复杂应力状态对混凝土梁外贴FRP条带抗剪贡献的影响

FRP剥离是外贴FRP抗剪加固混凝土梁主要的破坏模式之一。以往研究中往往简单的将面内剪切试验得到的FRP-混凝土界面粘结滑移关系应用于外贴FRP抗剪加固梁的剥离承载力计算。外贴FRP抗剪加固梁中FRP下的混凝土的应力状态与面内剪切试验情况有较大差别,这对FRP-混凝土界面的力学性能具有较大的影响。因此,以往的方法高估了FRP条带的抗剪贡献。该文研究了混凝土多轴应力状态对FRP-混凝土界面性能的影响,并根据试验研究结果,提出了U形FRP加固混凝土梁中FRP剥离应变的折减系数。与试验结果的对比计算分析表明:使用该折减系数修正后的设计公式更加合理。     

环境温差下FRP-混凝土界面粘结行为分析

为明确环境温差对纤维增强聚合物(FRP)加固混凝土构件的界面粘结性能的影响,基于粘结界面的双参数内聚力指数模型,建立了FRP-混凝土粘结结点在温差作用下的界面微分平衡方程,采用边界条件叠加的方法,解析推导了界面相对滑移、界面剪应力和FRP应力-应变分布计算公式。基于解析理论模型,提出了FRP-混凝土界面最大承载温差的计算方法,分析了粘结长度、温差变化和粘结层数对界面粘结性能的影响。结果表明:该文推导出的理论公式计算结果与界面试验结果吻合良好,建立的解析理论模型能够较好地预测温差作用下FRP-混凝土界面粘结行为;界面最大承载温差随粘结长度的增加存在上限值,且由于界面粘结性能的退化,FRP温度应力的最大值出现在达到界面最大承载温差之前;界面剪应力集中在粘结端部区域,受温差变化和FRP粘结层数影响较大,且当环境温差进入胶黏剂玻璃化转变区域后影响最为明显。该结论可用于强日照和高温车间等大温差环境下桥梁和建筑加固构件的温度应力分析和界面承载力评估。     

FRP片材与混凝土粘结性能的精细有限元分析

FRP-混凝土界面粘结性能是外贴FRP片材加固混凝土结构技术的关键问题。基于FRP与混凝土界面面内剪切试验,采用精细单元有限元模型对其界面粘结性能进行了研究。在该模型中,混凝土和FRP片材都使用非常小的单元加以模拟,通过调整混凝土材料的本构模型来考虑单元尺寸的影响。FRP单元和混凝土单元直接连接,通过混凝土单元的断裂破坏来模拟FRP和混凝土界面的宏观剥离破坏过程。通过与大量面内剪切试验结果对比,验证了该精细有限元模型的正确性,并基于精细有限元分析结果,对界面剥离破坏机理进行了讨论。     

FRP布加固混凝土框架子结构抗连续倒塌的精细有限元分析EI北大核心CSCD

外贴FRP布加固是一种有效提高既有建筑抗连续倒塌性能的手段,但现有FRP布加固方式存在降低结构抗震性能、加固施工不便等缺点。该文采用数值模拟方法分析了FRP布加固方式对现浇和装配式混凝土框架子结构抗连续倒塌与抗震性能的影响,并开展了优化方案研究。基于通用有限元软件LS-DYNA建立了FRP布加固混凝土框架子结构的连续倒塌精细数值模型,其中混凝土、钢筋与FRP布分别采用实体、梁与壳单元进行模拟,考虑了FRP布和钢筋的滑移、新旧混凝土界面的粘结失效和机械套筒处的钢筋截面损失。试验验证表明该方法可准确模拟试验试件的破坏模式和承载力发展。分析试验试件的不同粘贴方案结果发现:对现浇混凝土子结构,梁底与梁侧中性轴粘贴纵向FRP布并在梁端塑性铰区粘贴U形横向FRP布后,小变形下的结构倒塌抗力提升有限(最大仅2.6%)、基本不影响结构抗震性能,而对大变形下的结构倒塌抗力提升幅度可达49.5%;对于装配式混凝土子结构,在梁底、梁顶与梁侧底部外贴纵向布并在梁端塑性铰区粘贴U形横向FRP布可将小变形和大变形下的结构抗力最大提升24.2%和48.1%,使得装配式子结构在小变形下受力等同现浇结构,提升了原装配式子结构的抗震性能。对上述最优方案进一步的分析表明:保持FRP布用量不变而将塑性铰区内U形横向FRP布的分布范围和条数增加可提高大变形下的结构倒塌抗力,而不影响小变形下的加固效果。     

