Local Bond Stress Slip Research Articles

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.

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.

Three-Dimensional Finite-Element Modeling of the Thermomechanical Response of GFRP-Reinforced Concrete Slab Strips Subjected to Fire

The fire design of reinforced concrete (RC) members with glass fiber–reinforced polymer (GFRP) bars comprising lap splices and cold anchoring zones requires the explicit consideration of the GFRP–concrete bond degradation with temperature; however, few studies in the literature have addressed this issue. This paper presents numerical investigations in which three-dimensional finite-element models were developed to simulate fire resistance tests performed in GFRP-RC slab strips subjected to a fire load and current standards. The slabs comprised continuous and spliced reinforcements, as well as different concrete cover thicknesses and concrete strengths. In the models, the GFRP–concrete interaction was simulated by means of previously calibrated local bond stress–slip laws at different temperatures. This study provided new insights into the fire behavior of GFRP-RC flexural members, confirming that their fire resistance can be drastically reduced when lap splices, designed for ambient temperature conditions, are directly exposed to heat and that even adopting relatively low concrete cover, fire endurances above 120 min can be attained provided that the bars’ anchors remain sufficiently cold.

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.

Local Bond Stress-Slip Model of High-Strength Stainless Steel Wire Ropes in ECC

Engineered cementitious composites (ECC) reinforced with high-strength stainless steel wire ropes (HSSSWR) is a new composite that has attracted much attention. Comprehensive understanding of the local bond stress-slip relationship of HSSSWR in ECC is a significant aspect to popularize the application of this new composite. In this research, the local bond stress-slip relationship between HSSSWR and ECC was investigated experimentally and theoretically, considering the influences of bond lengths, nominal diameters of HSSSWR and compressive strength of ECC. In order to accurately predict the bond stress and slip at different positions along the embedded length, a local bond stress-slip model was proposed for HSSSWR-ECC interface, and the model parameters were determined based on the pull-out test results and microsegment analysis of HSSSWR in ECC by using a nested iteration procedure. Furthermore, the three-dimension (3D) nonlinear finite element (FE) modeling method by using the proposed model was used to predict the bond-slip performance of HSSSWR in ECC. Finally, the global load-slip relationships calculated by using the iterative procedure and the 3D FE modeling method were compared with test results, which validated the acceptability of the developed local bond stress-slip model and the FE modeling method.

Bond position function between deformed steel bars and early-age cementitious grout

Pullout tests for the deformed steel bars in early-aged cementitious grout by considering the variables of testing ages, cover thicknesses, and diameters of steel bar were conducted, and the local bond stress–slip relationship at different positions of the anchorage length of steel bar was studied. Results indicated that, with the increase of testing age, the load-slip curves exhibited a higher ultimate load and had steeper ascending and descending branches; however, the slippage at the ultimate load exhibited a decreasing trend. Moreover, with the increase of cover thickness and diameter of steel bar, the ultimate load of pullout specimens increased obviously, while the corresponding slips generally had no obvious correlations. According to an analysis of the measured rebar strain, the distributions of the steel stress and bond stress as well as the relative slip along the embedded length were obtained under different external loads. Steel stress transfer and bond stress distribution parameters were introduced to characterize the nonuniformity of the distributions of the steel stress and bond stress along the steel bar, and the effects of the testing age, cover thickness, and diameter of steel bar on these two parameters were analyzed. Results showed that the both of these two parameters increased with the increasing tensile load, testing age, and cover thickness and with the decreasing diameter of steel bar. Thereafter, the local bond stress-slip relationships along the anchorage length and position functions reflecting the variations of these relationships were proposed.

Modeling Local Bond Stress-Slip Relationships of Reinforcing Bars Embedded in Concrete with Different Strengths.

Although many different analytical models of local bond stress–slip have been proposed, considering the possible differences between materials in different countries, their applicability needs to be further explored. In this paper, the local bond stress–slip characteristics of reinforcing bars embedded in concrete with different strengths were experimentally studied. The experimental variables included the concrete strength (20, 40, and 60 MPa) and deformed rebar size (#4, #6, and #8). The experimental results of the bond stress–slip relationship were compared with the Euro-International Concrete Committee (CEB-Comité Euro-International du Béton)-International Federation for Prestressing (FIP-Fédération Internationale de la Précontrainte) Model Code and prediction models found in the literature. In addition, based on the test results, an empirical model of the bond stress–slip relationship was proposed. The evaluation and comparison results show that, regardless of the concrete strength grades, the predicted value calculated using the CEB-FIP Model Code will underestimate the bond strength of the specimens with different steel bar diameters. From this perspective, it is more conservative. In contrast, the proposed model can predict the test results with a reasonable accuracy.

