Bond Stress-slip Relationship Research Articles

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.

Study on local bond behaviors between steel bars and high-strength concrete exposed to early disturbances

This paper investigates the effects of early vibration exposure on the local bond behaviors between deformed steel bars and high-strength concrete (HSC). The effects of steel fiber and vibration intensity (ranging from static to peak particle velocity (PPV) of 10 cm/s) on the deterioration laws of bond performances and mechanisms were revealed. Finally, an empirical prediction model was developed to uniformly describe the bond stress-slip relations of specimens subjected to different vibration levels. Results indicated that early vibration could produce separation gaps and the surrounding viscous layers around steel bars, leading to the reduction/elimination of initial debonding resistances. Besides, the failure mode of pull-out specimens may alter due to the degradation of concrete. The bond stiffness of HSC is more vulnerable to environmental vibration (up to 62.6%), followed by bond strength (39.4%) and compressive strength (33.7%). The bond toughness of specimens, especially with the addition of steel fibers, is less sensitive to vibration (29.8%/6.8% for HSC/HSFRC). The increases in concrete strength grades and additions of fibers significantly enhance the threshold resistance to early vibration, making it a recommended solution for vibration scenarios. Using a consistent expression, the proposed empirical model reasonably reproduces bond stress-slip curves of specimens with/without vibration exposure.

Bond performance of fiber reinforced polymer bars in normal- and high-strength concrete

The structural performance of reinforced concrete (RC) members depends on the bond between the reinforcing bars and surrounding concrete. Unlike steel bars, fiber-reinforced polymer (FRP) bars are manufactured with different fiber types (glass, basalt, carbon, and aramid) and have different mechanical properties and surface configurations (sand-coated, deformed, grooved, sand-coated over helical wrapping). These differences impact the bond performance between the FRP bars and concrete. This paper investigates the bond performance of new generation and different FRP bars embedded in concrete. A total of 87 pullout specimens were constructed and tested until failure. The test results are presented and discussed. The test parameters include (i) fiber type (glass or carbon), (ii) surface configuration (sand-coated, deformed, or grooved), (iii) No. 4 to No. 8 (13 to 25.4 mm diameter) bar diameters), and (iv) concrete strength (normal- or high-strength concrete: NSC or HSC). Specimens with deformed 10 M or 15 M steel bars were also fabricated as controls and tested for comparison. The embedment length and block dimensions were also considered to ensure pullout-failure mode. The test results reveal that increasing the bar diameter generally reduced the bond strength of the FRP bars, while the bar diameter had insignificant impact on the steel bars. There was a positive relationship between the concrete compressive strength and the bond strength gained. The anticipated failure modes with the high concrete strength were at the fiber resin interface due to the high confinement generated by high-strength concrete. With the lower strength, failure usually occurred at the resin–concrete interface. In addition, the test results were used to assess different analytical models for bond stress-slip relationships.

Modeling of bond stress-slip relationships of mono-prestressing strands with H-anchorage dead end

Modeling of bond stress-slip relationships of mono-prestressing strands with H-anchorage dead end

Strain-based analysis of reinforced concrete pull-out tests under monotonic and repeated loading

This paper presents the results of an experimental campaign on standard pull-out specimens instrumented with refined strain measurement techniques. The quasi-continuous strain measurements allowed the derivation of experimentally substantiated local bond stress-slip relationships for static and repeated loading. It was shown that the force transfer between concrete and reinforcement along the bond length is not uniform and that redistribution occurs with increasing as well as repeated load. The variation of bond stresses along the bond length is of great importance for fatigue studies, as the fatigue damage depends on the level of the applied stress. In addition, the refined measurements revealed non-uniformities in the unloading behaviour. Thus, a slip reversal occurs mainly on the loaded side of the pull-out specimen. The experiments confirmed that the residual nominal bond strength after repeated loading does not significantly vary from the initial bond strength, and the redistribution capacity was identified as a possible cause. The comparison with the nominal bond-slip relationship obtained in a conventional test setup agreed well with the mean value of the refined measurements. However, local degradation processes, such as fatigue, cannot be investigated properly with conventional pull-out tests.

