Strengthening fire-damaged concrete by confinement with fibre-reinforced polymer wraps

Extensive research has shown that fibre-reinforced polymer (FRP) wraps are very effective for strengthening concrete columns for increased axial and flexural load and deformation capacity, and this technique is now widely used around the world. The study reported in this paper extends the FRP confinement technique to strengthening fire-damaged circular concrete columns. An experimental programme was undertaken to study the compressive strength and stress-strain behaviour of both unconfined and FRP-confined plain concrete cylinders after being heated to various elevated temperatures for up to four hours and cooled to room temperature. The results show that FRP confinement is highly effective for enhancing the load-carrying capacity of even severely fire-damaged concrete columns. The results also yield insights into the mechanics of FRP confinement of concretes of similar composition but with varying compressive strengths. A modified version of a pre-existing confinement model is proposed for use in designing FRP strengthening schemes for fire-damaged concrete columns.     

纤维增强复合材料(FRP)约束混凝土矩形柱应力-应变关系的研究

在国内外试验研究基础上,分析了纤维增强复合材料(FRP)约束混凝土矩形柱的特点,指出FRP约束后混凝土矩形柱的应力-应变关系曲线可能有软化段或硬化段。指出FRP约束混凝土矩形柱的转折点应力和应变主要与FRP侧向约束刚度和混凝土弹性模量比值有关,并提出相应的计算公式。指出FRP约束混凝土矩形柱的极限应力和应变主要与FRP侧向约束强度、FRP类型、矩形截面的转角、混凝土强度等参数有关,提出极限应力和极限应变可在等价FRP约束以矩形截面较大边为直径的等价圆柱体极限应力和应变基础上乘以相应的折减系数得到,该计算方法简单且较全面地考虑了各影响参数。最后,提出三个确定FRP约束混凝土矩形柱的应力-应变关系模型,各模型在一定的条件下均与试验结果符合较好。     

FRP约束混凝土圆柱有软化段时的应力-应变关系研究

试验研究了不同数量、不同类型(高强度、高弹模、高延性)FRP约束混凝土圆柱的应力-应变关系,发现其应力-应变关系曲线可能有软化段也可能没有软化段。指出FRP约束混凝土圆柱的轴向最大应力主要与侧向约束强度或侧向约束刚度有关,而轴向极限应变除了与该两个参数有关外,还与FRP的轴向极限拉应变有关。基于试验及搜集到的数据,提出判断FRP约束混凝土圆柱有无软化段的侧向约束强度与混凝土强度比界限值。基于对FRP侧向约束刚度和强度、FRP轴向极限拉应变、混凝土强度及弹性模量等参数的分析,提出了FRP约束混凝土圆柱有软化段时的峰值应力、峰值应变、极限应力及极限应变计算公式。最后,建议了两个确定FRP约束混凝土圆柱有软化段时的应力-应变关系模型。与大量试验数据的比较表明,本文建议的公式和模型与试验结果均符合较好。     

固化粉土小应变剪切模量与强度增长相关性研究

小应变剪切模量和无侧限抗压强度是表征固化土刚度和强度特性的两个重要参数。简要介绍了弯曲元测试技术的原理及其在试验中存在的问题,采用压电陶瓷弯曲元测试技术对水泥和木质素固化剂固化粉土试样在不同养护龄期下的小应变剪切模量进行了测试,同时对相应龄期下试样进行了常规无侧限抗压强度试验,通过引入归一化参数G28和UCS28对不同固化土的小应变剪切模量和无侧限抗压强度之间的相关关系进行分析,提出了固化土刚度与强度的相关性模型,可为地基处理中固化土的无损测试与加固效果评价提供新的方法。结果表明,水泥、木质素固化粉土的小应变剪切模量随养护龄期增加而增加,养护龄期28 d内增长显著,28 d后增长趋于平稳;相同类型固化土不论固化剂掺量多少,其小应变剪切模量随养护时间的发展在本质上是相同的;固化土归一化无侧限抗压强度表现出与小应变剪切模量相似的发展趋势;提出的固化土归一化模型可作为一种土体强度无损检测的新方法。     

