Deterioration of concrete due to the alkali–silica reaction (ASR) involves a reaction between alkaline ions in the cement pore solution and non-crystalline silica found in many aggregates. The product of reaction is a porous hydrophilic ASR gel, which causes expansion and cracking of concrete structure. Currently, there is no reliable method for non-destructive evaluation of ASR. We have investigated the feasibility of using relative dielectric permittivity obtained from X-band microwave backscattering of concrete. Advantages of this method of concrete evaluation are that measurements are non-contact and one-sided, and the approach is scalable to arbitrary-size concrete structures. The exploratory study was performed using an unrestrained set of accelerated ASR concrete prism specimens developed according to the ASTM1293 standard. One set of specimens contained no entrained air, while another set was contained entrained air. The specimens were removed from the environmental chamber at different times during a year-long study to create a set of different ASR maturity specimens. Strain measurements were performed on all specimens. Strain data both air entrained and non-air entrained sets were shown to be in close agreement with Larive model of isothermal unrestrained ASR expansion with similar fitting parameters. Relative dielectric permittivity of concrete specimens was correlated with strain data. Both sets show correlation between dielectric permittivity and strain, with stronger linear correlation observed for the air-entrained specimens. Development of such correlation would enable estimating expansion of actual concrete structures from microwave backscattering measurements, which could be used for risk stratification to guide and minimize coring.

中文翻译：

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用微波反向散射监测加速碱硅反应混凝土的介电常数

由于碱-二氧化硅反应 (ASR) 引起的混凝土劣化涉及水泥孔隙溶液中的碱性离子与许多骨料中发现的非结晶二氧化硅之间的反应。反应的产物是多孔亲水性 ASR 凝胶，它会引起混凝土结构的膨胀和开裂。目前，没有可靠的方法对 ASR 进行无损评估。我们已经研究了使用从混凝土的 X 波段微波反向散射获得的相对介电常数的可行性。这种混凝土评估方法的优点是测量是非接触和片面的，并且该方法可扩展到任意尺寸的混凝土结构。探索性研究是使用一组根据 ASTM1293 标准开发的无限制加速 ASR 混凝土棱柱试样进行的。一组样本不包含夹带空气，而另一组包含夹带空气。在为期一年的研究中，样本在不同时间从环境室中取出，以创建一组不同的 ASR 成熟度样本。对所有样品进行了应变测量。空气夹带和非空气夹带组的应变数据显示与具有类似拟合参数的等温无限制 ASR 膨胀的 Larive 模型非常一致。混凝土试件的相对介电常数与应变数据相关。两组均显示介电常数和应变之间的相关性，对于夹带空气的样品观察到更强的线性相关性。