Bond-slip Curves Research Articles

Experimental Evaluation of Bonding between CFRP Laminates and Different Structural Materials

AbstractThis study presents an analysis of carbon fiber reinforced polymers (CFRP)-to-parent material interfaces based on 40 single-lap shear tests intended to highlight the strength of the interfaces under fracture mode II. Three different substrates are analyzed: timber; concrete, and steel, using the same CFRP laminates and adhesive agent. The externally bonded reinforcement (EBR) technique was used throughout the study. The results show that the CFRP-to-timber interfaces had the highest strength but also showed that these interfaces need a longer bonded length in order to reach maximum strength, i.e., CFRP-to-timber interfaces had the longest effective bond length. The local nonlinear bond-slip curve of CFRP-to-concrete can be approximated to exponential curves, whereas the CFRP-to-timber or steel interfaces showed trilinear and bilinear bond-slip relations, respectively. Also, the CFRP-to-timber interfaces revealed the highest fracture energy.

Bond-slip behavior and embedment length of reinforcement in high volume fly ash concrete

To investigate bond-slip behavior of reinforcement in high volume fly ash concrete (HVFAC), 189 pull-out specimens are studied under monotonic static load in this paper. The main research variables involve the volume of fly ash, the type and diameter of the steel bars and the water-to-cement ratio (w/c). The tensile loading in this study is applied to steel bar, which increases stably by controlling the gradual increase of steel bar’s slip until end of the tests. For each specimen, the complete relationship curve to bond stress and slip are collected. Results indicate that the bond strengths of steel bars increased along with the decrease of the w/c ratio and decreased when the diameter of steel bar increased. Other results also show that the type of steel bar has a significant influence on bond and slip behavior and similar bond-slip relationship curves are presented in HVFAC, compared to conventional concrete (CC). To assess the feasibility of existing bond strength models in HVFAC, predictions from the models are compared with experimental results in the study. Based on the analyses and comparative results, a revised ultimate bond strength model and bond–slip relationship model are proposed to evaluate behavior of deformed steel bar in HVFAC. The first model is affected by the volume of fly ash and could evaluate the bond strengths well, the second one could monitor complete bond-slip curve reasonably. In addition, using the above revisions to the bond behaviors of bar in HVFAC, a simple calculation method for the embedment length of deformed steel bar in the concrete is recommended, because it has a stable design safety reserve.

Bond behaviour with reinforcement corrosion

The bond between steel and concrete has a profound influence on the structural behaviour of reinforced concrete structures in both service and ultimate conditions. In the present work, bond–slip behaviour between steel and concrete has been analysed in the presence of corrosion of a steel reinforcing bar. The results of pull-out RILEM type tests are presented. The tests have been performed on specimens with and without confinement in the presence of an increasing level of corrosion up to 20% of mass loss. Corrosion has been induced by means of an electrochemical procedure with a current density of 200μA/cm2. The local bond–slip curve between the steel and concrete, in the presence of a short embedment length between the two materials, has been obtained. The results show the influence of the corrosion level and the presence of confinement in terms of the bond–slip response.

Refining bond–slip constitutive relationship between checkered steel tube and concrete

In view of the problem of low average bond strength between steel tube and concrete, nine push-out test specimens of concrete filled checkered steel tube (CFCST) have been done to investigate the bond performance between checked steel tube and concrete. According to the experimental results of bond–slip curves, a basic bond–slip constitutive model is established. Considering the varying influential parameters of height of checkered pattern and concrete strength, the changing trends of the ultimate average bond strength are analyzed. Meanwhile, the distribution regularities of checkered steel tube strains, bond strengths and slippages along the longitudinal length, and bond–slip curves in different longitudinal positions are obtained. Based on the basic bond–slip constitutive model, two position functions are introduced to establish the bond–slip constitutive relationship. The experimental results show that the height of checkered pattern has a significant influence on ultimate average bond strength and that the basic bond–slip constitutive model matches well with our test curves. The introduction of the position functions laid the foundation for further finite element analysis of CFCST.

