Number Of Carbon Fiber Reinforced Polymer Layers Research Articles

Behaviour of CFRP wrapped RC square columns under eccentric compressive loading

This paper presents the effects of carbon fibre reinforced polymer (CFRP) volumetric ratios on structural and failure behaviour of CFRP wrapped reinforced concrete (RC) square columns under concentric, eccentric and flexural loadings. The CFRP volumetric ratio is considered as the ratio of volume of CFRP to the volume of concrete. The applicability of existing confinement models to predict the load-moment interaction diagrams of such columns in eccentric loadings is also presented in this paper. Total 20 columns were tested. Dimensions of all columns were 175x175x800 mm with 20 mm rounded corners. Variable considered in this study were number of CFRP layers (e.g. 0, 1, 2 and 3 layers) which corresponds to CFRP volumetric ratio of 0, 0.3, 0.6 and 0.9%, respectively, increasing eccentricities (e.g. 0, 25, 35 and 50 mm) of the applied load and pure flexural loading. Results show that ultimate load capacity and axial deformation increase with increase in CFRP volumetric ratios irrespective of eccentricities. However, load capacity and deformation decrease with increasing eccentricities for any given CFRP volumetric ratio. Hoop strains also increase with increase in CFRP volumetric ratios and eccentricities. Stiffness of CFRP wrapped columns also increases with increase in CFRP volumetric ratio up to 0.6% and decreases with increase in eccentricities. Ductility of CFRP wrapped columns increases with increase in CFRP volumetric ratios and eccentricities. The algorithms for calculation of critical points for load-moment (P-M) interaction diagrams using analytical as well as numerical stress integrations are developed and compared to those of the column tests. P-M interaction diagram developed using existing confinement models agree well with the experimental P-M interaction diagrams of CFRP wrapped RC square columns under eccentric loading.

Numerical analysis of central mixed-mode cracking in steel plates repaired with CFRP materials

Finite element method (FEM) was adopted to study the fatigue properties of steel plates with central mixed-mode (I/II) crack repaired with carbon fiber-reinforced polymer (CFRP) sheets. To simulate crack propagation in repaired and un-repaired specimens, a script was written in Python for automatic modelling. The numerical results were compared with the experimental results, and they fitted well in terms of crack growth trajectories and fatigue propagation lives. Thereafter, different parameters such as the number of CFRP layers, CFRP width, CFRP modulus, and load application angle were considered to investigate CFRP repairing efficiency. Results demonstrated that FEM effectively simulated the fatigue performance of steel plates with mixed-mode crack. CFRP sheets can reduce the effective stress intensity factor (SIF) range to prolong the fatigue life of the cracked steel plate. The aforementioned parameters significantly affected the fatigue performance.

The performance of steel conical shells reinforced with CFRP laminates subjected to uniform external pressure

Abstract This paper presents an experimental study into the behaviour of steel conical shells reinforced by carbon fibre reinforced polymer (CFRP) laminates subjected to uniform external pressure. The tests were conducted on nine conical shells with different slenderness ratios, which utilised different number of CFRP layers. It was concluded that the CFRP laminates increased the stiffness and buckling strength of conical shells, with the increase depending on the conical shell slenderness ratio. Additionally, existing analytical models for predicting the buckling strength of the unreinforced conical shells were visited and modifications were proposed for predicting the buckling strength of reinforced conical shells by CFRP laminates. Comparable predilections between the modified analytical models and the experiments were observed.

Experimental and numerical study on CFRP-lined prestressed concrete cylinder pipe under internal pressure

Prestressed concrete cylinder pipe (PCCP) has been widely used for water conveyance projects. However, prestressing wire breaks may result in the rupture of a pipe and cause a serious catastrophe. Application of carbon fiber reinforced polymer (CFRP) liners adhered to the inner concrete core is an effective method of internal repairing and strengthening of PCCP. In this study, a full-scale experimental test and corresponding finite element model evaluation were conducted to investigate the structural performance of CFRP-lined PCCP. In particular, the complex CFRP-concrete bonded interface was simulated by a cohesive element layer with a bilinear traction-separation response. In addition, the failure risk analysis method was introduced to determine the required number of CFRP layers for the distressed PCCP. The experimental results showed that the watertightness of liner can be improved by painting polyurea coating on the surface of CFRP laminate. It was shown by the simulation that for each additional layer of CFRP, the threshold value for the design pressure increases by 7 broken wires under the damage limit state and by 20 broken wires for the working pressure. The research presented in this paper can provide technical recommendations for the application of CFRP-lined PCCP and help utilities to better strengthen the distressed PCCP pipelines.

