A study of reinforced concrete bridge columns retrofitted by steel jackets

Abstract

In this paper, theoretical and experimental results of two as‐built circular reinforced concrete (RC) bridge columns and two columns retrofitted with steel jackets are presented. A constitutive model for concrete confined by a steel jacket is proposed. The proposed model is implemented into a sectional analysis to predict the lateral load‐deformation relationship of retrofitted columns. 2/5 scaled RC bridge columns are designed based on the standard details of the existing bridge columns mostly built in late 1980s and early 1990s, in Taiwan (Ministry of Transportation and Communication, 1987; 1995). The columns are expected to have a flexural failure mode during severe ground shaking. Displacement‐controlled cyclic loading tests were conducted to obtain the seismic performance of the columns. The experimental results showed that the bridge column retrofitted with steel jacketing could greatly improve seismic performance measured based by the strength and ductility. The analytical results showed that the proposed constitutive model, implementing sectional analysis, could well capture the lateral force‐displacement relationship of the bridge columns retrofitted with steel jackets.     

A theoretical study on shear strengthening of reinforced concrete beams using composite plates

The present paper deals with shear strengthening of reinforced concrete beams by means of thin fiber-reinforced composite plates. First, the theoretical model is presented. It is then applied to the shear strengthening of a T-beam. This beam is also used to investigate the effects of some design parameters on the ultimate shear strength.     

Behavior of full-scale reinforced concrete beams retrofitted for shear and flexural with FRP laminates

Four full-scale reinforced concrete beams were replicated from an existing bridge. The original beams were substantially deficient in shear strength, particularly for projected increase of traffic loads. Of the four replicate beams, one served as a control and the remaining three were implemented with varying configurations of carbon fiber reinforced polymers (CFRP) and glass FRP (GFRP) composites to simulate the retrofit of the existing structure. CFRP unidirectional sheets were placed to increase flexural capacity and GFRP unidirectional sheets were utilized to mitigate shear failure. Four-point bending tests were conducted. Load, deflection and strain data were collected. Fiber optic gauges were utilized in high flexural and shear regions and conventional resistive gauges were placed in eighteen locations to provide behavioral understanding of the composite material strengthening. Fiber optic readings were compared to conventional gauges.Results from this study show that the use of fiber reinforced polymers (FRP) composites for structural strengthening provides significant static capacity increases approximately 150% when compared to unstrengthened sections. Load at first crack and post cracking stiffness of all beams was increased primarily due to flexural CFRP. Test results suggest that beams retrofit with both the designed GFRP and CFRP should well exceed the static demand of 658 kN m sustaining up to 868 kN m applied moment. The addition of GFRP alone for shear was sufficient to offset the lack of steel stirrups and allow conventional RC beam failure by yielding of the tension steel. This allowed ultimate deflections to be 200% higher than the pre-existing shear deficient beam. If bridge beams were retrofit with only the designed CFRP failure would still result from diagonal tension cracks, albeit at a 31% greater load. Beams retrofit with only the designed shear GFRP would fail in flexure at the mid-span at an equivalent 31% gain over the control specimen, failing mechanism in this case being yielding of the tension steel. Successful monitoring of strain using fiber optics was achieved. However, careful planning tempered by engineering judgement is necessary as the location and gauge length of the fiber optic gauge will determine the usefulness of the collected data.     

考虑损伤的纤维增强混凝土剪力墙恢复力模型研究

通过理论分析与试验回归相结合的方法,对6个局部纤维增强混凝土(FRC)配筋剪力墙循环往复加载试验结果进行研究,提出了考虑轴压比、边缘约束区纵筋率、配箍特征值、FRC和钢筋力学性能以及钢套管等因素的骨架曲线特征点简化计算公式。同时,考虑加载历程对墙体性能退化的影响,通过引入一个基于试验拟合结果的损伤模型,定量描述了各试件滞回环卸载刚度和骨架曲线下降段强度退化规律,给出了恢复力模型滞回规则,并与试验墙进行了模拟对比,验证了模型的准确性。     