海水浸泡对FRP筋-珊瑚混凝土粘结性能的影响

开展了30℃海水浸泡条件下玻璃纤维增强树脂基复合材料（GFRP）筋、碳纤维增强树脂基复合材料（CFRP）筋与珊瑚混凝土粘结性能的试验研究,分析了纤维增强树脂基复合材料（FRP）筋-珊瑚混凝土粘结滑移曲线特征、破坏形态及粘结强度变化。试验结果表明,海水浸泡后FRP筋力学性能和粘结性能均表现为不同程度的降低。随浸泡时间增加,GFRP筋表层树脂与纤维间的孔隙率明显增大,并逐渐出现脱粘现象,纤维本身遭受到侵蚀,而CFRP筋仅表面基体有少许损伤,其耐久性明显优于GFRP筋;FRP筋-珊瑚混凝土粘结强度呈现出先增加后减小的趋势,且后期下降速率逐渐变小,部分GFRP筋-珊瑚混凝土试件的破坏模式逐渐由筋被拔出转变为筋材断裂;增加保护层厚度能有效地减缓海水对GFRP筋的侵蚀,有利于保持GFRP筋-珊瑚混凝土间的粘结性能。     

FRP加固混凝土梁粘结层剪应力分析

外贴FRP以提高混凝土结构抗弯承载力的加固技术的有效性在很大程度上取决于混凝土梁与FRP板之间的界面行为,这也是过去很多研究的重点。特别是针对FRP加固混凝土梁粘结层剪应力提出了很多解析解,但是这些研究很少适用于不对称荷载情形。该文运用最小余能原理得到了FRP加固简支混凝土梁粘结层剪应力分布的解析公式。此公式不仅适用于对称荷载作用的情形,还适用于两端支座处作用不等弯矩这种荷载情形。通过与已有结果进行比较,验证了该文方法的正确性。     

抗剪加固FRP与混凝土界面粘结性能的试验研究

为了深入理解抗剪加固FRP与混凝土界面的粘结性能,该文对侧面粘贴FRP的21根预置裂缝小梁进行了试验研究。试验设计考虑了FRP粘结长度、宽度、厚度及FRP纤维方向与裂缝张开方向的角度(简称“FRP纤维受拉角度”,用θ表示)对FRP与混凝土界面粘结性能的影响。试验结果表明:1) 所用的预置裂缝小梁可以有效对FRP与混凝土的界面粘结性能进行试验研究;2) FRP粘结长度、宽度、厚度及纤维受拉角度等因素对粘结强度有明显影响;3) FRP应变沿宽度方向的分布是不均匀的,这主要是由于裂缝不均匀开展导致FRP在裂缝开展快的一边先剥离。     

预应力FRP加固工程结构技术研究进展

围绕国内外预应力FRP加固技术的研究现状以及最新进展,从预应力FRP加固混凝土结构、预应力FRP加固钢结构、预应力FRP加固中关键技术的研究等方面进行了综述。试验研究表明：预应力FRP加固混凝土能显著提高构件的开裂荷载、屈服荷载和极限荷载,改善受弯构件在长期荷载的力学性能,提高构件的疲劳寿命;预应力CFRP加固钢梁后,其屈服荷载和极限荷载相对于对比梁都有明显的提高,其提高的程度随着预应力CFRP的用量和预应力水平的提高而增大;预应力CFRP加固对钢梁的刚度提高作用也比较明显,对低强度的钢材,提高效果更明显;采用预应力FRP加固工程结构的关键问题在于预应力的施加体系、预应力控制值、预应力损失和端部的锚固。     

