Microwave nondestructive detection of influenc of Alkali-Silica Reaction (ASR) in materials properties of concrete specimens

Alkali-silica reaction has been identified to be a cause deterioration in concrete structures. This reaction takes place when alkalis (e.g., potassium and sodium), hydroxyl ions in the pore solution and certain siliceous minerals (present in some aggregates) are present, resulting in what is known as the ASR gel. Expansion of this gel, in the presence of heat and moisture, causes stresses that lead to microcracking and deterioration of concrete. The criticality of this problem has been particularly highlighted in recent past with respect to nuclear power plant containment concrete structures. Although there have been many studies devoted to examining and developing testing techniques for detecting and assessing ASR, currently no real nondestructive and field-deployable technique exists. Microwave materials characterization techniques, capable of evaluating material properties (physical and chemical) of cementitious materials, including those with ASR, have shown great promise for this purpose. In this study, a microwave nondestructive testing (NDT) method, using an open-ended rectangular waveguide probe, for conducting reflectometry and determining complex dielectric properties of materials, was employed at R-band (1.72-2.6 GHz). Three previously-produced large concrete specimens, one devoid of ASR and a set of two with ASR, were examined. The results illustrate the capability of differentiating among these specimens and corroborating the trend of the results previously obtained for mortar with ASR. This paper describes the properties of these specimens, method of measurement and the subsequent findings.