在弯矩、剪力和反复扭矩复合作用下的碳纤维布加固RC箱梁抗扭性能试验研究

为研究碳纤维布加固弯矩、剪力和反复扭矩复合作用下的钢筋混凝土箱梁的抗扭性能,共设计制作了4根钢筋混凝土箱梁试件,其中3根采取碳纤维布加固、1根不加固作为对比试件。试验在自行研制的扭转试验装置上进行,对箱梁试件同步施加弯矩、剪力和反复扭矩作用。以加固方式和加固数量为主要研究参数,分析了箱梁试件的破坏机理、承载能力、变形能力和滞回性能等。通过各箱梁试件的碳纤维布和钢筋的应变变化规律,探讨了碳纤维布加固箱梁的抗扭工作机理;通过测得的各试件的扭矩-扭转角滞回曲线和骨架曲线,提出了碳纤维布加固钢筋混凝土箱梁的抗扭恢复力模型。从而为碳纤维布加固钢筋混凝土箱梁抗扭性能的理论研究和工程应用提供了重要的依据。     

Torsional strengthening of rectangular and flanged beams using carbon fibre-reinforced-polymers – Experimental study

The present experimental investigation deals with the torsional strengthening of concrete beams without stirrups using epoxy-bonded carbon fibre-reinforced-polymer (FRP) sheets and strips as external transverse reinforcement. The experimental program comprises 14 rectangular and T-shaped beams tested under pure torsion. Based on the measured values of the torsional moment at cracking and at ultimate, the corresponding twists, the behavioural curves and the failure modes of the beams, useful concluding remarks are indicated. The strengthened rectangular beams using full wrapping with continuous FRP sheets performed enhanced torsional behaviour and higher capacity than the strengthened beams with FRP strips. U-jacketed flanged beams exhibited premature debonding failure and substantial reductions of the potential torsional capabilities are reported. In general, FRP fabrics could effectively be used as external torsional reinforcement in under-reinforced concrete elements without steel transverse reinforcement.     

Thickness of shear flow zone in a circular RC column under pure torsion

This study proposes a rational method capable of analyzing the behavior of circular reinforced concrete (RC) columns under pure torsion with or without axial compression. The developed method is based on the concept of the rotating-angle softened truss model (RA-STM) and incorporates the recently updated material models. In particular, the most important factor in estimating torsional capacity of RC structures-thickness of shear flow zone-is investigated in-depth for the circular RC column and discussed together with other mechanical aspects. The concept of thickness of shear flow zone for rectangular RC members is theoretically well established, while it is not clearly understood for circular RC members. This is due to the lack of existing experimental and analytical research work in this area. Recently, a circular RC column under pure torsion was tested at Missouri S&T. Test results of the column were used to validate the proposed method in terms of overall column behavior and local behavior of each component (concrete, both longitudinal and transverse reinforcement). The comparisons proved that the proposed method was in reasonable agreement with experimental results. In addition, the concept of the proposed method can be applied for any arbitrary section and is free from mechanical assumptions such as concrete cover spalling.     

复材网格-UHTCC复合增强钢筋混凝土梁抗弯性能试验研究

文中进行7根复材（FRP）网格增强超高韧性纤维水泥基（UHTCC）复合加固钢筋混凝土梁的抗弯性能试验,将FRP网格类型、FRP网格增强率、FRP-UHTCC复合层黏结长度作为试验变量,分析各变量对FRP-UHTCC复合增强混凝土梁弯曲性能的影响。在试验研究的基础上,给出FRP-UHTCC复合增强混凝土梁的抗弯承载力计算方法。试验结果表明,FRP-UHTCC复合层与混凝土间没有发生相对滑移现象,可以有效抑制加固层端部剥离破坏,加固梁的破坏模式为FRP网格中纵向纤维筋被拉断破坏。BFRP格栅与UHTCC黏结基体没有发生脱黏现象,优于BFRP编织网与UHTCC的黏结效果。随着FRP网格增强率的增大,加固梁的抗弯承载力得到显著提高。与未加固的普通混凝土梁相比,加固梁的开裂、屈服和极限荷载最大提高幅度分别为97%、35%和33%。计算结果表明,预测值与试验值吻合较好,可以有效地预测FRP-UHTCC复合增强混凝土梁的抗弯承载力。     