GFRP-混凝土组合板界面抗剪连接性能的试验研究

FRP-混凝土组合板界面的可靠连接是保证其正常工作的基础。为考察组合板中的界面粘结性能,该文对采取4种连接方式的27个玻璃FRP（GFRP）-混凝土组合构件进行双剪推出试验,根据试验结果分析构件的破坏模式、承载力变化及界面滑移情况。研究结果表明:4种连接方式均获得了较高的粘结强度,可考虑作为GFRP-混凝土组合板的界面抗剪构造措施;GFRP-混凝土界面粘结强度与混凝土强度等级、界面粗糙程度、胶结剂的粘结能力、剪力键布置方式有关,且随混凝土强度等级的提高而有所增大。基于该文的试验设计和试验结果,对试验中的3个试件进行了三维非线性有限元分析。     

预应力碳纤维板加固受弯构件的试验研究

传统外部粘贴碳纤维增强复合材料加固技术无法充分发挥材料性能,加固效果有限。预应力碳纤维加固技术可有效解决上述缺陷,成为目前碳纤维加固研究的重点。作者研发了对碳纤维板进行预应力加固的设备,在此基础上进行了8根采用外部粘贴预应力碳纤维板加固的受弯构件的模型试验,研究了预应力碳纤维加固对试件承载力、使用阶段变形、碳纤维应用效率及延性的影响。试验结果表明:预应力碳纤维板加固可显著提高受弯构件开裂及屈服荷载,减小构件使用阶段内的变形,充分利用碳纤维材料性能;通过仔细设计构件延性可满足工程需要。     

Failure diagrams of FRP strengthened RC beams

Amongst various methods developed for strengthening and rehabilitation of reinforced concrete (RC) beams, external bonding of fibre reinforced plastic (FRP) strips to the beam has been widely accepted as an effective and convenient method. The experimental research on FRP strengthened RC beams has shown five most common modes, including (i) rupture of FRP strips; (ii) compression failure after yielding of steel; (iii) compression failure before yielding of steel; (iv) delamination of FRP strips due to crack; and (v) concrete cover separation. In this paper, a failure diagram is established to show the relationship and the transfer tendency among different failure modes for RC beams strengthened with FRP strips, and how failure modes change with FRP thickness and the distance from the end of FRP strips to the support. The idea behind the failure diagram is that the failure mode associated with the lowest strain in FRP or concrete by comparison is mostly likely to occur. The predictions based on the present failure diagram are compared to 33 experimental data from the literature and good agreement on failure mode and ultimate load has been obtained. Some discussion and recommendation for practical design are given.     

RC beams strengthened with NSM CFRP rods and modeling of peeling-off failure

The aim of the experimental programme developed in this work was to investigate the possibility of using Carbon Fibre-Reinforced Polymer (CFRP) rods to strengthen concrete structural members with the Near Surface Mounted reinforcement (NSM) technique. The global behaviour of reinforced cantilever concrete beams strengthened by the NSM technique and subjected to flexure is investigated. The specific problem of cantilever beams (strengthening outward pressure) was studied. The global behaviour of the cantilever concrete beams was compared with that of beams subjected to flexure with four points load test. A carbon–epoxy pultruded FRP (CFRP) rod of 6 mm in diameter was used. The study was carried out up to the failure load, and focused on the modifications in mechanical behaviour, cracking and failure mode of the beams. An analytical and Finite Element models to predict the peeling-off failure mode were compared.     

