Nondestructive and Noncontacting Testing of Hardened Mortar Specimens Using a Free-Space Method

Abstract

Microwave transmission (and reflection) properties measured from diverse sides of prepared hardened mortar specimens with different water-to-cement ratios (w/c=0.4 and 0.7) and sand-to-cement ratios (s/c=1.5 and 2.0) are shown. Measurements were carried out by a simple and inexpensive free-space setup at three discrete frequencies (f=8.5, 10, and 12 GHz) in X-band (8–12 GHz) during 3–36 months after specimens’ preparation. A recently developed approach (relative difference approach) is applied to predict the state and degree of aging (curing) of specimens using transmission properties obtained from various sides of specimens. It is observed that while reflection properties of specimens are nearly constant, their transmission properties and aging changed, except for the last few months, during 3–36 months. After approximately 30 months, aging of all specimens became constant. It is shown that, for measurements acquired from the same side of specimens with any s/c ratio, the mortar specimen with a higher w/c ratio (w/c=0.7) will lose free water faster than the other specimen (w/c=0.4) over approximately 3–9 months (first stage) of curing. In this stage, the effects of s/c ratio and the operating frequency on temporal differences in transmission properties are not considerable. On the other hand, the mortar specimen with a lower w/c ratio (w/c=0.4) will drop free water sooner than the other specimen (w/c=0.7) during approximately 9–30 months (second stage) of curing. In this stage, while the effect of s/c ratio on temporal differences in transmission properties is still negligible, that of the operating frequency is highly pronounced.