Bond-slip Curves Research Articles

Bond-slip model for horizontal reinforcing bars in reinforced brick masonry

The bond-slip performance and constitutive model of the interface between reinforcing steel bars and brick masonry are the basis for seismic performance analysis of reinforced masonry through finite element simulation. In this study, the gradual increase of the vertical load in masonry during construction is simulated using a convenient self-developed test setup that ensures that the specimen is under constant pressure based on the actual stress state of horizontal reinforcing steel bars in reinforced brick masonry. Both the vertical compressive stress and the bond length were considered as variables and were used to form 19 groups of specimens. The bond characteristics of reinforcing steel bars were investigated using pull-out tests. The bond-slip curves of the interface between the horizontal reinforcing steel bar and brick masonry were obtained from test data; then, a five-segment bond-slip model that considered the effect of the vertical compressive stress and the bond length on the bond performance is proposed. This study analysed the variation trend of the bond parameters of each feature point influencing the bond performance in the model. The initial bond stiffness of the micro-slip segment increased with the increase in the vertical compressive stress, and the ultimate bond stress increased significantly. The calculation formulas of the bonding parameters are obtained by regression statistics taking these factors into consideration. The predictions obtained from the model show good agreement with test results.

Bond Slip Law and Micropore Structure of Freeze-thaw Concrete

In order to study the influence of freeze-thaw cycles on the bond-slip relationship between concrete and steel bars, uniaxial compression and pull-out tests were carried out on concrete cubes experiencing 0, 25, 50 and 75 freeze-thaw cycles according to the existing stress models and the related concepts of ultimate bond force after freeze-thaw cycles. The bond stress and slip law between freeze-thaw concrete and steel bar are obtained. At the same time, the bond slip curve can be fitted by Bigaussian function. The results showed that with the increase of freeze-thaw times, the micro-pore structure on the surface and inside of concrete expands, the uniaxial compressive strength decreases, and the bond stress and slip between steel bar and concrete decrease.

Bond characteristics of deformed steel bar in early-age frozen concrete during service period

Concentric pull-out tests were conducted to investigate the bond characteristics of deformed steel bars in early-age frozen concrete during service time; different concrete strength grades and frost environments were considered in the investigation. The bond strength and bond stress-slip curves between the early-age frozen concrete and steel bar were presented. The effects of frost environment and the concrete strength grade on the bond properties of the steel bar during service period in early-age frozen concrete were analyzed. The deterioration of the bond properties of the deformed steel bars in early-age frozen concrete during the service period decreased with an increase in the concrete strength grade, which is observed as steeper ascending branches, greater bond strengths in the bond-slip curves, and greater bond stresses at the occurrence of free-end slippage. With the decreasing frost temperature or increasing frost duration, the bond properties of deformed steel bars in early frozen concrete during the service period were clearly negatively affected. In addition, for early-age frozen specimens with different frost onset times, the degradation of bond properties during the service period listed in descending order was 2, 0.5, 8, 24, and 72 h. Thereafter, based on the test results, a bond strength model for deformed steel bars in early-age frozen concrete was proposed. A good agreement was observed on comparing the calculated results of the model with experimental results obtained from different sources.

Analysis of the interface bond property between the concrete and steel bar under textile reinforced concrete confinement

Determining the interface bond property between the concrete and steel bar under textile-reinforced concrete (TRC) confinement has been one of the important problems to solve for evaluating the TRC-strengthened effectiveness. This study adopted a research method combining experimental studies and theoretical analysis to discuss the mechanisms by which the steel bar shape, concrete strength and TRC confinement affect the interface bond property based on center pull-out tests. Furthermore, the interface damage variable and the damage-slip equation were presented to analyze the interface damage process. The research results showed that TRC confinement can improve the interface bond property of the specimens using a deformed bar and change its failure mode from splitting to splitting-pull out. The concrete strength had a significant impact on the interface bond property and the ultimate interface bond strength, but the influence degree is greatly reduced after TRC confinement. Moreover, the secant slope between the points on the bond-slip curve and the origin of the coordinate as the damage variable can be used to describe the degree of interface damage. The damage analysis results show a good consistency with the test results and bond mechanism analysis.

