Fiber Reinforced Polymer Layers Research Articles

Numerical finite element study of strengthening of damaged reinforced concrete members with carbon and glass FRP wraps

Concrete spalling is considered as one of the most common weaknesses phenomena in concrete members. In this article, reinforced concrete (RC) column and beam members are subject to a variety of loads under damaged and strengthened conditions using carbon and glass fiber reinforced polymer (FRP) wraps. The main parameters in this study include the number of the FRP layers, the materials of the strengthening FRP layers, and the loading types. The imposed loads include pure bending moment, shear, and pure torsional moment, to enable studying the structural elements's behaviors under such states. The numerical finite element (FE) model was verified using experimental results, and 10 different case numerical FE models were analyzed. The analysis results demonstrated that using an FRP layer increases the shearing and torsional capacities. Adding another FRP layer does not significantly affect the models' behavioral specifications. In both RC beam and column, this strengthening method did not affect the torsional capacity, while managed to prevent sudden capacity loss and enhance ductility.

Analytical study on the effectiveness of hybrid FRP strengthening on behaviour of slender reinforced concrete square columns

Strengthening of reinforced concrete (RC) columns using external bonded (EB) fibre-reinforced polymer (FRP) composites has become popular since last three decades. The effectiveness of EB confinement reduces for slender columns due to the secondary moment arising from applied axial loads. The effectiveness of the hybrid FRP strengthening system for enhancing the performance of RC slender square columns is analytically investigated. Hybrid FRP strengthening uses a combination of EB FRP wrapping for confinement and longitudinal near-surface mounted (NSM) FRP laminates for bending. Hybrid FRP strengthening aims to reduce the second-order lateral displacement of the slender RC column and increase its axial capacity and flexural stiffness. A numerical modelling approach is presented to capture second-order effects in hybrid FRP strengthened slender RC square column based on Newmark's central difference scheme. Both material and geometric nonlinearities are accounted for in the calculations. The predictions are validated with test results available in the literature. After validation, a parametric study is carried out to evaluate the influence of slenderness ratio, the number of EB FRP layers, NSM laminate ratio, modulus of NSM laminate, and buckling of FRP composites. Results show that hybrid FRP strengthening is effective in improving the performance of slender RC columns.

SCFs in tubular X-connections retrofitted with FRP under in-plane bending load

This paper investigates the stress concentration factors (SCFs) in tubular X-connections retrofitted with fiber reinforced polymer (FRP) under in-plane bending moment. For this aim, a finite element (FE) model was generated and validated using available 13 experimental tests. After that, 276 FE models were created to investigate the effect of the FRP (layer number, orientation, and material) and the connection geometry (β, γ, and τ) on the SCFs. In these models, the contact between the FRP sheets and the steel members (chord, weld, and braces) was modeled. Results indicated that the rise of the FRP sheet number causes a notable drop in the SCFs. Also, the increment of the elastic modulus of FRP along the fibers causes a decrement of the SCF ratios. Moreover, the SCF in an X-connection retrofitted with CFRP can be down to 37% of that of the un-retrofitted connection. Despite the notable efficacy of the FRP sheets on the drop of the SCFs in the X-connections, there is not any study or equation on the X-joints with FRP. Therefore, a precise equation was proposed for quantifying the SCFs in X-connections with FRP. Also, the proposed equation was checked according to the UK DoE acceptance standard.

Shear Strength of Externally U-Bonded Carbon Fiber-Reinforced Polymer High-Strength Reinforced Concrete.

In this paper, we investigate the contribution of Fiber-Reinforced Polymer (FRP) to the load-carrying capacity of shear-strengthened Reinforced Concrete (RC) beams. Specifically, the investigation is focused on the FRP’s contribution in the presence and absence of shear stirrups. To this end, two sets of full-scale RC beam specimens were tested to failure in a simply supported setup. Set 1 consisted of specimens without shear stirrups whereas Set 2 included steel stirrups spaced at 170 mm. One and two layers of FRP discrete strips were bonded to the beams in a U-jacketing configuration. To investigate the contribution of FRP and its interaction with the stirrups, two different locations were considered when bonding the FRP strips: between the stirrups (referred to as Off-beams) and at the same level of the stirrups (referred to as On). Results of the experimental program showed that strengthening the beams with two layers of FRP does not necessarily translate to improved capacity. Furthermore, the location of FRP strips with respect to the location of shear stirrups has an influence on the beam’s overall behavior, especially its displacement ductility. This is an important parameter to consider to avoid premature failure of RC members. Test results were then used to assess the performance and accuracy of the predictions of ACI PRC-440.2-17 and fib-TG9.3. Both design codes were found to be conservative with an average prediction-to-test ratio of 0.7.

