Externally Bonded Research Articles

Shear Strength Model for RC Beams with U-Wrapped FRCM Composites

AbstractThe shear strength of reinforced concrete (RC) elements can be improved by applying externally bonded (EB) fiber-reinforced cementitious matrix (FRCM) composites. FRCM composites are genera...

Shear strengthening of hybrid externally-bonded mechanically-fastened concrete beams using short CFRP strips: Experiments and theoretical evaluation

Hybrid externally bonded (EB) mechanically-fastened (MF) technique for carbon-fiber-reinforced-polymers (CFRP) strips offers a high potential in shear strengthening to preclude debonding phenomenon and effectively utilize the maximum use of CFRP. This study investigated the failure envelope of reinforced concrete (RC) beams, designed deficient in shear, strengthened with hybrid EB/MF technique and reduced-size CFRP strips. The investigated parameters included strip length, number, spacing, and diameter of fasteners. The beams were subjected to three-point loaded testing with a shear span-to-depth ratio of 2.5. The strengthening had an axial rigidity of 1.8 GPa. The hybrid EB/MF technique for the reduced-size CFRP strips (up to 44% length reduction) inhibited crack propagation and could preclude the debonding phenomenon provided that adequate spacings of fasteners used. The results showed that the hybrid EB/MF specimens with reduced-size CFRP strips could achieve up to 65% shear load gain over the unstrengthened specimen. Also, a theoretical evaluation was conducted using other specimens from past studies to estimate the contribution of CFRP strips with and without steel stirrups. In addition, the ultimate shear loads were predicted, as per North-America’s guidelines, and compared with the experimental results. Finally, a set of recommendations was drawn based on the results of this study and the available literature.

Effect of Cyclic Damage on the Performance of RC Square Columns Strengthened Using Hybrid FRP Composites under Axial Compression

The effectiveness of hybrid fibre-reinforced polymer (FRP) strengthening is evaluated for rapid repair of the pre-damaged plain concrete (PC) and reinforced concrete (RC) columns. The objective of this study is to understand the efficiency of hybrid technique for completely restoring the initial stiffness, load carrying capacity and ductility of pre-damaged columns under cyclic compression loads. Two series of PC and RC square columns were cast. The columns were pre-damaged by loading up to 80% of peak load capacity for three cycles under pure compression. After cyclic damage, the columns were strengthened with two techniques, namely (a) near-surface mounted (NSM) carbon FRP (CFRP) laminates and (b) hybrid FRP technique, which uses a combination of NSM and externally bonded (EB) CFRP fabric. Analytical modelling was carried out for predicting the behaviour of columns with initial cyclic pre-damage. Additionally, a phased three-dimensional nonlinear finite element (FE) analysis was performed to validate the behaviour of pre-damaged columns with different strengthening techniques. Test results show that cyclic pre-loading and resulting damage causes a reduction in axial stiffness of all damaged specimens. Hybrid strengthening completely restored the stiffness and strength under compression. Prediction of analytical and FE analysis correlated well with the tests.

Crack based bond strength model of externally bonded steel plate and CFRP laminate to predict debonding failure of shear strengthened RC beams

Shear cracking can have significant effects on interfacial bond strength of externally bonded (EB) plates, and thus development of a bond strength model considering effects of cracking is vital to predict debonding failure of shear strengthened reinforced concrete (RC) beams. The main aim of this study was to develop bond strength models of EB steel plate and CFRP laminate, considering effects of cracks, to predict debonding failures of shear strengthened RC beams. In the experimental program, forty-eight RC prisms were fabricated and strengthened using steel plate and CFRP laminate; all prisms in this category were tested using a special indirect pull out test to incorporate cracks. To validate the models, four full-scale RC beams were shear strengthened using EB steel plate and CFRP laminate. Based on the results of cracked prisms, interfacial bond strength models of EB steel plate and CFRP laminate have been developed. The proposed models showed that the presence of cracks reduced the interfacial bond strengths of EB steel plate and CFRP laminate significantly, and the bond strength was also reduced because of increased length and width of EB steel plate and CFRP laminate. As compared to existing models, the proposed models showed lower interfacial bond strength of EB plates. The predicted debonding failure loads of steel plate and CFRP laminate shear strengthened RC beams using the proposed models were 3.4% and 4.7% differences when compared to experimental results, respectively. However, the models from existing research had shown significantly higher debonding failure loads as compared to those of experimental results.

