Carbon Fiber-reinforced Polymer Retrofit Research Articles

Experimental and Analytical Study on Postfire Reinforced Concrete Beams Retrofitted with CFRP in Flexure and Shear

In this study, experiments were performed on carbon fiber reinforced polymer (CFRP) retrofitted postfire reinforced concrete (RC) beams, followed by theoretical analyzes. Experiments were conducted on eleven RC beams, which were exposed to different fire durations and retrofitted with CFRP in flexure and shear. The experimental results indicated that fire shifted the flexure failure to the flexure-shear failure of postfire RC beams. CFRP retrofitted postfire RC beams experienced progressive peeling-off failure. FRP retrofitting significantly increased the yield deflection by 58.2−97.3% but decreased the ultimate deflection by 43.0−55.5% compared with that of the control beam. Consequently, the ductility was reduced by 69.7−74.7%, categorized as low ductility. CFRP retrofitting successfully increased the strengths of 30-min postfire beams by up to 23.1% higher than those of the control beam. Fire significantly decreased the stiffness of postfire beams by 46.4−49.2% compared with that of the control beam, whereas CFRP retrofitting did not fully recover the stiffness of postfire beams. Finally, a simple model of the moment capacity of postfire beams without/with CFRP retrofits was developed based on the practicability of limited data feasibly obtained from real fires. The proposed model, with its simplicity, practicability, and reasonable accuracy, can be a useful tool for structural engineers in the FRP retrofitting of postfire RC structures. Doi: 10.28991/CEJ-2023-09-07-05 Full Text: PDF

Performance of Postfire Reinforced Concrete Beams Retrofitted with External Bonded and Near-Surface Mounted CFRP: Experiments and Analyses

Fire imposes negative effects on reinforced concrete (RC) members, shifting postfire structures onto dangerous situations. This study aims at carbon fiber-reinforced polymer (CFRP) retrofit of postfire RC beams to recover their mechanical properties, providing technical information for structural engineers in practice. Toward this aim, experiments were performed for 16 RC beams, which were divided into four groups exposed to 30, 45, 60, and 75 min of ISO 834 fire. After exposure to fire, these beams were strengthened by external bonded (EB) and near-surface mounted (NSM) techniques using a similar amount of CFRP for comparison. These specimens were then loaded until failure. The results are compared with one another and compared with control specimens. It is found that both EB and NSM CFRP retrofitted beams failed in the form of peeling off of concrete, whereas no rupture of CFRP was observed. CFRP retrofit significantly changes the behavior of postfire RC beams, being brittle or exhibiting low ductility. The effectiveness of EB and NSM CFRP techniques is similar and it decreases with the increase in fire duration. CFRP retrofit increases the yield load-carrying capacity by 27.5%−40.9% for 30−60‐min postfire RC beams and recovers the yield load-carrying capacity for 75-min postfire RC beams. CFRP retrofit significantly increases the ultimate load-carrying capacity, whereas its increasing levels depend not only on the CFRP retrofit but also on the fire duration. Both CFRP retrofitting techniques successfully recovered the yield stiffness of postfire RC beams. The moment capacity of CFRP retrofitted postfire RC beams was theoretically analyzed, and a prediction model was proposed. The proposed model is simple, but it accurately predicts the moment capacity of retrofitted postfire beams; thus, it can be useful for structural engineers in practice.

Numerical Analysis on Dynamic Response of CFRP-Wrapped RC Columns under Lateral Impact Loading

