Direct Pull-out Tests Research Articles

Bond behavior between the deformed steel bar and different types of cementitious grout under reversed cyclic loading

To systematically investigate the bond behavior of deformed steel bars in cementitious grout, direct pullout tests were conducted on 15 sets of specimens by considering five types of cementitious matrixes and three different loading schemes. The failure modes, bond stress-slip curves, as well as the bond degradation and cumulative energy dissipation of specimens under reversed cyclic loading, were compared and analyzed. Results indicate that splitting pull-out or pull-out failure modes were observed, and pull-out failure was the most common failure mode of the specimen (accounting for 91 %) due to the installation of stirrups and the sufficiently short anchorage length. Meanwhile, regardless of the loading scheme, the general relationship between the magnitude of maximum bond stress of specimens composed of different cementitious materials was shown as C60＞CGM-270＞CGM-300＞CGM-340＞CGM-380, while there was no consistent pattern in relative slippage. Moreover, the relationship between the degradation of the maximum bond stress and stiffness of each specimen under reversed cyclic loading was shown as CGM-380 >CGM-340 ≈CGM-300 >CGM-270 >C60. The degradation mainly occurs in the first two cycles, and then gradually stabilizes. Furthermore, the cumulative energy dissipation relationship of specimens with different materials was shown as C60 >CGM-270 >CGM-300 >CGM-340 >CGM-380 under the reversed cyclic loading. The above investigations could provide theoretical support for the evaluation of ductility and energy dissipation of concrete members strengthened with cementitious grout under earthquake actions.

Bond behavior between bundled steel-FRP composite bars and grouted corrugated duct

The utilization of steel-fiber reinforced polymer (FRP) composite bars (SFCBs) reinforcement in precast concrete structures offers numerous benefits, including controllable post-yield stiffness and reduced residual displacement. Employing bundled reinforcement can improve construction efficiency. This study presents direct pull-out tests conducted on SFCBs embedded in grouted corrugated ducts. The results show that bond strength decreases with both the number of bars in a bundle (nb) and the actual embedded length (la). The bond strength of single bar SF-1b-3d with 3d embedded length is 24.21 MPa, while that of 2-bar bundled SF-2b-3d and 3-bar bundled SF-3b-3d is 17.41 MPa and 12.61 MPa, respectively. Additionally, the error in predicting the bond strength between bundled SFCBs and grouted corrugated duct using the equivalent diameter method is found to be less than 8.4%. Utilizing the mBPE constitutive model, an analytical hardening-slip model with a linear slip field assumption accurately predicts the full-range behavior, bond stress distribution, and SFCB's axial stress distribution. Furthermore, the effects of different parameters on yield slip sy, ultimate slip su, and critical anchorage length Ltr are investigated by considering different local bond-slip constitutive models (varying peak bond strength τm and its corresponding slip sm and ascending section coefficient (α) as variables. The yield slip sy varies from 0.1 mm to 1.3 mm, while the ultimate slip su can reach a maximum of 35.0 mm. Furthermore, the validity of existing models for evaluating the anchorage length of single SFCB and bundled SFCBs grouted in corrugated duct connections is discussed, and a formula to estimate the critical anchorage length of bundled SFCBs and grout considering the number of bars within one bundle (nb) is proposed.

Bond behavior and modeling of steel-FRP composite bars in engineered cementitious composites

The combination of the steel-FRP composite bar (SFCB) and engineered cementitious composite (ECC) holds favorable applications in terms of durable resilient structures. However, a clear understanding of the interfacial bond performance of this novel combination is currently lacking. Therefore, the bond behavior and mechanism of the SFCB-ECC interface were investigated through direct pullout tests. A total of 48 cube specimens were fabricated, with the test variables including bar type, bar diameter, embedment length, and matrix type. The experimental results indicated that the bond failure mode of SFCBs and FRP bars differed from that of deformed steel bars, attributed to the damage of resin-rich ribs in relatively high-strength ECC. The bond-slip curves revealed both the fundamental differences and similarities among steel bars, SFCBs, and FRP bars. The Poisson effect and shear lag effect increased with a decrease in bar stiffness and an increase in diameter. The combined impact of the interaction between the bar stiffness and diameter resulted in varying bond strengths for steel bars, SFCBs, and GFRP bars, despite their similar surface geometries. A theoretical model for the bond strength at the SFCB-ECC interface was derived using the thick-walled cylinder principle and subsequently validated with test data. Finally, a unified empirical model framework for predicting the bond strength of various bar-matrix combinations was proposed based on the theoretical model. The accuracy of this method was evaluated using a collected database.