Finite Element Modelling of Reinforced Concrete Beam Strengthened with Embedded Steel Reinforcement Bars

The increased of loads on existing reinforced concrete infrastructure and the lack of initial design and construction will induce flexural and shear failure. Several methods have been introduced to increase the shear capacity of existing reinforced concrete elements with FRP, involving the use of plates or fabric externally bonded (EB) to the webs of the bridge beams, prestressed straps wrapped around the beams or bars mounted within grooves prepared in the near-surface mounted (NSM) technique. Typical Indonesian concrete bridges consisted main girders connected with diaphragm beams where the distance between those girders are very close. In particular case, where the webs of the beams are difficult to access, a novel approach is introduced, namely deep embedment (DE) method. Three reinforced concrete beam models are prepared for this study. One specimen is the control specimen and identified, as Beam-CS and the other two are the strengthened specimens and identified as Beam-SS-3EB and Beam-SS-5EB. All specimens have the same dimensions and reinforcement configuration. Specimen Beam-SS-3EB was strengthened with three rows of 6 mm embedded plain steel bars while specimen Beam-SS-5EB was strengthened with five rows of 6 mm plain steel bars. The results showed that element size significantly affects the load-displacement curve behaviour. The similarity of the hysteresis curve in the FE analysis using the 25 mm element size suggested a reasonably good agreement between the analytical calculation and the prediction result from the FE analysis. Furthermore, maximum reaction force for Beam-SS-3EB and Beam-SS-5EB were 30.30 kN and 31.77 kN, respectively, represents an increase of 17.67% and 23.29% compared to that of the Beam-CS.

Developing an anchored near-surface mounted (NSM) FRP system for fuller use of FRP material with less epoxy filler

Near-surface mounted (NSM) FRP systems have a number of advantages over externally bonded (EB) FRP systems. Relying exclusively on epoxy-filler bond, however, conventional NSM FRP systems cannot ensure the tensile strength of FRP reinforcements to be fully developed. The liquid characteristics of epoxy filler before curing also narrow NSM FRP applications down to negative moment regions. In this study, an anchored NSM FRP system has been developed to fully develop the FRP tensile strength with a minimum epoxy filler. Anchorages are the extended parts of FRP reinforcements embedding into predrilled holes and then overlapping FRP material within anchor regions. Experiments were conducted to validate the proposed NSM FRP system and investigate the impacts of key parameters, i.e. location and density of grooves as well as the amount of epoxy. Experimental results also suggest a great potential extending the anchored NSM FRP system to positive moment regions.

Developing an anchored CFRP reinforcement for efficiently and readily strengthening reinforced concrete structures

Externally bonded (EB) CFRP laminates and near-surface mounted (NSM) CFRP reinforcements are two major methods in use of CFRP material for shear and flexural strengthening of reinforced concrete (RC) structures. Relying exclusively on CFRP-concrete bond, however, both methods cannot ensure the tensile strength of CFRP material to be fully developed, greatly limiting their efficiencies and applications. Although anchored EB CFRP laminates were able to make fuller use of CFRP material, the complicated installations might compromise their merits in strengthening RC structures. This study presents an efficiently and readily strengthening method, i.e. the anchored CFRP reinforcement. Experimental results suggested this proposed method was able to fully develop the tensile strength of CFRP reinforcement by using a much simpler installation. Results also demonstrated the impacts of patch arrangement, embedment depth and reinforcement section on the anchored CFRP reinforcement. By using those valuable experimental data, this efficient and simple method was expected to be well designed for strengthening RC structures.

Flexural behaviour of RC beams strengthened with combinations of two CFRP systems and P-SWRs

Externally bonded (EB) carbon fibre-reinforced polymer (CFRP) sheets, near-surface-mounted (NSM) CFRP bars and prestressed steel wire ropes (P-SWRs) are widely used for strengthening reinforced-concrete (RC) beams. Existing research has shown that it can be inefficient or infeasible to provide sufficient load-carrying capacity using one strengthening method because of the CFRP strain limitation at debonding or the upper bound on the amount of strengthening materials due to limited space for strengthening. Moreover, some already strengthened RC beams need to be further strengthened under heavier loading conditions. Therefore, using a combined strengthening scheme for one beam should be considered in both these situations. In this study, five concrete beams were strengthened and tested to explore the effects of different combined schemes on beam flexural behaviour. The results demonstrate that P-SWRs are of crucial importance in improving the cracking behaviour and ductility of beams originally strengthened with EB-CFRP sheets or NSM-CFRP bars. The strengthening scheme of NSM-CFRP bars and P-SWRs is found to be better than that of EB-CFRP sheets and P-SWRs. A recommendation for the appropriate selection of strengthening schemes is given based on the test results.

