Fiber Reinforced Polymer Layers Research Articles

Experimental studies and theoretical models for concrete columns confined with FRP composites: a review

PurposeAdvanced fibre-reinforced polymer (FRP) composites have been increasingly used over the past two decades for strengthening, upgrading and restoring degraded civil engineering infrastructure. Substantial experimental investigations have been conducted in recent years to understand the compressive behaviour of FRP-confined concrete columns. A considerable number of confinement models to predict the compressive behaviour of FRP-strengthened concrete columns have been developed from the results of these experimental investigations. The purpose of this paper is to present a comprehensive review of experimental investigations and theoretical models of circular and non-circular concrete columns confined with FRP reinforcement.Design/methodology/approachThe paper reviews previous experimental test results on circular and non-circular concrete columns confined with FRP reinforcement under concentric and eccentric loading conditions and highlights the behaviour and mechanics of FRP confinement in these columns. The paper also reviews existing confinement models for concrete columns confined with FRP composites in both circular and non-circular sections.FindingsThis paper demonstrates that the performance and effectiveness of FRP confinement in concrete columns have been extensively investigated and proven effective in enhancing the structural performance and ductility of strengthened columns. The strength and ductility enhancement depend on the number of FRP layers, concrete compressive strength, corner radius for non-circular columns and intensity of load eccentricity for eccentrically loaded columns. The impact of existing theoretical models and directions for future research are also presented.Originality/valuePotential researchers will gain insight into existing experimental and theoretical studies and future research directions.

Evaluation and comparison of GRP and FRP applications on the behavior of RCCs made of NC and HSC

This paper presents the results of axial pressure testing on reinforced concrete columns (RCCs) filled with confined normal concrete (NC) and high-strength concrete (HSC) using glass-fiber reinforced plastic pipes (GRP) casing as well as fiber reinforced polymer (FRP). This study aims to evaluate the behavior and mechanical properties of columns confined with GRP casing and FRP wrapping under pressure loads. The major parameters in the experiments were the type of concrete, the effect of GRP casing and FRP wrapping, as well as the number of FRP layers. 12 cylindrical RCCs (150*600) mm were prepared and divided into two groups, NC and HSC, and each group was divided into two parts. In each part, one column was without FRP strengthening layer, a column was wrapped with one FRP layer and another column with two FRP layers. All columns were tested under concentrated compression load. The results of the study showed that the utilization of FRP wrapping and GRP casing improved compression capacity and ductility of RCCs. The addition of one and two layers-FRP wrapping increased compression capacity in the NC group to an average of 18.5% and 26.5% and to an average of 10.2% and 24.8% in the HSC group. Meanwhile, the utilization of GRP casing increased the compression capacity of the columns by 4 times in the NC group and 3.38 times in the HSC group. The results indicated that although both FRP wrapping and GRP casing result in confinement, the GRP casing resulted in increased compression capacity and ductility of the RCCs due to higher confinement. Furthermore, the confinement effect was higher on columns made with NC.

A Stress-Strain Model Considering Bilinear and PostPeak Softening Behaviors for FRP-Confined RC Columns

Objectives: A model with more generalized scope than those in the literature has been proposed to predict the stressstrain response of Fiber-Reinforced Polymer (FRP) confined rectangular RC columns under axial compression. Methods/ Statistical Analysis: 255 columns with their experimental data were used as a basis for the model expressions. Their database included columns with aspect ratios ranging from 1.0 to 2.0. Furthermore, the cross sectional depth varied from 100 to 500 mm and the unconfined concrete strength varied from 18.3 to 51.5 MPa. Findings: Based on interpretation of the test results, bilinear and post-peak softening stress-strain responses are proposed as referencing models, respectively, for columns having a sectional depth of smaller or larger than 300 mm. The softening model is composed of a parabolic expression for describing the first stage of the response and two linear expressions for the second and third stages. The influences of aspect ratio and size of cross sections, number of FRP layers and internal hoop steel bars on the key features of the envelope stress-strain curve model (i.e. ultimate axial stress and strain) are all considered in the model. Application/ Improvements: Reasonable agreement was revealed between the analytical responses of the proposed model with the selected tested responses, confirming its ability to simulate the stress-strain behavior of FRP-confined rectangular RC columns of small or large sizes. Keywords: FRP-confined Concrete, Rectangular RC Columns, Stress-strain Model

