Fiber Reinforced Polymer Strips Research Articles

Evaluation of shear behavior of short-span reinforced concrete deep beams strengthened with fiber reinforced polymer strips

The application of externally bonded fiber-reinforced polymers (EB-FRP) is known to improve the shear behavior of reinforced concrete beams. Nevertheless, the studies available in the literature are not sufficient to explain the shear behavior of the beams that have low shear span to effective depth ratios (av/d) and are strengthened with FRP and the interaction between the horizontal steel shear reinforcement and FRP strips. To fill these gaps in the literature, an experimental study involving twenty-two tests of deep beams with a low av/d and with different amounts of vertical and horizontal steel shear reinforcement was carried out. Both unstrengthened and strengthened with fully wrapped FRP strips beams were tested where the number of FRP layers, the spacing between them and the type of FRP material were varied. The experimental results showed that the FRP strips improved the shear and ductility capacities of beams with low av/d. In addition, as the vertical shear steel reinforcement ratio in the beams strengthened with FRP strips increased, the percentage increases in the shear and deflection capacities decreased. Also, as the horizontal shear steel reinforcement ratio in the strengthened beams increased, the percentage increase in the shear capacity increased. Finally, a new equation was proposed to estimate the FRP contribution to the shear capacity. To prove the reliability of the proposed equation, a comparative study was conducted using twenty existing equations.

STUDY THE EFFECT OF EXTERNAL CRACK ON THE MECHANICAL PROPERTIES OF COMPOSITE MATERIALS

Increasing the strength of the matrix material rather than increasing stiffness is frequently the primary objective when designing composite materials. This is typically the case when the matrix is made of brittle, low-strength material. the resilience of this matrix substance Usually, it is determined by the initial flaws in the raw material. Depending on the microstructure of the composite material, these flaws or fissures may be large or minor. The orientations of the foil fibers, which can be customized to improve the performance of the laminate, have a significant impact on the properties of fiber reinforced polymer (FRP) laminates. This study's goal is to look at how matrix cracking affects the mechanical characteristics of FRP chips. The presence and absence of exterior cracks are examined in unidirectional glass fiber reinforced polymer strips. Samples were manually cast using plastic molds constructed in accordance with ASTM standards. It were able to comprehend the impact of external cracking on composite materials through the mechanical tests that were conducted since it causes the material to weaken. Consequently, it is seen as a location where tensions are concentrated.

A novel double bridging-slipping (DBS) concept to overcome deformation incompatibility of textile reinforced-engineering cementitious composite (TR-ECC)

Due to the deformation incompatibility between the engineering cementitious composite (ECC) matrix and embedded fiber reinforced polymer (FRP), conventional textile reinforced (TR)-ECC usually fails in premature FRP rupture when subjected to a tensile load. To address this issue, a novel double bridging-slipping (DBS)-based concept is proposed in this study. Apart from bridging-slipping capacity of incorporated short fibers in the ECC matrix, the presence of FRP strip can contribute to the other bridging-slipping mechanism for enhancing the tensile performance of the whole specimen. The embedment zone of FRP strips can bridge the crack tips of ECC matrix through the FRP-ECC interfacial bond stress, and the slippage of FRP strips in the uncracked zone can compensate for the excessive deformation of ECC matrix in the multi-cracking zone. Experimental results show that compared to the conventional TR-ECC, the proposed DBS-based TR-ECC with the same FRP volumetric ratio of 0.113 % can significantly improve the ultimate tensile strain and toughness by 148.3 % and 38.0 %, respectively. As DBS-based TR-ECC facilitates the combination of superior strength of FRP textiles and excellent toughness of ECC materials, it has a promising application in infrastructure projects.

