Bond-slip Curves Research Articles

Bond performance of FRP bar reinforced concrete under monotonic load and fatigue effect

In order to investigate the bonding properties of FRP-reinforced concrete under different loads, a series of FRP-reinforced concrete specimens were subjected to pull-out tests. Three types of reinforcement were used in the experiment: ordinary steel reinforcement, carbon fiber reinforcement (CFRP), and glass fiber reinforcement (GFRP). Pull-out tests were first conducted under monotonic load to determine the effect of concrete strength and reinforcement diameter on the bond performance of FRP bar-reinforced concrete and steel-reinforced concrete. The investigation of the variation of bond performance under different reinforcement materials was made. Second, a fatigue test was done, followed by further pull-out tests to get the bond stress-slip curve between concrete and reinforcement, and the influence of fatigue action on the bond stress-slip curve was further examined. The formulae to calculate the stress were developed, which can be applied to determine the bond failure pattern, bond strength, and bond-slip curve.

Bond performance between hybrid fiber-reinforced concrete and BFRP bars under freeze-thaw cycle.

This study applied the pull-out test to examine the influence of freeze-thaw cycles and hybrid fiber incorporation on the bond performance between BFRP bars and hybrid fiber-reinforced concrete. The bond-slip curves were fitted by the existing bond-slip constitutive model, and then the bond strength was predicted by a BP neural network. The results indicated that the failure mode changed from pull-out to splitting for the BFRP bar ordinary concrete specimens when the freeze-thaw cycles exceeded 50, while only pull-out failure occurred for all BFRP bar hybrid fiber-reinforced concrete specimens. An increasing trend was shown on the peak slip, but a decreasing trend was shown on the bond stiffness and bond strength when freeze-thaw cycles increased. The bond strength could be increased significantly by the incorporation of basalt fiber (BF) and cellulose fiber (CF) under the same freezing and thawing conditions as compared to concrete specimens without fibers. The Malvar model and the Continuous Curve model performed best in fitting the ascending and descending sections of the bond-slip curves, respectively. The BP neural network also accurately predicted the bond strength, with relative errors of predicted bond strengths ranging from 3.75% to 13.7%, and 86% of them being less than 10%.

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.

Bond properties of steel bar with engineered geopolymer composites under monotonic load

AbstractEngineered geopolymer composites (EGC) featuring extraordinary tensile ductility are a potential alternative to engineered cementitious composite. In this study, the local bonding properties of steel bars in EGC were investigated. Monotonic loading tests of the 120 specimens were performed using the pullout test method. The study examined the ultimate tensile strain of EGC, diameter and anchorage length of the steel bar on bond performance. The results showed that the EGC, with an ultimate tensile strain of 4.57%, had the highest peak bond strength. By comparing the test results of rebar to ordinary concrete bond performance provided in several previous literature, it was demonstrated that EGC has superior bond performance with rebar compared to ordinary concrete. Based on this, suggestions were provided for the design of rebar anchorage length in EGC. Finally, a mathematical model suitable for determining the bond–slip curve between EGC and steel bars is proposed.

Experimental investigation on bond behaviour of the GFRP bars with normal and high strength geopolymer concrete

The study presents the experimental investigation of bond performance of normal and high-strength geopolymer concrete and Glass Fibre Reinforced Polymer (GFRP) bars. The effects of bar diameters, embedment lengths, and concrete grades on maximum pullout load and failure characteristics are investigated. When the adhesive and residual strengths of GFRP bond specimens were compared to steel specimens, the former demonstrated remarkable performance despite having differing surface properties. An energy-based equivalent bond strength approach is proposed to examine pre-peak bond performance, which demonstrating that the decreasing rate of strength is faster for higher diameter and embedment, showing a drop in bond performance. Pullout failure was noted in smaller diameter bond specimens, where the shear strength between the ribs and the GFRP bar controlled the bond strength. Concrete split failure occurs for 12 mm specimens with embedment lengths greater than 5d in both GPC40 and GPC60. Specimens with 16 mm, excluding G-16-5d-60, demonstrated concrete split failure for all embedment lengths and concrete grades; bond strength is determined by the concrete shear strength. Enhancing the concrete’s compressive strength has a beneficial impact on the bond strength as it increases ranging, from 14.57 to 32.12%. Analytical models such as mBPE and CMR were employed to derive the mathematical expression for the bond stress versus slip curve. It was found that the curves of CMR model were typically closer to experimental curves than the mBPE model.

