Bond-slip Curves Research Articles

Bond behavior of square CFT using manufactured sand and recycled coarse aggregate

This paper experimentally investigated the bond behavior of concrete-filled square steel tubes using manufactured sand and recycled coarse aggregate (referred to as MS-RCA-CFT). A total of 48 specimens were tested to study the effect of the following parameters: (i) concrete compressive strength, (ii) width-to-thickness ratio, (iii) stone powder content, and (iv) type of sand. The test results indicated that MS-RCA-CFT members had similar bond behavior as conventional CFT members. The bond strength increased with increasing concrete compressive strength and decreasing width-to-thickness ratio. Specimens with a stone powder content of 5% to 10% had the highest bond strength, while a higher stone powder content (15% and 20%) had a negative impact on the bond strength. Specimens with manufactured sand exhibited higher bond strength compared with specimens with natural sand. The bond strength of MS-RCA-CFT tested in this research was compared with the bond strength of conventional CFT from literature. Applicability of several design codes in calculating the bond strength of MS-RCA-CFT members, RCA-CFT members, and conventional CFT members was evaluated. Lastly, an analytical model for the bond-slip curve was also evaluated.

Effects of adhesive property and thickness on the bond performance between carbon fiber reinforced polymer laminate and steel

The bonding between CFRP (Carbon Fiber Reinforced Polymer) laminate and steel plays a key role in the strengthening effects of CFRPs on a steel structure. In this paper, the toughness of the adhesives and the bonding thickness were investigated on the CFRP-steel bond performance using a single lap shear test. The properties of the adhesives, brittle (linear elastic) or tough (nonlinear elastic, lower modulus, higher elongation), led to apparent differences in the ultimate load, stiffness, the shape of the bond-slip curve, and the stress distribution of the CFRP-steel bond. For the tough adhesive, an increase of the bonding thickness from 0.5 mm to 1.5 mm improves the fracture energy but reduces the interface stiffness. On the contrary, for the brittle adhesive, the effects of the bonding thickness are negligible. According to the test and literature results, key parameters of the bond-slip models of the tough adhesive and brittle adhesive in the case of the delamination of CFRP laminate were determined.

Bond-slip behaviors between reinforced concrete and coated rebar via a new strain monitoring method and FEA simulation

Bond-slip behavior is a complex interaction between steel bar and concrete, which significantly affects the bearing capacity of the reinforced concrete structures. Considering the practical difficulty to accurately measure the strain of coated rebar in structure, a reliable in-situ test method of integrated strain gauges is proposed to measure the strain of coated steel in this study. Using this test method, the bonding properties between steel bar coated with home-made polymer modified cement-based coating and concrete are investigated. The influence of the rebar diameter, the thickness of cover concrete, and the compressive strength of concrete on the bond-slip property are discussed. The results show that the coating does not reduce or even improve the bonding strength of the coated steel bar and concrete. By increasing the concrete compressive strength or increasing the thickness of cover concrete, the bonding strength of the coated steel bar and concrete is improved. The bonding strength between coated rebar and concrete decreases with the increase of rebar diameter. The numerical model of the coated steel bar and the concrete beam is established by ANSYS software. The bond-slip curves of coated steel bar and concrete obtained by simulation can well describe the curves measured by the experiment. This study lays a foundation for future research on simulating the reinforced concrete beam with coated rebar under accidental load.

