钢筋混凝土矩形空心墩柱的FRP约束效应研究

为了正确评价纤维增强复合材料(FRP)约束效应对钢筋混凝土(RC)空心墩柱受力性能的影响,首先采用有限元数值模拟方法,对FRP类型、厚度、幅宽和间距参数影响下约束RC矩形空心柱的轴压荷载、约束混凝土压应变、箍筋和FRP拉应变的变化规律进行研究。基于模拟结果,提出一种考虑FRP布环箍和箍筋共同约束效应的侧向约束压力计算公式,进而建立FRP约束混凝土压应力-应变模型。进一步应用提出的FRP约束混凝土模型评价FRP加固矩形空心桥墩的受力性能,并与试验结果对比,两者吻合较好。     

RC桥墩的FRP延性加固设计方法研究

为了制定经济合理的钢筋混凝土(RC)墩柱的纤维增强复合材料(FRP)抗震延性快速加固方案,提出了基于目标位移延性系数提高指标的FRP抗震延性加固设计方法,并建立了加固墩柱宏观力学性能参数(位移、延性系数)与材料性能参数(FRP约束混凝土极限压应变、FRP配箍率、FRP有效极限抗拉强度、FRP有效极限拉应变)之间的力学计算模型。进一步应用所提出的计算模型,结合有限元数值模拟技术,得到FRP约束圆形和矩形墩柱受力性能的分析结果,并与试验结果进行对比。结果表明,提出的FRP延性加固设计方法可较正确地计算加固RC墩柱的受力性能、FRP片材配箍率或加固厚度,并验证了考虑箍筋和FRP共同约束作用的Seible FRP约束混凝土极限压应变计算模型的合理性。     

影响FRP约束混凝土柱力学性能的参数分析

为建立FRP约束混凝土柱合理的设计方法,必需明确各因素对FRP约束混凝土柱力学性能的影响。本文归纳了已有的影响FRP约束混凝土柱力学性能的参数的研究成果,并分析了研究中存在的不足。通过对大量研究成果的总结分析,探讨了FRP约束混凝土柱有待进一步解决问题,并展望了FRP约束混凝土柱的力学性能的发展前景。     

倒角半径与加载速率对FRP弱约束方形混凝土柱的承载能力影响

为研究倒角半径和应变速率对纤维增强复合材料(FRP)弱约束混凝土方柱抗压强度的影响规律,对20个FRP弱约束混凝土方柱进行了不同加载速率的轴压试验,探讨混凝土柱倒角半径和应变速率对FRP约束混凝土力学性能的影响,并绘制了相应的应力-应变关系曲线。结果表明:随着试件倒角半径的增加,FRP弱约束混凝土的抗压强度呈现增大的趋势;随着应变速率的增大,FRP弱约束混凝土的抗压强度也呈现增大的趋势;并采用试验结果对现有的抗压强度模型进行评估,发现不同加载速率下FRP强约束混凝土圆柱的抗压强度模型可以用于预测不同加载速率下FRP弱约束混凝土方柱的抗压强度。     

BFRP约束方形混凝土柱轴心受压强度模型

目前,FRP(纤维增强复合材料)约束混凝土强度模型大部分都是建立在FRP约束混凝土圆形柱基础之上.而FRP约束混凝土方形柱的侧向约束力受截面形状的影响较大,与圆形柱相比,由于拐角的应力集中导致的侧向约束应力不均匀等,使得FRP包裹方形柱的侧向约束效果明显降低,强度也随之下降.本文基于FRP约束圆柱体的强度公式,考虑方形柱的有效核心区、拐角效应的影响,引入对应的影响系数,建立了BFRP(玄武岩纤维复合材料)约束方形混凝土柱轴心抗压强度模型并与试验数据对比分析,结果表明,该模型具有良好的吻合度.     

