纤维增强复合材料加固钢筋混凝土单向板受弯性能研究

通过纤维增强复合材料（FRP）布加固钢筋混凝土单向板的受弯性能试验,研究了FRP布种类、层数、宽度、粘贴方式和锚固措施等因素对加固效果的影响。结果表明：在板底粘贴CFRP布和高强GFRP布均可明显提高钢筋混凝土单向板的受弯承载力和纵筋屈服后刚度。当采用横向粘贴1层U形CFRP布条作为端部锚固措施时,所有加固单向板试件的破坏模式均为跨中弯曲裂缝引起的剥离破坏。当FRP布宽度和层数相同时,采用CFRP布加固效果优于高强GFRP布加固。与对比试件相比,FRP布加固钢筋混凝土单向板试件的位移延性略有降低。高强GFRP布加固钢筋混凝土单向板的位移延性优于CFRP布加固钢筋混凝土单向板。FRP布的粘贴方式对FRP加固钢筋混凝土单向板的位移延性也有影响,单层FRP布加固单向板试件的延性较好。通过对试验结果的分析,提出了FRP布加固钢筋混凝土单向板构件受弯破坏模式的判别方法,验证了已有文献和GB 50608-2010《纤维增强复合材料建设工程应用技术规范》中给出的跨中弯曲裂缝引起的FRP剥离应变计算公式对FRP布加固混凝土单向板的适用性,建立了FRP布加固钢筋混凝土单向板受弯承载力计算方法。     

Design-oriented stress–strain model for FRP-confined concrete

External confinement by the wrapping of FRP sheets (or FRP jacketing) provides a very effective method for the retrofit of reinforced concrete (RC) columns subject to either static or seismic loads. For the reliable and cost-effective design of FRP jackets, an accurate stress–strain model is required for FRP-confined concrete. In this paper, a new design-oriented stress–strain model is proposed for concrete confined by FRP wraps with fibres only or predominantly in the hoop direction based on a careful interpretation of existing test data and observations. This model is simple, so it is suitable for direct use in design, but in the meantime, it captures all the main characteristics of the stress–strain behavior of concrete confined by different types of FRP. In addition, for unconfined concrete, this model reduces directly to idealized stress–strain curves in existing design codes. In the development of this model, a number of important issues including the actual hoop strains in FRP jackets at rupture, the sufficiency of FRP confinement for a significant strength enhancement, and the effect of jacket stiffness on the ultimate axial strain, were all carefully examined and appropriately resolved. The predictions of the model are shown to agree well with test data.     

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.     

锈蚀钢筋混凝土柱基于变形的性能指标研究

采用ABAQUS有限元软件,对按GB 50011-2010《建筑抗震设计规范》和GB 50010-2010《混凝土结构设计规范》设计的342个锈蚀钢筋混凝土柱进行有限元分析,研究钢筋的锈蚀率、轴压比、剪跨比、配箍率和纵向钢筋配筋率等参数对钢筋混凝土柱变形性能的影响。结果表明：当钢筋的锈蚀率较小时,构件的变形性能受锈蚀率影响不大;当钢筋的锈蚀率达到10%以后,构件的破坏形态发生改变,变形能力急剧退化;随着轴压比的增大,构件的变形能力降低;轴压比较小的构件变形性能受剪跨比影响比轴压比较大的构件明显;随着配箍率和纵向钢筋配筋率的增大,构件的变形能力有所提高;提出了锈蚀钢筋混凝土柱抗震性能等级和性能界限状态的划分方法,以塑性转角作为变形性能指标,对数值计算结果进行统计分析,得到了锈蚀钢筋混凝土柱各性能界限点的塑性转角统计特征值。研究成果可为钢筋混凝土结构全寿命的性能化抗震设计和抗震性能评估提供参考。     

BFRP加固高轴压比低强混凝土圆柱抗震性能研究

为研究纤维增强复合材料（FRP）加固高轴压比低强混凝土配筋圆柱的抗震性能,对6根加固柱和3根对比柱进行了低周反复侧向加载试验.结果表明：加固柱的破坏为弯曲破坏,构件的承载力、延性和耗能能力显著改善,对高轴压比低强混凝土柱的加固效果最好;在相同侧向约束应力下,玄武岩纤维布和碳纤维布加固柱承载力相近,但玄武岩纤维布加固柱延性和耗能能力更高.基于试验数据进行回归分析,提出了更广泛轴压比和混凝土强度范围下加固圆柱的峰值荷载和位移延性系数计算式.     