Interface bond behavior of tensioned glass fiber-reinforced polymer (GFRP) tendons embedded in cemented soils

This paper presents, evaluates, and interprets element pullout tests conducted to investigate the interface bond behavior of Glass Fiber-Reinforced Polymer (GFRP) tendons embedded in cemented soils. Pullout tests were conducted in conjunction with unconfined compression tests to characterize the strength of the cemented soils in which GFRP tendons were embedded. The effects of water content, cement content, and curing time of the cemented soil on the interface bond strength were investigated. Bond-slip curves of the tested specimens were developed considering a multi-segment function defined by characteristic points of interface bond stress peaks and troughs. Two sets of bond-slip curves were identified based on behavior trends observed in the residual strength phase. The ultimate bond strength of the GFRP tendons in cemented soils was found to strongly correlate with the compressive strength of the cemented soil. Ultimately, the bond-slip behavior was modeled using the cemented soil’s unconfined compressive strength and the GFRP tendon’s rib spacing. The proposed bond-slip model was developed using correlations of bond strength and against the parameters governing the behavior of cemented soil.

Dynamic interfacial bond behaviour between basalt fiber reinforced polymer sheets and concrete

An experimental investigation on the dynamic interfacial bond behaviour between basalt fibre (BFRP) sheets and concrete under different loading speeds (i.e. 8.33E−6 m/s, 0.1 m/s, 1 m/s, 3 m/s, 5 m/s, and 8 m/s) by using single-lap shear tests was carried out in this study. Experimental results including bond strength, strain time histories, strain distributions in the bonding areas, interfacial fracture energy, and bond-slip curves are presented in this study. The test results show that the interface is strain rate dependent. The interfacial fracture energy, bond strength, and interfacial bond shear stress-slip are sensitive to strain rate. Empirical bond-slip model including strain rate effect are established for the predictions of bond properties between BFRP sheets and concrete under dynamic loading.

Analysis of influence factors on interfacial bond between BFRP bars and seawater sea-sand concrete

In this paper, the influences of parameters such as the bond length, surface textures of reinforcement, reinforcement type and stirrups restraint were considered. Pull-out failure, splitting failure and splitting-pullout failure modes were observed during the test. The slip at the free end always lagged behind the slip at the loading end and the bond-slip curve of ribbed basalt fiber reinforced polymer (BFRP) bars included the micro-slip stage, slip stage, descent stage, and residual stage. Reducing the bond length and using ribbed-sand coated bars were beneficial to improve the bond performance. Increasing the bond length from 2.5 d to 5 d reduced the bond strength by 49.2%. The application of ribbed-sand coated bars instead of plain bars increased the bond strength by 1202.3%. The difference in bond strength between steel bars, BFRP bars and glass fiber reinforced polymer (GFRP) bars was small and the bond strengths of the three were much greater than that of carbon fiber reinforced polymer (CFRP) bars. This was mainly attributed to the different rib forms of the bars. The application of stirrups increased the bond strength by 11.5%, which indicated that the stirrup restraints can improve the bond behavior to a certain extent. Besides, the analysis of the bond-slip curve based on the energy perspective was consistent with test results.

Bond of epoxy-coated reinforcement to seawater coral aggregate concrete

The bond between epoxy-coated reinforcement (ECR) and seawater coral aggregate concrete (SCAC) were studied by using a series of pull-out tests. Five different parameters were considered in this test: types of concrete (SCAC and ordinary concrete (OC)) and reinforcement (ECR and ordinary reinforcement), the ratio of bond length to reinforcement diameter (3, 5 and 8), and concrete strength (C25 and C35) and thickness of concrete cover. The failure mode, bond-slip curve and strength of specimens were analyzed. Typical failure mode of pull-out specimen was splitting failure, while the failure surface of SCAC specimen was different from that of OC specimen. Test results indicated that the bond strength of specimen with SCAC was high compared to specimen using OC even under the similar concrete strength. Seawater coral aggregate concrete-ECR (SCAC-ECR) bond strength decreased with increasing bond length and decreasing concrete strength and thickness of concrete cover. The bond-slip curve of SCAC-ECR was similar to that of ordinary concrete (OC)-ordinary reinforcement (OR). However, SCAC and ECR reduced the bond-slip curve curvature. The decline of curve was improved by using ECR. Finally, an analytical expression for bond stress-slip curves was proposed. The obtained results provide a basis for the application of reinforced SCAC.

