轴压加载下高强钢筋混凝土柱尺寸效应试验研究

钢筋混凝土构件的尺寸效应主要是:1）混凝土材料本身的非均质性和力学非线性;2）钢筋/混凝土相互作用的高度复杂性共同导致的。随着建筑结构尺寸不断变大,高强混凝土的应用越来越广泛,其高脆性使得其尺寸效应行为更加明显。此外,钢筋混凝土构件大多处于复杂加载条件下,进而使得其尺寸效应问题变得尤为复杂。该文设计了最大截面尺寸为800 mm×800 mm的16组几何相似高强钢筋混凝土柱,对其在轴向受压单调加载及循环重复加载下的力学行为进行了试验研究,并对其名义轴压强度的尺寸效应进行了分析。结果表明:1）高强钢筋混凝土柱的破坏模式受到加载模式的显著影响;2）随试件尺寸变大,高强钢筋混凝土柱耗能能力减弱;3）较单调加载情况,轴向受压循环重复加载下高强钢筋混凝土柱的破坏表现出更强的脆性,且名义轴压强度减小,尺寸效应更显著;4）Ba?ant提出的“尺寸效应律”能够较好地描述大尺寸高强钢筋混凝土柱名义轴压强度的尺寸效应规律。     

轻骨料无腹筋混凝土梁剪切破坏及尺寸效应:细观模拟

轻骨料混凝土具有轻质、高强及保温隔热性能好等优点,被广泛应用于工程结构中。采用细观数值模拟方法,将普通及轻骨料混凝土看作由骨料颗粒、砂浆基质及界面过渡区组成的三相复合材料,建立了无腹筋混凝土梁剪切破坏行为模拟的三维细观力学分析模型,研究了不同尺寸普通及轻骨料无腹筋混凝土悬臂梁在单调加载下的剪切破坏模式与失效机制,揭示了名义剪切强度的尺寸效应规律。此外,结合模拟结果对相关设计规范抗剪承载力计算公式的准确性和安全性进行了初步探讨。研究结果表明:区别于普通混凝土梁,轻骨料混凝土梁由于骨料强度较低而首先发生破坏;不同尺寸混凝土梁的剪切破坏模式基本一致,梁的名义剪切强度展现出明显的尺寸效应;相比于普通混凝土梁,轻骨料混凝土梁剪切破坏表现出具有更强的尺寸效应。     

混凝土劈裂抗拉强度和弯曲抗压强度尺寸效应的细观数值分析

混凝土作为非均质材料,其材料性能存在随试件几何尺寸变化的尺寸效应。该文在细观层次上将混凝土看作由粗骨料、砂浆和二者界面过渡区组成的三相复合材料,采用刚体弹簧元数值方法模拟了混凝土的劈裂抗拉强度和弯曲抗压强度的尺寸效应,并与已有的试验结果进行了对比验证。结果表明:劈裂加载的试件破坏形态和劈裂抗拉强度与试验结果均具有良好的一致性,并且小尺寸试件所表现出的尺寸效应要明显于大尺寸试件;对不同尺寸四点弯曲钢筋混凝土梁开展细观数值分析得到跨中截面混凝土的弯曲抗压强度,随着梁有效高度的增加,名义弯曲抗压强度整体上呈现降低的趋势,但当梁有效高度大于240mm时趋于稳定。     

钢筋混凝土悬臂梁剪切破坏及尺寸效应律研究

相比于混凝土材料,钢筋混凝土构件的破坏模式与机制更为复杂,采用混凝土材料层次的尺寸效应理论难以描述钢筋混凝土构件破坏的尺寸效应行为。为研究钢筋混凝土悬臂梁剪切破坏的尺寸效应行为,从细观角度出发,建立了钢筋混凝土悬臂梁三维细观尺度数值分析模型。结合现有试验数据,验证了细观模拟方法的可行性与合理性,进而拓展模拟与分析了剪跨比及配箍率对钢筋混凝土悬臂梁剪切破坏尺寸效应行为的影响规律,发现:剪跨比对悬臂梁抗剪承载力有较大影响,对尺寸效应的影响很小;配箍率的增大提高了悬臂梁抗剪承载力,同时削弱了梁的抗剪强度尺寸效应。根据剪跨比及配箍率对悬臂梁抗剪强度的影响机制与规律,基于Bažant材料层次尺寸效应律,建立了钢筋混凝土悬臂梁抗剪强度尺寸效应理论公式。对比模拟结果及试验数据,验证了所提尺寸效应理论公式的准确性与合理性。     