Analytical approach for the flexural analysis of RC beams strengthened with prestressed CFRP

The objective of this paper is to propose a simplified analytical approach to predict the flexural behavior of simply supported reinforced-concrete (RC) beams flexurally strengthened with prestressed carbon fiber reinforced polymer (CFRP) reinforcements using either externally bonded reinforcing (EBR) or near surface mounted (NSM) techniques. This design methodology also considers the ultimate flexural capacity of NSM CFRP strengthened beams when concrete cover delamination is the governing failure mode. A moment–curvature (M–χ) relationship formed by three linear branches corresponding to the precracking, postcracking, and postyielding stages is established by considering the four critical M–χ points that characterize the flexural behavior of CFRP strengthened beams. Two additional M–χ points, namely, concrete decompression and steel decompression, are also defined to assess the initial effects of the prestress force applied by the FRP reinforcement. The mid-span deflection of the beams is predicted based on the curvature approach, assuming a linear curvature variation between the critical points along the beam length. The good predictive performance of the analytical model is appraised by simulating the force–deflection response registered in experimental programs composed of RC beams strengthened with prestressed NSM CFRP reinforcements.     

Strengthening RC beams with composite fiber cement plate reinforced by prestressed FRP rods: Experimental and numerical analysis

This paper deals with the development of a new strengthening system for reinforced concrete beams with externally-bonded plate made of composite fiber cement reinforced by rebars made of fiber-reinforced plastic (FRP) [1]. The proposed strengthening material involves the preloading of FRP rod before mortar casting. The paper presents experimental and numerical analysis carried out on many large-scale beams strengthened by well-known reinforcement techniques, such as externally bonded Carbon Fiber-Reinforced Plastic (CFRP) plate and the Near Surface Mounted (NSM) technique, which are compared to the proposed new strengthening material through four-point bending tests. Results are analyzed with regard to the load-displacement curve, bending stiffness, cracking load, yield strength and failure load. The developed numerical model is in agreement with the experimental results. It clearly shows the effects of prestressed FRP rod on cracking mechanisms and internal strength distribution in the analyzed beams.     

Assessment of CFRP strengthened RC beams through dynamic tests

The use of Fiber Reinforced Polymers (FRPs) for strengthening damaged RC beams has become common practice over recent years. Two methods adopted for repairing or strengthening such beams are FRP plates/sheets glued onto their concrete surface and FRP rods or strips inserted into grooves.This paper investigates experimental vibration monitoring of strengthening according to the two aforementioned methods through dynamic tests on six RC beam models strengthened using carbon FRP. Three beams were strengthened applying CFRP sheets on the tensile cracked surface after loading and three beams were strengthened by near surface mounted (NSM) CFRP rods. The experimental results include both the static tests to create damage and the dynamic tests of strengthened, measuring natural vibration modes and frequency values for free end beams.Comparison between experimental dynamic response and static behavior established that vibration monitoring is a convenient, non-destructive method for assessing strengthened beams under service loads. Further studies and tests must be developed in order to solve the issues that emerged following the analysis of the experimental data obtained.     

预应力铝合金筋嵌入式补强钢筋混凝土梁裂缝分析与计算

完成了7根预应力7075铝合金筋嵌入式补强混凝土梁试件的四点弯曲静载试验,应用非接触式数字图像相关法对混凝土加固梁的裂缝形成、分布、裂缝宽度和间距进行分析,研究了铝合金加固量、预应力以及预应力水平对嵌入式补强混凝土梁试件破坏模式和裂缝特性的影响。试验研究表明:铝合金筋嵌入式补强法可以显著提高混凝土梁的承载能力,施加预应力进一步增强加固梁的强度并延缓混凝土开裂和钢筋屈服;端部锚固有效避免了加固梁试件发生剥离破坏,提高高强铝合金强度利用率;施加预应力、增大加固量和提高预应力水平,均可以有效控制裂缝扩展,减小裂缝宽度和间距;根据中国《混凝土结构设计规范》(GB 50010?2010)对嵌入式非预应力/预应力铝合金筋补强混凝土梁的裂缝宽度和分布进行了计算,理论计算值与试验结果吻合良好,结果表明:中国混凝土结构设计规范给出的正常使用状态下最大裂缝宽度计算方法能够较好地考虑预应力、加固筋数量以及预应力水平对最大裂缝宽度的影响,适用于嵌入式补强钢筋混凝土受弯构件的裂缝计算与分析。     