Experimental Investigation and Statistical Modeling of FRP Confined RuC Using Response Surface Methodology

Scrap tires that are dumped to landfill is a serious problem in China and rest of the world. The use of rubber in concrete is an effective environmental approach to reduce the amount of scrap tires around the world. However, the loss in compressive strength of concrete is a major drawback of rubberized concrete. In this paper, the fiber reinforced polymer (FRP) confinement technique is used to overcome the drawbacks of rubberized concrete (RuC). A total of sixty six RuC cylinders were tested in axial compression. The cylinders were cast using recycled rubber to replace, a) 0-50 percent fine aggregate volume, b) 0-50 percent coarse aggregate volume, and c) 40-50 percent fine and coarse aggregate volume. Twenty seven cylinders of the latter mix were then confined with one, two and three layers of CFRP jackets. Concrete suffered a substantial reduction in compressive strength up to 80 percent by fine and coarse aggregate replacement with rubber content. However, CFRP jackets recovered and further enhanced the axial compressive strength of RuC up to 600% over unconfined RuC. SEM was performed to investigate the microstructural properties of RuC. Statistical models were developed on the basis of experimental tests for FRP confined RuC cylinders using response surface method. The effect of variable factors; unconfined concrete strength, rubber replacement type and number of FRP layers on confined compressive concrete strength was investigated. The regression analysis was performed to develop the response equations based on quadratic models. The predicted and experimental test results were found in good agreement as the variation between experimental and predicted values were less than 5%. Furthermore, the difference between predicted and adjusted R2 was found to be less than 0.2 which shows the significance of the statistical models. These proposed statistical models can provide a better understanding to design the experiments and the parameters affecting FRP-confined RuC cylinders.

Modeling the influence of interleaf layers in composite materials on elastic properties, thermal expansion, and chemical shrinkage

Within this paper, it is shown that the assumption of transversal isotropy usually applied to unidirectional layers is not valid, if interleaf layers are introduced to increase the impact performance of the material. An approach is suggested to consider the meso-structure of composite materials with interleaf layers in the calculation of the elastic properties, the thermal expansion coefficients, and the chemical shrinkage coefficients. The approach uses known rules of mixture in a first step, to determine the properties of the carbon fiber–reinforced polymer layer as well as those of the interleaf layer and combine them in a second step by new meso-scale rules of mixture. The approach is discussed using known material data and shows to provide plausible results. The effects of the material properties influenced by the meso-scale rules of mixture on the process induced distortions are then discussed exemplary considering the thermally induced spring-in. It is shown that the meso-structure has a considerable influence on the process-induced distortions and cannot be neglected in the prediction of it.

Nonlinear modeling of the dynamic response of pipe conveying fluid coated with FRP under seismic load: Comparison of RSM and Kriging approach

In order to increase the computational efficiency of pipes conveying fluid system with fibre reinforced polymer (FRP) layer under seismic and thermal loads, the ability of response surface (RSM) and kriging models is investigated. The two models of RSM and kriging are calibrated based on the analytical data points of maximum dynamical displacements, which are obtained by analytical analysis. Three input variables as thickness-to-radius ratio (h/R), temperature and the number of FRP layers are used to build the nonlinear models. The results show that the kriging model provides more accurate predictions compared to RMS. It is also found in the parametric study that the predictions by the kriging model closely follow the analytical results. By increasing h/R and FRP layers and decreasing temperature, it is found that the maximum displacements under seismic load of pipes are increased.

Research on Dynamic Response of Concrete‐Filled Steel Tube Columns Confined with FRP under Blast Loading

Recently, a concrete‐filled steel tube confined with fiber‐reinforced polymer (FRP) has become a hot research issue as a new type of structure. These studies mainly focus on its static performance and seismic and impact behaviour, with little research on its blast resistance performance. In this study, the dynamic response of concrete‐filled steel tube columns confined with FRP under blast loading was investigated. Numerical analysis was implemented using multimaterial ALE method in the finite element analysis program LS‐DYNA. The proposed numerical model was validated by the SDOF result and available experimental data. And the effects of the number of FRP layers, concrete strength, and cross section were also discussed in detail based on the proposed numerical model. The results indicate that the constraints of FRP effectively enhance the blast resistance of the column, and the vulnerable parts mainly occur at the middle and two ends of the column. The blast resistance of the column can be enhanced by increasing the number of FRP layers or concrete strength. These results could provide a certain basis for blast resistance design of concrete‐filled steel tubes confined with FRP.