SCFs in tubular X-joints retrofitted with FRP under out-of-plane bending moment

This paper presents a parametric study on the stress concentration factors (SCFs) on the chord member in tubular X-connections reinforced with fiber reinforced polymer (FRP) under out-of-plane bending moment. For this aim, a FE model was generated and validated using several available experimental tests. After that, a set of 276 FE models was created to evaluate the effect of the FRP (layer number, orientation, and material) and joint geometry (γ, τ, and β) on the SCFs. In these FE models, the contact between the FRP sheets and the steel members (chord, weld, and braces) was modeled. Results indicated that the rise of the FRP laminate number causes a notable drop of the SCFs, especially in the connections with big γ. Moreover, the increment of the elastic modulus of FRP along the fibers causes a notable decrease of the SCF. Results showed that, for certain geometrical parameters set, the SCF in an X-connection retrofitted with carbon fiber reinforced polymer (CFRP) can be down to 23% of the SCF in the associated un-retrofitted connection. Despite the notable efficacy of the FRP sheets on the drop of the SCFs in the X-connections, there is not any study or equation on the X-connections with FRP. Therefore, an equation was proposed for quantifying the SCFs in the X-connections with FRP.

Compressive behavior of heat-damaged square concrete prisms confined with basalt fiber-reinforced polymer jackets

Externally bonded fiber-reinforced polymer (FRP) composites are increasingly used to confine concrete columns as an effective strengthening technique. During the past two decades, a significant research effort has focused on the performance of FRP-strengthened concrete structures under fire exposure. However, there is still a lack of research on the structural behavior of heat- or fire-damaged concrete structures repaired with FRP composites. This paper presents the results of the first-ever experimental study on the compressive behavior of heat-damaged square concrete prisms that are either unconfined or confined by a promising type of FRP composites made of basalt fibers. A total of 51 specimens were prepared and tested under axial compression. The design variables included the heat-induced damage levels of concrete prisms after exposure to various elevated temperatures (200, 400, 600 and 800 °C) and the layers of basalt FRP (BFRP) jackets (2-, 3- and 4-layer) used for strengthening. The failure modes, compressive strengths, ultimate axial strains, axial stress–strain curves, and axial strain-to-hoop strain relationships of the specimens were investigated and compared. The external confinement of the BFRP jackets was proved to be effective in enhancing the strength and deformation capacities of heat-damaged concrete with a square cross-section. The strength growths and the ultimate axial strains of the BFRP-confined heat-damaged square concrete prisms were improved with the increase in the exposure temperature or the number of BFRP jacket layers. Two typical concrete confinement models were then modified by taking the residual mechanical properties of concrete into account to predict the compressive behavior of the BFRP-confined heat-damaged concrete with a square cross-section. The comparisons between the test results and the model predictions are presented, and the accuracy of the concrete confinement models are examined and discussed.

Influences of fiber reinforced polymer layer on the dynamic deflection of concrete pipes containing nanoparticle subjected to earthquake load

AbstractA numerical solution for dynamic analysis induced by earthquake load in concrete pipes containing SiO2 nanoparticles warped by fiber reinforced polymer (FRP) layer is utilized in the present paper on the basis of Mori–Tanaka model. The FRP layer is used as reinforcement for the concrete pipe. Utilizing the cylindrical higher order shell theory, the concrete pipe is mathematically simulated and the related governing relations are derived based on. Utilizing numerical methods of Newmark and Galerkin, the dynamic displacement of the concrete pipe is obtained. The influence of FRP layer and its thickness, volume fraction and agglomeration of nanoparticle, boundary conditions and length to radius ratio of the concrete pipe is studied on the dynamic deflection of the concrete pipe. Numerical outcomes illustrate that the FRP layer can reduces the dynamic deflection significantly.