Evaluation of bond–slip behaviour of embedded rebars through control field equations

Anchorage of steel bars is critical in reinforced concrete design to ensure composite action between concrete and rebars. The force–slip relationship of rebars is another factor governing the failure mode and rotation of structural joints. These two aspects can be predicted by bond–slip models and depend on bond behaviour at the reinforcement–concrete interface. However, current bond–slip models show either low accuracy and limited applications due to simplified assumptions or complicated computation due to nested iteration loops. This paper proposes analytical models derived from control field equations, which are characterised by low computational cost, high accuracy and broad application. The control field equations describe force equilibrium of embedded rebars by second-order differential equations and give accurate predictions by incorporating the local bond stress–slip relationship. The proposed models are appropriate for adequate and inadequate embedment lengths of rebars and can describe the evolution of the bond–slip distribution along embedment length. Closed-form solutions can be obtained for perfect anchorage and obviate iterative nested computations. If closed-form solutions are unavailable, owing to free-end slip for short embedment length, a mathematical function in Matlab is used to solve non-linear control field equations and complicated nested iteration is avoided. Finally, the proposed models show good agreement with published experimental results.

Local bond slip behavior of steel reinforcing bars embedded in ultra high performance fibre reinforced concrete

The local bond stress slip behavior is a fundamental property required for the analysis and design of concrete structures at both serviceability and at ultimate limit. The addition of fibers has been shown to significantly improve the bond between normal strength concrete and steel reinforcement but little work has investigated the bond between reinforcing and ultra high performance fibre reinforced concrete (UHPFRC). In this paper a series of 69 pull tests are carried out on UHPFRC with either short straight or long hooked steel fibers including mixes where the two fiber types have been blended. The results of this study were combined with the results from existing literature to regress a material model for the bond slip behavior between UHPFRC and ribbed steel reinforcing bars. Importantly, it is shown that models for normal strength fiber reinforced concrete cannot be extrapolated to UHPFRC.

Flexural strengthening of RC beams with externally bonded CFRP plate: experimental study on shear-peeling debonding

This paper reports on experimental investigations of externally bonded carbon fibre reinforced polymer (CFRP) plate debonding behaviour using beam specimens flexurally strengthened by CFRP plates induced by a relative vertical displacement imposed on the two sides of a shear crack in each beam under a four-point bending load. Three specimens, two of which were control specimens to confirm shear failure and one of which was a shear-deficient test specimen under high-level fatigue load, were strengthened with bottom-bonded CFRP plate and side-wrapped CFRP sheet. The variables investigated during the experimental programme were the displacement between the concrete substrate and the CFRP plate, the strain distribution of the CFRP plate and the local bond stress–slip relation. The test results showed the effectiveness of shear–peeling debonding in CFRP plates along with shear crack width opening based on fatigue load and local debonding with peeling off as an important factor affecting the fatigue life by shear failure in this case.

Effects of spiral confinement to the bond behavior of deformed reinforcement bars subjected to axial tension

When grouted pipe splice connections are employed, the structural performance of the connected precast concrete components is significantly influenced by the bond behavior between the main reinforcing bars and the surrounding grout. Owing to this important characteristic, the bond behavior, in particular the local bond-stress slip relationship and the bond strength, of the main steel reinforcement bars embedded in grout needs to be investigated accordingly. This paper presents the bond behavior of main deformed steel reinforcement bars confined by a grouted spiral connection. A total of 36 pullout specimens were tested under increasing axial tensile load to investigate the effects of spiral confinement to the connected main steel bars. Parameters covered in this study were spiral diameter and spiral pitch distance. The experimental results showed that the spiral configurations influence the bond performance due to the effect of confinement generated by the spiral diameter and pitch distance. As compared to spiral pitch distance, the spiral diameter provides more dominant confinement effect which subsequently increases the bond strength significantly.