Experimental and analytical assessment of fatigue damage in reinforced concrete tension members

In this paper, the effects of bond deterioration due to fatigue loading on the structural response of reinforced concrete tension members are studied. A comprehensive experimental campaign on reinforced concrete ties and pull-out specimens was carried out under monotonic and repeated loading. The use of distributed fibre optical sensing to assess the local strains allowed the bond deterioration mechanism under repeated loading to be observed. Previously applied load cycles caused a reduction in tension stiffening for load levels below the maximum cyclic load but did not significantly influence the deformation behaviour of the member near the ultimate load. The bond stresses at the maximum cyclic load decreased fairly linearly with the logarithm of load cycles, with the most pronounced reduction in bond stresses near the cracks. At the minimum cyclic load, the bond stress distribution along the tie changed and bond stresses reversal occurred close to the cracks. This caused negative tension stiffening in some cases, i.e., the mean strains exceeded those occurring in a bare reinforcing bar at the same load. The mean steel stresses and the steel stress amplitudes between the cracks increased with the number of load cycles due to the gradual reduction of the concrete contribution and bond degradation. Bond degradation occurred linearly on a semi-logarithmic scale, as expected for the generated bond stresses, which were below the fatigue bond strength. The test results highlight that accounting for shrinkage-induced strains is essential for a realistic evaluation of tension stiffening and residual deformations in particular.Based on the experimental observations and previously developed models for monotonic and cyclic loading, two approaches were used to predict the response of reinforced concrete tension members under repeated loading: the Local Fatigue Damage Model (LFDM), based on the local bond stress-slip relationship, and the adapted Tension Chord Model (TCM) which assumes constant bond stresses and mean strains. The results of both approaches agree well with the test results at the maximum load but yielded less satisfactory predictions at the minimum loading level where only the LFDM was able to be applied successfully. Further research is required to validate the negative bond stresses occurring upon slip reversal in the LFDM and the reduction factor proposed in the adapted TCM.

Cyclic bond deterioration of deformed steel bar in well-confined concrete

The cyclic bond deterioration mechanism of deformed steel bars embedded in concrete is critical for establishing the hysteretic bond stress-slip relationship. For a deformed steel bar, well-confined in concrete, the bond failure model under monotonic loading is governed by pullout, the bond deterioration trend under cyclic loading is remains unclear, especially in the case of applied lateral pressure. In this paper, the effect of uniaxial lateral pressure on the bond performance under different loading histories was studied. The test results indicate that the variation in bond parameters under varied lateral pressure is negligible. Therefore, the monotonic and reversed bond stress-slip constitutive relationship of a deformed bar failed by pullout, without lateral pressure confinement, can be used for simulating the bond response under different uniaxial lateral pressures. Cubic concrete specimens with small openings were used to quantitatively analyze the failure process under reversed loading through digital imaging correlation (DIC). The bond stress was observed to be nearly uniformly distributed along the interface at the initial loading stage. After increasing the pull-out or push-in loading to 75 %–85 % of the peak bond strength, we observed minute shear cracks between the interfaces. Subsequently, the length and width of the cracks were then gradually developed with the controlling slip and number of cycles, leading to reduced mechanical interaction and friction between the shear cracks.

Effects of bond-slip on flexural behavior of reinforced nano-material modified UHPC beams: Experimental and numerical investigation

A series of central pull-out tests were carried out to assess the bond performance between steel bar and nano-material modified ultra-high performance concrete (nano-UHPC). The effects of parameters, such as the type of concrete and steel bar, the diameter of steel bar, bond length, the thickness of concrete cover, the length and content of steel fiber, and the nanomaterial type and content, on the failure mode and the bond stress-slip relationship were analyzed in detail. Subsequently, a constitutive model was proposed based on the experimental results for the precise prediction of the bond stress-slip relationship. Furthermore, to illustrate the effectiveness of the constitutive model, a reinforced nano-UHPC beam under the monotonic lateral loading was tested, and numerically simulated by a precise finite element model incorporating the constitutive model. Parametric studies were also conducted to evaluate the effect of critical parameters of the constitutive model on the flexural behavior of the reinforced nano-UHPC beam. The experimental results revealed that there was a superior bond performance between the steel bar and nano-UHPC. The increase in the bond length and the ribbed steel bar diameter were not conducive to the bond performance, and so did the decrease in the concrete cover thickness, steel fiber length and content. Nano-SiO2 and nano-CaCO3 had a positive impact, whereas nano-Al2O3 showed a negative effect. The numerical results indicated that both the bond strength and modulus had a significant effect on the flexural behavior of the reinforced nano-UHPC beam, whereas the slope of descending segment of the bond stress-slip relationship had an insignificant effect.

