Carbon Fiber Reinforced Polymer Strips Research Articles

Experimental investigation and strength model of RC deep beams externally bonded by CFRP

This paper presents an experimental study on shear strengthening of reinforced concrete (RC) deep beams using externally applied carbon fiber reinforced polymer (CFRP) jackets. A total of 18 RC deep beams were tested using a four-point bending load. The examined parameter included span to depth ratio with or without CFRP strengthening. CFRP strips were applied perpendicular to the probable crack pattern as observed during testing for un-strengthened beams. The properties that is cracking behavior, ultimate strength, deflection, and energy dissipation capacity of beams were evaluated and discussed. The test results showed enhancement in shear strength up to 17% with CFRP. An increase in depth of RC deep beams caused a decrease in the rate of enhancement in shear strength for CFRP strengthened beams as compared to that observed in their counterparts. The experimental test results were compared with the existing design guidelines to assess their application for RC deep beams strengthened with CFRP. A strength equation based on regression analysis was developed to predict the experimental shear strength of RC deep beams strengthened with CFRP. This study showed that the RC deep beams strengthened with CFRP can effectively be used in structural engineering applications with enhanced shear strength and energy absorption capacity.

Seismic behavior of RC columns internally confined by CFRP strips

During past decades fiber-reinforced polymer (FRP) sheets have been externally bonded to structural elements to increase their axial, shear, or bending capacity. FRP bars also have been widely used to replace the steel reinforcements in columns subjected to a harsh environment. In this study, carbon fiber-reinforced polymer (CFRP) strips were used as the transverse reinforcement for concrete columns. Although FRP bars have already been used as the transverse reinforcement in concrete columns, the efficiency and feasibility of CFRP strips have not been investigated. CFRP strips are flexible; therefore, they can be easily shaped as spirals to confine the concrete core of columns. The efficiency of CFRP strips for the confinement of the concrete core was examined through a series of quasi-static cyclic tests on four full-scale columns that had similar size and longitudinal reinforcements. One of the columns was selected as the reference, and steel spirals transversally reinforced it. CFRP strips transversally reinforced the other three columns with different widths and spacing. The obtained results showed that the number of cracks in the CFRP-confined columns was less than the reference column. The length of cracks in the CFRPconfined columns was also relatively shorter. Besides, the CFRP-confined columns had a larger ultimate load, effective yield strength, and displacement ductility ratio compared with the reference column.

An experimental approach to the influence of partial rebar corrosion and concrete degradation on behavior of FRP-strengthened RC beams

Human factors are one of the main reasons for structural damage as they decrease the bearing capacity and also lead to an inaccurate estimation of the structure. Previous studies show that the use of CFRP in the damaged structures can significantly increase their bending and shear capacity. This study examines the capacity and cracks distribution in eight RC (reinforced concrete) beams (210 × 250 × 250 cm), each of which was rehabilitated with seven CFRP (carbon fiber-reinforced polymer) strips using the strip method. Each beam, except for the control specimen, experiences different types of concrete and rebar damages, which are finally compared with those of the control specimen. The results indicated that rebar damage in all the beams was significant and the effects of concrete damage were minimized by CFRP strips. Moreover, the force–displacement diagrams indicate the greatest force for the control specimen. Other specimens reached up to 80% of the force experienced by the control specimen. Finally, the parametric study showed that the influence of the crack width on decreasing the bearing capacity was more significant compared with the other parameters.

Experimental study of the bond interaction between CFRP and concrete under blast loading

Specific to blast loading, bond shear tests between carbon fiber reinforced polymer (CFRP) strips and concrete have not yet been reported in literature, to the best knowledge of the authors. Given the high potential of strengthening concrete with CFRP to increase blast resistance, it is necessary to better understand the dynamic interaction between concrete and CFRP under blast impulse. This article presents a new experimental setup developed in order to study blast driven bond interaction between CFRP and concrete. An evaluation of the bond at the interface between the CFRP strip and the concrete is conducted in order to identify the parameters that affect the bond strength under blast loading. Several retrofitted specimens with different bond strength are tested. An explosive driven shock tube (EDST) set-up is used to generate the blast loading. Piezo resistive strain gauges and digital image correlation (DIC) measurement are used to record the debonding process and the evolution of the strain along the bonded area. As such, the bond behaviour is studied for three different bond lengths and the effect of interaction of the blast wave within the concrete is highlighted. The experimental tests demonstrate that under blast loading, the debonding between the CFRP strip and the concrete occurs due to a combined effect: interface stresses caused by the induced force in the CFRP and additional stresses resulting from the propagation of the blast induced stress wave within the concrete.

