The Influence of Chemical Admixtures on Restrained Drying Shrinkage of Concrete

J. Brooks, X. H. Jiang
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引用次数: 25

Abstract

A consequence of drying shrinkage is intrinsic cracking due to some form of restraint. In thick sections of concrete, drying from the surface causes differential shrinkage and such internal restraint can be responsible for surface cracking because of the induced tensile stress. When thin drying concrete members are restrained externally, a time-dependant failure is likely unless drying shrinkage is minimized. Besides drying shrinkage, the potential for cracking depends on tensile creep and tensile strength or tensile strain capacity and such properties are not normally measured in the laboratory. The possible effects of chemical admixtures on the forgoing properties is also largely unknown. The current research is investigating the role of tensile creep in relieving the tensile stress induced by fully restraining the drying shrinkage of concrete with and without chemical and mineral admixtures. All the relevant properties contributing to the time-dependant strength are being measured using bobbin-shaped specimens previously developed for uniaxial creep determination. The present paper presents the findings for concretes with and without plasticizer and a new shrinkage reducing admixture. While the plasticizer has little influence on properties, the shrinkage reducing admixture significantly lowers the strength, elastic modulus, free drying shrinkage and creep. When restrained from the age of seven days all the concretes failed between 4 and 13 days, the concrete with the shrinkage reducing admixture failing at the lowest stress but after the longest time.
化学外加剂对混凝土抑制干燥收缩的影响
干燥收缩的结果是由于某种形式的约束而产生的内在开裂。在较厚的混凝土截面中,表面的干燥会引起不同的收缩,这种内部约束可能会导致表面开裂,因为诱发了拉应力。当薄型干燥混凝土构件受到外部约束时,除非干燥收缩最小化,否则很可能发生时间相关的破坏。除了干燥收缩,开裂的可能性取决于拉伸蠕变和拉伸强度或拉伸应变能力,这些特性通常不会在实验室测量。化学外加剂对上述性能的可能影响在很大程度上也是未知的。目前的研究是探讨拉伸蠕变在充分抑制混凝土干燥收缩中所起的作用。所有与时间相关的强度的相关特性都是用先前为单轴蠕变测定而开发的线轴形试样来测量的。本文介绍了在混凝土中添加增塑剂和不添加增塑剂以及一种新型减缩外加剂的研究结果。增塑剂对材料的性能影响不大,而减缩掺合料能显著降低材料的强度、弹性模量、自由干燥收缩和蠕变。当约束龄期为7 d时,所有混凝土均在4 ~ 13 d失效,掺加减缩外加剂的混凝土应力最小,但失效时间最长。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
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