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混凝土强度尺寸效应的实验研究
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摘要
尺寸效应是准脆性材料的一个普遍性质,混凝土作为一种准脆性材料也存在尺寸效应。混凝土强度的尺寸效应是指混凝土的强度不仅由材料的性质决定,还会随着试件或构件的尺寸的大小而变化,而不是一个定值。随着混凝土材料的运用越来越广泛,结构高度和跨度的增加,混凝土结构尺寸也越来越大,但受到试验研究条件的和认识水平的限制,目前我们大尺寸试件的设计还是参考小尺寸试件研究成果而建立的设计理念和方法、计算公式等,当它们用于现代实际工程中时,将缺乏科学依据的推导。虽然我们在混凝土强度尺寸效应的研究领域中取得一些成绩,但还没有一个统一的衡量尺寸效应的标准或指标,目前为止还没有把尺寸效应考虑到行业规范和标准中去。针对上述情况,本文开展了以下几个方面工作:
     (1)本文进行了混凝土抗压强度和劈裂抗拉强度两方面的尺寸效应实验研究,抗压试验考虑了立方体、棱柱体和圆柱体三种试件形式;劈裂受拉试验考虑了立方体和圆柱体两种试件形式。
     (2)混凝土抗压强度的尺寸效应实验中,各种试件形式考察了边长或直径为150mm、250mm、350mm、450mm四组不同试件尺寸。实验结果表明,无论哪种试件形式混凝土的抗压强度都存在尺寸效应,且随着试件尺寸的增加,混凝土抗压强度在减小。同时采用Bazant基于应力重分布的尺寸效应公式及其修正式和Carpinteri多种分形理论尺寸效应公式拟合了实验结果,两种理论均能较好的说明实验结果。
     (3)混凝土劈裂受拉强度的尺寸效应实验中考察了边长为150mm、250mm、350mm、450mm、550mm、750mm六组不同立方体试件尺寸和直径为150mm、250mm、350mm、450mm、550mm五种圆柱体试件尺寸。实验结果显示,混凝土的劈裂受拉强度存在尺寸效应,且随着试件尺寸的增加,混凝土抗压强度在减小,但无论试件形式如何当劈裂面积达时1000mm2时,尺寸效应接近消失,劈拉强度趋近一个定值。同时采用Weibull统计尺寸效应理论分析了实验结果,研究表明混凝土劈裂受拉强度与劈裂面积存在很好的相关性。
     (4)通过对试件形式对混凝土强度尺寸效应影响的比较,发现无论是混凝土的抗压强度实验还是劈裂抗拉强度试验,试件的形式对混凝土的强度尺寸效应存在影响,虽然在大体趋势上表现相同,但是在具体尺寸上试件形状对混凝土强度的尺寸效应的影响有差异。
Size effect is a general property of quasi-brittle materials. Concrete as a quasi-brittle materials, there is also size effect happening. The size effect of concrete strength is affected not only by the property of the material, but also by the size of specimen, which is not a constant value. With more and more concrete materials being widely used, and the increase of structure height and span, the sizes of the concrete structures are also increased. However, the designing concepts, methods and formulas of large-size specimens are also based on the research findings of the small-size specimens currently because of the limitation of understandings and experimental conditions. This will be lack of scientific proof when they are used in modern practical projects. Although we have got some achievements in the research field of the size effect of concrete strength, there is no uniform standard or index to judge the size effect. And we still haven't taken the size effect into account in the codes of conduct so far. According to the above problems, the several main aspects of theoretical and experimental research are as follows:
     (1) The size effect of concrete compressive strength and splitting tensile strength are considered in this experimental research of this article. The concrete compressive strength test considered cube, prism and cylinder these three specimen form. While the splitting tensile test only considered cube and cylinder these two kinds of specimens.
     (2) In the size effect test of concrete compressive, four groups of cubes and prism with side length of 150mm,250mm,350mm,450mm,and four groups of cylinders with diameter of 150mm,250mm,350mm,450mm were considered respectively. The results show that size effect exists in all the three forms of concrete specimens of compressive strength and that the concrete compressive strength decrease with the increase of specimens'size. The author also use Bazant's size effect formula and modified formula based on the stress distribution and Carpinteri fractal size effect formula to fitting the experimental results, which fits well.
     (3) In the size effect test of concrete splitting tensile strength, six groups of cube specimens with side lengths of 150mm,250mm,350mm,450mm,550mm,750mm, and five cylinder specimens with diameters of 150mm、250mm、350mm、450mm、550mm were considered respectively. The result show that size effect exists in the concrete splitting tensile strength and that the concrete splitting tensile strength decrease as its size increase. But no matter what the specimens'form is, when the splitting area reaches 1000mm2, the size effect almost disappears, and splitting tensile strength reaches a constant value. At the same time, using Weibull's statistical size effect theory to analysis experimental results, a good correlation can be found between splitting tensile strength of concrete and splitting area.
     (4) By comparing the influence of the specimens'form on the size effect of concrete strength, the specimens'form affect the concrete strength, which can be found on the concrete compressive strength test and the splitting test. Although the general trends perform the same, the influences that the different shapes acted at the size effect of concrete strength perform differently.
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