State-of-the-art review on the bond properties of corroded reinforcing steel bar

Abstract This review summarizes the latest research progress in regard to the bond behavior of corroded reinforcing steel bar. The literature survey indicates that the previous studies mainly focus on the deterioration of bond strength. The primary factors influencing bond strength, including main bar corrosion, concrete cover, concrete type, stirrups, stirrup corrosion, and corrosion rate, have been well recognized. However, the existing models for bond strength are mostly developed based on specific test results and are not generalized. The corrosion-induced surface crack width is an ideal parameter for the evaluation of bond strength, but models in this respect are still scarce. As for the bond-slip behavior, literature studies reveal that the bond-slip mechanism is hardly affected by corrosion. Currently, discrepancies exist regarding the corrosion influence on the key parameters that shape the bond-slip curves. Preliminary comparisons of the existing bond-slip models show that reliable and generalized models remain to be developed based on larger database. Studies on the bond behavior of corroded steel bar under cyclic loading are quite few. The limited test results indicate that the repeated loading without changing sign shows no apparent influence on bond strength, whereas the cyclic reversed loading exacerbates the corrosion-induced bond deterioration. Knowledge gaps and research areas where further investigations are required are outlined.

Estimations of the debonding process of aged joints through a new analytical method

The estimation of the long-term durability of adhesively bonded interfaces between Fiber Reinforced Polymers (FRP) and concrete substrates is crucial because degradation potentiates FRP premature debonding. One of the main reasons for mistrusting the use of FRP composites is the premature debonding phenomenon, which, associated to degradation, has been preventing their widespread use. In this research work, an analytical model is proposed that introduces ageing to estimate the effects of degradation of Glass (G) FRP externally bonded to concrete. Cycles were used to experimentally accelerate ageing of beam specimens, namely, (i) salt fog cycles; (ii) wet-dry cycles with salted water; (iii) temperature cycles between −10 °C and +30 °C; and (iv) temperature cycles between +7.5 °C and +47.5 °C. Based on the experimental results obtained and a corresponding bond-slip curve, the analytical model predicts the complete debonding process between FRP composites and a substrate. Consequently, the temporal evolution of the degradation of the bonded interfaces can be calculated and compared with the initial situation prior to exposure. The effects of the environmental conditions are reported and compared.

Analytical model for the bond stress-slip relationship of deformed bars in normal strength concrete

The lateral confinement provided by concrete cover and stirrups has a significant influence on the bond behavior of reinforcing bars. This paper presents a systematical study on the local bond stress-slip behavior of deformed steel bars based on test results of beam-end specimens and test data reported in the literature. The bond-slip mechanism is discussed in detail and a parameter K representing the confining ability of concrete cover and stirrups is proposed. Through parameter K the bond failure mode can be predicted. With K as the governing parameter, a mathematical model for peak slip is proposed. The proposed model enables a smooth transition from splitting failure to pull-out failure. Furthermore, the nonlinear characteristics of bond stress-slip curves under splitting failure mode, which has not been well recognized in previous studies, are thoroughly discussed. By dividing the post-peak bond stress into two components, a mechanism-based mathematical model for the nonlinear descending branch of bond-slip curves is formulated. Finally, a bond-slip model for deformed steel bars is proposed, which is applicable for both splitting failure and pull-out failure modes. Comparisons with experimental results available in the literature show that the proposed model can provide good predictions for the bond-slip behavior of deformed steel bars with varying confinement levels.

Performance of CFRP-steel joints enhanced with bi-directional CFRP fabric

Carbon fiber reinforced polymers (CFRPs) have gained popularity and become recognized as a prominent material for engineering applications in the last few decades owing to their high strength-to- weight ratio, resilience and corrosion resistance. The key concern is the bond between the CFRP and steel elements. The type of CFRP utilized in strengthening systems is dependent on the amount of deterioration to which a structure is subjected. This paper reports the results of an experimental analysis of five sets of specimens, consisting of 75 samples in total, to investigate the bond properties between steel and CFRP using a new anchorage configuration. The anchorage system is utilized to delay or counter the effect of debonding of the joints. The results show that the utilization of bidirectional normal modulus CFRP sheets in conjunction with laminates increases the ultimate strength of the joint by approximately 89%. The bond-slip curve is also derived in this study.