纤维布(FS)约束混凝土矩形柱轴心受压试验及非线性有限元分析

通过29个纤维布(FiberSheet,FS)约束混凝土柱和2个混凝土对比柱的轴心受压试验,以及FS约束混凝土柱的三维非线性有限元分析,研究FS约束混凝土矩形截面柱的轴心受压性能和FS的约束机理。FS约束混凝土柱有斜剪破坏和轴压破坏两种破坏形态;FS约束混凝土的轴压应力-应变曲线有三种形状;增大截面长宽比会减弱FS的约束效果;单独缠绕CFS比复合缠绕CFS和GFS对强度提高有利,但复合缠绕且GFS不少于1层时,对增大变形有利;轴向压应力大于0.6～0.8倍峰值应力后,FS的横向拉应变增大,对混凝土起约束作用;角部FS横向拉应变及截面角部混凝土的水平应力最大;可以将截面分为强约束区和弱约束区;角部的圆弧半径与1/2边长之比越大,强约束区的面积越大;纤维特征值对强约束区面积大小的影响不大,但是纤维特征值大,截面的水平应力也大。     

Nonlinear analysis of circular concrete-filled steel tubular short columns under axial loading

The confinement effect provided by the steel tube in a circular concrete-filled steel tubular (CFST) short column remarkably increases the strength and ductility of the concrete core. The reliable prediction using nonlinear analysis methods for circular CFST columns relies on the use of accurate models for confined concrete. In this paper, accurate constitutive models for normal and high strength concrete confined by either normal or high strength circular steel tubes are proposed. A generic fiber element model that incorporates the proposed constitutive models of confined concrete is created for simulating the nonlinear inelastic behavior of circular CFST short columns under axial loading. The generic fiber element model developed is verified by comparisons of computational results with existing experimental data. Extensive parametric studies are conducted to examine the accuracy of various confining pressure models and the effects of the tube diameter-to-thickness ratio, concrete compressive strengths and steel yield strengths on the fundamental behavior of circular CFST columns. A new design formula accounting for concrete confinement effects is also proposed for circular CFST columns. It is demonstrated that the generic fiber element model and design formula adequately predict the ultimate strength and behavior of axially loaded circular CFST columns and can be used in the design of normal and high strength circular CFST columns.     

FRP网格约束混凝土圆柱的抗震性能

对纤维增强聚合物(FRP)网格约束混凝土圆柱进行了轴压荷载试验;给出了FRP网格侧向约束强度和侧向约束刚度计算公式及FRP网格约束混凝土圆柱的应力-应变关系模型确定方法.在此基础上,进一步完成了FRP网格加固钢筋混凝土圆柱在低周反复荷载作用下的试验,重点探讨了FRP网格加固用量及FRP网格中纵筋是否锚入柱底座对试件承载力和变形能力的影响.结果表明,FRP网格约束后混凝土圆柱的强度和延性有明显提高,约束后的应力-应变关系曲线有无软化段主要与FRP约束量有关,FRP网格加固能明显提高钢筋混凝土结构的承载力和延性等抗震性能.     

镀锌铁丝约束混凝土应力-应变本构研究

为给复杂高层、大跨混凝土结构小缩尺比模型试验中常采用的铁丝约束混凝土的本构模拟提供依据,该文设计了素混凝土、铁丝约束混凝土、钢筋约束混凝土共计13个圆柱,对其进行轴压试验,对比研究铁丝约束混凝土的破坏历程、应力-应变曲线与钢筋约束混凝土的区别,并分析铁丝与普通钢筋对核心混凝土约束作用的区别。结果表明:模型试验中常采用的镀锌铁丝弹性模量仅钢筋的60%左右;与素混凝土相比,铁丝约束混凝土的强度和变形能力均得到显著提高;对于螺旋配箍的钢筋约束混凝土,按Mander模型模拟更为准确;对于螺旋配箍的铁丝约束混凝土,Park模型能很好地估计其峰值压应力,但过低估计其峰值压应力对应的应变和极限压应变;修正后的Park模型能很好地模拟铁丝约束混凝土的应力-应变行为。     

