Externally Bonded Reinforcement Research Articles

Enhancing Flexural Strength of RC Beams with Different Steel–Glass Fiber-Reinforced Polymer Composite Laminate Configurations: Experimental and Analytical Approach

This study intended to measure the efficiency of different strengthening techniques to advance the flexural characteristics of reinforced concrete (RC) beams using glass fiber-reinforced polymer (GFRP) laminates, including externally bonded reinforcement (EBR), externally bonded reinforcement on grooves (EBROG), externally bonded reinforcement in grooves (EBRIG), and the near-surface mounted (NSM) system. A new NSM technique was also established using an anchorage rebar. Then, the effect of the NSM method with and without externally strengthening GFRP laminates was studied. Twelve RC beams (150 × 200 × 1500 mm) were manufactured and examined under a bending system. One specimen was designated as the control with no GFRP laminate. To perform the NSM method, both steel and GFRP rebars were used. In the experiments, capability, as well as the deformation and ductileness of specimens, were evaluated, and a comparison was made between the experimental consequences and existing standards. Finally, a new regression was generated to predict the final resistance of RC beams bound with various retrofitting techniques. The findings exhibited that the NSM technique, besides preserving the strengthening materials, could enhance the load-bearing capacity and ductileness of RC beams up to 42.3% more than the EBR, EBROG, and EBRIG performances.

Flexural strengthening of RC beams by advanced concrete canvas

AbstractIn this study, concrete canvas (CC) layers were used as a novel method to strengthen reinforced concrete (RC) beams because of their flexibility, rapid installation, fast curing, and rapid strengthening. Due to the importance of the CC mechanical properties, various combinations of cement including ordinary (OPC) and High Alumina (HAC), as well as silica fume, were investigated to determine the optimal mixture for strengthening RC beams. The results indicate that substituting 75% of HAC with OPC significantly boosted flexural strength by 33%, and increased tensile strength in the warp and weft directions by 25% and 21%, respectively. The study examined six RC beam specimens in a four‐point flexural test, assessing their flexural properties, ductility, cracking behavior, and energy dissipation. The number of CC layers and the bonding method, which involved either the conventional externally bonded reinforcement (EBR) technique or the externally bonded reinforcement on grooves (EBROG) technique were studied. The experimental results revealed the superiority of the EBR method over the EBROG technique due to the uniform bond between CC and the concrete surface. Using the EBR technique, improvements of 35% and 51% were observed in the load‐carrying capacity of the beams strengthened with one and two CC layers, respectively. It is while for the beam with an improved loading capacity of 51%, the crack width decreased from 5.3 to 2.1 mm, and the energy dissipation increased by three times compared to the control specimen. Theoretical analysis also was performed to calculate the load‐carrying capacity of the RC beams strengthened with CC layers.

Experimental Studies and Application of Fiber-Reinforced Polymers (FRPs) in Civil Infrastructure Systems: A State-of-the-Art Review.

The application of FRPs in civil infrastructure has increased, particularly in the last 20 years. FRPs have gained importance because of their resistance to harsh environments, high strength-to-weight ratio, and good corrosion resistance, and they are faster and easier to apply than other traditional methods. The rehabilitation of structures is the main area in which FRPs have been developed, because they have allowed for compliance with architectural restraints in historic structures. This review is a compilation of the research conducted on the laboratory and field applications of FRPs, highlighting the different applied methods, installation difficulties, and failure modes of FRPs. Moreover, this review compares studies on the types of fibers such as CFRPs, GFRPs, and AFRPs, and their effects would affect the mechanical properties of civil infrastructure and the durability characteristics of civil infrastructure in challenging environmental conditions. In addition, this review focuses on the modification of the mechanical properties of structural elements using different methods of installing FRPs, including externally bonded reinforcement (EBR), and their main problem: debonding failure before the ultimate load.