Bond behavior between steel bars and carbon nanotube modified concrete

The effect of different carbon nanotube (CNT) dosage, concrete cover thickness, steel bar diameter on bond behavior between steel bars and CNT modified concrete was experimentally studied. Beam specimens were prepared and tested for better simulation of the stress state of concrete in the vicinity of steel bars in real structures. The test results indicated that the initial bond strength, defined at a slippage of 0.01 mm, and the peak bond stress can be increased by approximately 37.2% and 49.7% at a CNT dosage of 0.1 wt% and 0.15 wt%, respectively. It was also found that the peak bond stress increased nearly linearly with the ratio, c/d, of the concrete cover thickness to the reinforcing bar diameter, and for a given c/d ratio, varied with the CNT dosage synchronously with the uniaxial tensile strength of concrete. The bond slip corresponding to the peak bond stress and the two shape parameters of the ascending and descending branches of the bond slip curve remained nearly constant over different CNT dosages and c/d ratios. Based on the synchronousness between the peak bond stress and the tensile strength, a time dependent bond slip model for CNT modified early-age concrete was proposed. The generated knowledge can be used for cracking control of young concrete members.

Material and glass-fibre-reinforced polymer bond properties of geopolymer concrete

Geopolymers are recognised for their environmental benefits and excellent chloride and sulfate resistance. However, information on the material characteristics and bond behaviour of geopolymers based on fly ash (FA) and slag is still relatively scarce. This paper presents a comprehensive study on the integration of two modern construction materials – geopolymer concrete (GPC) based on FA and ground granulated blast-furnace slag and glass-fibre-reinforced polymer (GFRP) bars. The aim of the study was to assist concrete structural design by identifying the key preparation parameters such as hardening and bond behaviour. It was found that the hardening of the GPC was much faster than that of ordinary Portland cement (OPC) concrete at an early age and the stress–strain curves, elastic moduli and splitting tensile strengths of the GPC were accurately predicted by existing models developed for OPC concrete. The GFRP bars in the GPC outperformed those in the OPC concrete in terms of normalised bond strength. The GPC reinforced with both GFRP and steel rebars exhibited similar bond–slip curves to the OPC concrete. Overall, the behaviour of the GFRP-reinforced GPC was found to be analogous to that of the OPC concrete while providing adequate compressive, tensile and bond strength for the construction of foundations of low-rise buildings.

On mitigating rebar–concrete interface damages due to the pre-cracking phenomena using superabsorbent polymers

This study intends to determine the effects of incorporating superabsorbent polymers (SAP) within concrete on the bond properties of steel reinforcing bars (rebar) embedded in uncracked and pre-cracked concrete. An experimental program is conducted to check the performance of SAP, as a healing agent inside the concrete, in mitigating internal damage at the rebar-concrete interface due to the pre-cracking phenomena. Two types of SAP with different particle sizes (0.15 and 0.50 mm) and chemistries are considered in the experimental program. Pull-out test results show improved bond properties of steel rebars embedded in uncracked and healed concrete containing lower dosages of SAP. However, concrete containing a high dosage of SAP shows lower bond strength, compared to normal concrete, due to the presence of macro voids. A considerable healing effect is observed for the initial bond-slip curve portion, the bond strength, and the energy absorbed by the bond mechanism, within the cracks of pre-cracked SAP-modified concrete subjected to wet-dry cycles. This study shows that SAP can significantly increase the autogenous healing performance of concrete at rebar-concrete interfacial damage sites.