FLEXURAL CAPACITY ASSESSMENT OF BRICK AGGREGATED PRE-CRACKED RC BEAM STRENGTHENED WITH CARBON FIBER POLYMER

Ageing and improvements to design code has led to many existing RC structures made of locally available brick aggregates are now found structurally deficient and are in need of rehabilitation. This research emphases on flexural capacity assessment and investigation of failure modes of Carbon Fiber Reinforced Polymers (CFRP) strengthened brick aggregated RC beams. Flexural performance of the RC beam specimens are evaluated using four point bending method. Six RC beams (initially cracked) with CFRP strengthening were tested by varying (i) type of CFRP, (ii) reinforcing area, (iii) anchorage type; and (iv) number of CFRP layers. Two beams were tested as control specimens. Unidirectional carbon fiber sheet (Tow Sheet) and individually hardened continuous fiber strands woven into sheet form (Strand Sheet) were used. Simple flexure failure was obtained for unstrengthened RC beams while end plate and interfacial debonding were observed for the initially cracked CFRP strengthened RC beams. Strengthening of pre-cracked beams using Strand Sheet gave better performance compared to Tow sheet. Overall flexural strength improvement of CFRP strengthened beams varied from 12% to 34% with respect to unstrengthened beams depending on strengthening methods.

FLEXURAL CAPACITY ASSESSMENT OF BRICK AGGREGATED PRE-CRACKED RC BEAM STRENGTHENED WITH CARBON FIBER POLYMER

Ageing and improvements to design code has led to many existing RC structures made of locally available brick aggregates are now found structurally deficient and are in need of rehabilitation. This research emphases on flexural capacity assessment and investigation of failure modes of Carbon Fiber Reinforced Polymers (CFRP) strengthened brick aggregated RC beams. Flexural performance of the RC beam specimens are evaluated using four point bending method. Six RC beams (initially cracked) with CFRP strengthening were tested by varying (i) type of CFRP, (ii) reinforcing area, (iii) anchorage type; and (iv) number of CFRP layers. Two beams were tested as control specimens. Unidirectional carbon fiber sheet (Tow Sheet) and individually hardened continuous fiber strands woven into sheet form (Strand Sheet) were used. Simple flexure failure was obtained for unstrengthened RC beams while end plate and interfacial debonding were observed for the initially cracked CFRP strengthened RC beams. Strengthening of pre-cracked beams using Strand Sheet gave better performance compared to Tow sheet. Overall flexural strength improvement of CFRP strengthened beams varied from 12% to 34% with respect to unstrengthened beams depending on strengthening methods.

High-strain rate compressive behavior of CFRP confined concrete: Large diameter SHPB tests

The behavior of a material under high-strain rate compression can be different from quasi-static one and can be determined by using the so-called Split-Hopkinson Pressure Bar (SHPB). The aim of this study is to investigate the high-strain rate compressive behavior of Carbon Fiber Reinforced Polymer (CFRP) confined concrete in comparison with unconfined concrete. A total of 112 specimens with a diameter of 150mm were tested under quasi-static (with a length to diameter ratio of 0.5 and 2) and under high-strain rate axial compression (with a length to diameter ratio of 0.5) using a universal testing machine and a SHPB with a large diameter of 155mm, respectively. The specimens were characterized by two different strength classes, namely C30 and C60, and by a different number of CFRP layers ranging from 0 (plain cylinder) to 3. Tests were executed up to a strain rate of 100s-1. The measurements were validated by checking the consistency of the large diameter SHPB and by comparing the results of plain concrete with the data available in the literature. The results indicate that the strength of CFRP confined concrete is not significantly sensitive to strain rate effects.