Bond behavior between steel-FRP composite bars and engineered cementitious composites in pullout conditions

Steel-FRP composite bar (SFCB) reinforced engineered cementitious composites (ECC) members are promising to apply in structural engineering due to their excellent mechanical properties and durability. The bond behavior between SFCB and ECC was investigated by direct pullout tests. The experimental variables included the diameter of SFCB, bond length, type of matrix, compressive and tensile strengths of ECC, and type of reinforcement. All ECC specimens failed in a ductile manner with ECC keys being crushed and sheared off. The bond strength and bond toughness of ECC specimens were higher than those of concrete specimens possessing similar compressive strengths. An interfacial bond-slip model was proposed to predict the bond behavior of SFCB embedded in ECC. An iteration procedure was performed utilizing the developed bond-slip model to analyze the distributions of bond stresses and slips. Notably, an increase in bond length and the yielding of SFCB led to heightened non-uniformity in bond stress and slip distributions. Furthermore, equations were derived to predict the embedment lengths of SFCB applicable to various scenarios.

Enhancement of bond characteristics between sand-coated GFRP bar and normal weight and light-weight concrete using an innovative anchor

To prevent the concrete bar from slipping, a common approach is to ensure an adequate development length or bend it upwards. However, in the case of fiber reinforcement polymer (FRP) bars, on-site bending is not feasible. Therefore, employing a mechanical end-anchor offers an alternative solution to enhance the bond in reinforced concrete. This study investigates the influence of end-anchor shape on ribbed sand-coated glass fiber-reinforced polymer (GFRP) bars, utilizing four different types of mechanical end-anchors. Additionally, the effects of concrete type and compressive strength are evaluated by considering normal-weight concrete (NC) and light-weight concrete (LC) with compressive strengths ranging from 26 MPa to 38 MPa. The study involves the preparation and testing of 60 specimens using a direct pull-out test. The findings demonstrate that the utilization of the proposed mechanical end-anchors enhances the developed tensile stress of the GFRP bar by 10% to 50%, consequently significantly increasing the ultimate capacity of the reinforced concrete section. Furthermore, optimizing stress distribution and improving the bond behavior between the bar and concrete can be achieved by increasing the number of end-anchors and adjusting the distance between them, while maintaining the total length and diameter.

Effect of Using Glass Fiber Reinforced Polymer (GFRP) and Deformed Steel Bars on the Bonding Behavior of Lightweight Foamed Concrete

The study aims to conduct a direct pull-out test on fifty-four cube specimens considering different variables, including the type of reinforcement (sand-coated glass fiber-reinforced polymer (GFRP) and ribbed steel bars); the type of concrete (normal weight concrete NWC and lightweight foamed concrete LWFC); the diameter of the reinforcing bars (10 mm; 12 mm; and 16 mm) and the bonded length (3∅, 4∅, and 5∅). The hybrid fiber hooked-end steel (0.4% by volume) and polypropylene (0.2% by volume), respectively were used to improve the properties of LWFC by converting the brittle failure to ductile. The results showed that in the case of strengthened foamed concrete (FC), the bond strength with steel bars was greater compared to that with the GFRP bars. The bond strength ratio between the GFRP and steel bars of the FC specimens was found to vary between 37.8–89.3%. Additionally, in all specimens of FC, pull-out failure was witnessed with narrower crack width compared to NWC. Furthermore, mathematical equations have been proposed for predicting the bond strength of FC with steel and GFRP bars and showed good correlation with the experimental results.

