FRP与钢双管约束混凝土应力-应变关系理论模型

在平面应变条件下对FRP-混凝土-钢混合双管柱进行力学分析,考虑混凝土和钢管的弹塑性,通过FRP管有无达到极限状态判断试件是否破坏,提出FRP与钢双管约束混凝土应力-应变关系理论模型,理论计算曲线与试验曲线吻合较好。在理论模型基础上对FRP与钢双管约束混凝土应力-应变曲线进行参数分析,考察FRP管类型、空心率、钢管径厚比、混凝土强度以及钢管强度对混凝土应力-应变曲线的影响,研究表明FRP管类型、空心率和混凝土强度对应力-应变曲线有影响,钢管径厚比和钢管强度几乎没有影响。     

FRP约束混凝土强弱约束判断准则

基于数值模拟和以往研究,分析了不同强弱约束状态下的FRP约束混凝土行为.分析结果表明:(1)约束混凝土应力-应变曲线根据强弱约束状态不同而发生变化,处于强约束条件下的混凝土,极限强度大于无约束混凝土强度,对于加固量较小的弱约束混凝土,破坏时的应力值等于或者小于无约束混凝土的强度;(2)计算非圆形截面的约束应力、约束刚度、极限约束应力时要考虑形状等效系数kse,约束混凝土极限状态下的FRP断裂应变小于FRP极限拉应变,对于混凝土圆柱,FRP断裂应变可等效为环形劈裂试验值,对于非圆形截面,需计入形状等效系数kse的影响;(3)定义极限约束应力与无约束混凝土强度比值为约束比,约束比决定混凝土强弱约束状态.通过对收集到的试验数据进行回归,提出了强弱约束判断准则.     

FRP约束混凝土的应力-应变关系

通过修正已有文献中提供的约束混凝土体积应变计算模型,并基于受定侧压力作用下混凝土的应力-应变关系模型,采用数值方法,全过程计算了具有被动约束特征的圆形截面纤维增强塑料(FRP)约束混凝土的应力-应变关系。结果表明,无论是对于具有强化特征还是具有软化特征的FRP约束混凝土,计算结果和实验结果及其他文献报道的实验结果均吻合良好。     

纤维增强聚合物复合材料-钢复合圆管约束混凝土轴压性能预测模型

纤维增强聚合物复合材料（FRP）-钢复合圆管约束混凝土由于具有很好的综合性能,近年来得到了广泛的研究,尚缺乏完整的应力-应变关系全曲线计算模型。本文在综合作者现有研究成果的基础上,广泛收集了96个FRP-钢复合圆管约束混凝土柱的轴心受压试验结果,系统分析了试件参数的影响规律,提出了FRP-钢复合圆管约束混凝土的应力-应变关系曲线的完整计算模型,包括峰值应力、峰值应变、极限应力和极限应变的计算方法,并建议了应力-应变关系全曲线预测模型,模型较好地预测了FRP-钢复合圆管约束混凝土的应力-应变关系曲线的特征,预测结果与试验结果吻合较好。建议模型具有较好的通用性和准确性。      

武器间接命中条件下FRP约束混凝土抗多次打击能力

在3种约束比下FRP约束混凝土圆柱体试件准静载和快速加载试验的基础上,进行多次打击试验及多次打击后的准静载试验 。结果表明:FRP约束混凝土是一种抗多次打击能力较好的抗压复合材料,经每次打击后的刚度虽逐渐衰减,但有收敛趋势,其残余刚度随约束比ξ及最大应力比η的增加而减小。经受多次打击后的FRP约束混凝土与未经受多次打击的试件相比,其准静载下的初始弹性模量降低,强度基本不变,极限应变提高,其中弱约束混凝土延性提高得尤其显著,强约束混凝土应力-应变曲线中线性强化段的斜率随刚度衰减程度的增加而增加,最终FRP强约束混凝土有变成线弹性材料的趋势,为进一步研究FRP约束混凝土在防护结构中的应用提供条件。      

