Evaluation of Microwave Reflection Properties of Cyclically Soaked Mortar Based on a Semiempirical Electromagnetic Model
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Detection of chloride ingress and evaluation of its distribution and temporal movement in reinforced concrete structures is an important practical issue. Steel reinforcing bars embedded in good quality concrete are normally protected from corrosion. However, the presence of a sufficient concentration of free chloride ions in the region of the reinforcing steel can initiate the process of corrosion. Therefore, it is important to be able to detect ingress of chloride ions and their distribution in cement-based materials. Moreover, it is important to obtain this information nondestructively. In recent years, near-field microwave nondestructive evaluation methods, using open-ended rectangular waveguide probes, have proven effective for evaluating many important properties of cement-based materials, including the detection of salt, added to the mixing water and when entering these materials through exposure to salt solution. Additionally, successful electromagnetic modeling of the interaction of microwave signals with moist cement-based materials has provided the necessary insight for evaluating the distribution and movement of moisture within these materials, leading to the current study involving ingress of sodium chloride solution. To this end, a mortar cube was subjected to cycles of wetting in a sodium chloride bath with a salinity of 2.8%, followed by episodes of drying. Subsequently, the microwave reflection properties of the cube were measured at 3 and 10 GHz using open-ended rectangular waveguides for several cycles, each lasting about 35 days. A semiempirical electromagnetic model, representing the cube as a stratified structure with a nonuniform dielectric property profile, was then developed to simulate the measured reflection properties. The simulated and the measured results at both frequencies and for all cycles were in good agreement. Subsequently, the effect of ingress of salt solution in terms of the temporal distribution of moisture along with the dissolved salt (i.e., pore solution) within the cube for every cycle was also estimated. This paper presents a brief description of the measurement approach and a detailed description of the model and its results.