Fiber-reinforced Polymer Rupture Research Articles

Influence of FRP thickness and confining effect on flexural performance of HB-strengthened RC beams

This paper presents details of an experimental study and an analytical model that investigates the influence of fiber reinforced polymer (FRP) thickness and confining effect on the flexural performance of hybrid bonded (HB) FRP strengthened reinforced concrete (RC) beams. In the experimental study, a total of ten 150 × 250 × 2400-mm RC beams, with and without HB strengthening, were tested under four-point bending. Each specimen varied in the thickness of FRP plate and the confining effects of HB fastener. Fastener detachment and FRP rupture were identified as the dominant failure modes. The load-carrying capacity increased when thicker FRP plate and higher confining effect were used. The best ductility performance was reached when the fastener detachment and the FRP rupture occurred simultaneously. An analytical model is developed to predict the load-carrying capacity of HB-strengthened RC beams, showing satisfactory precision as compared to the experimental data. The parametric study shows that increasing the FRP thickness increases the load-carrying capacity when the failure mode is FRP rupture, whereas increasing the confining effect increases the load-carrying capacity when the failure mode is FRP rupture. A design method is proposed to obtain satisfactory load-carrying capacity and ductility performance.

Reliability assessment for flexural FRP-Strengthened reinforced concrete beams based on Importance Sampling

Due to the uncertainties raised by model error, materials (e.g., concrete, fiber reinforced polymer (FRP) and steel) and environment (e.g., loads), research aimed at reliability analysis and risk assessment of FRP-strengthened structures is necessary. In this paper, reliability analysis of flexural FRP-strengthened reinforced concrete (RC) beams with the ultimate limit state is studied. Four failure modes are considered: concrete crushing, FRP rupture, intermediate crack-induced (IC) debonding and end debonding. First, the computational models for each failure type are assessed in terms of model error based on a comprehensive database that contains 624 FRP strengthened RC beams. Then, the stochastic information for mechanical properties of the FRP and concrete materials is further identified through a series of mechanical tests on 600 concrete compression blocks and 50 FRP tensile specimens. Subsequently, the reliability index is estimated by Importance Sampling (IS) for its satisfied accuracy and efficiency. The results show that: (1) The probability of failure is different for each failure mode. It is necessary to consider all the possible failure modes to provide a comprehensive guide in the design process. (2) Based on the partial safety factor design format, the effect of the FRP partial safety factor γf on the reliability index for concrete crushing, IC debonding and end debonding is negligible. The calibration of γf is only valid for FRP rupture in order to meet the target reliability index. (3) Based on the resistance reduction factor design format, it is effective to calibrate resistance reduction factor ϕ for concrete crushing, FRP rupture, IC debonding and end debonding. Each failure mode should be designed with corresponding ϕ. (4) Additional anchors can improve the reliability index for IC debonding and helps to transfer to other failure modes for end debonding.

Experimental Behavior and Design-Oriented Analysis of Sandwich Beams with Bio-Based Composite Facings and Foam Cores

Sandwich beams made of bio-based fiber-reinforced polymer (FRP) composite facings and foam cores were studied. The FRP facings consisted of a plant-based bidirectional flax fiber fabric (400 g/m2) and a bio-based epoxy resin (30% bio content), and the foam cores were made of 75-mm-thick closed cell polyisocyanurate. A total of nine sandwich beam specimens (1,200 mm long and 150 mm wide) were prepared and tested under three-point bending. The parameters of the study were core density (32, 64, and 96 kg/m3) and facing thickness (one, two, and three layers of flax fabric). Three failure mechanisms were observed during testing, including top face wrinkling/crushing, core shear, and tensile rupture of the bottom face. It was shown that the foam with the density of 96 kg/m3 was stiff and strong enough to achieve the tensile rupture of the flax FRP (FFRP) facing. Also, a nonlinear behavior was observed for the sandwich beams. A bilinear stress-strain model for FFRP facing was proposed and, based on that, closed-form moment-curvature and load-deflection equations of the sandwich beams were derived for design applications. The proposed design-oriented model was verified against the test data of this study and an independent study capturing the stiffness, strength, and nonlinearity of the test specimens.