Ductile strengthening using externally bonded and near surface mounted composite systems

Externally bonded fiber reinforced polymers (FRP) has been established as an effective technique for strengthening concrete members. Other techniques, like near surface mounted (NSM) FRP bars, and steel reinforced polymers (SRP) have emerged as viable alternatives. In this study, four composite-based strengthening systems were used to provide equivalent flexural performance, namely: externally bonded CFRP sheets, NSM prefabricated CFRP strips, externally bonded SRP sheets and NSM stainless steel bars. The strengthening design was based on achieving approximately 38% increase in flexural capacity over the unstrengthened control beams. The mode of failure by design was brittle failure controlled by concrete crushing at 0.003 strain. However, the experimental program was designed to demonstrate the effectiveness of transverse anchoring reinforcement to control premature debonding failure modes and fully utilize the high strength of the composite systems. A more ductile behavior was also observed as a result of transverse strengthening and concrete confinement effects. Accordingly, an increase of approximately 50% in flexural strength is accomplished.     

Experimental investigation on fire endurance of insulated concrete beams strengthened with near surface mounted FRP bar reinforcement

Fiber reinforced polymer (FRP) composites are known to be susceptible to deterioration at elevated temperature. To evaluate the feasibility of achieving a fire-rated FRP system an investigation was undertaken to examine and document the performance of near surface mounted (NSM) FRP strengthened concrete beams under fire conditions. Twelve reinforced concrete beams were strengthened in flexure with NSM FRP bars and insulated with different insulation systems. The specimens were subsequently exposed to a standard fire while subjected to full service load. Tests results on fire indicated that insulated NSM FRP strengthened beams can achieve a fire endurance of at least 2 h. Moreover structural testing to failure at room temperature of the fire testes beams has shown that well insulated members are able to retain (part of) their original strengthened flexural capacity.     

预应力FRP加固混凝土结构技术研究与应用

介绍了笔者进行的预应力芳纶纤维布和碳纤维筋加固混凝土结构的一些主要研究成果,内容包括:预应力芳纶纤维布永久锚具的开发;预应力芳纶纤维布的应力松弛损失研究;预应力芳纶纤维布加固混凝土梁的受弯、受剪性能研究;温度对芳纶纤维布配套粘结材料的力学性能影响研究;体外预应力碳纤维筋局部加固混凝土梁的力学性能研究;碳纤维筋预应力粗纤维混凝土梁的抗震性能研究;预应力纤维布加固混凝土结构的工程应用等。     

预应力CFRP加固钢筋混凝土受弯构件的抗力概率模型研究

预应力CFRP 加固混凝土结构技术因其在材料性能利用方面的优越性能已成为CFRP 加固的热点方向,其中预应力CFRP 加固结构的可靠性是这一方向研究的重要内容。该文分析了影响预应力CFRP 板加固钢筋混凝土受弯构件承载力的主要变量的概率特征,考虑CFRP 板尺寸效应和应力分布的影响,采用Weibull 分布推导了CFRP 板的极限强度概率分布函数,根据预应力CFRP 张拉工艺,分析了预应力损失随机变量,建立了在不同失效模式(破坏形态)下的受弯构件抗力概率模型,并开展了参数敏感性分析,获得各个失效模式下抗力概率模型的主要影响因素。研究结果表明:抗力概率模型是预应力CFRP 加固结构的失效概率计算与可靠度校准的重要内容之一,各参数的影响规律与各失效模式的破坏形式密切相关。