This paper presents a numerical study examining the dynamic response and resistance mechanism of reinforced concrete (RC) columns strengthened with or without carbon-fiber-reinforced polymer (CFRP) wraps under lateral impact loading by using the software LS-DYNA. First, the information of eight column models was briefly introduced as part of the laboratory experimental program from the literature. Secondly, finite element (FE) models were established in terms of the geometries of impact tests. Then, a detailed comparison between numerical results and experimental results was made, and FE models showed a relatively high simulation accuracy. Subsequently, a series of parametric analyses were carried out with a focus on the effects of axial compression ratio, the boundary condition at the column top, the layer number of CFRP wraps, and the impact velocity and impact height on the dynamic responses of plain and strengthened columns. The results demonstrated that the CFRP retrofit mechanism was not activated during the initial Stage-I when the impact force rapidly increased to the first peak and then decreased to zero. CFRP strengthening came into play in the second stage, Stage-II, and affected the response of the shear force and moment along the column height, as well as had a great influence on the control of shear damage. The dynamic response of RC columns was more sensitive to the impact velocity than to other parameters, regardless of whether CFRP wrapping was applied. The axial compression ratio would have a different influence on the column failure mode if the impact velocity was varied. The variation in impact height and boundary condition at the column top had little influence on the damage mode of strengthened columns.

Mitigation effects of air-backed RC slabs retrofitted with CFRP subjected to underwater contact explosions

Reinforced concrete (RC) structures are commonly used in ocean engineering, and can be subjected to various blast scenarios. To evaluate the strengthening effects of the carbon fiber reinforced polymer (CFRP) on the blast resistance of air-backed RC slabs to underwater contact explosions, a numerical approach based on the coupled Lagrange-Euler (CLE) method is proposed. First, two blast tests with available experimental results are utilized to verify the numerical method. Then, numerical investigations are conducted on air-backed slabs retrofitted with CFRP subjected to underwater contact explosions using the verified analytical method. The retrofitting effectiveness of CFRP on air-backed RC slabs against underwater contact explosions are analyzed contrastively. In addition, the effects of various factors, namely, the retrofitted scheme, retrofitted thickness, retrofitted material, and charge weight, on the blast resistance of FRP retrofitted air-backed slabs are investigated. The results demonstrate that the CFRP retrofitting can significantly improve the blast resistance of air-backed slabs against underwater contact explosions by reducing deformation and catching flying debris.

Reinforced concrete beams retrofitted with UHPC or CFRP

This study explores the reinforced concrete beams retrofitted with ultra-high-performance concrete (UHPC) or carbon fiber-reinforced polymer (CFRP). The main test items and methodologies are as follows:(i) Use basic mechanical tests to evaluate the feasibility of UHPC as a strengthening material.(ii) Probe into the retrofit effect of making small-sized reinforced concrete beams and using UHPC or CFRP.The results of the study show that Poisson’s ratio of UHPC to ordinary concrete is between 0.19 and 0.21, which is suitable for cement mortar or concrete repair and has good compatibility. Using UHPC or CFRP to retrofit reinforced concrete beam members, the ultimate load increases by 35% and 56.6%, respectively. However, the displacement of CFRP is only increased by 4 mm, while the displacement can be increased by 26.8 mm after UHPC (2% steel fiber) retrofit. The unit price of a reinforced concrete beam strengthened with CFRP is 2–4 times higher than that of a UHPC retrofit. Therefore, UHPC has better toughness, displacement, and fracture energy than CFRP retrofit. The UHPC is suitable for the strengthening and economic benefit of reinforced concrete structures.

Damaged Tubular T-joints Retrofitted with Carbon Fiber Reinforced Polymer Subjected to Monotonic and Cyclic Loadings

Owing to seismic, wave, or wind actions, tubular joints used in onshore and offshore civil infrastructures may require retrofitting to withstand both static and cyclic loadings. In the present study, bearing capacities, failure modes, and ductilities of damaged tubular T-joints retrofitted with carbon fiber reinforced polymer (CFRP) were investigated. Two-step loading was applied: first, a pre-determined load was applied to induce chord deformation; this was followed by unloading and CFRP retrofitting. Subsequently, the specimens were reloaded to failure. Results showed that the retrofitted specimens exhibited a maximum capacity similar to and a plastic deformation slightly larger than that of the bare specimen when these subjected to static loading; the retrofitted specimens, which were not severely damaged, exhibited a higher compressive ductility index than that of the bare specimen when these subjected to cyclic loading.

