Dual-phase steel re-bars for high-ductile r.c. structures, Part 2: Rotational capacity of beams

The article presents an application of an innovative type of reinforcing bars, made by dual-phase (DP) steel, for the design of high-ductile r.c. structures in seismic areas.

In particular a comparison of the capacity, in terms of ductility, of r.c. structural elements reinforced using dual-phase (DP) steel bars with that of identical structures reinforced with the traditional Tempcore steel bars, currently used in practice, has been carried out.

To this purpose an analytical procedure enabling the calculation of the plastic rotational capacity of r.c. beams has been set up, obtaining the bending moment–curvature law through an expressly developed model in order to reproduce “tension-stiffening” phenomenon.     

Study of the corrosion of reinforcement in concrete elements used for the repair of monuments

The conservation of cultural heritage is a demanding and imperative task. Of particular interest is the prevention of damage in monuments in load bearing elements made of reinforced concrete. In this case, the remediative intervention should be done with special caution, especially in places of the structure where earthquakes pose immediate threat for the integrity of historical monuments. The present work focuses on the problems associated with corrosion of steel in reinforced concrete, considered as a composite material consisting of steel and inorganic constituents. The study is concerned with S500s Tempcore^® steel and the associated corrosion phenomena, responsible for the degradation of the mechanical properties of the reinforced concrete. Corrosion of steel bars in reinforced concrete is attenuated due to either of the incomplete coverages of the metal with concrete, leaving it exposed to the aggressive marine environment or from galvanic corrosion. The latter is due to the presence of different types of metallic elements introduced by the welding process employed often in pre-earthquake and post-earthquake repairs in order to strengthen load bearing elements. In a microscopic scale the development of special conditions may lead to major damages of the structures. In the present work, we report on the results of mechanical stress measurements done on S500s Tempcore^® steel bar specimens. Prior to the stress measurements, the specimens were subjected to accelerated corrosion by exposure in salt spray chamber for time periods up to 90 days. Specimens in which welding side-by-side was applied were also exposed at the same conditions and the respective mechanical stresses were measured. The effect of increasing corrosion damage on the strength properties of the steel S500s Tempcore^® was moderate. The effect of salt spray exposure on the tensile ductility however was appreciable. All measurements have shown that corrosion had an adverse effect on the materials strength, while the ductility of the specimens was drastically reduced. The embrittlement of reinforcing steel due to corrosion was confirmed both by the gradual decrease of the specimens’ ductility and the examination of the morphology of the cross-section of the respective specimens by scanning electron microscopy.     

Effect of degree of corrosion on the properties of reinforcing steel bars

This paper reports results of a study conducted to assess the effect of degree of corrosion of reinforcing steel bars on their mechanical properties. Reinforcing steel bars, 6 and 12 mm in diameter, that were corroded in reinforced concrete specimens were removed and tested in tension. Results indicated that the level of reinforcement corrosion does not influence the tensile strength of steel bars, calculated on the actual area of cross-section. However, when the nominal diameter is utilized in the calculation, the tensile strength is less than the ASTM A 615 requirement of 600 MPa when the degree of corrosion was 11 and 24% for 6- and 12-mm diameter steel bars, respectively. Furthermore, reinforcing steel bars with more than 12% corrosion indicates a brittle failure.     

Numerical simulation of the probability of corrosion initiation of RC elements made of reinforcing steel with improved corrosion performance

Abstract

Steel reinforcing bars in concrete are protected from corrosion by a thin oxide film that is created on the steel surface in the highly alkaline environment of the concrete. Corrosion process begins once this oxide film is destroyed, for example, by chloride ions penetrating from the element surface into the concrete and to the steel rebar. Critical chloride concentration is highly variable owing to numerous factors such as the type of reinforcing steel used. The TransChlor® software is used to simulate the time history of chloride ion ingress and corrosion initiation into a typical reinforced concrete structural element with four different types of reinforcing steel, namely, carbon steel, ferritic stainless steel (1.4003) with rolling skin, duplex stainless steel (1.4462) and ferritic stainless steel (1.4003) with pickled surface. A climate model is used to simulate two different exposure conditions of the structural element, namely direct and mist exposure, representatives for winter conditions in mountainous and lowland areas. A probabilistic method is applied to consider uncertainty of material properties in the model. The results show that reinforcing steel bars with enhanced corrosion performance significantly improve the durability of reinforced concrete.     