Width effect of interfacial bond characteristics

The bond properties of an interface in composite structures are generally considered as local mechanical characteristics. Through experimental testing and analytical study, it is shown in this work that the interfacial bond characteristics are generally size dependent, which involve both Mode II and Mode III fractures. For fiber-reinforced polymer (FRP) externally bonded (EB) to concrete members, this size effect is called the width effect. Test results in this work show that there is a central region on EB FRP-to-concrete bond face where only Mode II fracture is involved, and hence, the bond properties are size independent. The measured bond test results in the central region are used to determine interfacial bond properties that are size independent. The global test results or overall bond responses are subsequently used to determine the width effect and its model, through rational reasoning and regression of test results. The proposed model includes new factor that has not been considered in the width effect modeling, and consequently shows a better performance.

Finite element modeling of reinforced concrete beams externally strengthened in flexure with side-bonded FRP laminates

Reinforced concrete (RC) beams are often strengthened through externally bonded (EB) fiber-reinforced polymer (FRP) systems. In such applications, improving the bending capacity of RC beams is of main interest and which is achieved via bonding EB-FRP systems to the soffit of beams. Access to soffit of beams can be restricted by existing structural members or limited (in case a RC beam span over neighboring compartments). In those scenarios, RC beams can still potentially be strengthened in flexure through longitudinal bonding of EB-FRP systems to the sides of beam's web. This paper examines critical parameters that influence the effectiveness of side bonded EB-FRP systems through a newly developed finite element (FE) model. This model utilizes state-of-art simulation techniques and is capable to trace the flexural behavior of RC beams externally strengthened with side-boned FRP laminates throughout all loading stages till failure. The model was validated by comparing with experimental data and the predicted and measured results were in good agreement. The validated model was then utilized to study the effect of concrete compressive strength, FRP material type, FRP size, and steel reinforcement ratio on the behavior of strengthened RC beams. Outcomes of this numerical investigation show the effectiveness of side-bonding FRP systems as an alternative to conventional soffit strengthening systems to improve the flexural capacity of RC beams.

Experimental and Analytical Approach for Prediction of Out-of-Plane Capacity of Reinforced Masonry Walls Strengthened with Externally Bonded FRP Laminate

AbstractThis experimental study has shown the effectiveness of fiber-reinforced polymer (FRP) external bonding (EB) in enhancing the flexural capacity of reinforced masonry (RM) walls subjected to ...

STRENGTHENING OF RC BEAMS USING NEAR SURFACE EMBEDDED-FRCM

Fibre reinforced cementitious matrix (FRCM) systems are mostly externally bonded (EB) for the strengthening of reinforced concrete (RC) and masonry structures. In this paper, the relatively new concept of near-surface embedded (NSE) FRCM, has been introduced for the flexural strengthening of beams. The process of the application of NSE-FRCM strengthening technique involves the removal of the concrete layer at beam soffit, being the most deteriorated in actual practices. Experimental evidence of the flexural strengthening efficacy of this technique is provided here. Eight RC beams were prepared and tested under four-point loading with the consideration of two test parameters: (a) FRCM material (polyparaphenylene benzobisoxazole (PBO)/carbon/ glass); and (b) the reinforcement ratio (0.5% representing flexure-deficient beams and 1.28% representing typical under-reinforced beams). The strengthening led to gains in ultimate loads that ranged between 31.4% and 84.3%.

A state-of-the-art review: Near-surface mounted FRP composites for reinforced concrete structures

Abstract The use of fibre reinforced polymer (FRP) in civil construction applications with near-surface mounted (NSM) method has gained considerable popularity worldwide and can produce confident strengthening and repairing systems for existing concrete structures. The response of concrete structural members rehabilitated using FRP with externally-bonded (EB) method under monotonic and fatigue loading has been well reported. On the other hand, the monotonic and fatigue response of NSM FRP-rehabilitated concrete structures is less known. This paper reviews current research on concrete members retrofitted by NSM FRP system and exposed to monotonic and fatigue loading. It provides an outline of FRP composites and the fatigue behaviour of concrete, steel and FRP materials. In addition, the review focuses on the bond characteristics between NSM FRP, adhesive and concrete substrate, and on the flexure behaviour of NSM FRP-strengthened and repaired reinforced concrete (RC) beams. Furthermore, the paper reviews the failure modes, factors affecting NSM FRP systems, guidelines and codes for the use of FRP in concrete structures, damage accumulation and residual strength, stress limits in steel reinforcement, and the limitations of using epoxy and polymer-based cementitious adhesives for the strengthening of concrete members.