Numerical modeling of deteriorated concrete bridge bent caps with externally bonded CFRP retrofit

Deterioration of old concrete bridge bent caps due to spalling and rebar corrosion is a common occurrence. Carbon fiber-reinforced polymer (CFRP) laminate external retrofit is a viable option for regaining such bent cap capacity and serviceability. In this study, a three-dimensional numerical modeling of significantly deteriorated bent caps from a 78-year old cast-in-place bridge in Forney, Texas, was performed. The purpose was to determine the effectiveness of the repair and strengthening process. Numerical models of the bent caps in the original undamaged, damaged, epoxy mortar repaired and CFRP strengthened states were created and calibrated using strain data obtained from full-scale live load testing. The calibrated model strains showed good agreement with those from the load testing. With short span lengths and deep sections, the model strains were small and the effect of the repair/strengthening at modest levels. The neutral axes of the bent cap sections, found from numerical modeling, moved slightly downward after the repair/strengthening process. The model with no section loss showed the highest tensile capacity, and subsequent levels of concrete spalling resulted in progressively smaller capacity. After the first crack initiation, inelastic behavior set in along with crack opening, which explains the increased rate of strains. Truck live loads were adequately represented in the developed numerical models.

Fatigue repair of underwater navigation steel structures using Carbon Fiber Reinforced Polymer (CFRP)

The nation’s Steel Hydraulic Structures (SHS) have significant deterioration caused by the effects of complex phenomena including corrosion and fatigue cracking caused by mixed mode loading. Current methods of repair of SHS, adopted primarily from the bridge engineering industry, have proven ineffective because they were developed for Mode 1 loading (tension) only. Various studies have been carried out to assess the use of CFRP for the rehabilitation of the aging and deteriorated infrastructure systems in the United States, however no research has been conducted on submerged steel structures. This paper presents the evaluation and implementation of an innovative retrofit method of CFRP to repair and strengthen the strut arms of a tainter valve. The evaluation of the retrofit method required a Fitness-for-Service fatigue assessment followed by a Finite Element Analysis (FEA) of the tainter valve with and without the retrofit method. Finally, the article presents the installation procedures used to install the CFRP retrofit and describes the advantages of the innovative repair techniques resulting in two to four folds increase of fatigue life.

Flexural Assessment of Blast-Damaged RC Beams Retrofitted with CFRP Sheet and Steel Fiber

This study presents the effects of blast-induced local damages on the flexural strength of blast-damaged and repaired specimens. In the experimental program, blast-damaged specimens were repaired with steel fiber reinforced cementitious composite (SFRCC) as well as carbon fiber-reinforced polymer (CFRP) sheets and tested for flexural strength measurements. The test parameters included shear reinforcement (amount and spacing), steel fiber content (0, 1.0 vol%), and retrofitting with CFRP sheets. The test results indicated that the use of higher amounts of stirrups demonstrated insignificant benefits in preventing local damages. However, it was shown that the use of small-diameter steel bars for stirrups with small spacing could decrease the local damages more effectively compared to the large-diameter steel reinforcement. For the residual strength of the damaged specimens, the specimens using more stirrups could resist over 60% of their original flexural strength. CFRP retrofitting showed insignificant enhancement in ductility of intact, damaged, and repaired specimens. However, it distributed the blast load and protected debris scattering. The addition of steel fibers results in increased ductility and enhanced blast resistance against local damages. All specimens, excluding control specimen, that repaired with SFRCC showed higher flexural strength to their original strength. Therefore, it can be concluded that replacing damaged concrete cover with SFRCC is adequate for repairing the blast-damaged RC members.

