Structural performance of corroded RC beams repaired with CFRP sheets

Corrosion of reinforcement is a serious problem and is the main cause of concrete structures deterioration costing millions of dollars even though the majority of such structures are at the early age of their expected service life. This paper presents the experimental results of damaged/repaired reinforced concrete beams. The experimental program consisted of reinforced concrete rectangular beam specimens exposed to accelerated corrosion. The corrosion rate was varied between 5% and 15% which represents loss in cross-sectional area of the steel reinforcement in the tension side. Corroded beams were repaired by bonding carbon fiber reinforced polymer (CFRP) sheets to the tension side to restore the strength loss due to corrosion. Different strengthening schemes were used to repair the damaged beams. Test results showed detrimental effect of corrosion on strength as well as the bond between steel reinforcement and the surrounding concrete. Corroded beams showed lower stiffness and strength than control (uncorroded) beams. However, strength of damaged beams due to corrosion was restored to the undamaged state when strengthened with CFRP sheets. On the other hand, the ultimate deflection of strengthened beams was less than ultimate deflection of un-strengthened beams.     

Repair of shear-deficient normal weight concrete beams damaged by thermal shock using advanced composite materials

The use of advanced composite materials such as Fiber Reinforced Polymers (FRPs) in repairing and strengthening reinforced concrete structural elements has been increased in the last two decades. Repairing and strengthening damage structures is a relatively new technique. The aims of this study was to investigate the efficiency and effectiveness of using Carbon Fiber Reinforced Polymer (CFRP) to regain shear capacity of shear-deficient normal weight high strength RC beams after being damaged by thermal shock. Sixteen high strength normal weight RC beams (100 × 150 × 1400 mm) were cast, heated at 500 °C for 2 h and then cooled rapidly by immersion in water, repaired, and then tested under four-point loading until failure. The composite materials used are carbon fiber reinforced polymer plates and sheets. The experimental results indicated that upon heating then cooling rapidly, the reinforced concrete (RC) beams exhibited extensive map cracking without spalling. Load carrying capacity and stiffness of RC beams decreased about 68% and 64%, respectively, as compared with reference beams. Repairing the thermal damaged RC beams allowed recovering the original load carrying without achieving the original stiffness. Repaired beams with CFRP plates with 90° and 45° regained from 90% to 99% of the original load capacity with a corresponding stiffness from 79% to 95%, whereas those repaired with CFRP sheet on the web sides and a combination of CFRP plates and sheet regained from 102% to 107% of the original load capacity with a corresponding stiffness from 81% to 93%, respectively. Finally, finite element analysis model is developed and validated with the experimental results. The finite element analysis showed good agreement as compared with the experimental results in terms of load–deflection and load–CFRP strain curves.     

模拟不中断交通状况下粘贴碳纤维布加固钢筋混凝土梁试验研究

为了调查动荷载作用下碳纤维布与钢筋混凝土梁的粘贴性能及加固效果,进行了5根模拟交通荷载(等幅正弦波形动荷载)作用下粘贴碳纤维布加固钢筋混凝土梁和1根保持荷载下粘贴碳纤维布加固钢筋混凝土梁以及2根对比梁的试验研究。试验中考虑了混凝土等级、配筋率、有无锚固条、粘贴长度、荷载幅值5个变化参数。试验结果表明,在模拟交通荷载的作用下,碳纤维布加固的钢筋混凝土梁粘贴性能满足要求,粘贴效果良好。在模拟交通荷载后的静载作用下,试验梁的抗弯承载力提高较多,加固效果明显,进一步验证了粘贴效果。     

碳纤维布加固已承受荷载的钢筋混凝土梁抗弯性能试验研究及抗弯承载力计算

进行了6根碳纤维布加固已承受荷载的钢筋混凝土梁和2根对比混凝土梁的抗弯性能试验研究,分析了碳纤维布加固已承受荷载的钢筋混凝土梁的破坏机理,研究了荷载历史对加固梁极限荷载的影响。试验结果表明,粘贴碳纤维布可以有效地提高加固梁的抗弯承载能力。无论荷载历史如何,只要梁承受的初始荷载相同,梁破坏时的极限荷载基本相同。梁端锚固对加固梁的极限荷载影响不明显。根据不同的破坏模式,提出了碳纤维布加固已承受荷载的钢筋混凝土梁的承载力计算方法,给出了工程实用计算公式。     

