Fiber Reinforced Polymer Strengthening Research Articles

Experimental and numerical study on the torsional behavior of rectangular hollow reinforced concrete columns strengthened By CFRP

This paper reported a series of hysteretic torsion experiment to investigate the torsional behavior of rectangular hollow reinforced concrete (RHRC) column strengthened by fiber reinforced polymer (FRP). Six RHRC column specimens with different number of longitudinal reinforcements, spacing of stirrup and strengthening method using FRP were designed. One was not strengthened, four were strengthened with CFRP, one was strengthened with CFRP and GFRP. The experimental results showed that the primary failure modes of specimens were the spalling of surface concrete with the detachment of FRP. In details, under the hysteretic torsional load, the interaction between adhesive and concrete caused the intersecting diagonal cracks in the internal concrete. Compared with the hysteretic curve of specimen without FRP strengthening, FRP strengthening can significantly improve the initial stiffness by 50 % and peak torsional strength by 70 %. For RHRC column without strengthening, the fullness was poor because of the weak torsional energy dissipation. The FRP strengthening can also enhance the torsional energy dissipation and seismic behavior of RHRC column. To predict the complex torsional behavior of RHRC column strengthened by FRP, a finite element (FE) model and a constitutive model were developed. The FE model considered potential cracks in concrete and FRP–concrete interface based on the application of the cohesive zone model (CZM), whereas the constitutive model accounted for interface damage and plasticity. The results of the performed simulations indicated that the proposed model can effectively represent the hysteretic mechanical behavior of columns under torsional load, which cannot be achieved using conventional FE methods.

Clustering classifier of FRP strengthened concrete beams using superpixels and principal component analysis

Nowadays, the application of Fiber Reinforced Polymer (FRP) materials in concrete structures is more and more extended because of their advantages. However, premature and brittle failure caused by debonding of FRP materials from the concrete surface may be critical and needs to be detected at its earliest stages to avoid possible catastrophic events. Digital image correlation (DIC) has been used in the past for the detection of abnormalities from surface digital images. In this work, an efficient computer-aided diagnosis (CAD) system for damage identification for this type of FRP strengthening is developed. It is based on the combination of cells of strain maps built with a DIC system and an automated clustering classifier optimized with the use of superpixels and principal component analysis. Additionally, preprocessing of the images is also done with some filters to enhance the accuracy of the methodology. The procedure has been validated with experimental results on a progressively damaged FRP externally strengthened beam. Results show that the proposed detection scheme achieves an effective automated classification method for the damage detection of this kind of strengthened specimens. Furthermore, the optimal choice of the subset size, number of superpixels and principal components contributed to the success of the method. Simultaneously, an attempt to quantify damage from the proposed method and its validation with electromechanical impedance method has been developed.

Experimental analysis of the bond-slip characteristics between CFRP and natural stone masonry units

Purpose Although the reinforcement of concrete and brick masonry with fiber-reinforced polymer (FRP) has been extensively researched, its application and impact on natural stone, especially in historic preservation, have received less attention. This study aims to examine the bond-slip characteristics of carbon fiber-reinforced polymer (CFRP) with two types of natural stone masonry, aiming to enhance their effectiveness in reinforcing historic structures. The stones studied include one from the Chouf-Lekdad region (A) and another from a historic structure in Sétif City (B). Both stones were strengthened using CFRP and carbon fiber fabric (CFF) through near-surface mount (NSM) and external bonding (EBR) techniques. Design/methodology/approach The interaction was assessed during the pull-out test by analyzing the stress transfer mechanisms, adhesion and deformation. This study also examines the effects of the following parameters on the bond between CFRP and stone: type of stone (A and B), type of reinforcement (plat CFRP and CFF), various notch shapes and sizes (bp, tp and Lb), and reinforcement techniques (NSM and EBR). Findings This study demonstrated the practicality and effectiveness of enhancing natural stone masonry of old buildings by integrating NSM and EBR techniques with CFRP. With a bond length of 30 mm, the pull-out force correlates with the strength of the stone. This indicates the importance of stone strength in obtaining better adhesion. The CFF–resin interface is more cohesive than the CFRP plate–resin interface because the resin penetrates the flexible CFF strip, ensuring better adhesion. In contrast, the CFRP plate interface is rigid and smooth. The results suggest that natural stone–CFRP adhesion is more effective than CFRP bonded to concrete and brick masonry due to the stone's strong resistance. Originality/value This experimental investigation provides new study into the bond-slip behavior of CFRP-reinforced natural stone masonry, filling the gap in existing research. The findings offer useful direction for creating FRP strengthening solutions that are specifically adapted to the properties of natural stone used in historic constructions. This study helps to improve preservation procedures by guiding the selection of reinforcing techniques, such as NSM versus EBR, and finding ideal bond lengths. This work's novelty stems from its ability to improve the structural integrity of culturally significant buildings while preserving their historical authenticity.