Behavior study of NC and HSC RCCs confined by GRP casing and CFRP wrapping

This paper presents the results of axial compression testing and numerical modeling on reinforced concrete columns (RCC) with normal concrete (NC) and high-strength concrete (HSC), RCC confined by glass-fiber reinforced plastic pipes (GRP) casing as well as carbon fiber reinforced polymer (CFRP), The major parameters evaluated in the experiments were the effects of concrete type, GRP casing and CFRP wrapping, as well as the number of CFRP layers. 12 cylindrical RCC (150×600 mm) were prepared and divided into two groups, NC and HSC. Each group was divided into two parts; with and without GRP casing. In each part, one column was without CFRP strengthening layer, a column was wrapped with one CFRP layer and another column with two CFRP layers. All columns were tested under concentrated compression load. Numerical modeling was performed using ABAQUS software and the results of which were compared with experimental findings. A good agreement was found between the results. Results indicated that the utilization of CFRP wrapping and GRP casing improved compression capacity and ductility of RCC. The addition of one and two layer-FRP wrapping increased capacity in the NC group to an average of 18.5% and 26.5% and in the HSC group to an average of 10.2% and 24.8%. Meanwhile, the utilization of GRP casing increased the capacity of the columns by 3 times in the NC group and 2.38 times in the HSC group. The results indicated that although both CFRP wrapping and GRP casing increased confinement, the GRP casing gave more increase capacity and ductility of the RCC due to higher confinement. Furthermore, the confinement effect was higher on NC group.

Additive Manufactured Sandwich Composite/ABS Parts for Unmanned Aerial Vehicle Applications.

Fused deposition modelling (FDM) is one of most popular 3D printing techniques of thermoplastic polymers. Nonetheless, the poor mechanical strength of FDM parts restricts the use of this technology in functional parts of many applications such as unmanned aerial vehicles (UAVs) where lightweight, high strength, and stiffness are required. In the present paper, the fabrication process of low-density acrylonitrile butadiene styrenecarbon (ABS) with carbon fibre reinforced polymer (CFRP) sandwich layers for UAV structure is proposed to improve the poor mechanical strength and elastic modulus of printed ABS. The composite sandwich structures retains FDM advantages for rapid making of complex geometries, while only requires simple post-processing steps to improve the mechanical properties. Artificial neural network (ANN) was used to investigate the influence of the core density and number of CFRP layers on the mechanical properties. The results showed an improvement of specific strength and elastic modulus with increasing the number of CFRP. The specific strength of the samples improved from 20 to 145 KN·m/kg while the Young’s modulus increased from 0.63 to 10.1 GPa when laminating the samples with CFRP layers. On the other hand, the core density had no significant effect on both specific strength and elastic modulus. A case study was undertaken by applying the CFRP/ABS/CFRP sandwich structure using the proposed method to manufacture improved dual-tilting clamps of a quadcopter UAV.

A Numerical Investigation on the Structural Behavior of Deficient Steel Frames Strengthened using CFRP Composite

Carbon fiber reinforced polymers (CFRP) is one of the materials that is used to strengthen steel structures. Most studies on CFRP strengthening steel on structures have been done on beams and steel columns. No independent study has studied the effect of CFRP strengthening on the structural behavior of steel frames having initial deficiency.The deficiency in steel structures may be created due to the errors caused by construction and others.This study aims to carry out a numerical study on the efficiency of CFRP sheet on strengthening square hollow section (SHS) steel frames having initial deficiency. Seven specimens, five of which were strengthened using CFRP sheets, were analyzed. ANSYS software was used to analyze the SHS steel frames. The results showed that the coverage length, the width, and the number of CFRP layers have a significant effect on increasing and recovering the ultimate load capacity of the SHS steel frames having initial deficiency.

Flexural-strengthening efficiency of cfrp sheets for unbonded post-tensioned concrete T-beams