Fire behavior and design of steel columns reinforced by prestressed CFRP strips

This study analyzes the behavior of steel columns reinforced by prestressed (PS) carbon fiber reinforced polymer (CFRP) strips exposed to fire. First, tests are performed to study the CFRP material degradation under high temperatures. The elastic modulus and strength of the CFRP decrease with increasing temperature. Second, a thermal–mechanical coupling analysis is conducted with finite element method (FEM) to study the behavior of composite members exposed to fire. Under compressive preloading, the columns are exposed to the ISO 834 standard fire curve. The fire exposure time of columns with different initial CFRP prestressing forces is obtained. All the specimens fail because of global buckling either due to the slacking of the concave side of the CFRP or the plastic deformation of the steel. Subsequently, parametric studies are performed considering different initial CFRP prestressing forces, preloadings and column slenderness values. Finally, a design method is proposed for steel columns with PS CFRP that are exposed to fire, followed by a design example. Columns with protections can achieve high buckling capacities at room temperature and long fire exposure time.

Bond Behaviour of Near-Surface Mounted Strips in RC Beams-Experimental Investigation and Numerical Simulations.

This paper presents an investigation of the bond mechanism between carbon fibre reinforced polymer (CFRP) laminates, concrete and steel in the near-surface mounted (NSM) CFRP-strengthened reinforced concrete (RC) beam-bond tests. The experimental program consisting of thirty modified concrete beams flexurally strengthened with NSM CFRP strips was published in. The effects of five parameters and their interactions on the ultimate load carrying capacities and the associated bond mechanisms of the beams are investigated in this paper with consideration of the following investigated parameters: beam span, beam depth, longitudinal tensile steel reinforcement ratio, the bond length of the CFRP strips and compressive concrete strength. The longitudinal steel reinforcement was cut at the beam mid-span in four beams to investigate a better assessment of the influence of the steel reinforcement ratio on the bond behaviour of CFRP to concrete bond behaviour. The numerical analysis implemented in this paper is based on a nonlinear micromechanical finite element model (FEM) that was used for investigation of the flexural behaviour of NSM CFRP-strengthened members. The 3D model based on advanced CFRP to concrete bond responses was introduced to modelling of tested specimens. The FEM procedure presents the orthotropic behaviour of the CFRP strips and the bond response between the CFRP and concrete. Comparison of the experimental and numerical results revealed an excellent agreement that confirms the suitability of the proposed FE model.

Retrofitting of SCC Deep Beams With Circular Openings Using CFRP

The main objectives of this study are: encouraging the production and use of self-compacting concrete, use of materials which are lightweight, easy to use, and highly efficient in the retrofitting of reinforced concrete buildings. Six deep beams specimens (L= length of 1400mm, h= height of 400mm, and b= width of 150mm) were cast using self-compacting concrete. The location of the openings is in the middle of assumed load path. Five patterns were adopted to arrange carbon fiber reinforced polymer (CFRP) strips. The cylinder compressive strength of the concrete was approximately equal for all beams and was about (44 MPa) at 28 days age. All the beams have the same steel reinforcement for shear and flexure. There have been many tests for fresh and hardened concrete. The reinforced concrete deep beams were tested up to (60%) of the ultimate load of control beams to simulate degree of damage, and then released the load. After that, the beams were retrofitted using (CFRP) strips, and then the beams were tested to failure. The study was focused on determining the vertical mid-span deflection, ultimate load, the load that causes first shear and flexural cracks, and mode of failure. The results showed that, the best increase in the ultimate failure load was (27.27%) and achieved using the inclined strips pattern and the pattern of vertical and horizontal strips together. Reduction in the deflection values for the retrofitted beams compared to the control beam by about (12-13%) due to restrictions imposed by CFRP strips and the...