A Study on the Bond Behavior of Corroded Reinforced Concrete Containing Recycled Aggregates

This paper investigated bond-slip characteristics of chloride-induced corroded reinforced concrete incorporating different levels of recycled concrete aggregates (RCA). Pullout tests were adopted to evaluate the bonding and debonding behaviors of the embedded rebar experiencing different corrosion levels. Both high- and low-strength concrete were considered. Bond-slip curves were recorded to determine the influences of rebar corrosion levels and RCA replacements on the bond strength and debonding energy of the specimens. Test results indicate that increasing rebar corrosion level gradually weakens the antisliding ability of reinforced recycled aggregate concrete (RAC) except for a small level corrosion and the degradation rate of ultimate bond strength increases with a decrease of compressive strength at 0.5% rebar corrosion. The results also demonstrate that the ultimate bond strength of reinforced RAC slightly decreases with an increase of RCA replacement. However, the relative bond strength between uncorroded rebar and RAC is little affected by RCA content, while it decreases with an increase of RCA replacement in high-strength specimens after rebar corrosion. The debonding energy between deformed rebar and RAC is found decreasing with the increment of the rebar corrosion level and increasing with an increase of RAC content.

Bond–slip behaviour between self-compacting concrete and carbon-fibre-reinforced polymer sheets

The bond–slip behaviour between self-compacting concrete (SCC) and carbon-fibre-reinforced polymer (CFRP) sheets was investigated using double shear pull-off concrete specimens. Concrete blocks of size 150 × 150 × 100 mm were made from two aggregate types (limestone and basalt), two aggregate gradations (12 mm and 19 mm) and three water-to-cement (w/c) ratios (0·30, 0·35 and 0·40). CFRP sheets were attached to the concrete specimens using three different bond widths (50, 100 and 150 mm) and four bond lengths (60, 80, 100 and 120 mm). The ends of the CFRP sheets were adhered to the concrete specimens at two parallel sides using a special epoxy; the middle was left free for the wrapping of an aluminium cylinder of 100 mm diameter before pull-off testing. The obtained results and literature data were employed in the development of a statistical model to estimate the ultimate shear strength and slippage between CFRP sheets and SCC. The results showed that the effect of aggregate type is insignificant, although the basalt aggregate samples showed higher bond strength and corresponding slippage than the limestone samples. The trend behaviour of the bond–slip curves for double shear pull-off specimens showed no noticeable difference between SCC mixtures made from two different aggregate gradations, but the resulting values showed a slight increase in bond strength and slippage for the concrete specimens made from the larger aggregate particles. Water-to-cement ratio was found to have a significant effect on bond strength and corresponding slippage: bond strength and slippage increased with a reduction in w/c ratio. There was a noticeable decrease in bond stress and a noticeable increase in corresponding slippage for increasing bond length and bond width. The statistical model developed shows an excellent fit to the data used, as indicated by high coefficients of determination.

A new discrete method to model unidirectional FRP-to-parent material bonded joints subjected to mechanical loads

Nowadays fiber reinforced polymer (FRP) composites play an important role in the strengthening of structures. Different methods can be used to apply these materials: the externally bonded reinforcement (EBR), and the near surface mounted (NSM) using strips and NSM rods. There are only a few studies comparing these methods or presenting an efficient model to simulate these strengthening techniques. This study looks mainly at the analysis of the interface between FRP-to-parent material bonded joints. The paper examines, through a new discrete model based on axial and shear springs, the performance of FRP-to-parent material bonded joints for EBR or NSM techniques using strips or composite rods. In order to implement the model a routine in MATLAB was developed and several bond–slip curves were assumed. The results revealed that load–slip curves or bond stresses, strains or slippages along the bonded length obtained from several bond–slip curves are similar to the analytical and other numerical solutions found in literature. In what concerns the adhesion between two different materials, and assuming the same bond characteristics for the three fiber strengthening techniques, the NSM system using FRP strips had the highest maximum load transmitted to the FRP strip combined with the lowest effective bond length. The results obtained from the proposed model were also very accurate with that obtained from an analytical solution found in literature that simulates the debonding phenomenon of FRP-to-concrete interfaces between to adjacent cracks.

