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 reflecti on coefficients, decreased as a function of increasing w/c. This is op posite to what is expected for physical mixing (no chemical interactio n or hydration). The presence of curing as evidenced by this result in dicates that microwaves could be used to monitor the amount of curing of a concrete member. The variation in the reflection coefficient of t hese samples as a function of w/c followed a trend similar to the vari ation of compressive strength as a function of w/c. Subsequently, a co rrelation between compressive strength and reflection coefficient of t hese blocks was obtained. This result can be used to directly and nond estructively estimate the compressive strength of a cement paste block , thus opening up the possibility of nondestructively testing the comp ressive strength of concrete.