Experimental Investigation of CFRP Strengthened Unreinforced Masonry Walls with Openings

ABSTRACT Several conventional rehabilitations and strengthening techniques are available depending on the purpose required for unreinforced masonry (URM) walls. Carbon Fiber Reinforced Polymer (CFRP) retrofitting is one of these techniques which increases the shear capacity and overall post-strengthening performance of the walls. This study aims to experimentally explore shear behavior of URM walls with openings that were strengthened by CFRP strips. For this purpose, five solid clay brick masonry wall specimens having a central opening with dimensions of 315 × 230 mm were fabricated and then strengthened using double-sided diagonal CFRP strips with different thickness and surface area. All these specimens were tested under combined in-plane monotonic lateral load and constant axial load. It was concluded that for a given FRP reinforcement amount, a single-layer four-strip configuration results in slightly higher shear strength when compared with a double-layer two-strip configuration. Furthermore, in-plane shear strength of CFRP strengthened URM walls with opening was also estimated by existing analytical models in the literature.

Investigation of impact behavior of reinforced concrete beam to column connection strengthened with carbon fiber‐reinforced polymer strips

AbstractReinforced concrete (RC) members may expose to impulsive dynamic loads due to the reasons such as the explosions occurring in the interior or exterior part of them, rockfall, the vehicle crash to the bridges, the collision of masses with the effects of floods and landslide. Many studies have investigated the effects of impulsive dynamic loads on the beam, column, and slab RC structural elements have been investigated in the literature. However, the authors have not encountered any study focused on the impact behavior of beam to column connections of the frames constructing the bearing system of reinforced concrete structures. Therefore, an experimental study has been planned to investigate RC beam impact behavior to column connections strengthened with carbon fiber‐reinforced polymer (CFRP) strips. The concrete compressive strength, shear reinforcement spacing, CFRP strip spacing, and input impact energy applied to test specimens were taken as experimental variables. The time histories of impact load acting on test specimens, accelerations, displacements, and the strains measured from CFRP strips have been recorded in experiments. The experimental variables' effect on dynamic responses of RC beam to column connections strengthened with CFRP strips subjected to impact load has been interpreted in detail. The study's scope and improved numerical analysis procedure have also been introduced to verify experimental results. Good agreement between numerical and experimental results demonstrated that the presented numerical procedures could be safely used for evaluation of impact behavior of RC beam to column connections strengthened with CFRP strips.

Behavior and modeling of CFRP nonuniformly wrapped circular seawater sea-sand concrete (SSC) columns under axial compression

An experimental study was performed in this paper to investigate the compressive behavior of carbon fiber-reinforced polymer (CFRP) nonuniformly wrapped seawater sea-sand concrete (SSC) cylinders. The effect of test variables on the mechanical performance was comprehensively discussed, including the clear spacing and thickness of external CFRP strips. The test results indicate that the failure process of tested specimens is less brittle compared with that of the fully or partially FRP-confined columns. Besides, the average ultimate stresses and strains of the specimens nonuniformly wrapped with two-layer external CFRP strips increase by 6.7%-27.4% and 5.9–41.6% respectively with the reduction of clear spacing ratio. Moreover, when the thickness of external CFRP strips double increases, the maximum increment of strength and strain at ultimate condition becomes 42.8% and 63.8% respectively. It is also shown that the FRP efficiency factor of confined specimens with nonuniform wrapping scheme is the highest among various strengthening strategies under the same FRP volumetric ratio. Finally, a new nonuniform confinement effective coefficient, as well as a analytical stress–strain model, is developed based on the available test results. Compared with several existing predicted models, the proposed model can provide more accurate and reliable predictions.

