RC beam strengthened with pre-stressed CFP under the secondary load

Feasibility of using pre-stressed carbon fiber plates to strengthen reinforced concrete beams was studied. Based on the characteristics of carbon fiber plates, we developed a pre-stress clamp and a device for applying the pre-stress. Contrast tests were conducted between ordinary carbon fiber plates and a pre-stressed carbon fiber plate and between secondary loaded carbon fiber plates and a concrete beam strengthened with a secondary loaded carbon fiber plate. On this basis, we analyzed the failure pattern, the width of cracks and their distribution, the cracking load, the yield load, the limit load and the relation between load and deflec- tion. The results indicate that using pre-stressed carbon fiber plates to strengthen concrete beams is feasible and effective.

Experimental Study of RC Beams Strengthened with CFRP Sheets Under Sustaining Loads

Seven reinforced concrete （ RC ） beams strengthened in flexure using carbon fiber reinforced polymer （CFRP） sheets subjected to different sustaining loads were tested. The effects of initial load and load history on the ultimate strength of strengthened RC beams were examined by externally bonded CFRP sheets. The main experimental parameters included different levels of sustaining load at the time of strengthening, and load history. Experimental results show that sustaining load levels at the time of strengthening have important influences on the ultimate strength of strengthened RC beams. If the initial load is the same, the ultimate strength of RC beams strengthened with CFRP sheets is almost the same regardless of load history at the time of strengthening.

Numerical study of RC beams under various loading rates with LS-DYNA

Having an accurate understanding of concrete behavior under effects of high strain rate loading with the aim of reducing incurred damages is of great importance. Due to complexities and high costs of experimental research, numerical studies can be an appropriate alternative for experimental methods. Therefore, in this research capability of the finite element method for predicting concrete behavior at various loading conditions is evaluated by LS-DYNA software. First, the proposed method is presented and then is validated in three stages under different conditions. Results of load-lnidspan displacement showed good agreement between experimental and finite element results. Capability of finite element method in analyses of beams under various rates of loading was also validated by low error of the results. In addition, the proposed method has reasonable ability to evaluate reinforced concrete beams under various loading rates and different conditions.

The Failure Mechanism of CFRP Reinforced RC Beam under Impact Loads

The influence of carbon fiber reinforced plastic(CFRP)on dynamic mechanical properties of reinforced concrete(RC)beam was studied by drop hammer impact test system.The impact behaviors of beam,including failure mode,impact force peak value and peak deflection were analyzed.The experimental results show that bonding CFRP can reduce the crack width and change the failure mode of the beam.The length of CFRP has a certain influence on the impact force and deflection,and the peak inertia force of most beams is roughly in the range of 1/2-5/6 of the peak impact force.In addition,dynamic increase factor(DIF)increases with the increase of CFRP length,and its maximum value can reach 2.11.

Experimental research and numerical simulation of RC beams strengthened with bonded steel plates

It is a common method to strengthen the damaged RC structures with bonded steel plates. At present the ultimate bearing ca- pacity of RC structures strengthened with bonded steel plates is calculated mostly using the theory based on the test. Four beams, including one reference beam, two strengthened concrete beams in primary force and secondary force respectively, and one strengthened concrete beam which was not anchored enough, were tested under four-point bending (4PB) in order to get the data of strain of longitudinal bars, bonded bottom steel plate in tension and deflection of beams in the middle span. The experimental program was supported by a three-dimensioned finite analysis using ABAQUS. At the end of experiments and finite analysis, it is concluded that the investing strengthening technique can significantly improve the load-carrying capacity and the phenomenon of stress concentration at the end of interface, as well as the damage at interface, can be well simulated with cohesive element provided by ABAQUS.

ANALYTICAL ANALYSIS OF INTERFACIAL STRESSES IN FRP-RC HYBRID BEAMS WITH TIME-DEPENDENT DEFORMATIONS OF RC BEAM

In this paper, the effect of time-dependent deformations （such as shrinkage and creep） on the interracial stresses between an RC beam and FRP plate is presented. For this end, a closed-form solution for such stresses in externally FRP plated RC beams including creep and shrinkage effects is presented. The developed model is formulated to predict the interfacial stresses at time ＇t＇, in which the RC beams have been already subjected to creep and shrinkage effects. The adherend shear deformations have been included in the present theoretical analysis by assuming a parabolic shear stress through the thickness of the RC beam and the FRP panel. Contrary to some existing studies, the assumption that both RC beam and FRP panel have the same curvature is not used in the present investigation. This research is helpful for the understanding on mechanical behavior of the interface and design of the FRP-RC hybrid structures.