FRP约束钢筋混凝土圆柱力学性能的试验研究

以长细比和偏心率为变化参数,共进行了16个圆形截面钢筋混凝土柱试验,其中8个试件在环向包裹了FRP(纤维增强复合材料),另8个试件未包裹FRP,以作力学性能对比。试验结果表明,由于FRP的约束作用,包裹FRP试件较相应未包裹FRP试件的承载力提高14%～68%,同时延性也有不同程度的提高,证明FRP约束对提高钢筋混凝土柱(包括长柱及偏压柱)力学性能的有效性。最后,提供了环向包裹FRP后圆形截面钢筋混凝土柱承载力计算方法,计算结果和试验结果基本吻合。     

Fiber element modeling for seismic performance of bridge columns made of concrete-filled FRP tubes

Concrete-filled fiber reinforced polymer (FRP) tubes (CFFT) are an alternative to reinforced concrete, providing for rapid construction with comparable strengths and higher ductility. Despite encouraging test data, it is not clear as to whether traditional analytical tools may be used for prediction of structural performance of CFFT, especially in seismic applications. This paper reports on modeling of CFFT either as cast-in-place reinforced or precast post-tensioned column in conjunction with a reinforced concrete (RC) footing. The model is verified against two earlier experimental programs at the member-level and subassembly-level. The model was then used to conduct a parametric study of different column configurations under a constant axial load and a reversed cyclic lateral load. Moreover, seismic performance of a typical CFFT column is compared with its RC counterpart under three different ground acceleration records. The study shows that seismic analysis of CFFT columns is possible using available analytical tools for conventional RC columns. Also, CFFT columns demonstrate superior performance over their RC counterparts in response to wide-ranging ground acceleration records. Fiber architecture of the FRP tube could be optimized for strength and ductility. Internal steel reinforcement and a minimum thickness of FRP tube are deemed necessary to provide adequate ductility and system integrity in seismic applications.     

Stress–strain model for FRP-confined concrete under cyclic axial compression

One important application of fibre-reinforced polymer (FRP) composites in construction is as FRP jackets to confine concrete in the seismic retrofit of reinforced concrete (RC) structures, because FRP confinement can enhance both the compressive strength and ultimate strain of concrete. For the safe and economic design of FRP jackets, the stress–strain behaviour of FRP-confined concrete under cyclic compression needs to be properly understood and modelled. This paper presents a stress–strain model for FRP-confined concrete under cyclic axial compression. The model consists of the following major components: (a) a monotonic stress–strain model for FRP-confined concrete developed by the authors in a previous study for predicting the envelope curve; (b) new algebraic expressions for predicting unloading and reloading paths; and (c) predictive equations for determining the permanent strain and stress deterioration, with the effect of loading history duly accounted for. The capability and accuracy of the proposed model in predicting the complete stress–strain history of FRP-confined concrete under cyclic axial compression are demonstrated through comparisons between predictions of the proposed model and test results.     

Experimental study on seismic strengthening of RC columns with wrapped CFRP sheets

Eight specimens, including two strengthened after being loaded to yield level to imitate strengthening with some damage and one strengthened under a sustained axial load to imitate strengthening under service condition, were tested under constant axial load and lateral cyclic load to investigate seismic performance of RC columns strengthened with carbon fiber reinforced polymer sheets (CFRP sheets). The ductility enhancement with the confinement of CFRP sheets was studied by the strain development and distribution in the CFRP sheets. Based on the experimental results, a confinement factor of CFRP and an equivalent transversal reinforcement index were suggested. Thus, the seismic design method of the current Chinese seismic design code for RC columns can be directly used in determining the amount of CFRP required for seismic strengthening.     

Effects of strain hardening of reinforcement on flexural strength and ductility of reinforced concrete columns

In the evaluation of flexural strength of reinforced concrete (RC) columns, the elastic–perfectly plastic constitutive model is generally used for steel reinforcement, which ignores the strain hardening effect. While some engineers believe that the flexural strength so obtained is on the safe side, others are concerned that underestimation of member strength could lead to inaccurate prediction of overall structural behaviour especially under extreme events. In any case, better understanding of the possible over‐strength and its effects on flexural ductility and failure mechanism is necessary. In this paper, the effects of strain hardening of reinforcement on the flexural strength and ductility of reinforced normal‐ and high‐strength concrete columns are studied based on rigorous full‐range moment–curvature analysis. The study has identified if and how various parameters affect the strain hardening effect, which include axial load ratio, concrete strength, confining stress, reinforcement ratio and the tensile‐strength‐to‐yield‐stress ratio of steel. The effects of strain hardening can be quite significant for RC columns under relatively low axial load and relatively high confining stress. Copyright © 2009 John Wiley & Sons, Ltd.     