Local bond stress–slip relationship of ribbed reinforcing bars embedded in UHPC: Experiment, modeling, and verification

This paper provides an experimental and analytical investigation on the bond behavior of reinforcing bars embedded in ultra-high performance concrete (UHPC) via a series of pullout tests. The local bond stress–slip relationship and global bond slip behavior were tested using specimens with embedment lengths of 2d, 5d, and 7d, where d is the diameter of the reinforcing bar. Four curing procedures and two loading ages for the UHPC were adopted to achieve different compressive strengths with the same mixture. During the test, the tension force and slip at the free end of the reinforcing bar were measured. The strain distribution along the anchorage length was measured using a distributed optical fiber sensor glued to an internal groove on the bar. The experimental results show that the ascending part of the local bond stress–slip relationship is nearly linear. A simplified local bond stress–slip model is proposed and verified using the tested strain distribution and the slip of the reinforcing bar in long specimens. This model is used to discuss the distribution form of the slip and bond stress along the anchorage length and the theoretical development length for pullout failure. The concrete surrounded reinforcing bar was subjected to the tension–compression–compression triaxial stress state, determining the higher bond strength of the UHPC compared with conventional concrete. When the anchorage length is no longer than two times the diameter of the bar, the slip and bond stress can be assumed to have a uniform distribution, whereas when the anchorage length is less than 7.5 times the diameter of the bar, the slip and bond stress can be assumed to have a linear distribution.

Bond Degradation Mechanism and Constitutive Relationship of Ribbed Steel Bars Embedded in Engineered Cementitious Composites under Cyclic Loading

In order to investigate the bond degradation mechanism and constitutive relationship of ribbed steel bars in engineered cementitious composites (ECCs) under cyclic loading, 12 groups of specimens were tested in this paper. The design parameters included ECC compressive strength, ECC flexural toughness, cover thickness, and anchorage length. The results indicated that the degradation of the bond behavior of the ribbed steel bars in the ECCs under cyclic loading was mainly caused by the degradation of the properties of the ECC material itself, concentrating on the development of cracks in the ECC, the extrusion and shear failure of the ECC between the steel bar ribs, and the continuous grinding of the ECC particles on the shear failure surface. The degradation of the bond stress-slip curves under cyclic loading was mainly reflected by the degradation in the ultimate bond strength and unloading stiffness. According to the monotonic loading test results, a monotonic bond stress-slip relationship model was proposed. On this basis, through building the hysteretic rules of the bond stress-slip curves under cyclic loading, a calculation model was proposed to predict the bond stress-slip constitutive relationship between the ribbed steel bars embedded in the ECCs under cyclic loading. Finally, the validity of the proposed model was verified by a comparison between the model curves and the tested curves.

Comparative study on the bond performance of near-surface mounted fiber-reinforced polymer bars

This paper presents the experimental and analytical results of a large study on the bond performance of near-surface mounted (NSM) fiber-reinforced polymer (FRP) bars. The investigated parameters included the bar material (basalt-FRP (BFRP), glass-FRP (GFRP), carbon-FRP (CFRP), and stainless steel (SS) bars), the bar surface configuration (deformed and sand-coated bars), the filling adhesive (NSM-Gel, Sikadur-30, and Sika Grout-214), and the bonded length of the bar (6, 12, and 24 times its diameter). Sixty-six C-shape concrete specimens were tested under direct pullout loading configuration. The bond strength, the free-end and loaded-end slip, the strains in the NSM bar, and the modes of failure of the tested specimens are reported and discussed. The NSM-Gel adhesive outperformed other adhesives in developing the bond strength of the tested specimens regardless of their bar material. Both the deformed and sand-coated NSM-BFRP and GFRP bars showed almost similar bond strengths while the NSM-CFRP bars showed the highest strengths. The images obtained from the scanning electron microscope confirmed the obtained results in terms of the modes of failure and the bond failure mechanism. Analytically, the BPE model was calibrated using the experimental results to describe the bond stress-slip relationships of the FRP bars.

