Strengthening of RC T-section beams with low strength concrete using CFRP composites subjected to cyclic load

Various methods are developed for strengthening reinforced concrete beams against shear. Nowadays, external bonding of different composite members to RC beams was very popular and successful technique internationally. This study presents test results on strengthening of shear deficient RC beams by external bonding of carbon fiber reinforced polymer (CFRP) straps. Six RC beams with a T-section were tested under cyclic loading in the experimental program. Width of the CFRP straps, arrangements of straps along the shear span, and anchorage techniques that were applied at the ends of straps were the main parameters that were investigated during experimental study. Shear deficient beams with low strength concrete were strengthened by using CFRP straps for obtaining ductile flexural behavior. The test results confirmed that all CFRP arrangements improved the strength, stiffness and energy dissipation capacity of the specimens significantly. The failure mode and ductility of specimens were proved to differ according to the CFRP strap width and arrangement along the beam. Experimental results were compared with the analytical approaches that were suggested by ACI-440 Committee Report.     

预载水平及剪/弯承载力比对剥离破坏的影响

通过碳纤维片材加固钢筋混凝土简支梁的受弯试验及文献中相关试验,研究了预载水平及剪/弯承载力比对纤维片材初始剥离荷载的影响。对本文试验及文献中发生剥离破坏的碳纤维片材加固钢筋混凝土梁的试验研究,分析结果表明:(1)随剪/弯承载力比的增大,加固梁纤维片材的初始剥离荷载也随之增大;但当剪/弯承载力比值较小,即受剪承载力富裕度较小的情况下,纤维片材可能会在低于设计极限承载力时发生剥离破坏;(2)加固时预载水平的大小对碳纤维片材加固钢筋混凝土梁的极限荷载和CFRP片材初始剥离时的荷载影响不大。因此加固设计时必须充分考虑加固构件的受剪承载力对剥离破坏的影响,以避免纤维剥离这一脆性破坏模式的发生。     

Retrofitting of reinforced concrete beams using composite laminates

This paper presents the results of an experimental study to investigate the behaviour of structurally damaged full-scale reinforced concrete beams retrofitted with CFRP laminates in shear or in flexure. The main variables considered were the internal reinforcement ratio, position of retrofitting and the length of CFRP. The experimental results, generally, indicate that beams retrofitted in shear and flexure by using CFRP laminates are structurally efficient and are restored to stiffness and strength values nearly equal to or greater than those of the control beams. It was found that the efficiency of the strengthening technique by CFRP in flexure varied depending on the length. The main failure mode in the experimental work was plate debonding in retrofitted beams.     

Effective methods of using CFRP bars in shear strengthening of concrete girders

This paper presents the results of an experimental study that investigated the shear strength contribution of carbon fiber reinforced polymer (CFRP) bars attached with concrete beams using a near surface mounted (NSM) technique. In this research, four concrete beams were cast with regular steel reinforcement in flexure. The control beam had typical shear steel and the other three beams were strengthened in shear with CFRP bars. Strain gauges were attached with the shear reinforcement of all four beams at various shear critical locations. Strains during loading to failure of the beams were recorded using a data acquisition system. The performance of the NSM technique was found to be very effective with no occurrence of delamination, debonding or fracture of FRP. Effective strains in the NSM CFRP bars were determined through analyzing the collected strain data. A new formula to calculate the nominal shear strength provided by NSM CFRP bars has also been proposed.     

Damaged RC beams repaired by bonding of CFRP laminates

This paper summarizes the results of experimental studies on damaged reinforced concrete beams repaired by external bonding of carbon fiber reinforced polymer (CFRP) composite laminates to the tensile face of the beam. Two sets of beams were tested in this study: control beams (without CFRP laminates) and damaged and then repaired beams with different amounts of CFRP laminates by varying different parameters (damage degree, CFRP laminate width, concrete strength class). All beams were tested in four-point bending over a span of 1800 mm. The tests were carried out under displacement control. The most investigated parameter in this experimental study is damage degree (ratio between pre-cracked load and load capacity of control beam). Repairing damaged RC beams with externally bonded CFRP laminates were successful for different degrees of damage. The observed failure modes were peeling off and interfacial debonding. These failure modes depend only on the laminate width.The results indicate that the load capacity and the rigidity of repaired beams were significantly higher then those of control beam for all tested damage degrees. The authors remarked that for a load capacity improvement, reinforcement with a CFRP having about a half width of the beam is satisfactory. Finally, the contribution of CFRP laminates on the load capacity and rigidity of repaired RC beams is significant for any concrete strength class.     