Numerical modelling of the effects of elevated service temperatures on the debonding process of FRP-to-concrete bonded joints

There are many issues concerning the performance behaviour of FRP-to-concrete interfaces at elevated service temperatures (EST). At EST, i.e. slightly above the glass transition temperature (Tg), some properties associated with the FRP composites, such as the stiffness, strength or the bond characteristics, degrade. This is a crucial issue and there are only a few studies that take into account such effects on FRP-to-concrete interfaces at EST. This paper examines, through a numerical analysis, the performance of FRP-to-concrete bonded joints at EST using a new discrete model based on truss elements and shear springs. The External Bonded Reinforcement (EBR) systems subjected to EST are analyzed. The numerical discrete model was implemented in a MATLAB routine and the bond–slip curves of the interfaces at EST were obtained from a model found in literature. The numerical results revealed that the interface at EST behaves similarly to one with two equal mechanical loads applied at both ends of the FRP plate. The load–slip curves or bond stresses, strains or slippages along the bonded length obtained from several bond–slip curves at different temperatures were obtained. Two different single-lap shear tests were simulated at steady-state (steady temperature followed by load increase) and transient state (steady load followed by temperature increase). Regarding the influence of the temperature on the adhesion between the FRP and concrete, the results showed that an increase in the temperature at an earlier situation, i.e. during a period where temperature had no influence in the concrete deformations, leads to an increase in the effective bond length of the interface affecting the initial strength of the interface.

Bond Behavior between Near-Surface-Mounted CFRP Strips and Concrete at High Temperatures

AbstractThis paper presents an experimental study concerning the bond behavior at high temperatures between concrete and carbon fiber–reinforced polymer (CFRP) strips installed according to the near-surface-mounted (NSM) technique. Double-lap shear tests were performed on concrete blocks strengthened with CFRP strips installed into slits and bonded with either an epoxy adhesive (series EP) or a mixed grout composed of epoxy and cement binders (series MG). Specimens were first heated up to predefined temperatures (from 20 to 150°C, measured in the adhesive) and then were loaded up to failure. The obtained results allow drawing the following conclusions: (1) the axial strains along the bonded length became closer to linear (at high temperature) because of the softening (glass transition) of the bonding materials; (2) the effective bond length increased for elevated temperature; (3) the bond-slip curves exhibited considerable and consistent stiffness and maximum shear stress reduction with temperature; (4) t...

Bond Performance of Basalt Fiber-Reinforced Polymer Bars to Concrete

This paper presents the test results of a study on the bond behavior of basalt fiber-reinforced polymer (BFRP) bars to concrete. Thirty six concrete cylinders reinforced with BFRP bars and twelve cylinders reinforced with glass fiber-reinforced polymer (GFRP) bars were tested in direct pullout conditions. Test parameters included the FRP material (basalt and glass), the bar diameter, and the bar embedment length in concrete. Bond-slip curves of BFRP and GFRP bars revealed similar trends. BFRP bars developed average bond strength 75% of that of GFRP bars. All BFRP specimens failed in a pullout mode of failure along the interfacial surface between the outer layer of the bar and the subsequent core layers. The influence of various parameters on the overall bond performance of BFRP bars is analyzed and discussed. The well-known BPE and modified-BPE analytical models were calibrated to describe the bond-slip relationships of the bars. Test results demonstrate the promise of using the BFRP bars as an alternative to the GFRP bars in reinforcing concrete elements.