非对称集中荷载下无腹筋RC梁受剪性能试验研究

非对称集中荷载作用下钢筋混凝土无腹筋简支梁具有两个不同的剪跨比,而两剪跨段受剪承载力与剪力作用之间的相对大小存在不确定性.通过开展6根非对称和4根对称集中荷载作用下钢筋混凝土无腹筋简支梁受剪性能试验研究,获取了破坏形态、荷载-跨中位移曲线和纵向受拉钢筋应变,并分析了《混凝土结构设计规范》(GB 50010—2010)公式、修正压力场理论、基于截面应变分析的抗剪模型和Zsutty统计公式的适用性.结果表明,对于小剪跨比恒为1.0的无腹筋简支梁,大剪跨比为2.0~4.0的梁均在大剪跨段发生剪切破坏;当大剪跨比由3.0增大至3.5时,梁的极限承载力出现由大剪跨段控制转变为小剪跨段控制的现象《.混凝土结构设计规范》(GB50010—2010)预测非对称集中荷载作用下钢筋混凝土无腹筋简支梁发生剪切破坏的位置与试验结果相反,而Zsutty统计公式的预测效果最好.

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.

粘铝合金板加固RC梁抗弯性能试验及正截面承载力

为了探讨铝合金板厚度与端部U形铝合金箍板构造对RC梁破坏机理及抗弯性能的影响,对16根铝合金板加固RC梁进行了4点弯曲静力加载试验。在试验梁底铝合金板的表面以及试验梁跨中截面的顶面、侧面粘贴应变片,在试验梁底跨中、梁顶两端支座布置百分表。为了推导铝合金板加固RC梁的正截面承载力计算式,基于试验得到的破坏机理,通过引入6个基本假定,分析了5种不同破坏模式的相对受压区高度。建立了各破坏模式下铝合金板加固RC梁的正截面承载力计算式,并将其与引入文献中的试验结果进行了对比分析。结果表明:试件破坏形态包括适筋破坏、端部铝合金板与结构胶剥离、端部铝合金板与梁剥离及中部铝合金板剥离4种情况;当端部设置U形铝合金箍板时,可有效抑制端部铝合金板发生剥离,使钢筋与铝合金板的材料性能得到充分发挥,试件承载力与延性明显提升;当铝合金板厚度分别为2~6 mm时,试件承载力随铝合金板厚度的增加而提高;端部设置U形铝合金箍板可有效防止端部铝合金板发生剥离,但试件仍可能会出现中部铝合金板剥离现象;当试件出现适筋破坏特征时,试件抗弯承载力的计算值与试验值比值均在1以下。本研究所提计算式具有明显的规律性和适用性,可为工程实际设计提供参考。

配筋率与质量比对FRP-RC梁冲击响应与损伤的影响

基于考虑应变率效应的三维数值模型,对玄武岩纤维增强复材筋混凝土(BFRP-RC)梁的抗冲击性能进行了研究.分析了冲击体与FRP-RC梁质量比、配筋率等因素对冲击响应的影响规律.结果表明,随着质量比增大,FRP筋混凝土梁的破坏模式从弯曲破坏转变为剪切破坏,但是配筋率的改变并不影响梁的破坏模式.由于BFRP筋弹性模量低,并且没有屈服阶段,故即使在梁损伤比较严重的情况下,BFRP筋的应变和BFRP筋梁的位移也会发生较大恢复.在冲击过程中,冲击力时程曲线的形状与质量比之间的规律不随配筋率而改变.此外,以剩余承载力为基准,建立了以频率变化、损伤因子和峰值位移为指标的损伤评估方法,为实际工程中FRP筋混凝土梁构件的灾后处置提供理论基础.

弯-剪-扭复合受力CFRP布加固RC梁抗扭性能细观数值分析

为了研究弯-剪-扭复合受力条件下碳纤维增强复合材料(carbon fiber reinforced polymer, CFRP)布加固钢筋混凝土(reinforced concrete, RC)梁的抗扭性能,建立体现混凝土材料非均质性、钢筋-混凝土黏结滑移关系和CFRP布-混凝土相互作用关系的细观数值分析模型。由于缺乏CFRP布加固RC梁在弯-剪-扭复合受力作用下失效破坏的典型物理试验,采用复合对比验证法对数值模型的合理性进行验证,即分别与单独受剪、单独受扭以及受弯-扭复合作用下RC梁和CFRP布加固RC梁的试验结果进行对比验证。进而,基于建立的细观数值分析模型探究了配纤率和扭弯比对CFRP布加固RC梁在弯-剪-扭复合受力作用下抗扭力学性能的影响。结果表明:1)采用CFRP布加固能够显著提高RC梁的峰值扭矩;2)随配纤率增加,RC梁损伤分布范围逐渐变大,CFRP布应变逐渐减小,梁整体峰值扭矩增大,但其增大幅度减小;3)随扭弯比增加,RC梁裂缝分布愈加集中,裂缝与水平方向夹角逐渐减小,梁整体峰值扭矩增大,CFRP布应变减小。建立的考虑弯-剪-扭复合受力作用的数值模拟方法可为后续研究复杂应力状态下CFRP布加固RC梁尺寸效应行为等奠定基础。

