Bond Behavior Research Articles

Double shooting method for FRCM reinforced systems in debonding problems

The debonding process in FRCM (Fiber Reinforced Cementitious Matrix) strengthened brittle substrates, such as concrete and masonry, is characterized by complex failure modes that include debonding at both the external and internal interfaces of the composite, as well as damage to the mortar matrix. This study addresses this problem classically as a mode-II fracture process by accounting for the one-dimensional non-linear interfacial shear stress-slip relationship combined with matrix axial non-linearity. The approach employed considers the longitudinal displacements of both the inner and outer layers of mortar, along with that of the fiber textile, as unknowns, and analytically solves the second-order ordinary differential equation (ODE) system that governs the debonding problem. Then, the boundary value problem (BVP) is transformed into an initial value problem (IVP) and a double shooting method is proposed to find displacements and stresses along the bonded length in the presence of non-linearity. The appropriate initial values of matrix layer displacements, satisfying the required boundary conditions, are efficiently determined through a two-dimensional bisection method working on rectangular and triangular patches. The softening behavior of both mortar layers and interfaces between mortar and fiber are taken into account using jagged constitutive relationships to enhance convergence. The numerical calculation speed, robustness, and key simulation parameters are thoroughly discussed. Moreover, the proposed numerical approach is compared with existing experimental data and models, on both concrete and masonry FRCM-reinforced specimens. The influence of mortar failure and interfacial strength is also explored. The model demonstrates its ability to effectively reproduce the global bond behavior observed in experimental studies, while capturing the local behavior and simulating various failure modes observed in the tests.

Study on bond behaviour of corroded reinforced concrete beam – finite element analysis

Study on bond behaviour of corroded reinforced concrete beam – finite element analysis

Fatigue and post-fatigue behavior of FRCM-concrete specimens under direct shear and bending conditions

The combined effect of cyclic loading and steel rebar corrosion can determine the premature deterioration of reinforced concrete (RC) structures subjected to dynamic and environmental actions. The use of fabric-reinforced cementitious matrix composites (FRCM) is a promising solution to extend the fatigue life of RC structures. Externally bonded FRCMs are applied to the tension side of structural members to reduce the rebar stress level, thus delaying fatigue crack nucleation and/or propagation. A key aspect for the design of FRCM strengthening of these structures is the matrix-fiber bond behavior under fatigue loading. In this paper, a critical review of currently available studies on the fatigue behavior of FRCM-strengthened RC beams is presented. Then, the results of 25 bond tests on PBO FRCM-concrete specimens are provided and discussed. These tests include both single-lap direct shear and modified beam tests performed in quasi-static and cyclic mode. The results obtained show that the cyclic load may induce progressive debonding at the matrix-fiber interface with rupture of fiber filaments. Modified beam tests are more affected by these phenomena than direct shear tests. The rupture of fiber filaments is confirmed by the results of post-fatigue tests that show capacities lower than those of corresponding quasi-static tests.

Bond shear modulus in reinforced concrete at high temperature: A design‐oriented approach

AbstractModeling bond behavior in either ordinary or high‐temperature conditions requires the knowledge of bond shear modulus—called also slip modulus or simply bond stiffness—that has received so far scanty attention because of the greater interest for bond as a guarantee of equilibrium at the Ultimate Limit State (and in fire conditions) than as a means to guarantee both equilibrium and compatibility at the Serviceability Limit State (and in fire/post‐fire conditions). The limited knowledge of bond shear modulus makes it difficult to numerically model such phenomena as tension stiffening, that controls the structural behavior in both ordinary and fire conditions. The general trends identified by examining eleven experimental campaigns with anchored bars covering 27 different cases and temperatures ranging between 20 and 800°C are the starting point of the design‐oriented laws proposed in this study about bond shear modulus as a function of concrete residual strength and temperature. A simple shear‐lag model is introduced for bond shear modulus at room temperature, as its evaluation from test data is no simple matter due to initial chemical adhesion and different test procedures. Bond shear modulus is shown to be a decreasing function of concrete residual compressive strength and of the maximum temperature reached by the bar‐concrete system. Design charts are proposed to allow the designer to identify the value of the bond stiffness on the basis of the max. temperature, of concrete residual strength and of bar diameter, making it possible to realistically model tension stiffening in fire‐damaged RC structures.