碳纤维布加固弯剪扭复合受力的钢筋混凝土箱梁抗扭性能的模型试验

对碳纤维布(CFS)加固复合受力的钢筋混凝土箱梁的抗扭性能进行试验研究。设计制作4根矩形钢筋混凝土箱梁模型和1套抗扭试验装置,对其中3根箱梁采用不同方式的CFS加固,并对4根箱梁进行不同弯剪扭复合受力下抗扭性能的模型试验;根据试验结果对比分析加固后箱梁承载力的提高和刚度的变化,从而得出CFS的加固效果;深入研究加固后箱梁纵筋和箍筋的应力变化以及CFS的应变情况,从而得到CFS加固提高箱梁抗扭承载力的工作机理;应用变角度空间桁架模型对箱梁模型的抗扭承载力进行理论计算,并与试验结果进行比较,理论值与试验值符合较好,其比值的平均值为1.076,标准差为0.0692,变异系数为0.0643。最后得出结论,CFS加固能有效地提高弯剪扭复合受力下箱梁的抗扭承载力,利用变角度空间桁架模型能较精确地计算CFS加固的箱梁的抗扭承载力。     

碳纤维布加固弯剪扭复合受力的钢筋混凝土箱梁抗扭性能的理论分析

对碳纤维布(CFS)加固的弯剪扭复合受力的钢筋混凝土箱梁的抗扭性能进行理论分析。文中针对4根CFS加固弯剪扭复合受力的钢筋混凝土箱梁的抗扭性能的模型试验,基于修正的斜压场理论,集中考虑剪扭应力相加面的应力分布,并将混凝土软化系数提高25%,建立CFS加固弯剪扭复合受力的钢筋混凝土箱梁的抗扭承载力的计算方法。通过对4根箱梁试件的极限抗扭承载力的理论计算,并将计算结果与模型试验结果进行对比和分析,发现两者符合较好,其比值的平均值为0.9215,标准差为0.0320,变异系数为0.0347;对箱梁试件的试验破坏截面与理论破坏截面位置不一致的现象给出很好的解释;并与其他计算方法所得的计算结果取得一致。最后得出结论:修正的斜压场理论模型概念明确、安全可靠,可以作为CFS加固弯剪扭复合受力的钢筋混凝土箱梁的极限抗扭承载力的设计计算方法。     

Evaluation of debonding failure of reinforced concrete girders strengthened in flexure with FRP laminates using finite element modeling

The use of Fiber-Reinforced Polymer (FRP) materials dates back to the early 1940s when they were used in aerospace and naval applications. During the 1970s and early 1980s, FRP started being used in civil engineering applications for new construction, but more importantly for repair and strengthening of existing structures. However, experimental research showed that the typical failure mode of reinforced concrete (RC) structures strengthened with FRP composite materials is due to the debonding that occurs at the interface between concrete and FRP. The bond between FRP and concrete is therefore the key factor controlling the behavior of these structures since it limits the full use of the FRP strength. The paper evaluates the effect of the debonding failure on the response of FRP-strengthened RC beams. A nonlinear RC beam element with bond-slip between the concrete and the FRP laminates is developed and used to analyze several test specimens and to investigate their corresponding failure mode. The model was also used to study the reduction factor of FRP tensile strength of simply supported strengthened RC girders due to debonding failure. This reduction factor proved to be affected by several parameters: (a) the bond strength between FRP and concrete interface; (b) the concrete strength; (c) the thickness of FRP; (d) the modulus of FRP; (e) the width of FRP laminate; and (f) the development length of the FRP sheet. A large number of beam specimens were analyzed in order to conduct a thorough evaluation of debonding failure of RC beams strengthened with FRP laminates. Based on these studies, new equations that account for the aforementioned parameters were proposed to address the reduction in FRP strength due to debonding failure.     