Experimental and numerical study on FRP-upgraded RC beams with large rectangular web openings in shear zones

This paper studies experimentally and numerically the effect of large rectangular web openings in the shear zone on the response of unstrengthened and upgraded reinforced concrete (RC) beams. Studied parameters were opening size and strengthening scheme. Seven simply supported RC beams were prepared and tested under the action of two point loads. Beams comprised of one reference specimen without opening (i.e. solid beam) and six beams constructed with two large rectangular web openings, symmetrically located near supports. Out of the six beams with openings, two specimens were unstrengthened and four beams were strengthened with two fiber reinforced polymer (FRP) strengthening schemes. In one of the schemes, beams were strengthened using two layers (0°/90°) of carbon FRP (CFRP) sheets. However, the second scheme was a hybrid system comprising of glass FRP (GFRP) sheets anchored with bolted steel plates. The numerical models were prepared and the results of finite element (FE) analysis were validated with experiments. The validated numerical analysis was then used for some useful parametric studies in which the effect of different parameters was investigated.

Interlaminar shear strength of fibre metal laminates after thermal cycles

Abstract Fibre Metal Laminates are characterized by low density, high static and fatigue strength, high impact resistance and good corrosion resistance. Currently some new solutions of FML which will contain carbon fibre reinforced polymer layers with aluminium (CARALL laminates) are tested. The purpose of the study was to investigate the influence of the thermal cycles on interlaminar shear strength and microstructure of aluminium-carbon fibres laminates modified by additional glass interlayer. Six different variants of laminates were tested. The 1500 thermal cycles were used. Before and after thermal cycles, the Interlaminar Shear Strength tests were conducted to compare the shear properties of CARALL and CARALL with thin glass interlayer. No delamination, cracks of matrix or fibres were observed after thermal cycles. The interlaminar shear tests have shown that the strength is not dependent on thermal cycles. However, some differences in microstructure of FML after ILSS were noted. Laminates without thermal cycles were damaged more often by delamination at the metal/composite interphases in the middle region of sample (directly under the stamp), whilst the laminates after thermal cycles were damaged also by delamination at the metal/composite interphases but the delamination mostly originated from the edges of the samples. National Science Centre, Poland UMO-2014/15/B/ST8/03447.

Compressive behavior of ultra-high performance fiber-reinforced concrete (UHPFRC) confined with FRP

This study presents the results of an experimental program on the compressive behavior of fiber reinforced polymer (FRP) confined ultra-high performance fiber-reinforced concrete (UHPFRC). A total of 38 specimens were prepared and tested under axial compression. In addition to FRP confined UHPFRC, FRP confined ultra-high performance concrete without fiber addition (UHPC), high strength concrete (HSC), and normal strength concrete (NSC) were also tested to investigate their comparative performances. The test results indicate that the FRP confined UHPFRC can exhibit ductile behavior if sufficient FRP confinement is provided. However, due to their ultra-high strength as well as the unique microstructure, FRP confined UHPFRC is likely to exhibit more brittle behavior than FRP confined NSC and HSC. Compared to FRP confined NSC and HSC, the confinement efficiency is less for FRP confined UHPFRC. Sudden stress reduction or stress fluctuations are observed shortly after the initial peak stress (axial stress at the first peak point) for FRP confined UHPFRC. Based on the confinement level, the stress-strain behavior of FRP confined UHPFRC may experience a second ascending branch or a continuous descending branch after the sudden stress reduction or stress fluctuations. The influences of FRP layers, FRP types, and fiber addition on the compressive behavior of FRP confined UHPFRC are observed to be significant. Moreover, existing stress-strain models available for FRP confined UHPFRC are evaluated by using a database collected in this study.