500MPa级钢筋混凝土结构的研究进展

在混凝土构件中使用500MPa级高强度钢筋可以明显地减少用钢量,显著提高工程质量,并能取得较好的社会经济效益.介绍了当前500MPa级钢筋混凝土结构的国内外研究现状,总结了国内500MPa级钢筋混凝土不同构件的试验,对500MPa级钢筋混凝土不同构件的主要试验研究成果作了阐述.同时对未来500MPa级钢筋混凝土结构的研究提出了建议.     

连续玻璃纤维筋的基本力学性能研究

连续玻璃纤维筋具有较好的力学性能和抗侵蚀能力 ,用其代替钢筋可解决混凝土结构的耐久性问题。本文通过试验测试了连续玻璃纤维筋的基本力学性能指标。在此基础上 ,对其代替钢筋的可行性进行了分析。结果表明 ,通过适当的设计 ,连续玻璃纤维筋完全可用于预应力混凝土结构中 ,发挥其优良作用     

钢纤维自应力混凝土加固梁抗弯疲劳性能试验研究

通过对4根三分点加载方式下加固梁的抗弯静力和疲劳试验,研究了钢纤维自应力混凝土叠合层对加固梁的正截面疲劳性能影响。通过分析梁的开裂弯矩、跨中挠度、拉区钢筋应变以及梁裂缝的发展规律,考察了钢纤维自应力加固梁在弯曲重复荷载作用下的疲劳损伤过程。试验结果表明,钢纤维自应力混凝土叠合层可延缓加固梁的开裂,显著降低裂缝宽度,明显提高梁的疲劳性能,采用钢纤维自应力混凝土进行旧混凝土简支梁桥变连续体系加固是一种有效的方法。     

钢筋锚固长度与钢筋混凝土梁的抗弯承载力

在混凝土结构课程学习中,学生往往因为对锚固长度概念的理解不足而对保证钢筋混凝土梁抗弯承载力的各项构造措施产生疑惑.文章首先通过理论模型导出钢筋锚固长度的计算公式,再根据锚固区钢筋中的实际应力水平确定钢筋混凝土梁的抗弯承载力,并绘制相应的抵抗弯矩图,进而分析支座处钢筋的锚固长度、负弯矩区纵向受力钢筋的切断点位置,澄清了教学中对此问题的模糊认识,有利于加深学生对钢筋锚固长度以及混凝土结构中与之相关构造措施的理解.     

轴心受压钢筋混凝土柱与外包钢柱有限元对比分析

采用有限元分析软件ANSYS对普通钢筋混凝土轴心受压柱和湿式外包钢加固轴心受压柱进行了相关力学研究,对两种柱在加载过程中混凝土裂缝的发展及混凝土、钢筋和外包钢的应力分布进行了对比分析.研究表明,在提高构件的延性和承载力方面,外包钢骨架发挥了较好的作用.     

BFRP筋/钢筋混合配筋混凝土梁抗弯试验与有限元分析

通过抗弯性能试验,分析了玄武岩纤维增强筋(BFRP筋)/钢筋混合配筋混凝土梁的承载力、破坏形态、挠度及裂缝发展情况。在试验基础上利用ANSYS软件建立了BFRP筋/钢筋混合配筋混凝土梁的三维有限元数值分析模型,并与试验结果进行了对比。在数值分析模型基础上研究了FRP筋类型及其布置方式对混合配筋混凝土梁抗弯性能的影响。结果表明:BFRP筋/钢筋混合配筋混凝土梁的破坏形式均为钢筋屈服后受压区混凝土被压碎;配筋面积比Af/As越大,初裂荷载越小,裂缝总数越少,挠度逐渐增加;建立的数值分析模型能较准确地模拟BFRP筋/钢筋混合配筋混凝土梁的抗弯性能;BFRP筋、AFRP筋(芳纶纤维增强筋)及GFRP筋(玻璃纤维增强筋)与钢筋混合配筋混凝土梁的荷载-挠度曲线比较接近;与BFRP筋相比,CFRP筋(碳纤维增强筋)/钢筋混合配筋混凝土梁的开裂荷载和极限荷载分别高14.2%和9.3%,最大挠度小35%左右;BFRP筋的布置方式(单层或双层)对混合配筋混凝土梁抗弯性能的影响不大。     