Effect of wet-dry cycles on the bond behaviour of concrete elements strengthened with NSM CFRP laminate strips

The near-surface mounted (NSM) strengthening technique is capable of effectively increase the bearing capacity of structural concrete elements. This technique which basically consists of placing FRP reinforcements inside small grooves cut in the concrete cover, has been widely investigated in terms of structural performance and ability to improve the flexural and shear behaviour of reinforced concrete beams and columns. However, little research has been carried out concerning to the NSM long-term performance and durability. Motivated by the need of increasing the knowledge on the expected durability of the NSM technique using CFRP laminates, this paper presents an experimental program in which direct pull-out tests are carried out for evaluating the bond behaviour of specimens aged through wet-dry cycles. A total of 30 specimens are tested, analysing the effect of the bond length, the groove width, the groove depth and the aging effect on the bond behaviour. Digital image correlation method is also used to identify the bond resistant mechanism developed in an element strengthened using NSM technique. Finally, using the experimental results, an analytical–numerical strategy is applied to establish the local bond stress–slip relationship.

Measurement of bar strain during pull-out tests: use of electrical resistance gauge methods under large displacement

Two issues make measuring the local bond stress–slip relationship difficult: first, the bar is embedded within the concrete, making measurement of local strain values complex; second, yielding of the steel bar influences the bond stress–slip behaviour. Global bond stress–slip behaviour is generally obtained by measuring the slip that occurs at the free end of the bar during pull-out testing. Several distinct mounting methods have been used to install electrical resistance strain gauges on embedded bars in concrete to obtain local strain data. These methods are reviewed in relation to their ability to deliver reliable and robust data at both small and large values of slip. The use of such strain gauges is found to provide a viable method for assessing the variation in strain along embedded steel bars during pull-out testing. The use of surface-mounted strain gauges leads to high failure rates due to physical damage to either the gauges or their connections. The use of near-surface- and central-groove-mounted strain gauges enables measurement of local bar strain over large displacements. Using data from near-surface-mounted strain gauges, it is shown that in the post-peak region the bond stress increases linearly from the free end of the bar.

Local Bond Stress-Slip Relationship between Carbon Fiber-Reinforced Polymer Plates and Concrete under Fatigue Loading

Local Bond Stress-Slip Relationship between Carbon Fiber-Reinforced Polymer Plates and Concrete under Fatigue Loading

Bond between glulam and NSM CFRP laminates

With the aim of evaluating the bond behaviour between glulam and carbon fibre reinforced polymer laminates strips, an experimental program using pull-out tests was carried, when the near-surface strengthening technique is applied. Two main variables were studied: the bond length and the type of pull-out test configuration. The instrumentation included the loaded and free-end slips, as well as the pull-out force. Based on the obtained experimental results, and applying an analytical–numerical strategy, the local bond stress–slip relationship was determined. In this work the tests are described, the obtained results are presented and analysed, and the applicability of an inverse analysis to obtain the local bond law is demonstrated.

Bond behavior between glulam and GFRP’s by pullout tests

To evaluate the bond behavior between glulam and GFRP rods, applied according to the near-surface mounted strengthening technique, an experimental program composed of beam and direct pullout tests was carried. In this experimental program three main variables were analyzed: the GFRP type, the GFRP location into the groove, and the bond length. From the monitoring system it was registered the loaded and free end slips, and the pullout force. Based on these experimental results, and applying an analytical–numerical strategy, the local bond stress–slip relationship was calculated. In this work the tests are described, the obtained results are presented and discussed, and the applicability of the inverse analysis to obtain the local bond law is demonstrated.

Embedding NSM FRP Plates for Improved IC Debonding Resistance

The use of near surface mounted (NSM) fiber reinforced polymer (FRP) strips for strengthening reinforced concrete structures shows great promise as the strains at intermediate crack (IC) debonding are generally much greater than for externally bonded FRP strips. In this research, the NSM technique is taken a step further by embedding the NSM strip, i.e., is by providing cover to the strip. It is shown that embedment can increase the IC debonding resistance by up to three times. Importantly, embedment allows: substantially larger strains at IC debonding and, hence, greater ductility; the use of larger cross sections of FRP plate; and, through providing cover to the NSM, may be the first step in enhancing the fire resistance. In this paper, 20 new pull tests are described from which mathematical expressions are developed for the effect of embedment on both the IC debonding resistance and its associated local bond stress–slip (τ–δ) relationship.