Study on the bond properties between BFRP bars and hybrid fibers reinforced recycled concrete under freeze-thaw cycles

To study the bond properties between basalt fiber reinforced polymer (BFRP) bars and hybrid fibers which were basalt fiber (BF) and polypropylene fiber (PF) reinforced recycled concrete under freeze-thaw cycles, conducting center pull-out tests to study the effects of three factors on the bond properties: the number of freeze-thaw cycles, the volume fractions of single fiber, and the volume fractions of hybrid fibers. Based on the data obtained from the test, establishing a four-stage bond-slip constitutive relationship model. The results showed that the failure modes were pull-out failure and splitting failure. The bond strength decreased when adding the single fiber, with a maximum reduction of 13.18%, but the peak slip increased, with a maximum increase of 69.92%. When the volume fraction of BF was 0.3%, it achieved the optimal effect. The bond strength and peak slip increased when adding hybrid fibers, with maximum increases of 13.47 and 130.08%, respectively. However, excessive fiber content will reduce the increase of bond strength. The bond strength between BFRP bars and hybrid fiber-reinforced recycled aggregate concrete (HFRAC) increased when the number of freeze-thaw cycles increased but decreased when the number of freeze-thaw cycles exceeded 50. The four-stage bond-slip constitutive relationship model fitted well with the bond-slip curves. Compared with other fiber-reinforced recycled concrete specimens, this model fitted better with the curves of HFRAC specimens after freeze-thaw cycles and had the best fitting effect for the internal crack slip stage of the curves.

A Sustainable Steel-GFRP Composite Bars Reinforced Concrete Structure: Investigation of the Bonding Performance

Steel-fiber reinforced polymer (FRP) composite bars (SFCBs) can enhance the controllability of damage in concrete structures; thus, studying the interfacial bonding between them is fundamental and a prerequisite for achieving deformation coordination and collaboration. However, research on the interfacial bonding performance between SFCBs and concrete remains inadequate. This study conducted central pullout tests on SFCB-concrete specimens with different concrete strengths (C30, C50, and C70), bar diameters (12, 16 and 20 mm), and hoop reinforcement constraints, analyzing variations in failure modes, bond-slip curves, bond strength, etc. Additionally, finite element simulations were performed using ABAQUS software to further validate the bonding mechanism of SFCB-concrete. The results showed that the failure mode of the specimens was related to the confinement effect on the bars. Insufficient concrete cover and lack of hoop restraint led to splitting failure, whereas pullout failure occurred otherwise. For the specimens with pullout failure, the interfacial damage between the SFCB and concrete was mainly caused by the surface fibers wear of the bar and the shear of the concrete lugs, which indicated that the bond of the SFCB-concrete interface consisted mainly of mechanical interlocking forces. In addition, the variation of concrete strength as well as bar diameter did not affect the bond-slip relationship of SFCB-concrete. However, the bond strength of SFCB-concrete increased with the increase of concrete strength. For example, compared with C30 concrete, when the concrete strength was increased to C70, the bond strength of the specimens under the same conditions was increased to 50–101.6%. In contrast, the bond strength of the specimens decreased by 13.29–28.71% when the bar diameter was increased from 12 to 14 mm. These discoveries serve as valuable references for the implementation of sustainable SFCB-reinforced concrete structures.

Experimental analysis of the effect of bar anchorage length on bond behavior in pull-out test

The article presents bond tests performed using the pull-out method. The experiments were designed to evaluate the effect of anchorage length on the bond in this test. The analysis shows that the length of the bar-concrete interface significantly influences the bond behavior. It determines the type of bond failure, affects the distribution and value of stresses in the bar, and projects the bond-slip curve, which is a key result of the pull-out test

Finite element analyses of failure of fiber reinforced polymer-concrete interface considering the concrete tension–compression softening effect

The concrete near the shear failure surface of the FRP (fiber reinforced polymer)-concrete interface is subjected to both tensile and compressive biaxial stress. However, existing finite element methods of the interfacial failure behavior of FRP-concrete interface to describe the tension–compression softening effect of concrete failure are not sufficiently accurate, and some detailed results may not be reflected. To address these issues, a finite element analysis method is proposed that accurately considers the tension–compression softening effect on concrete failure. The effectiveness of the method is demonstrated through analysis of literature data from experiments on plain concrete failure, FRP-concrete interface shear, and three-point bending. The method accurately reflects the generation of interfacial discontinuous micro-cracks, the oscillation singularity of stress field near the interfacial crack tip, the phenomenon of interface mismatch, the dispersion observed in the bond-slip curves, and the phenomenon of interfacial shear stress reversal. Moreover, this finite element analysis method is applied to the double shear experiments of the fully-bonded FRP-concrete interface under different boundary conditions, and the reasons for the different failure modes observed in the specimens are explained. The results indicate that considering the biaxial effect on concrete failure strength in numerical analysis can more accurately and comprehensively describe the shear failure behavior of the FRP-concrete interface.