Predicting bond behavior of HB FRP strengthened concrete structures subjected to different confining effects

This paper presents an analytical model to predict the bond behavior of hybrid bonded (HB) fiber reinforced polymer (FRP) strengthened concrete structures considering different confining effects. The bond strength of HB-strengthened concrete structures subjected to uniaxial loading is attributed to the friction-type bond from steel plate and the chemical bond provided by epoxy-based resin. The friction-type bond is assumed to develop before any interfacial slip occurs, and remain constant when the interfacial slip is nonzero; for the chemical bond between FRP and concrete, a tri-linear bond-slip relation is adopted. The loading process of HB-strengthened structures consists of three stages, and the stage-wise solution to slip, bond stress, and axial force of HB FRP are derived based on a governing ordinary differential equation (ODE). In this study, the experimental program includes twelve HB-strengthened concrete specimens, using different confining effects (twisting moments on the steel bolts). Test results indicated that the ultimate pullout strength increases and the failure mode switches from debonding to FRP rupture as the twisting moment increases. Moreover, by comparison to three series of experimental results, the proposed model can precisely predict failure mode, ultimate strength, load-slip relation and bond stress distribution. The twisting moment on bolts is found to have the most significant improvement on the ultimate tensile strength. Finally, the critical twisting moment without yielding FRP rupture is derived.

Behavior of Hollow-Core FRP-Concrete-Steel Columns under Static Cyclic Flexural Loading

This paper presents the seismic behavior of hollow-core fiber-reinforced polymer (FRP)-concrete-steel (HC-FCS) columns comparable with the conventional RC column. The typical HC-FCS column consists of a concrete shell sandwiched between an outer FRP tube and an inner steel tube. The HC-FCS column represents a compact engineering system; the steel and FRP tubes act together as stay-in-place formworks. The steel tube acts as a flexural and shear reinforcement. This paper studies three large-scale columns—one RC column having a solid cross section and two HC-FCS columns. Each column has an outer diameter of 610 mm (24 in.) and a shear span-to-diameter ratio of 4.0. The steel tube is embedded into the reinforced concrete footing with an embedded length of 1.6 times the steel tube diameter, whereas the FRP tube only confines the concrete shell and truncates at the top of the footing. The HC-FCS columns exhibits high lateral drift reaching 15.2% and fail gradually due to concrete crushing and local steel tube buckling, followed by FRP rupture. The reference RC column fails at a drift of 10.9% due to rebar fracture. Simple beam theory overpredicts the flexural strength of the columns by an average of 9%.

Full-range FRP failure behaviour in RC beams shear-strengthened with FRP wraps

Reinforced concrete (RC) beams can be strengthened in shear by externally bonded (EB) fibre reinforced polymer (FRP) composites in the forms of side-bonded FRP strips, FRP U-jackets or FRP wraps. The shear failure of almost all RC beams shear-strengthened with FRP wraps and some of the beams strengthened with FRP U-jackets, is due to the rupture of FRP. For an FRP wrapped beam with FRP bonded to the beam surface, debonding of EB FRP from concrete substrate usually precedes the ultimate FRP rupture failure, therefore the failure process of the beam is associated with both FRP debonding and rupture failures. Despite extensive research in the past decade, there is still a lack of understanding of how the failure of FRP wraps in such an FRP-strengthened beam progresses and how it affects the shear behaviour of the beam. This paper presents an analytical study on the progressive failure of FRP wraps in such strengthened beams. In this study, the debonding and the subsequent rupture processes are derived and the FRP contribution to the shear capacity of the beam is quantified. The analytical solution is verified by comparing its predictions with the predictions of a finite element model. An additional merit of the analytical solution is that the development of FRP shear contribution with the opening of the shear crack can be quantitatively determined, providing a useful tool for further investigation on the shear interaction among different components (FRP shear reinforcements, steel shear reinforcements, and concrete) in RC beams shear-strengthened with FRP.