钢筋混凝土柱纯扭破坏尺寸效应数值分析

在地震作用下,钢筋混凝土柱时常会受到扭矩的作用,而扭矩的存在会改变钢筋混凝土柱的破坏模式。为探究钢筋混凝土柱纯扭破坏的尺寸效应行为,采用三维细观数值模拟方法,考虑了混凝土细观组分的非均质性及钢筋与混凝土间的粘结滑移作用,建立了钢筋混凝土柱的纯扭作用数值模型。模拟分析了结构尺寸、纵筋率、配箍率和截面形状对钢筋混凝土柱抗扭破坏的影响。结果表明：钢筋混凝土柱扭转破坏表现为脆性特征,名义抗扭强度表现出明显的尺寸效应;纵向配筋对扭转强度尺寸效应影响不大;方形截面柱比圆形截面柱具有更强的尺寸效应;箍筋可以提高扭转强度,且可以削弱名义抗扭强度的尺寸效应。最后,修正了Ba?ant尺寸效应律,建立了全结构尺寸范围内的名义抗扭强度预测公式。     

结构尺寸对钢筋混凝土短柱抗震性能影响:细观分析

地震作用下,钢筋混凝土短柱由于承受竖向和水平荷载共同作用而往往产生脆性压-剪破坏行为,该脆性破坏可能表现出尺寸效应行为。借助混凝土细观结构特征,考虑非均质性以及钢筋/混凝土非线性相互作用的影响,建立了钢筋混凝土短柱力学行为研究的三维细观尺度数值模型。在模拟结果与试验结果吻合良好的基础上,研究了轴压比对不同结构尺寸尤其是大尺寸钢筋混凝土短柱抗震性能,如抗剪承载力、滞回特性、延性性能和耗能能力等参数的影响。最后,借助相关试验和模拟数据,基于经典的Bažant尺寸效应律思想提出了能够反映尺寸效应影响钢筋混凝土短柱抗剪承载力修正公式。     

钢筋混凝土柱偏心受压力学性能的细观数值研究

钢筋混凝土构件的宏观力学性能由其组分-钢筋和混凝土两部分的力学性能决定。结合混凝土细观结构形式,认为混凝土是由骨料颗粒、砂浆基质及界面过渡区组成的复合材料,假定钢筋与混凝土之间完好粘结,基于钢筋混凝土柱偏心受压试验,建立了钢筋混凝土柱偏心受压加载下力学特性及破坏行为研究的细观尺度力学分析模型。通过对混凝土方形和矩形试件进行受压力学特性模拟,采用反演法确定了界面的力学参数,进而模拟了钢筋混凝土柱偏心受压加载下的宏观力学性能。结果表明,相比于宏观尺度模型,细观数值分析模型能够充分体现材料的非均质性,能够较好的模拟试件的宏观力学性能,并且能够细致的描述裂缝发展及试件破坏过程,与试验结果吻合良好。该文建立的细观尺度分析模型与方法,为钢筋混凝土构件层次宏观力学非线性及其尺寸效应研究提供了理论支持。     

基于三维细观数值方法的钢筋混凝土悬臂梁抗剪行为研究

从细观角度出发,结合混凝土内部组成的微/细观结构特征,考虑钢筋与混凝土之间的非线性粘结-滑移行为,建立了钢筋混凝土悬臂梁力学行为研究的三维细观尺度数值分析模型与方法。基于建立的数值方法,对已有关于钢筋混凝土悬臂梁在循环往复荷载作用下的抗剪破坏试验进行模拟,模拟结果与试验结果的良好吻合表明了细观数值方法的可行性。进而在此基础上,拓展模拟了不同箍筋率下钢筋混凝土悬臂梁的剪切破坏过程,揭示了箍筋对悬臂梁滞回特性及抗剪承载力的影响规律。研究表明:1）细观数值分析模型能较好的模拟构件的宏观力学性能,且能细致的描述裂缝在混凝土中的发展过程;2）随着配箍率的增加,梁抗剪承载力及延性增加,但随着配箍率增加,钢筋混凝土梁抗剪承载力增加趋势减缓。建立的细观尺度数值方法可为有/无腹筋混凝土梁抗剪/抗弯破坏尺寸效应提供理论支持。     

