Bond-slip Curves Research Articles

Bond-Slip Relationship for CFRP Sheets Externally Bonded to Concrete under Cyclic Loading.

The objective of this paper was to explore the bond–slip relationship between carbon fiber-reinforced polymer (CFRP) sheets and concrete under cyclic loading through experimental and analytical approaches. Modified beam tests were performed in order to gain insight into the bond–slip relationship under static and cyclic loading. The test variables are the CFRP-to-concrete width ratio, and the bond length of the CFRP sheets. An analysis of the test results in this paper and existing test results indicated that the slope of the ascending segment of the bond–slip curve decreased with an increase in the number of load cycles, but the slip corresponding to the maximum shear stress was almost invariable as the number of load cycles increased. In addition, the rate of reduction in the slope of the ascending range of the bond–slip curve during cyclic loading decreased as the concrete strength increased, and increased as the load level or CFRP-to-concrete width ratio enhanced. However, these were not affected by variations in bond length if the residual bond length was longer than the effective bond length. A bilinear bond–slip model for CFRP sheets that are externally bonded to concrete under cyclic loading, which considered the effects of the cyclic load level, concrete strength, and CFRP-to-concrete ratio, was developed based on the existing static bond–slip model. The accuracy of this proposed model was verified by a comparison between this proposed model and test results.

Bond Behavior of Wet-Bonded Carbon Fiber-Reinforced Polymer-Concrete Interface Subjected to Moisture

The use of carbon fiber-reinforced polymer (CFRP) composite materials to strengthen concrete structures has become popular in coastal regions with high humidity levels. However, many concrete structures in these places remain wet as a result of tides and wave-splashing, so they cannot be completely dried before repair. Therefore, it is vital to investigate the effects of moisture on the initial and long-term bond behavior between CFRP and wet concrete. This research assesses the effects of moisture (i) during CFRP application and (ii) throughout the service life. Before CFRP bonding, the concrete blocks are preconditioned with a water content of 4.73% (termed “wet-bonding”). Three different epoxy resins are applied to study the bond performance of the CFRP-concrete interface when subjected to moisture (95% relative humidity). A total of 45 double-lap shear specimens were tested at the beginning of exposure and again after 1, 3, 6, and 12 months. All specimens with normal epoxy resins exhibited adhesive failure. The failure mode of specimens with hydrophobic epoxy resin changed from cohesive failure to mixed cohesive/adhesive failure and to adhesive failure according to the duration of exposure. Under moisture conditioning, the maximum shear stress (τmax) and corresponding slip (smax) of the bond-slip curve first increased and then decreased or fluctuated over time. The same tendency was seen in the ultimate strain transmitted to the CFRP sheet, the interfacial fracture energy (Gf), and the ultimate load (Pu). Analytical models of Gf and Pu for the CFRP-concrete interface under moisture conditioning are presented.

Bond characteristics of CFRP-to-steel joints

Carbon Fiber Reinforced Polymer (CFRP) composites have a large potential for strengthening and retrofitting steel parts but due to their premature debonding from steel, further data and research are still required for wider application in such situations. In the present paper, the bond characteristics of CFRP-to-steel joints in pull-pull loaded conditions were studied. Monotonic loading of the double strap joints with different bond lengths was applied and the failure modes and interfacial bond-slip curves were obtained. A tri-linear bond-slip model is proposed and it was derived from the experimental data. A closed-form solution approach is also proposed based on the tri-linear bond-slip model. The strength of the CFRP-to-steel interface, the distribution of the relative displacements between bonded materials, the strains developed in the CFRP laminate and the bond stresses along the interface are reported and the closed-form solution is compared with the experimental results. Two cases are selected for presentation: (i) one with the bond length greater than the effective bond length; and, inversely, (ii) one with bond length which is shorter than the effective bond length. The results predicted by the closed-form solutions are shown to be accurate enough when compared to the experiments.