CALCULATION METHOD FOR DETERMINATION OF ADHESION LEVEL OF COMPOSITE REINFORCEMENT WITH CONCRETE

The scientific work is devoted to the interaction process of concrete and composite reinforcement, which is characterized by “adhesion-slip” dependence. It is known, that composite reinforcement does not behave in the same way as traditional steel reinforcement, because in some cases their mechanical properties differ significantly. CFRP/FGRP/BFRP products have higher strength, but a lower modulus of elasticity, so direct replacement of steel with such reinforcement is not always possible according to many constructional requirements. Adhesion forces create a complex stress-strain condition in concrete interacting with reinforcement. This condition leads to the distribution of loads along the axis of reinforcement, and, as a result, the longitudinal forces on reinforcement become variable along the entire length of the rod. A detailed analysis of the existing approaches to the problem of adhesion level of concrete and composite reinforcement is performed in article. It was determined that the complex multiparameter state of the interaction of concrete and composite reinforcement is characterized by the corresponding curves of “adhesion-slip” dependence, which can be obtained by two experimental methods (beam test method and direct pull-out test method). A theoretical research of the adhesion level of concrete and composite reinforcement (beyond the limits of cracks formation) was carried out, connected with the analysis of the distribution of deformations of concrete and reinforcement along the span of the element. Current analysis is based on the determination of a number of differential equations with a step-by-step description of adhesion level problems. The results of research can be used in future during the design and calculation of concrete structures reinforced with different types of composite reinforcement (based on basalt, glass, carbon fibers etc.), however, it is necessary to conduct further experiments into the long-term operation (behavior) of composite reinforcement over time under the influence of various factors, to establish a number of rheological aspects. Keywords: adhesion, calculation, algorithm, composite reinforcement, concrete, slip.

Bond durability between anchored GFRP bar and seawater concrete under offshore environmental conditions

The lower bond strength of FRP bars to concrete compared to steel bars has remained an unsolved barrier to the widespread use of FRP-reinforced concrete under extreme loading. Additionally, the degradation of the bond between FRP reinforcement and concretes in aggressive environments adds to the existing concern. In this study, an innovative anchorage system comprised of polypropylene pipe was used to strengthen the bond between seawater concrete and GFRP bars after 250 days of exposure to offshore environmental conditions. As material factors, two types of GFRP bars (sand-coated and ribbed) and two types of concrete (normal and seawater concrete) were evaluated. Four distinct environmental conditions were used to assess the samples: (i) ambient environment (control), (ii) tap water, (iii) seawater, and (iv) wet-dry cycles in seawater. According to the findings of the direct pull-out tests, the suggested anchor system strengthens the bond and shifts the failure mode from bond failure to bar rupture. Additionally, after exposure to 250 days of seawater wet-dry cycles, GFRP-reinforced seawater concrete lost 5% of its maximum bond strength (developed bar tensile stress). All other samples exposed to different environmental conditions either increased or decreased in bond strength by less than 5% after 250 days, compared to the control samples.

Cyclic bond behavior and bond stress-slip constitutive model of rebar embedded in hybrid fiber reinforced strain-hardening cementitious composites

In this study, direct pullout tests were conducted to illustrate the influences of fiber types (polyethylene (PE) fiber, steel (ST) fiber and hybrid PE/ST fiber), rebar diameter (20 mm, 25 mm, 32 mm), matrix strength (50 MPa and 70 MPa), and loading procedure (monotonic and cyclic loading) on the bonding performance between rebar and strain-hardening cementitious composites (SHCC). The effects of design parameters on bond behavior under monotonic loading, as well as degradation laws of bond strength, bond stiffness, and energy dissipation capacity under cyclic loading are discussed in detail. Experimental results indicate the synergic effect of hybrid fiber is beneficial to crack arrest, bond strength, and bond strength and bond stiffness retention. Compared with those under monotonic loading, SHCC specimens under cyclic loading exhibit similar ascending behavior while an obvious performance degradation at descending. Additionally, ST fiber behaved more effectively than PE fiber in improving the energy dissipation capacity. Furthermore, a cyclic bond stress-slip constitutive model with acceptable accuracy is developed to predict the cyclic bond behavior of rebar embedded in SHCC.