Compressive behavior of cylindrical rubber buffer confined with fiber reinforced polymer

This paper presents a new composite buffer for mitigating the lateral displacement of structures under seismic loading. The buffer consists of a cylindrical rubber wrapped with fiber reinforced polymer composite. The uniaxial compressive stiffness of the buffer can be controlled by varying either the number of fiber reinforced polymer layers or the wrapping scheme of fiber reinforced polymer. First, a test program is carried out to investigate the impact of various parameters on the compressive stiffness and strength of the new buffer including thickness of fiber reinforced polymer, wrapping scheme, and method of wrapping of fiber reinforced polymer. Next, a theoretical formulation is derived to describe the constitutive behavior of fiber reinforced polymer wrapped rubber under uniaxial compression using strain energy density function of the Yeoh N-order polynomial model. Finally, a finite element model is developed to analyze the new composite buffer and the numerical results are validated using the experimental results. The results of the study show that the Yeoh model is able to simulate the behavior of rubber under compression. The new composite buffer exhibits significantly higher stiffness and strength than that of pure rubber. Wrapping scheme plays an important role in defining the mechanical behavior of the buffer. The study also shows good agreement between the numerical simulation and the experimental results.

Composite mechanics-based design approach for FRP-strengthened walls

ABSTRACTThis study deals with development of a composite mechanics-based design approach for flexural strengthening of masonry walls using different layers of fiber-reinforced polymer (FRP) strips externally bonded and/or near surface mounted (NSM) FRP bars. To verify the developed design approach, experimental study was also conducted on three groups of walls. Six burnt clay brick masonry walls of size 780 × 480 × 230 mm were fabricated. Out of six, four masonry walls are strengthened with combination of longitudinal carbon fiber-reinforced polymer strips and horizontal NSM bars. The flexural responses of these masonry walls were predicted using three-point static loading. To evaluate the effect of using horizontal NSM FRP bars in conjunction with the longitudinal FRP strips on the response of the strengthened wall, an analytical approach using classical lamination theory has been presented. The developed analytical approach shows good agreements with the experimental results in terms of load carrying capacity and mid-span deflection. This analytical approach will help in setting up the design procedure to evaluate the response of a wall that is strengthened with FRP bars/strips provided in multiple layers. A design example has also been provided for easy understanding of the design approach.

Assessment and enhancement of the behavior of lightly RC wall elements subjected to pure shear

Observed damage from the past earthquakes has indicated that several structural walls before the 1970s are non-seismically designed and performed poorly, exhibiting unexpected failure modes such as shear failure due to inadequate detailing and/or poor confinement of concrete core. The shear behavior alone is complex, because it influenced by a host of parameters often interdependent. This paper aims to assess and enhance the non-linear behavior of lightly reinforced concrete wall or panel elements subjected to pure shear. The existing softened membrane model was chosen as a basic model to evaluate the structural behavior and the work strategy was given as follows: first, identification of the range in which reinforced concrete panels were considered as lightly reinforced elements, based on the non-ductile behavior, Experimental data of reinforced concrete panel element A1 conducted by Pang and Hsu and the recommendation of Euro code 2. Next, the influence of the following design parameters on the shear behavior of lightly reinforced panel elements was investigated: the steel grid orientation; the diameter of longitudinal reinforcement, the type of reinforcement, and the crack angle; Finally, to enhance the shear performance, one case that reinforced with ribbed bars exhibited a non-ductile behavior has been selected to strengthen it with one, two, or three layers of fiber-reinforced polymer as an alternative to the traditional methods, using the so-called “softened membrane model-FRP”.

The Effects of Carbon Fiber Reinforced Polymer Strengthening on Cylindrical Steel Storage Tanks under Bending Shear Load

Large thin-walled cylindrical steel storage tanks play important roles in the development of economy and infrastructures. Storage tanks subjected to internal pressure can cause buckling modes, especially elephant foot bulge at the bottom of the tanks. The design of tanks against buckling is very necessary. In this paper, the load-carrying capacity and the failure modes of the storage tanks subjected to internal pressure under bending shear load will be calculated by the finite element analysis. The results will be compared with the design method from Architectural Institute of Japan. Moreover, this paper will also consider the effects of the use of carbon fiber reinforced polymer layers to strengthen storage tanks against buckling and the increase of the load-carrying capacity of the tanks under bending shear load.