There has been a limited number of studies about the flexural behavior of unbonded post-tensioned concrete (UPC) beams strengthened with carbon fibre reinforced polymer (CFRP) and these studies have not systematically examined the effect of CFRP sheets on the tendon strain as well as the strengthening efficiency. Moreover, current design guides for the FRP strengthening techniques have not provided any design procedure for UPC structures. This study, thus, investigates the influence of CFRP sheet ratio on the flexural behavior of CFRP-strengthened UPC T-beams and quantifies its effect upon tendon behavior in this kind of UPC beams. The testing program consisted of nine large-scale UPC T-beams strengthened by different layers of CFRP sheets with or without CFRP U-wrapped anchors. The experimental results have shown that the use of CFRP sheets and CFRP U-wrapped anchors significantly affected the tendon strain. The FRP reinforcement ratio governed the flexural capacity, the crack width, the mid-span displacement, and the ductility of the beams in which the strengthening efficiency reduces with the increased number of CFRP layers. The configuration of the CFRP U-wrapped anchors affected the strain of the CFRP sheets, the failure mode and thus the beam behavior. In addition, semi-empirical equations were proposed to estimate the actual strain of unbonded tendons in which the effect of the CFRP sheets and CFRP U-wrapped anchors have been taken into consideration. The proposed equations, which are simple to use, yield reliable predictions with a small variation.

INVESTIGATING THE INFLUENCE OF STEEL SURFACE ROUGHNESS AND MULTIPLE CFRP LAYERS ON THE BONDING BEHAVIOUR OF A SINGLE-STRAP JOINT

Three series of steel-plate single-strap joints bonded by Carbon-Fibre Reinforced Polymer (CFRP) sheets, which are used with an adhesive material, are investigated in this study. Experimental and finite element (FE) analyses using ABAQUS software are employed for this purpose. The main objective of this study is to provide further understanding of the bonding mechanism of strengthened single-strap joints under a pure-tension load. The experimental tests specifically investigated the effects of using multiple CFRP layers with normal/high steel surface roughness on the bonding failure mode, bond-load capacity, and effective bond length. Furthermore, the FE analysis was used to establish the shear stress-slip relationships of the suggested specimens. The results showed that the bond capacity (ultimate bond load) of single-strap joint did not increased much when the CFRP bonding length pass the effective bond length (limit). The bonding capacity (ultimate bond load) of single-strap joint increased only of about 14% when increase d the CFRP layers from two to four. Moreover, the behaviour of the shear stress-slip relationship was not significantly affected by increasing the number of CFRP layers.

Compressive behavior of post-heated circular CFST short columns externally strengthened with CFRP sheets

Due to the increasing usage of concrete-filled steel tube (CFST) members in structural engineering, there will be more chances of fire hazards on these structures in the near future. Externally bonding carbon fiber reinforced polymer (CFRP) composites has emerged as a popular method for repairing damaged steel-reinforced concrete members. However, limited research is available to evaluate the behavior of CFRP strengthened CFST members. This paper presents the results of an experimental investigation on the compressive behavior of the post-heated circular CFST short columns externally strengthened with CFRP sheets. A total of twenty-one specimens are tested to investigate the influence of temperature and the number of CFRP layers on the mechanical behavior of repaired specimens. The ultimate strength is obtained from monotone static tests. The extensometer technique based on gauge strain is used to measure strains of electric gauges glued to the external surface of specimens. The results indicate that the increase in the number of CFRP layers leads to a significant change in the mechanical properties of post-heated CFST columns. Furthermore, it is shown that increases in the number of CFRP layers remarkably enhance the ultimate strength and initial stiffness of specimens subjected to the same heat treatments, while deteriorate the ductility. Based on extensive experimental analysis, simplified formulae are proposed to estimate the compressive ultimate strength of all specimens tested, providing reasonably good correlation with the experimental results. Besides, the proposed formulae are compared with some existing empirical models, and validity of the proposed formulae is evaluated.