An Experimental Evaluation of Development Length of Reinforcements Embedded in Geopolymer Concrete

Geopolymer concrete is an emerging construction material that uses a by-product material such as fly ash to completely replace the ordinary Portland cement. This material is being studied extensively and shows promise as a greener substitute for ordinary Portland cement. This paper evaluates the bond strength and development length of reinforcements embedded in geopolymer concrete with reinforcing steel using pull-out tests. The test according to EN 10080 was carried out on 27 specimens for three kinds of geopolymer concrete of 20, 30 and 40 MPa compressive strength and 10, 16 and 35 mm diameter reinforcing bars. The tests show that the bond strengths in geopolymer concrete were decreased as the diameter of reinforcement increased as in ordinary concrete and the values were greater than those in ordinary concrete. Also, the bond-slip curves were obtained which have similar shape with those of ordinary concrete. The equation for the determination of development length based on this experiment was proposed.

Factors influencing the performance of externally bonded reinforcement systems of GFRP-to-concrete interfaces

Fibre reinforced polymer (FRP) composites may prematurely debond from the surface of concrete, i.e. before its elastic resistance is exhausted. This is a very common situation and can be aggravated if additional factors are not taken into account. These factors include the type of surface preparation, the exposure to aggressive environmental action, the tensile concrete strength or fatigue and creep loading to which the structural element may be subject. An experimental programme based on double shear tests was undertaken to analyse the influence of some of these factors on the performance of the interface between composite glass fibres (GFRP) and concrete. The results allowed the determination and comparison of maximum loads transmitted to the GFRP plates and maximum bond stresses obtained considering various surface treatments and aging conditions. Bond–slip curves were also determined. The experimental results are compared with those obtained from a numerical analysis.

Local bond–slip behavior between cold-formed metal and concrete

Composite action in systems consisting of steel and concrete depends on an effective shear-transfer mechanism between the two materials. Such mechanism for smooth steel surfaces inside concrete will be limited to the bond–slip behavior at the steel/concrete interfaces. This research investigates the bond–slip behavior of galvanized cold-formed (light gauge) steel profiles embedded in normal weight normal strength concrete. A new innovative pull-out test is presented which is convenient to set up and perform and reduces the undesirable parameters to the minimum. Global bond–slip curves for different values of concrete strength are obtained from such tests. Through an innovative procedure, mathematical equations and selected points from the experimental global bond–slip curves are used to develop a bi-linear local bond–slip model which represents the discussed bond–slip behavior. By curve fitting, empirical equations are proposed to determine the suggested model’s parameters based on the concrete compressive strength. Finally, validity of the proposed model is explored by two methods: (1) by comparing the results from analytical equations with test results, (2) by comparing the results from finite element modeling with test results. An excellent agreement has been observed in both verification methods.

Bond Stress-Slip Behavior: Case of GFRP Bars in Geopolymer Concrete

The use of geopolymer concrete reinforced with fiber-reinforced polymer (FRP) bars is anticipated to address the concerns on the usage of traditional reinforced concrete structures, such as the corrosion of internal steel reinforcement, costly repair and rehabilitation, and development of sustainable infrastructures. To gain wide acceptance in the construction market, the bond between geopolymer concrete and the FRP bar should be investigated first because it is a critical factor that influences the behavior of structures, specifically its strength and long-term durability. In this study, the bond performance of sand-coated glass fiber-reinforced polymer (GFRP) bars into geopolymer concrete with a compressive strength of 33 MPa was investigated under a direct pullout test. The effects of parameters such as bar diameter (12.7, 15.9, and 19.0 mm) and embedment length (5, 10, and 15 db, where db is the bar diameter) were evaluated. The results showed that the maximum average bond stress obtained is around 23 MPa. As GFRP bar diameter increases, the average bond stress decreases. Similarly, the average bond stress decreases as the bond length becomes longer. The specimens with shorter embedment length failed because of pullout of the bars, whereas those with longer embedment lengths failed because the concrete split. The results further revealed that the geopolymer concrete reinforced with GFRP bars have a bond strength similar to that of steel-reinforced geopolymer concrete. Finally, bond-slip models for the ascending branch up to maximum bond stress of the bond-slip curves for GFRP bars and geopolymer concrete were proposed.