A multiscale experimental study of CFRP-UHPFRC interfacial bond behaviour: Failure mechanisms and a fibre volume dependent bond-slip law

Carbon fiber reinforced polymer (CFRP) materials are being increasingly used with ultra high performance fiber reinforced concrete (UHPFRC) to construct composite structural members making use of their unique properties. As the interfacial bond behaviour between CFRP and UHPFRC plays a key role in the design of such members, it was investigated in detail by multiscale experiments of single shear pull-off tests of 26 CFRP-UHPFRC bonded joints with different bond lengths and widths of CFRP laminates and 0.5–4% volume fraction (Vsf) of straight high-strength steel fibers (length = 13 mm, diameter = 0.2 mm). 12 CFRP-normal strength concrete (NSC) bonded joints were also tested for comparison. The surface strain evolution and the interfacial debonding process were captured and analysed by the digital image correlation (DIC). The complex microscale failure mechanisms, such as the mortar cracking, the CFRP-UHPFRC interfacial debonding, and the steel fibers’ slipping and pulling out of the mortar, were visualised and analysed by the 3D micro X-ray Computed Tomography (μXCT) scanning. It was found that the debonded layer of mortar under the laminates was much thinner than that in the CFRP-NSC bonded joints, due to the lack of coarse aggregates in UHPFRC. The peak pulling forces for the specimens with Vsf = 0.5%, 1%, 2%, and 4% were 9%, 21%, 28%, and 29% higher than those without steel fibers, respectively. An empirical bond strength model and an interfacial bond-slip model were proposed based on the test results, accounting for Vsf, the material properties, and the laminate to UHPFRC width ratio. Finally, the proposed bond-slip model was applied in finite element modelling to simulate the flexural behaviour of an FRP-UHPFRC composite deck in the literature, with the interfacial debonding failure and the ultimate loads successfully predicted.

Compressive and Bonding Performance of GFRP-Reinforced Concrete Columns

The use of glass-fiber-reinforced polymer (GFRP) bars as an alternative to steel bars for reinforcing concrete (RC) structures has gained increasing attention in recent years. GFRP bars offer several advantages over steel bars, such as corrosion resistance, lightweight, high tensile strength, and non-magnetic properties. However, there are also some challenges and uncertainties associated with the behavior and performance of GFRP-reinforced concrete (GFRP-RC) structures, especially under compression and bonding behavior. Therefore, there is a need for comprehensive experimental investigations to validate the effectiveness of GFRP bars in concrete columns. This paper presents a study that aims to address these issues by conducting experimental tests on GFRP-RC columns. The experimental tests examine the mechanical properties of GFRP bars and their bond behavior with concrete, as well as the axial compressive behavior of GFRP-RC columns with different reinforcement configurations, tie spacing, and bar sizes. The findings reveal that GFRP bars demonstrate a comparable, if not superior, compressive capacity to traditional steel bars, significantly contributing to the load-bearing capacity of concrete columns. The study concludes with a set of recommendations for further exploration, underscoring the potential of GFRP bars in revolutionizing the construction industry.

Experimental investigation of dynamic bond behaviors between LRS-FRP and concrete