Flexural behaviour of concrete beams reinforced with different grade steel and strengthened by CFRP strips

Externally bonded (EB) carbon fibre-reinforced polymer (CFRP) strips can be used to increase load-bearing capacity of reinforced concrete (RC) elements or to improve serviceability conditions of the structures. Such a strengthening technique can be applied both in new buildings, as well as in existing ones, erected years or even decades ago. Therefore, the elements to be strengthened could differ considerably because of the physical parameters of the materials used during construction. The bulk of experimental studies on debonding phenomena of EB-strengthened RC structures focus on factors related to the concrete substrate and composite material. The assumptions presented in the existing international codes and guidelines, as well as in numerous theoretical bond models, neglect the influence of steel grade or yield strength on bond properties, and in turn, the load-bearing capacity of the connection between the composite and concrete substrate as well.Four RC beams were flexurally strengthened with CFRP laminates and tested. Low-strength concrete and four steel grades with different yield points were used to cast the test elements. The results indicate that CFRP laminates with the externally bonded reinforcement are an effective technology to increase cracking, service, and maximum loads of RC beams, even when made with poor-quality concrete. Therefore, it is noted that the yield point of steel, and therefore, the grade of steel is the next parameter influencing the bond efficiency of CFRP laminates in the RC structures to be studied.

Torsional strengthening of reinforced concrete beams with externally bonded composites: A state of the art review

The use of externally bonded (EB) fiber reinforced composites to strengthen reinforced concrete (RC) structures has been explored extensively in recent decades. While many studies have been conducted on the flexural, shear, and axial strengthening of RC members, far fewer studies have been conducted on torsional strengthening. Thus, the knowledge on the behavior of RC members strengthened in torsion with EB composites is rather limited. The aim of this paper was to present a comprehensive review and evaluation of torsional strengthening of RC beams using EB composites. A detailed survey of the literature was conducted, and a database of experimental tests was developed. The database includes beams reinforced with EB fiber reinforced polymer (FRP) and EB fiber reinforced cementitious matrix (FRCM) composites. The effectiveness of the strengthening system was examined in terms of geometrical and mechanical characteristics of the RC beam, composite type, and composite wrapping configuration. Different modes of failure of the strengthened beams were also discussed. Additionally, analytical and numerical methods developed to predict the torsional response of RC beams strengthened with EB composites were summarized and discussed. Finally, design provisions were examined based on the knowledge gained from this study.

Near surface embedded-FRCM for flexural strengthening of reinforced concrete beams

Fibre reinforced cementitious matrix (FRCM) systems are mostly externally bonded (EB) for the strengthening of reinforced concrete (RC) and masonry structures. In this paper, the relatively new concept of near-surface embedded (NSE) FRCM, has been introduced for the flexural strengthening of beams. The process of the application of NSE-FRCM strengthening technique involves the removal of the concrete layer at beam soffit, being the most deteriorated in actual practices. Experimental evidence of the flexural strengthening efficacy of this technique is provided here. Fourteen RC beams were prepared and tested under four-point loading with the consideration of three test parameters: (a) FRCM material (polyparaphenylene benzobisoxazole (PBO)/carbon/glass); (b) strengthening technique (NSE and EB); and (c) the reinforcement ratio (0.5% representing flexure-deficient beams and 1.28% representing typical under-reinforced beams). The strengthening led to gains in ultimate loads that ranged between 30.1% and 93.3%. The strengthening techniques are compared based on an introduced strengthening efficiency factor.

Shear Strengthening of Reinforced Concrete Beams Using CFRP Wraps

The need to retrofit existing reinforced concrete (RC) structures have increased over the decades due to corrosion of steel reinforcement inside the concrete, neglect and overuse, and increased loading. Experimental and numerical studies in this research field showed that using fiber-reinforced Polymer (FRP) materials to strengthen RC members in shear, flexure, and column confinement applications is an effective method to retrofit RC structures. This strengthening technology has numerous advantages over conventional steel plating, such as providing high strength-to-weight ratio, versatility, durability, and ease of use to strengthen RC members. The purpose of this paper is to study the effect of strengthening shear deficient RC beams with externally bonded (EB) carbon fiber-reinforced polymer (CFRP) sheets with different wrapping configurations. Two shear strengthening wrapping schemes of CFRP sheets will be investigated; U-Wrapped and completely Wrapped. Although the completely wrapped scheme is more effective, however, due to its limitations, the U-Wrapped scheme became the most commonly used method in shear strengthening of RC structures. The drawback of this strengthening scheme is the premature failure caused by debonding of the CFRP laminates, without utilizing its full strength. The main aim of this study is to compare the performance of a U-Wrapped T-beam with two completely wrapped rectangular beams. The first strengthened rectangular specimen (WBR1) has a depth equivalent to the T-beam’s web height, while the second strengthened rectangular specimen (WBR2) has a depth equivalent to the T-beam’s total depth. In addition, a control T-beam and two rectangular beams were cast and tested as benchmark specimens. The experimental test results showed that the beam strengthened by U-Wrapped CFRP sheets increased the control beam’s shear strength by 114.82%, while the increase in shear capacity of the completely wrapped equivalent WBR1 and WBR2 beams over their control unstrengthened specimens was 69.28% and 201.63%, respectively. In addition, the completely wrapped scheme provided more ductility compared to that of the U-Wrapped T-beam specimen, that failed by CFRP sheets debonding in a brittle manner. Thus, it could be concluded that an ideal way to increase the shear capacity and ductility of RC beams is to completely wrap the beams using CFRP sheets. However, if the completely wrapping scheme is not possible due to geometrical obstructions, U-wrapping scheme could be effective in increasing the shear capacity of RC beams but will fail in a brittle mode by sheet debonding without utilizing its full strength. Anchoring the CFRP U-Wraps could be a viable solution to enhance the performance of strengthened RC beams that should be investigated in future research studies.

