Observations on the electrical resistivity of steel fibre reinforced concrete

Steel fibre reinforced concrete (SFRC) is in many ways a well-known construction material, and its use has gradually increased over the last decades. The mechanical properties of SFRC are well described based on the theories of fracture mechanics. However, knowledge on other material properties, including the electrical resistivity, is sparse. Among others, the electrical resistivity of concrete has an effect on the corrosion process of possible embedded bar reinforcement and transfer of stray current. The present paper provides experimental results concerning the influence of the fibre volume fraction and the moisture content of the SFRC on its electrical resistivity. The electrical resistivity was measured by alternating current (AC) at 126 Hz. Moreover, an analytical model for the prediction of the electrical resistivity of SFRC is presented. The analytical model is capable of predicting the observed correlation between the fibre volume fraction and the electrical resistivity of the composite (the SFRC) for conductive fibres and moisture saturated concrete. This indicates that the steel fibres were conducting when measuring the electrical resistivity by AC at 126 Hz. For partly saturated concrete the model underestimated the influence of the addition of fibres. The results indicate that the addition of steel fibres reduce the electrical resistivity of concrete if the fibres are conductive. This represents a hypothetical case where all fibres are depassivated (corroding) which was created to obtain a conservative estimate on the influence of fibres on the electrical resistivity of concrete. It was observed that within typical ranges of variation the influence of the moisture content on the electrical resistivity was larger than the effect of addition of conductive steel fibres, but also that the relative impact on the electrical resistivity due to conductive steel fibres increased when the moisture content of the concrete was reduced.     

钢筋与结构型合成纤维对混凝土抗冲击性能混杂效应的分析

为了研究钢筋与结构型合成纤维混杂后对混凝土抗冲击性能的影响,采用改进的自由落球冲击试验装置,对素混凝土、钢筋混凝土、结构型合成纤维增强混凝土以及钢筋-结构型合成纤维混杂增强的混凝土试件的抗冲击性能进行了试验研究,分析了钢筋、结构型合成纤维以及钢筋与结构型合成纤维混杂后对混凝土抗冲击性能的影响及其增强机理。同时,利用Weibull分布理论分析了试件初裂冲击次数和破坏冲击次数的分布规律。研究表明:结构型合成纤维可以提高混凝土抗冲击性能;对于提高混凝土的抗冲击性能,钢筋与结构型合成纤维表现出显著的正混杂效应;两参数Weibull分布能较好的描述钢筋-结构型合成纤维混凝土抗冲击次数的分布特征。     

Shear behaviour of steel fibre reinforced self-consolidating concrete beams based on the modified compression field theory

A series of steel fibre reinforced self-consolidating concrete (SFRSCC) beams have been tested to investigate the influence of steel fibres and the combined effect of fibres and stirrups on the deflection and cracking, ultimate loads and failure pattern. The experiment indicates that the shear strength increases clearly with the increasing of fibre content. The combination of steel fibres and stirrups demonstrates a positive composite effect on the ultimate load, ductility and failure pattern of concrete beam. This study also examines the feasibility of applying the modified compression field theory (MCFT) for the suitable assessment of shear resistance in fibre and steel rebar reinforced self-consolidating concrete beams. For fibre reinforced concrete member, a theoretical method is proposed based on the MCFT. The proposed ultimate shear capacity model was verified by the comparison with different test results.     

浅谈钢筋混凝土中钢筋锈蚀修补技术

研究钢筋混凝土结构的修补原理、技术与材料对保证钢筋混凝土结构工程的长期、耐久性及使用寿命有着重要的指导意义及实用价值。本文介绍了钢筋混凝土中钢筋锈蚀的有关修补技术。     

An experimental study on the tensile strength of steel fiber reinforced concrete

This paper deals with steel fiber reinforced concrete mechanical static behaviour and with its classification with respect to fibers content and mix-design variations. A number of experimental tests were conducted to investigate uniaxial compressive strength and tensile strength. Different mixtures were prepared varying both mix-design and fiber length. Fibers content in volume was of 1% and 2%. Mechanical characterization was performed by means of uniaxial compression tests with the aim of deriving the ultimate compressive strength of fiber concrete. Four-point bending tests on notched specimens were carried out to derive the first crack strength and the ductility indexes. The tensile strength of steel fiber reinforced concrete (SFRC) was obtained both from an experimental procedure and by using an analytical modelling. The experimental tests showed the different behaviour of SFRC with respect of the different fiber content and length. Based on the experimental results, an analytical model, reported in literature and used for the theoretical determination of direct tensile strength, was applied with the aim of making a comparison with experimental results. The comparison showed good overall agreement.     