Design method for concrete columns strengthened with prestressed CFRP sheets

To improve upon the bearing capacities of short circular concrete columns strengthened with carbon fiber reinforced polymer (CFRP) sheets, this paper conducted experimental and numerical investigations to study the influencing factors on the compressive strength of CFRP-strengthened concrete columns, including the preloading of columns, the prestressing applied to CFRP sheets and the number of layers of CFRP sheets. The test columns consisted of an un-strengthened control column in addition to 10 preloaded and non-preloaded columns strengthened in compression with one or two CFRP layers at prestress levels of either 300MPa or 600MPa. These columns were tested under an axial load. The failure modes, stress changes, bearing capacities and load–longitudinal displacement behaviors of the tested specimens were recorded and discussed. Furthermore, the finite element models (FEM) of 10 strengthened columns were developed and validated by comparing the load–longitudinal displacement curves obtained through finite element analysis (FEA) with those retrieved experimentally. A parametric study was subsequently performed using the validated FEM to investigate the effects of preloading, prestressing and the number of CFRP layers. The parametric study demonstrated that the strain lag caused by preloading was negatively related to the compressive strength of the CFRP-strengthened columns. In addition, prestressing could reduce the negative effects of preloading and could improve on the utilization of CFRP tensile strength. Furthermore, the yield load of CFRP-strengthened concrete columns was improved correlative with an increment of the number of layers and the prestress levels of the CFRP sheets. Conversely, whereas the ultimate load only improved with an increment of the number of layers of CFRP sheets, it had no obvious relationship with the prestress levels of the CFRP sheets and was instead related to their ultimate tensile strength. Based on the test data, the bearing capacity formulas for preloaded and non-preloaded short circular concrete columns strengthened with prestressed CFRP sheets were proposed. Subsequently, the presented formulas were confirmed to be reasonably accurate by comparing the bearing capacities calculated numerically with those measured experimentally.

Fatigue behaviour of CFRP strengthened open-hole steel plates

Open-hole details have gained widely application in bolted connections subjected to fatigue loading. Although the use of carbon fibre reinforced polymer (CFRP) can be efficient in strengthening critically stressed tension area, its benefit in improving the fatigue life of open-hole details has not been extensively evaluated. This paper presents the results of an experimental and finite element analytical study on the fatigue behaviour of CFRP strengthened open-hole steel plates. The merits resulted from CFRP strengthening are investigated and compared with some others from referred open-hole fabrication methods and these with artificial initial cracks or notches as reported in the literature. Typical failure modes, local strain evolution and stiffness degradation behaviour of the test open-hole details are reported. The fatigue life results are evaluated and compared against the design curves of the AASHTO. The results show that the use of single and triple layered CFRP strengthening results in fatigue life enhancement of nearly 20% and 60% respectively in contrast to bare steel open-hole plates. Incorporating the stress ratios allowing for fibrous rupture and steel-adhesive interface failure, the fatigue life of CFRP strengthened specimens can be properly evaluated. Finally, the geometric and configuration parameters of CFRP on the stress concentration of the open-hole details are studied based on finite element modelling. It is demonstrated that the studied stress concentration can be reduced with the increase of the modulus of carbon fibre and the number of CFRP layers. The contribution of the effective width of CFRP to the decrease of stress concentration is also discussed.

Durability of CFRP-Wrapped Concrete Exposed to Hydrothermal Environment

In recent years, it has become increasingly common to strengthen concrete by wrapping carbon fibre reinforced polymer (CFRP) laminates to improve its compressive strength and ductility. However, studies on the durability of CFRP-confined concrete are relatively scarce, which could significantly hinder the application of CFRP in structural reinforcement. An experimental study was conducted to investigate the effect of exposure to natural hydrothermal environment on the confinement system of CFRP-wrapped concrete in this study. Three kinds of specimens were prepared and tested after natural exposure to the subtropical environment in South China, including 35 CFRP flat coupons, 35 epoxy adhesive flat coupons and 60 CFRP-wrapped concrete cylinders. The test parameters included the exposure duration (0, 6, 12, 18 and 30 months) for all specimens and the number of CFRP layers (0, 1, 2 and 3) for the CFRP-wrapped concrete specimens. Based on the experimental results, a compressive strength model for the CFRP-confined concrete exposed to hydrothermal environment was proposed. The results show that hydrothermal environment had a significant effect on the mechanical properties of the epoxy adhesive, but a relatively slight effect on that of the CFRP. Due to the deterioration of the CFRP, the compressive strength of CFRP-wrapped concrete gradually decreased with the increase of exposure duration. After a 30-month exposure, the compressive strength of CFRP-wrapped concrete has a decrease of approximately 10%.