FRP-confined concrete filled PVC tubes: A new design concept for ductile column construction in seismic regions

In their recent work Fakharifar and Chen (2016) [1] have introduced and validated a new ductile design concept of confined concrete-filled polyvinyl-chloride tubular (CCFPT) columns through experimental and analytical studies. Confinement was provided with the use of a polyvinyl-chloride (PVC) tube and fiber-reinforced polymer (FRP) wrappings or FRP with a sandwiched layer of foam to enhance impact energy reduction due to potential FRP rupture and PVC fracture. In the present paper, additional flexural tests on CCFPT columns were undertaken to further investigate the structural behavior of the proposed system for column construction in seismic regions. Axial and flexural behavior of CCFPT columns was investigated with compressive and flexural tests, respectively, and compared with those of their corresponding concrete-filled polyvinyl-chloride tubular (CFPT) and FRP-wrapped (FW) columns. Test results obtained from 152×305mm (6×12in.) stub columns under axial loads and 152×1524mm (6×60in.) flexural beams under four-point loads indicated that the CCFPT columns can significantly enhance strength over the CFPT columns and enhance ductility over the FW columns. The transverse confining pressure from FRP wrapping and the interface property between the FRP and PVC proved critical in CCFPT design. Furthermore, the idea of introducing a cushioning foam layer between FRP layers and PVC to lessen the brittleness of FRP rupture in seismic regions proved effective.

Cyclic lateral response of FRP-confined circular concrete-filled steel tubular columns

Concrete-filled steel tubular (CFT) columns are widely used as columns in many structural systems and a common failure mode of such tubular columns is inelastic outward local buckling near a column end. The use of fibre-reinforced polymer (FRP) jackets/wraps for the suppression of such local buckling has recently been proposed and has been proven to possess excellent potential in both retrofit/strengthening and new construction. This paper presents the results of an experimental study into the behaviour of large-scale FRP-confined CFT (CCFT) columns under combined axial compression and lateral loading. The test parameters included the stiffness of the FRP jacket and the loading scenario. The test results showed that the FRP jacket can effectively delay or even prevent outward local buckling at the end of a cantilevered CFT column, leading to significantly improved structural performance under combined constant axial compression and cyclic lateral loading. Compared to monotonic lateral loading, cyclic lateral loading was found to introduce more severe localized deformation near the column end and may lead to earlier FRP rupture within that region.

Corner strengthening of square and rectangular concrete-filled FRP tubes

This paper presents the results of an experimental study that investigated the application of a corner strengthening technique to square and rectangular concrete-filled fiber-reinforced polymer (FRP) tubes (CFFTs). Effects of the strengthening technique were investigated through axial compression tests undertaken on a series of CFFT specimens having different corner radii and sectional aspect ratios. The results indicate that the compressive behavior of square and rectangular CFFTs can be improved through corner strengthening, which significantly reduces or completely eliminates temporary post-peak strength softening observed in some CFFTs. The results also indicate that the premature FRP rupture at corners due to stress concentrations can be prevented through sufficient strengthening of the specimen corners. It is found that increasing the amount of corner strengthening leads to an increase in the confinement effectiveness of CFFTs and results in a more uniform confining pressure distribution along specimen cross-section. It is also found that corner strengthening provides a more significant improvement in the compressive behavior of CFFTs with lower original confinement effectiveness caused by a smaller corner radius or a larger aspect ratio.

