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

使用近场微波技术检测和分析混凝土中加速氯离子渗透

• 批准号: 9817695
• 负责人: Reza Zoughi
• 金额: $ 26.76万
• 依托单位: Colorado State University
• 依托单位国家: 美国
• 项目类别: Continuing Grant
• 财政年份: 1999
• 资助国家: 美国
• 起止时间: 1999-06-15 至 2001-03-31
• 项目状态: 已结题
• 来源: https://www.nsf.gov/awardsearch/showAward?AWD_ID=9817695&HistoricalAwards=false
• 关键词: Detection; Profiling; Accelerated; Chloride; Penetration

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.

混凝土通常为钢筋提供足够的防腐保护。当钢被包裹在混凝土中时，由于混凝土的高pH值，在钢-混凝土界面形成一层保护性的氧化铁膜。这层薄膜保护钢材不受腐蚀。然而，氯离子在钢筋混凝土中的侵入会使这层保护膜被破坏。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。一旦钢材开始腐蚀，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压力。氯化物可以通过多种方式加入混凝土中。它可以由单个混凝土成分引入；骨料、水泥、外加剂和混合水；或者暴露在除冰盐、海水或含盐空气环境中。由于氯化物可以通过许多机制引入，限制任何一种成分（例如，混合水）并不总能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，以产生任何嵌入钢的腐蚀潜力的指示。目前，测量氯化物渗入混凝土的标准试验有两种；即“快速氯化物渗透试验”和“90天积水试验”。前一种测试不一定能复制氯离子在现实生活中的传输，也没有可靠的科学依据。该试验的结果也被证明会受到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。该试验不是在现场进行的，具有破坏性。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。该试验的结果至少需要90天才能得出，但随着新的高强度混合物的出现，由于氯化物从该试验中缓慢进入，结果可能需要更长的时间。一种能够预测氯离子在混凝土中渗透的新测试方法，对于混凝土和建筑行业具有不可估量的价值。对于联邦公路管理局（FHWA）和运输部来说尤其如此，在用除冰盐处理过的路面上，存在氯化物进入和钢铁腐蚀的主要问题。提出了一种采用开放式矩形波导探头的近场微波无损检测技术，用于预测混凝土基材料中氯化物含量和氯化物渗透程度。研究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。本调查的目的是检测氯在水泥基材料中的存在和含量，并密切估计氯在这些材料中渗透的程度或深度。研究小组已经成功地利用这种微波无损检测技术，研究了水泥基材料（硬化膏体、砂浆和混凝土）的近场微波反射特性与其w/c比、s/c比、粗骨料体积和粒径分布以及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存在于混合水、水泥浆和砂浆中的氯化物含量，以及氯化物对这些标本curin的影响。它的目的是使水泥膏体、砂浆和混凝土在压力和控制温度下使用一个充满氯化物溶液的封闭罐中加速氯化物渗透。在连续暴露于氯化物溶液后，将测量这些样品在几个微波频率下的反射特性。然后将结果与氯化物含量以及它渗透到的深度相关联。利用离散模型描述氯化物在这些试样中的渗透方式，并利用现有的电磁模型对结果进行建模，更重要的是预测氯化物在这些材料中的渗透量和深度。