Full-Range fatigue behavior of NSM CFRP-to-Concrete bonded joints

The interfacial bond characteristics and failure mechanism of near-surface mounted (NSM) fiber reinforced polymer (FRP) composites in concrete under fatigue loading are less understood compared to their bond behavior under static loading. In this paper, degradation of the local bond strength of FRP-to-concrete bonded joints subjected to direct pull-out fatigue loading is presented for both carbon FRP (CFRP) rods and strips under different fatigue load ranges, i.e., 10–50%, 10–60%, and 10–70%, of the corresponding static load-carrying capacity. The interfacial debonding between FRP and adhesive was observed during fatigue cycles for CFRP rod specimens to different extents. Specimens using CFRP rods with sand-coated and spirally wound surface treatment presented a higher local bond strength and a slower degradation rate, as compared to the ones using CFRP rods with roughened or sand-coated treatment. Using epoxy adhesives with higher strengths benefitted the fatigue bond performance of the tested specimens in terms of local bond strength and stable behavior. Higher fatigue load ranges substantially accelerated the local bond degradation resulting in a much shorter fatigue life of the bonded joints.

A state-of-the-art review on reinforced concrete slabs strengthened by frp sheets under monotonic, impact and repeated loading

Composites made of fiber-reinforced polymer (FRP) have gained popularity as new high-performance material for concrete buildings in now days. The advantages of FRP composites include high strength, lightweight, and corrosion resistance. This paper’s primary objective is to assess the affect of FRP strips on the strengthening and repair of plain and reinforced concrete slabs. under the monotonic, impact, and repeated loading. According to studies’ observations, applying FRP strips to RC slabs significantly affects the final load and deflection. The number and positioning of FRP sheets improve the performance of slabs. The failure load magnitude of strengthened slabs would increase as slab thickness, concrete compressive strength, and sheet thickness increase. The ultimate load capacity was increased by (27-52%), depending on the FRP strengthening strategy used. GFRP sheets could be employed for improving or enhancing the impact strength of concrete structures. Externally attached GFRP sheets gave good resistance for reinforced concrete slabs against typical explosive blast.

Experimental and finite element analysis of reinforced concrete multi-cell box girders retrofitted with carbon fiber reinforced polymer strips under torsion

This study expands the state of the art in studies that assess torsional retrofit of reinforced concrete (RC) multi-cell box girders with carbon fiber reinforced polymer (CFRP) strips. The torsional behavior of non-damaged and pre-damaged RC multi-cell box girder specimens externally retrofitted by CFRP strips was investigated through a series of laboratory experiments. It was found that retrofitting the pre-damaged specimens with CFRP strips increased the ultimate torsional capacity by more than 50% as compared to the un-damaged specimens subjected to equivalent retrofitting. This indicated that the retrofit has been less effective for the girder specimen that did not develop distortion beforehand as a result of pre-loading. From experimental observations, when the girder specimen was cracked and the transverse steel reinforcement bars started straining, the CFRP strips started to work more effectively and contributed together with the transverse steel reinforcement to resist torsion load. Additionally, Finite Element (FE) simulations were developed using Abaqus to model the torsional behavior of the retrofitted girders, the results of which were validated with the experimental data. With the numerical model, the effect of concrete compressive strength, transverse reinforcement spacing, and CFRP strip spacing were examined. The effectiveness of the CFRP strips in enhancing the torsional strength of the girder was found to increase with increasing the spacing between the transverse reinforcement.

Torsional behavior of FRP-strengthened reinforced concrete members considering various wrapping configurations: Theoretical analysis and modeling

Reinforced concrete (RC) structural members such as spandrel, flanged and curved beams, slab beams, and bridge girders may be subjected to significant torsional loading. Despite extensive research on the strengthening of RC members, very little is known about the torsional behavior of RC members strengthened with fiber-reinforced polymer (FRP) sheets. Therefore, very limited guidelines apply to the design of externally bonded FRP systems for torsional strengthening, and no design provisions exist for the U-wrapped and 2-sided strengthening systems in torsion. This study aims to contribute to this growing area of research by developing a new analytical model to predict the full torsional response of strengthened RC members with many conventional configurations such as spiral, transverse and longitudinal FRP, continuous and intermittent strips along the beam length, side bonded, and full-wrapped systems. Moreover, this study provided an analytical model accurately predicting the contribution, efficiency, and failure mode of side-bonded strengthening systems. Additionally, the influence of various parameters such as FRP stiffness and transverse steel reinforcement ratio is taken into account in the constitutive law of concrete and internal reinforcement for the first time. The good agreement between the test results of 32 collected FRP-strengthened RC beams from the literature and the theoretical ones, which were predicted through programming proves the validity and accuracy of the presented theory.