钢纤维-橡胶/混凝土抗剪性能试验

抗剪强度和剪切韧性是反映构件在复合受力状态下承载能力及耗能能力的重要指标。为研究钢纤维(SF)-橡胶/混凝土的剪切性能,设计了14组SF-橡胶/混凝土试件,通过双面剪切试验,研究了SF体积分数掺量、橡胶掺量和水胶比对SF-橡胶/混凝土试件的抗剪性能及剪切破坏形态的影响。研究表明:SF的桥联作用及其与橡胶颗粒的协同作用可显著改善混凝土的抗剪性能。SF对SF-橡胶/混凝土试件的抗剪性能起主导作用,SF-橡胶/混凝土试件的抗剪强度、峰值变形及剪切韧性相比普通混凝土及橡胶/混凝土试件均显著提高,且增幅随SF掺量的增加而增大,剪切破坏呈现出明显的延性特征。当SF体积分数为1.5vol%时,橡胶掺量(等体积取代砂取代率)为10%的SF-橡胶/混凝土试件的抗剪强度、峰值变形相比橡胶/混凝土分别提高了78%、63%。橡胶对SF-橡胶/混凝土试件的抗剪性能也起到辅助作用,SF-橡胶/混凝土试件的剪切韧性及延性相比SF/混凝土试件进一步增加。采用水胶比优化设计后,随着橡胶掺量的增加,SF-橡胶/混凝土的抗剪强度、峰值变形及峰值前剪切韧性可基本保持不变,而峰值后韧性指标进一步增加,增幅可高达96%。根据试验结果,考虑橡胶及SF掺量的影响提出了SF-橡胶/混凝土的抗剪强度计算式。      

Crack damage mitigation and shear behavior of shear-dominant reinforced concrete beams repaired with strain-hardening cement-based composite

This research aims to study: (1) the crack damage mitigation and shear behavior of reinforced concrete (RC) beams that have been repaired using strain-hardening cement-based composite (SHCC) via experimental testing and (2) the contribution of the SHCC layers to the shear strength of the repaired RC beams via predictions. Five cantilever RC beams with a shear span-to-depth ratio of 2.8 were subjected to cyclic concentrated loading. The study variables include two types of tensile performance of the SHCC (with low or high strength in tension) and two repair methods (patching and layering). The experimental results show that the use of a SHCC layer leads to a substantial increase in the shear strength and ductility of the RC beams after the peak load. During the tests, all of the SHCC repaired beams showed delamination along the interface between the concrete and SHCC, and the shear resistance started to drop. However, the results also indicate that SHCC layers can be effective repair material for enhancing the control of cracking to help protect the concrete from the migration of aggressive agents in severe environments. In order to predict the shear strength of RC beams that have been repaired with SHCC, two methods were used in this study; one is based on Dinh's proposed model that considers the shear strength in both the compression and tension zones, and the other method considers the shear strength of the reinforcement, such as a stirrup or fiber-reinforced polymer (FRP) laminate that considers only the tensile strength across cracks. These two methods were able to predict the contribution of the SHCC layer to the shear strength of the RC beams, and the predicted shear strength values were very similar between the two methods.     

基于纤维模型的型钢混凝土组合剪力墙滞回性能分析

为更深入研究型钢混凝土剪力墙的抗震性能,在已有该类剪力墙滞回性能试验结果基础上,采用OpenSees软件中非线性纤维梁-柱单元进行低周往复加载的数值模拟,通过直接在截面层次定义非线性剪切恢复力方法模拟纤维截面的抗剪。计算结果与试验结果总体吻合较好,表明该纤维模型法能较好模拟型钢混凝土剪力墙的抗剪承载力、捏缩效应及刚度退化。该方法可为此类结构体系的弹塑性分析提供参考。     