Simulation of the Behavior of Corrosion Damaged Reinforced Concrete Beams with/without CFRP Retrofit

Harsh environmental conditions along with aggressive chemical agents are known as one of the main reasons behind damages observed in reinforced concrete members. Corrosion of reinforcement worldwide is one of the leading causes of damages occurred in reinforced concrete over the lifespan. There are many critical energy and transportation infrastructures located on coastal regions exposed to high humidity and chloride content where they are highly prone to reinforcement corrosion. This calls for retrofit methods, which safeguard not only the strength but also the durability of corrosion deteriorated reinforced concrete structures. Carbon fiber polymers considering their mechanical and chemical properties are recognized as one of the main retrofit techniques. In this study, the influence of different levels of corrosion on the structural behavior of reinforced concrete beams is studied. ABAQUS software package is employed to simulate the nonlinear behavior of reinforced concrete beams with tensile reinforcements and stir-ups corrosion degrees of 20% and 40%. The structural behavior of original damaged specimen as well as the same specimen strengthen with carbon fiber reinforced polymer (CFRP) is studied. The purpose of the retrofit is compensate for the loss of shear and flexural capacity of the member due to corrosion. Different variants for the arrangement of CFRP strips are studied and compared. The result of the current research further uncaps the efficiency of fiber polymers to secure strength and durability of corrosion damaged reinforced concrete members.

Modeling shear behavior of reinforced concrete beams strengthened with externally bonded CFRP sheets

Extensive research work has been performed on shear strengthening of reinforced concrete (RC) beams retrofitted with externally bonded carbon fiber reinforced polymer (CFRP) in form of strips. However, most of this research work is experimental and very scarce studies are available on numerical modelling of such beams due to truly challenging nature of modelling concrete shear cracking and interfacial interaction between components of such beams. This paper presents an appropriate model for RC beam and to simulate its cracking without numerical computational difficulties, convergence and solution degradation problems. Modelling of steel and CFRP and their interfacial interaction with concrete are discussed. Finally, commercially available non-linear finite element software ABAQUS is used to validate the developed finite element model with key tests performed on full scale T-beams with and without CFRP retrofitting, taken from previous extensive research work. The modelling parameters for bonding behavior of CFRP with special anchors are also proposed. The results presented in this research work illustrate that appropriate modelling of bond behavior of all the three types of interfaces is important in order to correctly simulate the shear behavior of RC beams strengthened with CFRP.

CFRP Strip Cable Retrofit of RC Frame for Collapse Resistance

AbstractA half-scale, three-story, two-bay, and three-span reinforced concrete model frame retrofitted with carbon fiber–reinforced polymer (CFRP) strip cables was tested under sudden removal of two side middle columns in the long direction on the ground floor to study dynamic responses, resisting mechanism, and CFRP retrofit effects. The CFRP strip cables were anchored to concrete slabs by specially designed anchorage. The experimental results are compared with previous tests of the same frame without CFRP retrofit. The comparison shows that the CFRP strip cables were effective in preventing the total collapse of the frame after the loss of columns. With additional CFRP strip cables, the frame structure sustained the applied gravity load as moment-resisting frames without a transition to catenary action. To further demonstrate the effects, the CFRP strip cables were released, causing the frame structure to deflect downward and eventually collapse. A simplified method was proposed to calculate the average...

In-Plane Seismic Strengthening of Nonductile Reinforced Concrete Shear Walls Using Externally Bonded CFRP Sheets

AbstractThis study evaluated the effectiveness of using externally bonded carbon fiber–reinforced polymer (CFRP) sheets to enhance the seismic performance of nonductile reinforced concrete shear wall specimens representative of low aspect ratio walls. The specimens were designed to replicate structural deficiencies typically found in older concrete structures constructed during the 1960s and 1970s. The nonductile reinforcement details included insufficient shear reinforcement, lack of boundary elements, and low concrete compressive strength. The specimens were repaired or strengthened using vertical and horizontal CFRP sheets and then cyclically tested to simulate seismic loading. A tube anchor system was used to transfer the tensile force carried by the CFRP sheet to the foundation of the specimens. Experimental results showed that the CFRP retrofitting system was able to restore the original strength and initial stiffness of severely damaged specimens and significantly increase the flexural strength, du...