An analytical model to predict bond behavior of fiber reinforced polymer bonded to concrete with an end anchorage system

The end mechanical anchorage system has been widely applied in the conventional externally bonded reinforcement (EBR) technology, to hinder the premature debonding failure of fiber reinforced polymer (FRP) laminate, especially for the reinforcement structures with prestressed FRP due to higher interfacial shear stress at the ends. Through the introduction of the hinge as the substitution of end anchorage, this paper presents a numerical model based on the bilinear bond-slip model and ordinary differential equations in elastic, elastic-softening, and elastic-softening-debonding stages. The bond behavior of the FRP-concrete interface under the end anchorage in different stages is predicted, obtaining the analytical expressions of interfacial load-slip relation, interfacial shear stress distribution, and the stress distributions of FRP. Additionally, two experimental programs from this group and previous literature were involved in this study to validate the accuracy of the numerical model. The comparison shows they are in good agreement. Moreover, it indicated that the strength increase is linear with the torque applied in the end anchorage, but bonding strength is not the case.

Experimental and theoretical study on the flexural behaviour of RC beams strengthened with externally bonded reinforcement on grooves CFRP laminates

This paper presents a study on the flexural capacity of Reinforced Concrete (RC) beams strengthened with Externally Bonded Reinforcement on Grooves (EBROG) Carbon Fibre Reinforced Polymer (CFRP) laminates. Thirteen single-shear tests and seven RC beams strengthened with CFRP are examined, comparing Externally Bonded Reinforcement (EBR) and EBROG techniques with different groove distributions. The results showed that EBROG improved the bond and flexural performance. Comparison with existing prediction models reveals that while the literature models accurately predict EBR results, they underestimated the bending capacity for EBROG due to calibration based on EBR bond parameters. Consequently, a numerical model is developed to simulate Intermediate Crack Debonding (ICD) for various bond-slip laws, applicable to the different strengthening techniques. The proposed model is validated using experimental flexural tests and calibrated bond-slip laws obtained from single-shear tests, demonstrating accurate prediction of ICD failure for both EBR and EBROG.

Analysis of FRP-Strengthened Reinforced Concrete Beams Using Electromechanical Impedance Technique and Digital Image Correlation System.

Fiber-reinforced polymer (FRP) strengthening systems have been considered an effective technique to retrofit concrete structures, and their use nowadays is more and more extensive. Externally bonded reinforcement (EBR) and near-surface mounted (NSM) technologies are the two most widely recognized and applied FRP strengthening methods for enhancing structural performance worldwide. However, one of the main disadvantages of both approaches is a possible brittle failure mode provided by a sudden debonding of the FRP. Therefore, methodologies able to monitor the long-term efficiency of this kind of strengthening constitute a challenge to be overcome. In this work, two reinforced concrete (RC) specimens strengthened with FRP and subjected to increasing load tests were monitored. One specimen was strengthened using the EBR method, while for the other, the NSM technique was used. The multiple cracks emanating in both specimens in the static tests, as possible origins of a future debonding failure, were monitored using a piezoelectric (PZT)-transducer-based electromechanical impedance (EMI) technique and a digital image correlation (DIC) system. Clustering approaches based on impedance measurements of the healthy and damaged states of the specimens allowed us to suspect the occurrence of cracks and their growth. The strain profiles captured in the images of the DIC system allowed us to depict surface hair-line cracks and their propagation. The combined implementation of the two techniques to look for correlations during incremental bending tests was addressed in this study as a means of improving the prediction of early cracks and potentially anticipating the complete failure of the strengthened specimens.

Bond behavior of prestressed FRP sheet-concrete joints: Comparison of EBROG and EBR methods

When strengthening reinforced concrete (RC) elements with prestressed fiber-reinforced polymer (FRP), proper transfer of the prestressing force to the concrete surface must be assured. The current study investigated the bond behavior of specimens prepared with prestressed FRP sheets as well as the types of grooves used by employing a novel prestress-lap shear test. The prestressed and non-prestressed specimens were tested using a prestress-lap shear test and single-lap shear test, respectively, and the results were compared. A new groove classification technique was developed and results for the use of 5@15 mm (square grooves of 5×5 mm distanced center-to-center at 15 mm) and 10@20 mm grooves were compared with those for the conventional externally bonded reinforcement (EBR) method. The prestressing force was processed at 0%, 20%, and 30% of the ultimate strain. At all prestressing levels, specimens prepared in the 5@15 mm groove class performed better than those in the 10@20 mm groove class. The 5@15 mm groove class also required 53% less filling resin compared to the 10@20 mm groove class. At all prestressing levels, the specimens prepared through the grooving method (GM) using the externally bonded reinforcement on grooves (EBROG) technique performed better than those prepared by the EBR. The GM method allowed the 30% prestress force to be transferred to the substrate surface by engaging more depth of the concrete. The combined use of prestressed FRP sheets and GM to strengthen concrete specimens improved the overall performance of the joint compared to the specimen strengthened with non-prestressed FRP sheets and EBR.