Evaluation Method for Bond-Splitting Behavior of Reinforced Concrete with Corrosion Based on Confinement Stress of Concrete against Corrosion Expansion

In this study, the bond behavior of corroded reinforced concrete (RC) was investigated by conducting pull-out tests on RC specimens with different corrosion crack widths. The confinement stress of concrete was evaluated by an expansion simulation using a non-explosive demolition agent, from which a confinement stress equation for RC with corrosion cracking was proposed. Bond strength linearly decreased with decreasing confinement stress in the concrete. The rate of decrease was independent of longitudinal corrosion crack width at the concrete surface, cover thickness, and reinforcement diameter. Furthermore, from the relationship between bond strength and confinement stress in concrete, equations for predicting bond stress and slip curve were proposed. Overall, the predicted values agreed well with the experimental results of other studies.

Influence of water-to-binder ratio on top-bar effect and on bond variation across length in Self-Compacting Concrete specimens

The present study is investigating the impact of five different water-to-binder (w/b) ratios on bond capacity of Self-Compacting Concrete (SCC) specimens, with a focus on top-bar effect and on bond variation across horizontal specimens with a single casting point at one edge. One 600mm high vertical specimen and one 1780mm long horizontal specimen with transverse reinforcement bars that were distributed over height and across length, respectively, were cast and tested by pull-out for each of five SCC mixtures, and compared to three Normally Vibrated Concrete (NVC) mixtures with similar aggregate grading curves and compressive strength. The bond–slip curves were evaluated on the basis of analytical models for NVC, proposed by major international building codes and literature. SCC develops an improved bond capacity, a less intense top-bar effect and a lower scatter compared to NVC. The analytical bond–slip model curves appear to underestimate bond stiffness and overestimate the assumed maximum bond stress. Water increase leads to lower stresses and a more intense top-bar effect, which is though always less significant in SCC. The code provisions concerning the top-bar effect of rebars close to the poor bond conditions zone seem to overestimate the actual bond reduction. Bond capacity of SCC is not decreased across length up to the measured rebar distance (1.60m) from the casting point and the scatter of the resulting stresses is significantly lower compared to NVC, especially for higher w/b ratios.

Analytical and experimental investigation for bond behaviour of newly developed polystyrene foam particles’ lightweight concrete

Structural lightweight concrete (LWC) is of high importance to the construction industry, as it is cost effective and highly advantageous. A new kind of LWC was produced at the Department of Structural Engineering of Ain Shams University in 2005, which combines the advantages of normal density concrete, cellular concrete and high workability concrete through partially replacing the normal weight aggregates with polystyrene foam particles. This leads to concrete’s unit weight reduction while maintaining adequate strength. The latter material can therefore be produced using standard methods familiar to the construction industry with a dry unit weight of 18.50kN/m3, which in turn leads to self weight reduction of 15–20% and the associated decrease in the structure’s overall cost, hence, providing a feasible challenge to normal weight concrete (NWC). The bond behaviour of structural polystyrene foam lightweight concrete (PF-LWC) was investigated analytically and experimentally. The experimental program incorporated two phases. The first phase was performed on the standard pull-out specimens to compare its results with the commonly conducted bond testing and to abstract the bond slip curve of the standard pull-out test specimens. The second phase deal with a deduced beam-end specimen to assess the behaviour of bond between reinforcing bars and concrete in flexure members. Then analytical investigation of the obtained experimental results was performed to develop a model capable of assessing the structural bond behaviour of PF-LWC flexural members. The defining parameters of the bond stress–slip curve were modified for NWC and PF-LWC using the best fit technique to the experimental results in order to add the bar diameter as a variable in the bond stress–slip relationship. The defining parameters of the CEB-FIP 1990 bond stress–slip curve [1] were modified for NWC and PF-LWC using the best fitting technique.

