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蚀保护。 当钢被包裹在混凝土中时，由于混凝土的高pH值，在钢-混凝土界面处形成保护性氧化铁膜。这层膜保护钢不受腐蚀。 然而，氯离子侵入钢筋混凝土中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，钢将通过电化学过程腐蚀。 一旦钢筋开始腐蚀，混凝土就会变质。 这是因为腐蚀的副产品占据了比钢本身更大的体积，这对周围的混凝土施加了很大的应力。 氯化物可以通过多种方式引入混凝土中。 它可能通过单独的混凝土成分、骨料、水泥、外加剂和混合水引入;或通过暴露于除冰盐、海水或含盐空气环境引入。 由于氯化物可以通过许多机制引入，因此对任何一种成分（例如混合水）进行限制并不总是限制混凝土中氯化物的总量。 因此，重要的是能够测量混凝土的氯化物含量，以指示任何嵌入式钢材的腐蚀潜力。 目前，有两种标准测试用于测量氯离子渗透到混凝土中;即“快速氯离子渗透测试”和“90天积水测试”。 前一种测试不一定能复制现实生活中氯离子的运输，也没有可靠的科学依据。 该试验的结果也被证明受化学外加剂和添加剂的影响，因为它们可能含有一些氯化物。 该测试不是在现场进行的，并且是破坏性的。 使用后一种测试需要很长时间才能获得结果。 该试验将混凝土试样在给定的时间内置于氯化物溶液中。 该测试的结果至少需要90天才能得出，但随着新的高强度混合料的出现，由于该测试中氯化物的缓慢进入，结果可能需要更长的时间。 一种新的测试方法，可以预测氯离子渗透在混凝土中使用的现浇混凝土，将是非常宝贵的混凝土和建筑行业。 对于联邦公路管理局（FHWA）和交通部来说尤其如此，在用除冰盐处理的路面中，氯化物侵入和钢材腐蚀一直存在重大问题。 提出了一种近场微波无损检测技术，采用开口矩形波导探头，预测氯离子含量和氯离子渗透混凝土基材料的程度。 调查组建议使用加速技术将氯化物引入水泥净浆、砂浆和混凝土中。 本研究的目的是检测水泥基材料中氯化物的存在和含量，以及密切估计氯化物在这些材料中渗透的程度或深度。 研究小组已成功地利用这种微波无损检测技术研究水泥基材料（硬化浆体、砂浆和混凝土）的近场微波反射特性，并将其与水灰比、硫灰比、粗骨料体积和粒度分布以及抗压强度相关联。 在初步的基础上，他们已经能够检测氯离子含量，存在于混合水中，水泥浆和砂浆中，以及氯离子对这些样品固化的影响。 其目的是在压力和受控温度下使用充满氯化物溶液的封闭罐使水泥浆、砂浆和混凝土经受加速氯化物渗透。 在连续暴露于氯化物溶液后，将在几个微波频率下测量这些试样的反射特性。 然后将结果与氯化物含量以及其渗透的深度相关联。 使用离散模型描述的方式，其中氯化物已经渗透在这些标本，现有的电磁模型将被用来模拟的结果，更重要的是预测氯化物的量和渗透到这些材料的深度。

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.