Evaluating the behavior and bond properties of FRP spike anchors under confined conditions and elevated temperature

The use of fiber-reinforced polymer (FRP) anchors in combination with the conventional externally bonded reinforcement (EBR) installation technique is an effective method to prevent or postpone the debonding of FRP sheets. However, their behavior under confined conditions and elevated temperatures is still unknown. In such a framework, in the first phase, the current research aims to assess pullout test results for single FRP anchors with dowels ranging from 50 to 75 mm in confined and unconfined conditions. To achieve this goal, three concrete slabs with the dimension of 1550 × 1250 × 250 mm3 were cast. Experimental results showed that the bond strength of FRP anchors in confined conditions increased by about 50% compared to the unconfined conditions. Additionally, a comparison was made between the experimental results and the literature models. In the second phase, eleven pullout tests were conducted on the cylindrical specimens with the dimension of 150 × 200 mm2 (diameter × height) under confined conditions and elevated temperatures by considering different sustained load levels.The results showed as the temperature increases and FRP anchor is constantly loaded, it fails in a shorter time before the adhesive reaches its glass transition. Finally, a bond strength vs. temperature model was developed for FRP anchors in confined conditions and high temperatures.

Flexural Strength of Damaged RC Beams Repaired with Carbon Fiber-Reinforced Polymer (CFRP) Using Different Techniques

In this study, an experimental program was developed to investigate the flexural behavior of pre-damaged reinforced concrete (RC) beams that had been repaired and strengthened using carbon fiber-reinforced polymer (CFRP) laminates under a monotonic load. Two techniques were used: externally bonded reinforcement (EBR) and near-surface-mounted (NSM) reinforcement, to repair and strengthen the tested beams. The experimental program involved casting and testing nine simply supported RC rectangular beams; one beam was considered as the reference beam and did not undergo additional strengthening, and the remaining beams were strengthened using CFRP laminates. These eight beams were divided into two main groups for the purposes of strengthening: beams for which the EBR technique was used, and beams for which the NSM technique was used. The primary variables observed in the EBR and NSM groups included four damage percentages obtained according to the preload (20, 40, 60, and 80%) from the ultimate load carried by the reference beam. The experimental results show that decreasing the damage percentage leads to an increase in ultimate strength from about 3.6% to 17.2% for the beams repaired using the EBR technique and from 27.6% to 57% for the beams repaired using the NSM technique; additionally, the NSM method was more effective than the EBR method in terms of the flexural strength and mode of failure. However, using CFRP laminates enhances the flexure capacity of strengthened RC beams.

Bio-Based Pultruded CFRP Laminates: Bond to Concrete and Structural Performance of Full-Scale Strengthened Reinforced Concrete Beams.

This paper presents an experimental study about the use of innovative bio-based pultruded carbon-fiber-reinforced polymer (CFRP) laminates for structural strengthening. The bio-based laminates were produced in the framework of an applied research project (BioLam) using a resin system with 50% (wt.%) bio-based content, obtained from renewable resources. In the first part of the study, their tensile and interlaminar shear properties were characterized and compared with those of conventional oil-based CFRP laminates. In the second part of the study, the bond behavior to concrete of both types of CFRP laminates applied according to the externally bonded reinforcement (EBR) technique was assessed by means of single-lap shear tests performed on CFRP-strengthened concrete blocks; the experimental results obtained from these tests were then used in a numerical procedure to calibrate local bond vs. slip laws for both types of laminates. The final part of this study comprised four-point bending tests on full-scale EBR-CFRP-strengthened reinforced concrete (RC) beams to assess the structural efficacy of the bio-based laminates; these were benchmarked with tests performed on similar RC beams strengthened with conventional CFRP laminates. The results obtained in this study show that the (i) material properties, (ii) the bond behavior to concrete, and (iii) the structural efficacy of the developed bio-based CFRP laminates are comparable to those of their conventional counterparts, confirming their potential to be used in the strengthening of RC structures.