Influence of elevated temperatures on the bond performance of CFRP-steel lap joints bonded by nanosilica-modified adhesive with mixed curing agents

Elevated temperatures may influence greatly the mechanical properties of adhesives, and it is difficult for adhesives with a single curing agent to meet the requirements of adequate heat resistance and toughness simultaneously. First, this paper proposes a nano-SiO2-modified epoxy adhesive (GY34) using mixed curing agents to improve the bond performance of CFRP-steel joints at a high temperature. Then, studies are taken to examine the impact of elevated temperatures on the mechanical performance of GY34 with mixed curing agents and the interfacial properties of CFRP-steel double-lap joints bonded by GY34. After that, models are developed to predict the bearing capacity and bond-slip of CFRP-steel double-lap joints at different temperatures using the test data. The results indicate the toughness is optimal at 70 °C when the curing agents C105 and D230 are mixed with a mass ratio of 1:2. The bearing capacity of the CFRP-steel lap joint bonded with GY34 is much higher than the cases only bonded with a single C105 or D230, respectively. Moreover, the load-displacement curve has a significant ductile development stage as the temperature rises below the glass transition temperature (Tg, S), beyond which, the bearing capacity declines. In addition, the length of shear stress transmission and the effective bonding length increase significantly at high temperatures. The bond-slip curve bonded with GY34 has a trilinear trapezoid shape at different temperatures, and the peak shear stress and stiffness decline at various elevated temperatures. Finally, the relative slip and interfacial fracture energies increase first and then decrease with the rise of temperature.

Grout sleeve splicing of steel-fiber reinforced polymer composite bars: Tensile behavior and bond mechanism

In reinforced concrete structures exposed to harsh environments, steel corrosion poses a significant problem. Ductile and durable steel-fiber reinforced polymer composite bars (SFCBs), which can effectively address this issue, are a perfect alternative to conventional steel bars. However, the lack of suitable connection methods for SFCB has hindered their widespread adoption in civil engineering. To address this limitation, this paper proposes the use of grout-filled steel sleeves as a solution for connecting SFCB. The study designed 120 specimens of grout sleeve splices for SFCB to investigate their performance under unidirectional tensile forces. The experimental variables included the ratio of grout thickness to SFCB diameter, SFCB diameter, anchorage length, and grout strength. The results revealed various failure modes and found that specimens with a smaller ratio of grout thickness to SFCB diameter and greater grout strength demonstrated higher peak stress. The peak stress first increases and then slowly declines with an increase in anchorage length. Additionally, the peak stress decreased with increasing SFCB diameter, and the peak slip remained constant. Meanwhile, the bond mechanism between SFCB and grout was revealed. Finally, the study derived a theoretical bond-slip curve between SFCB and grout, as well as the development anchorage length.

Investigation of the bond properties between hybrid fiberreinforced concrete and BFRP bars after exposure to high temperature

This research investigated the bond properties at high temperatures and with hybrid fiber addition between basalt fiber reinforced polymer (BFRP) bars and hybrid fiber-reinforced concrete (HFRC). The tensile strength and appearance morphology of BFRP bars were analyzed. Used scanning electron microscopy (SEM) to examine the microscopic morphology and used the bond-slip constitutive model to fit the bond-slip curves. When BFRP bars after exposed to 300 °C, their tensile strength dropped by 42%. With the increasing temperature, the bond strength and stiffness of normal concrete (NC) specimens decreased by 27.3 % and 67.5 %, respectively, while HFRC specimens decreased by 20.8 % and 55 %, respectively. Hybrid fibers increased the bond strength and stiffness of HFRC specimens by 27.1 % and 49.1 %, respectively. The best fitting models were the Malvar model and the Continuous Curve model.