钢纤维混凝土遮弹层抗弹丸侵彻效应试验研究与分析

钢纤维混凝土遮弹层抗常规武器侵彻效应问题,是防护工程界亟待解决的一个崭新课题。为研究这种新型防护材料的抗侵彻性能,利用Φ12.7mm弹道炮-测速靶系统对混凝土及钢纤维混凝土进行了弹道冲击对比试验,获得了弹丸着靶速度及对应的最大侵彻深度、弹坑直径、靶体破坏形态等试验参数,并利用高速摄影系统记录了靶体的动态破坏过程。针对现有经验公式均不能反映钢纤维混凝土材料高韧性影响的不足,引入钢纤维混凝土材料韧度R,对试验数据进行了回归分析,导出了侵彻深度工程计算公式。计算结果与试验数据对比表明,预估公式计算精度较高,公式中相关参数简单易于确定,且能反映钢纤维混凝土的高强高韧性特点,在实际工程应用中具有重要的参考价值。     

On the reduce of interfacial shear stresses in fiber reinforced polymer plate retrofitted concrete beams

One major problem when using bonded fiber reinforced polymer (FRP) plate is the presence of high interfacial shear stresses near the end of the composite (edge effect) which might govern the failure of the strengthening schedule. It is known that the decrease of plate thickness reduces the magnitude of stress concentration at plate ends. Another way is to use a plate end tapering. In this paper, the analytical solution of interfacial shear stresses obtained has been extended by a numerical procedure using the modal analysis of finite element method (FEM) in a retrofitted concrete (RC) beam with the FRP plate with tapered end, which can significantly reduce the stress concentration. This approach allows taking into consideration the variation of elastic properties of adhesive and plate as well as the complicated geometrical configurations and effects of thermal loads.     

Analysing the shielding effect of a concrete wall reinforced with steel plate using generalised surface integral equations

The electromagnetic shielding effect of a two-dimensional concrete wall reinforced with steel plate is analysed using a generalised surface integral equation (GSIE) method developed from Huygens? equivalence principle. The wall is divided into small blocks. Each block is analysed independently at first, and its electromagnetic characteristics are expressed by an associated generalised transition matrix defined on the reference surface of that block. In order to find the electromagnetic characteristics of the whole system, a GSIE formulation is established on the reference surface of all blocks, which can be solved efficiently using characteristic basis functions.     

Predictions of comulative damage for concrete and reinforced concrete

Nonlinear damage evolution equation is suggested to predict fatigue life of plain concrete subjected to sequential, constant amplitude loading. This approach seems to eliminate the nonconservative aspect of the linear damage law (the Miner hypothesis) and is based on the observed shape of the damage growth relationship. For serviceability based design of reinforced concrete structures, a simple nonlinear relationship is developed to relate the increases in deflection and crack width with the cycle-ratio. The results compare favorably with experimental data.     

混合联肢部分外包组合剪力墙抗震性能试验研究

混合联肢部分外包组合剪力墙结构是一种典型的两阶段耗能结构体系。依据现行设计规范设计的混合联肢部分外包组合剪力墙结构对结构弹塑性阶段的整体性能缺乏考虑,在设防地震和罕遇地震作用下结构的屈服顺序难以控制。基于混合联肢部分外包组合剪力墙结构体系“小震不坏,中震可修,大震不倒”的三水准抗震设防性能目标,提出结构两阶段耗能的塑性设计方法。以目标位移和理想破坏模式作为预测结构弹塑性受力状态性能目标,针对不同地震水准下的性能目标,考虑耦连比对结构合理耗能机制的影响,将结构的能力曲线等效为三线型,改进了传统基于能量平衡的塑性设计方法,保证结构在设防地震及罕遇地震下实现理想屈服顺序,避免薄弱层产生。采用建议的设计方法设计了一个12层结构算例,并采用ABAQUS对算例进行推覆分析和弹塑性时程分析。结果表明,采用基于两阶段耗能的塑性设计方法预测的中震及大震下结构顶点位移角与数值分析结果误差分别为0.5%和12.1%,最大层间位移角满足结构弹塑性层间位移角的要求。算例结构能够实现“强节点弱构件、强墙肢弱连梁”合理的屈服顺序,保证结构预期的失效机制和不同地震水准作用下的性能目标,验证了基于两阶段耗能的塑性设计方法的有效性。

     

