Near-Field Microwave Non-invasive Determination of NaCl in Mortar

R. Zoughi, Missouri University of Science and Technology
A. D. Benally
Karl J. Bois

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In recent years near-field microwave nondestructive testing and evaluation techniques have shown great promise for evaluating different properties of cement-based structures. An important issue regarding the inspection of these structures is the ability to determine the presence and evaluate the content of chloride in them. Chlorides can be introduced in these structures in many different ways including when salts are present in their mixing water. Consequently, for this investigation, two sets of 8"*8"*8" (203 mm) cubic mortar specimens were prepared each with a water-to-cement (w/c) ratio of 0.5 and 0.6, respectively, and both sets with sand-to-cement ratios of (s/c) of 1.5. Four specimens were produced for each set with different salt (NaCl) contents added to the mixing water, producing salt-to-cement (NaCl/c) ratios of 0.0, 1, 2 and 3%, respectively. For the purpose of compressive strength measurement, cylindrical specimens of these mortar specimens were also prepared and tested for their compressive strength. Microwave reflection properties of these specimens were measured at S-band (2.6-3.95 GHz) and X-band (8.2-12.4 GHz), employing open-ended rectangular waveguide probes. It is shown that the magnitude of reflection coefficient is a useful parameter for detecting different chloride levels in these specimens. Moreover, the influence of chloride on the curing properties (i.e. setting time) and the compressive strength of these specimens are subsequently shown to be well correlated to the measured magnitude of reflection coefficient of these specimens at S-band. It is also shown that this correlation is unambiguous as a function of w/c which possesses significant practical ramifications.