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。     

The effect of steel fibres on the earthquake-resistant design of reinforced concrete structures

The results of an experimental investigation are presented, studying the effect of fibres on the behaviour of reinforced-concrete (RC) structures designed in accordance with Eurocode 8. Twelve two-span continuous RC columns, eight with and four without steel fibres, were tested to failure, under constant axial force and monotonic or cyclic lateral displacement. Specimens without fibres suffered in some cases premature brittle failure, reflecting the incompatibility between post-peak concrete behaviour and the theoretical model underlying RC design. It was shown that it is possible to correct for this incompatibility through the use of steel fibres, resulting in a behaviour that satisfied current performance requirements for strength and ductility.     

Preliminary study on bullet resistance of double-layer concrete panel made of rubberized and steel fiber reinforced concrete

In this study, the impact resistance of double-layer concrete panels made of rubberized and steel fiber reinforced concrete subjected to direct fire weapon (11 mm or 0.45 magnum bullet size) is investigated. Concrete panels with dimensions of 400 × 400 × 50 mm are subjected to impact forces from 11 mm-diameter bullets at a distance of 10 m. Three types of concrete panels are tested: single-layer steel fiber reinforced concrete (SFRC), single-layer crumb rubber concrete (CRC), and double-layer CRC/SFRC. For a double-layer CRC/SFRC, the CRC layer of 12.5 mm is added to the front surface by partially replacing part of the SFRC panel. The CRC layer is expected to act as a cushion layer to absorb impact energy from the bullet and to reduce damage to the concrete panel.     

Experimental and analytical assessment of the behavior of stainless steel reinforced concrete beams

This paper reports the results of a comprehensive experimental test program on high performance and standard AISI 316L stainless, steel reinforced concrete beams. Experimental results were compared with theoretical analysis data using current reinforcement mechanical feature standards and experimental testing-based ones, to identify any resistance, ductility and membrane collapse mode difference. It is also proved the importance of establishing a specific standard on the use of stainless steel reinforcement, in order to enable its more widespread, structural use, and not only for its corrosion resistance.

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Résumé Dans ce travail sont présentés les résultats d'un vaste programme d'essais expérimentaux réalisés, sur des poutres en béton armé normal et à haute résistance; toutes les poutres sont renforcées de barres d'acier inoxydable. Les résultats obtenus à partir de l'analyse expérimentale ont étés comparés avec ceux obtenus par une analyse théorique dans laquelle on a utilisé les valeurs des caractéristiques mécaniques de l'acier, soit standardisées, soit obtenues à partir de l'expérimentation, afin de mettre en évidence pour les poutres, les différences en termes de résistance, de ductilité et de mode de rupture.

     

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。     

Studies on the corrosion resistance of reinforced steel in concrete with ground granulated blast-furnace slag--An overview

The partial replacement of clinker, the main constituent of ordinary Portland cement by pozzolanic or latent hydraulic industrial by-products such as ground granulated blast furnace slag (GGBFS), effectively lowers the cost of cement by saving energy in the production process. It also reduces CO2 emissions from the cement plant and offers a low priced solution to the environmental problem of depositing industrial wastes. The utilization of GGBFS as partial replacement of Portland cement takes advantage of economic, technical and environmental benefits of this material. Recently offshore, coastal and marine concrete structures were constructed using GGBFS concrete because high volume of GGBFS can contribute to the reduction of chloride ingress. In this paper, the influence of using GGBFS in reinforced concrete structures from the durability aspects such as chloride ingress and corrosion resistance, long term durability, microstructure and porosity of GGBFS concrete has been reviewed and discussed.     

Influence of fiber chemical coating on the acoustic emission behavior of steel fiber reinforced concrete

The current study focuses on the effect of chemical coating on the acoustic emission (AE) characteristics monitored during the fracture process in steel fiber reinforced concrete (SFRC). Different shapes of chemically treated and un-treated steel fibers are used to create specimens which are subjected to four point bending up to failure. Sensitive AE indices demonstrate that the coating gives distinct characteristics to the interface bonding between the fiber and the concrete matrix, which are evident mainly during the pull-out stage, after the moment of macroscopic crack formation. Specifically, AE average frequency and RA value, which defines the rising angle of the waveforms indicate that coating results in extensive matrix cracking in addition to the friction between fiber and concrete which characterizes the uncoated fibers. AE analysis can be used for interpretation of the fracturing stage and characterization of the fracture mode. It is shown that the surface conditioning of the fibers leaves a clear fingerprint on the AE signals, shedding light into the processes that occur during failure in SFRC.     