A new finite element formulation for the lateral torsional buckling analyses of orthotropic FRP-externally bonded steel beams

Abstract The present study develops a new finite element (FE) formulation for the lateral torsional buckling (LTB) analyses of steel beams exteriorly bonded with orthotropic fiber-reinforced polymer (FRP) layers. The formulation considers shear deformations, partial material interaction, local and global warping deformations, and orthotropic FRP properties. The buckling responses of multispan FRP-bonded steel beams predicted by the present solutions are excellently validated against experiment and numerical solutions. As observed, the FRP strengthening is highly effective for the LTB resistance of steel beams. However, the LTB responses are strongly dependent on the orthotropic properties and strengthening lengths of FRP layers. The effects of shear deformations, span ratios, and loading conditions on the LTB responses are also quantified in the present study.

Evaluation of various FRP strengthening configurations for RC beam-column joints

There are several methods for strengthening reinforced concrete joints. Due to the unique properties of FRP (Fiber Reinforced Polymer) composites, the use of FRP laminates is one of the most commonly used techniques. The high cost of preparing concrete surfaces and attaching FRP laminates is a restricting factor for its application in reinforced concrete joint retrofit. Therefore, determining proper configurations that reduce the needed FRP material, as well as the surface preparation required for attaching FRP is an important factor in decreasing the strengthening cost. The proper arrangements of FRP strips can improve their performance in the rehabilitation and retrofit of reinforced concrete joints. To determine the proper configuration of FRP strips, finite element modeling was employed. The connection specimens are modeled in ABAQUS general-purpose finite element software and are classified into 10 general groups. To improve the performance of FRP strip configuration, the determined arrangement is investigated for different thicknesses and different widths of the FRP strips, and their results are compared with those of the connection specimen strengthened with full FRP coverage. The analysis results indicated that the load-bearing capacity of the connection retrofitted by the combined configuration of X-shape and orthogonal strips of FRP is close to that of the specimen with full FRP coverage. In this suitable configuration, the required FRP strips are reduced by about 30%, which decreases the cost and construction works needed for concrete joint rehabilitation.

Damage Detection In Concrete Beams Strengthened With Frp Using Principal Component Analysis And K-Means Clustering

Fiber reinforced polymers (FRP) strengthening systems have been considered as an effective technique to retrofit concrete structures and their use nowadays is more and more extended. However, one of their main disadvantages is a possible brittle failure mode provided by a sudden debonding of the FRP. Digital image correlation (DIC) has been used in the past for the detection of abnormalities from surface digital images. In this work, an efficient computer-aided diagnosis (CAD) system for damage identification at its earliest stages for this type of FRP strengthening is developed. For it, a clustering classifier is implemented from the cells of the strain maps built with a DIC system. Additionally, to optimize the procedure, principal component analysis (PCA) is used for feature selection.

Flexural behavior of historical RC beams strengthened with hybrid FRP sheets

Historical reinforced concrete (RC) buildings are culturally and aesthetically significant and require conservation due to aging. Strengthening RC buildings often requires the use of Fiber Reinforced Polymer (FRP) sheets, however distinct material properties of historical RC buildings have not been systematically considered yet. This study investigates the flexural behavior of historical RC beams strengthened by hybrid FRP sheets. A total of 11 beam specimens with varying FRP sheet configurations were tested using a four-point bending test to assess their flexural behavior, including failure mode, cracking pattern, load-deflection behavior, strain distribution, and ductility. The results showed that hybrid FRP sheets increased the maximum bearing capacity of strengthened specimens up to 30.2 %. An optimal fiber sheet configuration (T-3) is identified. 45-degree incline installation contributes to a 4.5 % increase in the bearing capacity of specimens. The average ductility loss of hybrid FRP sheets strengthened specimen (except for double-layer strengthened specimen) is 42 %, while an increase of sheet layer post adverse effect on ductility. Strain distribution conforms to the plane-section assumption. Additionally, a maximum bearing capacity calculation model was proposed, aiding the understanding of FRP strengthening in historical RC buildings.