CFRP加固钢筋混凝土方柱抗震性能尺寸效应的细观分析

为研究CFRP加固钢筋混凝土方柱在地震作用下的破坏模式,该文考虑混凝土材料的非均质性、钢筋-混凝土间的粘结滑移作用,建立了CFRP加固钢筋混凝土方柱三维细观数值模型。在验证数值模型与试验结果吻合良好的基础上,扩展工况探讨了轴压比、CFRP体积配置率对CFRP加固钢筋混凝土方柱抗震性能及名义抗剪强度尺寸效应的影响。结果表明:一定轴压比范围内柱的承载力随轴压比增大而提高,但其延性会降低;该文工况中,CFRP加固钢筋混凝土柱的名义抗剪强度随试件尺寸增大呈降低趋势,存在着明显尺寸效应行为;在经典材料层次尺寸效应律基础上,提出了CFRP加固混凝土方柱名义抗剪强度尺寸效应理论公式(适用范围轴压比小于0.4),模拟结果证实了公式的合理性。     

地震作用下钢筋混凝土悬臂梁抗弯性能及尺寸效应试验研究

随着建筑结构尺寸的不断增大,准确的预测地震荷载作用下钢筋混凝土梁抗震性能及抗弯强度随构件尺寸增大的变化趋势对评估结构的安全性具有重要的意义。设计了剪跨比为4的5组几何相似的钢筋混凝土悬臂梁,对其在低周往复荷载作用下的抗弯性能进行试验研究,旨在探讨悬臂梁的破坏模式、延性、耗能能力,刚度退化及抗弯强度等与构件尺寸的关系。研究表明:(1)不同尺寸梁的破坏模式基本相同,均在梁的固定端发生累积压缩破坏;(2)随着梁尺寸增加,其延性有所增强,但耗能能力及刚度退化性能基本相同;(3)低周往复荷载作用下梁的名义抗弯强度存在明显的尺寸效应;(4)Bazant尺寸效应理论能较好的描述钢筋混凝土悬臂梁在低周往复荷载作用下的尺寸效应规律。     

Effect of specimen size on flexural compressive strength of reinforced concrete members

It is important to consider the effect of member size when estimating the ultimate strength of a concrete flexural member, because the strength always decreases with an increase of member size except for well-reinforced members. Research conducted previously in this area include axial compressive strength size effect on cylindrical specimens and flexural compressive strength size effect on C-shaped specimens, notched cylindrical specimens, and axially loaded double cantilever beam (DCB) specimens. Since the most widely used flexural member type is reinforced concrete (RC) beams, it is logical to extend the study of flexural compressive strength size effect to flexural loaded RC beam members. Previously, several researchers have reported from their studies that flexural compressive strength size effect does not exist. However, the analyses show that the specimens used for the study had limited size variation and the neutral axis depth variations were too similar to show distinct size effect. Therefore, this study enforced distinct neutral axis depth variations for all of the tested specimens.In this study, the size effect of a RC beam was experimentally investigated. For this purpose, a series of beam specimens subjected to four-point loading was tested. RC beams with three different effective depths were tested to investigate the size effect. The shear-span to depth ratio and the thickness of the specimens were kept constant to eliminate the out-of-plane size effect.The test results are curve fitted using Levenberg–Marquardt’s Least Square Method (LSM) to obtain parameters for Modified Size Effect Law (MSEL) by Kim et al. The analysis results show that the flexural compression strength and ultimate strain decrease as the specimen size increases. Comparisons with existing research results considering the depth of neutral axis were also performed. They also show that the current strength criteria-based design practice should be reviewed to include member size effect.     