Numerical analysis, experimental tests and non-linear analytical models in the study of bent timber beams reinforced with FRP strips

The paper presents selected methods for modelling bent timber beams strengthened with fibre-reinforced polymers (FRP). The assumptions are described, and the main analytical models representing the non-linear bending behaviour of wood are shown. Based on the models presented, analytical approaches considering reinforcement in the tension zone are proposed. The problem of determining the appropriate material parameters of the wood used in analytical calculations and numerical models is discussed. The main numerical methods used in timber analyses are presented. The Hill yield criterion and the generalised Hill yield criterion are discussed in more detail. A method for retrospective estimation of material properties of wood taking into account non-linear material models and fitting the generalised Hill yield criterion is described. Experimental bending tests of technical-scale beams reinforced with composites are presented. The numerical analysis was carried out and discussed. The generalised Hill yield criterion was applied to represent the behaviour of the wood, distinguishing the different yield stresses in compression and tension. The suitability of using non-linear models and advanced strength criteria in the numerical analysis of timber elements is discussed.

Strengthening of RC beams in shear with novel near-surface mounted (NSM) U-shaped fiber-reinforced polymer (FRP) composites

Over the last three decades, various researchers have proven the high efficiency of the near-surface-mounted (NSM) fiber-reinforced polymer (FRP) method for strengthening reinforced concrete (RC) beams. The shear strengthening of RC beams by embedding FRP bars or strips onto the side concrete cover is a primary application of NSM FRP investigated in numerous studies. These studies revealed that despite the high bond efficiency between the NSM FRP and concrete, NSM FRP shear-strengthened beams frequently failed in debonding modes, such as FRP–concrete interfacial debonding or side concrete cover separation. Such debonding failures limit the strength utilization and strengthening efficiency of the FRP material. Anchoring measures have been proposed for NSM FRPs to mitigate such debonding failures. However, the manufacturing and installation processes of these anchoring devices are complicated. Moreover, interfacial debonding between the NSM FRPs and anchoring devices can still occur. Therefore, this study proposes a novel NSM U-shaped FRP (NUF) device for the shear strengthening of RC beams. The proposed NUF device was fabricated using a fiber sheet through a wet lay-up process. The device can effectively increase the bond efficiency between the FRP and strengthened beam. The concept, manufacturing, and installation processes of this device are discussed. Subsequently, an experiment was performed on seven full-scale RC beams. The test results showed that the proposed NUF device is able to postpone/prevent the debonding failure between NSM FRP and concrete thus largely increase the strengthening efficiency of NSM FRP method.

Estimation of the Ultimate Strength of FRP Strips-to-Masonry Substrates Bond

Fiber-Reinforced Polymers (FRP) were developed as a new method over the past decades due to their many beneficial mechanical properties, and they are commonly applied to strengthen masonry structures. In this paper, the Artificial Neural Network (ANN), K-fold Cross-Validation (KFCV) technique, Multivariate Adaptive Regression Spline (MARS) method, and M5 Model Tree (M5MT) method were utilized to predict the ultimate strength of FRP strips applied on masonry substrates. The results obtained via ANN, KFCV, MARS, and M5MT were compared with the existing models. The results clearly indicate that the considered approaches have better efficiency and higher precision compared to the models available in the literature. The correlation coefficient values for the considered models (i.e., ANN, KFCV, MARS, and M5MT) are promising results, with up to 99% reliability.