高强钢棒螺旋箍筋约束混凝土圆形截面柱受力性能试验研究

为研究高强钢棒螺旋箍筋约束混凝土柱的受力性能,设计了32个约束混凝土柱,对其进行轴心受压试验。试件主要设计参数包括混凝土强度等级为C50、C60、C70、C80,高强钢棒抗拉强度标准值为800、970MPa,体积配箍率为0.9%、1.2%、1.6%、2.0%。试验结果表明：约束混凝土柱在轴向荷载作用下呈腰鼓形破坏或单折剪切破坏,对于确定的非约束混凝土抗压强度和箍筋抗拉强度,约束箍筋体积配箍率较小时发生单折剪切破坏,体积配箍率较大时发生腰鼓形破坏;约束箍筋拉应变随混凝土弹性模量与非约束混凝土抗压强度比值增大而增大,随箍筋体积配箍率增大而减小;约束混凝土柱的体积配箍率大于某一量值时,会出现约束混凝土达到抗压强度时箍筋拉应变小于其屈服应变的情况。基于试验结果,建立了用于判别腰鼓形破坏和单折剪切形破坏的界限,并给出了相应的计算式;建立了约束混凝土柱发生轴压破坏时约束箍筋拉应变计算公式及箍筋受拉屈服对应的最大体积配箍率计算公式,为合理设计高强钢棒螺旋箍筋约束混凝土圆形截面柱提供参考。     

Analysis of circular concrete-filled double skin tubular slender columns with external stainless steel tubes

This paper investigates the strength and behaviour of concrete-filled double skin steel tubular (CFDST) slender columns under axial compression. The lean duplex stainless steel material (EN 1.4162) which has recently gained significant attention is considered herein as the external jacket of such columns. Finite element (FE) analyses of several CFDST columns are conducted. Careful consideration is taken in the modelling for the concrete behaviour, for which both of the compressive and the tensile behaviours and the non-linear behaviour due to cracking are fully considered. The accuracy of the current FE models is ensured through the comparison with the existing columns in literature. A parametric study is then conducted to investigate the behaviour of such columns under different affecting factors; the slenderness ratio, the concrete confinement effect, the hollow ratio, the concrete compressive strength and the thickness ratio. The behavioural differences between intermediate length and very long CFDST columns are carefully addressed. Analytically obtained ultimate strengths are compared with design strengths calculated by European and American specifications. European design strength is found to give better predictions compared to the American specifications. However, it is shown that both strengths cannot be used in design because they overestimate the ultimate strengths and thereby do not satisfy the safety requirements. Therefore, a modification is suggested to the European design model which is shown to be able to estimate the compressive resistance of the CFDST columns more accurately than other methods.     

Analytical model for predicting axial capacity and behavior of concrete encased steel composite stub columns

The axial compressive capacity and force-deformation behavior of concrete encased steel stub columns were analytically investigated. An analytical model was developed for predicting the force-deformation response for composite stub columns with various structural steel sections and volumetric lateral reinforcement. Constitutive relationships were established for materials used in the composite cross section, which included unconfined concrete, partially and highly confined concrete, structural steel section, and longitudinal reinforcing bar. The axial capacity of composite stub columns can be determined from strengths contributed from each material component following the stress-strain relationship. Analytical results show that the axial load-carrying capacity and force-deformation behavior measured in the experiments can be accurately predicted. In addition to the lateral reinforcement, the structural steel section can provide a confinement effect on the concrete and enhance the axial capacity and post-peak strength.     