Bond-slip behaviour of corroded reinforcement and ultra-high toughness cementitious composite in flexural members

Bonding between ultra-high toughness cementitious composite (UHTCC) and corroded reinforcement has a great effect on the mechanical behaviour of reinforced UHTCC members subjected to chloride attack and of corroded structural members repaired with UHTCC. This paper presents an experimental study on bonding between UHTCC and post- and pre-corroded rebar using bending tests. The test parameters included corrosion ratio and bond length. Failure and cracking pattern, bond-slip curves, bond strength, and bond toughness were compared. The bond strength decreased with increasing bond length, and increased initially and then decreased with increasing corrosion ratio. The critical corrosion ratio, at which the bond strength became lower than the initial strength, was approximately 12–13.5% for post-corrosion specimens and 13.5–18% for pre-corrosion specimens. The residual bond strength of pre-corrosion specimens was superior to that of post-corrosion specimens. Rebar corrosion obtained at these corrosion levels had hardly any negative effect on the bond toughness index.

Influence of Roughness on Shear Bonding Performance of CFRP-Concrete Interface.

The potential of Fiber Reinforced Polymer (FRP) in the reinforcement of concrete structures has been shown in many studies and practical applications. However, few works have focused systematically on the development of quantitative criteria to measure surface roughness and relate this parameter to the bonding mechanical property. Moreover, some researchers have declared that, if the concrete interface is rougher, the bond performance between FRP and concrete will be increase, However, there is no answer to how rough the surface is. There are limited application standards for engineers to conduct in FRP reinforcement projects. This work evaluated several concrete specimens with three different strengths and six types of interface roughness. A single shear test was conducted to study the influence of surface roughness on the interfacial bonding performance of a carbon fiber-reinforced composite (CFRP)-concrete beam. The results show that, among the six interfaces, a concrete interface with the roughness of 0.44 has the best interfacial bonding performance. An interfacial appearance with the cement mortar almost cleaned away, and almost one fifth of the single coarse aggregate bared will get the best bond performance. Roughness parameters significantly influenced the effective bond length. The effective bond length of the six interfaces experienced an overall decreasing trend as the roughness increased. The bond–slip curves of concrete interfaces with roughness of 0.25–0.44 did not significantly change the rigidity within the brittle region. The rougher the interface was, the shorter the brittle region was. After entering a plasticity stage, the bond–slip curves for the six types of interfaces all declined with different slopes, and the max slip values were 0.04–0.35 mm when debonding failure occurred.

Development of a simple bond-slip model for joints monitored with the DIC technique

The monitoring of structures has undergone important advances with the improvements of digital cameras available on the market. Thus, the Digital Image Correlation (DIC) technique has become a viable way of studying engineering problems. Recently it has been used in the debonding failure process between the reinforcement and the substrate. The methods or methodologies that should be followed to obtain the results associated to the debonding phenomenon using the DIC technique need to be better understood and studies on this topic are scarce. The present work therefore proposes a new and inexpensive method to monitor the interfacial behaviour between a reinforcement material and a substrate by combining the use of the DIC technique and a simplified nonlinear bond-slip model. For the validation of the proposed method, a series of single-lap shear tests with a sufficient long bond length carried out by the authors are used. Based on the slip distribution obtained from the DIC technique, it was found that a third-degree polynomial function can be used to approximate the interfacial bond-slip curve of the joint. The validation of the model is made with several analytical solutions using the proposed bond-slip model.

Bond behavior between recycled aggregate concrete and deformed bars under uniaxial lateral pressure

This paper investigates the bond behavior of deformed bars embedded in recycled aggregate concrete (RAC) subjected to uniaxial lateral pressure. Sixty pull-out specimens with different levels of recycled coarse aggregate (RA; i.e. 0%, 30%, 50%, 70% and 100%) were manufactured, and then these specimens were submitted to test the bond-slip curves under different levels of uniaxial lateral pressure (i. e. 0, 0.1fcu, 0.2fcu, and 0.3fcu). The effects of lateral pressure and RA content on bond strength, peak slip, residual bond strength and bond energy density were analyzed; the bond-slip relationship between RAC and deformed bars was also established. The results demonstrated that the bond strength of reinforced RAC is comparable with that of reinforced natural aggregate concrete (NAC). It is feasible to adopt RA in structural application but the bond ductility should be improved before it can be reliable reused. With the incremental of lateral pressure, the peak slip, relative bond strength and the bond energy density gradually increased, while the relationship between these parameters and RA content could not be clearly observed. In addition, the parameter value for descending bond-slip curve was abruptly decreased after lateral pressure was applied, demonstrating that the lateral pressure increased the bond ductility of RAC specimens.