存在薄弱界面的碳纤维布复合材料加固钢筋混凝土梁剥离试验研究

置换受损保护层并外贴碳纤维复合材料（CFRP）加固锈蚀钢筋混凝土梁时,可能因新老混凝土黏结不良产生水平薄弱界面,影响其受力性能。基于此,设计10个带有水平弱界面的CFRP加固钢筋混凝土梁试件,对比分析不同界面黏结强度、加固量、保护层厚度及U形箍约束条件下加固梁的受力性能及CFRP的剥离破坏行为。结果表明,薄弱界面的存在易导致中部保护层剥离过早发生,并向端部发展,削弱了加固体系的整体性,无约束条件下纵向CFRP发生整体剥离。界面削弱越严重,加固量越大,保护层厚度越小,界面剪切传递能力越弱,剥离破坏越易发生和发展。U形箍约束能防止纵向CFRP整体剥离破坏,保证其有效受力,但不能完全防止局部剥离沿薄弱界面发展,易导致纵向CFRP在复合受力条件下过早断裂,且存在较高的端部锚固破坏的风险。     

Assessing the effectiveness of embedding CFRP laminates in the near surface for structural strengthening

Near surface mounted (NSM) is a recent strengthening technique based on bonding carbon fiber reinforced polymer (CFRP) bars (rods or laminate strips) into pre-cut grooves on the concrete cover of the elements to strength. To assess the effectiveness of the NSM technique, an experimental program is carried out involving reinforced concrete (RC) columns, RC beams and masonry panels. In columns failing in bending the present work shows that the failure strain of the (CFRP) laminates can be attained using the NSM technique. Beams failing in bending are also strengthened with CFRP laminates in order to double their load carrying capacity. This goal was attained and maximum strain levels of about 90% of the CFRP failure strain were recorded in this composite material, revealing that the NSM technique is also very effective to increase the flexural resistance of RC beams.The effectiveness of externally bonded reinforcing (EBR) and NSM techniques to increase the flexural resistance of masonry panels is also assessed. In the EBR technique the CFRP laminates are externally bonded to the concrete joints of the panel, while in the NSM technique the CFRP laminates are fixed into precut slits on the panel concrete joints. The NSM technique provided a higher increase on the panel load carrying capacity as well as a larger deflection at the failure of the panel.The performance of EBR and NSM techniques for the strengthening of RC beams failing in shear is also analyzed. The NSM technique was much more effective in terms of increasing the beam load carrying capacity as well as the beam deformability at its failure. The NSM technique was easier and faster to apply than the EBR technique.     

CFRP网格-聚合物水泥砂浆加固RC梁的#br# 抗剪性能与计算公式

为揭示不同网格单位加固量对碳纤维增强复合网格(carbon fiber reinforced polymer grid,简称CFRP grid) 聚合物水泥砂浆(polymer cement mortar,简称PCM)复合加固钢筋混凝土(reinforced concrete,简称RC)梁抗剪性能的影响,构建CFRP网格-PCM加固RC梁的承载力计算方法,文章首先对7根采用CFRP网格 PCM复合加固RC梁进行四点弯曲静力加载试验,并在试验研究的基础上,提出基于横竖双向网格实际抗剪贡献的CFRP网格-PCM加固RC梁的承载力计算公式。研究结果表明：采用CFRP网格-PCM复合加固RC梁能显著提高其抗剪承载力,其中CFRP网格对于抗剪承载力的提高发挥主要作用,而PCM仅起到黏结剂的作用;加固梁的抗剪承载力与CFRP网格单位加固量呈正相关,但CFRP网格的协同变形性与CFRP网格单位加固量呈负相关;所提出的抗剪承载力计算公式与试验结果吻合良好。     