Axial capacity and stiffness of post-heated circular and square columns strengthened with carbon fiber reinforced polymer jackets

The exposure of reinforced concrete (RC) columns to elevated temperatures is detrimental to its axial capacity and stiffness. Selecting the proper strengthening technique to repair heat-damaged RC columns is critical to ensure that the axial capacity and stiffness are restored. This paper examines the performance of different strengthening schemes using carbon fiber reinforced polymer (CFRP) jackets and near-surface mounted (NSM) steel bars in repairing circular and square RC columns damaged due to the exposure to high elevated temperature. The tested columns were divided into three categories, un-heated and un-strengthened columns, post-heated and un-strengthened columns, and post-heated and strengthened columns. The heated columns were exposed to 600 °C for 3 h. Two strengthening schemes were evaluated. The first strengthening scheme consisted of unidirectional CFRP strips in combination with NSM steel bars. The second scheme consisted of strengthening with unidirectional CFRP strips only. All the columns were tested under axial compression until failure. The test results showed that exposure to elevated temperatures caused a reduction in the load-carrying capacity of 38% in circular columns and 29% in square columns. Both strengthening schemes were effective in restoring and exceeding (by approximately 16%) the initial load-carrying capacity of heat-damaged RC columns. The use of CFRP strips in combination with NSM steel bars was more effective in restoring the initial stiffness of the post-heated columns compared to using CFRP strips alone. The initial stiffness recovery was more pronounced in circular columns compared to square columns. Circular columns exhibited 15% higher initial stiffness when compared to the control columns before heat-exposure. For square columns, the initial stiffness was equivalent to the initial stiffness of the control RC columns before heat-exposure.

Experimental and numerical investigation on cyclic behavior of masonry infilled RC frames retrofitted with partially bonded CFRP strips

This paper investigates experimentally and numerically the behaviors of retrofitted masonry infilled reinforced concrete (RC) frames using totally bonded or partially bonded diagonal carbon fiber reinforced polymers (CFRP) strips under cyclic loads. Five 1/2-scaled, one bay, one storey nonductile RC frames were built and tested as bare and infilled reference specimens, and as x-cross CFRP sheets retrofitted specimens. Retrofitting the RC frames was based on the use of two different techniques; (a) totally bonded CFRP strips (b) partially bonded CFRP strips. Significant findings were noted while comparing the retrofitted specimens with reference specimens in terms of inter-storey drift, lateral load capacity, energy dissipation capacity, stiffness, and the observed damages. Moreover, using CFRP anchors delayed the debonding failure mode. A two-dimensional (2D) finite element model (FEM) of the tested frames was also developed to predict the behaviors of these specimens under cyclic loading. Element types and materials constitutive model of cyclic behavior were described. The infill-frame interaction, in addition to the interactions between CFRP and RC frames, were accurately mimicked. The suggested (2D) FEM was compared with experimental test results. Results of the FEM showed good agreement with test results, showing high accuracy in relation to ultimate load capacities and modes of failure. Bonded and partially bonded CFRP stripes showed similar lateral strength and stiffness.

Experimental research on hybrid anchorages of prestressed composite strips for structural strengthening

The prestressed unidirectional carbon fibre reinforced polymer (CFRP) strips are currently often used for flexural strengthening of concrete structures. To ensure reliable strengthening, strips have to be anchored at their ends to the surface. However, anchoring of unidirectional CFRP strips is difficult because of their weak transversal mechanical properties. Although several CFRP strip anchorage approaches have been developed to date, only very few were applied on-site. The paper presents the development of hybrid bonded/bolted anchorages of CFRP strips used in a novel prestressing system. The anchorages are made of thin steel plates, clamped with the high-strength friction grip (HSFG) bolts and epoxy adhesive, to fix the CFRP strips. The paper describes the results of full-scale tests carried out on a series of axial tensile specimens. The developed anchorage has an ultimate tensile strength of about 200 kN, i.e. 70% of the ultra-high CFRP strips’ strength, which is sufficient for strengthening purposes. After a series of tests in the laboratory, the anchorages have been implemented on-site and an example is presented in the paper.

Strengthening of Continuous Reinforced Concrete Deep Beams with Large Openings Using CFRP Strips.

To accommodate utilities in buildings, different sizes of openings are provided in the web of reinforced concrete deep beams, which cause reductions in the beam strength and stiffness. This paper aims to investigate experimentally and numerically the effectiveness of using carbon fiber reinforced polymer (CFRP) strips, as a strengthening technique, to externally strengthen reinforced concrete continuous deep beams (RCCDBs) with large openings. The experimental work included testing three RCCDBs under five-point bending. A reference specimen was prepared without openings to explore the reductions in strength and stiffness after providing large openings. Openings were created symmetrically at the center of spans of the other specimens to represent 40% of the overall beam depth. Moreover, finite elements (FE) analysis was validated using the experimental results to conduct a parametric study on RCCDBs strengthened with CFRP strips. The results confirmed reductions in the ultimate load by 21% and 7% for the un-strengthened and strengthened specimens, respectively, due to the large openings. Although the large openings caused reductions in capacities, the CFRP strips limited the deterioration by enhancing the specimen capacity by 17% relative to the un-strengthened one.