Bond strengthening of lap spliced reinforcement using external FRP jackets: An effective technique for seismic retrofit of rectangular or circular RC columns

Confinement of concrete at the critical hinging regions is one of the most effective means for improving the seismic behavior of reinforced concrete (RC) members. Confinement reduces the bond degradation of the steel reinforcement and limits the concrete damage within the confined zone leading to increased energy absorption and dissipation capacity of the structure. Recent experimental and analytical studies undertaken on the use of FRP composites for seismic upgrading of RC members [FRPRCS08 Proceedings. In: Triantafillou TC, editor. 8th International symposium on fiber-reinforced polymer reinforcement for concrete structures. Patrace, Greece: University of Patrace; July 2007. p. 16–8] have clearly identified the advantages of this technology when compared to conventional strengthening methods. However, unfortunately, no guidelines have yet been developed by code committees for seismic retrofit using FRP composites. This paper provides a synthesis of the results of a series of experimental and analytical studies undertaken in the main part by the writer and in part by other investigators on the use of external FRP jackets for bond strengthening of developed/spliced steel bars in tension and its implications on the static and seismic response of concrete members. Results of the experimental programs are briefly discussed, and design expressions for evaluating the minimum thickness of FRP jacket required for seismic bond strengthening are presented and compared. The accuracy of the design expressions was verified against experimental data. In addition, a generalized model of the local bond stress–slip response of steel bars embedded in FRP confined concrete corresponding to splitting mode of bond failure is described. The model is composed of a monotonic envelope curve and bond degrading curve and can be used for evaluating the seismic behavior of the hinging zones in reinforced concrete members when confined externally with FRP jackets in comparison with the response of unconfined concrete.

Finite element analysis of thin GFRC panels reinforced with FRP

Glass fibre reinforced concrete (GFRC) incorporating fibre reinforced polymer (FRP) bar reinforcement is potentially an ideal composite material for the manufacture of thin structural elements due to its superior durability over GFRC containing conventional steel reinforcement. GFRC without any bar reinforcement has only been used for small units and short spans due to its relatively low flexural strength. Until now, no work has been reported on the use of FRP bars in GFRC. The first part of the paper deals with the stress–strain characteristics of GFRC. In the second part the bond strength of GFRC with both steel and FRP reinforcing bars is determined from a series of 24 pullout tests from which the characteristics of the local bond stress–slip response was established. The results show that the bond of FRP bars in GFRC is, in general, better than the bond in normal concrete, and that conventional numerical models can be used to model the behaviour. The last part of the paper investigates the performance of a 3 m span thin GFRC permanent formwork panel, reinforced with FRP, both experimentally and analytically with finite element (FE) analysis. It is concluded that the behaviour of thin GFRC elements incorporating FRP reinforcement can be predicted by FE analysis in which the GFRC stress–strain characteristics and bond characteristics are modelled with robust spring elements.

Local and global bond characteristics of steel reinforcing bars

The development of an accurate local bond stress–slip ( τ – δ ) relationship for steel reinforcing bars has been hindered for two reasons: because steel reinforcing bars are internally encased, it is difficult to strain gauge the reinforcing bar to quantify the change in strain over a given length and, therefore, to directly measure the local τ – δ relationship; and because of the presence of yielding, which generally occurs before debonding and significantly influences the behaviour of the reinforcing bar. To overcome these difficulties associated with quantifying the local τ – δ relationship of steel reinforcing bars, theory developed to obtain the local τ – δ relationship from the global load slip ( P – Δ ) response of FRP plated reinforced concrete structures has been modified in this research to account for short embedment lengths. Furthermore, in this paper, a link between the global load–slip ( P – Δ ) response and the local τ – δ relationship of steel reinforcing bars, that is independent of embedment length, is presented for the first time. Additionally, the new theory is used to derive the local τ – δ relationship from test results and to illustrate the effect of corrosion on the local τ – δ relationship which quantifies the amount of corrosion to cause bar debonding in RC beams.