UHPC-PINN-parallel micro element system for the local bond stress–slip model subjected to monotonic loading

The bond stress between Ultra-High-Performance Concrete (UHPC) and steel bars is an essential issue in designing reinforced UHPC structures. Although, to date, considerable studies have been directed on the Local Bond Stress-Slip (LBSS) relationship, not enough progress has been achieved in the parametric investigation of the Parallel Micro Element System (PMES) on the LBSS relationship at the steel bar-UHPC interface. Therefore, the presented research applied the two-parameter Weibull Cumulative Distribution Function (WCDF) and conducted a series of monotonic pullout tests on 144 specimens considering the most influential parameters on the LBSS, such as concrete cover (c), UHPC compressive strength (fUHPC′), steel bar diameter (db), and bond length (lb). The Finite Element Method (FEM), ABAQUS, calibrated the experimental results to provide a reliable database. The Genetic Algorithm (GA) and the Physics-Informed Neural Network (PINN) were adopted to solve the inverse problem of this research, predict the results, and check the accuracy of the PMES. It can be concluded that PINN's application, compared to GA, leads to more precise results. Observing the specimens R16C1L2F155 and R18C1L3F155, for instance, PINN provided more consistent values for the friction coefficient (β) [0.2549, 0.8165] and the LBSS relationship stiffness (G) [453.1250, 249.9757], while GA failed to acquire not only β and G but also the hyper-parameters of (a) [0.3016, 0.3088] and (m) [0.3035, 0.3635], respectively. This research developed the application of GA and PINN to solve an inverse problem in structural engineering.

Dynamic beam-end tests: Investigation using split Hopkinson bar

The bond between concrete and steel reinforcement is a highly investigated topic under both quasi-static and dynamic loading conditions. Strain rate sensitivity and dynamic increase factor are the key aspects affecting the resulting response of the structure and represent crucial parameters for modelling the impact-related events. Measuring the bond properties under dynamic conditions, however, is a complex task requiring advanced experimental techniques and methodology. In this paper, the bond stress-slip relationships during dynamic pull-out was measured using an innovative technique based on a tensile split Hopkinson bar and a near-to-full-scale beam-end specimen representing an end-part of a reinforced concrete beam.A tensile split Hopkinson bar was used to generate a tensile stress wave with a long duration that pulls a reinforcement bar with a short bond length out of the concrete block. As a very complex wave propagation behavior was observed during the impact, the forward and backward propagating waves have to be separated and subsequently used to properly calculate the bond stress-slip relationship. Two redundant methods utilizing strain gauge signals and digital image correlation for separation of the longitudinal stress waves were employed to solve the problem. The proof-of-concept and applicability of the method which are the main focus of the contribution were demonstrated with a numerical simulation performed in LS-DYNA. An exemplary evaluation of the results of the real tests performed at medium impact velocity with full pull-out of the rebar was performed. The presented method is considered suitable for the evaluation of the dynamic bond stress-slip relationship and produced high-quality results during the initial impact phase and reasonable quality results up to the full pull-out.