Behavior of CFRP Laminates Bonded to a Steel Substrate Using a Ductile Adhesive

Strengthening of steel structures with adhesively bonded carbon fiber–reinforced polymer (CFRP) laminates (or plates) has received extensive research attention over the past few years. Existing studies have revealed that debonding of CFRP plates from a steel substrate is one of the main failure modes in such CFRP-strengthened steel structures. To better understand and model debonding failures, the behavior of CFRP-to-steel bonded joints needs to be well understood. Recent tests conducted by the authors’ have demonstrated that the bond strength of such bonded joints depends significantly on the properties of the adhesive used, and more specifically on the interfacial fracture energy rather than the tensile strength of the adhesive. The study has also showed that the bond-slip curves for nonlinear ductile adhesives have an approximately trapezoidal shape. This paper resents an analytical solution for the full-range behavior of CFRP-to-steel bonded joints with a ductile nonlinear adhesive. Predictions from the analytical solution are presented to explain the different stages of debonding failure and are compared with test data to demonstrate the validity of the analytical solution.

Bond stress-slip behaviour of steel reinforcing bar embedded in hybrid fiber-reinforced concrete

Experimental study was carried out to evaluate the application of metallic fibers to confine the concrete in column-beam joint. Pullout tests on cubic specimens of size 200 × 200 × 200 mm with deformed steel bar embedded for a fixed length of 5 times the diameter of tested deformed bar were performed under monotonic applied displacement at the tip of the bar (displacement controlled tests). The key variable of the experimental study was the different confining conditions; by ordinary reinforcing steel bars, and by metallic fibers. To confine the concrete by the fibers, two different types of metallic fibers were used; amorphous metallic fibers and carbon steel hooked-end fibers. The fibers were investigated both in mono and hybrid forms. The dosage of fibers in mono fiber-reinforced concrete was kept 40 kg/m3. In case of hybrid fiber-reinforced concrete, two concrete compositions were investigated: one containing 20 kg/m3 of each fiber and second containing 40 kg/m3 of each type of fiber. The pullout tests results revealed that both types of fibers contribute to ameliorate the peak bond stress and also to improve the ascending branch of the bond stress versus slip curve in terms of stiffness. Comparison of the bond stress-slip response of anchored bar in concrete confined by the ordinary reinforcing steel bars and by the metallic fibers demonstrated that the concrete confined by the fibers in hybrid form showed better performance than the concrete confined by the ordinary steel bars in terms of peak bond stress and toughness. Since the hybrid form of investigated metallic fibers realize same effect as that by ordinary steel, the congestion of steel in seismic resistant column-beam joint can be avoided by replacing some percentage of steel ratio to confine the concrete by the metallic fibers.

Bond Properties of Plain Steel Bar in Concrete with Machine-Made Sand

To meet the requirement of machine-made sand application in concrete structures, it is necessary to understand the bond properties of steel bar with machine-made sand concrete (MSC). Therefore, the experimental studies were carried out on the bond of plain steel bar with MSC by the central pull-out test method. Three specimens were cast as one group, 6 groups were tested considering the changes of strength grade of MSC and ordinary concrete. The bond-slip curves were measured and analyzed. The results show that the bond slip begins at the tensile side and transfers gradually to the free end before the entire slip turns up along the interface of plain steel bar and surrounded concrete, the largest average bond stress, i. e. the bond strength of plain steel bar corresponds to the initial entire slip of plain steel bar. With the increasing strength grade of MSC and ordinary concrete, the difference of slip at tensile side and free end becomes greater. Comparing that only appears in ordinary concrete with higher strength, the larger slips turn up while the bond stress reaches the largest for the plain steel bar in MSC. Larger scatter of bond strength is between specimens in the same group. Some plain steel bars yields with the beginning of entire slip along the interface.