Experimental Study on the Confinement of Concrete Cylinders with Large Rupture-Strain FRP Composites

Large rupture strain (LRS) fiber-reinforced polymer (FRP) composites, typically formed from polyethylene naphthalate (PEN) and polyethylene terephthalate (PET) fibers, generally exhibit ultimate rupture strains >5%. Such fibers are particularly suited to the confinement of concrete columns on account of their LRS and sufficient elastic modulus. There are currently a limited number of studies on LRS FRP-confined concrete, particularly with high- and ultrahigh-strength concrete, so their behavior across a range of variables is still unknown. To improve this understanding, this paper systematically investigates the influence of fiber type, fiber thickness, and concrete strength on the behavior of FRP-confined concrete. To achieve this objective, the current investigation presents the results of 66 circular FRP-confined cylinders that are loaded concentrically. Three main parameters are investigated, namely, fiber type (i.e., PEN, PET, carbon, glass, and aramid), concrete strength (i.e., normal, high, and ultrahigh strength), and fiber thickness. The results show that regardless of fiber type, the stress–strain response is bilinear when the concrete is sufficiently confined. However, when there is insufficient confinement provided to the concrete core, the stress–strain response becomes trilinear. This trilinear response is more pronounced for LRS FRP-confined specimens because the confinement stiffness of the LRS FRP jacket is lower than that of a traditional FRP-confined specimen with an equivalent confinement ratio. Increasing the confining hoop stiffness (i.e., increasing FRP layers) reduces the magnitude of strength reduction after initial concrete cracking. It is also evident that as the unconfined concrete strength increases, the minimum confinement stiffness ratio necessary to prevent strength reduction after initial concrete cracking increases.

Stress concentration factors in tubular X-connections retrofitted with FRP under compressive load

In the present study, the stress concentration factors (SCFs) in circular hollow section X-connections retrofitted with fiber reinforced polymer (FRP) under compressive load are evaluated. After validation of the FE model with several available experimental data, a set of 279 FE models was created to parametrically investigates the efficacy of the FRP and connection geometry on the SCFs and SCF ratios. In the FE models, the contact between the FRP layers and steel members (chord, weld, and braces) was modeled. Results showed that the rise of the FRP sheet number causes a considerable drop in the SCFs. Moreover, the use of FRP causes the propagation of stresses in steel and FRP. Moreover, the rise of the elastic modulus of FRP along the fibers causes the more drop in the SCFs. Despite the notable efficacy of the FRP on the drop of the SCFs in the X-joints, there is not any study or formula in X-connections with FRP. Hence, the FE results were used to propose two precise formulas for quantifying the SCFs in X-connections with FRP layers at the saddle and crown points. Also, the proposed equations were checked according to the UK DoE acceptance standard.

Static capacity of tubular X-joints reinforced with fiber reinforced polymer subjected to compressive load

In the present paper, the initial stiffness, the ultimate capacity, the capacity ratio, and the failure mechanisms of the tubular X-joints reinforced with Fiber Reinforced Polymer (FRP) under compressive load are investigated. For these aims, a finite element (FE) model was generated and verified with the available experimental data. Afterward, 109 finite element (FE) models were created to investigate the efficacy of the FRP layers (length, number, and orientation) and the connection geometry (β, τ, and γ) on the static performance of the reinforced X-joints. In the FE models, the effects of the weld profile and the contact between the FRP and the members (chord, weld, and braces) were considered. Results showed that the FRP can remarkably enhance the stiffness, ultimate capacity, and improve failure mechanisms. Despite the notable effect of the FRP on the performance of the joints, there is not any study on the X-joints with FRP. Hence, after the parametric study, the FE results were used to propose a theoretical formula, based on the yield body model, to predict the ultimate strength of X-joints with FRP. In addition, the proposed formula is confirmed by the acceptance criteria of the UK Department of Energy.