预应力碳纤维复合板加固钢筋混凝土梁的抗弯响应预测

现有的试验研究显示,碳纤维复合材料用量不同时,采用预应力碳纤维复合板加固钢筋混凝土梁可能出现3种弯曲破坏模式(包括受压破坏、受拉破坏、剥离破坏)。基于应变和力平衡的协调性,提出理论公式对3种破坏模式下的名义抗弯强度进行预测。当出现剥离破坏时,提出预应力碳纤维复合板中的受拉应变的极限。此外,考虑预应力碳纤维复合板的作用,提出开裂弯矩、裂缝宽度和加强梁的挠度计算方法。对5根采用预应力碳纤维复合板加固的钢筋混凝土梁进行试验,并通过非线性有限元分析验证所提出的理论公式。同时,预测值与其他研究者的试验结果也进行了比较。     

基于三维细观分析方法的CFRP加固无腹筋混凝土梁剪切强度尺寸效应研究

以CFRP侧贴加固无腹筋钢筋混凝土悬臂梁为研究对象,从细观角度出发,考虑混凝土细观非均质性及CFRP-混凝土之间的相互作用,建立了单调加载下CFRP侧贴无腹筋钢筋混凝土悬臂梁三维细观尺度数值分析模型。通过仅改变CFRP条带的厚度来改变CFRP的配纤率,进而以CFRP的配纤率为主导参数,并且在之前试验工作的基础上,扩展模拟了尺寸和配纤率对梁的剪切破坏机理和失效模式的影响,研究了外贴CFRP加固钢筋混凝土悬臂梁剪切破坏尺寸效应行为。结果表明: CFRP侧贴加固无腹筋钢筋混凝土梁的名义剪切强度尺寸效应明显,CFRP布加固对小尺寸梁贡献最大,其有效性随着梁尺寸的增加而减小; 配纤率的增大提高了悬臂梁的名义抗剪强度,但同时也削弱了抗剪强度的尺寸效应; 当配纤率较大时,峰值剪切应力的增长趋势明显减缓,即层数过多时,加固效果不明显。     

Behavior and capacity of RC beams strengthened in shear with NSM FRP reinforcement

A recent and promising method for shear strengthening of reinforced concrete (RC) members is the use of near-surface mounted (NSM) fiber-reinforced polymer (FRP) reinforcement. In the NSM method, the reinforcement is embedded in grooves cut onto the surface of the member to be strengthened and filled with an appropriate binding agent such as epoxy paste or cement grout. Only a few studies have been conducted to date on the use of NSM FRP reinforcement for shear strengthening of RC beams. These studies identified some critical failure modes related to debonding between the NSM reinforcement and the concrete substrate. However, more tests need to be conducted to identify all possible failure modes of strengthened beams. Moreover, virtually no test results are available on the behavior of shear-strengthened beams containing steel shear reinforcement, and on the effect of variables such as the type of epoxy used as groove filler. This paper illustrates a research program on shear strengthening of RC beams with NSM reinforcement, aimed at gaining more test results to fill the gaps in knowledge mentioned above. A number of beams were tested to analyze the influence on the structural behavior and failure mode of selected test parameters, i.e. type of NSM reinforcement (round bars and strips), spacing and inclination of the NSM reinforcement, and mechanical properties of the groove-filling epoxy. One beam strengthened in shear with externally bonded FRP laminates was also tested for comparison purposes. All beams had a limited amount of internal steel shear reinforcement to simulate a real strengthening situation. Test results are presented and discussed in the paper.     