Mechanical behaviour of corroded RC beams strengthened by NSM CFRP rods

Corrosion of steel in reinforced concrete leads to several major defects. Firstly, a reduction in the cross-sectional area of the reinforcement and in its ductility results in premature bar failure. Secondly, the expansion of the corrosion products causes concrete cracking and steel–concrete bond deterioration and also affects the bending stiffness of the reinforced concrete members, causing a reduction in the overall load-bearing capacity of the reinforced concrete beams. This paper investigates the validity of a repair technique using Near Surface Mounted (NSM) carbon-fibre-reinforced polymer (CFRP) rods to restore the mechanical performance of corrosion-damaged RC beams. In the NSM technique, the CFRP rods are placed inside pre-cut grooves and are bonded to the concrete with epoxy adhesive.Experimental results were obtained on two beams: a corroded beam that had been exposed to natural corrosion for 25 years and a control beam, (both are 3 m long) repaired in bending only. Each beam was repaired with one 6-mm-diameter NSM CFRP rod. The beams were tested in a three-point bending test up to failure. Overall stiffness and crack maps were studied before and after the repair. Ultimate capacity, ductility and failure mode were also reviewed. Finally some comparisons were made between repaired and non-repaired beams in order to assess the effectiveness of the NSM technique. The experimental results showed that the NSM technique improved the overall characteristics (ultimate load capacity and stiffness) of the control and corroded beams and allowed sufficient ductility to be restored to the repaired corroded elements, thus restoring the safety margin, despite the non-classical mode of failure that occurred in the corroded beam, with the separation of the concrete cover due to corrosion products.     

Behavior of full-scale reinforced concrete beams retrofitted for shear and flexural with FRP laminates

Four full-scale reinforced concrete beams were replicated from an existing bridge. The original beams were substantially deficient in shear strength, particularly for projected increase of traffic loads. Of the four replicate beams, one served as a control and the remaining three were implemented with varying configurations of carbon fiber reinforced polymers (CFRP) and glass FRP (GFRP) composites to simulate the retrofit of the existing structure. CFRP unidirectional sheets were placed to increase flexural capacity and GFRP unidirectional sheets were utilized to mitigate shear failure. Four-point bending tests were conducted. Load, deflection and strain data were collected. Fiber optic gauges were utilized in high flexural and shear regions and conventional resistive gauges were placed in eighteen locations to provide behavioral understanding of the composite material strengthening. Fiber optic readings were compared to conventional gauges.Results from this study show that the use of fiber reinforced polymers (FRP) composites for structural strengthening provides significant static capacity increases approximately 150% when compared to unstrengthened sections. Load at first crack and post cracking stiffness of all beams was increased primarily due to flexural CFRP. Test results suggest that beams retrofit with both the designed GFRP and CFRP should well exceed the static demand of 658 kN m sustaining up to 868 kN m applied moment. The addition of GFRP alone for shear was sufficient to offset the lack of steel stirrups and allow conventional RC beam failure by yielding of the tension steel. This allowed ultimate deflections to be 200% higher than the pre-existing shear deficient beam. If bridge beams were retrofit with only the designed CFRP failure would still result from diagonal tension cracks, albeit at a 31% greater load. Beams retrofit with only the designed shear GFRP would fail in flexure at the mid-span at an equivalent 31% gain over the control specimen, failing mechanism in this case being yielding of the tension steel. Successful monitoring of strain using fiber optics was achieved. However, careful planning tempered by engineering judgement is necessary as the location and gauge length of the fiber optic gauge will determine the usefulness of the collected data.     

CFRP布加固方案对RC梁剪切性能及尺寸效应影响分析EI北大核心CSCD

采用同时考虑混凝土材料非均质性、钢筋与混凝土之间的相互作用以及CFRP布与混凝土之间的相互作用影响的三维细观数值模拟方法,建立了CFRP布加固RC梁剪切破坏力学分析模型。在验证了细观数值方法合理性的基础上,设计并建立了12根CFRP布加固RC梁细观模型,探究相同CFRP配纤率(用布量)前提下,不同CFRP布加固方案对单调荷载作用下RC梁的剪切性能及尺寸效应的影响。结果表明:CFRP布应变分布与裂缝位置紧密相关,越靠近裂缝位置的CFRP布应变越大,提供的抗剪贡献越多;在CFRP配纤率一致的前提下,CFRP布宽度大厚度小的加固方案优于CFRP布厚度大宽度小的加固方案;CFRP布U型加固RC梁剪切强度存在尺寸效应现象,但相同CFRP配纤率下,不同CFRP布加固方案对名义抗剪强度尺寸效应的影响较小,可以忽略。     