Bond behavior of high-strength recycled aggregate concrete-filled large square steel tubes with different connectors

To solve the problem of insufficient interface bond strength of concrete-filled square steel tubes (CFSSTs), 16 large CFSST specimens with eight different types of connectors (no additional connectors, built-in studs, built-in circular ribs, built-in vertical ribs, built-in reinforcement cage, and various combinations of the above) were designed for push-out testing. Specimens were constructed using high-strength concrete or high-strength recycled aggregate concrete (RAC). The bond behavior and slip failure mechanism were analyzed by comparing the force-slip curves, slip displacements, and steel tube strains. The bond strength calculation method and bond-slip constitutive relationship of the interface under multiple parameters were obtained. Experiments revealed the following: bond-slip curves for CFSST without additional connectors were composed of an ascending section and a residual section; for tubes with different connectors, the peak values of the curves were higher; the performance-price ratio of studs and circular ribs for improving bond strength was high, and the combined use of studs and circular ribs can work well; the combination of circular ribs and vertical ribs can significantly improve bond strength and energy dissipation capacity; RAC had better bond behavior than normal concrete in HSCFSST without additional connectors or built-in vertical ribs. The predicted force-slip curves agreed well with the test curves, providing a theoretical basis for CFSST applications.

Bond performance of Steel-CFRP bar reinforced coral concrete beams

Steel-Carbon Fiber Reinforced Polymer (SCFRP) bar provides a new way to solve the shortcomings of poor ductility and low stiffness of FRP reinforced concrete members. In order to understand the bond performance between SCFRP bars and coral concrete, 39 pull-out tests of SCFRP reinforced coral concrete is performed. Different parameters such as the types, diameters and bond lengths of SCFRP reinforcements were investigated. The failure modes, failure mechanism, stress process and bond-slip relationship of the specimens were analyzed. The failure modes of steel bar pull-out can be divided into steel core pull-out before yielding and steel core pull-out after yielding. The concrete splitting failure modes can be divided into coral concrete splitting before steel core yielding and Coral concrete splits after steel core yielding. There are four characteristic points in the bond slip curve: elastic slip point, yield slip point, peak slip point and residual slip point. The bond slip model of SCFRP reinforced coral concrete was developed in this paper using coefficients of slip before steel core yields and slip after steel core yields are 0.86 and 1.0 respectively, which can describe the bond slip behavior between SCFRP and concrete more accurately.

Effect of functionalized nano-SiO2 addition on bond behavior of adhesively bonded CFRP-steel double-lap joint

The mechanical properties of adhesives significantly influence the bond behavior of steel structures strengthened by carbon fiber-reinforced polymer (CFRP) composites. In order to improve the toughness of the adhesive and the ductile behavior of adhesively bonded CRFP-steel joint, functionalized nano-sized silica (hereafter refer as nano-SiO2) toughening agent were added into the adhesive. Firstly, the effects of functionalized nano-SiO2 toughening agent on the mechanical properties of the adhesives after curing at room temperature were tested and analyzed. Then, a total of 19 adhesively bonded CFRP-steel double-lap joints were fabricated and tested under shear loading to investigate the effect of functionalized nano-SiO2 on the bonding behaviors, including the failure modes, bond–slip relationships, bond strength and so on. The test results indicate that tensile stress–strain relationship of the adhesive changes from linear to nonlinear with the increase of functionalized nano-SiO2 toughening agent, the elongation at break, strain energy and shear strength of functionalized nano-SiO2-toughened adhesive (ZR14-weight ratio of 1% functionalized nano-SiO2) was improved by 202.88%, 292.10% and 133.12% respectively in comparison to the adhesive without functionalized nano- SiO2 toughening agent. Moreover, the tensile strength, elongation at break, and strain energy of functionalized nano-SiO2-toughened adhesive ZR14 are much higher than those of commercial adhesives (e.g. Sika30, 330) without Nano-SiO2. In addition, the failure mode of CFRP-steel joints changed from interface failure to CFRP plate delamination, when the weight ratio of functionalized nano-SiO2 to adhesive increasing from 0 to 1%. The bond-slip curves could be simplified into a bilinear model (triangle-shape) for CFRP-steel joints without and with only weight ratio of 0.5% functionalized nano-SiO2, however, the bond-slip curves change to simplified trilinear model (trapezoidal shape) for the joints with 1% functionalized nano-SiO2 and the toughness of bonding interface is significantly improved. In all, the CFRP-steel lap joints bonded by adhesive with appropriate amount functionalized nano-SiO2 (i.e., wt. of 1%) shows superior interfacial performance.

