which the non-uniform corrosion of tensile reinforcement and confinement conditions (characterized by the thickness-diameter ratios of 1.9–3.8 and stirrup confinement index of 0–8.3%) are investigated. In the 3D numerical model, fine modeling was adopted to consider the influences of corrosion on the steel-concrete interface and the non-uniform corrosion was considered by separating the tensile bars into two parts with inconsistent diameter loss. Meanwhile, a two-stage numerical analysis method was utilized, to reflect the corrosion-induced concrete cracking by applying forced displacement and subsequently analyzed their flexural bond performance. In comparison to the existing experiments, the proposed 3D numerical model is proved to be reasonable. In addition, the simulation results show that the increment of the (residual) bond strength generated by the increasing thickness-diameter ratio increases with the mass loss rate, while the increases of stirrup confinement index cause much more increases in the (residual) bond strength for the corroded RC beams than the un-corroded beams. Furthermore, a bond stress-slip relationship is proposed taking the non-uniform corrosion and confinement conditions into account, which correlates well with the experiments.

Investigation of bond behavior between waterborne epoxy-coated steel reinforcing bars (WECR) and concrete

Understanding the bond behavior of waterborne epoxy-coated steel reinforcing bars (WECR) and concrete is a prerequisite for its application in concrete structures. In this study, a total of forty-eight pull-out specimens were tested to investigate bond behavior between WECR and concrete. The failure modes and bond stress-slip relationships were analyzed and discussed in detail. The formula for the ultimate bond stress between WECR and concrete was obtained. The critical anchorage length of WECR was calculated by reliability analysis. Finally, the detailed 3D finite element model was established to simulate the effect of coating thickness on bonding behavior. The results showed that, compared with ordinary reinforcement, the ultimate bond stress between WECR and concrete decreased by about 10% and the slip increased by about 1 mm. The critical anchorage length of WECR was about 1.1 times that of ordinary reinforcement, and it was recommended to take 1.25 times in the structural design. The change in rib height and rib angle due to coating is the main reason that affects the bonding performance.

Bond Behavior of Plain Bars in Concrete under Reversed Cyclic Loading.

Plain bars with a diameter of 10 mm are widely used in reinforced concrete buildings, and the bond behavior between the bars and concrete has an essential effect on the seismic performance of concrete structures. Thus, to assess the safety of old buildings and repaired buildings with normal concrete, it was necessary to further investigate the bond performance of the plain bars in the concrete. The bonding tests under monotonic and reversed cyclic loading were carried out on the specimens reinforced with plain bars, and the influences of concrete grade and embedment length on the bond behavior were taken into consideration. The results indicate the maximum bond stress under reversed cyclic loading is less than that under monotonic loading, and this is the same for corresponding slip for the same test parameters. The concrete compressive strength positively affects the maximum bond stress, whereas the embedment length has a negative effect. Based on the elasticity analysis and test data fitting, the expressions of bond stress at characteristic points on the bond stress-slip curves were carried out. Consequently, the bond stress-slip model was established, which could be applied to calculate the bond stress-slip relationships under monotonic and reversed cyclic loading. By comparison between the test curves and proposed model, a good agreement is observed, which indicates that the proposed model can be used to predict the bond stress-slip curve of plain bars in concrete.

Bond durability performance of sand-coated BFRP bars in high-strength fiber reinforced concrete

The ever-increasing utilization of fiber-reinforced polymer (FRP) bars has led to a surge in research regarding their bonding properties with concrete. In this study, the bond durability behavior between high-strength fiber-reinforced concrete (FRC) and sand-coated basalt FRP (BFRP) bars was examined under three different environmental conditions including immersion in seawater solution at 50 °C for 3, 6, and 9 months, immersion in the sulfuric acid solution for 3, 6, and 9 months with an ambient temperature range of 20 to 30 °C, and exposure to 270 freeze–thaw cycles. A total of 60 pull-out samples were prepared using either basalt (B) fibers or polypropylene (PP) fibers. Plain concrete pull-out samples were served as controls. Test results revealed that all exposures have degraded the bond, leading to pull-out failure in all of the specimens. Exposure to 50 °C seawater was found to cause the most significant degradation. However, the use of basalt fibers helped to mitigate the loss of bond strength in the specimens when compared to plain concrete specimens. For example, after 3, 6, and 9 months of exposure to 50 °C seawater, bond strength retentions of 73.58%, 56.04%, and 34.47% were observed for basalt FRC (BFRC) specimens, compared to 39.64%, 49.37%, and 29.61% for plain concrete specimens. On the other hand, a lower retention in bond strength of 16.28% was recorded for polypropylene FRC (PPFRC) specimens at 9 months of exposure to 50 °C seawater. The bond stress-slip relationship of the tested bars was accurately predicted using two calibrated analytical models: the Eligehausen, Popov, and Bertero (BPE) model and the Cosenza, Manfredi, and Realfonzo (CMR) model. Notably, both models showed a strong correlation with the observed bond stress-slip relationships in the experiments.

Bond-slip behavior between reactive powder concrete and H-shaped steel

The mechanical performance of the steel reinforced reactive powder concrete (SRRPC) structural components significantly depends on the steel-concrete interfacial bond behavior. This paper presents an experimental and analytical investigation on the bond-slip behavior between the reactive power concrete and the H-shaped steel. A total of 11 specimens, considering the cover thickness, embedded length, stirrup ratio and bonding part (whole section, flange and web), were tested through the push-out configurations. Results showed that the cover thickness has the most significant impact on the interfacial properties. Moreover, the SRRPC exhibited the highest residual bond strength compared with the other steel reinforced concretes. The five-stage model and the modified tri-linear model were proposed to describe the average bond stress-slip relationship. It was found that the five-stage model had an excellent accuracy while the modified tri-linear model can reasonably predict the average bond stress-slip relationship. Finally, a comprehensive analytical model was developed to investigate the local longitudinal strain of the shape, bond stress and relative slip.

Experimental investigation on realistic model for local bond stress-slip relationship between ribbed GFRP bars and concrete

Due to the lack of relevant research, the effect of the lateral confinement provided by stirrups and concrete cover on the bond behavior between glass fiber-reinforced polymer (GFRP) bars and concrete is ambiguous, resulting in the lack of a practical bond slip theory of GFRP bars. Therefore, it is impossible to accurately analyze the load response of GFRP-reinforced concrete (RC) structures from the stress level. In this paper, the local bond behavior of ribbed GFRP bars was systematically analyzed by the beam-end tests. The test parameters include concrete strength, stirrup spacing, and concrete cover. In addition, a bond stress-slip model considering the bonding mechanism was proposed on the basis of the beam-end tests and previous research. The results show that, the development of concrete cracks is restricted due to the existence of stirrups, and the specimens tend to fail in pull-out failure with strong confinement of stirrups. With the increase in the lateral confinement level, the failure mode of specimens transits gradually and smoothly from splitting failure to pull-out failure. Meanwhile, the peak slip of specimens with splitting failure also increases, and its maximum value does not exceed the peak slip of the corresponding specimens with pull-out failure. The confining capacity of stirrups and concrete cover was unified by a proposed parameter K. Then, a bond stress-slip model based on parameter K was established to predict the bond behavior of ribbed GFRP bars.