钢骨混凝土异形柱-钢梁节点抗震性能试验研究

为了研究钢骨混凝土异形柱-钢梁节点的抗震性能,进行了4个T形钢骨混凝土柱-钢梁节点和4个L形钢骨混凝土柱-钢梁节点的拟静力试验。试验考虑了混凝土强度等级、核心区配箍率和轴压比等参数的影响,对骨架曲线、承载力、核心区剪切变形、延性和耗能能力等抗震性能指标进行了分析。结果表明,在低周往复荷载作用下,钢骨混凝土异形柱-钢梁框架节点滞回曲线饱满,表现出良好的延性性能和耗能能力,典型破坏形态为节点核心区剪切斜压破坏和节点区焊缝失效破坏;高轴压力下节点具有较高的承载能力但延性性能降低;混凝土强度越高,节点承载能力越大,但延性性能越差;增大核心区配箍率对试件的延性和承载力有明显的提高,并能改善试件屈服后的耗能能力。     

Experimental and theoretical investigation of the shear resistance of steel fibre reinforced prestressed concrete X-beams—Part I: Experimental work

This is the first part of two papers on the experimental (Part I) and theoretical (Part II) resistance of steel fibre reinforced precast concrete beams. Short steel fibres have been introduced into prestressed concrete X beams in order to study their behaviour under shear loads. The X beams, which have circular web profiles, were chosen to represent longitudinal sections from 215 mm deep prestressed precast hollow cored floor units, which are known to fail in shear in a brittle manner. No shear links were used. Round hooked end high strength steel (HS), and thin amorphous metal (AM) fibres were used in volume fractions up to 2.0%. The maximum flexural strength of fibre reinforced concrete (FRC) was 10.28 N/mm², some 50% greater than plain concrete. In the plain concrete beams the ratio η of the ultimate shear resistance to the cracking resistance was 1.0, as expected. For the fibre beams η=1.43 to 1.52 for the HS fibres and η-1.23 for the AM fibres. Theoretical and empirical equations were developed using modified FRC principal tensile stress methods to predict ultimate shear strength and are given in Part II.

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Résumé Il s'agit de la première partie d'un article sur la résistance des poutres en béton préfabriqué renforcé de fibres d'acier, présentant les expériences (partie I) et la théorie (partie II). De courtes fibres d'acier ont été utilisées dans des poutres X précontraintes pour étudier leur comportement sous l'action de forces de cisaillement. Les poutres X, qui ont des ames de profil circulaire, ont été choisies pour représenter des sections longitudinales des dalles alvéolées précontraintes de 215 mm d'épaisseur, qui sont connues pour leur mode de rupture casant sous l'action des efforts tranchants. L'armature de cisaillement n'a pas été utilisée. Des crochets ronds en acier à haute adhérence et de minces fibres métalliques amonrphes ont été utilisés jusqu'à des fractions de 2% du volume. La résistance à la tension maximale sous l'effet de flexion du béton de fibres d'acier était de 10,28 N/mm², soit 50% de plus que pour le béton normal. Pour les poutres en béton normal, le rapport η entre la résistance ultime au cisaillement et la résistance à la fissuration était de 1,0, comme prévu. Pour les poutres en fibres d'acier ce rapport était entre 1,43 et 1,52 pour l'acier à haute adhérence et 1,23 pour les fibres métalliques amorphes. Des équatins théoriques et empiriques ont été développées à l'aide de méthodes de contrainte de tension principlale du béton de fibres d'acier modifiées pour la prédiction de la résistance ultime au cisaillement; celles-ci sont données dans la deuxième partie.

     

Performance evaluation of corrosion-affected reinforced concrete bridge girders using Markov chains with fuzzy states

A methodology for performance evaluation of reinforced concrete bridge girders in corrosive environments is proposed. The methodology uses the concept of performability and considers both serviceability- and ultimate-limit states. The serviceability limit states are defined based on the degree of cracking (characterized by crack width) in the girder due to chloride induced corrosion of reinforcement, and the ultimate limit states are defined based on the flexural load carrying capacity of the girder (characterized in terms of rating factor using the load and resistance factor rating method). The condition of the bridge girder is specified by the assignment of a condition state from a set of predefined condition states. Generally, the classification of condition states is linguistic, while the condition states are considered to be mutually exclusive and collectively exhaustive. In the present study, the condition states of the bridge girder are also represented by fuzzy sets to consider the ambiguities arising due to the linguistic classification of condition states. A non-homogeneous Markov chain (MC) model is used for modeling the condition state evolution of the bridge girder with time. The usefulness of the proposed methodology is demonstrated through a case study of a severely distressed beam of the Rocky Point Viaduct. The results obtained using the proposed approach are compared with those obtained using conventional MC model. It is noted that the use of MC with fuzzy states leads to conservative decision making for the problem considered in the case study.     