基于贝叶斯理论的FRP约束矩形混凝土柱轴压极限强度研究

基于贝叶斯理论的模型参数识别方法,不仅可以考虑模型误差的影响,还可以得到模型中参数的最有可能值,同时定量描述各参数的不确定性。为获得更合理的FRP约束矩形(含方形)混凝土柱轴压极限强度模型,广泛整理了已有试验数据,并对已有极限强度模型进行评估,归纳出四种典型的应力-应变曲线形态以及三种典型的极限强度模型形式。在对已有平均断裂应变率计算公式进行修正的基础上,进一步建立了基于贝叶斯理论的极限强度模型参数识别框架,采用MATLAB编程获取到模型参数最有可能值和协方差矩阵。基于所识别的最有可能值,进一步优化得到了与FRP约束圆形混凝土柱轴压强度模型相统一的模型。与部分已有极限强度模型对比发现,新提出的模型在强度预测上更为准确,模型形式更加科学合理。     

约束钢筋混凝土矩形空心墩的FRP布拉应变试验研究

为了确定影响约束钢筋混凝土(RC)空心墩的纤维增强复合材料(FRP)有效拉应变的参数,建立其正确的计算模型,对10个FRP约束RC矩形空心墩进行了恒轴压、水平单向反复荷载作用下的拟静力试验。分析了不同约束试验空心墩的FRP拉应变实测值,结果表明:影响FRP布拉应变的参数包括约束区损伤位置、FRP类型和配箍率、剪跨比和约束墩的破坏模式。与不同FRP有效拉应变计算值进行对比,验证了弯曲破坏下约束矩形空心墩的FRP实测值与剥离破坏下FRP应变计算结果较接近,而与环向包裹粘贴或FRP拉断破坏模式下的FRP计算值相差较大。     

FRP约束钢管混凝土长柱承载力研究

已有研究表明,钢管混凝土柱和FRP约束混凝土柱承载力的计算方法不适用于FRP约束钢管混凝土柱承载力的计算。为研究FRP约束钢管混凝土长柱的承载力,本文以钢管约束效应系数ξs和FRP约束效应系数ξf为主要参数,考虑混凝土强度等级对FRP约束钢管混凝土柱承载力提高效果的调整系数,建立了FRP约束钢管混凝土短柱承载力的计算模型。在此基础上,分别引入钢管混凝土稳定系数φs和FRP约束混凝土柱稳定系数φf,建立FRP约束钢管混凝土长柱的承载力计算模型。并将模型的计算结果与试验数据进行比较。     

FRP环向加固木柱轴心受压性能试验研究

提供了15根FRP环向加固木柱的轴心抗压性能试验数据,详细探讨了受载后试件的工作机理和破坏模式,试件的设计参数为FRP的层数和FRP的类型,分析了各设计参数对加固木柱承载力和峰值应变的影响。试验结果表明,FRP环向加固木柱可提高木柱的抗压承载力,改善木柱的延性。在极限荷载以前,加固木柱的荷载-应变关系曲线基本保持线性变化,在极限荷载以后曲线为近似理想塑性。加固木柱的承载力和峰值应变随加固层数的增加而增加。3层GFRP可提高木柱承载力和峰值应变分别达21.82%和94.95%。试件的极限荷载和轴向应变随环向FRP的弹性模量的增加而增加,但增幅逐渐变缓。加固木柱达到极限荷载时,环向加固层没有出现拉断现象,其环向应变并未达到环向加固层的极限应变,仅为FRP极限拉应变的10%左右。木柱的破坏始于木纤维的弯曲变形,环向FRP可有效约束这种变形的发展,这是改善木柱轴心受压性能的主要原因。所有试件的破坏模式都表现为木柱产生错动变形,被完全压皱破坏。     

反复荷载作用下FRP约束混凝土柱受压性能的研究综述

FRP作为一种新型高性能材料,由于其可以增加构件的强度和延性,近年来在土木工程中应用广泛,尤其适用于结构抗震加固。目前,国内外对FRP约束混凝土柱的研究主要集中于单向荷载作用,而对反复荷载作用下的研究相对较少,因此对反复荷载作用下FRP约束混凝土的受压性能进行更加深入的研究具有重要的理论意义和工程实践价值。总结了国内外学者关于FRP约束素混凝土柱和钢筋混凝土柱在反复荷载作用下受压性能的研究成果,得出了以下结论:1单向荷载作用下的应力-应变曲线可以作为反复荷载下的包络线;2反复加卸载会造成应力损失,唯一性理论不成立;3FRP布的约束效果随着截面尺寸的增加而减小。最后对今后拟开展的研究工作提出了建议与展望。     

Effectiveness of cement and plaster layers in protection of FRP confined concrete exposed to high temperatures