Bond Behaviors Between Full-Recycled-Aggregate Concrete and Deformed Steel-Bar

Introduction: Full-recycled-aggregate concrete (aRAC) is a new concrete reusing 100% fine and coarse recycled aggregates produced from waste concrete of demolished concrete structures. As there is a lack of studies on the bond behavior between steel bar and aRAC, findings of this study are of significance for the structural application of aRAC. Method: This paper presents the pull-out test results of 24 groups aRAC specimens with deformed steel bar, and discusses the effects of aRAC strength, bond length and lateral constructional stirrups on the bond behavior between deformed steel bar and aRAC. Results and Conclusion: The bond stress and slip at key points of bond-slip curve are analyzed in relation to the tensile strength of aRAC and the bond length of steel bar. The bond-slip relationship between deformed steel bar and aRAC is proposed based on the test and analysis of this study.

Theoretical model for the bond–slip relationship between ribbed steel bars and confined concrete

This paper conducts a theoretical research into bond–slip relationship between ribbed steel bars and concrete. According to whether the concrete is cracked or not, the ascending part of the bond–slip curve is divided into two sections which are calculated by two different mechanical models. Based on the wedging action mechanism, the slip is introduced as displacement boundary conditions into the model, which is the main innovation of this study. The boundary conditions of the model are changed by the changing slip, so the bond stress values corresponding to all levels of slip are obtained. According to the rebar geometrical parameters specified in National Standard GB1499‐2007, the spacing of the adjacent ribs is fitted as a linear function of the rebar diameter. Then characteristic slip with the rib spacing as the main parameter is adopted in this study. Also, a negative exponential function is proposed for the descending part of the bond–slip curve. Specially, parameters of slipping path are therefore suggested for engineering applications. Comparison between calculated values and test results shows that the theoretical model can reasonably predict the local bonding problems with different steel bar diameters and concrete strength grades.

Bond–slip behavior of superelastic shape memory alloys for near-surface-mounted strengthening applications

The use of superelastic shape memory alloy (SMA) bars in the near-surface-mounted (NSM) strengthening application can offer advantages such as improved bond behavior, enhanced deformation capacity, and post-event functionality. This study investigates bond characteristics and load transfer mechanisms between NSM SMA reinforcement and concrete. A modified pull-out test specimen that consists of a C-shaped concrete block, where the NSM reinforcement are placed at the center of gravity of the block, was used for experimental investigations. The effects of various parameters such as epoxy type, bonded length, bar diameter, and mechanical anchorage on the bond behavior were studied. The slip of the SMA reinforcement relative to concrete was measured using an optical measurement system and the bond–slip curves were developed. Results indicate that the sandblasted SMA bars exhibit satisfactory bond behavior when used with the correct filling material in NSM strengthening applications, while the mechanical anchorage of SMA bars can significantly increase the bond resistance.

Corrosion-induced cracking and bond behaviour of corroded reinforcement bars in SFRC

This paper investigates, experimentally and numerically, the effect of fibre reinforcement on the initiation of corrosion-induced cracks in concrete and the bond behaviour of corroded reinforcement bars in fibre reinforced concrete. The fibres, due to their confining effect, contributed to delay crack initiation, improve the post-peak bond behaviour and retain the initial splitting strength for corrosion levels of up to 8%. The mechanisms for delayed crack initiation were explained through 3D finite element analyses of the experiments whereas a 1D model, using experimental bond-slip curves as an input, was employed to quantify the beneficial effect of fibres on the reinforcement anchorage length.

Bond-Slip Behavior of Basalt Fiber Reinforced Polymer Bar in Concrete Subjected to Simulated Marine Environment: Effects of BFRP Bar Size, Corrosion Age, and Concrete Strength

Basalt Fiber Reinforced Polymer (BFRP) bars have bright potential application in concrete structures subjected to marine environment due to their superior corrosion resistance. Available literatures mainly focused on the mechanical properties of BFRP concrete structures, while the bond-slip behavior of BFRP bars, which is a key factor influencing the safety and service life of ocean concrete structures, has not been clarified yet. In this paper, effects of BFRP bars size, corrosion age, and concrete strength on the bond-slip behavior of BFRP bars in concrete cured in artificial seawater were investigated, and then an improved Bertero, Popov, and Eligehausen (BPE) model was employed to describe the bond-slip behavior of BFRP bars in concrete. The results indicated that the maximum bond stress and corresponding slip decreased gradually with the increase of corrosion age and size of BFRP bars, and ultimate slip also decreased sharply. The ascending segment of bond-slip curve tends to be more rigid and the descending segment tends to be softer after corrosion. A horizontal end in bond-slip curve indicates that the friction between BFRP bars and concrete decreased sharply.