Investigation on shear behavior of the interface between engineered cementitious composites and normal-strength concrete

Engineering cementitious composites (ECC), possessing excellent mechanical properties for good durability, can be applied to repair and strengthen reinforced concrete structures. In the present study, the interface shear behavior between ECC and normal-strength concrete was studied. Thirty bonded joint specimens were fabricated and subjected to push-out direct-shear tests. The construction methods of ECC mixture (precast and cast-in-place), ECC strength, and interface bonding methods (i.e., Epoxy resin, UHPC, cast-in-place ECC, cast-in-place ECC with chiseling, and bolts) were investigated. The results indicated that interface bonding methods have a significant influence on the failure modes and interface shear strength between ECC and existing concrete. There were three failure modes: interface debonding, interface debonding converted to concrete fracture, and concrete matrix destruction failure. Epoxy resin bonded specimens exhibited the highest shear strength, and cast-in-place ECC bonded specimens showed the lowest shear strength. Moreover, the shear performance of the ECC and the normal-strength concrete interface was analyzed in terms of interfacial shear strength and shear stiffness, bond-slip curve, and shear transfer mechanism. Finally, an analytical model for estimating the interfacial shear strength considering the interfacial bonding methods was established, with its predictions having high accuracy.

Bond-slip behavior of deformed rebar in grouted duct connection: Experiment, theoretical analysis and cohesive-zone element model

In precast bridge piers, the grouted corrugated duct (GCD) connection can offer advantages of accommodating large construction tolerances and being cost-effective. However, this connection is relatively new, and its bond strength and effective bond length are a concern in structural design. To fill the research gap of the bond behaviors of the GCD connection, experimental tests of 36 pull-out specimens with/without GCD were conducted in this paper. The influencing factors of bond strength were investigated, including duct, rebar diameter and anchorage length. The results revealed that the GCD connection required a shorter anchorage length and exhibited a plumper bond-slip curve and higher bond strength compared to the specimen without GCD connection. Then, an analytical model based on thick-walled cylinder theory was proposed to construct the bond-slip curve of GCD connection, and to identify the criteria for mortar strength. To simulate the bond-slip relationship in finite element model, a cohesive-zone element model realized by the zero-thickness user defined element was developed at the interface between rebar and dummy rebar. Finally, a comparison was made among the solution of proposed bond-slip model, finite element analysis, and the tested data, demonstrating good consistency between the three.

Simulation of the degraded (steel - concrete) bond strength due to corrosion via modeling pull out tests

Steel corrosion is recognised as major degradation factor of structural capacity and durability of reinforced concrete (RC) structures, especially in marine environment. RC structures with corroded reinforcement present reduced performance due to loss of the cross-sectional area of reinforcement, cracks in concrete and loss of bond between steel and the surrounding concrete. In the present study, simulation of the degraded bond strength on corroded RC elements was elaborated developing a three-dimensional (3D) model through finite element analysis (FEA) in ABAQUS, taking into account the recommendations of fib Model Code 2010. As a basis for the model validation, an existing experimental study on the effect of corrosion and stirrups spacing on bond behavior was considered. In the abovementioned study the bond mechanism of two groups of corroded RC specimens in confined (stirrups Φ8/120mm) and unconfined (without stirrups) conditions were examined upon the completion of eccentric pull-out tests. In order to model the complex conditions of the corroded steel-concrete interface, the cohesive behavior was adopted by appropriately modificated parameters, for each corrosion level. The bond-slip curves extracted from the developed model were shown to be in a good agreement with the relevant experimental results. In particular, the bond strength of confined RC specimens deviates at about 10% among the analytical and experimental outcomes, for both uncorroded and corroded conditions up to 8.3% corrosion level. In case of the unconfined specimens, the bond strength prediction was satisfactory up to 5% corrosion level, whereas for greater corrosion level, large deviation was observed. Regarding the prediction of the relative slip between steel and concrete, further investigation is required as the development of slip is influenced by a plethora of factors.

Study on the Bonding Performance of BFRP Bars with Seawater Sand Concrete.

A total of 66 sets of pullout specimens were prepared to investigate the bonding properties of basalt fiber-reinforced polymer reinforcement (hereinafter referred to as BFRP) with seawater sand concrete (hereinafter referred to as SSC). The volume dosages of mono-doped glass fibers and mono-doped polypropylene fibers were 0.1%, 0.2%, and 0.3%; the total volume dosage was set to be constant at 0.3%; and the doping ratios of the hybrid fibers were 1:2, 1:1, and 2:1. The effect on the bonding performance of BFRP reinforcement with SSC was studied on the condition of the diameter D of the BFRP reinforcement being 12 mm; the bond length of SSC being 3D, 5D, and 7D; and the surface characteristics of the reinforcement being sandblasted and threaded. The research showed that due to internal cracks in the matrix, salt crystals in the pores, chloride salts with high brittleness and expansion, as well as sulfate corrosion products such as "Frederick salts" in SSC, the concrete became brittle, resulting in more brittle splitting failures during the pullout test. Doped fibers can increase the ductility effect of concrete, but the bonding effect between the threaded fiber reinforcement and the SSC was not as good as that of the sandblasting group. When the bond length was 5D, the bonding effect between the BFRP reinforcement and SSC was the best, and the bonding performance of the experimental group with doped fibers was better than that of the threaded group. Finally, by combining the ascending segment of the Malvar model with the descending segment of the improved BPE model, a constitutive relationship model suitable for the bond-slip curve between BFRP reinforcement and SSC was fitted, which laid a theoretical foundation for future research on SSC.