基于修正混凝土塑性损伤模型的大应变FRP约束混凝土有限元分析

精确的有限元分析(FEA)依赖于材料的准确定义。通过编写用户子程序UMAT实现了大断裂应变纤维增强聚合物(LRS FRP)在纤维方向上拉伸特性的定义。基于Abaqus中混凝土塑性损伤模型的理论框架,提出了一种改进的混凝土塑性损伤模型用于定义LRS FRP约束混凝土的材料特性。这些改进包括：通过LRS FRP约束混凝土的实验数据校准了与屈服准则相关的参数K;硬化/软化准则与约束刚度相关;流动法则与轴向塑性应变相关。采用修改后的材料模型进行FEA,结果表明：FEA预测的应力-应变曲线与实验结果吻合。基于FEA的结果,讨论了矩形柱截面上应力分布的不均匀性,根据约束效果可分为有效约束区域和弱约束区域,且在约束有效区域上应力分布不均匀性随着截面比(长边/短边)的增加而增加。     

钢骨约束混凝土的约束机制及其应力-应变模型建立

目前,钢骨混凝土（SRC）柱的试验结果与数值分析结果之间多无法良好吻合;究其原因,在于缺乏对钢骨约束混凝土的约束机制认识及与之相应的钢骨约束混凝土模型的建立。为此,该文在以往钢骨约束混凝土试验基础上,深入研究了带翼缘十字形钢骨对混凝土的约束机制,提出了钢骨对混凝土产生的实际侧向约束应力的简化分布形式,并以此将钢骨约束混凝土划分为钢骨强约束混凝土、钢骨弱约束混凝土和钢骨无约束混凝土。同时,通过计算有效侧向约束应力,确定了各钢骨约束区域可表征钢骨约束作用强弱的混凝土强度提高系数;借鉴经典的Mander模型,通过修正模型关键参数方法以建立各钢骨约束区域混凝土的应力-应变模型。引入该应力-应变模型的有限元分析和试验对比结果表明,该文所提出的钢骨约束混凝土的约束机制和所建立的钢骨约束混凝土应力-应变模型是合理且有效的。     

玄武岩纤维布约束高温损伤混凝土方柱轴压力学性能试验

对36个玄武岩纤维布增强聚合物基复合材料（BFRP）约束的高温损伤混凝土方柱和15个不同高温损伤的对比试件进行了轴压试验。试验表明,玄武岩纤维布横向约束能改变高温损伤后混凝土方柱的破坏形态,显著提高混凝土方柱的轴压强度和变形能力。其中三层玄武岩纤维布包裹的200℃、400℃、600℃和800℃高温损伤混凝土方柱轴压强度分别提高了48%、130%、206%和389%,轴向变形分别提高了433%、344%、319%和251%。采用典型的纤维增强聚合物基复合材料（FRP）约束常温未损伤混凝土轴压力学性能的设计模型预测FRP约束高温损伤混凝土的轴压强度和变形时存在较大的偏差。通过构建柱状膜结构静水压力平衡模型和约束混凝土方柱与FRP体积应变能平衡模型,分别改进了FRP约束混凝土方柱轴压极限应力和极限应变计算模型的基本形式。基于该基本形式和试验数据,分别确定了BFRP约束高温损伤混凝土方柱轴压极限应力和极限应变计算中与温度相关的参量,提出了适用于高温损伤混凝土方柱的轴压极限应力和极限应变的设计模型。      

基于长短期记忆网络的FRP约束混凝土圆柱循环轴压应力-应变预测模型

纤维增强复合材料(Fiber reinforced polymer, FRP)已被广泛应用于既有混凝土结构的加固改造和新建结构中。FRP约束混凝土柱在地震作用下通常会受到轴压的往复循环作用,研究FRP约束混凝土在循环轴压作用下的应力-应变特性对于FRP在实际工程中的应用具有重要意义。该文提出了一种用于建模循环轴压下FRP约束混凝土柱应力-应变特性的神经网络预测模型,该模型采用长短期记忆(Long short-term memory,LSTM)单元对循环应力-应变曲线中的滞回特性进行建模,构件的物理参数被有效地集成在网络的输入中。该模型能以端到端的方式进行高效的训练且不依赖任何专家经验。制作了一个包含166个FRP约束普通混凝土柱的循环轴压数据库,在该数据库上对模型的准确性和鲁棒性进行了充分的评估,结果表明测试集平均预测误差仅为0.32 MPa。此外,对网络结构和超参数的影响进行了详细的讨论,结果表明该模型具有出色的预测性能。     