An experimental investigation into the bond-slip behaviour between CFRP composite and lightweight concrete

Epoxy bonded fibre reinforced polymer (FRP) composites are widely used for the retrofit of ailing reinforced concrete structures, for both shear and flexure. The behaviour of retrofitted concrete structures is governed by the bond strength and the material characteristics of the epoxy bonded FRP and the concrete. Previous studies show that lightweight concrete (LWC), which uses Pulverised Fuel Ash (Lytag) instead of coarse granite aggregates, has significantly lower tensile strength and aggregate interlock compared to normal weight concrete. Performance of shear retrofitted concrete elements is primarily governed by the aggregate interlock and tensile strength. Thus the study of FRP enhancement techniques in LWC is paramount for limit state design. Many studies have been conducted to understand the bond-slip behaviour between normal weight concrete (NWC) and FRP composites, where the increasing interfacial (shear) and normal stresses with increasing plastic deformation lead to FRP debonding and/or FRP rupture failures. This paper presents the experimental pull-off test results obtained from lightweight concrete prisms with various configurations of epoxy bonded Carbon FRP (CFRP) sheets. The experimental results show that the LWC can successfully be applied in the strengthening of lightweight concrete structures. However, the lightweight concrete prisms failed due to a diagonal crack within the concrete materials. This was due to a lower tensile strength compared to normal weight concrete specimens where peeling or rupture of FRP is the dominant failure mechanism.

Implementation of Bond Durability in the Design of Flexural Members with Externally Bonded FRP

Externally bonded fiber-reinforced polymer (FRP) composites are becoming a method of choice for the repair/strengthening of civil structures, some of which are situated in environments detrimental to FRP-concrete bond durability. Based on analysis of an extensive database of durability test results from notched-beam three-point bending tests, a bond durability factor (BDF) of 0.60 was identified as an appropriate estimate of bond durability for wet-layup carbon fiber-reinforced polymer (CFRP). This factor was compared to a normalized database of durability data from the literature, and it was found that a 0.60 BDF agrees well with these data. A modified procedure for determining the ultimate design strain primarily in bond-critical applications (such as external flexural and shear FRP reinforcement), is proposed. This procedure makes a distinction between durability properties associated with the FRP rupture and debonding failure modes. When used for practically feasible strengthening ratios and in conjunction with BDF of 0.60, the proposed procedure yields overall ultimate flexural-member design strengths that are 0–15% lower than those determined using current design procedures.

Basalt fiber reinforced polymer grids as an external reinforcement for reinforced concrete structures

This paper presents series of experimental studies on newly developed basalt fiber reinforced polymer (FRP) grids for the flexural strengthening of reinforced concrete beams. The study includes tensile behavior of the basalt FRP grids with three types of thickness, bonding behavior between the basalt FRP grids and the concrete with epoxy resin, and flexural behavior of the reinforced concrete beams strengthened with basalt FRP grids. The results revealed that the modulus of elasticity of the basalt FRP grids ranged between 40 and 43 GPa, and their tensile strength ranged between 815 and 931 MPa. The bond stress–slip behavior of the basalt FRP grids and concrete exhibited a ductile failure manner in comparison to that of FRP sheets and concrete; the bond strength of the tested basalt FRP grids was in the range of 3.26–6.06 MPa, and its slip at peak ranged between 0.15 and 0.4 mm. Beams strengthened with basalt FRP grids failed by crushing of compressive concrete or fiber reinforced polymer rupture and no debonding failure was observed for any of the tested beams.

Effects of confining stiffness and rupture strain on performance of FRP confined concrete

FRP (fiber reinforced polymer) confinement has been proven to be effective in enhancing the structural performance of concrete columns. However, the effectiveness of FRP confinement is dependent on various structural parameters, including the concrete strength, FRP confining stiffness and FRP rupture strain, and is not easy to predict as revealed by the large differences in the existing design formulas. Herein, a newly developed axial and lateral stress–strain model is used to evaluate the effects of these parameters on the yield strength/strain, ultimate strength/strain and ductility of FRP confined concrete. The new model is first verified by comparing with test results published in the literature and then used to perform a parametric study, based on which design formulas for estimating the performance of FRP confined concrete are developed. It will be seen at the end that the confining stiffness is more fundamental than the confining stress at rupture and that the rupture strain has independently significant effect on the ultimate strength/strain and ductility.