In recent years, fiber-reinforced polymers (FRPs) materials have shown great potential as materials for repair and reinforced concrete structures such as beams or columns by externally bonding FRP sheet(s) onto the surface of substrate concrete structures. However, the performance of FRP systems exposed to fire is a serious concern due to the combustibility of FRPs. This study introduces the results of an experimental investigation on the behavior of the circular columns of concrete under a load of axial compression, confined by an envelope of composite materials (carbon fiber and glass fiber) and protected by a layer of mortar cement or plaster coating, after they have been subjected at various temperature (23, 120, and 350 °C). The specific objectives of this study are verifying the applicability and the effectiveness of the proposed technique to improve the behavior of concrete in fire resistance and evaluate the effect of composite materials and the layer coating type used. The results indicated that protecting heat circular confined columns, with a layer of mortar cement or plaster has a significant effect on the axial strength and the ductility. It was shown that the ultimate load and axial strain of heated columns can be restored up to the original level or greater than those of unheated columns. However, the effect of a layer of plaster is more significant than a layer of mortar cement. So this coating system would enhance fire resistance of the FRP, safety and reliability of FRP reinforced concrete structures.     

Behavior of sisal fiber concrete cylinders externally wrapped with jute FRP

The use of environmentally friendly natural fibers as building materials is benefit to achieve a sustainable construction. This article performs a study on the use of natural jute fibers as reinforcement of concrete and natural sisal fibers in fiber reinforced polymer (FRP) composites as concrete confinement, i.e., sisal fiber reinforced concrete (SFRC) composite column wrapped by jute FRP (JFRP) (SFRC‐JFRP). Uniaxial compression test was conducted to assess the compression performance of the composite columns as axial structural member. A total of 24 specimens were tested. The effects of JFRP wrapping thickness and sisal fiber inclusion on the compressive performance of the composite columns were investigated. Results indicate that JFRP confinement significantly increases the compressive strength and ductility of both PC and SFRC with an increase in JFRP thickness. Besides, the inclusion of sisal fiber further enhances the strength as well as the efficiency of confinement under uniaxial compression. Also, the models for ultimate strength and ultimate strain of PC‐JFRP and SFRC‐JFRP are proposed. POLYM. COMPOS., 38:1910–1917, 2017. © 2015 Society of Plastics Engineers     

Comparative behavior under compression of concrete columns repaired by fiber reinforced polymer (FRP) jacketing and ultra high-performance fiber reinforced concrete (UHPFRC)

This paper summarizes the experimental results from a comprehensive research program to study the fundamental stress–strain behavior of damaged concrete repaired by two techniques: increased concrete section and bonding fiber reinforced polymer (FRP). In this work, two types of FRP composite jackets were used, carbon fiber reinforced polymer (CFRP) and glass fiber reinforced polymer, and two types of concretes were used to repair the damaged concrete by increased concrete section: ordinary concrete and ultra high-performance fiber reinforced concrete (UHPFRC). Fifteen circular columns of concrete (110 × 220) cm³ were initially pre-damaged up to intense cracking, repaired by increased concrete section and by bonding FRP, and tested under uni-axial compression by loading up the damage. The impact of different design parameters, including plain concrete strength, types of composites, and type of concrete used for increasing section, was considered in this study. The strength enhancement and ductility improvement of specimens are discussed. A simple model is presented to predict the compressive strength of repaired damaged concrete columns. A significant strength and an increase in ductility were achieved, particularly when the columns were repaired by increasing section with UHPFRC and by bonding CFRP. These preliminary tests indicate that the use of UHPFRC is an effective technique for rehabilitating damaged concrete columns, highly competitive with the repaired concrete by wrapping specimens with FRP composite jackets.     

Experimental and analytical investigations of creep of epoxy adhesive at the concrete–FRP interfaces

This paper presents the results of experimental and analytical investigations on the long-term behavior of epoxy at the interface between the concrete and the fiber-reinforced-polymer (FRP). Double shear experiments under sustained service load were performed on nine specimens composed of two concrete blocks connected by FRP sheets bonded to concrete using epoxy. The primary investigation parameters included the ratio of shear stress to ultimate shear strength, the epoxy thickness and the epoxy time-before-loading. Loading was sustained for periods up to nine months. We show that the magnitude of shear stress to ultimate shear strength and the epoxy time-before-loading could be the most critical parameters affecting creep of epoxy at the concrete–FRP interfaces. It was also found that the creep of epoxy can result in failure at the interfaces due to the combined effect of relatively high shear stress to ultimate shear strength and thick epoxy adhesive. This can have an adverse effect on the designed performance of reinforced concrete (RC) structures strengthened with FRP. Based on the experimental observations, rheological models were developed to simulate the long-term behavior of epoxy at the concrete–FRP interfaces. It is shown that the long-term behavior of epoxy at the interfaces can be properly modeled by analytically for both loading and unloading stages.     