Unified analytical approach for determining shear capacity of RC beams strengthened with FRP

This paper presents a rational model to predict the ultimate load capacity of reinforced concrete (RC) beams strengthened by a combination of longitudinal and transverse fiber reinforced polymer (FRP) composite plates/sheets (flexure and shear strengthening system). The model is based on the truss analogy and the theory of plasticity and is opportunely refined in order to incorporate some critical aspects, such as variable angle crack, non-uniform FRP stress distribution over the shear crack, shear span/depth ratio. It is a general and unified model that allows consideration of all the main possible failure mechanisms of strengthened RC beams, related to flexural-shear interaction, shear web-crushing and pure flexural mechanisms. The model is validated against a large number of beam tests reported in the literature, involving a wide range of geometrical and mechanical characteristics. The numerical investigation shows a very satisfactory correlation between predicted and experimental data.     

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。     

Strengthening of RC beams in shear using GFRP inclined strips – An experimental study

Though there have been a number of studies on shear strengthening of RC beams using externally bonded fiber reinforced polymer sheets, the behaviour of FRP strengthened beams in shear is not fully understood. This is partly due to various reinforcement configurations of sheets that can be used for shear strengthening and partly due to different failure modes a strengthened beam undergoes at ultimate state. Furthermore, the experimental data bank for shear strengthening of concrete beams using FRP remains relatively sparse due to which the design algorithms for computing the shear contribution of FRP are not yet clear. The objective of this study is to clarify the role of glass fiber reinforced polymer inclined strips epoxy bonded to the beam web for shear strengthening of reinforced concrete beams. Included in the study are effectiveness in terms of width and spacing of inclined GFRP strips, spacing of internal steel stirrups, and longitudinal steel rebar section on shear capacity of the RC beam. The study also aims to understand the shear contribution of concrete, shear strength due to steel bars and steel stirrups and the additional shear capacity due to glass fiber reinforced polymer strips in a RC beam. And also to study the failure modes, shear strengthening effect on ultimate force and load deflection behaviour of RC beams bonded externally with GFRP inclined strips on the shear region of the beam.     

钢-混凝土组合梁的受扭试验与分析

为了研究钢-混凝土组合梁的抗扭性能,完成了4根不同配箍率的组合梁纯扭试验,借助结构分析软件,采用三维8结点实体单元的有限元模型,对组合梁纯扭试件在弹性阶段的变形、截面应力分布情况进行了分析。试验和分析结果表明:受扭承载力主要由混凝土翼板提供,翼板的截面尺寸是影响极限承载力的主要因素,其中翼板厚度对组合梁受扭承载力的影响更为显著;而配箍率对组合梁受扭承载力的影响并不大。其它条件相同时,当配箍率为0.54%左右时,组合梁受扭承载力将达到最大。提出了组合梁弹性抗扭刚度计算公式和开裂扭矩计算公式;本文采用变角空间桁架模型,并结合已有的试验成果,提出了可供设计参考的极限承载力计算公式。公式计算结果与试验值吻合良好。     

RC beams shear-strengthened with FRP: Stress distributions in the FRP reinforcement

Reinforced concrete (RC) beams may be strengthened for shear with externally bonded fibre reinforced polymer (FRP) composites through complete wrapping, U-jacketing or bonding on their sides only. The two main shear failure modes of such strengthened beams are FRP rupture and debonding. In both modes of failure, the contribution of the bonded FRP reinforcement to the shear capacity of the beam depends strongly on the stress (or strain) distribution in the FRP at the ultimate limit state. This paper presents a numerical study of the FRP stress distribution at debonding failure in U-jacketed or side-bonded beams using a rigorous FRP-to-concrete bond–slip model and assuming several different crack width distributions. Numerical results indicate that Chen and Teng’s early simple assumption [Chen JF, Teng JG. Shear capacity of FRP-strengthened RC beams: FRP debonding. Constr Build Mater 2003;17:27–41] for the stress distribution in the FRP results in satisfactory predictions for the effective FRP stress in most cases for both U-jacketed and side-bonded beams. However, it may become unconservative for side-bonded beams that have only light flexural steel reinforcement.     