Bond of FRP rods in damaged RC beams strengthened with NSM technique

In strengthening reinforced concrete (RC) elements with composite materials, the role of adherence between concrete and FRP assumes great importance as it guarantees the effectiveness of the consolidation method. In this paper the bond mechanisms between FRP rods and concrete second the Near Surface Mounted (NSM) technique have been analyzed by experimental results of investigation and a theoretical linear model. The experimental investigation consists of direct pull-out tests on RC elements with circular GFRP and CFRP rods inserted into a groove. Through pull-out tests main parameters of bond have been presented. The experimental results are compared with those obtained by the theoretical model.

Study of the influence of the surface characteristics of steel bars on the steel-concrete interaction

The steel-concrete adherence is of fundamental importance for the operation of reinforced concrete elements since they provide the transfer of stresses between the materials. The geometry of the steel bars is one of the main factors that interfere with adherence behavior. This paper makes an experimental study of adherence using steel bars with different surface characteristics, applying direct pull-out tests. From the experimental investigation, it was concluded that the steel bars' geometry interferes with the steel-concrete interface's performance. After identifying the necessary parameters, numerical simulations were performed using the Finite Element Method to verify the numerical results with the experimental ones and thus validate the modeling. Furthermore, the Mohr-Coulomb and Drucker-Prager criteria made it possible to visualize the stresses in the concrete. Finally, after the analyses, it was found that the numerical model developed was close in parts to the experimental model and that further refinement is needed to accurately represent the physical effects present in direct pull-out tests.

Enhancement of the bond behaviour between sand coated GFRP bar and normal concrete using innovative composite anchor heads

Anchor heads could efficiently address the bond weakness between Fibre-Reinforced Polymer bars and concrete. This experimental study enhanced the bond behaviour between GFRP bars and concrete using three innovative anchorage systems made from glass fibre cloth and epoxy resin. A direct pullout test was used to study the bond-slip performance between the bar and the concrete. Test variables were GFRP bar diameter (3 diameters), concrete compressive strength (20.4 and 40.2 MPa), and anchor system (three different types). Based on the test results, in low-strength concrete (i.e. 20.4 MPa) samples, the anchor system efficiency was not promising, and the failure occurred between the concrete and anchors. However, for higher strength concrete (i.e. 40.2 MPa) samples, the ultimate developed tensile load increased between 14 and 68% for different bar sizes and anchorage systems compared to the unanchored control specimens.

Analytical and Numerical Modeling of the Pullout Behavior between High-Strength Stainless Steel Wire Mesh and ECC.

Bond behavior is a key factor in the engineering application of composite material. This study focuses on the constitutive model of the bond behavior between high-strength stainless steel strand mesh and Engineered Cementitious Composites (ECC). In this paper, the effects of strand diameter, bond length and transverse steel strand spacing on bond behavior were studied based on 51 direct pullout tests. Experimental results showed that the high-strength stainless steel strand mesh provided specimens an excellent ductility. Based on the experimental data, the existing bond–slip model was revised using the theory of damage mechanics, which fully considered the influence of the steel strand diameter on the initial tangent stiffness of the bond–slip curve. The results of the model verification analysis show that errors are within 10% for most parameters of the bond–slip model proposed, especially in the ascending section, the errors are within 5%, indicating that the calculated results using the revised model are in good agreement with the test results. In addition, the revised model was applied to the finite element analysis by using the software ABAQUS to simulate the pullout test, in which the spring-2 nonlinear spring element was used to stimulate the bond behavior between steel strand meshes and ECC. The simulation results show that the numerical analysis fits the experimental result well, which further verifies the accuracy of the model and the feasibility and applicability of the numerical analysis method.

