Microwave nondestructive determination of sand-to-cement ratio in mortar

Microwave nondestructive testing methods have shown great promise for the inspection of cement-based materials. Previously it has been shown that the magnitude of reflection coefficient from cement paste specimens can be correlated to their water-to-cement (w/c) ratios and more importantly to their compressive strengths. For mortar specimens, the sand-to-cement (s/c) ratio plays an important role in its physical, mechanical, and microwave reflection properties. To determine the w/c ratio and other properties of mortar specimens, one also needs to know its s/c ratio. To illustrate the ability of microwave reflection measurements for this purpose, two sets of four mortar specimens are produced with w/c ratios of 0.50 and 0.60 and with s/c ratios of 1.0, 1.5, 2.0, and 2.5. The microwave reflection coefficients of these specimens are measured using an open-ended rectangular waveguide sensor, in the G- (3.95-5.85 GHz), J- (5.85-8.2 GHz), and X-band (8.2-12.4 GHz) frequency ranges. It is shown that a simple relationship between the standard deviation of the magnitude of the reflection coefficient of mortar and its s/c ratio exists, and a more sensitive relationship exists for higher frequencies (waveguide bands). This approach may be extended to nondestructively determine the aggregate volume content and size distribution in concrete specimens.