预加载RC梁在升降温全过程中承载性能的试验研究

为了研究钢筋混凝土梁在服役状态下遭受火灾后承载性能的变化,对7根钢筋混凝土梁进行了高温和加载试验,测试了在250℃、450℃和650℃加热过程和降温过程中梁的温度、极限弯矩和极限挠度等参数;分析了温度及冷却方式对预加载梁承载性能的影响。结果表明:在预加极限弯矩的39%荷载状态下,随着温度的升高,自然冷却梁的极限弯矩逐渐降低为未受高温时梁的96.45%.93.41%和91.38%。喷水冷却梁的极限弯矩逐渐降低为未受高温时梁的96.45%、95.69%和94.68%。达到极限荷载时,自然冷却和喷水冷却梁的跨中挠度均随着所受温度的升高而逐渐增大,受250℃、450℃和650℃后自然冷却梁的挠度分别为10.03mm、16.11mm和22.32mm;而喷水冷却梁的挠度分别为15.89mm、16.63mm和17.43mm。预加载梁所经受的温度越高,则其承载性能降低的越多。

带非对称摩擦连接的RCS梁柱节点抗震性能与构造参数研究

基于非对称摩擦连接(AFC)的耗能特点,提出了一种新型的钢筋混凝土柱-钢梁(RCS)梁柱节点,对3组采用AFC连接的RCS梁柱节点进行往复加载试验,考虑了螺栓预紧力、摩擦垫片材料等构造因素对节点抗震性能的影响.结果表明:3组带摩擦连接RCS试件均为梁铰机制破坏,满足“强节点,弱构件”的抗震设计要求;3组试件的滞回曲线具有明显的双平台耗能,滞回曲线饱满稳定,有利于实现两阶抗震设防目标;耐磨钢材料与黄铜材料的两组试件的承载力相近,滞回曲线特征相似,都具有良好的耗能能力;随着螺栓预紧力的增加,试件获得的承载力、刚度也随之大,耗能能力得到了提升,在实际应用中,可通过合理控制预紧力来满足预期所需的滑移荷载与耗能能力.

功能可恢复RCS混合框架结构研究进展

钢筋混凝土柱-钢梁(RCS)混合框架结构以其结合了混凝土优异的抗压性能及钢材优异的抗弯性能而受到广泛关注,而可恢复功能结构以其震后快速恢复使用功能的能力,也成为地震工程界研究的新热点。功能可恢复RCS混合框架结构可以在弯矩较大的梁端和柱脚部位设置可更换构件,实现结构的功能可恢复能力。文中简述了近年来功能可恢复RCS混合框架结构研究进展,重点关注各类型功能可恢复钢梁、功能可恢复摇摆柱脚的构造研究,介绍了功能可恢复RCS混合框架结构性能分析研究进展。最后,对功能可恢复RCS混合框架结构仍需深入研究的问题进行了展望。

Dynamic responses of reinforced concrete beams under double-endinitiated close-in explosion

The reinforced concrete(RC) structural component might suffer a great damage under close-in explosion.Different from distant explosions, blast loads generated by the close-in explosion are non-uniformly distributed on the structural component and may cause both local and structural failure. In this study,an experimental study was conducted to investigate the dynamic responses of RC beams under doubleend-initiated close-in explosions. The experimental results show that the distribution of blast loads generated by the double-end-initiated explosion is much more non-uniform than those generated by single-point detonation, which is caused by the self-Mach-reflection effects. A 3 D finite element model was developed and validated in LS-DYNA by employing the modified K&C model. Intensive numerical calculations were conducted to study the influences of the initiation way, scaled distance and longitudinal reinforcement ratio on the dynamic responses and failure modes of RC beams. Numerical results show that the RC beam suffers greater damage as the cylindrical explosive is detonated at its double ends than the scenario in which the cylindrical explosive is detonated at its central point. RC beams mainly suffer flexural failure and flexure-shear failure under the double-end close-in explosion, and the failure modes of RC beams change from the flexural damage to flexure-shear damage as the scaled distance or the longitudinal reinforcement ratio decreases. The direct shear failure mode is not usually observed in the double-end-initiated explosion, since the intense blast loads is basically concentrated in the midspan of RC beam, which is due to self-Mach-reflection enhancement.