The dynamic bonding behaviour at the interface between a large-rupture-strain fibre-reinforced polymer (LRS-FRP) and concrete was investigated via drop weight impact tests of notched beam specimens. The main experimental variables included the concrete strength grade, number of LRS-FRP layers, and loading rate. With increasing loading rate, the location of the peeling damage at the interface between the LRS-FRP and concrete shifted from the interior of the concrete layer to the concrete–binder layer interface, binder layer, and even binder layer–FRP interface. The ultimate debonding load of the LRS-FRP strips increased with increasing strain rate, reaching more than 1.7 times the static value at a strain rate of approximately 4/s, while the effective bond length did not show a clear increasing or decreasing tendency with increasing strain rate. For the local bond-slip relationship, both the peak interfacial bond stress and interfacial fracture energy increased with increasing strain rate, and the influences of the concrete strength and number of FRP layers on the strain rate sensitivity were insignificant. Based on the experimental results, improved models to predict the ultimate debonding load and effective bond length at the LRS-FRP–concrete interface under both static and dynamic loading conditions were proposed. Furthermore, dynamic increase factor (DIF) formulas for the peak interfacial bond stress and interfacial fracture energy for bond-slip modelling of the interface between a polyethylene naphthalate (PEN)-FRP and concrete were established.

Structural behavior of concrete slabs made of seawater sea sand and reinforced with GFRP reinforcement under flexural loading

Seawater sea sand concrete (SSC) structures reinforced with glass fibre-reinforced polymer (GFRP) reinforcing bars offer a promising alternative to conventional reinforced concrete, especially in the face of depleting clean water and river sand. However, comprehensive studies on the flexural performance and cracking behaviour of SSC structures reinforced with GFRP reinforcing bars are currently limited. This study delves into the influence of longitudinal reinforcement ratio on the flexural and cracking behaviour of SSC slabs reinforced with GFRP reinforcing bars under quasi-static loads, as well as the bond behaviour of GFRP reinforcing bar and SSC. The experimental program included 9 large-scale slabs and 30 pull-out specimens, revealing that the longitudinal reinforcement ratio significantly affects the flexural performance, particularly at higher load levels. Increased reinforcement ratio improved flexural stiffness, reduced deflection and crack width, enhancing load-carrying capacity and energy absorption. Although bond strength decreased at the serviceability load (up to 47%), a marginal reduction was observed at the ultimate state. The study proposes a semi-empirical equation for the effective moment of inertia of SSC slabs, accounting for the influence of reinforcement ratio on cracking moment and reduced bond strength of GFRP reinforcing bars. This equation aligns well with experimental results, demonstrating low error and coefficient of variation.

Additively manufactured steel reinforcement for small scale reinforced concrete modeling: Tensile and bond behavior

Small scale (∼1:30–1:40) Reinforced Concrete is useful for centrifuge testing. However, manufacturing the reinforcing cages by hand at this scale is practically difficult. This paper suggests that small scale reinforcement can be manufactured using a metal 3D printer. Mechanical properties of 3D printed submillimeter rebars are discussed and compared to properties of typical prototype rebars. Different model concrete mix designs are tested to identify optimal mixes. Pullout tests of rebars with different surface rib configurations embedded in different concrete mixes are discussed.Based on the test results, by modulating the printing parameters it seems feasible to obtain 3D printed submillimeter bars that can be used as physical models of prototype rebars. A gypsum-based model concrete was more similar to prototype concrete than cement-based mixes. Most importantly, bond slip behavior that is comparable to full-scale concrete could be achieved, something that is vital and has never been reported before.

Bond behavior of CFRP reinforcement systems with concrete under static and fatigue loading

In this research, the single lap shear tests were used to investigate the bond behavior of carbon fiber reinforced polymer (CFRP) sheets bonded to concrete blocks under static and fatigue loads. The magnitude of the fatigue load was considered as a parameter that varied from a minimum load of 10% of Pu to a maximum load of 60%, 70% and 80% of the ultimate static load. For both loading scenarios, the applied load versus slips curves and the profiles of the interface zone strain were presented. In addition, comparisons, and analyses of the failure mode of samples subjected to static and fatigue loading are presented. The experimental results demonstrate that the loading level has a significant impact on the failure mode, fatigue life and stiffness degradation of CFRP-concrete joints during the fatigue loading.Finally, a prediction model for the fatigue life of the CFRP-concrete joint was developed as part of this investigation.