Effect of compression casting method on the axial compressive behavior of FRP-confined seawater sea-sand concrete columns

The application of the novel compression casting method is beneficial to improve the compressive strength and durability of concrete. However, the compressed concrete fails rapidly after the peak load and exhibits stronger brittleness than the normal concrete, which could impair the seismic performance of the columns. As one of the passive confining measures, wrapping fiber-reinforced polymer (FRP) could be a practical technique to improve the ductility of such brittle columns. In this regard, this study experimentally investigated the axial compression behavior of both the partially and fully FRP-confined compression-cast seawater sea-sand concrete (SSC) columns. The test results indicated that the compression casting method enhanced the compressive strength but weakened the axial deformation capacity of the unconfined and FRP-confined SSC specimens. The compressive strength of unconfined concrete was improved by 38.5% due to compression casting. The measured average FRP rupture strain of the FRP-confined compression-cast SSC specimens was 6.3% smaller than that of FRP-confined normal SSC specimens due to the sudden appearance of the main oblique cracks, indicating poorer FRP utilization efficiency. Meanwhile, the average ultimate axial strain enhancement ratio of FRP-confined SSC specimens decreased from 12.04 to 7.83 after compression casting. The FRP hoop strain of the fully FRP-wrapped columns distributed more uniformly compared to the partially FRP-wrapped columns as the mean value of FRP efficiency factor increased from 0.807 to 0.928. With increasing FRP strip spacing, the post-peak segment of the stress–strain curve transferred from strain-hardening to strain-softening regardless of the confining stiffness. Finally, several existing stress–strain models were employed to examine their accuracy in anticipating the ultimate conditions of the FRP-confined compression-cast SSC columns.

Retrofitting and rehabilitation of masonry structures using FRP composites

Fibre Reinforced Polymer (FRP) is used to reinforce a range of structural elements, including masonry walls, slabs, beams, and columns. By attaching FRP strips along the axis of bending at the soffit portion of beams and slabs, flexure can be reinforced. Wrapping around the cross-section of beams in a U-shape can strengthen beams under flexure and shear. The ultimate stress and strains of FRP composites are far higher than most of the alternatives available for structural strengthening. The density of FRP composites is low, thus adding less additional dead weight to the structure. Compression members like columns may be strengthened by externally wrapping them with FRP, or by casting them in FRP stay-in-place formwork. The externally bonded FRP imparts passive confinement to the column, improves the axial stiffness, reduces lateral expansion for given load levels, allows the column to reach higher stress and strain levels than the unwrapped columns. Recent years have seen a substantial increase in the adoption of seismic retrofit using FRP materials among the civil engineering community. The current state of using FRP materials as a retrofitting strategy for structures not built to withstand seismic activity is presented in this study in a representative overview. It provides an overview of the applications and range of FRP materials in seismic retrofitting plans for RC structures. The study also discusses the benefits, design principles, and restrictions of FRP applications for seismic retrofit. A case study revealed that using the FRP strengthening method improved the performance of the existing structure. Aesthetic appearance of the building maintained and also improved the strength aspects. After rehabilitation the structure requires less maintenance due to adherence of FRP laminates.

Eccentric compression behavior of concrete-filled steel tube columns strengthened by CFRP/steel strip

Concrete-filled steel tube (CFST) column structures have been widely applied in structural engineering because of their superior bearing capacity and ductility. During the service lives of CFST columns, due to the change in the service function, the increase in the load, and the corrosion of the steel tubes, some existing CFST columns face the strengthening demand of increasing bearing capacity and durability. In this study, two efficient strengthening methods for CFST columns were proposed, namely, winding carbon fiber-reinforced polymer (CFRP) or welding steel strips around the columns. A total of 11 specimens were tested to investigate the behavior of the strengthened CFST columns under eccentric compression according to parameters such as the height-diameter ratio, fiber-reinforced polymer (FRP) layer, eccentric distance, and steel strip thickness. The results indicated that the ultimate strength and initial stiffness decreased with increasing eccentric distance. The arrangement of the FRP and steel strips effectively improved the eccentric carrying capacity and ductility. The improvement in the ultimate load due to FRP confinement exceeded 34.31%, and that due to the steel strips exceeded 13.47%. Finally, considering the effect of the eccentric distance and height-diameter ratio on the behavior of the structure under compression, a model for predicting the bearing capacity under eccentric load was proposed and verified by a database that included various design parameters.