RC梁柱节点抗剪机理及承载力计算方法综述

对国内外关于框架结构中梁柱节点抗震性能研究成果进行总结归纳,分析了加载过程中节点的破坏形态及其抗剪机理,总结了典型的梁柱节点抗剪承载力计算模型,并基于各国规范对比了现有的抗剪承载力计算模型的适用性。对比了美国规范ACI 318-19、欧洲规范EC2的抗剪承载力计算的差异,指出了《混凝土结构设计规范》(GB 50010—2010)(2015年版)建议的梁柱节点抗剪承载力计算结果相对保守。建议适量增加节点核心区配箍率,从而提高钢筋与核心区混凝土的粘结性能,减少纵筋的粘结滑移现象。

Pushover analysis of reinforced concrete frames considering shear failure at beam-column joints

Since most current seismic capacity evaluations of reinforced concrete （RC） frame structures are implemented by either static pushover analysis （PA） or dynamic time history analysis, with diverse settings of the plastic hinges （PHs） on such main structural components as columns, beams and walls, the complex behavior of shear failure at beam-column joints （BCJs） during major earthquakes is commonly neglected. This study proposes new nonlinear PA procedures that consider shear failure at BCJs and seek to assess the actual damage to RC structures. Based on the specifications of FEMA-356, a simplified joint model composed of two nonlinear cross struts placed diagonally over the location of the plastic hinge is established, allowing a sophisticated PA to be performed. To verify the validity of this method, the analytical results for the capacity curves and the failure mechanism derived from three different full-size RC frames are compared with the experimental measurements. By considering shear failure at BCJs, the proposed nonlinear analytical procedures can be used to estimate the structural behavior of RC frames, including seismic capacity and the progressive failure sequence of joints, in a precise and effective manner.

箍筋弯曲部断裂分布对RC长梁受剪性能影响

[目的]探究碱集料反应(AAR)导致的箍筋弯曲部断裂劣化对RC长梁抗剪性能的影响.[方法]采用人工方法模拟箍筋弯曲部断裂,通过三点加载调查试验梁宏观受剪性能,并在试验的基础上使用有限元软件Diana模拟试验梁的受剪承载行为.[结果]研究结果表明:相比完好梁,支点附近箍筋下端弯曲部断裂以及箍筋下端弯曲部全部断裂的缺陷梁的受剪承载力分别下降了10.0%和12.6%;仅加载点附近箍筋下端弯曲部断裂的缺陷梁下降了0.1%;支点附近箍筋下端和加载点附近箍筋上端弯曲部均断裂的缺陷梁则下降了18.6%.[结论]支点附近箍筋下端弯曲部断裂弱化了拱机制,降低了混凝土抗剪贡献;加载点附近箍筋上端弯曲部断裂降低了侧肢在与主斜裂缝相交位置的抗拉力,从而减弱了箍筋参与桁架机制的抗剪贡献,而下端弯曲部断裂对抗剪的影响并不明显;当支点附近箍筋下端弯曲部和加载点附近箍筋上端弯曲部同时断裂时,拱机制和桁架机制均受到削弱,承载力下降最为显著.

UHPC-RC节段组合梁的抗弯性能分析

为研究低预应力度条件下,不同节段接缝的超高性能混凝土-钢筋混凝土(ultra-high performance concrete-reinforced concrete,UHPC-RC)组合梁的抗弯性能,文章基于ABAQUS有限元软件,建立UHPC-RC节段组合梁的基准有限元模型,并利用已有的试验数据验证该模型的合理性。节段接缝分别为平面干接缝、平面胶接缝、齿键干接缝和齿键胶接缝,在基准有限元模型的基础上,进一步研究节段接缝类型对组合梁抗弯性能的影响。研究发现,在四点受弯加载方式下,接缝的几何特征(平面或齿键)对组合梁的极限荷载和开裂荷载影响较小,环氧树脂胶的使用对组合梁的抗弯性能影响较大。

UHPC加固RC梁受力性能试验研究

为研究超高性能混凝土(UHPC)在RC梁加固中的适用性,通过双面剪切试验对UHPC-普通混凝土界面粘结性能进行分析。分别对RC梁受压、受拉区进行UHPC薄层加固,对未加固RC梁和受压、受拉区加固RC梁进行四点弯曲试验,对比正截面抗弯性能。结果表明:界面处理后,界面粘结性能大于普通混凝土抗剪强度。确定人工凿毛5 mm的界面处理方式。RC梁采用UHPC加固后,抗弯承载能力显著提高,受压区加固RC梁后受拉钢筋屈服荷载、试验峰值荷载分别提高了54.3%、52.2%,受拉区加固RC梁后受拉钢筋屈服荷载、试验峰值荷载分别提高了85.7%、96.9%;受拉钢筋屈服时受压、受拉区加固RC梁裂缝宽度较未加固RC梁分别减小了49.4%、72.3%;相同裂缝宽度0.2 mm下,受压、受拉区加固RC梁承载能力较未加固RC梁分别提高了1.4倍、6.5倍。提出UHPC加固RC梁极限抗弯承载力计算公式,该公式计算值与试验实测值吻合良好,且偏安全,可以指导UHPC加固RC梁设计。