Confining of square RC columns with FRP sheets using corner strip–batten technique

The application of fiber reinforced polymer (FRP) composites has been successfully promoted for external strengthening of reinforced concrete (RC) columns. Recent studies showed that when these jackets are formed in a wet lay-up process for confinement of columns, an average rupture strain of FRP in specimens substantially fall below those of flat coupon tensile tests. The problem is more important in rectilinear columns because of premature rupture of FRP at corners due to stress concentration.In this paper, a new method is introduced for confinement of square concrete columns as using FRP strips at corners and FRP battens at sides. In this method, FRP battens do not experience any curvature in confining of section and are stretched as completely flat strips similar to flat coupons. To compare the new proposed method with other FRP-confining techniques, sixteen square RC columns with 133×133mm cross section and 500mm height were experimentally tested under uniaxial compression. The test parameters included continuity or discontinuity of corner strips along height of column, the volume of fibers used, and the number of confining layers. The experimental results clearly demonstrated that confining battens in the new method of corner strip–batten are uniformly stretched under the tension stresses of confinement; therefore, more uniform distribution of confining pressure on section occurs and the stress concentration at corners is eliminated. Thus, in the proposed method, better performance of FRP in confinement was observed and the compressive behavior of the strengthened column was significantly improved compared to those confined using conventional FRP wraps.

Limit analysis on FRP-strengthened RC members

Reinforced concrete (RC) members strengthened with externally bonded fiber-reinforced-polymer (FRP) plates are numerically investigated by a plasticity-based limit analysis approach. The key-concept of the present approach is to adopt proper constitutive models for concrete, steel reinforcement bars (re-bars) and FRP strengthening plates according to a multi-yield-criteria formulation. This allows the prediction of concrete crushing, steel bars yielding and FRP rupture that may occur at the ultimate limit state. To simulate such limitstate of the analysed elements, two iterative methods performing linear elastic analyses with adaptive elastic parameters and finite elements (FEs) description are employed. The peak loads and collapse mechanisms predicted for FRP-plated RC beams are validated by comparison with the corresponding experimental findings.

A Mechanistic 1D Finite Element Model for Nonlinear Analysis of FRP-Strengthened Reinforced Concrete Beams

An efficient frame finite element (FE) is developed and applied for nonlinear analysis of reinforced concrete beams flexurally strengthened with externally bonded fibre reinforced polymer (FRP) strips/plates. The developed nonlinear model is able to capture different failure modes associated with concrete crushing and cracking, yielding of steel, FRP rupture and intermediate crack-induced debonding of FRP sheets/plates. The element is formulated using force interpolation concept and a simple approach based on application of discrete springs and linear interpolation of bond forces along the element axis is adopted to take account of shear-slip. The developed formulation and analytical tool are employed to predict the loading capacity and the load-deflection response of reinforced concrete beams strengthened with FRP and the numerically simulated responses agree well with the corresponding experimental results. The major features of this frame element are its simplicity and efficiency compared with more complex FEs which makes it a suitable tool for practical use in design-oriented parametric studies.

A modified constitutive model for FRP confined concrete in circular sections and its implementation with OpenSees programming

OpenSees is a well-recognized open source platform with high compatibility, and it has a well-developed fiber element method to cope with nonlinear structural analysis. Fiber reinforced polymer (FRP) confined concrete can effectively improve the seismic performance of concrete structures. However, sophisticated constitutive models for FRP confined concrete are not available in the current version of OpenSees. In this paper, after reviewing several typical FRP confined concrete constitutive models, a modified constitutive model for FRP confined concrete in circular sections was proposed based on Lam and Teng (2003)’s model with four main modifications including the determination of FRP rupture strain, ultimate condition, envelope shape, and hysteretic rules. To embed the proposed constitutive model into OpenSees is a practical solution for engineering simulation. Hence, the secondary development of OpenSees New UserMat was briefly demonstrated and a set of critical steps were depicted in a flow chart. Finally, with the numerical implementations of a series of FRP confined concrete members covering a wide range of load cases, FRP confinement types and geometric properties, the utility and accuracy of the proposed model compared with Lam and Teng (2003)’s model and new material secondary development in OpenSees were well validated.