Modeling of the interface between the concrete and the fibers grid in concrete slab

Abstract. Fiber grid reinforcements are widely used in construction, especially in the rehabilitation of structures; short fibers are also used in concrete. However, reinforcing mortars with textiles (fiber gratings) which offers an improved concrete, and a wide variety of new and exciting architectural forms, is still a relatively new application that shows a need for research. Tests on the use of Glass Fiber Grids (GFG) and Polypropylene Grids (PPG) in the concrete matrix which can be an interesting solution to improve the bearing capacity and change the failure mode of slabs subjected to a load of punching have been realized. The results showed a significant increase in the bearing capacity, a better distribution of the cracks, as well as an improvement of the deflexion before failure; compared to unreinforced reference slabs. In order to understand the behaviour of the concrete-GFG interface of the tested slabs, a modeling of this latter was carried out using a constitutive law expressing the shear stress between the two materials as a function of the relative sliding between the surfaces in contact, taking into account the mechanical characteristics of both materials. The analysis of the bond-slip curve obtained has shown that the use of the glass fiber grid inside the concrete matrix allows a better behaviour of the interface between constituent materials with respect to the glued fiber-reinforced polymer (FRP) plates on the surface of the slab.

Double lap shear test on steel fabric reinforced cementitious matrix (SFRCM)

The present work deals with the experimental characterization of the mechanical bond behaviour of a galvanized steel fabric reinforced cementitious matrix (SFRCM) laminated on concrete support. The specimens, made of two low strength concrete blocks connected with a galvanized steel fabric embedded in a geo-polymeric mortar layer, have been tested according to double lap test (DLT) set-up. Six different groups of specimens have been tested varying both the lamination length and the steel fabric density. In order to reproduce the load-slip or bond-slip curves, a tri-linear bond slip model together with its parameters identification has been proposed.For some specimens, the slip profile and the slip distribution have been analysed and split into the substrate laminate slip and inner laminate layers slip. This distinction has been used as a measure of the fabric-matrix compatibility. In addition, the effect induced by the rigid blocks rotation occurred during the DLT has been argued. The DIC optical system monitoring has been used to asses both the force-slip distribution and the crack opening displacement (COD).

Modeling of the interface between the concrete and the fibers grid in concrete slab

Fiber grid reinforcements are widely used in construction, especially in the rehabilitation of structures; short fibers are also used in concrete. However, reinforcing mortars with textiles (fiber gratings) which offers an improved concrete, and a wide variety of new and exciting architectural forms, is still a relatively new application that shows a need for research. Tests on the use of Glass Fiber Grids (GFG) and Polypropylene Grids (PPG) in the concrete matrix which can be an interesting solution to improve the bearing capacity and change the failure mode of slabs subjected to a load of punching have been realized. The results showed a significant increase in the bearing capacity, a better distribution of the cracks, as well as an improvement of the deflexion before failure; compared to unreinforced reference slabs.
 In order to understand the behaviour of the concrete-GFG interface of the tested slabs, a modeling of this latter was carried out using a constitutive law expressing the shear stress between the two materials as a function of the relative sliding between the surfaces in contact, taking into account the mechanical characteristics of both materials. The analysis of the bond-slip curve obtained has shown that the use of the glass fiber grid inside the concrete matrix allows a better behaviour of the interface between constituent materials with respect to the glued fiber-reinforced polymer (FRP) plates on the surface of the slab.

Reliability Analysis of Bond Behaviour of CFRP–Concrete Interface under Wet–Dry Cycles

Effective bonding between adherents plays a key role in retrofitting concrete structures in civil engineering using fibre-reinforced polymers (FRPs). To ensure structural safety, it is critical to develop design codes, which account for uncertainties of materials, the environment, and load, to estimate bond behaviour under long-term exposure to harsh environments. Therefore, a reliability analysis was performed to study the bond behaviour of FRP–concrete interface under wet–dry cycles and sustained loading. Thirty double-lap, shear-bonded carbon FRP (CFRP)–concrete composite specimens were tested after wet–dry cycles and sustained loading exposure. The fracture energy Gf of the bond behavior between CFRP and concrete was directly obtained from the measured local bond-slip curves. Five widely used test methods were adopted to verify the possible distribution types of Gf. Based on the best fit distribution of Gf, a reliability index β was then calculated for the specimens. The effects of wet–dry exposure and sustained loading on β were analysed separately. The effects of the mean and standard deviation of the load on β were compared. It was found that the mean had a greater impact on reliability than the standard deviation, but neither changed the regulation of the exponential reduction of β with increasing wet–dry cycle time. Their impact was significant for a small number of wet–dry cycles but insignificant for more than 4000 wet–dry cycles.

