Detection and Profiling of Accelerated Chloride Penetration in Concrete Using Near-field Microwave Techniques

混凝土通常为钢筋提供足够的防腐保护。当钢筋被包裹在混凝土中时，由于混凝土的高pH值，在钢-混凝土界面上形成了一层保护性的氧化铁膜。这层膜保护钢材不受腐蚀。然而，氯离子侵入钢筋混凝土会导致这种保护膜被破坏。此外，当混凝土中存在水分和氧气时，钢材会通过电化学过程进行腐蚀。一旦钢筋开始腐蚀，混凝土就会变质。这是因为腐蚀的副产物比钢材本身占据了更大的体积，这对周围的混凝土施加了巨大的压力。氯离子可以以多种方式引入混凝土中。它可以通过单独的混凝土成分、集料、水泥、外加剂和混合水引入；也可以通过暴露在除冰盐、海水或盐水空气环境中而引入。由于氯可以通过许多机制引入，限制任何一种成分(例如混合水)并不总是限制混凝土中的氯总量。因此，重要的是能够测量混凝土的氯离子含量，以指示任何埋入的钢筋的腐蚀潜力。目前，用以量度氯离子渗入混凝土的标准试验有两种，即“氯离子快速渗透试验”和“90天积水试验”。前一种试验不一定复制现实生活中氯离子的传输，也没有可靠的科学基础。由于化学外加剂和添加剂可能含有一些氯化物，因此这项试验的结果也受到化学外加剂和添加剂的影响。这种测试不是在现场进行的，是破坏性的。使用后一种测试需要很长时间才能得到结果。测试使混凝土试件在给定的时间内经受氯化物溶解。这项试验的结果至少需要90天才能产生，但随着新的高强度混合料的出现，由于这次试验中氯的缓慢进入，结果可能需要更长的时间。一种新的测试方法可以预测氯离子在混凝土中的渗透性，用于现浇混凝土，对混凝土和建筑业具有非常重要的价值。对于联邦公路管理局(FHWA)和交通部来说尤其如此，在这些地方，使用除冰盐处理的人行道上存在着氯离子渗入和钢筋腐蚀的重大问题。提出了一种利用开放式矩形波导探头的近场微波无损检测技术，用于预测混凝土材料中的氯离子含量和氯离子渗透程度。调查小组建议使用加速技术在水泥浆、砂浆和混凝土中引入氯化物。这项调查的目的是检测水泥基材料中氯化物的存在和含量，并密切估计氯化物在这些材料中的渗透程度或深度。研究小组已成功地利用这种微波无损检测技术研究并关联了水泥基材料(硬化浆体、砂浆和混凝土)的近场微波反射特性与其水灰比、S/水泥比、粗集料体积和粒度分布以及它们的抗压强度之间的关系。在初步的基础上，他们已经能够检测到混合水中的氯含量，水泥浆体和砂浆中的氯含量，以及氯对这些样品的氯化物的影响。它的目的是使水泥浆体、砂浆和混凝土在压力和温度受控的情况下，使用装满氯离子溶液的封闭储罐加速氯离子渗透。在连续暴露在氯化物溶液中后，将在几个微波频率下测量这些样品的反射特性。然后，结果将与氯化物含量以及它渗透的深度相关联。使用描述氯化物在这些样品中渗透的方式的离散模型，以及现有的电磁模型将被用来模拟结果，更重要的是预测氯化物的量及其在这些材料中的渗透深度。

Concrete normally provides reinforcing steel with adequate corrosion protection. When steel is encased in concrete, a protective iron oxide film forms at the steel-concrete interface due to the high pH of the concrete. This film protects the steel from corrosion. However, the intrusion of chloride ions in reinforced concrete can cause this protective film to be destroyed. Additionally when moisture and oxygen are present in the concrete, the steel will corrode through an electrochemical process. Once the steel begins to corrode, the concrete will deteriorate. This occurs because the by products of corrosion occupy a greater volume than the steel itself, which exerts a substantial stress on the surrounding concrete. Chloride can be introduced into concrete in many ways. It may be introduced by the individual concrete ingredients; aggregates, cement, admixtures and mixing water; or through exposure to deicing salts, seawater, or salt air environments. Since chloride can be introduced through many mechanisms, placing limits on any one ingredient (e.g. mixing water) will not always limit the total amount of chloride in the concrete. Therefore, it is important to be able to measure the chloride content of concrete to yield an indication of the potential of corrosion of any embedded steel. Currently, there are two standard tests used to measure the chloride penetration into concrete; namely, the "rapid chloride permeability test" and the "90-day ponding test". The former test does not necessarily replicate the transport of chloride ions in real-life situations, nor does it have a sound scientific basis. The results of this test have also been shown to be influenced by chemical admixtures and additives since they may contain some chloride. This test is not conducted in-situ and is destructive. Using the latter test takes a long time to obtain results. The test subjects a concrete, specimen to a chloride sloution over a given amount of time. Results from this test take at least 90 days to produce, but with the advent of newer high strength mixes, results may take longer due to the slow ingress of chloride from this test. A new test method that can predict the chloride ion penetration in concrete for use with in-place concrete, would be invaluable to the concrete and construction industry. This is especially true for the Federal Highway Administration (FHWA) and the Department of Transportation where there have been major problems with chloride ingress and steel corrosion in pavements treated with deicing salts. A near-field microwave nondestructive testing technique, employing open-ended rectangular waveguide probes, is proposed to predict chloride content and the extent of chloride penetration in concrete based materials. The investigating team proposesto use accelerated techniques to introduce chloride in cement paste, mortar and concrete. The objectives of this investigation are to detect the presence and the content of chloride in cement based materials in addition to closely estimating the extent or depth to which chloride has penetrated in these materials. The investigating team has been successfully using such a microwave nondestructive testing technique to study and correlate the near-field microwave reflection properties of cement based materials (hardened paste, mortar and concrete) to their w/c ratio, s/c ratio, coarse aggregate volume and size distributions, ans their compressive strenths. On a preliminary basis they have been able to detect chloride content, present in the mixing water, in cement paste and mortar as well as the influence of chloride on the curin of these specimens. It is intended to subject cement paste, mortar and concrete to accelerated chloride penetration using an enclosed tank filled with chloride solution under pressure and controlled temperature. Upon successive exposure to chloride solution, the reflection properties of these specimens will be measured at several microwave frequencies. The results will then be correlated to the chloride content as well as the depth to which it has penetrated. Using a discrete model describing the manner by which chloride has penetrated in these specimens, and existing electromagnetic model will be used to model the results and more importantly predict the amount of chloride and its penetrated depth into these materials.