Axial–flexural interaction in circular FRP-confined reinforced concrete columns

External confinement by fibre reinforced polymers (FRPs) is now a widely implemented technique to strengthen reinforced concrete (RC) columns. To date, the vast majority of experiments on FRP-confined concrete have considered short, unreinforced, small scale concrete cylinders, with aspect ratios of less than three and tested under concentric axial load. In practice, RC columns invariably have aspect ratios larger than three and are subjected to inadvertent load eccentricities or combined axial–flexural (P–M) loading. Limited available research suggests that the effectiveness of FRP confinement is reduced under P–M loading. This paper presents the results of a systematic test program on circular FRP-confined RC columns of realistic slenderness under eccentric axial loads to study the mechanics and performance of these types of members. Test data are compared against theoretical P–M interaction diagrams, as well as against recently published design guidelines for P–M interaction in FRP-confined RC columns.     

空间曲面纤维网格制作及加固水下混凝土柱试验研究

平面纤维（FRP）网格以其卓越的性能在特定结构加固方面具有较好的应用优势,但其对小尺寸曲面、异形混凝土结构构件的约束加固并不适用。提出了空间曲面纤维网格制作工艺,制作了曲面纤维网格制品,测试了纤维网格拉伸的力学性能;提出了空间曲面纤维网格加固水下混凝土柱的施工工艺,并进行了轴压试验研究,试验结果表明：空间曲面纤维网格加固水下混凝土试件的轴压性能明显优于未加固混凝土圆柱试件,随着纤维网格纤维用量的增加,加固试件的承载能力和变形能力能有效提高,并取得和陆上试件相近的加固效果,根据纤维网格约束量的不同,水下加固试件的应力-应变曲线表现为软化、强化与临界行为,采用现有FRP约束混凝土模型能够较好地预测空间曲面纤维网格加固水下混凝土的极限应力与极限应变。     

Exploring the impact of chloride-induced corrosion on seismic damage limit states and residual capacity of reinforced concrete structures

A non-linear finite element (FE) framework for time-dependent capacity assessment of corroded rectangular reinforced concrete (RC) columns is developed. The proposed non-linear FE model includes the impact of corrosion on inelastic buckling and low-cycle fatigue degradation of longitudinal reinforcement. The proposed non-linear FE model is validated against a set of experimental data and then extended to evaluate the impact of corrosion on damage limit states to be used in seismic performance and evaluation of corroded structures. This is done through a parametric study on hypothetical RC columns, varied in axial force ratios, mass loss ratios, cover crack widths and confinement levels. Moreover, the application of the proposed model in seismic collapse capacity assessment of corroded structures is shown through non-linear dynamic analyses of prototype columns. Results show that, depending on the axial force ratio, corrosion changes the failure mechanism of the columns. The results of this study suggest that in seismic fragility analysis of corroded structures, the damage limit states should be considered as time-variant parameters.     

FRP加固钢筋混凝土柱经历模拟地震作用后的残余性能试验研究

本文对8根大比例的FRP加固钢筋混凝土桥柱模型在轴压比为0.2的恒定轴压力下进行拟静力试验,再对其中5根试验柱在经历有限的地震作用后进行长期荷载作用下的轴压试验。试验表明,FRP加固柱的徐变变形远小于对比柱的徐变变形;带损伤的加固柱在长期荷载作用下的变形发展受加固柱的损伤程度和持荷大小的控制,并与FRP的弹性模量有关;在试验条件下,带损伤的加固柱长期轴向变形呈稳定态势。采用AEMM和F ind ley模型为基础,考虑损伤程度、密封状态、三向应力状态和应力重分布等因素的影响,建立了损伤加固柱的徐变计算模型,并进行了验证。分析表明,损伤对加固柱的徐变发展和徐变破坏时限有显著影响。     

FRP加固钢筋混凝土圆柱侧向变形能力研究

结构变形能力定量计算是基于性能的抗震设计的重要内容,能确保结构满足不同的性能目标要求.研究纤维增强复合材料(FRP)加固钢筋混凝土(RC)圆柱侧向变形能力计算方法.通过数值分析对FRP加固RC圆柱截面弯矩—曲率关系进行计算,计算结果显示截面极限曲率明显小于试验实测值,两者差异程度受轴压比控制.根据计算结果和试验结果,提...     