Mechanical behavior of corroded reinforcing steel bars S500s tempcore under low cycle fatigue

The results of an experimental study for assessing the effects of the gradually accumulating corrosion damage due to laboratory salt spray corrosion on the mechanical behavior of reinforcing steel bars Grade S500s tempcore, when subjected to alternating cycles in the region of low cycle fatigue are presented. The accelerated laboratory salt spray method as well as the low cycle fatigue, simulate the natural corrosion in coastal areas and the seismic loads of structures made from reinforced concrete respectively, in a satisfactory way.The low cycle fatigue test results indicate that the corroded steel bars exhibit a gradual reduction in both the load bearing ability as well as the available energy. Tensile tests confirmed the gradual occurring embrittlement due to corrosion of the steel bars. Furthermore, a reduction of the number of cycles to failure under conditions of low cycle fatigue was observed. As the two significant degradation factors corrosion and low-cycle fatigue are functions of time, it seems that the reliability of steel bars S500s tempcore is time dependent as well.     

内置钢构架钢骨混凝土转换深受弯构件试验研究

以实际工程为背景,对两榀1/6缩尺内置钢构架钢骨混凝土转换深受弯构件模型进行试验研究,分析这种新型转换构件在竖向集中荷载作用下的力学性能和破坏形态,研究构件在受力过程中裂缝的开展规律、钢筋和钢构架的应变变化特点、传力模式。研究结果表明内置钢构架钢骨混凝土转换深受弯构件具有承载力高、刚度大、变形能力强等诸多优点,内置钢构架钢骨混凝土转换深受弯构件的破坏形态有显著的改善,避免了普通钢筋混凝土转换梁(尤其是深受弯构件)的脆性破坏形态,表现出良好的塑性。内置钢构架钢骨混凝土转换深受弯构件充分利用了深受弯构件的拉力拱传力机理,设置钢构架实现了以最短、最直接的传力路线和方式传递上部荷载。     

冷挤压套筒连接RC装配式剪力墙抗震性能试验研究

为解决RC装配式剪力墙钢筋连接施工和质量检验困难的问题,提出了一种采用冷挤压套筒钢筋连接方式的装配式剪力墙构造形式.为探明此连接方式装配式剪力墙的可行性及其抗震性能,完成7个装配式剪力墙试件和2个现浇剪力墙对比试件的拟静力试验.对试件的水平荷载-侧移曲线及其特征点、钢筋应变进行了分析.结果 表明:装配式剪力墙试件和现浇...     

Tensile behavior of corroded reinforcing steel bars BSt 500s

The results of an experimental study for assessing the effects of the gradually accumulating corrosion damage due to laboratory salt spray corrosion on the mechanical behavior of reinforcing steel bars Class BSt 500_s tempcore are presented. The experiments have shown that the corrosion exposure causes an appreciable mass loss which increases with increasing duration of exposure. This leads to a significant increase of the applied stress. In addition, a moderate reduction of the material’s tensile strength values and a significant reduction of the tensile ductility were observed. For laboratory salt spray exposure times, which are realistic for simulating natural corrosion, the tensile properties of the steel bars drop to values lying below the limits which are set in the standards presently in force for using steels in reinforced concrete members.     

The impact of fire and fire extinguishing method on the corrosion behavior of the steel bars in concrete pore solution

Exposure of reinforced concrete structural elements to high temperatures, e.g. fire, remains one of the serious potential risks to such structures. This exposure can lead to losses in concrete structural capacity, possible plastic deformation of embedded steel and loss of bond between reinforcing steel and concrete. The level of temperature and the method of extinguishing the fire alter the microstructure of reinforcing steel bars. This microstructure alteration changes the corrosion behavior of steel in such structures, when exposes to chloride ions. The present study was undertaken to determine the impact of exposure of steel to high temperature and the method of extinguishing the fire on its corrosion behavior in alkaline environment of concrete in presence of chloride ions. Results indicated that exposure of steel to high temperatures, regardless of its level; made steel more susceptible to corrosion compared to the reference case, when steel was kept at room temperature. The worst case happened for the steel exposed to 850 °C for an hour, then cooled rapidly in water, where the corrosion rate increased approximately 60% in chloride contaminated concrete pore solution.     