高强不锈钢绞线网/ECC约束高强混凝土受压本构模型

为将新型复合材料“高强不锈钢绞线网/ECC约束素混凝土”用于实际工程结构,基于高强不锈钢绞线网/ECC约束高强混凝土(简称HSME约束高强混凝土)复合材料轴心受压试验结果,分析ECC强度、核心混凝土强度以及横向钢绞线体积配网率等对其受压性能的影响规律。试验结果表明：HSME能够有效约束核心混凝土轴心受压,破坏模式具有明显的延性性能,根据试验数据绘出HSME约束高强混凝土复合材料受压应力-应变曲线,可以分为三个阶段：弹性阶段、弹塑性阶段和下降段。根据各阶段曲线的数学特征,建立HSME约束高强混凝土复合材料受压本构关系的全过程模型表达式。引入ECC特征值和横向钢绞线特征值,对本构模型的各参数进行分析,提出HSME约束高强混凝土复合材料的开裂压应变、峰值应力、峰值压应变和极限压应变等参数的表达式。将各参数代入所建立的受压本构关系绘出其应力-应变曲线,模型结果与试验所得应力-应变曲线吻合良好,开裂压应变与极限压应变的计算值与试验值对比范围分别为0.949～1.068和0.938～1.039。表明所提出的受压本构模型能够较好地反映HSME约束高强混凝土复合材料的应力-应变关系。     

考虑拉伸刚化效应的FRP筋/混凝土轴拉构件变形计算方法

考虑拉伸刚化效应是精确计算纤维增强树脂复合材料(FRP)筋/混凝土构件变形和裂缝的基础。提出了考虑拉伸刚化效应的FRP筋/混凝土拉伸构件变形计算的解析方法。首先,对修正Eligehausen黏结滑移模型(修正BPE模型)进行简化提出四线性黏结-滑移模型。根据该模型推导了拉伸构件在不同拉伸荷载阶段的FRP筋、混凝土应力和变形及黏结力和滑移量的分布表达式。结合混凝土开裂判别方法,提出了FRP筋/混凝土拉伸构件的全过程变形计算方法。通过与已有文献试验结果对比验证了本文方法的准确性。对影响拉伸刚化的一些参数进行了敏感性分析。结果表明,混凝土强度和配筋率对拉伸刚化效应影响不大,FRP筋弹性模量是影响拉伸刚化效应的主要因素。      

Prediction of tensile capacity based on cohesive zone model of bond anchorage for fiber-reinforce dpolymer tendon

In this paper, analytical solutions based on a cohesive zone model (CZM) are developed for the bond fiber-reinforce dpolymer (FRP) tendon anchorage under axial load. With bilinear cohesive laws, the analytical solutions of tensile capacity of anchorages are derived. The concept of the minimum relative interface displacement s_m is introduced and used as the fundamental variable to express all other parameters, such as external tensile load. Experimental and analytical results show that the thickness of the anchoring material is main factor affecting tensile capacity. The characteristic bond strength depends mainly on the properties of the bonding agent-anchoring material, the geometry and surface conditions of the tendon, and the radial stiffness of the confining medium. A comparison of the calculated and experimental results showed good agreement. Formulas based on fracture energy of the tension load capacity derived in the present work can be directly used in the design of FRP tendon anchorage.     