Bond deterioration of corroded steel in two different concrete mixes

This paper investigated the effects of rebar corrosion on bond performance between rebar and two different concrete mixes (compressive strengths of 20.7 MPa and 44.4 MPa). The specimen was designed as a rebar centrally embedded in a 200 mm concrete cube, with two stirrups around the rebar to supply confinement. An electrochemical accelerated corrosion technique was applied to corrode the rebar. 120 specimens of two different concrete mixes with various reinforcing steel corrosion levels were manufactured. The corrosion crack opening width and length were recorded in detail during and after the corrosion process. Three different loading schemes: monotonic pull-out load, 10 cycles of constant slip loading followed by pullout and varied slip loading followed by pull-out, were carried out on the specimens. The effects of rebar corrosion with two different concrete mixes on corrosion crack opening, bond strength and corresponding slip value, initial slope of bond-slip curve, residual bond stress, mechanical interaction stress, and energy dissipation, were discussed in detail. The mean value and coefficient of variation of these parameters were also derived. It was found that the coefficient of variation of the parameters of the corroded specimens was larger than those with intact rebar. There is also obvious difference in the two different concrete mixes for the effects of rebar corrosion on bond-slip parameters.

Bond behaviour of GFRP reinforced geopolymer cement concrete

Bond plays a key role in the performance of reinforced concrete structures. Glass fibre reinforced polymer (GFRP) reinforcing bar and Geopolymer cement (GPC) concrete are promising alternative construction materials for steel bars and Ordinary Portland Cement (OPC) concrete respectively. In this study, the bond behaviour between these two materials is investigated by using beam-end specimen tests. The bond behaviour of 15.9 mm diameter sand-coated GFRP bar was investigated. An embedment length of six and nine times the bar diameter were used. The free end and the loaded end bond-slip-relationships, the bond failure mode and the average bond stress were used to analyse each of the specimens. Additionally, the distribution of tensile and bond stress along the embedment length was investigated by installing strain gauges along the embedment length in some of the specimens. Test results indicate that a significant difference exists between the free end and loaded end bond-slip curves, which is due to the lower elastic modulus of the GFRP bars. Furthermore, it was found that the tensile and bond stress distribution along the embedment length is nonlinear and the nonlinearity changes with the load.

An Experiment on the Bond-Slip Behaviors of Pre-Tensioned Prestressed UHPC Members

An Ultra High Performance Concrete (UHPC) exhibits very high compressive and tensile strength and excellent durability comparing to those of the conventional concrete. The applications of UHPC to precast and/or prestressed concrete result the optimum utilization of high strength characteristics. This paper experimentally investigates the bond-slip behaviors between prestressing strands and UHPC in pretnesioned members. The diameters of strands, 12.7mm and 15.2mm, the cover depths and tensioning forces are chosen as variables, which are known to affect the bond behaviors. The bond-slip curves for various specimens are obtained and the equations for bond strengths of UHPC are derived based on experiments.

Analysis of the bond behaviour between prestressed strands and concrete in fire

In this paper a robust model has been developed to predict the average bond stress-slip relationship between the strands and concrete of prestressed concrete structural members. Two bond-slip curves have been proposed to represent the bond-slip characteristics for the three-wire and seven-wire strands. This model considers the variation of concrete properties, strands’ geometries and the type of strand surface, smooth or indented. The degradation of materials and bond characteristic at elevated temperatures are also included in the model. The proposed model has been validated against previous experimental results at both ambient and elevated temperatures.