Bond-slip behaviour between GFRP/steel bars and seawater concrete after exposure to environmental conditions

Corrosion resistant FRP reinforced seawater concrete structures are attractive alternatives to conventional steel reinforced normal concrete structures. In this experimental study, the bond-slip durability of glass fibre reinforced polymer (GFRP) and steel bars embedded in seawater concrete after exposure to environmental conditions has been studied. Specimens with bars embedded in normal concrete were also tested for comparison. In total, 48cubic specimens were constructed, conditioned, and tested under a direct pull-out test. Four environmental conditions, including the ambient weather, immersion in tap water, immersion in seawater, and seawater wet-dry cycles were used in this study. The results showed the maximum bond strength reductions of about 6 % and 10 % of steel reinforced normal concrete after 250 days of exposure to seawater and seawater wet-dry cycles, respectively compared to the specimens conditioned at ambient weather. The corresponding strengths reductions were 8 % and 13 % for steel reinforced seawater concrete. However, due to the compressive strength increment of specimens exposed to tap water immersion (better curing than ambient weather), a slight bond strength increments up to 4 % was found in both normal and seawater concretes reinforced with steel bars. With respect to GFRP reinforced concrete samples, similar to steel bars, bond strength increments of about 5 % and 13 % after 250 days of immersion in tap water were obtained for normal and seawater concretes, respectively. However, small reductions of 2 % and 3 % were observed in GFRP reinforced normal concretes after exposure to seawater and wet-dry cycles, respectively. The corresponding values were 20 % and 8 % for GFRP reinforced seawater concrete specimens.

Bond behaviors of BFRP bars in concrete using carbon fiber rib anchorage

The geometry of Fiber Reinforced Polymer (FRP) reinforcement considerably affects the load-carrying capacity of structural member. Utilization of additional rib anchorage is expected to be the more effective approach to enhance the bond strength between basalt-FRP (BFRP) bar and concrete. In this paper, A total of 20 tested specimens (including 16 anchored specimens and 4 reference specimens) are fabricated to carry out direct pull-out tests. The influences of concrete strength, anchorage length and anchorage diameter on the bond performance of BFRP bar-to-concrete interface are investigated. Finally, an improved constitutive model is proposed to describe the pull-out behaviors of BFRP bars embedded in concrete using carbon fiber rib anchorage. Results indicate that the bond strength between BFRP bar and concrete can be significantly enhanced by additional rib, increasing by 17.0% compared with reference specimen. The proposed model includes fewer undetermined parameters than existing model, and its prediction accuracy is verified by the relative error ranging between 0.02% and 5.48% in comparison with experimental result. The enhancement effects of additional rib are sensitive to anchorage configurations, an additional rib with length-to-diameter ratio of 4.0 is suggested theoretically in this paper.

Experimental Study on the Flexural Behavior of Lap-Spliced Ultra-High-Performance Fiber-Reinforced Concrete Beams.

In this paper, the structural performance of lap-spliced beams were investigated by the testing of 18-UHPFRC beams with and without a lap-spliced region and steel fiber. All test specimens were subjected to flexural load by using the four-point bending test. It was shown that test steel fiber inclusion in the ultra-high-strength matrix significantly increased the strength of the lap-spliced beams. The maximum strength of the specimen increased linearly with the increase in steel fiber volume fraction. The hybrid-type UHPFRC, with lower steel fiber volume fraction, using two types of steel fiber was effective in improving the bond strength. In order to verify the safety of lap splice length design methods by current code provisions and UHPFRC design recommendations, the average bond stress at the ultimate state was used to calculate the lap splice length, which makes reinforcement yielding, and this value, compared with design lap splice length. Most of the design equations were under the estimated bond stress of UHPFRC. However, the AFGC recommendation, which considers the tensile strength of UHPFRC, overestimates the bond stress of UHPFRC because this recommendation was developed from the direct pull-out test results.