Shear capacity of FRP-strengthened RC beams: FRP debonding

Many studies have been undertaken on shear strengthening of reinforced concrete (RC) beams by externally bonding fibre-reinforced polymer (FRP) composites. These studies have established clearly that such strengthened beams fail in shear mainly in one of two modes: FRP rupture; and FRP debonding, and have led to preliminary design proposals. This paper is concerned with the development of a simple, accurate and rational design proposal for the shear capacity of FRP-strengthened beams which fail by FRP debonding. Existing strength proposals are reviewed and their deficiencies highlighted. A new strength model is then developed. The model is validated against experimental data collected from the existing literature. Finally, a new design proposal is presented.     

考虑混凝土贡献的修正变角桁架模型

为提高有腹筋RC（钢筋混凝土）浅梁抗剪承载力的预测准确性和稳定性,提出一种考虑混凝土贡献的修正变角桁架模型。首先,基于变角桁架模型得到箍筋的抗剪贡献值计算公式,并结合塑性下限定理确定斜裂缝倾角的取值|其次,通过混凝土浅梁受压区的劈裂破坏模式得到混凝土项的抗剪贡献值计算公式,并利用尺寸效应系数考虑了混凝土项的脆性破坏特征|最后,将箍筋项和混凝土项提供的抗剪承载力进行叠加,建立有腹筋RC浅梁抗剪承载力计算公式。基于有腹筋RC浅梁抗剪承载力试验数据库ACI-DAfStb对本文、GB50010、ACI及AASHTO规范中抗剪计算公式的预测准确性进行对比评估,结果表明：文中抗剪承载力公式不仅能够体现配筋率、剪跨比和混凝土强度等主要抗剪参数的影响规律,还能考虑尺寸效应和斜裂缝倾角对抗剪承载力的修正|同时,在抗剪承载力预测准确性和稳定性方面,相对规范方法有所较高。     

玄武岩纤维布加固钢筋混凝土梁受弯受剪承载力计算分析

基于现行混凝土结构加固设计规范关于粘贴纤维增强复合材料加固法的计算理论,分别对玄武岩纤维布和碳纤维布加固钢筋混凝土梁的正截面受弯承载力、斜截面受剪承载力进行计算分析,以探讨玄武岩纤维布对钢筋混凝土梁的加固效果。计算结果表明,设计人员可以方便地按现行加固设计规范对玄武岩纤维受弯、受剪加固进行计算。     

碳纤维增强复合网格-聚合物水泥砂浆加固RC梁的抗弯性能试验研究

为研究碳纤维增强复合(CFRP)网格和聚合物水泥砂浆(PCM)复合加固钢筋混凝土(RC)梁的抗弯性能,对5个RC梁试件进行抗弯性能试验,分析CFRP网格-PCM复合加固RC梁的抗弯破坏机理,研究网格不同层数和不同单位加固量对RC梁抗弯性能的影响。基于抗弯承载力的既有计算模型,引入剥离应变建立改良计算模型,并采用其他学者的9根FRP网格加固RC梁的受弯试验数据,验证改良计算模型的准确性。研究结果表明：CFRP网格-PCM对RC梁的抗弯加固效果明显,单位加固量较高的试件具有更高的承载能力,但其更易发生剥离破坏;在单位加固量相当的条件下,单层网格与双层网格呈现出相同的抗弯性能,双层网格重叠布置的加固方式是有效的;抗弯承载力的既有计算模型对试验结果拟合效果较差,所建立的改良计算模型拟合程度较好,能更好地反映CFRP网格-PCM复合加固层的实际受力状态。     