Improving bonding behavior between basalt fiber-reinforced polymer sheets and concrete using multi-wall carbon nanotubes modified epoxy composites

Premature debonding failure is a critical problem in the reinforcement of concrete structures using fiber reinforced polymer (FRP) sheets with externally bonded reinforcement (EBR) technique. The usual anchoring methods are likely to cause damage to the concrete. The effect of incorporating various amounts of carboxyl (COOH)-functionalized MWCNTs in the epoxy resin on the bonding behavior of the basalt fiber-reinforced polymer (BFRP)-concrete joints was detailed studied by single-shear tests and the digital image correlation (DIC) technique. Experimental results indicated that adding the functionalized MWCNTs into the epoxy considerably enhanced the bonding properties. In comparison with the BFRP-concrete joints using neat epoxy, the effective bond length, bond strength, ultimate global slip, interface fracture energy, and BFRP strain of the BFRP-concrete joints using 0.8 wt% MWCNTs modified epoxy increased by 96 %, 55 %, 39 %, 172 %, and 114 %, respectively. The scanning electron microscope (SEM) images of debonded BFRP surface revealed that the MWCNTs could penetrate into concrete along with epoxy resin, and the MWCNTs pull-out and crack-bridging could indicate a reinforcing effect to prevent premature adhesive failure. This study demonstrated the great promise of the MWCNTs modified epoxy composites toward practical engineering application in reinforced concrete (RC) structures. Data availability statementThe raw/processed data required to reproduce these findings cannot be shared at this time as the data also forms part of an ongoing study.

Analysis of Strengthening RC Beams Using Stainless Steel Continuous Reinforcement Embedded at Ends Using ANSYS

Abstract: An innovative system for the flexural strengthening of RC structures designated continuous reinforcement embedded at ends (CREatE) is presented in this research work. The main charateristics and procedures for the application of this new strengthening techniques were described. To evaluate the performance and efficiency of this technique, a set of RC T-beams was subjected to a four pointed bending test setup. Different application arrangements and different amount of reinforcement were considered, and the CREatE technique was tested under monotonic and cyclic loading histories. The tests were modeled using the nonlinear finite element method (FEM) to predict the performance of the RC T beam, which allowed analysing, in detail and with good agreement with the experiments, the influence of the CREatE technique on the (1) strains developed in the concrete, (2) cracking patterns, and (3) strains developed in the stirrups. Apart from the expected increases in the flexural stiffness and load bearing capacity of the T beam, the result showed that the use of the CREatE technique led to higher ductility indexes in the displacement compared with traditional techniques. Moreover, with the CREatE technique, premature debonding of the reinforcement material from the concrete tensioned surface commonly observed in externally bonded reinforcement (EBR) strengthening system was eliminate.

Durability of bond of EBR CFRP laminates to concrete under real-time field exposure and laboratory accelerated ageing

The durability of bond between carbon fibre-reinforced polymer (CFRP) laminates and concrete with the externally bonded reinforcement (EBR) technique was investigated under real-time field exposure (RTFE) and accelerated ageing. The experimental program, over two years, includes four outdoor environments inducing carbonation, freeze–thaw attack, extreme temperatures, and airborne chlorides from the ocean. A laboratory environment (20 °C/55% RH) was used as reference environment. Additionally, a water-immersion environment (20 °C) was also considered. Relatively low values of bond degradation were observed, where the maximum pullout force varied between −4 % and +16 % under RTFE, while on water–immersed specimens, the maximum pullout force decreased by ∼8 %.