Experimental and analytical investigation of reinforced high strength concrete continuous beams strengthened with fiber reinforced polymer

Carbon and glass fiber reinforced polymer (CFRP and GFRP) are two materials suitable for strengthening the reinforced concrete (RC) beams. Although many in situ RC beams are of continuous constructions, there has been very limited research on the behavior of such beams with externally applied FRP laminate. In addition, most design guidelines were developed for simply supported beams with external FRP laminates. This paper presents an experimental program conducted to study the flexural behavior and redistribution in moment of reinforced high strength concrete (RHSC) continuous beams strengthened with CFRP and GFRP sheets. Test results showed that with increasing the number of CFRP sheet layers, the ultimate strength increases, while the ductility, moment redistribution, and ultimate strain of CFRP sheet decrease. Also, by using the GFRP sheet in strengthening the continuous beam reduced loss in ductility and moment redistribution but it did not significantly increase ultimate strength of beam. The moment enhancement ratio of the strengthened continuous beams was significantly higher than the ultimate load enhancement ratio in the same beam. An analytical model for moment–curvature and load capacity are developed and used for the tested continuous beams in current and other similar studies. The stress–strain curves of concrete, steel and FRP were considered as integrity model. Stress–strain model of concrete is extended from Oztekin et al.’s model by modifying the ultimate strain. Also, new parameters of equivalent stress block are obtained for flexural calculation of RHSC beams. Good agreement between experiment and prediction values is achieved.     

Structural behaviour of RC beams with external flexural and flexural–shear strengthening by FRP sheets

This paper presents experimental research on reinforced concrete (RC) beams with external flexural and flexural–shear strengthening by fibre reinforced polymer (FRP) sheets consisting of carbon FRP (CFRP) and glass FRP (GFRP). The work carried out has examined both the flexural and flexural–shear strengthening capacities of retrofitted RC beams and has indicated how different strengthening arrangements of CFRP and GFRP sheets affect behaviour of the RC beams strengthened. Research output shows that the flexural–shear strengthening arrangement is much more effective than the flexural one in enhancing the stiffness, the ultimate strength and hardening behaviour of the RC beam. In addition theoretical calculations are developed to estimate the bending and shear capacities of the beams tested, which are compared with the corresponding experimental results.     

Long-term behavior of existing low-strength reinforced concrete beams strengthened with carbon fiber composite sheets

This paper presents the results of an experimental study of the short- and long-term behavior of low-strength reinforced concrete (RC) beams strengthened with carbon fiber reinforced polymer (CFRP). A numerical analysis model was developed and verified for the calculation of the long-term deflection and maximum allowable long-term load of such RC beams. A parametric study was also conducted and it was found that the maximum allowable long-term load of a CFRP-strengthened beam was dominated by the deflection of RC beam when the cubic compressive strength of concrete was less than a certain value. For concrete of higher strength, the maximum allowable long-term load was dominated by the stress levels of the steel bars. It was also found that the yielding load of the strengthened beams increased significantly with areas strengthened by CFRP sheets and steel bars, while the maximum allowable long-term load only increased slightly.     

Flexural behavior of reinforced concrete (RC) beams retrofitted with hybrid fiber reinforced polymers (FRPs) under sustaining loads

This paper reports experimental studies of reinforced concrete (RC) beams retrofitted with new hybrid fiber reinforced polymer (FRP) system consisting carbon FRP (CFRP) and glass FRP (GFRP). The objective of this study is to examine effect of hybrid FRPs on structural behavior of retrofitted RC beams and to investigate if different sequences of CFRP and GFRP sheets of the hybrid FRPs have influences on improvement of strengthening RC beams. Toward that goal, 14 RC beams are fabricated and retrofitted with hybrid FRPs having different combinations of CFRP and GFRP sheets. The beams are loaded with different magnitudes prior to retrofitting in order to investigate the effect of initial loading on the flexural behavior of the retrofitted beam. The main test variables are sequences of attaching hybrid FRP layers and magnitudes of preloads. Under loaded condition, beams are retrofitted with two or three layers of hybrid FRPs, then the load increases until the beams reach failure. Test results conclude that strengthening effects of hybrid FRPs on ductility and stiffness of RC beams depend on orders of FRP layers.     