Strain distribution of basalt FRP-wrapped concrete cylinders

This paper presents the results of an experimental study on the distribution of strain on a unidirectional basalt fiber-reinforced polymer (FRP) wrapped around concrete cylinders. A total of 12 cylinders (150 mm x 300 mm) were wrapped with 2, 4, and 6 layers of basalt FRP (BFRP) and the distribution of hoop strain under axial compression load was studied using multiple strain gauges. The new aspect of this study is the use of BFRPs as a new construction material for wrapping concrete elements with a focus on the distribution of hoop strain towards refining design strain of the wrap. Also, the effect of number of BFRP layers on the premature rupture of the wrap with respect to flat coupon test was evaluated in the form of a strain efficiency factor. It was concluded that the maximum hoop strain was not necessarily associated with the ruptured areas of the wrap. Also, an analysis of variances showed that the difference between hoop strains in the overlap and non-overlap regions was non-significant and an average hoop strain can represent the overall dilation of the specimens. The average strain efficiency factor was found ranging from 0.61 to 0.86. The test data was added to a large database of concrete cylinders wrapped with unidirectional FRPs and after statistical evaluations a refined strain efficiency factor of 0.70 was proposed instead of the current factor of 0.55 in ACI 440.2R-17 for design applications.

Effects of various aluminum surface treatments on the basalt fiber metal laminates interlaminar adhesion

Fiber metal laminates (FMLs) are a new class of advanced materials consisting of metal and fiber reinforced polymer layers. Surface treatment of the metal layers can have significant effects on the interlaminar properties and thus the overall properties of FMLs. The aim of this study was to apply four different surface treatment methods on the aluminum layers and investigate their effects on the flexural behavior of a FML made up of Al2024, basalt fibers and epoxy Epon 828. To study the surface morphologies and fracture surface of samples, scanning electron microscopy (SEM), optical microscopy (OM) and profilometry were utilized. Results of the four methods applied, showed that the mechanical abrasion and alkaline etching treatments were not found suitable for enhancing the flexural properties due to weak interfacial adhesive bonding. OM images revealed that the adhesive failure was the failure type of mechanical abrasion and alkaline treated samples. On the other hand, Forest Products Laboratory etching (FPL-etching) and anodizing treatments caused significant improvements in the flexural properties. The improvements are attributed to the formation of surface oxide layers containing microscale pits. The microscale pits provided appropriate sites for the penetration of the polymer resin, which caused strong interfacial bonding by mechanical interlocking mechanism. Moreover, SEM and OM images indicated that the main failure type of FPL-etching and anodized samples was the cohesive failure.

Axial Behaviour of Glass Fibre Reinforced Polymer-Confined Reinforced Concrete Short Columns

Fibre-reinforced polymer (FRP) jacketing is an effective confinement tool for reinforced concrete (RC) columns. To avoid the excessive stress concentration in square-shaped columns, a jacketing technique with rounded column edges needs to be practiced. The load-carrying capacity of RC square short columns wrapped with glass FRP (GFRP) on axial loading was investigated. The effi­ciency of confinement offered by improved jacketing schemes with different corner radii of the column, number of layers of FRP and various vertical distri­bution methods of jacketing are compared. The experimental study was con­ducted on 31 columns of size 140 mm x 140 mm x 1200 mm with one, two and three layers of FRP distributed over one-fourth, one-third and full length of the column. The corner radii of the columns were 0,12.5, 25 and 37.5 mm. Spe­cimens were loaded to the failure modes in concentric uniaxial compression, and the behaviour of the specimens was analysed. An analytical model for the ultimate load-carrying capacity of RC square columns confined by GFRP is proposed from the test results.