The problem of shear in RC beams strengthened with CFRP laminates

The effectiveness of externally bonded reinforcement of a strengthened Reinforced Concrete (RC) beam subjected to a shear-dominant loading regime is not well-established. The aim of this paper is to clarify the structural performance of RC beams without any internal shear reinforcement but strengthened with Carbon Fibre Reinforced Polymer (CFRP) laminates when the primary mode of failure of the un-strengthened beam is in shear. Four RC beams were specifically designed without and with an externally anchorage system, which was carefully detailed to enhance the benefits of the strengthening lamina and counteract the destructive effects of shear forces. All the four beams were identical in terms of their geometry, internal reinforcement and concrete strength but varied in their test loading regime to highlight the role of shear. All the beams were tested under four point bending and extensively instrumented to monitor strains, cracking, load capacity and failure modes. The structural response of the four beams is then critically analysed in terms of deformability, strength and failure processes under a shear loading regime. It is shown that with a carefully designed anchorage system, a predominantly brittle shear failure of a strengthened beam can be transformed to an almost ductile failure with well-defined enhancement of structural performance in terms of both deformation and strength. The results presented in this paper should enable engineers to totally avoid shear failure in strengthening beams with little or even no internal shear reinforcement.     

Effect of bond enhancement using carbon nanotubes on flexural behavior of RC beams strengthened with externally bonded CFRP sheets

This paper studied the effect of incorporation of carbon nanotubes (CNTs) in carbon fiber reinforced polymer (CFRP) on strengthening of reinforced concrete (RC) beams. The RC beams were prepared, strengthened in flexure by externally bonded CFRP or CNTs-modified CFRP sheets, and tested under four-point loading. The experimental results showed the ability of the CNTs to delay the initiation of the cracks and to enhance the flexural capacity of the beams strengthened with CFRP. A nonlinear finite element (FE) model was built, validated, and used to study the effect of various parameters on the strengthening efficiency of CNTs-modified CFRP. The studied parameters included concrete strength, flexural reinforcement ratio, and CFRP sheet configuration. The numerical results showed that utilization of CNTs in CFRP production improved the flexural capacity of the strengthened beams for U-shape and underside-strip configurations. The enhancement was more pronounced in the case of U-shape than in the case of use of sheet strip covers on the underside of the beam. In case of using underside-strip, the longer or the wider the sheet, the higher was the flexural capacity of the beams. The flexural enhancement of RC beams by strengthening with CNTs-modified CFRP decreased with increasing the rebar diameter and was not affected by concrete strength.     

Shear strengthening of RC beams with web-bonded continuous steel plates

This paper presents the results of an experimental program and a parametric study conducted on RC beams strengthened in shear with web-bonded continuous steel plates. An experimental investigation was conducted to assess the effectiveness of web-bonded continuous steel plates for shear strengthening of RC beams having internal stirrups. A two-dimensional nonlinear finite element model was developed to simulate the overall behavior of beams with epoxy bonded steel plates. In order to develop a design methodology for beams with web-bonded steel plates, a parametric study was conducted. Main parameters considered were concrete strength, plate thickness, plate depth-to-beam depth ratio, yield strength of steel plates, beam size and the internal shear reinforcement ratio. A formula to compute the shear strength of such beams was proposed by adding up the concrete contribution, shear reinforcement contribution and the contribution of steel plates. The validity of proposed formula was checked against the results from parametric study and the experiments. It was found that the proposed formula predicts the ultimate shear strength of RC beams with web-bonded continuous steel plates very well.     

预应力CFRP布加固T形RC梁正截面承载力分析与计算

采用碳纤维布施加预应力后加固混凝土梁,可以充分发挥碳纤维材料的高强度特点,显著减小梁的裂缝和挠度,提高加固效率。对预应力CFRP布加固钢筋混凝土T形梁的各种破坏模式、界限和理论计算进行了较系统的分析和总结,并给出了判别破坏模式的方法和计算预应力碳纤维布加固混凝土T形梁正截面的计算公式,计算出的结果与试验结果较吻合。     

碳纤维布加固钢筋混凝土梁的试验分析

本试验预制了6根混凝土适筋梁,梁净跨1.8 m,加固方法采用直接粘贴碳纤维布并沿试件长度方向等间距布置,两端未加任何锚固措施。采用竖向静力两点加载体系,合适的剪跨,采用改变混凝土强度、碳纤维布的粘结层数等参数进行了较系统的试验。测得的主要试验数据有:梁跨中挠度、加载千斤顶荷载、梁底纵筋跨中的应变、碳纤维布应变分布与受拉钢筋的应变分布。     