Shear performance of short-span FRP-reinforced concrete beams strengthened with CFRP and TRC

This paper presents an experimental program to study the shear performance of short-span GFRP (glass fiber reinforced polymer) bar-reinforced concrete beams strengthened in shear with CFRP (carbon fiber reinforced polymer) laminates or TRC (textile reinforced concrete). Seven short-span concrete beams reinforced with GFRP bars, which have an aspect ratio of 1.7, are strengthened in shear with various schemes. Two un-strengthened beams, which have limited and no transverse reinforcements, are used as the controlled specimens. Two commonly used strengthening methods, namely CFRP strip and TRC are applied to the tested specimens. The number of textile layers is investigated in this experimental program. The U-wrapping strengthening configuration is used for both CFRP and TRC methods. The experimental results indicate that both TRC and CFRP schemes effectively enhance the shear strengths of short-span FRP-reinforced concrete beams. The number of textile layers slightly affects the shear strengths of the tested beams because the failure is controlled by the crushing of the concrete struts in short-span beams. A simple strut-and-tie method is used in the last part of the paper to predict the shear strengths of the tested specimens.

Anchoring of the main CFRP sheets with transverse CFRP strips for optimum upgrade of RC Beams: Parametric experimental study

This study illuminates the impact of reasonable anchoring of carbon fiber-reinforced polymer (CFRP) composites as an adaptive flexural strengthening technique for reinforced concrete (RC) beams. To assess the usefulness of the proposed anchoring technique, 38 RC beams were fabricated and tested. The specimens were varied in terms of three major parameters including: number of layers of the main CFRP sheets (0, 1, and 2); length of the main CFRP sheets (none, 400, 700, and 1000 mm); and width of the anchoring CFRP strips (none, 50, and 100 mm). The experimental results showed that the proposed anchoring technique significantly enhances the flexural behavior RC beams in terms of its major characteristics. The attained enhancement was governed by the efficiency of the transverse end-anchoring, which allowed the main CFRP sheets to provide higher contribution to the flexural behavior without immature de-bonding due to cracking; even when the nominal capacity was reached. With increased length of the main CFRP sheets, they became more capable of bridging the emerging flexural cracks; which also true when increasing the width of the anchoring CFRP strips. The effects of the studied parameters were evaluated for the ultimate load and deflection, performance factor, energy absorption, displacement, and ductility. The results revealed that with the proposed anchorage system, the contribution of CFRP sheets to the flexural strength can be improved by about 25%, in addition to substantial enhancements in the other performance characteristics.

Experimental and numerical analysis of compression and bending strength of old wood reinforced with CFRP strips

The reinforcement and repair of historical wooden structures is very important because they reflect the socio-economic structure and architectural construction techniques of the period it was built. In this study, carbon fiber-reinforced polymer (CFRP) strips were used to improve the mechanical behaviors of old wood (OW) samples that were damaged over time because of various environmental effects and biological factors. To determine the effect of reinforcement on the strength values of OW and new wood (NW) samples under bending and compression loads, CFRP strips were applied at tension surfaces of bending samples and around the perimeter of compression samples. The effect of reinforcement on mechanical properties of OW and NW samples was evaluated with a statistical analysis program. Moreover, the ductility and energy dissipation capacities of experimental samples was calculated from the load-deflection curve obtained under bending loading. Reinforcement with CFRP has significantly improved the mechanical strength and stiffness of OW samples. The bending strength values increased 28%, 34% and 59% for new wood (NW), reinforced old wood (FROW) and reinforced new wood (FRNW) with respect to the non-reinforced OW, respectively. The modulus of elasticity values of NW, FROW and FRNW were 15%, 19% and 34% higher than OW, respectively. The 3-dimensional numerical model was developed and validated by experimental tests carried out on wood samples with and without reinforcement arrangements. Wood behaviors were modeled using uniaxial stress-strain curves. In numerical analysis, load-deflection curves, equivalent stresses and normal stresses of the beams were obtained. A good correlation was obtained between numerical and experimental results.