Bond Behavior of Glass Fine Aggregate Reinforcement Concrete after Chloride Erosion under Deicing Salt

This paper reports on the bond behavior of glass fine aggregate reinforced concrete (GFARC) under chloride erosion, considering the chloride solution and glass fine aggregate (GFA) exchange rates as variable parameters. The 16 groups of specimens are designed to conduct central pull-out tests after chloride erosion. The experimental results are analyzed, such as the τ–s curve, ultimate bond strength, peak slip, and bond stiffness. The results indicated that the degree of reinforcement corrosion in GFARC is low under the action of chloride corrosion. Compared with natural aggregate-reinforced concrete (NARC), the ultimate bond strength and bond stiffness of GFARC improve under the same chloride corrosion. The ultimate bond strengths of 25% GFARC, 50% GFARC, and 75% GFARC increased by 7%, 7.85%, and 17.31%, respectively, under natural conditions. Under 3.5% chlorine erosion, the GFARC group increased by 4.67%, 4.83%, and 13.53%, respectively. Under 5% chlorine erosion, the GFARC group increased by 5.54%, 6.24%, and 12.64%, respectively. Glass fine can improve the bonding performance between concrete and steel bars, and its effect is related to the replacement rate. The shape and chemical characteristics of glass sand play an important role in this process and became more prominent with the deepening of the effect. Through the analysis of the experimental results, this paper further elaborated on the bonding mechanism of GFARC under the influence of chloride corrosion. The research indicates that the use of GFA has a great advantage in improving the bond performance under chloride erosion.

Structural performance of RC beams strengthened with hybrid bonded CFRP

Debonding at the interface between externally-bonded (EB) carbon fiber-reinforced polymer (CFRP) composites and the concrete limits the performance of CFRP-strengthened reinforced concrete (RC) beams. To mitigate the interface debonding and improve the material utilizing efficiency of CFRP, a hybrid-bonding (HB) technique was developed. HB CFRP system combines the adhesive bonding and mechanical fastening (i.e., anchoring the CFRP plate through bolted cover steel plate bonded on the surface of the CFRP plate). When interfacial debonding occurs, the friction between the steel cover plate and the CFRP plate and the reacted normal stress at the interface enhanced d the concrete aggregates interlocking and the bond strength of CFRP-to-concrete interface. Additionally, the normal stresses can be also activated through pre-tightening the anchors. Five HB CFRP-strengthened RC beams, two conventional EB CFRP-strengthened RC beams, and two un-strengthened RC beams were tested. Two different steel cover plates lengths and three different pre-tightening torques were adopted. The test results demonstrated that the HB technique improved the local bond behavior in the anchored regions, resulting in higher strength utilization efficiency of CFRP and increased load-carrying capacity of the strengthened beams. Such improvement was more significant when longer steel cover plates and higher pre-tightening torques were deployed.

A dual dynamically cross-linked hydrogel promotes rheumatoid arthritis repair through ROS initiative regulation and microenvironment modulation-independent triptolide release

High oxidative stress and inflammatory cell infiltration are major causes of the persistent bone erosion and difficult tissue regeneration in rheumatoid arthritis (RA). Triptolide (TPL) has become a highly anticipated anti-rheumatic drug due to its excellent immunomodulatory and anti-inflammatory effects. However, the sudden drug accumulation caused by the binding of “stimulus-response” and “drug release” in a general smart delivery system is difficult to meet the shortcoming of extreme toxicity and the demand for long-term administration of TPL. Herein, we developed a dual dynamically cross-linked hydrogel (SPT@TPL), which demonstrated sensitive RA microenvironment regulation and microenvironment modulation-independent TPL release for 30 days. The abundant borate ester/tea polyphenol units in SPT@TPL possessed the capability to respond and regulate high reactive oxygen species (ROS) levels on-demand. Meanwhile, based on its dense dual crosslinked structure as well as the spontaneous healing behavior of numerous intermolecular hydrogen bonds formed after the breakage of borate ester, TPL could remain stable and slowly release under high ROS environments of RA, which dramatically reduced the risk of TPL exerting toxicity while maximized its long-term efficacy. Through the dual effects of ROS regulation and TPL sustained-release, SPT@TPL alleviated oxidative stress and reprogrammed macrophages into M2 phenotype, showing marked inhibition of inflammation and optimal regeneration of articular cartilage in RA rat model. In conclusion, this hydrogel platform with both microenvironment initiative regulation and TPL long-term sustained release provides a potential scheme for rheumatoid arthritis.