Impact response characterization of shear CFRP strengthened RC beams by Fourier and wavelet methods: tests and analyses

Abstract The current experimental study investigates impact response features of reinforced concrete (RC) beams with dominant shear failure mode. Six built nonstrengthened and strengthened RC beams with a shear span-to-depth ratio of 2.5 were subject to repeated low-velocity impacts. Projectile energy is equal to the amount required for collapse. Retrofit consists of carbon fiber-reinforced polymer (CFRP) sheets attached to the exterior surface in vertical-unidirectional and cross-ply orientation. Laser linear variable differential transformer (LVDT) and micro-electro-mechanical systems (MEMS) accelerometers record the resultant deflection and acceleration to inspect the relationship between retrofit and damage. Fast Fourier transform (FFT) and continuous wavelet transform (CWT) extract the response content specimens to monitor the frequency change for each case. According to extracted results, CFRP skin provides notable stiffness on the upward shift of active frequency range and peak spectrum amplitude maintained for impact repeats. The active domain in FFT spectrum is sensitive to failure type (shear or flexural). CWT analysis ridges show a clear difference between failure type samples. CFRP retrofit controls cracking and deflection, which is shown as increased acceleration in records, especially for the cross-ply wrap. Effective stiffness recovers during initial impacts and then starts degrading because of the rupture of horizontal fibers in cross-ply retrofit, but the stiffness is stable for unidirectional retrofit during all impacts.

Effects of CFRP retrofit on impact response of shear-deficient scaled reinforced concrete beams

An experimental procedure is carried out to investigate response features of semi-deep reinforced concrete (RC) beams with dominant static shear failure subjected to low-velocity impact dynamic load both in intact and retrofitted cases. Built specimens have scaled geometry and material properties preserving physical similarity with full-scale members and are strengthened using externally bonded carbon fiber reinforced polymer (CFRP) sheets. Conducted tests gather reliable and robust data revealing notable stiffness and strength recovery within skin attachment. Desired performance criterion is detected here as concrete flexural cracking along span only when brittle shear collapse is prevented. Peak absorbed impulse capacity is attained for unidirectional carbon fiber retrofit. Stiffness is recovered initially during first impacts and then starts degrading due to rupture of horizontal fibers in cross-ply retrofit, but provided stiffness is stable for unidirectional retrofit during all impacts.

Hysteretic Behavior of Tubular Steel Braces Having Carbon Fiber Reinforced Polymer Reinforcement Around End Net Sections

This study presents an experimental investigation into the seismic retrofit of tubular steel braces using carbon fiber reinforced polymer (CFRP) members. CFRP retrofitting of net sections for compact tubes are proposed for delaying potential local net section failure. A total of almost full-scale three (TB-1, TB-2, and TB-3) compact steel tubular specimens were designed per AISC specifications, constructed, and cyclically tested to fracture. Retrofitted braces, when compared to the reference specimen, developed fuller hysteretic curves. Increase in cumulative hysteretic energy dissipation and the elongation in fracture life in the specimen retrofitted with CFRP plates and CFRP sheet wraps at net sections are observed during testing. This resulted in a maximum of 82.5% more dissipated energy for compact tube specimens. Also, this retrofit provided a longer experimental fracture life (maximum 59% more). Due to fracture initiation during the last cycles, significant reductions in strength and stiffness have been obtained. No significant change (maximum 10%) in the brace stiffness was observed, which could be desirable in seismic retrofit applications. Pushover analysis per FEMA 356 for the bare specimen shows that FEMA does not represent actual brace behavior in the compression side although pushover and experimental results are in good agreement in the tension side.

Seismic Response and CFRP Retrofit of Poorly Detailed Shear Walls

AbstractThe reversed cyclic loading behavior of full-scale shear wall specimens was investigated before and after retrofit. The wall specimens were designed and detailed to simulate nonductile reinforced concrete construction of the 1960s, having lap splices of the longitudinal reinforcement in the potential plastic hinge region, and having inadequate confinement of the boundary regions. The lap splices in these walls failed in a brittle manner before yielding occurred in the main flexural reinforcement. The use and effectiveness of carbon fiber-reinforced polymer (CFRP) wrap for improving the lap splice behavior and the shear strength of the walls were investigated. The walls were tested under reversed cyclic loading with loading applied near the tip of the walls. The retrofit technique improved the displacement ductility, energy dissipation, and prevented premature failure of the lap splices. The target ductility level of 2.0 was achieved with this CFRP retrofit.