Bond behaviour at high temperatures between concrete and CFRP or steel strengthening bars applied according to the embedded through-section (ETS) technique

The embedded through-section (ETS) technique is an efficient shear-strengthening method for reinforced concrete (RC) structures, in which fibre reinforced polymer (FRP) or steel bars are bonded in holes predrilled in RC members. Compared to conventional FRP strengthening methods, ETS systems are expected to present better performance when exposed to high temperatures/fire, due to the insulation provided by concrete to both adhesive and FRP/steel components. Yet, the mechanical/bond properties of the strengthening materials are highly affected when the glass transition temperatures (Tg) of their components is attained/exceeded; however, there are no studies available about the influence of elevated temperatures on the performance of ETS systems. This paper presents an experimental study that addresses the effects of elevated temperatures on the bond behaviour of ETS systems – pull-out tests were conducted on steel rebars or sand-coated carbon-FRP (CFRP) rods bonded with epoxy adhesive on concrete cylinders at temperatures up to 90 °C. The results obtained confirm that the strength and stiffness of the bonded interfaces decrease when the temperature in the adhesive approaches/exceeds its Tg; moreover, with sand-coated CFRP rods, failure was also influenced by the bond between the sand coating and the CFRP rods core, and thus the average bond capacity was significantly reduced for temperatures below the Tg of the epoxy adhesive. Bond-slip relations were calibrated for different elevated temperatures for the steel/CFRP-concrete interfaces using data obtained from the pull-out tests; these bond laws can be used to simulate the mechanical behaviour of ETS-strengthened-RC members subjected to elevated temperatures.

INNOVATIVE TORSIONAL STRENGTHENING OF RC T-BEAMS USING NSM FRP ROPES

This study addresses the critical need for effective torsional strengthening techniques in reinforced concrete (RC) structures, with a focus on T-beams. Existing methods, particularly those utilizing externally bonded fiber-reinforced polymer (FRP) composites, have been extensively studied for axial, bending, and shear loads, but torsional behavior remains a complex and underexplored area. The paper introduces a novel approach employing FRP ropes configured as closed stirrups, aiming to enhance torsional capacity while addressing practical challenges associated with conventional methods. Experimental investigations on T-shaped RC beams reveal promising results, demonstrating the effectiveness of FRP ropes in closed stirrup form. The findings not only contribute to a deeper understanding of this specific strengthening technique but also offer practical insights for designing and retrofitting RC structures susceptible to torsional loading. The study's outcomes have the potential to influence design practices, contribute to sustainable infrastructure development, and inform future guidelines for optimizing FRP strengthening strategies in T-beams.

INNOVATIVE TORSIONAL STRENGTHENING OF RC T-BEAMS USING NSM FRP ROPES

This study addresses the critical need for effective torsional strengthening techniques in reinforced concrete (RC) structures, with a focus on T-beams. Existing methods, particularly those utilizing externally bonded fiber-reinforced polymer (FRP) composites, have been extensively studied for axial, bending, and shear loads, but torsional behavior remains a complex and underexplored area. The paper introduces a novel approach employing FRP ropes configured as closed stirrups, aiming to enhance torsional capacity while addressing practical challenges associated with conventional methods. Experimental investigations on T-shaped RC beams reveal promising results, demonstrating the effectiveness of FRP ropes in closed stirrup form. The findings not only contribute to a deeper understanding of this specific strengthening technique but also offer practical insights for designing and retrofitting RC structures susceptible to torsional loading. The study's outcomes have the potential to influence design practices, contribute to sustainable infrastructure development, and inform future guidelines for optimizing FRP strengthening strategies in T-beams.

Influence of deteriorated stirrups on the shear performance of RC beams incorporating fly ash and enhanced with carbon fiber-reinforced polymer strengthening

Influence of deteriorated stirrups on the shear performance of RC beams incorporating fly ash and enhanced with carbon fiber-reinforced polymer strengthening

Investigation of the Effect of CFRP and GFRP Sheet Length on the Final Capacity of Reinforced Concrete Beams of Buildings in Coastal Areas

In the last two decades, the use of FRP (Fiber Reinforced Polymer) composites for flexural, shear, torsion, and enclosing reinforcement of various structural components has grown exponentially. In this paper, the effect of using FRP composite with carbon fibers and glass with different lengths on the behavior of reinforced concrete beams is investigated. Eight reinforced concrete beams with a length of 150 cm, a width of 15 cm, and a height of 20 centimeters were modeled by composite fibers with lengths of 80 cm, 100 cm, 120 cm and 140 centimeters. Then, the sample reinforced concrete beam was modeled in ABAQUS® , which the results were compared with the laboratory samples. It were observed that with increasing the length of FRP sheets, the strength of reinforced concrete beams compared to the sample concrete beam, increased by about 61%, 82%, 86%, and 87%, respectively. It was due to the high cost of FRP sheets and compressive strength required in these sheets that used optimally. Therefore, the length of FRP strengthening sheets can be optimized to reduce the cost of FRP sheets and use them efficiently.

