MICROWAVE NONDESTRUCTIVE ESTIMATION OF CEMENT PASTE COMPRESSIVE STRENGTH

Dielectric and reflection properties of four cement paste samples with water-cement ratios (w/c) of 0.31, 0.40, 0.50, and 0.60 were measured daily for 27 days using microwave frequencies of 5, 9, 13, and 17 GHz. The dielectric properties of these samples, and hence their reflection coefficients, decreased as a function of increasing w/c. This is opposite to what is expected for physical mixing (no chemical interaction or hydration). The presence of curing as evidenced by this result indicates that microwaves could be used to monitor the amount of curing of a concrete member. The variation in the reflection coefficient of these samples as a function of w/c followed a trend similar to the variation of compressive strength as a function of w/c. Subsequently, a correlation between compressive strength and reflection coefficient of these blocks was obtained. This result can be used to directly and nondestructively estimate the compressive strength of a cement paste block, thus opening up the possibility of nondestructively testing the compressive strength of concrete.