Experimental Study on the Seismic Performance of Columns Reinforced by the CFRP Bar and Sheet

To analyze the influence of CFRP (Carbon Fiber Reinforced Polymer) reinforcement ratio and CFRP sheet on the self-centering performances of concrete circular columns, five concrete circular columns were tested under the low-cyclic reversed load in this work. The five concrete circular columns included 1 RC (Reinforced Concrete) column, 2 CFRP bars reinforced concrete circular columns and 2 CFRP bars reinforced concrete circular columns with CFRP sheet strengthening partially. The hysteretic curves, skeleton curves, stiffness degradation, strength degradation, residual deformations, energy dissipation and ductility of the five circular columns were obtained and analyzed to verify the improvement of the seismic performances of the specimens reinforced with CFRP bars and CFRP sheets. At the same time, the reference opinions for practical applications of CFRP bars and CFRP sheets in structures can be provided. The test results showed that the bearing capacity and deformation capacities of the concrete circular columns reinforced with CFRP bars were 25.5% and 25% higher than that of the RC column, respectively. The deformation capacities, energy consumption capacities and deformation recovery capacities of CFRP bars reinforced concrete circular columns with CFRP sheet strengthening partially were 21.5%, 40% and 78.5% higher than that of the RC column, respectively.

     

Shear repair of RC beams using epoxy injection and hybrid external FRP

This experimental study aims at investigating the behavior of reinforced concrete (RC) beams strengthened by unidirectional and hybrid bidirectional fiber-reinforced polymer (FRP) sheets and subjected to cyclic loading. RC beams tested under cycled loading were subsequently repaired using both epoxy injection and external FRP sheets, and then re-tested under monotonic loading. Six RC beam specimens, two of which were control specimens and four were shear deficient, were upgraded with side-bonded FRP sheets in the first phase of the experimental program. In the second phase, three of the damaged beams were repaired using epoxy injection and unidirectional carbon fiber polymer (CFRP) sheets. The repairing method, FRP type, and FRP wrapping scheme were the test variables investigated. Test results show that the repair schemes imparted significant mechanical improvements in terms of ultimate shear capacity and ductility. The simultaneous application of epoxy injection and externally bonded FRP sheets was found to be a highly effective repair technique.     

Strengthening and repair of concrete foundation beams with carbon fiber composite materials

For several years Carbon Fiber Reinforced Polymer (CFRP) have been gradually replacing steel plates in the reinforcement of concrete structures that are damaged or need increased resistance. It has long been established that the addition of CFRP laminates to structures such as beams and slabs, increases their bending and shearing strength considerably. However, the behaviour of foundation beams with these reinforcements is not clear. The essential issue in the analysis of reinforced structures with composite materials is to understand the individual behaviour of each material and its interaction with the remaining ones. In this paper, bending and shearing strength of concrete foundation beams with CFRP reinforcement are analyzed through the study of their load capacity variation and beam deflections. Different height/span beam relations are considered. The numerical modeling is performed by the Finite Elements Method with the Abaqus program. Non-linear models are used for concrete and soil, and a linear elastic model is adopted for composites materials.     

Nonlinear finite element modeling of concrete deep beams with openings strengthened with externally-bonded composites

This paper aims to develop 3D nonlinear finite element (FE) models for reinforced concrete (RC) deep beams containing web openings and strengthened in shear with carbon fiber reinforced polymer (CFRP) composite sheets. The web openings interrupted the natural load path either fully or partially. The FE models adopted realistic materials constitutive laws that account for the nonlinear behavior of materials. In the FE models, solid elements for concrete, multi-layer shell elements for CFRP and link elements for steel reinforcement were used to simulate the physical models. Special interface elements were implemented in the FE models to simulate the interfacial bond behavior between the concrete and CFRP composites. A comparison between the FE results and experimental data published in the literature demonstrated the validity of the computational models in capturing the structural response for both unstrengthened and CFRP-strengthened deep beams with openings. The developed FE models can serve as a numerical platform for performance prediction of RC deep beams with openings strengthened in shear with CFRP composites.     

Crack damage mitigation and shear behavior of shear-dominant reinforced concrete beams repaired with strain-hardening cement-based composite