Investigation on the Flexural Behavior of Corroded Concrete Beams Repaired by CFRP Sheet Under Different Corrosion Levels

The paper presents an investigation on the flexural behavior of corroded reinforced concrete (RC) beam strengthened with Carbon Fiber Reinforced Polymer (CFRP) sheets. Different levels of corrosion are considered as a new method for classifying the corrosion levels of the corroded RC members. Twenty RC beams with five corrosion levels were fabricated by accelerated corrosion equipment, and then the CFRP were used to strengthen these members, considering the effect of different CFRP layers and different strengthened schemes, finally the static flexural load test was carried out. The test results show that the bearing capacity of the CFRP strengthening corroded RC beams is effectively enhanced, the crack width is restrained, and the flexural stiffness is improved. The ultimate flexural capacity of the specimens strengthened by one layer of CFRP sheet raises by 30% to 50% than that of the unstrengthened ones, with the increase of the corrosion level, the enhancement decreased. With the increase of the number of CFRP layers, the bearing capacity increases, while the increasing ratio reduces with the increase in the number of layers. For specimens with obvious cracks, the effectiveness of the strengthening method by bonding CFRP after cutting or chiseling off concrete method (RM1 or RM2) is higher than the method of bonding CFRP directly (DM). It is suggested that the corroded RC members with Level A, B or C1 can be strengthened by DM, while the other level members should be strengthened by RM1 or RM2. The formulae in current design code are used to predict the flexural bearing capacity of the RC beam strengthened with CFRP and their results are compared with experimental ones. A revised formula that can give a better prediction for multi-layer CFRP strengthening members is also proposed.

Finite Element Modeling of Axially Loaded CFRP-Confined Rectangular Reinforced Concrete Columns

This paper investigates numerically the behaviour of rectangular RC columns strengthened with carbon fiber reinforced polymer (CFRP) composites under uniaxial loading. For this a reason, a parametric study is conducted and the effects of CFRP layers number, compressive strength of unconfined concrete, and fiber orientation on the behaviour of such columns have been studied. The number of CFRP layers has been changed from one to five layers while the fibers are oriented transversely. Compressive strength of unconfined concrete has been increased from 26 MPa to 45 MPa. In addition, three different fiber orientations are considered. The results show that an increase in the number of CFRP layers would enhance the ultimate strength of specimens. Although increasing the number of layers would not increase the ultimate strength of specimens exponentially, but the rate of strength gain would also decrease. Moreover, it is shown that lateral strains increase as the layer number increases. The effect of unconfined concrete strength on the ultimate strength is less for low strength concrete than high strength concrete. Evaluating the effect of fiber orientation shows that the maximum ultimate strength is obtained from transverse orientation and as the angle of orientation increases, the ultimate strength decreases.

Enhancing the structural performance of steel channel sections by CFRP strengthening

The aim of this test study was to achieve an in-situ strengthening technique to retrofit the deteriorated tall steel structures using carbon fiber-reinforced polymer (CFRP). A total of 20 structural steel channel sections with various retrofitting configurations were tested to arrive at an optimal strengthening technique to improve the buckling mode of failure of structural steel channel section under flexural loading. An approach of transforming the open cross section to closed cross section was used to increase the torsional resistance of channel sections. The various parameters such as type of CFRP fabric, wrapping configuration (skin and closed wrap) and number of CFRP layers were also included in this study. The results indicate that the closed wrapped configuration with an additional (final) layer of bi-directional CFRP fabric provides the higher strength and stiffness compared to control and skin wrapped specimen due to the confinement of hoop directional fibers. This study also proves that the lateral torsional buckling (LTB) mode of failure of open cross section specimens can be improved by appropriate CFRP strengthening techniques.

Durability of CFRP strengthened circular hollow steel members under cold weather: Experimental and numerical investigation

The durability of carbon fibre reinforced polymer (CFRP) strengthened steel circular hollow section (CHS) member under prolonged cold weather has not been explored sufficiently. Furthermore, various parameters that may affect the durability of such members have not been revealed yet. This paper presents an analytical and experimental study with a finite element (FE) approach to study the effects of prolonged cold weather (+3°C) exposure on CFRP strengthened CHS member subjected bending. It was found that a prolonged exposure about 12month cold weather immersion adversely affected the durability of strengthened beams by reducing strength and stiffness. Durability design factor has been recommended. The proposed analytical models and FE models were found in good agreement with the experimental results. The results of parametric studies showed that the durability design factors varied from section to section, CFRP with higher tensile modulus and higher number of CFRP layers performed better in cold weather.