U型FRP加固钢筋混凝土梁受剪剥离性能的有限元分析

采用FRP布对梁进行抗剪加固,可以有效的解决梁因配箍率不足而导致的受剪承载力偏低的问题。根据文献[1]中7根试验梁的参数,针对工程中常用的U型FRP受剪加固形式,建立三维有限元分析模型,采用商业有限元计算软件ANSYS,数值模拟了加载全过程和受剪剥离受力性能,根据试验结果确定了FRP-混凝土界面粘结剥离强度,并建议了合适的裂面剪力传递系数。根据有限元分析结果,作者又进一步研究了U型FRP布的应变分布、分担剪力的贡献、剥离破坏的过程,以及加固量、FRP类型和粘贴面积率对加固梁受剪承载力的影响。在有限元分析的基础上结合试验结果,建议了U型粘贴加固的受剪剥离承载力计算方法。     

疲劳加载下纤维复合材料的剩余强度

回顾了纤维复合材料在疲劳加载下的剩余强度的描述方法和试验结果, 并按照纤维复合 材料中疲劳损伤发展的行为, 提出了一个描述纤维复合材料的剩余强度的模型, 该模型考虑了疲 劳损伤在纤维复合材料中产生和扩展的特征, 试验结果与理论描述吻合较好。      

Stress–strain and deflection relationships of RC beam bonded with FRPs under sustained load

Fiber-reinforced polymer (FRP) systems that have a strong resistance against long-term deformation must provide improved serviceability to reinforced concrete (RC) members under sustained loads. Consequently, there is a need to develop a method for accurately predicting the time-dependent behavior of RC beams that are externally bonded with FRPs. However, there are very few previous studies that have been carried out or experimental results available, on the time-dependent behavior of RC beams externally bonded with FRP. In order to enable a reasonable prediction, correlations should first be clarified between the stress–strain relationship of the concrete, the reinforcement and the FRP that changes over time. By using these correlations, deflections under sustained loads should then be forecast. In this study, RC beams were fabricated for this purpose. Carbon reinforced polymer (CFRP) and glass reinforced polymer (GFRP) materials were bonded to the tension face of the two respective RC beams. The beams were then placed under sustained loads for 300 days. For the specimens that were externally bonded with FRPs and for the conventional specimen, the strain of the compression and tension reinforcement and the strain of FRP and deflection were measured respectively for comparison. In order to theoretically predict the time-dependent behavior of the RC Beam externally bonded with FRPs, creep coefficients for concrete and shrinkage strains were calculated by using the CEB-FIP and the ACI-209 Codes. For the method used to forecast the stress–strain relationships of the concrete, reinforcement and FRPs that change over time were theoretically clarified and were then compared with the experimental results. The deflection of the RC Beams externally bonded with FRP was predicted by using the ACI 318 Standard, EMM, AEMM, Branson’s method, and Mayer’s method. They were also compared to the experimental results. Subsequently, in the case of RC Beams externally bonded with FRPs under sustained loads, the proposed method proved that it is possible to accurately predict long-term deformations.     

Effect of different loading conditions on the mechanical behavior of [0/±45/90]s woven composites

The main objective of this paper is to investigate the behavior of [0/±45/90]_s woven FRP composites under tension, bending, and combined bending/tension loading conditions. First, the mechanical properties of the composite were determined experimentally using the ASTM testing standards. Bending properties were determined using 3-point and 4-point bending tests. The results showed that the woven composites performed better under bending loading than under tension loading. Finally, special test fixtures were designed to facilitate the study of the effect of the combined bending/tension loading. The bending moments were applied using offset shims of various thicknesses placed between the plane of the specimen and the loading axis. At the beginning, the load–strain diagrams at the specimen center showed the domination of bending strains, tension on one surface and compression on the other. With the advance of the loading process, the tension strain dominated and the strain on both sides were almost equal. The failure under combined bending/tension loading was due to the high stresses near the fixture. However, in pure bending, the material failed at the center because of the excessive delamination on the compressive side.     