Analysis of interfacial bonding characteristics of CFRP-concrete under fatigue loading

The paper presents the results of an experimental and analytical study involving the bond properties of nine specimens externally strengthened with carbon-fiber-reinforced polymer (CFRP) sheets under fatigue loading. The purpose of this study is to investigate the influence of concrete strength and fatigue amplitude on the bond performance of CFRP-concrete interface under fatigue loading. Firstly, modified beam specimens with different concrete strengths were designed and constructed. Secondly, the specimens were tested under fatigue loads with different fatigue amplitudes. The test results show that the fatigue life increased with an increase in concrete strength, but decreased as the fatigue load amplitude increased, and a formula was proposed to predict the fatigue life. Moreover, the concrete strength and fatigue loading amplitude have little effect on the effective bond length under fatigue loading. The test results of bond-slip relationship show that the bond-slip curves continuously degraded with an increase in the number of load cycles, that is, the maximum shear stress decreased with an increase in the number of load cycles. Finally, a bond-slip model was developed considering the effects of concrete strength and fatigue loading amplitude. The comparison between experimental results and this model demonstrated that this developed bond-slip model can be used to describe the bond-slip relationship of the CFRP-concrete interface under fatigue loading.

Bond behaviour between NSM CFRP laminate and concrete using modified cement-based adhesive

Near-surface mounted (NSM) techniques have been successfully utilized for the strengthening of concrete structures with CFRP composites. Although the adhesive most commonly used for strengthening of structures is epoxy, this adhesive is suitable only for low temperature environments due to the deterioration of its mechanical properties at elevated temperatures, in addition to its other disadvantages such as flammability and emission of toxic fumes. Substituting the epoxy with cementitious adhesive is therefore of great interest. However, few investigations on the strengthening of concrete structures using cementitious bonding materials have been reported, particularly for NSM CFRP strengthening systems. Since the bond between NSM FRP reinforcement and the concrete substrate plays a major role in the efficiency of NSM strengthening systems, in this paper, the bond properties of NSM CFRP laminate using modified cement-based adhesive are investigated. 15 concrete specimens were studied, and parameters including bond length, strain distribution, and bond-slip curve were considered. Finite element simulation was conducted to verify the experimental results using ATENA Software from Cervenka. The results showed excellent bond properties with reasonable correlation between experimental and finite element analysis results.

Influence of Nanoclay on Concrete Subjected to Freeze-Thaw Cycles and Bond Behavior between Rebar and Concrete

This paper explores the effects of nanokaolinite clay (NKC) on the behavior of cement-based materials concrete. The resistance of NKC modified cement-based materials to freezing-thaw cycles and the corrosion processes of rebar embedded in the concrete were investigated. Ordinary Portland cement was partially substituted with NKC in ratios of 0%, 1%, 3%, and 5% by weight. The Rapid Freeze-Thaw Cabinet was used to measure the resistance of ordinary Portland cement concrete and concrete with clay to deterioration caused by repeated cycles of freezing and thawing, compressive strength were measured at regular intervals. The corrosion conditions of the rebar embedded in the concrete were studied by an electrochemical accelerated penetration system, pullout tests were performed to assess the bond properties including bond-slip curve, ultimate bond strength between concrete and rebar before and after corrosion. It is revealed that the introduction of NKC improves the freeze-thaw resistivity values and bond behavior in the concrete specimens compared to the control concrete; the corrosion of the rebar embedded in the concrete is impeded efficiently.

Determination of the bond–slip behavior of CFRP-to-steel bonded interfaces using digital image correlation

The bond–slip behavior between carbon fiber reinforced polymer (CFRP) plates and steel is crucial for predicting the structural behavior of CFRP-strengthened steel members. In this paper, the CFRP-to-steel bonded joints with different bond lengths and adhesive thicknesses were tested to investigate the bond–slip behavior of CFRP-to-steel bonded interfaces. The digital image correlation (DIC) technique was used to measure the displacement and strain data. The test results, including the failure mode, ultimate load, displacement distribution, and CFRP strain distribution, are presented and discussed. The non-linear bond–slip curves of CFRP-to-steel bonded interfaces are determined by a fitting procedure for CFRP strain distributions. A bilinear bond–slip relationship is further proposed to simplify the non-linear bond–slip curve. The test results indicate that the adhesive thickness has an influence on the failure mode, ultimate load, and bond–slip behavior. Similar to a CFRP-to-concrete bonded interface, an effective bond length also exists for a CFRP-to-steel bonded interface. The development of the interfacial shear stress is analyzed based on the determined non-linear and bilinear bond–slip curves. Finally, two theoretical models are employed for the prediction of the ultimate load, effective bond length and bond–slip curve. This study shows that the DIC method is suitable for investigating the bond–slip behavior of CFRP-to-steel bonded joints.