Experimental study and numerical simulation of the damage mode of a square reinforced concrete slab under close-in explosion

Terrorist attacks using improvised explosive devices on reinforced concrete buildings generate a rapid release of energy in the form of shock waves. Therefore, analyzing the damage mode and damage mechanism of structures for different blast loadings is important. The current study investigates the behavior of one-way square reinforced concrete (RC) slabs subjected to a blast load through experiments and numerical simulations. The experiments are conducted using four 1000 mm × 1000 mm × 40 mm slabs under close-in blast loading. The blast loads are generated by the detonations of 0.2–0.55 kg trinitrotoluene explosive located at a 0.4 m standoff above the slabs. Different damage levels and modes are observed. Numerical simulation studies of the concrete damage under various blast loadings are also conducted. A three-dimensional solid model, including explosive, air, and RC slab with separated concrete and reinforcing bars, is created to simulate the experiments. The sophisticated concrete and reinforcing bar material models, considering the strain rate effects and the appropriate coupling at the air–solid interface, are applied to simulate the dynamic response of RC slab. The erosion technique is adopted to simulate the damage process. Comparison of the numerical results with experimental data shows a favorable agreement. Based on the experimental and numerical results, the damage criteria are established for different levels of damage. With the increase of the explosive charge, the failure mode of RC slab is shown to gradually change from overall flexure to localized punching failure.     

Experimental programme and analytical study of bond stress distributions on a composite plate bonded to a reinforced concrete beam

An experimental investigation of the bond stress along the concrete-epoxy-plate interface is presented. The main variables were the plate end cut-off and the different corrosion ratios induced to the lower reinforcement of the beams. This was supported with a parametric study for several variables based on a simplified model. The results showed that the plate breadth to thickness ratio, (b_pl/t_pl), was a very significant factor, which affects the bond stress concentration at the plate end. It was also found that the plat end cut-off may affect the bond stress concentration. However, more investigation is still needed to address this issue.     

Experimental and numerical study of the fatigue behaviour of composite patch reinforced cracked steel plates

One-side, low cost carbon/epoxy patches were laminated on large scale specimens, which were tested and subsequently modelled with the finite element method. An R = ?1 cyclic loading was applied and patch strains and crack propagation rate were monitored. The experimental results were very encouraging, demonstrating that patches can effectively slow down the crack growth and extend the specimens’ life by a factor of up to 2, despite their relatively low stiffness ratio. Based on some simplified assumptions, FE modelling proved adequate in accurately predicting the crack propagation rate.     

高延性混凝土加固高轴压比剪力墙抗震性能试验研究

为提高高轴压比下(低矮)剪力墙的抗震性能,提出采用高延性混凝土(HDC)面层对其加固。设计了3片剪跨比为1.1的混凝土剪力墙,其中1片为对比试件,其余2片分别采用HDC面层和钢筋网HDC面层进行加固。通过拟静力试验,研究剪力墙试件的破坏形态、变形能力、耗能能力及刚度退化特性。试验结果表明:采用HDC面层加固的剪力墙试件,加固层裂而不坏,与内部墙体协同工作性能良好,可对内部混凝土形成一定的约束作用,改善了剪力墙的脆性剪切破坏特征;HDC面层能有效提高剪力墙的受剪承载力、变形能力和耗能能力;在HDC面层中配置钢筋网片使加固面层斜裂缝开展延缓,可充分发挥HDC良好的拉伸性能和耐损伤性能,使加固试件在破坏阶段的刚度退化缓慢。基于软化桁架模型,考虑HDC加固层贡献,提出了加固试件的受剪承载力公式。