Bond-Slip Performance between High-Strength Steel Wire Rope Meshes and Engineered Cementitious Composites

Engineered cementitious composites (ECC) are distinguished by ultrahigh ductility and multiple cracking. High-strength stainless steel wire rope (HSSSWR) meshes have been successfully applied to strengthening existing structures. Taking advantage of both ECC and HSSSWR meshes, HSSSWR mesh–reinforced ECC promises to be an innovative strengthening material for significantly improving overall performance of existing structures. In order to capture the bond performance between HSSSWR meshes and ECC, direct pull-out tests on HSSSWR meshes embedded in ECC were performed considering the effects of spacing of transverse wire ropes, bond length, and nominal diameter of HSSSWR. The influence laws of the considered factors on the bond behavior between HSSSWR meshes and ECC were investigated. The failure characteristics and bond-slip relationship between HSSSWR meshes and ECC were revealed through analyses of experimental results. A bond-slip model was proposed to predict the bond-slip behavior of HSSSWR meshes in ECC, which was verified to be acceptable by comparing it with test results.

Bond enhancement for BFRP bar in concrete by using a resin-filled FRP tube anchorage

A novel additional anchorage system made out of epoxy resin-filled FRP tubes was proposed for FRP bars in concrete. A total of 33 specimens were carried out via the direct pull-out test to investigate the effectiveness of the developed anchorage in enhancing the bond between BFRP bars and concrete. The investigated variables consisted of the bond length of FRP bar, anchorage length, and the anchorage spacing. Due to the improved bond transfer mechanism for BFRP bar in concrete by using the proposed anchorage, the bond indexes, including the initial bond stiffness, ultimate bond strength, and the residual bond strength, were found to have the maximum improvements of 165.1%, 39.7%, and 162.9%, respectively. In addition, two current bond stress-slip constitutive models for unanchored FRP bars were modified to simulate the pull-out process of the anchored BFRP bars with good accuracy.

Investigation into corrosion-induced bond degradation between concrete and steel rebar with acoustic emission and 3D laser scan techniques

Corrosion-induced bond degradation of deformed steel rebar in concrete is experimentally investigated with acoustic emission (AE) and 3D laser scan technique. Concrete specimens were fabricated and subjected to direct pullout test after being corroded to different levels. The number and width of cracks present during the corrosion tests and the pull-out tests were recorded. The energy released during the pullout tests were captured with AE probes, and the frequency characteristics was analyzed. After pullout tests, the surface morphology of corroded steel rebars was determined with a 3D laser scanner. A modified bond deterioration model was proposed and the parameters associated with the model were analyzed. Results indicated that two types of AE signals were acquired during pullout tests: concrete cracking in high frequency range of 35 ~ 41 kHz and steel-concrete friction in low frequency range of 3 ~ 15 kHz. The bond strength and the bond-slip characteristics depend upon the level of corrosion as well as the number and width of cracks. The reduction factor of the bond-slip model exponentially decreases as a function of the average cross-sectional area loss and linearly decreases with an increase of the rib area loss.

Assessment of frictional-bond strength at the interface of single SSF in cementitious composite and prediction of accompanying pressure of surrounding matrix

ABSTRACT This paper deals with the study of bond in fiber-reinforced concrete composites and the effect of fiber/concrete interfacial friction on the fiber bond capacity. The paper presents the results of an experimental campaign including direct pull-out test of straight steel fiber-SSF embedded in concrete matrix, aimed at investigating the effect of some parameters, such as bonded length, yielding strength of steel fiber, fiber/concrete roughness, surrounding pressure of concrete matrix, and Poisson’s ratio of concrete. Two analytical models based on the experimental results were proposed. Groups of single fiber embedded in concrete were prepared using different values of bonded length. The results showed that the frictional-bond strength, during fiber-slip stage, can be considered as constant despite changing the fiber embedment length. Furthermore, the SSF/concrete system can be considered as an elastic system, whereas the stress–displacement relation is semilinear. Finally, the results showed good improvements in the efficiency of frictional-bond strength when using fresh concrete with higher self-compacting, greater roughness at the interface of SSF/hardened-concrete, and fiber with lower yield strength. While no significant effect of Poisson’s ratio on the frictional-bond strength was noticed.