Bond behavior of GFRP bar-concrete interface: Damage evolution assessment and FE simulation implementations

This study presents a bond damage assessment approach for the glass fiber-reinforced polymer (GFRP) bar-concrete interface. The damage evolution equations are proposed based on the strain equivalence principle of damage mechanics, where the variations of the secant modulus of the bond-slip curve are utilized to evaluate the interface deterioration against slip. Numerical analyses are conducted with the ANSYS finite element (FE) program to simulate the bond behavior of pullout test. Nonlinear material behaviors of the GFRP composite and concrete matrix with respect to plain concrete and fiber-reinforced concrete (FRC) are implemented using appropriate constitutive models. The interfacial bond-slip behavior is implemented using nonlinear spring elements. Numerical predictions are validated by the experimental results and compared with the widely used analytical models accounting for the FRP-concrete bond. Upon this, the bond damage evolution curves are derived thereafter, which are found to strictly follow the lognormal distribution or Weibull distribution. Further comparisons of different specimens are performed to investigate the critical factors and their impacts on the damage evolution, as well as those on the critical bond damage corresponding to the bond strength.

Bond–slip behaviour of steel bars in low-strength concrete

This paper presents the bond–slip models for deformed and plain bar types in low-strength concrete. The newly developed bond-strength models were used to account for the effect of different deficient variables typically found in non-engineered reinforced-concrete structures in developing countries, such as low-strength concrete, insufficient development length and low concrete cover. Residual bond strength was evaluated from experimental bond–slip curves by defining approximate criteria for calculating residual slip. Suitable bond–slip relationships are identified to define local bond–slip relations for both bar types in low-strength concrete. Based on the experimental findings, the strain distribution profile over the development length is proposed, which can be used to evaluate the bar elastic slip in low-strength concrete by employing the macro model. The proposed bond–slip models can be used in the inelastic analysis of non-engineered reinforced concrete structures.

Bond-Slip Relation of Pretensioned Concrete Members for Various Tensioning Forces

The bond between prestressing (PS) steel and concrete plays an important role in pretensioned concrete members because the pretension force is directly transferred from PS steel to concrete by cutting or releasing the PS steels at the end of the members. While in capacity design approaches, designing for bond is considered to be a question of adequate strength along the bar length, in performance-based design approaches, the level of deformation as well as bond strength must be known and the bond-slip curves may give enough information. The purpose of this paper is to derive bond-slip curves for various tensioning forces within Korean design Code experimentally. For this purpose several series of tests have been conducted with different cover depths and tensioning forces. A realistic bond-slip curve is proposed which may be the base for more rational design of pretensioned concrete members.

Experimental study on the bond durability between steel-FRP composite bars (SFCBs) and sea sand concrete in ocean environment

Bond durability tests on steel-fiber reinforced polymer (FRP) composite bars (SFCBs) with sea sand concrete and river sand concrete in simulated ocean environments are conducted in this paper, and steel bars are also used for comparison. Two types of environments, seawater immersion and seawater wet–dry cycling, are adopted. Sixty-six pullout specimens are prepared, and the variations of bond performance after 30, 60 and 90days of aging are tested and comparatively analyzed. The test results showed that, with respect to the bond–slip curves, wave-shaped descending branches were observed for SFCBs; the bond strength between SFCBs and sea sand concrete in a wet–dry cycling environment at all ages were improved. However, with regard to the steel bars in the same condition, the surfaces were rusted, and the bond strengths showed a decreasing trend. Immersion in 40°C seawater reduced the bond performance of SFCBs with concrete, whereas the rust process of steel bars slowed down due to the lack of fresh oxygen.