CFRP-to-steel bonded joints subjected to cyclic loading: An experimental study

Pseudo-cyclic and cyclic loading were applied to CFRP-to-steel bonded joints built with two different CFRP laminates. In this paper, the strength capacity and bond-slip curves are presented and compared. The modes of failure are also described and associated with the types of material used, and the observed performances are correlated. The analysis of the results showed a threshold value for loading and amplitude level, below which the cyclic loading caused no detectable damage. For cycles above that limit, the region of the joints around the loaded end presented degradation reflected on the bond-slip stiffness and on the increase of residual deformation. It was found that the normalized dissipated energies either obtained from the bond-slip relationship or from the load-slip response had the same trend. The experimental data allowed also to establish a relationship between the damage developed within the interface and the normalized slip. A preliminary estimate of fatigue limit based on those data is suggested.

Proposal and assessment of an efficient test configuration for studying lap splices in reinforced concrete

Connecting reinforcement bars in their longitudinal direction is often needed in reinforced concrete structures. These connections must guarantee a correct stress transmission between bars without excessive cracking or inadmissible slips, and they are commonly executed by overlapping the reinforcement bars in a procedure known as a lap splice. In the absence of a standard or widely recognized test, three or four point bending tests are often used for the experimental study of lap-splices in tension. This test configuration shows inherent limitations, large costs and, usually, significant scatter.In this paper the authors have selected, fine-tuned and evaluated a specific test configuration for studying lap splices in reinforcement bars submitted to tensile forces. The proposed experiment was designed to be flexible and affordable in terms of cost, time consumption and required testing equipment. Consequently, it is possible to carry out multiple reruns of the experiments, minimizing the uncertainty and providing a good starting point for applying statistic methodologies. In contrast to the regular test configurations, the selected setup makes possible to directly obtain the bond-slip curves of the spliced bars. The experimental work carried out to assess the reliability, sensitivity and scatter of the proposed configuration shows promising results.

Pull-out behaviour of recycled aggregate based self compacting concrete

The use of recycled aggregate in concrete is gaining much attention due to the growing need for sustainability in construction. In the present study, Self Compacting Concrete (SCC) is made using both natural and recycled aggregate (crushed recycled concrete aggregate from building demolished waste) and performance of recycled aggregate based SCC for the bond behaviour of reinforcement is evaluated. The major factors that influence the bond like concrete compressive strength (Mix-A, B and C), diameter of bar (Db=10, 12 and 16 mm) and embedment length of bar (Ld=2.5Db, 5Db and full depth of specimen) are the parameters considered in the present study in addition to type of aggregates (natural and recycled aggregates). The mix proportions of Natural Aggregate SCC (NASCC) are arrived based on the specifications of IS 10262. The mix proportions also satisfy the guidelines of EFNARC. In case of Recycled Aggregate SCC (RASCC), both the natural coarse and fine aggregates are replaced 100% by volume with that of recycled aggregates. These mixes are also evaluated for fresh properties as per EFNARC. The hardened properties like compressive strength, split tensile strength and flexural strength are also determined. The pull-out test is conducted as per the specifications of IS 2770 (Part-1) for determining the bond strength of reinforcement. Bond stress versus slip curves were plotted and a typical comparison of RASCC is made with NASCC. The fracture energy i.e., area under the bond stress slip curve is determined. With the use of recycled aggregates, reduction in maximum bond stress is noticed whereas, the normalised maximum bond stress is higher in case of recycled aggregates. Based on the experimental results, regression analysis is conducted and an equation is proposed to predict the maximum bond stress of RASCC. The equation is in good agreement with the experimental results. The available models in the literature are made use to predict the maximum bond stress and compare the present results.

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.