侧立面嵌贴碳纤维增强塑料板条加固混凝土梁受剪承载力研究

为研究加固钢筋混凝土梁的受剪承载力,按照沿梁轴向非对称布置箍筋的方式浇筑13根钢筋混凝土梁试件,在箍筋布置较少侧混凝土梁侧立面保护层上嵌入或外贴碳纤维增强塑料板条对混凝土梁进行受剪切补强。对其进行弯曲试验,研究内嵌碳纤维增强塑料板条补强混凝土梁的破坏形态、承载力等情况,并与相应位置外贴等量碳纤维增强塑料板条的混凝土梁进行比较,分析内嵌或外贴在混凝土梁上的碳纤维增强塑料板条应变变化情况及补强混凝土梁受剪承载力影响因素和受剪承载力计算方法。研究表明,与对比梁相比,内嵌碳纤维增强塑料板条补强混凝土梁受剪承载力提高18.8%~45.8%,外贴碳纤维增强塑料板条补强混凝土梁受剪承载力提高12.5%~13.3%,提出的承载力计算式计算结果与试验值吻合较好。     

嵌入式碳纤维片材抗剪加固钢筋混凝土T型梁的性能

通过试验研究介绍采用碳纤维片材嵌入式技术抗剪加固钢筋混凝土T型梁的效果。试验考虑了嵌入片材的3种角度(45°,60°和90°)以及3个不同的碳纤维片材用量。嵌入式加固梁的配箍率为0·10%。片材最高用量按照对比梁中钢筋配箍率为0·28%时所能够承担的最大荷载进行设计。结果表明:斜片材的效果优于竖直片材的效果;片材用量的增加导致梁的抗剪承载力增加;片材对梁抗剪能力的贡献受到混凝土抗拉强度的限制;梁的破坏模式受片材加固量的影响。对于每一种片材加固量,同时根据外贴碳纤维布技术,对相同构件采用连续U型碳纤维布箍加固后进行试验,对比了两种碳纤维材料加固技术的效果。结果表明:嵌入式加固技术更有效,不仅提高了梁的抗剪能力,同时,碳纤维材料的抗拉强度得到更充分的发挥。除了碳纤维片材用量最高的梁以外,采用嵌入式加固梁在破坏时的变形方面比外贴加固技术更有效。ACI和FIB的分析公式中预测的外贴加固对抗剪加固体系的贡献比相应的试验值更大,根据Nanni等人的公式可知,嵌入式加固技术使得碳纤维片材的贡献大约达到试验值的61%。     

CFRP加固钢筋混凝土梁二次受力抗剪试验研究

通过4个不同持续荷载下的CFRP加固梁抗剪试验,其中1个对比梁,3个不同荷载下的加固梁.试验结果表明,加固梁的极限抗剪承载力得到了提高,裂缝宽度和跨中挠度有所减小.发现现有的抗剪承载力计算公式远大于试验值,这是不安全的,进而提出了考虑二次受力的抗剪承载力计算公式,和试验结果吻合良好,具有一定的参考价值.     

CFRP加固钢筋混凝土直剪承载力试验

设计15个外贴碳纤维增强复合材料(CFRP)的Z型试件进行直剪加载试验,试验参数包括直剪面钢筋配筋率（0.46%~1.2%）和CFRP加固率（0%~0.3%）。依据试验结果提出加固后钢筋混凝土直剪承载力预测表达式,并基于分量模型分析其加固机理。研究结果表明:直剪承载力随CFRP加固率增大而提高,提高幅度为6%~50%;相同CFRP加固率时,直剪承载力提高幅度随配筋率的增加而降低,承载力提高源于CFRP提供了附加的侧向夹紧力;当外贴CFRP和内埋直剪钢筋提供相等的侧向夹紧力时,两者对直剪承载力的贡献作用相近;所提出的承载力预测表达式具有较好的适用性。