CFRP Anchor for Preventing Premature Debonding of Externally Bonded FRP Laminates from Concrete

AbstractTo delay or prevent debonding of externally bonded fiber-reinforced polymer (FRP) laminate from concrete substrate, a new carbon FRP (CFRP) anchor is developed. The monolithically built anchor comprises two legs and a wide head plate, with the legs inserted into adhesive-filled drilled holes in the concrete and the head plate bonded to the surface of the FRP laminate and the adjacent concrete. Full-scale reinforced concrete T-beams were strengthened in flexure with different amounts of CFRP laminate and tested in four-point bending. The anchor number and spacing were varied to find the most effective configuration. It was determined that the configuration involving nearly uniform spacing of the anchors along the laminate and the placement of the laminate strips between the anchor legs were most effective to allow beams with up to eight plies of the laminate to fail by rupture of FRP rather than debonding. When comparing the efficiency factor of the proposed anchor with those of other anchors repor...

Effect of corner radius and aspect ratio on compressive behavior of rectangular concrete columns confined with CFRP

This paper investigates the behavior of rectangular concrete columns confined with carbon fiber reinforced polymer (CFRP) composites, considering the effect of corner radius and cross section aspect ratio. Nonlinear behavior of confined concrete and orthotropic behavior of fiber-reinforced polymer (FRP) composite have been incorporated in the finite element modeling. The CFRP rupture was modeled in the state of biaxial stresses by using Tsai–Wu’s failure criterion. Therefore, the effects of axial stresses on rupture of FRP as well as the effects of hoop stresses were taken into consideration. Furthermore, to provide more precise modeling and analysis, Tsai–Wu’s criterion and a modified Drucker–Prager plasticity model were applied concurrently in simulating. The finite element analysis was verified by simulating CFRP confined cylindrical and rectangular columns. Concrete columns with cross section aspect ratios of 1.00, 1.25, 1.54 and 2.00, and different corner radii including 5, 15, 25 and 38 mm were modeled. Corner radii of 45, 60 and 75 mm were also modeled in square sections. Results showed that both decreasing the corner radius and increasing the aspect ratio, decrease the compressive strength and ductility enhancement of the confined column. Results of simulating rectangular confined columns led to a design-oriented model for predicting the compressive strength of these sections with different corner radii. The proposed simple model is shown to be in good agreement with the existing test results in the literature, showing the applicability of the model for design purposes for FRP-confined concrete columns.

Intermediate crack-induced debonding in RC beams externally strengthened with prestressed FRP laminates

Intermediate crack-induced debonding is often a dominant failure mode in fiber-reinforced polymer (FRP)-strengthened reinforced concrete (RC) beams in flexure. It has been extensively studied for RC beams externally strengthened with unstressed FRP laminates. However, very little work has been done on FRP debonding for RC beams strengthened with prestressed FRP. This article presents a sectional analysis model for predicting the flexural capacity of RC beams strengthened with prestressed FRP laminates with due consideration of different failure modes. The focus is placed on the effective strain in the prestressed FRP at the ultimate states of intermediate crack-induced debonding or rupture of debonded FRP. Through back-calculation analysis of 51 RC beams strengthened with post-tensioned FRP laminates, a model for predicting the effective strain of prestressed FRP for ultimate strength prediction based upon sectional analyses was developed and validated through comparisons with test results.