Bond behavior between reinforcement and ultra-high toughness cementitious composite in flexural members

Bonding between reinforcement and ultra-high toughness cementitious composite (UHTCC) is of great significance for reinforced UHTCC members. In this study, bond tests were carried out on short and slender UHTCC beams to simulate bond conditions in flexural members. The experimental results indicated that, differently from the brittle failure seen in concrete specimens, a ductile pull-out failure of reinforcement was obtained for all the UHTCC specimens. The descending branches of the bond-slip curves were correlated with the shear-span ratios and cover-depth to rebar-diameter ratios of specimens. The effect of shear-span ratios on bond strength for UHTCC beams was far less than that for concrete beams. Slender UHTCC beams had greater bond strength than short beams. Based on test data from different types of specimens in this study and previous studies, the effect of cover-depth to rebar-diameter ratios, bond-length to rebar-diameter ratios, and bond test methods was analyzed, and a bond strength model was proposed which includes the effect of these parameters.

Bond-slip behavior of large high-strength concrete-filled circular steel tubes with different constructions

To investigate the bond-slip behavior of the interface in large high-strength concrete-filled circular steel tubes (HSCFCSTs), 18 specimens with 6 different constructions (HSCFCSTs with no additional construction, with built-in studs, with built-in circular ribs, with built-in vertical ribs, and with various combinations of the above) were examined using push-out tests. Specimens were constructed from high-strength concrete or recycled aggregate concrete (RAC). The bond-slip behavior and slip-failure mechanisms were analyzed through comparison of load-slip curves, slip displacements, and steel tube strain to generate predicted load-slip curves based on multiple parameters. Experiments revealed that the bond-slip curves of HSCFCSTs with no additional constructions contained an ascending section, a rapidly descending section, and a residual section. For tubes with special constructions, the descending section was either postponed or not visible, the interfacial bonding properties were enhanced when using special constructions, the concrete strength and bond strength were positively correlated, and the RAC had better bonding behavior than normal concrete. The predicted load-slip curves agreed with the test curves and provided a theoretical basis for HSCFCST applications.

A numerical investigation of the cracking behaviour of reinforced-concrete tie elements

The cracking behaviours of reinforced-concrete (RC) ties are investigated by conducting virtual experiments using non-linear finite-element analysis. The assumptions in the model are verified by benchmarking the classical experiments of B. Bresler and V. V. Bertero as conducted in 1968 and P. J. Yannopoulos, conducted in 1989, which shows good agreement in the comparison of steel strains, development of crack widths and crack spacing. Furthermore, virtual experiments on four different RC ties show that the size of the cover and not the bar diameter governs the crack spacing and thus implicitly the crack width. An increase of the bar diameter has a beneficial effect in reducing the steel stress and the associated steel strains, which in turn reduces the crack width. Finally, a single bond–slip curve is sufficient in describing the average bond transfer of an arbitrary RC tie.

The bonding strength of GFRP bars embedded within concrete under direct pull-out test

Most of the reinforced concrete (RC) structures in the marine environment attacked with corrosion of steel reinforcements that required costly repairs or replacements. The utilization of FRP bars as a non-corrosion material is one of the solutions to increase the durability of RC structures in the marine environment. The innovation of using FRP bars should be developed to improve the quality and reduce the construction cost such as by removing the concrete cover and shear rebar. By removing the concrete cover, the depth of the beam can be minimized without reducing the effective depth of the beam. To substitute the conventional shear rebar, the GFRP sheet strip will be attached to the shear span after concrete hardened. However, the problem is that the bonding area between FRP bars and concrete reduced due to the removal of concrete cover. Therefore, before applying this innovation to the beams’ structure, the pull-out test should be conducted in order to clarify this behavior. The influence of the location of FRP bars and the effect of the U-Wrap GFRP sheet on the bond-slip curves will be analyzed. It is expected from this study that the bonding strength between FRP bars and concrete does not reduce significantly and hence, the innovation for constructing RC beams without a concrete cover on the tensile zone can be applied.