Accurate cover crack modelling of reinforced concrete structures subjected to non-uniform corrosion

Abstract

Concrete cover cracking caused by reinforcement corrosion is a significant durability problem of reinforced concrete (RC) structures. Extensive research has been carried out in the last few decades while most were focused on corrosion of a single reinforcing bar. Very little research has examined the whole cover cracking of RC structures due to multiple reinforcement corrosion. This article develops a numerical model to predict the structural failure of the whole cover of concrete induced by corrosion of multiple reinforcing bars. Moreover, a non-uniform corrosion model is established based on experimental results, in contrast to conventional uniform assumption. Two typical cover failure modes under the non-uniform corrosion of multiple reinforcing bars are identified and discussed. The effects of cover thickness, reinforcement spacing, fracture energy of concrete, etc., on cover cracking patterns and crack width are also investigated. The derived numerical model is verified by comparing the results with those from experiments in literature. Accurate prediction of concrete cover cracking can allow timely maintenance of existing structures and rational design for new buildings which prolongs the service life of the RC structures.     

Residual behavior of steel rebars and R/C sections after a fire

The mechanical properties of various steel bars exposed to high temperature (“residual” properties) are experimentally investigated up to 850 °C, with reference to a number of steel and bar types (carbon and stainless steel; quenched and self-tempered bars; hot-rolled and cold-worked bars; smooth and deformed bars). The aim is to clarify to what extent the thermal sensitivity of the different bars affects the ultimate capacity of a typical R/C section subjected to an eccentric axial force, past a fire (“residual” capacity). As usual in the design of R/C sections under combined bending and axial loading, the ultimate behavior is represented through the “M–N envelopes”, where the materials strength decay due to high temperature is taken into account. The results show that quenched and self-tempered bars (QST), very popular in Europe, are more temperature-sensitive above 600 °C than the carbon-steel bars extensively used in the States and nowadays rarely used in Europe. Furthermore, the best response is exhibited by the stainless-steel bars, provided that they are hot rolled, as it is generally the case for medium- and large-diameter bars. Similar conclusions can be drawn for the sections reinforced with the different bar types.     

海洋环境下混凝土中钢筋的防腐蚀设计

海洋环境下钢筋的锈蚀是影响混凝土结构耐久性的重要原因。通过简单分析海洋环境混凝土中钢筋锈蚀机理，提出了采用钢筋阻锈剂、环氧树脂涂层钢筋、热浸镀锌钢筋、不锈钢钢筋、纤维塑料筋，外加电流阴极保护法和牺牲阳极保护法等钢筋的防腐蚀方法。并综合分析比较了这些方法的工程适用性。     

悬链线效应下钢筋与混凝土黏结锚固性能试验研究

为研究悬链线效应下锚固钢筋与混凝土之间的黏结破坏,以及在RC框架结构连续性倒塌过程中钢筋拉断和黏结失效的先后顺序,分析锚固长度、钢筋布置方式对黏结锚固性能的影响。将钢筋锚固于钢筋混凝土柱中,通过自制的自平衡加载装置,实现对框架结构梁端破坏而钢筋未拉断时形成悬链线受力机制的钢筋进行加载。试验结果表明：锚固钢筋与混凝土之间发生劈裂破坏,而锚固钢筋并没有被拉断,所以在RC框架结构抗倒塌设计中不宜采用直锚的锚固形式,而应该采用其他有效的锚固形式。此外,单排钢筋布置有利于提高锚固钢筋与混凝土之间的平均黏结应力,较双排钢筋布置下的约提高15%。     

Structural models of critical regions in old-type r.c. frames with smooth rebars

Structural assessment of existing reinforced concrete constructions under gravity loads and seismic actions has a high social and economical impact; actually in many European countries, most of the buildings dates back to 1960s and 1970s and cannot ensure satisfactory seismic response, since many areas have been later classified as seismic or since design has been carried out according to obsolete codes. These structures are generally reinforced with smooth bars that exhibit poor bond and need specific anchoring end details. In the present paper, some key aspects of structural models of smooth reinforcement for old-type r.c. frame analysis are reported. Results of experimental tests on smooth reinforcement and circular hook anchoring devices are also used to discuss some aspects of behavioural models of beam to column critical regions.