Investigating the effect of steel micro‐ and macro‐fibers on the bond behavior of steel rebar embedded in ultra‐high performance concrete

AbstractIn recent years, the use of ultra‐high performance concrete (UHPC) has been widely considered in special structures and retrofitting of existing structures. Since most of the relations proposed by the regulations are based on the behavior of concrete with normal strength concrete, it is necessary to conduct more experimental studies to predict the behavior of UHPC. One of these notable behaviors is the bond and cohesion between rebar and UHPC, which plays a critical role in the design and seismic behavior of structures. In the present study, a series of experimental tests of uniaxial compressive strength, splitting (Brazilian) tensile strength, elastic modulus, and pull‐out of ribbed steel bars from ultra‐high performance fiber‐reinforced concrete (UHPFRC) were performed. The tests were done considering the four‐volume fractions of hooked‐end steel macro‐fibers, straight steel micro‐fibers, a hybrid combination of these two fibers, three different diameters of ribbed steel bars, and several embedded lengths. Moreover, direct tensile and compressive stress–strain curves, Young's modulus, bonding properties of rebar with UHPFRC, and fracture mechanisms were considered. The results indicated that applying micro‐fibers may enhance the bond stress between rebar and UHPC better than macro‐fibers. Also applying hybrid fibers resulted in the concurrent enhancement of bond stress and the optimum point slip of UHPC in the stress–slip diagram. This finding maybe attributed to the satisfactory performance of micro‐fibers in reducing the growth speed of micro‐level cracks and the suitable effect of macro‐fibers in decreasing the progression speed of macro‐level cracks. Furthermore, it was revealed that the increase in rebar diameter and embedding length leads to the reduction of maximum bond stress in UHPFRC.

Simplified Procedure to Determine the Cohesive Material Law of Fiber-Reinforced Cementitious Matrix (FRCM)-Substrate Joints.

Fiber-reinforced cementitious matrix (FRCM) composites have been largely used to strengthen existing concrete and masonry structures in the last decade. To design FRCM-strengthened members, the provisions of the Italian CNR-DT 215 (2018) or the American ACI 549.4R and 6R (2020) guidelines can be adopted. According to the former, the FRCM effective strain, i.e., the composite strain associated with the loss of composite action, can be obtained by combining the results of direct shear tests on FRCM-substrate joints and of tensile tests on the bare reinforcing textile. According to the latter, the effective strain can be obtained by testing FRCM coupons in tension, using the so-called clevis-grip test set-up. However, the complex bond behavior of the FRCM cannot be fully captured by considering only the effective strain. Thus, a cohesive approach has been used to describe the stress transfer between the composite and the substrate and cohesive material laws (CMLs) with different shapes have been proposed. The determination of the CML associated with a specific FRCM-substrate joint is fundamental to capture the behavior of the FRCM-strengthened member and should be determined based on the results of experimental bond tests. In this paper, a procedure previously proposed by the authors to calibrate the CML from the load response obtained by direct shear tests of FRCM-substrate joints is applied to different FRCM composites. Namely, carbon, AR glass, and PBO FRCMs are considered. The results obtained prove that the procedure allows to estimate the CML and to associate the idealized load response of a specific type of FRCM to the corresponding CML. The estimated CML can be used to determine the onset of debonding in FRCM-substrate joints, the crack number and spacing in FRCM coupons, and the locations where debonding occurs in FRCM-strengthened members.