Detection in RC Beams Damaged and Strengthened with NSM CFRP/GFRP Rods by Free Vibration Monitoring

This paper intends to deepen the topic of damage detection based on non-destructive tests (NDT) for the assessment of the dynamic behavior of RC beams damaged and strengthened both with near-surface mounted (NSM) Carbon and GlassFRP rods. The NSM strengthening with fiber-reinforced polymer (FRP) rods of damaged reinforced concrete (RC) beams is a viable alternative to the traditional strengthening with externally bonded (EB) FRP strips or sheets. In this paper, static tests were foreseen on RC beams to create cracking, and successively, the RC beams strengthened with NSM CFRP and GFRP rods were still investigated using free vibration tests at different loading levels until failure. The purpose of this research is to compare the response of two different types of strengthening of damaged RC beams based on the strength of CFRP and GFRP rods until failure modes. At different steps of loading, the behavior of beams under experimental vibrations has been monitored by frequency response function (FRF) diagrams. Finally, a discussion of the results is presented.

Effect of concrete strength and tensile steel reinforcement on RC beams externally bonded with fiber reinforced polymer composites: A finite element study

Due to the continual deterioration of concrete infrastructures, the construction industry faces challenges in terms of their serviceability, structural strength, and durability. Therefore, efficient rehabilitation strategies are required to enhance their performance. Externally bonding fiber reinforced polymers (FRP) have evolved as a simple and convenient retrofit technique when compared to conventional techniques of strengthening. FRPs can be powerful toolsfor strengthening concrete structural members in shear and flexure because of their exceptional mechanical characteristics. Flexural strengthening is generally achieved by externally bonding FRP strips or laminates in the soffit side of beams, resulting in improved ultimate strength and bending capacity. This study aims to conduct a numerical parametric investigation on reinforced concrete (RC) beam specimens flexurally strengthened by externally bonding FRP. Three-dimensional finite element models of four RC beams strengthened with FRP and with different concrete compressive strength and flexural steel reinforcement were developed to predict their load capacity. Models were developed and simulated using finite element software called ABAQUS. The beam behavior was evaluated with the help of different material models available in the ABAQUS library. Study considerations included concrete damage plasticity, Hashin's damage in CFRP, and perfect bonding between CFRP and concrete interfaces. The parametric study included two groups of beams: one with varying concrete compressive strength (i.e., 25 MPa and 50 MPa) and the other with varying tensile steel reinforcement (5#14 mm diameter bars and 2# 22 mm diameter bars). The ABAQUS results were verified using experimental data from the previousliteratures.The computed numerical resultsand experimental results demonstrated reasonable accuracy for load–displacement curves and ultimate load capacity. It was found that the ultimate load increased significantly in high-strength (M50) beams with a higher number and smaller diameter (5#14 mm) tensile bars. In conclusion, RC beams with more compressive strength and tensile steel reinforcement boost the effectiveness of the external bonding technique.

Seismic performance of exterior RC beam-column junction strengthened with Stainless Steel Wire Mesh (SSWM)

Strengthening of concrete structures is necessary to enhance their performance in terms of serviceability and strength requirements as well as to increase the life span of the structure. In the past, different types of fiber-reinforced polymer (FRP) materials are widely employed for strengthening concrete structures. However, FRP-strengthened structural elements exhibit brittle failure as well as debonding of FRP strips or wraps. Due to debonding of FRP, its tensile strength is not fully utilized. To explore an alternative to FRP, Stainless Steel Wire Mesh (SSWM) is chosen in the present study. SSWM is a locally available material having lower tensile strength compared to FRP but having good bond characteristics with concrete and ductile behavior. The present study mainly focuses on experimental investigations to evaluate the behavior of beam-column junction strengthened using SSWM. In this experimental study, the performance of reinforced concrete beam-column junction is evaluated under cyclic loading. The reduced (one-third) scale test specimen representing an external beam-column junction of a 6-story regular moment resisting frame structure was tested under cyclic loading till its ultimate capacity. Subsequently, the damaged beam-column junction was strengthened using SSWM and its performance was again examined under cyclic loading. Minor cracks developed in the specimen were first filled with grout. Subsequently, epoxy-based adhesive SIKADUR 30 LP epoxy was used for bonding SSWM on the surface of the beam-column junction. The performance of test specimen was evaluated in terms of hysteretic response, displacement ductility, energy dissipation, degradation of stiffness, and failure modes of specimens. Results of experimental investigations show that SSWM effectively contributes to load resistance and enhances ultimate load carrying capacity and ductility of the beam-column junction without brittle or any significant debonding from concrete surface. Therefore, SSWM can be an economical alternative of FRP material for strengthening of concrete structures.

