Comparative measurements are reported of the the moisture content, during air drying, of concrete slabs as measured by (a) a commercial electrical resistance "moisture meter" complying with the requirements of Australian Standard CA37-1966 (b) progressive weight loss to the atmosphere followed by oven drying. Wide divergence between the results from (a) and (b) were observed for all four concretes tested.
Lesser, but significant, differences were observed in the resistance-moisture relationships of otherwise identical concretes made with different brands of portland cement.
Resistance results from fixed depth electrodes showed a significant thickness effect; this disappeared if the electrode depths were made a fixed proportion of slab thickness. This is explained on the basis of diffusion theory.
No correlation could be found between cement composition and the resistancemoisture relationships of the concrete. This suggests that the physical structure of the cement paste may play a significant part in determining this relationship. It thus appears that the use of electrical resistance meters to determine the moisture content of concrete may produce very misleading results unless the meter is calibrated with concrete having the same characteristics as that to be tested.