Design-oriented stress–strain model for concrete prisms confined with FRP composites

A good modeling of the stress–strain behavior of FRP-confined concrete prism is necessary for the design of rectangular columns retrofitted with FRP composites. Existing stress–strain models for FRP-confined concrete prisms are mostly presented based on the concept of steel-confined concrete columns. Based on the results of more than one hundred specimens, the mechanical behavior of FRP-confined concrete prisms are studied in this paper. It is found that the stress–strain relationship of FRP-confined prism has either a strain-hardening or a strain-softening response, which mainly depends on the confinement strength of FRP, corner radius of cross-section, etc. Equations to predict the transitional stress and strain of FRP-confined concrete prisms are presented. By reducing the corresponding ultimate strength and strain of equivalent concrete cylinders confined with equivalent FRP, the ultimate strength and strain of FRP-confined concrete prisms can be predicted rationally. Three design-oriented models, which can be applied to various conditions, are suggested. The feature of those models is simple, and they agree well with extensive experimental results.     

Deep neural network with high‐order neuron for the prediction of foamed concrete strength

The article presents a deep neural network model for the prediction of the compressive strength of foamed concrete. A new, high‐order neuron was developed for the deep neural network model to improve the performance of the model. Moreover, the cross‐entropy cost function and rectified linear unit activation function were employed to enhance the performance of the model. The present model was then applied to predict the compressive strength of foamed concrete through a given data set, and the obtained results were compared with other machine learning methods including conventional artificial neural network (C‐ANN) and second‐order artificial neural network (SO‐ANN). To further validate the proposed model, a new data set from the laboratory and a given data set of high‐performance concrete were used to obtain a higher degree of confidence in the prediction. It is shown that the proposed model obtained a better prediction, compared to other methods. In contrast to C‐ANN and SO‐ANN, the proposed model can genuinely improve its performance when training a deep neural network model with multiple hidden layers. A sensitivity analysis was conducted to investigate the effects of the input variables on the compressive strength. The results indicated that the compressive strength of foamed concrete is greatly affected by density, followed by the water‐to‐cement and sand‐to‐cement ratios. By providing a reliable prediction tool, the proposed model can aid researchers and engineers in mixture design optimization of foamed concrete.     

FRP约束混凝土圆柱无软化段时的应力-应变关系研究

在国内外大量试验研究的基础上,比较分析了FRP、箍筋及钢管约束混凝土圆柱性能的差异。指出FRP约束混凝土圆柱的极限强度主要与FRP侧向约束强度、未约束混凝土强度及FRP形式等有关,并提出FRP约束圆柱后极限强度计算方法。指出已有的FRP约束混凝土圆柱极限应变计算方法误差均较大,本文提出新的极限应变计算方法:首先,发现FRP约束混凝土圆柱的极限泊松比趋向一恒定值,并建议了极限泊松比的计算公式,然后根据应变相容可确定FRP约束混凝土圆柱的极限应变。最后,建议了确定FRP约束混凝土圆柱无软化段时应力-应变关系的三折线模型,与已有的试验结果比较表明,模型简单且精度高,能适用于AFRP、GFRP、CFRP等不同类型及FRP管和FRP布等不同形式FRP约束的混凝土圆柱。     

建筑垃圾细料生产流动化回填材料的性能

以灰砂比0.03、0.05和0.08,粉砂比0、0.05、0.1、0.15和0.2为设计参数,对建筑垃圾回填材料进行设计。通过试验对回填材料的流动性(流动度、泌水率)、无侧限抗压强度以及应力应变曲线、本构关系模型和弹性模量等进行研究。研究结果表明:回填材料的流动度受水固比影响较大,两者接近线性关系;流动度在200~250mm范围,泌水率在4%~8%之间;回填材料抗压强度与灰砂比和水固比之间存在很好的幂指数关系;回填材料应力应变曲线形状与普通混凝土的相似,在此基础上提出回填材料的本构关系模型;回填材料无侧限抗压强度与弹性模量之间存在很好的指数关系。     