Studies on ductility and evaluation of minimum flexural reinforcement in RC beams

Some experimental investigations on ductility and prediction of minimum flexural reinforcement in reinforced concrete (RC) beams are reported. The minimum flexural reinforcement was evaluated using optimum ductility in RC beams. Beams of size 100 mm, 200 mm and 400 mm were tested, which were designed with varying percentages of flexural reinforcement i.e. 0.15, 0.30, 0.60 and 1.0. The beams were tested under four-point loading to study the flexural behaviour under uniform bending moment. The experimentally obtained average compressive strength of concrete was 30 MPa. The influence of beam size (depth) on cracking and normalised ultimate flexural strength, ductility and overall average rotation has been studied. The cracking in RC beams is complex phenomenon in small size beams, while the cracking strength decreases as the depth increases beyond 200 mm. The flexural strength of RC beams, from the present study, appears to decrease as the depth increases. The ductility of RC beams increases as the percentage of flexural reinforcement increases. The ductility number has been derived from dimensional analysis using fracture mechanics principles. The ductility of RC beams decreases as the depth of beams increases. An optimum percentage of flexural reinforcement has been established using optimum ductility number, N_p, which is equal to 0.20. The minimum flexural reinforcement was found to decrease as the beam depth increases, and decreases as the yield strength of reinforcement increases.     

CFRP布加固方案对RC梁剪切性能及尺寸效应影响分析EI北大核心CSCD

采用同时考虑混凝土材料非均质性、钢筋与混凝土之间的相互作用以及CFRP布与混凝土之间的相互作用影响的三维细观数值模拟方法,建立了CFRP布加固RC梁剪切破坏力学分析模型。在验证了细观数值方法合理性的基础上,设计并建立了12根CFRP布加固RC梁细观模型,探究相同CFRP配纤率(用布量)前提下,不同CFRP布加固方案对单调荷载作用下RC梁的剪切性能及尺寸效应的影响。结果表明:CFRP布应变分布与裂缝位置紧密相关,越靠近裂缝位置的CFRP布应变越大,提供的抗剪贡献越多;在CFRP配纤率一致的前提下,CFRP布宽度大厚度小的加固方案优于CFRP布厚度大宽度小的加固方案;CFRP布U型加固RC梁剪切强度存在尺寸效应现象,但相同CFRP配纤率下,不同CFRP布加固方案对名义抗剪强度尺寸效应的影响较小,可以忽略。     

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.     

高强钢绞线网——聚合物砂浆加固混凝土及预应力混凝土梁的抗弯性能试验研究

该文侧重研究了采用高强钢绞线网——聚合物砂浆加固技术三面加固普通钢筋混凝土(RC)梁和预应力混凝土(PRC)梁的抗弯性能.进行了6 片RC 梁及5 片PRC 梁的抗弯静载试验,探讨了不同初始损伤程度、有效预应力大小和加载方式等对高强钢绞线网——渗透性聚合物砂浆加固梁的抗弯性能影响.结果表明:采用高强钢绞线网——聚合物砂浆加固技术加固RC/PRC梁能够有效抑制裂缝的开展,能有效地提高RC/PRC 加固梁的抗弯承载能力和抗弯刚度;重复加载会导致加固梁刚度一定程度的退化,初始损伤程度大小不会显著改变加固后RC/PRC梁的抗弯承载力;该文研究对于大量既有RC/PRC梁的抗弯加固具有一定的参考价值.     

Flexural and shear strengthening of RC beams with composite materials – The influence of cutting steel stirrups to install CFRP strips

Experimental, numerical and analytical investigations have revealed that Carbon Fibre Reinforced Polymer (CFRP) strips with larger cross section height improve the effectiveness of the Near Surface Mounted (NSM) technique for the flexural strengthening of existing reinforced concrete (RC) beams. However, this height is limited to the concrete cover thickness of the longitudinal steel bars, since the application of strips of cross section height larger than the cover thickness requires that the bottom arm of the steel stirrups be cut. This work aims to assess the influence, in terms of shear resistance, of cutting the bottom arm of steel stirrups to install NSM strips for the flexural strengthening of RC beams. The obtained results showed that, for monotonic loading, cutting the bottom arm of steel stirrups led to a decrease of the beam’s load carrying capacity of less than 10%. Due to the high effectiveness of the adopted NSM flexural strengthening systems, shear can be a predominant failure mode for these beams. To avoid this type of failure mode, strips of wet lay-up CFRP sheets with U configuration were used, resulting in effective strengthening solutions for RC beams. In the present paper the experimental program is described, and the obtained results are presented and discussed.