Bond-Slip Model of Corrosion-Resistant Rebar and Fiber-Reinforced Seawater Sea-Sand Concrete

Abstract To study the bond performance between fiber-reinforced seawater sea-sand concrete and corrosion-resistant rebars (glass fiber–reinforced polymer [GFRP] bars and epoxy resin–coated rebars) in extremely hot and humid marine environments, bond anchorage tests between reinforcement and seawater sea-sand concrete are conducted to study bond performance. The failure morphology of reinforced concrete was investigated, and the bond-slip curves of different types of rebar and concrete were compared. According to the research results, the addition of basalt and polypropylene fibers to seawater sea-sand concrete changed the distribution of bonding stress, which was conducive to an improvement in the anchorage efficiency of reinforcement and concrete. The chemical bonding performance between the fiber-reinforced seawater sea-sand concrete and rebar is improved, wherein the bond stress increases by 94.01 % at the beginning of the slip, and the residual bond stress increases by 91.72 %. Compared with ordinary seawater sea-sand concrete, the bonding strength between ordinary reinforced rebar and fiber-reinforced seawater sea-sand concrete increased by 20.75 %, and the bonding strength between the GFRP bar and fiber-reinforced concrete decreased by 15.37 %. Based on the friction mechanism of rebars and concrete, bonding strength models of ordinary rebar, epoxy resin–coated rebar, and GFRP bars with fiber-reinforced seawater sea-sand concrete are presented.

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 Behavior between CFRP and Concrete with a Convex-Circular Arc Interface

The bond performance of CFRP to concrete plays a vital role in CFRP strengthening on concrete structures. In this paper, an experimental study was carried out to investigate the bond performance of CFRP to concrete with a convex-circular arc interface. The main factors were the curvature of the concrete surface and the bond length and the layers of the CFRP laminate. Based on the experimental results, the failure mode of the bond specimens, the variation of the bond capacity, the CFRP strain, and the bond–slip constitutive model are analyzed. The results showed that most of the specimens failed to peel off the interface concrete, and the bond capacity tended to increase with the increase in bond length when the bond length was within an effective value. When the interface curvature increased to 1/0.8 m, the bond capacity tended to increase due to the CFRP exerting a certain pressure on the concrete surface. The prediction formula of the bond capacity between the CFRP and concrete is proposed considering the influence of the interface curvature. The bond–slip curves are given out based on the finite differential analysis of the strain distribution of CFRP laminates. The accuracy and applicability of the proposed model are verified with a comparison to the test results and other existing models.

Study on the bond properties between basalt fiber-reinforced spontaneous combustion coal gangue concrete and BFRP bars

Microstructural characterization of spontaneous combustion coal gangue (SCCG), the hydration products and mechanism of spontaneous combustion coal gangue concrete (SCCGC) were discerned through microscopic analysis. The bond performance was assessed employing a central pull-out test on samples variably substituted with SCCG (0%, 25%, 50%, 75%, and 100%) and augmented with basalt fiber (BF) (0%, 0.1%, 0.15%, and 0.2%). The failure mode and bonding mechanism were also revealed by this test. The bond-slip curves were fitted by various bond-slip constitutive models and a suitable model was found for each section. As indicated by the results, SCCGC possessed a lower carbon content and higher Al and Si element contents. These elements would undergo secondary hydration reactions with CH, which could enhance the strength of the ITZ and the compactness of the bond interface between BFRP bars and concrete. The failure modes were splitting and pull-out. An inverse correlation was observed between bond strength and the increment in SCCG aggregate substitution, ranging from a decline of 2.6% to 23.1%. As the BF content increased, the bond strength and peak slip increased by 3.9% ∼ 19.7% and 4.0% ∼ 14.6%, respectively. Furthermore, the reinforcing effect of BF on bond strength increased from 3.9% ∼ 10.3% to 8.8% ∼ 19.7% as the SCCG replacement rate increased, which was noticeable. The Malvar model and the Continuous Curve model were the best fitting models for the ascending and descending sections of bond-slip curves, respectively, while the residual stage was well fitted by the Hao Qingduo model.