Compression behavior and modeling of FRP-confined high strength geopolymer concrete

Fiber-reinforced polymer (FRP) has proven its efficiency as a strengthening material of ordinary concrete. However, the research on the effect of these materials on geopolymer concrete is still less understood. Due to the excellent mechanical and environmental properties of geopolymer concrete, the compression behavior of confined high strength geopolymer concrete (HSGC) is experimentally and analytically investigated and compared with high strength cement concrete (HSC). The confined cylindrical test specimens of HSGC and HSC were tested in compression. The cylinders were confined using three schemes of FRP, namely one and two layers of Glass FRP (GFRP) and one layer of carbon FRP (CFRP). The stress–strain behavior of the HSGC is compared with HSC for different levels and types of confinement. For low confining pressure, there was a definite peak followed by descending portion of the stress–strain curve, whereas for high confining pressure, initially, the stress increases nonlinearly with strain, and after reaching a certain level of stress, it becomes asymptotic to a positive slope straight line. A single equation is proposed for predicting all curves of HSGC and HSC for low as well as high confining pressures. The validation of the model is also successfully shown with the data taken from the literature for a wide range of confinement levels.

Experimental study and numerical analysis on seismic performance of FRP confined high-strength rectangular concrete-filled steel tube columns

The fiber reinforced polymer (FRP) has attracted increasing attention as a strengthening method for concrete-filled steel tube (CFT) columns because of its lightweight and high strength. Nevertheless, related investigations on seismic performance of FRP confined high-strength rectangular CFT columns are rarely reported. Herein, a series of experimental and numerical analysis on the seismic performance of FRP confined rectangular CFT columns using high-strength thin-walled steel tubes and high-strength concrete are presented considering the following parameters: aspect ratio, axial compression ratio, number of FRP layers and fiber direction. The seismic performance of specimens under reversed cyclic lateral loading was evaluated by discussing the failure modes, hysteretic behaviors, capacity degradation, stiffness degradation, ductility, and strains. The numerical models were developed and verified by the test results, which could accurately simulate the mechanical behaviors and the failure of specimens. The observations drawn from this research might provide a good basis for the application of high-strength material and FRP-strengthened CFT structures.

Behavior and modeling of double-skin tubular columns filled by ultra-lightweight cement composites (ULCCs)

Double skin tubular columns (DSTCs) filled by ultra-lightweight cement composites (ULCCs), as primary load-bearing elements, are a practical and reasonable structural asset in lightweight civil infrastructure. This DSTC-ULCC system combines the advantages of three materials, 1) outer fiber-reinforced polymer (FRP) 1–3 ply jackets, 2) filled ULCCs, and 3) inner steel tubes to achieve lightweight, high strength and superior ductility. This article presents an experimental study on the mechanical behavior of DSTCs with FRP and ULCCs under axial compressive loading. The experiment designed a total of 26 DSTCs that were all filled with ULCCs. The key parameters were the number of FRP layers and the inner steel tube properties, (i.e., thickness and diameter of the steel tube). The test results indicate that the properties of the inner steel tube not only influences the effective FRP rupture strain ratio but also the ultimate axial strain of confined ULCCs. This calls attention to the inward buckling behavior for larger void ratios; moreover, thinner steel tubes result in more axial deformation of ULCC-filled DSTCs. In addition, ULCCs showed a much lower elastic modulus compared with normal concrete, which helped cause the change of transition stress of ULCCs in DSTCs, and FRP confinement can effectively increase the elastic length of the stress–strain curves for ULCCs. The experimental results are subsequently compared with predictions from an FRP-confined ULCC model, which led to a new ultimate strain model proposed herein for ULCC-filled DSTCs. The comparison demonstrates that the model can provide reasonably accurate predictions of the stress–strain curves of ULCCs in hollow DSTCs.