Numerical Study of GFRP-reinforced Concrete Beams with Straight and Hooked-end Bar Lap Splices

Glass fibre-reinforced polymer (GFRP) has been increasingly used as the main reinforcement in concrete structures due to its durability and resistance to corrosion. However, there is a lack of data regarding the bond behaviour of GFRP bars with 180-degree hooked ends used in flexure concrete elements. To investigate the effects of the hooked ends of GFRP bars on the bond strength with concrete, beams reinforced with GFRP bars with straight and hooked-end lap splices at the midspan were modelled using ABAQUS/Standard software. The finite element models were validated using the experimental results of four full-size concrete beams. Two beam specimens had straight-end bars, while the other two had hooked-end bars. Sensitivity analysis was performed to find the proper values for model verification, such as dilation angle and mesh density. According to the outcomes of the parametric study conducted in this research, changing the length of the bar lap splices in the range of 15 to 40 times the bar diameter had a negligible effect on the beam’s overall behaviour. Increasing the number of reinforcing bars from 3 to 5 increased the beam flexural strength by about 33%, whereas increasing the diameter of the bars from 10 to 20 mm doubled the beam strength.

Bond behavior of the interface between concrete and basalt fiber reinforced polymer bar after freeze-thaw cycles

Bond behavior of the interface between concrete and basalt fiber reinforced polymer bar after freeze-thaw cycles

Study on the bond performance of steel bars in early-age concrete considering the effect of circumferentially lateral pressures

The interface performance between steel bars and concrete is the fundamental necessary parameter in the bearing capacity evaluation of reinforced concrete structures, especially for those during construction in some developing areas where the plain bars were still adopted as the reinforcing materials. The age-correlation of the chemical adhesive force and frictional resistance at the plain bars-concrete interface needs to be investigated in detail but has not yet been deeply understood. Therefore, a systematic study including both the experimental and theoretical investigations on the bond behavior of plain steel bars in early-age concrete with lateral pressures was performed on the pullout specimens, with consideration of the influencing parameters including the bar diameters, the concrete strengths, and the ages. On the base of the experimental results, the calculation models for predicting the bond parameters of plain bars in early-age concrete under lateral pressures were derived and verified with the test results, and then a suited local bond-slip model was presented. Finally, a comparison of enhanced early-age bond strength caused by lateral pressure obtained from the experimental and the theoretical approaches was provided with reasonable agreement.

Experimental and finite element study of bond behavior between seawater sea-sand alkali activated concrete and FRP bars

The bond performance of fiber reinforced plastic (FRP) bars in seawater sea sand alkali activated concrete (SSASC) was investigated using pull-out test specimens. The effect of different concrete strengths, anchorage lengths, FRP bar types, and diameters on the bond performance were investiaged. Special emphasis was given to the applicability of the existing bond-slip constitutive models to FRP reinforced SSASC, which was investigated by calibrating well-known models to the experimental results. The correlation between the experimental curve and different bond-slip constitutive models is analyzed by calculating the R2 value. To verify the applicability of finite element analysis to SSASC and FRP bars system, numerical simulation was performed in ABAQUS finite element simulation software by setting the same conditions as the experimental variables. The results indicate that the Fan Xiaochun’s model, which is the modifications of the MBPE model, has a high degree of correlation and is more suitable for SSASC and FRP bars system. The simulation results using ABAQUS finite element software are in good agreement with the experimental results.

Temperature-Controlled Molecular Bonding Hysteresis: Interphase Dynamics of a Nanoparticle-Modified Polymer Network.

This study demonstrates the existence of temperature-induced molecular bonding hysteresis at nanoparticle-polymer interfaces in a highly cross-linked epoxy-based polymer, modified with core-shell rubber nanoparticles. This thermally induced bond hysteresis manifests itself in a hysteresis-like change of the strength of the electrical bond polarization between epoxy molecules and surface molecules of the core-shell nanoparticles. This kind of dynamic bond behavior can be controllably switched from one bond state to the other by a sufficient temperature change. The related optical remanence is evidenced by a refractive index hysteresis independent of the temperature change using the new experimental technique of temperature-modulated optical refractometry (TMOR). From the investigation of quasi-static and dynamic thermal expansion separately, TMOR allows for the conclusion that the observed hysteresis is caused by the specific refractivity and not the dipole number density.