Shear strengthening of RC beams with NSM FRP strips: Concept and behaviour of novel FRP anchors

The high efficiency of the near-surface mounted (NSM) fibre-reinforced polymer (FRP) technique for improving the shear capacities of reinforced concrete (RC) beams has been proved by numerous studies. However, premature debonding failures including interfacial debonding and concrete cover separation, which could largely limit the utilization of FRP strength, were frequently observed in RC beams strengthened in shear with NSM FRP. Proper anchoring measures of NSM FRPs can mitigate or even prevent such debonding failures and thus enhance the FRP strengthening efficiency, but relevant research on anchoring measures for NSM FRP shear-strengthened beams is still limited. Against the above background, a novel FRP anchor for NSM FRP shear-strengthened beams is proposed in this paper. A test program consisting of eight full-scale RC beams is carried out to investigate the effectiveness of the proposed FRP anchor system. It can be seen from the test results that the proposed FRP anchor is capable of mitigating or preventing the FRP debonding failures and thus significantly enhancing the load-carrying and deformation capacities of RC beams strengthened in shear with NSM FRP strips, and the configuration of the anchor can largely influence the performance of strengthened beams.

Short-Term Creep Effect on Strain Transfer from Fiber-Reinforced Polymer Strips to Fiber Bragg Grating-Optical Fiber Sensors.

In this study, the short-term creep effect (STCE) on strain transfer from fiber-reinforced polymer (FRP) strips to fiber Bragg grating-optical fiber (FBG-OF) sensors was investigated. Thirty OF sensors attached to FRP strips were investigated through three primary test parameters: bond length (40, 60, 80, 100, 120, and 150 mm); adhesive type (epoxy resin, CN adhesive, and epoxy resin combined with CN adhesive); and bonding method (embedded and external bonding methods). The strain transfer ability of the OF sensors was evaluated based on the strain ratio of the OF sensor to the FRP strip under different sustained stresses of 20, 40, 50, and 60% of the FRP ultimate tensile strength (fu). From the test results, it was found that the debonding phenomenon occurred at the interface between the FBG-OF sensor and the adhesive and was clearly observed after applying a load for three days. It was also found that the CN adhesive showed better strain transfer compared to the other adhesive types. Regarding the OF sensors bonded by epoxy resin, in order to maintain strain transfer ability under a high level of sustained stress (0.6fu), minimum bond lengths of 100 and 120 mm were required for the embedded and external bonding methods, respectively.

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.

Pultruded carbon fibre reinforced polymer strips produced with a novel bio-based thermoset polyester for structural strengthening

This paper presents the development and industrial manufacturing of a pultruded high-performance carbon fibre (CF) reinforced polymer (CFRP) composite strip using a bio-based unsaturated polyester (UP) resin containing more than 50% of its weight derived from renewable raw materials. The bio-based CF/UP strengthening strip, comprising unidirectional CF strands, was successfully produced and its mechanical and thermomechanical behaviours were assessed, and compared to an equivalent carbon fibre reinforced epoxy-vinyl ester (CF/VE) strip produced with conventional petroleum-based high-performance epoxy-vinyl ester resin and the same fibre architecture. The bio-based CF/UP composite strip presented 2095 MPa, 167 GPa, and 1.4% of tensile strength, modulus of elasticity, and strain at failure, respectively. These figures met or exceeded the mechanical properties of its equivalent conventional CF/VE counterpart tested under the same conditions. Furthermore, the bio-based CF/UP composite strip presented a glass transition temperature of 78 °C (defined from the onset of the storage modulus decay), enabling a maximum service temperature of ∼60 °C, which will not likely be exceeded in most climates, highlighting its potential for structural strengthening applications, and offering a key advantage in terms of sustainability.