Experimental investigation on bond behavior of CFRP-concrete interface with end anchorage in hygrothermal environments

Externally bonded reinforcement (EBR) of reinforced concrete (RC) structures with fiber reinforced polymer (FRP) is unable to meet the requirements of the engineering. Because premature debonding failure limits the usage scenario of the EBR method, which cannot fully utilize the high tensile performance of FRP. More and more studies focus on the improvement of the EBR method by extra anchors on the FRP-concrete interface to improve its ultimate strength. Considering the influence of environmental conditions on the FRP-concrete interface with anchorage, the application of extra anchorage under hygrothermal environments has to be investigated. The paper optimizes the existing end mechanical anchorage to avoid damage to carbon fiber reinforced polymer (CFRP) and concrete substrate. The compressive pressure exerted on the CFRP laminate can be controlled by adjusting the torque on the end anchorage. A series of end-anchoted double-lag shear tests under different hygrothermal conditions were carried out for invesitgating the bond behavior of CFRP-concrete interface. The test results showed that the load increment of the ultimate strength of the CFRP-concrete interface decreased with the increase in temperature and relative humidity. Correspondingly, friction surfaces due to the compression provided by end anchorage changed from concrete-concrete interface to the concrete-adhesive or adhesive-CFRP interfaces. The relationship between friction coefficient and temperature and relative humidity was built. The shear stress and slip distribution curves were calculated from the strain distribution functions fitted from the strain data measured by DIC. Fracture energy for the CFRP-concrete bond was estimated, and the bond-slip relationship for the hygrothermal conditions was constructed.

Bond Behavior of Prestressed CFRP Strips-to-Concrete Joints Using the EBROG Method: Experimental and Analytical Evaluation

Fiber-reinforced polymer (FRP) composites are widely employed as externally bonded reinforcement (EBR) systems for strengthening reinforced concrete members. More recently, a new technique, referred to as externally bonded reinforcement on grooves (EBROG), has been proposed, which is based on using a number of grooves throughout the concrete substrate to enhance the bond strength between the FRP composite and concrete. This study investigates the influence of groove depth on the resulting debonding process that can be observed in prestressed carbon FRP strips. To do so, prestressing force release tests were conducted on a series of EBR and EBROG FRP strips bonded to concrete specimens. Test results demonstrated that the fracture process leading to debonding of the EBROG specimens developed in a significantly different manner with respect to the case of EBR specimens. Specifically, fractures ran through deeper layers of concrete as the grooves became deeper. Numerical analyses are also proposed to scrutinize the actual bond–slip law characterizing both EBR and EBROG specimens. It was shown that a trilinear bond–slip law is appropriate for simulating the interface behavior in EBROG specimens.

Computer Simulations of End-Tapering Anchorages of EBR FRP-Strengthened Prestressed Concrete Slabs at Service Conditions

This article examines numerically the behavior of prestressed reinforced concrete slabs strengthened with externally bonded reinforcement (EBR) consisting of fiber-reinforced polymer (FRP) sheets. The non-linear finite element (FE) program Abaqus® is used to model EBR FRP-strengthened prestressed concrete slabs tested previously in four-point bending. After the calibration of the computational models, a parametric study is then conducted to assess the influence of the FRP axial stiffness (thickness and modulus of elasticity) on the interfacial normal and shear stresses. The numerical analysis results show that increasing the thickness or the elastic modulus of the FRP strengthening affects the efficiency of the FRP bonding and makes it susceptible to earlier debonding failures. A tapering technique is proposed in wet lay-up applications since multiple FRP layers are often required. It is shown that by gradually decreasing the thickness of the FRP strengthening, the concentration of stress along the plate end can be reduced, and thus, the overall strengthening performance is maximized. The tapering is successful in reducing the bond stress concentrations by up to 15%, which can be sufficient to prevent concrete rip-off and peel-off debonding failure modes. This article contributes towards a better understanding of the debonding phenomena in FRP-strengthened elements in flexure and towards the development of more efficient computational tools to analyze such structures.