Parametric study and formula for SCFs of FRP-strengthened CHS T/Y-joints under out-of-plane bending load

In this paper, the stress concentration factors (SCFs) in tubular T/Y-connections strengthened with fiber reinforced polymer (FRP) under OPB are investigated. For this aim, 263 FE models of T/Y-joints with FRP were generated and analyzed. In the FE models, the contact between the FRP sheets and the members was modeled. By using the generated FE models, the effect of the FRP layers, the brace inclination angle, and the joint geometry on the SCF ratios of the reinforced to the corresponding un-reinforced joints were investigated. Results showed that the SCF of an FRP strengthened joint can be down to 29% of the SCF of the associated un-strengthened joint. Moreover, the increase of the FRP layer number results in a considerable decrease of the SCFs. Also, this phenomenon is more significant in the joints with larger γ. Moreover, the increase of the elastic modulus of FRP along the fibers results in the remarkable decrease of the SCF ratios. Finally, the FE results were used to propose a design equation for determining the SCFs in T/Y-joints reinforced with different FRP materials under OPB load.

Behavior of composite columns repaired by CFRP and subjected to uniaxial and biaxial eccentric load

ABSTRACT This paper presents an experimental program to study the behavior of short high strength concrete composite columns subjected to either uniaxial or biaxial eccentric loading. The paper also includes a study on a suitable repairing technique for those columns by using carbon fiber reinforced polymers. The experimental program divided into two phases, the first consists of studying the behavior of six high strength reinforced concrete composite columns, three of them subjected to uniaxial eccentric loading as well as the other three subjected to biaxial eccentric loading, while the second phase deals with the behavior of five columns from the first phase after repairing them by carbon fiber reinforced polymers and subjecting them to the same eccentric loading. The main variables were the load direction, the number of sheets of carbon fiber reinforced polymers used and their arrangements. A comparison between the behavior of each column before and after repairing, using different numbers and arrangements of sheets of carbon fiber reinforced polymer layers was made. The comparison showed that wrapping composite steel columns with carbon fiber reinforced polymers is considered a very effective technique for repairing.

U-type mechanical clamping to anchor the CFRP composites in strengthening of RC beams subjected to bending

The present study investigates the performance of U-type clamp anchorage on external strengthening of reinforced concrete beams using carbon fibre-reinforced polymer composites. Unidirectional carbon fibre was used to strengthen the beam having a cross-section of 150 × 200 mm2 and the fabricated U-type clamp was used as a clamping device to enhance the fibre-reinforced polymer (FRP) bonding. To test the beam, four-point bending system was adopted and the experimental parameters were the number of FRP layers. The failure mode of strengthened beams with U-type clamp was very similar (rupture of FRP composites) to the beams without U-type clamp. However, the delamination of FRP was evaded by counteracting the peeling stress development at the edge of fibre. Since the beams strengthened with U-type clamp exhibited more linear behaviour than that of the beams strengthened without U-type clamp, there may be a higher possibility for abrupt/rapid/brittle mode of failure and the ductility index of the beam with U-type clamp confirmed the above behaviour. However, the introduction of U-type clamp enhanced the flexural stiffness and ultimate strength by counteracting the debonding, and the beam with U-type clamp exhibited a maximum of 58.33% and 20.37% enhancement in flexural stiffness and ultimate strength than that of the beam without U-type clamp, respectively. The theoretical strength value of all strengthened beams was evaluated using models proposed in the previous research and compared with experimental results. It is inferred that the use of U-type clamp in FRP strengthening provides economic and structural benefits compared to a beam without U-type clamp.

Crack monitoring and damage assessment of BFRP-jacketed concrete cylinders under compression load based on acoustic emission techniques

Fiber reinforced polymer (FRP) composites have been widely used in strengthening of existing concrete columns. However, the concrete cracks inside the FRP-strengthened columns caused by external loading cannot be observed by traditional field inspection techniques due to the coverage of FRP composites. In the present study, axial compression loading tests were performed on two unjacketed concrete and six BFRP-jacketed concrete cylinders. Acoustic emission (AE) techniques were utilized to monitor the cracking process of concrete specimens. The conventional AE hit and AE amplitude analysis were effective in detecting the first occurrence of concrete crack. The first noticeable concrete crack and final major crack can be detected by the crack intensity analysis. Then, five stages of cracking process of unjacketed concrete and BFRP-jacketed concrete specimens can be determined by a combination of the AE hit analysis and crack intensity analysis. AE analysis revealed that the cracking process of BFRP-jacketed concrete differed from that of unjacketed concrete, due to the presence of BFRP composites: (1) the time range of AE stable developing stage of BFRP-jacketed concrete was longer than that of the unjacketed concrete and the time range increased with an increase in FRP layers; (2) the crack intensity analysis parameters, which are based on AE signal strength, (i.e., severity index (Sr) and historic index (Hs)) of BFRP-jacketed specimens presented noticeable higher values at the final failure stage; (3) the RA value-average frequency (RA-AF) analysis revealed that the occurrence of shear cracks was delayed in the BFRP-jacketed concrete. The present study demonstrated that AE monitoring can effectively capture the characteristics of the cracking process of FRP-jacketed concrete.