Mechanics of bonds in an FRP bonded concrete beam

Fibre-reinforced plastic (FRP) materials have been recognised as new innovative materials for concrete rehabilitation and retrofit. Since concrete is poor in tension, a beam without any form of reinforcement will fail when subjected to a relatively small tensile load. Therefore, the use of the FRP to strengthen the concrete is an effective solution to increase the overall strength of the structure. The attractive benefits of using FRP in real-life civil concrete applications include its high strength to weight ratio, its resistance to corrosion, and its ease of moulding into complex shapes without increasing manufacturing costs. The speed of application minimises the time of closure of a structure compared to other strengthening methods. In this paper, a simple theoretical model to estimate shear and peel-off stresses is proposed. Axial stresses in an FRP-strengthened concrete beam are considered, including the variation in FRP plate fibre orientation. The theoretical predictions are compared with solutions from an experimentally validated finite element model. The results from the theory show that maximum shear and peel-off stresses are located in the end region of the FRP plate. The magnitude of the maximum shear stress increases with increases in the amount of fibres aligned in the beam's longitudinal axis, the modulus of an adhesive material and the number of laminate layers. However, the maximum peel-off stress decreases with increasing thickness of the adhesive layer.     

Monotonic and cyclic behaviour of isolated FRP anchors loaded in shear

Using anchors made of fibre reinforced polymers (FRP) is an increasingly accepted method to delay the delamination of FRP sheets from the concrete surface and to enhance the capacity of FRP strengthened concrete structures. For many applications, FRP anchors are primarily loaded in shear. When used for seismic retrofitting schemes, the anchors are subjected to cyclic loads which may lead to premature fatigue failure. To date, however, shear strength of FRP anchors has experienced much less attention than their tension resistance. This paper documents tests on isolated FRP anchors which were conducted to determine the seismic shear capacity of FRP anchors and to propose design rules. To this end, a test setup was developed which allows direct and reverse loading of FRP anchors.     

A design model for punching shear of FRP-reinforced slab-column connections

The overall aim of this paper is to develop a unified design method for the punching shear resistance of slab-column connections irrespective of the type of internal reinforcement. In the first part of the paper a design model for the punching shear resistance of concrete slab-column connections reinforced with fibre-reinforced polymers (FRP) is proposed. This design model is based on the authors’ theoretical analysis for such slabs, which considers the physical behavior of the connections under load. The effects of the inherent linear brittle response, the lower elastic modulus and the different bond features, as compared to steel, of the FRP reinforcement are all accounted for in the present study. The proposed model does not incorporate any fitting factors to match the theory to the trend of the available FRP slab test results. The excellent agreement between the predicted and published test results should give confidence to engineers and designers in using FRP as a sound structural reinforcement for slab-column connections.

It is then shown that the proposed design model for FRP slabs and the previous model of the authors for steel reinforced slabs are both identical in nature and structure, thus constituting a unified approach to design for punching shear in slabs. On the basis of the unified model comparison and correlation between an FRP slab and a reference steel reinforced slab, confirmed by the available test results, are presented. The unified model also enables the development of a more rational and reliable equivalent steel reinforcement ratio which can be applied to existing code equations for steel reinforced slabs to estimate the punching resistance of FRP-reinforced slabs.     

Glulam beams reinforced with FRP externally-bonded: theoretical and experimental evaluation

The glued- laminated lumber (glulam) technique is an efficient process for the rational use of wood. Fiber-reinforced polymer (FRPs) associated with glulam beams provide significant improvements in strength and stiffness and alter the failure mode of these structural elements. In this context, this paper presents guidance for glulam beam production, an experimental analysis of glulam beams made of Pinus caribea var. hondurensis species without and with externally-bonded FRP and theoretical models to evaluate reinforced glulam beams (bending strength and stiffness). Concerning the bending strength of the beams, this paper aims only to analyze the limit state of ultimate strength in compression and tension. A specific disposal was used in order to avoid lateral buckling, once the tested beams have a higher ratio height-to-width. The results indicate the need of production control so as to guarantee a higher efficiency of the glulam beams. The FRP introduced in the tensile section of glulam beams resulted in improvements on their bending strength and stiffness due to the reinforcement thickness increase. During the beams testing, two failure stages were observed. The first was a tensile failure on the sheet positioned under the reinforcement layer, while the second occurred as a result of a preliminary compression yielding on the upper side of the lumber, followed by both a shear failure on the fiber-lumber interface and a tensile failure in wood. The model shows a good correlation between the experimental and estimated results.