Experimental-Numerical Analysis of the Effect of Bar Diameter on Bond in Pull-Out Test

Bar diameter is one of the basic factors affecting bond behavior, which is still of interest due to opposing opinions regarding its effect on bond behavior in the pull-out test. This paper presents an experimental and numerical bond analysis of ribbed reinforcing bar in concrete. The aim was to experimentally evaluate the effect of bar diameter on the bond behavior in the pull-out test and to perform numerical simulations of the conducted experiments in ABAQUS to verify their convergence to the obtained experimental results. The experiments used concrete of C35/45 grade and B500SP reinforcing steel bars of three diameters: 10, 12, and 16 mm. FEA simulations employed the Concrete Damaged Plasticity (CDP) material model and the Contact Cohesive Behavior (CCB) method to model the concrete–bar interface. The study shows that bar diameter significantly affects the bond, both on the bond stress–slip relationship and the type of bond failure, as well as on the bond strength. FEA simulations correctly reflected the bond behavior observed in the specimens. The analytical models presented estimates that were too conservative regarding the maximum bond stress relative to the experimental results.

An Experimental Investigation and Computer Modeling of Direct Tension Pullout Test of Reinforced Concrete Cylinder

Executing the obligation of strengthened concrete is essential in investigating load exchanges from concrete to the inner reinforcing bar. The bond–displacement conduct and extreme pullout quality for pullout samples are essential information related to the durability of RC structures. The slip in the interface is basically due to a contrast in stresses between concrete and reinforcement. This distinction brings about the start of the split in encompassing concrete. This study examined the simple pullout solid 3D cylinder model strengthened by a reinforced steel bar, considered a line element for bond–slip conduct. The non-linear finite element model utilizing ANSYS software was established to concentrate on the concrete and steel reinforcement bond. Material nonlinearity because of cracking, crushing of concrete, and the steel reinforcing bar’s yielding were investigated. Test results showed that: a prediction model for early-age bond stress–slip relationship between steel bars and concrete was proposed based on modeling, which showed good agreement with test results. The precision of this model is explored by contrasting the finite element numerical analysis and that anticipated from test consequences of pullout examples. Immense homogeneity between the model and test results was found. This study could provide more accurate bond properties for structural analysis and design.

Enhancing bonding behavior between carbon fiber-reinforced polymer plates and concrete using carbon nanotube reinforced epoxy composites

Externally bonded carbon fiber reinforced polymer (CFRP) composites have been widely used as repairing or strengthening materials for deteriorated reinforced concrete (RC) structures. This study discusses whether carbon nanotube (CNT) can be used as an additive to enhance bonding strength between concrete and CFRP. In this study, two CNT types: single walled carbon nanotubes (SWCNTs) and multi-walled carbon nanotubes (MWCNTs), were mixed with two different types of epoxies. CNTs were added at 0.25%, 0.5%, and 1.00% by weight of epoxy to obtain the optimal performance. Single-shear test were conducted to investigate bonding behavior including bond strength, ultimate slip, effective bond length, bond stress-slip relationship, interfacial fracture energy, and crack formation between CFRP plate and concrete. The results showed that epoxy modified by 0.5% SWCNTs and 1.0% MWCNTs by weight of epoxy enhance the ultimate load and slip, bond strength, and interfacial fracture energy when compared to plain epoxy. However, high density epoxy modified with SWCNTs/MWCNTs showed diminished bonding properties.

Pre‐ and post‐yield bond, tension‐stiffening, and cracking in ultra‐high performance fiber reinforced concrete

AbstractIn reinforced concrete elements, tension‐stiffening strongly influences deflections and crack‐widths at the serviceability limit, and plastic hinge rotation at the ultimate limit. Modeling has shown the impact of tension‐stiffening at the ultimate limit to be particularly important when ultimate failure is governed by reinforcement rupture, which has been is common in ultra‐high performance fiber reinforced concrete (UHPFRC) elements. It is further known that the magnitude of tension‐stiffening and crack opening is proportional to both the reinforcement ratio of the tension chord and the diameter of the reinforcing bar. Despite this understanding, very little testing has been undertaken to quantify the bond between larger diameter reinforcement and UHPFRC and also on the resulting impact to tension‐stiffening and concrete cracking. To address these issues, in this paper a series of bond and tension‐stiffening tests are undertaken on ultra‐high performance concretes both with and without steel fibers. The result of the experiments is used to develop a local bond stress slip relationship applicable to both pre‐ and post‐yield. When implemented in a partial‐interaction model, it is shown that the new bond model can be used to predict pre‐ and post‐yield tension‐stiffening and crack‐widths.