Mechanical behavior of FRP confined steel tubular columns under impact

This paper presents experimental and analytical results of fiber reinforced polymer (FRP) confined steel tubular columns under transverse impact loads. Influences of applied impact energy, thickness of FRP jacket and impact position were discussed in detail, and then the impact responses of FRP confined steel tubes were compared with bare steel tubes. The test results revealed that the FRP jacket contributes to prevent outward buckling deformation of steel at the clamped end and inward buckling of steel at the impact position. For the given applied impact energy, specimens wrapped with one layer and three layers of FRP have the lower peak impact loads than those of the bare steel tubes, whereas specimens wrapped with five layers of FRP exhibit the higher peak impact loads. All the FRP confined steel tubular specimens displayed a longer duration time than the bare steel tubes under the same magnitude of impact energy, and the specimen wrapped with one layer of FRP had the longest duration time. In addition, increasing the applied impact energy leads to the increase of peak impact load and duration time, whereas increasing the distance of impact position from the clamped end results in the decrease of peak impact load and the increase of duration time. The dynamic analysis software Abaqus Explicit was used to simulate the mechanical behavior of FRP confined steel tubular columns, and the numerical results agreed well with the test data. Analytical solution for lateral displacement of an equivalent cantilever beam model subjected to impact load was derived out. Comparison of analytical and experimental results shows that the maximum displacement can be precisely predicted by the present theoretical model.

Performance of FRP confined and unconfined geopolymer concrete exposed to sulfate attacks

In this study, the effects of magnesium sulfate on the mechanical performance and the durability of confined and unconfined geopolymer concrete (GPC) specimens were investigated. The carbon and basalt fiber reinforced polymer (FRP) fabrics with 1-layer and 3-layers were used to evaluate the performances of the specimens under static and cyclic loading in the ambient and magnesium sulfate environments. In addition, the use of FRP materials as a rehabilitation technique was also studied. For the geopolymerization process of GPC specimens, the alkaline activator has selected a mixture of sodium silicate solution (Na2SiO3) and sodium hydroxide solution (NaOH) with a ratio (Na2SiO3/NaOH) of 2.5. In addition to GPC specimens, an ordinary concrete (NC) specimens were also produced as a reference specimens and some of the GPC and NC specimens were immersed in 5% magnesium sulfate solutions. The mechanical performance and the durability of the specimens were evaluated by visual appearance, weight change, static and cyclic loading, and failure modes of the specimens under magnesium sulfate and ambient environments. In addition, the microscopic changes of the specimens due to sulfate attack were also assessed by scanning electron microscopy (SEM) to understand the macroscale behavior of the specimens. Results indicated that geopolymer specimens produced with nano-silica and fly ash showed superior performance than the NC specimens in the sulfate environment. In addition, confined specimens with FRP fabrics significantly improved the compressive strength, ductility and durability resistance of the specimens and the improvement was found higher with the increased number of FRP layers. Specimens wrapped with carbon FRP fabrics showed better mechanical performance and durability properties than the specimens wrapped with basalt FRP fabrics. Both FRP materials can be used as a rehabilitation material in the sulfate environment.

Strain pattern detection of composite cylinders using optical fibers after low velocity impacts

The strain pattern distributed on composite cylinders after impacts was detected using optical fibers for the first time. The sensing optical fiber was implemented on the composite cylinders using aluminum (Al)-coated optical fiber, polyimide-coated optical fiber, or standard single mode fiber (SSMF) of polymer-coating. The residual strain of this sensing fiber was measured by a Brillouin optical correlation domain analysis (BOCDA) sensor system, using phase modulation and single side band modulation methods. Impact events of 10, 20, and 40 J energies as barely visible impact damages (BVID) were applied on the cylinders, and these fibers were deployed on the cylinder surface, or were embedded in the cylinders. For the surface deployment, Al-coated fiber exhibited the highest residual strain due to Al plastic deformation, and the strain was three times higher than the lowest SSMF value. For the embedment deployment, these fibers were individually embedded in the cylinder, which was composed of sixteen carbon fiber reinforced polymer layers using a filament winding process. In contrast to the surface deployment, the embedded SSMF gave as high residual strain as that of the Al-coated fiber. These similar residual strains of the three embedded optical fibers suggested that they came from permanent material damage inside the material. Impact events with fiber embedment caused the optical signal in the Al-coated fiber to suffer serious additional insertion loss, but the SSMF did not show loss. So the general fiber for optics communications was successfully demonstrated as a distributed residual strain sensor to detect BVID impacts of composite materials. The strain values were maintained for at least 15 months after impact events. This residual strain sensor is economical, because special fibers like Al-coated fiber were not needed, and the sensor is efficient, because it could sense the strain for long distance, without additional impact-driven insertion loss.