Investigation of the parameters affecting the behavior of RC beams strengthened with FRP

Three-point bending tests were carried out on nineteen Reinforced Concrete (RC) beams strengthened with FRP in the form of completely wrapping. The strip width to spacing ratios, FRP type, shear span to effective depth ratios, the number of FRP layers in shear, and the effect of stirrups spacing were the parameters investigated in the experimental study. The FRP contribution to strength on beams having the same strip width to spacing ratios could be affected by the shear span to effective depth ratios and stirrups spacing. The FRP contributions to strength were less on beams with stirrups in comparison to the tested beams without stirrups. Strengthening RC beams using FRP could change the failure modes of the beams compared to the reference beam. In addition to the experimental study, a number of equations used to predict the FRP contribution to the shear strength of the strengthened RC beams were assessed by using a limited number of beams available in the literature. The effective FRP strain is predicted by using test results, and this prediction is used to calculate the FRP contribution to shear strength in ACI 440.2R (2017) equation. Based on the statistical values of the data, the proposed equation has the lowest coefficient of variation (COV) value than the other equations.     

FRP加固SRHC梁界限破坏时抗弯承载力计算

提出FRP(纤维增强复合材料)加固SRHC(钢骨高强混凝土)受弯构件的概念,解决FRP加固后SRHC柱的破坏机理、受力性能问题,提出其界限破坏时的简化抗弯承载力计算公式。在以往FRP加固RC(钢筋混凝土)梁力学性能分析结果的基础上,采用新的叠加方法对FRP加固SRHC梁的受力过程、破坏特征、受力特点进行研究。根据中和轴与钢骨的截面位置不同,建立了相应的抗弯承载力计算方法,并提出了界限破坏时的受压区高度。将受压区高度与钢骨上翼缘至混凝土受压边缘的距离进行比较,以此来确定FRP加固后梁在界限破坏时的不同抗弯承载力的适用公式,给实际应用带来方便。     

CFRP片材加固混凝土梁收缩徐变效应研究

按换算截面法推导了FRP片材加固混凝土梁的短期与长期挠曲变形计算公式,在此基础上编制了相应的分析计算程序,该分析方法能较准确预测开裂混凝土梁的长期挠曲变形。制作了10根钢筋混凝土梁,其中6根为普通钢筋混凝土梁,另外4根为CFRP片材加固梁,并对试件的短期与长期受力性能进行了试验观测,持续荷载作用下对试件进行了近700d的收缩徐变试验。运用本文计算方法预测了试件的长期挠曲变形,理论计算值与本文试验结果吻合较好。研究表明:碳纤维片材对混凝土梁的长期挠曲变形影响很小,但碳纤维片材能有效地控制持续荷载作用下裂缝的进一步开展。研究结果可为CFRP加固混凝土结构工程设计与理论分析提供参考。     

碳纤维增强复合网格-聚合物水泥砂浆加固RC梁抗剪性能试验研究

为研究碳纤维增强复合(CFRP)网格和聚合物水泥砂浆(PCM)复合加固工字形截面钢筋混凝土(RC)梁的抗剪性能,对3个试件进行抗剪性能试验和有限元模拟,分析了CFRP网格-PCM加固RC梁的抗剪破坏机理,研究了不同加固方式对试件抗剪性能的影响。研究结果表明：采用CFRP网格-PCM对RC梁进行抗剪加固可有效抑制斜裂缝的发展,能够较大幅度提高RC梁的抗剪承载力;相比仅腹部加固的试件,腹部和腋部都加固的试件的二次刚度、极限荷载均有所提高,且CFRP网格变形减小,与混凝土界面的黏结能力增强;在有限元分析中,采用混凝土的CDP模型和Spring2弹簧单元来预测CFRP网格加固混凝土的抗剪承载力是可行的;建立了基于杆状材料有效应变的抗剪计算方法,该方法可有效预测加固RC梁的抗剪承载力,为结构加固设计提供参考。