Modelling the influence of age of steel fibre reinforced self-compacting concrete on its compressive behaviour

Steel fibre reinforced self-compacting concrete (SFRSCC) can combine the benefits of self-consolidating concrete technology with those derived from adding steel fibres to quasi-brittle cement based materials. In a recent applied research project joining pre-casting industry, private and public research institutions, a method was developed to design cost-competitive SFRSCC of rheological and mechanical properties required for the prefabrication of SFRSCC façade panels. To assure safe demoulding process of the panels, the influence of the concrete age on the compression behaviour of the SFRSCC should be known. For this purpose, series of tests with specimens of 12 h to 28 days were tested in order to analyze the age influence on the compressive strength, strain at peak stress, Young’s modulus, and compressive volumetric fracture energy. The experimental program was divided in two groups of test series, one with SFRSCC of a volumetric fibre percentage of 0.38% and the other with 0.57%. To apply the obtained data in the design and numerical analysis framework, the influence of the age on these SFRSCC properties was modelled. This work describes the carried out experimental program, presents and analyzes the obtained results, and provides the derived analytical expressions.     

基于荷载时程分析法的钢筋混凝土与钢板混凝土墙冲击响应对比分析

为对比核电站核岛厂房钢筋混凝土结构(RC)与钢板混凝土结构(SC)外墙的抗冲击性能,基于荷载时程分析法,用显示非线性动力分析软件ANSYS/LS-DYNA仿真分析1/7.5比例飞机模型撞击RC、SC墙的冲击实验。将RC、SC墙破坏模式、混凝土碎片残余速度及背部钢板变形等计算结果与飞射物-靶体相互作用分析法计算结果及实验结果以及同厚度不同结构类别墙的计算结果进行对比。结果表明,基于荷载时程分析法计算结果有一定保守性,与实验结果吻合较好,且SC墙抗冲击性能优于RC墙,尤其背部钢板能有效约束混凝土撞击方向的运动及限制混凝土碎片飞溅。用于抗飞机撞击的SC结构墙体厚度可适当减薄。     

Behaviour of bond between concrete and steel in a large post-yelding field

The bond-slip relationship between concrete and steel is very important in evaluating the non-linear behaviour of reinforced concrete structures. In this paper, the problem is analysed for reinforced concrete elements in tension. Two different analytical approaches are applied in order to estimate their performances. This research underlines the importance of bond-slip modeling in a steel post-yielding field by comparison among three well-known, different bond-slip relationships. Moreover, the results of simple experimental tests on reinforced concrete tensile elements are shown to assess the reliability of previously proposed models in the elastic range and to confirm that an appropriate model for the post-yielding range of behaviour is necessary.

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Résumé La relation d'adhérence entre l'acier et le béton est très importante dans l'analyse du comportement non linéaire des structures en béton armé. Dans cet article, on étudie le problème pour des éléments de béton armé en traction. Deux méthodes analytiques différentes sont utilisées pour évaluer leurs performances. Cette recherche souligne l'importance de la modélisation de l'adhérence en présence de fortes déformations plastiques de l'acier, en comparant trois relations analytiques bien connues. Les résultats d'un simple essai expérimental sur des éléments en béton armé en traction permettent d'évaluer l'efficacité des modèles d'adhérence présentés auparavant, qui semblent efficaces dans le domaine élastique, et de confirmer la nécessité d'établir un modèle approprié pour ce qui concerne les déformations plastiques de l'acier.

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Significance of oxygen concentration on the quality of passive film formation for steel reinforced concrete structures during the initial curing of concrete

Reinforced concrete prisms with varying surface conditions (sealed, partially sealed, and unsealed) and curing environments (in air and submerged in water) were used to vary the oxygen concentration in laboratory experiments. The experiment results (current rate and half-cell potential) of these tests were used to establish the time-dependent influence of oxygen concentration on passive film formation and corrosion of the reinforcement steel embedded in concrete under various defined conditions. It was found that the adequate availability of free oxygen during the initial curing stage is crucial for the proper formation of the passive layer. This suggests ways to improve passive layer development, and thus the durability of reinforced concrete structures, through the use of curing procedures that control both moisture and oxygen availability.     

Impact of corrosion on mechanical properties of steel embedded in 27-year-old corroded reinforced concrete beams

This paper deals with the impact of corrosion on the mechanical properties of steel in reinforced concrete. Steel bars were extracted from a 27-year-old corroded reinforced concrete beam that had been exposed to a chloride environment. Bars with different degrees of corrosion and with different corrosion pit depths were tested in tension. A comparison was made between nominal and true stress for corroded and control steel specimens. It was noted that the degree of corrosion strongly affected the mechanical properties of the steel, particularly the ultimate stress and strain. Interestingly, the true yield strength of all the corroded steel bars remained almost constant while their true ultimate strength was considerably increased. A reduction of the ultimate elongation appeared to be the major effect of corrosion and affected the compliance with standards.