This research aims to study: (1) the crack damage mitigation and shear behavior of reinforced concrete (RC) beams that have been repaired using strain-hardening cement-based composite (SHCC) via experimental testing and (2) the contribution of the SHCC layers to the shear strength of the repaired RC beams via predictions. Five cantilever RC beams with a shear span-to-depth ratio of 2.8 were subjected to cyclic concentrated loading. The study variables include two types of tensile performance of the SHCC (with low or high strength in tension) and two repair methods (patching and layering). The experimental results show that the use of a SHCC layer leads to a substantial increase in the shear strength and ductility of the RC beams after the peak load. During the tests, all of the SHCC repaired beams showed delamination along the interface between the concrete and SHCC, and the shear resistance started to drop. However, the results also indicate that SHCC layers can be effective repair material for enhancing the control of cracking to help protect the concrete from the migration of aggressive agents in severe environments. In order to predict the shear strength of RC beams that have been repaired with SHCC, two methods were used in this study; one is based on Dinh's proposed model that considers the shear strength in both the compression and tension zones, and the other method considers the shear strength of the reinforcement, such as a stirrup or fiber-reinforced polymer (FRP) laminate that considers only the tensile strength across cracks. These two methods were able to predict the contribution of the SHCC layer to the shear strength of the RC beams, and the predicted shear strength values were very similar between the two methods.     

Repairing corroded RC beam with near-surface mounted CFRP rods

This experimental program investigates the possibility of using Carbon-Fiber-Reinforced Polymer (CFRP) rods to repair RC beams damaged by steel corrosion. The 6-mm-diameter carbon-epoxy pultruded FRP rods were implemented in undamaged areas of the concrete cover by using the Near Surface Mounted reinforcement (NSM) technique. The corroded beam tested was stored in a chloride environment under service loading for 23 years, which was representative of real structural conditions. The corroded beam showed longitudinal corrosion cracks more than 3 mm in width along the tension reinforcing bars. The repaired corroded beam was tested in three-point flexure up to failure. Then, concrete was removed around the reinforcing bars in order to measure the real local steel cross-section distribution. The maximum reduction in steel cross-section measured in the tension area was about 36% and was located at mid-span. In spite of this heavy corrosion, the bearing capacity measured on the repaired beam was equal to that of the uncorroded control beam. In our experimental conditions, the NSM technique was applicable for repairing corroded concrete structural members.     

Fatigue behaviour of the bond interface between carbon fibre‐reinforced polymer sheets and concrete

Externally bonded carbon fibre‐reinforced polymers (CFRPs) have been applied to retrofit and strengthen civil structures. In this study, four‐point bending beams were manufactured and tested to examine the fatigue behaviour of the CFRP–concrete interface. The results indicated that the specimens exhibited debonding failure in the concrete beneath the adhesive layer under static loading. However, when cyclic loads were imposed on the small beams, debonding failure may occur in the adhesive layer. Moreover, fitting expressions were proposed to predict the shear stress–slip relationship between the CFRP sheets and concrete and the flexural strength of the CFRP‐strengthened beams under static loads, and good agreement with the test data was obtained. Finally, a fatigue life prediction model was also presented to capture the fatigue life of the CFRP–concrete interface under cyclic loads. The calculation results showed that the fatigue strength of the CFRP–concrete bond interface was approximately 65% of the ultimate load capacity.     

Behaviour of hybrid FRP–UHPC beams in flexure under fatigue loading

The fatigue behaviour of innovative hybrid FRP–UHPC beams under flexural loading is investigated in this paper. The beams were made up of a pultruded GFRP hollow box section beam with a cast-in-place UHPC layer on top and either a CFRP or SFRP sheet bonded along the bottom. Four hybrid beams were tested under variable amplitude loading, to determine the effect of cyclic loading on flexural strength and stiffness. Analysis also included the development of a modified S–N curve and evaluation of fatigue damage using the Palmgren–Miner rule. It was found that insignificant loss in strength and stiffness occurred for all beams, where the fatigue damage estimated using the Palmgren–Miner rule overestimated the fatigue life. It was postulated, by comparison, that the hybrid beams reinforced with CFRP sheets may perform better under fatigue loading than the beams reinforced with SFRP sheets.     

Flexural response predictions of reinforced concrete beams strengthened with prestressed CFRP plates

Existing experimental studies showed that the reinforced concrete (RC) beams strengthened with prestressed carbon fiber-reinforced polymer (CFRP) plates had three possible flexural failure modes (including the compression failure, tension failure and debonding failure) according to the CFRP reinforcement ratio. Theoretical formulas based on the compatibility of strains and equilibrium of forces were presented to predict the nominal flexural strength of strengthened beams under the three failure modes, respectively, and a limitation on the tensile strain level developed in the prestressed CFRP plate was proposed as the debonding failure occurred. In addition, the calculation methods for cracking moment, crack width and deflection of strengthened beams were provided with taking into account the contribution of prestressed CFRP plates. Experimental studies on five RC beams strengthened with prestressed CFRP plates and a nonlinear finite element parametric analysis were carried out to verify the proposed theoretical formulas. The available test results conducted by other researchers were also compared with the predicted values.