Cracking and failure mode behavior of hybrid FRP strengthened RC column members under flexural loading

AbstractStrengthening of reinforced concrete (RC) members in bridges and buildings is often required to improve flexural performance. Strengthening using fiber‐reinforced polymer (FRP) composites has become popular as FRPs offer multiple advantages compared with conventional strengthening practices like steel jacketing and concrete enlargement. External bonding (EB) using carbon FRP (CFRP) laminates/fabric has become a common strengthening practice. The effectiveness of the EB strengthened system is reduced due to possible debonding of FRP from concrete substrate. The near‐surface mounting (NSM) of FRP laminates is efficient in reducing the debonding failure, but it is not effective under dominant compression loading. Hybrid FRP (HYB) strengthening combines the advantages of both the NSM and EB techniques. HYB strengthening can enhance the performance of RC members under all loading combinations. The main objective of this study is to understand the flexural crack opening behavior of control and hybrid FRP strengthened RC column members under pure bending. The digital image correlation (DIC) analysis shows that the crack width and its propagation are significantly reduced due to hybrid FRP strengthening. The crack widths measured using the DIC are compared with the analytical predictions based on Eurocode 2 and fib bulletin 90. HYB FRP strengthening increased the strength and post‐cracking stiffness of RC elements under flexure without compromising the ductility. In addition, the HYB strengthening distributed the damage under flexural loading, unlike localized damage observed in control RC elements.

Strengthening of two-way slabs with fiber-reinforced polymer composites: A new system using grooving technique and fans

The flexural capacity of a reinforced concrete (RC) two-way slab can be efficiently enhanced using fiber-reinforced polymer (FRP) composites if FRP debonding is postponed. On the other hand, with the increase in flexural capacity, the vulnerability of the slab to punching shear failure increases. In this context, the current study examines FRP strengthening of lightly reinforced slabs using the externally bonded reinforcement on grooves (EBROG) technique, which has already been proven successful in postponing FRP debonding. Four slab specimens are tested in two groups under concentric loading. The first group consists of two specimens reinforced with headed shear studs: one reference specimen and one specimen strengthened in flexure with FRP. The second group includes two specimens without internal punching shear reinforcement: one reference specimen and one specimen strengthened with a new simultaneous flexural and punching shear strengthening system, taking advantage of FRP fans. According to the results, flexural strengthening of the slab in Group 1 increased its ultimate load capacity by 77%, and the proposed strengthening system in Group 2 led to a 94% enhancement of the ultimate load capacity. A calculation method based on the yield-line theory is presented to predict the load capacity of the test specimens. According to this method, graphs simplifying the estimation of the flexural capacity of FRP-strengthened slabs are proposed.

Finite Element Investigation of the Ultimate Capacity of FRP Strengthening Soffit Curved Girders

In recent decades, fiber reinforced polymer (FRP) has been increasingly used to reinforce curved reinforced concrete girders. The main objective of this research is to study the effect of curvature on the performance of curved reinforced concrete (RC) girders reinforced with fiber reinforced polymers (FRP). Five models with a length of 6 m and different degrees of curvature (1,2,3,4,5) mm/m were used, each with dimensions similar to the models used in cross-sectional area and effective extension similar to those of the source model. The modeling was in Ansys software and the girders were simulated to obtain the load deflection with respect to the mid span, failure load and failure pattern, in order to understand the effect of variable curvature on the performance of this type of structural element, it was applied under one central load from the middle of the girder length until failure. It was observed that the curvature pitch of 5 mm/m reduced the load-bearing capacity of the c RC curved girders by 7.33%.