钢-混凝土组合空间框架拟动力有限元分析

基于混凝土的塑性-损伤本构模型与钢材的弹塑性混合强化本构模型,采用ABAQUS有限元分析软件,对圆钢管混凝土柱-钢筋混凝土框架梁2层空间框架的三维实体精细有限元模型进行拟动力分析。模型中考虑了钢管与核心混凝土的约束作用以及连续地震作用下引起的塑性耗能累积效应。对钢管混凝土柱轴压比、钢管横向变形系数以及各关键点的钢管与核心混凝土、框架梁、环梁与楼板中的钢筋和混凝土的应力-应变滞回曲线以及框架结构的塑性耗能分布情况进行分析。结果表明：连续工况地震作用使框架结构产生塑性耗能累积效应,进而使楼层的位移响应明显增大,层间刚度减小;连续工况地震作用后,钢管和所有钢筋都屈服,钢管、环梁钢筋的约束作用使核心混凝土、环梁混凝土强度超过其轴心抗压强度,楼板混凝土因受拉而开裂;短边柱的轴压比有明显波动,罕遇地震作用下其值大于角柱、中柱和长边柱的轴压比;钢管混凝土柱为主要的塑性耗能构件,环梁次之。     

BFRP约束损伤钢筋混凝土圆柱轴压力学性能研究

为了研究钢筋、玄武岩增强复合材料(BFRP)包裹层数及预损伤对BFRP约束钢筋混凝土(RC)圆柱轴压力学性能的影响,对22个直径242mm、高726mm的混凝土圆柱(包括20个RC圆柱以及2个素混凝土圆柱)开展轴压试验。试验结果表明:纵筋对BFRP约束RC圆柱的极限强度有较明显贡献,箍筋对BFRP约束RC圆柱极限强度和极限应变均有较明显贡献;BFRP约束可有效改善损伤RC圆柱的极限强度和极限应变,但无法恢复RC圆柱因损伤而降低的初始刚度;损伤会降低BFRP约束RC圆柱的极限强度和初始刚度,且降低的程度随着损伤水平的增加而增加。最后,提出了BFRP约束损伤RC圆柱强度模型及极限应变模型,模型预测值与试验值吻合较好。     

玻璃纤维布加固混凝土柱的轴心受压试验及有限元分析

通过7根玻璃纤维布加固混凝土柱和1根混凝土对比柱的轴心受压试验及有限元分析,探讨了玻璃纤维布加固混凝土柱的受压性能及其加固机理.分析表明,玻璃纤维布加固混凝土柱有斜剪破坏和轴压破坏两种破坏形态;加固后的混凝土柱能明显提高其极限承载力,且提高程度与纤维布加固层数、加固间距和构件截面形状有关,存在着使承载力提高最大的最优加固层数、最优加固间距和合理截面形状.对比试验和有限元分析说明,合理选择有限元分析模型,可以较好地预测玻璃纤维布加固混凝土柱的轴心受压性能.     

碳纤维布约束混凝土矩形柱的抗压性能研究

对碳纤维布约束的混凝土柱(从方形截面逐步过渡至圆形截面)进行了轴压性能试验。试验研究了碳纤维布约束混凝土柱的破坏形态及轴压应力与轴向、横向应变之间的变化曲线,确定了碳纤维布的有效应变系数。探讨了FRP材料约束混凝土矩形柱的侧向约束机理;提出了新的有效约束面积模型及侧向约束压力计算公式。基于现有试验研究成果,建立了碳纤维布约束混凝土矩形柱的抗压强度与峰值应变计算模型。针对碳纤维布充足约束混凝土矩形柱的应力-应变反应,提出了双线性理论模型,理论计算曲线与试验曲线基本吻合。     

钢管钢筋混凝土柱的轴心抗压性能研究

从以前关于钢管钢筋混凝土柱的研究可以明确看出,钢管钢筋混凝土柱优于钢管混凝土柱。这主要是因为钢管和钢筋共同施加侧压于混凝土芯,从而改善了混凝土芯的性能。为了弄清混凝土芯约束性能的影响,并提出一个关于钢管钢筋混凝土柱轴心抗压强度的评估方程,使用参数研究法将数值结果与试验结果相匹配,从而对钢管钢筋混凝土柱进行数值分析。根据分析结果,对钢管钢筋混凝土中混凝土约束性能和钢管应力状态进行讨论。最后,提出一个和钢管钢筋混凝土柱荷载分担比例有关的﹑非常规的﹑评估约束性能的方程,并通过试验数据验证了钢管钢筋混凝土柱轴心抗压强度的评估方程。