Fracture prevention and repair via FRP-to-concrete joint using externally-bonded reinforcement on grooves (EBROG) technique

Fracture prevention and repair via FRP-to-concrete joint using externally-bonded reinforcement on grooves (EBROG) technique

Ultrasonic evaluation for the detection of contact defects of the timber and fiber-reinforced polymer

Fiber-reinforced polymer (FRP) composites have been used tremendously to repair and rehabilitate timber structures due to their formability, ease of use, and high specific strength. The bond quality between FRP and timber substrate is critical for complete composite action. In this paper, a comprehensive set of linear and nonlinear ultrasonic methods was performed to investigate the bond between carbon-FRP (CFRP) and timber. For this purpose, 126 specimens of reinforced timber were prepared. Two techniques were considered to bond CFRP and timber: (1) externally bonded reinforcement (EBR), and (2) externally bonded reinforcement on the groove (EBROG). The effect of the number of CFRP layers, adhesive types, the extent of artificial surface defects, and the depth of the groove were contemplated. By analyzing linear methods, differences in stiffness and modulus of elasticity of CFRP and timber led to velocity variation and attenuation phenomena. Higher-order harmonics were also used to investigate the nonlinear behavior of ultrasonic waves at the bond. The second harmonic generation detected surface damage at CFRP and timber bond. Single-lap shear tests were employed to determine the bond strength of CFRP and timber and evaluate the accuracy of the linear and nonlinear methods. Finally, using the linear and nonlinear features extracted from the ultrasonic response signals and available physical parameters, three artificial neural networks (ANNs) were developed to predict three important CFRP-to-timber bond characteristics quantitively: (1) to predict the groove depth, (2) to predict the volume of voids, and (3) to predict the CFRP-to-timber bond strength. The three proposed models achieved an average R 2 of 0.95, 0.96, and 0.92 in predicting the groove depth, voids volume, and bond strength, respectively.

Strengthening Existing Concrete Structures with Fibre Reinforced Polymers (FRP) in the New Version of Eurocode 2

This paper aims to introduce the content of Annex J “Strengthening of Existing Concrete Structures with CFRP” of Eurocode 2 [1]. This is first time that the design of adhesively bonded reinforcement with CFRP has been introduced in the European regulations through an informative annex. Annex J considers two different bonded strengthening techniques: externally bonded reinforcement (EBR) that consists of bonding CFRP strips or sheets to the surface of concrete elements, and near surface mounted reinforcement (NSM) that consists of embedding a CFRP strip or rod to a slot cut in the concrete cover. Since, the content of Annex J is new, asummary and background related to all aspects required for designing CFRP strengthened systems for concrete structures, are given in this paper.

Development of probabilistic FRP-to-concrete bond strength models for externally-bonded reinforcement on grooves: Bayesian approach

• FRP-to-concrete bond strength models are developed for EBROG joints. • An experimental database containing 304 test samples is collected. • Bayesian inference is adopted for probabilistic model calibration. • Analytical expressions are derived for probabilistic model prediction. • Traditional models are compared to illustrate the superior of proposed ones. Recent years have witnessed the superiorities of a grooving technique, i.e., the externally-bonded reinforcement on grooves (EBROG), over the traditional technique, i.e., externally-bonded reinforcement (EBR), in terms of strengthening reinforced-concrete members by fiber-reinforced polymer (FRP) composites. Similar to EBR, the prediction of interfacial bond strength is critical for the EBROG type of FRP-to-concrete joints. There have been a series of experiments conducted to study the bond strength of EBROG joints, however, the corresponding prediction models (especially those capable of uncertainty quantification) are rarely available. Therefore, this paper aims to develop probabilistic models able to both predict the bond strength of EBROG joints and quantify the associated uncertainties. Firstly, an experimental database containing 304 bond strength tests for EBROG joints with longitudinal grooves is collected. Secondly, two widely-accepted deterministic bond strength models are selected as candidates to be calibrated by including a model correction factor. Thirdly, the factor is expressed by power and exponential functions of the influencing variables, meanwhile the deterministic model parameters are replaced with probabilistic ones. Finally, the probabilistic models are determined by means of Bayesian inference and Markov chain Monte Carlo. Moreover, an approximate analytical expressions are derived to achieve the probabilistic bond strength prediction. The experimental database is used to investigate the model performances, and the results show that: 1) the developed models can predict the FRP-to-concrete bond strength of EBROG joints with sufficient predictive accuracies and reasonable uncertainty quantifications; 2) the groove geometry has significant influence on the model correction factor; and 3) the developed probabilistic models is only applicable to EBROG joints with longitudinal grooves, but not to ones with transverse/diagonal grooves.