Stress concentration factors in tubular T/Y-connections reinforced with FRP under in-plane bending load

Stress concentration factors (SCFs) in steel tubular T/Y-connections strengthened with fiber reinforced polymer (FRP) are investigated. In the first step, a finite element (FE) model was developed and verified using data of several available experimental tests and empirical formulas. After that, 134 FE models of T/Y-joints with and without FRP were created and analyzed under in-plane bending (IPB) load. In the FE models, the contact between the FRP layers and the members (chord, brace, and weld) was defined. Results showed that the increase of the FRP layer number leads to the notable decrease of the SCFs. Also, the SCFs of a T/Y-joint reinforced with FRP can be down to 40% of the SCF of the corresponding unreinforced joint. Finally, FE results were used to propose two parametric formulas for determining the SCFs in the T/Y-joints strengthened with FRP subjected to IPB load. The proposed formulas were checked according to the UK Department of Energy acceptance criteria.

Confinement of concrete in double skin tubular members under axial compression loads

A hybrid double-skin tubular members (DSTM) consist of three composite materials in one section, include fiber-reinforced polymer (FRP) tube outward, steel tube inward and concrete is in between. These three materials give the member unique properties like protect member from harsh environment, reduce the amount of concrete by the presence of an inner steel tube that allowed the passage of services. Therefore, consider sustainable structure elements, as well as the rising strengthening of members and excellent corrosion resistance. This paper presents an experimental and analytical study of twenty-four circular columns with two types; concrete-filled FRP tube (CFFT) and double skin tubular column (DSTC), with a dimension of 100 × 310 mm which constructed and testing under an axial compressive load till failure, with four variables: numbers of layers of outer FRP tube, the compressive strength of concrete, thickness to diameter ratio and void ratio of inner steel tube. The experimental results showed that the concrete was effected confinement in hybrid DSTC led to very ductile behavior of stress–strain relationship and the number of outer FRP layers was the most effective parameters on DSTCs behavior. The nominal confinement ratio was affected by the thickness of outer FRP tube and compressive strength of concrete inside DSTCs.

Numerical analysis of the shear behavior of FRP-strengthened continuous RC beams having web openings

Web openings are usually constructed in continuous reinforced concrete (RC) beams to accommodate the utility pipes and cables. This paper investigates numerically shear strengthening of continuous RC beams having web openings with FRP layers. The finite element analysis software ABAQUS/standard was used to construct non-linear finite element model (FEM) to analyze continuous RC beams having web openings strengthened with different schemes of fiber reinforced polymers (FRP) layers. The FEM was verified using the available experimental studies of both FRP-strengthened simply supported and continuous RC beams with/without openings found in literature before conducting the numerical investigation. The numerical investigation was carried out using the validated numerical FEM to investigate the impacts of crucial parameters such as opening location, opening area and strengthening configuration on the shear behavior of continuous RC beams strengthened with FRP layers. Three different locations were suggested along the length of the RC beam to construct the web openings. On the other hand, the effects of five different opening area on the load capacity and failure patterns were investigated. Moreover, two different FRP strengthening configurations were considered either strengthening above and below the opening or strengthening the whole RC beam height. Results showed that continuous RC beams containing openings at zone II group was the most affected among analyzed locations. The reduction in load capacities of this group ranged from 7.3 to 66.1% compared to the solid beam. The ultimate loads of specimens strengthened over the whole RC beam height exhibited the highest values among analyzed specimens.