基于长宽比变化的FRP加固混凝土轴压矩形柱的试验研究

制作了16根FRP加固混凝土矩形短柱进行轴压试验,按长宽比值不同分成4组,每组4个构件,其中1个未加固,2个无间隙粘贴FRP环向围束加固,1个有间隙粘贴FRP环向围束加固。通过考虑柱截面长宽比、FRP材料类型、粘贴方式等因素的变化,分析探讨了加固前后柱的承载力、延性、破坏形式等。试验表明,长宽比值在提高柱的极限承载力方面影响较大,而在提高柱的延性方面影响较小。根据试验数据对我国混凝土加固规范推荐的公式进行对比,提出了FRP有间隙粘贴环向围束的强度模型。     

FRP板加固钢筋砼梁正截面强度分析

根据平面假设对外贴纤维增强复合材料加固钢筋混凝土梁进行了弹塑性变形全过程分析,分析了不同配筋率情况下,加固率与极限弯矩、极限曲率的关系.结果表明:外贴FRP材料可以提高钢筋混凝土梁的极限承载力,但开裂前对钢筋混凝土梁的受力性能影响很小.对于较小的配筋率的钢筋混凝土梁,根据加固率的不同,可能发生FRP板拉断的破坏模式,也可能发生压区混凝土压碎的破坏模式.但对于较大配筋率的钢筋混凝土梁,不管加固率的大小,均不可能发生FRP板拉断的破坏模式.加固效果对相对小配筋率的梁更显著,随着配筋率及加固率的增加,加固钢筋混凝土梁的极限抗弯能力的增加率逐渐减小.     

Analysis of the nonlinear creep behavior of concrete/FRP-bonded assemblies

This paper investigates the creep behavior of adhesively bonded concrete/fiber-reinforced polymer (FRP) joints, through experimental and modeling approaches. The first part proposes a methodology for predicting the long-term creep response of the bulk epoxy adhesive; such a procedure consists of (1) performing short-term tensile creep experiments at various temperatures and stress levels, (2) building the creep compliance master curves according to the time–temperature superposition principle in order to assess the long-term evolution for each stress level, and (3) developing a rheological model whose parameters are identified by fitting the previous master curves. In our case, it was found that master curves (and, consequently, parameters of the rheological model) are dependent on the applied stress level, highlighting the nonlinear creep behavior of the bulk epoxy adhesive. Therefore, evolution laws of the model parameters were established to account for this stress dependence. The second part focuses on the creep response of the concrete/FRP assembly in the framework of a double lap joint shear test configuration. Experiments showed that creep of the adhesive layer leads to a progressive evolution of the strain profile along the lap joint, after only one month of sustained load at 30% of the ultimate strength. Besides, a finite element approach involving the abovementioned rheological model was used to predict the nonlinear creep behavior of the bonded assembly. It confirmed that creep modifies the stress distribution along the lap joint, especially the stress value at the loaded end, and leads to a slight increase in the effective load transfer length. This result is of paramount interest since the transfer length is a key parameter in the design of FRP-bonded strengthening systems. Moreover, instantaneous and long-term calculated strain profiles were found in fair agreement with experimental data, validating the modeling approach.     

新型玄武岩FRP布加固混凝土结构性能试验

为研究新型玄武岩纤维增强复合材料(FRP)布加固混凝土圆柱结构性能,制作了12根试件并进行轴心受压试验,通过改变不同粘贴层数,分析了混凝土柱承载力、延性的变化,以及玄武岩FRP布约束混凝土作用机理。结果表明,粘贴玄武岩FRP布加固结构,可以有效改善结构的力学性能,提高混凝土极限强度和极限应变,提高加固墩柱的承载能力;随着玄武岩FRP布层数增加,混凝土的延性得到较大改善,外贴1~3层玄武岩FRP布加固时,混凝土柱的延性提高幅度约为30.9%~80.7%;玄武岩FRP布约束力主要是在混凝土受压后达到屈服应力时产生;采用玄武岩FRP布进行结构加固,并不是粘贴层数越多加固效果越好。