Novel NSM configuration for RC column strengthening-A numerical study

Retrofitting of structures has gained importance over the recent years. Particularly, Reinforced Cement Concrete (RCC) column strengthening has become a challenge to the structural engineers, owing to the risks and complexities involved in it. There are several methods of RCC column strengthening viz. RCC jacketing, steel jacketing and Fiber Reinforced Polymer (FRP) wrapping etc., FRP wrapping is the most promising alternative when compared to the others. The large research database shows FRP wrapping, through lateral confinement, improves the axial load carrying capacity of the columns under concentric loading. However, its confining efficiency reduces under eccentric loading. Hence a relative newer technique called Near Surface Mounting (NSM), in which Carbon FRP (CFRP) strips are epoxy grouted to the precut grooves in the cover concrete of the columns, has been thrust domain of research. NSM technique strengthens the column nominally under concentric load case while significantly under eccentric case. A novel configuration of NSM in which the vertical NSM (VNSM) strips are being connected by horizontal NSM (HNSM) strips was numerically investigated under both concentric and eccentric loading. It was found that the configuration with 6 HNSM strips performed better under eccentric loading than under concentric loading, while the configuration with 3 HNSM strips performed better under concentric loading than under eccentric loading. Hence an optimum of 4 HNSM strips is recommended as strengthening measure for the given column specifications. It was also found that Aluminum alloy cannot be used instead of CFRP in NSM applications owing to its lower mechanical properties.

Behavior of glulam timber beam strengthened with carbon fiber reinforced polymer strip for flexural loading

In the last 20 years, the use of wooden structures and their dimensions have gradually increased. The wood application has increased in different structures such as multistory buildings, sports, industrial facilities, road and railway bridges, power transmission lines, and towers. The widespread use and size of wood structures have increased the research on developing special types of wood products supported by composite materials. Laminated wood elements are the leading composite wood materials. Laminated wooden beams allow making much larger openings than standard solid wood structural elements. The development of the sizes and usage areas of wooden structures has increased the capacity of glulam structural elements and reveals the need to improve their performance. Carbon fiber reinforced polymers (CFRPs) are the most suitable options for increasing the bearing capacity values of glulam beams and improving general load–displacement behaviors. In this study, the use of CFRP strips in different layouts to increase glulam wooden beams and the application of CFRP fan-type anchors in the CFRP strip endpoints are the studied variables. Anchored and non-anchored glulam wooden beams reinforced with CFRP strips with different layouts were tested using a three-point bending test. The ultimate load capacity, initial stiffness, displacement ductility ratio, energy dissipation capacity, failure mechanisms, and general load–displacement behavior of wooden beam test specimens were obtained and interpreted as a result of the experiments.

The development of nature-inspired gripping system of a flat CFRP strip for stress-ribbon structural layout

Abstract The elegant stress-ribbon systems are efficient in pedestrian bridges and long-span roofs. Numerous studies defined corrosion of the steel ribbons as the main drawback of these structures. Unidirectional carbon fiber-reinforced polymer (CFRP) is a promising alternative to steel because of lightweight, high strength, and excellent corrosion and fatigue resistance. However, the application of CFRP materials faced severe problems due to the construction of the anchorage joints, which must resist tremendous axial forces acting in the stress-ribbons. Conventional techniques, suitable for the typical design of the strips made from anisotropic material such as steel, are not useful for СFRP strips. The anisotropy of СFRP makes it vulnerable to loading in a direction perpendicular to the fibers, shear failure of the matrix, and local stress concentrations. This manuscript proposes a new design methodology of the gripping system suitable for the anchorage of flat strips made from fiber-reinforced polymers. The natural shape of a logarithmic spiral Nautilus shell describes the geometry of the contact surface. The continuous smoothly increasing bond stresses due to friction between the anchorage block and the CFRP strip surface enable the gripping system to avoid stress concentrations. The 3D-printed polymeric prototype mechanical tests proved the proposed frictional anchorage system efficiency and validated the developed analytical model.