Bond Behavior between Steel Rebar and RCA Concrete after Exposure to Elevated Temperatures

To explore the bond behavior between steel rebar and recycled coarse aggregate (RCA) concrete after exposure to elevated temperatures, an experimental study was carried out. The results demonstrated that the bond strength of RCA concrete pullout specimens decreased greatly with increasing temperature. As the exposure temperature elevated, the slope of the ascending portion of the bond‐slip curves gradually declined, and the descending portion of the curves tended to flatten. A modified model was developed to predict the bond strength between RCA concrete and steel rebar after exposure to elevated temperature, and the predicted results showed a very good fit in the experimental test results. Besides, the proposed bond‐slip relations for steel rebar in RCA concrete after elevated temperatures showed satisfactory agreement with test results.

Measurement and Correction of Interface Shear Stiffness between Slab and Base for Concrete Pavement

Interface parameters could affect the adhesion and durability performance of cement concrete pavements. In order to improve the pavement performance, the interface shear stiffness between the slab and the base of concrete pavement were investigated accurately. A horizontal shear tester was developed which could simulate the vertical pressure. The bond-slip curve of the specimens that the interface treated by three different measures was determined, such as emulsified asphalt, geotextile and asphalt concrete. Then, the correction of interface vertical pressure was made for average shear stiffness based on simple iterative method, and the equivalent shear stiffness of three types of specimens was recommended. The results showed that the specimen treated by emulsified asphalt required the maximum shear force, and asphalt concrete was minimum during the static friction stage. The contact states between slab and base are significantly correlated with the interface treatments. So, the interface treatments selection needed to find a balance between flexural tensile stress and thermal stress of the slab. In the sliding friction stage, the shear resistances of three types of interface treatment methods were significantly weakened and the average shear stiffness declined dramatically. It is very different for interface shear stiffness when the pavement structure in different using stages. The interface parameter must consistent with the using state of the pavement during the analysis process of the pavement structure.

Experimental Studies on Bond Performance of BFRP Bars Reinforced Coral Aggregate Concrete

Basalt fiber reinforced polymer (BFRP) rebars reinforced coral aggregate concrete is a new type of concrete used in ocean engineering. In order to investigate the bond performance between BFRP rebars and coral concrete, 30 pull-out tests were carried out in 10 groups with different diameters of BFRP rebars, bonding lengths and strength of the coral concrete. The results show that good bonding between BFRP rebars and coral concrete were achieved. The main failure modes can be categorized as BFRP rebars pull out destruction, splitting failure of coral concrete and BFRP rebars fracture. The bond slip (tau{text{-}}s) curves of the BFRP rebars and coral concrete were obtained during the tests. It was found to be similar to the common concrete using fiber reinforced polymer (FRP) bars. The bond-slip relation can be roughly divided into micro-slip phase, slip phase, decline phase, and the residual stress stage. The bond between BFRP rebars and coral concrete increases with the increase of the bond length and diameter of BFRP rebars, but the average bond stress will decrease. Moreover, increasing the strength of coral concrete is effective to improve the bond performance of BFRP rebars. In this paper, the continuous bond slip model (Gao et al. in J Zhengzhou Univ 23:1–5, 2002) was used to represent the tau{text{-}}s constitutive relationship of BFRP rebars and coral concrete. The analysis show that the proposed model has a high degree of accuracy in representing tau{text{-}}s curve of BFRP rebars and coral concrete.