Experimental and theoretical study on flexural behavior of hybrid bonded CFRP RC beams

Fiber reinforced polymer (FRP) composites are widely used to strengthen reinforced concrete (RC) structures due to their high mechanical and durability properties compared to traditional materials. However, intermediate crack debonding (ICD) failure limits the effectiveness of strengthening RC beams in flexure with externally bonded (EB)-FRP laminates. Anchorage of the FRP can mitigate this debonding, enhancing the efficiency of FRP strengthening. Hybrid bonding (HB)-FRP emerged as a viable method for delaying/preventing debonding of EB-FRP RC beams. HB combines adhesive bonding from EB with mechanical fastening through metallic plates, increasing resistance to debonding. While previous research explored the bonding capacity of the HB connection using FRP sheets in single-shear tests, few simplified approaches exist for predicting the bending capacity of strengthened beams. In this paper, the feasibility of HB systems to delay ICD of RC flexural elements strengthened with carbon FRP (CFRP) pre-cured laminates is assessed through experimental and theoretical studies. For this purpose, three RC beams strengthened with CFRP are tested, one using EB and two using HB with different anchor spacings. Results include load-deflection response, flexural capacity and failure mode. Experimental results are compared to literature provisions. Results show that the HB technique consistently improves the flexural performance of the beam in all considered cases.

A new bonding agent for the Near-surface mounted Fibre-reinforced polymer strengthening system for concrete structures

Over the past few decades, the implementation of Near Surface Mounted (NSM) Fibre Reinforced Polymers (FRP) has become a promising strengthening method for improving the performance of concrete structures. Despite the common use of epoxy adhesives in NSM FRP systems, concerns have arisen due to their drawbacks, including the emission of toxic fumes and reduced mechanical strength when subjected to elevated temperatures and moisture. To address these issues, cement-based adhesives have been introduced as a substitute bonding agent with the goal of enhancing the performance of concrete structures in high-temperature settings and minimizing environmental and health hazards. In this paper, a new cement-based adhesive was introduced for the NSM FRP system for concrete structures. The efficiency of this adhesive was assessed through an experimental investigation involving pull-out testing using a single-lap shear test setup to study the bond behaviour of the NSM Carbon Fibre-Reinforced Polymer (CFRP) laminate for concrete structures. The results highlighted the potential of the developed cement-based adhesive as a promising alternative for strengthening systems requiring ductile bond performance, achieving approximately 65% of the maximum bond stress compared to epoxy adhesives in the NSM FRP system.

Overcoming the brittleness of shear failure: A new FRP-RSC strengthening philosophy

Due to the linear elastic stress–strain behavior of carbon fiber reinforced polymer (CFRP), the ductility of CFRP shear-strengthened reinforced concrete (RC) beams will be significantly reduced. Compared with shear capacity enhancement, increasing the ductility of CFRP shear-strengthened RC members is a much more challenging task, and a satisfactory solution has yet to be found. This paper introduces a new methodology that combines the brittle fiber reinforced polymer (FRP) with ductile rubber support composite (RSC) to form the FRP-RSC strengthening system. The variable stiffness RSC can adjust the stiffness of the strengthening system, which is achieved by the changed shear transfer actions in FRP, transverse reinforcement and concrete. Eight specimens were tested to evaluate the effectiveness of the FRP-RSC strengthening method. Experimental tests demonstrated that the displacement ductility with this FRP-RSC technique was 2.15 times that of the conventional externally bonded (EB) FRP strengthening, reaching remarkably 5.24. The mechanisms for enhancements in ductility, deformability and shear capacity were revealed. Poor matches were observed between the predicted FRP shear strength contribution (Vf) per existing models and test data. A new model that can predict Vf of FRP-RSC strengthened RC beams was developed, and better agreements between predictions and experimental results were obtained.

Assessment of shear capacity in NSM prestressed FRP strengthening with anchorage conditions

Efforts to achieve a defect-free flat groove in near-surface-mounted prestressed fiber-reinforced polymer (FRP) strengthening systems have proven challenging. Despite attempts, attaining an ideal groove remains formidable. In this system, anchorage device installation involves injecting epoxy resin into groove defects to achieve planarity. Structural behavior analysis and optimization of the anchorage device were conducted based on device type, anchor type, and defect depth through shear loading tests. The mechanical anchor demonstrated an ultimate load of 261 kN, with failure modes including concrete cracking, concrete compression failure, and anchor tensile failure. The chemical anchor exhibited an ultimate load of 390 kN, with its final failure attributed to anchor tensile failure. The ultimate load capacity of anchorage device surpassed that of a carbon FRP(CFRP) bar by up to 62.5%. In a parametric study, the shear strength of the anchorage device was explored concerning concrete compressive strength and anchor diameter. The ultimate load of the proposed model with low concrete compressive strength proved insufficient compared to the strength of CFRP rebar.

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.