Axial compression–bending interaction behavior of severely damaged RC columns rapid repaired and strengthened using hybrid FRP composites

This paper investigates the axial compression and bending interaction behavior of reinforced concrete (RC) columns repaired and strengthened with different FRP (Fiber Reinforced Polymer) techniques after severe damage. The columns were initially damaged under different axial compression to bending load ratios. The loads were applied at different eccentricity (e) to depth (h) ratios to have (i) pure compression (e/h = 0), (ii) eccentric compression (e/h = 0.63) and (iii) pure flexure (e/h = ∞). After initial damage, the specimens underwent severe concrete core degradation with the main steel reinforcement reaching its yield strain. Later, the specimens were repaired using high strength cement grout (HSCG) and further strengthened using (i) near surface mounting (NSM) of carbon FRP (CFRP) laminates and (ii) hybrid FRP technique with a combination of NSM laminates and external bonded (EB) fabrics. After that, the columns were loaded again until failure. An axial compression (P) – bending moment (M) interaction diagram was developed experimentally for the severely damaged columns. Also, the interaction curves were developed analytically using strain compatibility procedure with the modified constitutive models. Experimental results reveal that the specimens repaired and strengthened using hybrid FRP technique can effectively restore the original capacity under different eccentric load levels. The analytical predictions showed a close match when compared to the experimental observations.

Shear strength components of adjustable hybrid bonded CFRP shear-strengthened RC beams

This paper presents an experimental study on shear behavior of full-scale rectangular reinforced concrete (RC) beams under shear-strengthening of carbon fiber-reinforced polymer (CFRP) U-strips. The test involved five RC beams with transverse reinforcement, including one control beam, one shear-strengthened by externally bonded (EB) CFRP U-strips only, and the remaining three strengthened in shear by a hybrid bonded (HB) CFRP system under different normal pressures applied on the CFRP U-strips. The transverse reinforcing steel bars and CFRP U-strips were extensively instrumented with strain gauges, which enabled the accurate quantitative assessment of shear strength contributions from CFRP (Vf), stirrups (Vs) and concrete (Vc) during the whole loading process of a shear test. Further, by comparing Vs and Vc for beams with and without CFRP strengthening, the interactions can be evaluated and clarified. In addition, key features of shear behaviors, e.g. failure mode, load-deflection curves, shear capacity and ductility, shear crack angle, and strain development and distribution of stirrups as well as CFRP strips, are presented, compared and analyzed.

Effect of transverse groove on bond behavior of FRP-concrete interface: Experimental study, image analysis and design

This study conducts a comprehensive investigation into the effect of transverse grooves on the bond behavior of fiber-reinforced polymer (FRP)-concrete interface. Twenty-seven concrete block specimens were tested under single shear to experimentally evaluate the effect of groove characteristics (such as clearance, width, and depth) on bond behavior. The ultimate load increased when a larger ratio (i.e., larger width and smaller clearance) of groove was used. As the groove depth increased, the ultimate load first increased and then decreased. The transverse groove yielded several peak bond stress distributions and higher bond strength. The following image analysis reveals that transverse groove enhanced the bond strength of the concrete-epoxy interface and switched the fracture surface from the concrete-epoxy interface to other interfaces, improving bond strength. Finally, a design method is proposed to predict the ultimate load of externally bonded (EB)-strengthened concrete structures using transverse groove. The contributions of different failure fracture surfaces are considered, and the empirical relationship between the percentage of each failure mode and groove characteristics is established. The comparison between the experimental and predicted ultimate load shows that the proposed design method provides close prediction of ultimate load.