Comparison of CFRP and Alternative Seismic Retrofitting Techniques for Bare and Infilled RC Frames

The opportunities provided by the use of modern repair schemes for the seismic retrofit of existing RC structures were assessed on a comparative experimental study of carbon fiber-reinforced polymer (CFRP) and more-conventional seismic retrofitting techniques for the repair of reinforced concrete members and masonry walls of bare and infilled RC frames, respectively, damaged because of cyclic loading. Four 1-story, one-bay, one-third-scale frame specimens are tested under cyclic horizontal loading up to a drift level of 4%—two bare frames with spirals or stirrups as shear reinforcement, respectively, and two infilled frames with weak infills and spirals or stirrups as shear reinforcement, respectively. The applied repair techniques are mainly based on the use of thin epoxy resin infused under pressure into the crack system of the damaged RC joint bodies or on the additional use of CFRP plates to the surfaces of the damaged structural RC members as external reinforcement and the use of a polymer modified cement mortar or two-sided diagonal CFRP fabrics for the damaged infill masonry walls. After repair, specimens were retested in the same way. Conclusions concerning the comparison of the effectiveness between conventional and CFRP seismic retrofitting applied techniques on the basis of maximum cycles load, loading stiffness, and hysteretic energy absorption capabilities of the tested specimens are drawn.

CFRP Rehabilitation of Concrete Frame Joints with Inadequate Shear and Anchorage Details

The research presented in this study involves full-scale experimental evaluation of carbon fiber-reinforced polymer (CFRP) rehabilitation for existing beam-column joints designed for gravity load with common pre-1970s deficient reinforcement details when subjected to cyclic loading. Numerous studies have demonstrated effectiveness of externally bonded fiber-reinforced polymer (FRP) materials for retrofitting the deteriorating RC structures. Although these materials are widely used in bridges, their applications in buildings have been somewhat limited. In particular, the experimental investigations on external FRP retrofit of deficient beam-column joints have not thoroughly been investigated and they are mainly on scaled-down specimens. The failure of these subassemblies, which possess lack of shear reinforcement within the joint core and shortly embedded positive beam reinforcement, would possibly result in catastrophic collapse of reinforced concrete frame structure during an earthquake event. Recognizing the urgent need to upgrade these structural subassemblies, the current investigation uses CFRP retrofit techniques to enhance the performance of such deficient joints. Experimental variables studied entail the developed CFRP retrofit configurations, and magnitude of the applied column axial load. Comparative analysis of the lateral loads versus drift hysteresis loops, stiffness degradation, and total dissipated energy curves of three as-built and three corresponding CFRP-retrofitted RC joints revealed that significant improvement in the shear capacity of the upgraded joints occurred. More importantly, the slippage of short embedded beam positive reinforcement into the joint was substantially controlled due to the developed CFRP retrofit. The results demonstrate the effectiveness of CFRP retrofit configurations in enhancing the structural performance of actual size connections.

Strut-and-Tie Method for CFRP Strengthened Deep RC Members

In recent years, a great deal of research on the carbon fiber-reinforced polymer (CFRP) composite retrofitted systems has been done. The investigation of CFRP retrofit systems, however, has focused on flexural and shear strengthening for slender concrete members. Very limited work has been done on CFRP strengthened deep concrete structures. For analysis of CFRP strengthened deep reinforced concrete (RC) members, the strut-and-tie method (STM) is also a powerful analysis tool since a bonded CFRP element acts as an additional tension tie. In this paper, a practical analysis and design process for CFRP strengthened deep RC members using the STM is presented. In addition, seven effective factor models accounting for reduction of strength in cracked concrete were also investigated. A total of 17 experimental deep beam test results were compared with the proposed STM approach results. It has been shown that the proposed STM approach with an effective factor model depending on the strut angle provides the best agreement with the test results.