初始损伤对CFRP加固混凝土梁受弯性能的影响

通过9根碳纤维布加固的具有初始损伤钢筋混凝土梁、6根碳纤维布直接加固钢筋混凝土梁及3根对比钢筋混凝土梁的抗弯性能试验，分析了初始荷载、初始裂缝及加载历史等对加固梁裂缝发展、屈服荷载、刚度、极限荷载的影响。试验结果表明，采用碳纤维布加固钢筋混凝土梁可以有效的提高其抗弯性能。初始荷载、初始裂缝及加载历史等对加固梁的裂缝发展、刚度、屈服荷载和极限荷载均有不同程度的影响。     

Load carrying capacity and ductility of circular concrete columns confined by ferrocement including steel bars

In order to significantly increase not only the load carrying capacity but also the ductility of existing circular concrete columns, a method to strengthen the columns with ferrocement including steel bars (FS) jacket was proposed. To evaluate the behaviour of the FS strengthening columns more objectively and thoroughly, a comparatively experimental study on uniaxial compression behaviours of concrete columns wrapt with three different confining systems, namely bar mat-mortar (BM), FS, and fibre reinforced polymer (FRP), was carried out. Fifty-one specimens were produced. The load–strain responses, failure modes, ultimate loads and ductilities of various strengthened columns were investigated. The experiment results showed that the mortar cover crack spaces of FS columns were basically equal to the wire mesh spacing, leading to the occurrence of much more cracks compared with that of BM columns. As a result, on the premise that the concrete compressive strength of the FS columns increased 30% compared with that of the BM columns, the ductility of the former reached about twice as that of the latter. When the confined concrete strength of FRP strengthened columns was similar to that of FS strengthened columns, the ductility, energy absorption capacity and deformation capacity of the former were obviously lower than those of the latter. In the light of a formula for calculating the ultimate load of concrete-filled steel tube (CFST), an empirical formula for predicting the ultimate load of the FS strengthened concrete column was proposed. The calculation results by using the proposed formula show a good agreement with the test results.     

预应力碳纤维布加固混凝土方形截面短柱轴心受压试验

为了研究预应力碳纤维(CFRP)布加固混凝土方形截面短柱轴心受压力学性能,根据配筋率的不同设计了3组共计12根混凝土方形截面短柱,每组包括3根预应力CFRP布加固的混凝土柱和1根普通混凝土柱。通过静载试验获得了各试件受压极限承载力、位移和预应力CFRP布应力增量等试验数据,得到了荷载作用下试件变形曲线及破坏形态,分析了预应力CFRP布应力水平和纵筋配筋率对混凝土方形截面短柱加固后的力学性能的影响。研究结果表明:采用预应力CFRP布约束混凝土柱一方面能够限制混凝土横向变形,使柱中混凝土从加固时刻起即受到环向应力,提高了混凝土极限压应变,从而显著提高柱的极限承载力,承载力提高幅度在60%以上;另一方面,由于受压柱的延性与截面配筋率有关,加固柱配筋率较低时,延性较好;反之则延性较差。     

Experimental evaluation of the fire behaviour of insulated fibre-reinforced-polymer-strengthened reinforced concrete columns

In recent years, there has been an increase in the use of fibre reinforced polymer (FRP) materials for strengthening of concrete structures. However, since FRP materials are combustible, and because they are typically applied to the exterior of structural members in these strengthening applications, concerns exist regarding the behaviour of such FRP strengthening systems in fire. There is currently little information available on the fire endurance of FRP-strengthened concrete systems. This paper presents results from full-scale fire resistance experiments on three insulated FRP-strengthened reinforced concrete (RC) columns. A comparison is made between the fire performances of FRP-strengthened RC columns and conventional unstrengthened RC columns tested previously. Data obtained during the experiments is used to show that the fire behaviour of FRP-wrapped concrete columns incorporating appropriate fire protection systems is as good as that of unstrengthened RC columns. Thus, satisfactory fire resistance ratings for FRP-wrapped concrete columns can be obtained by properly incorporating appropriate fire protection measures into the overall FRP-strengthened structural system. Fire endurance criteria and preliminary design recommendations for fire safety of FRP-strengthened RC columns are briefly discussed.