NRMCA has completed a research project on a proposed ASTM Standard Practice on Measuring the Electrical Resistance of Fresh Concrete. The practice describes procedures for sampling and measuring the resistance of fresh concrete during setting and subsequent hardening under sealed conditions in the laboratory or in the field. The scope of the practice indicates that the measured resistance of concrete can be used to estimate the water-to-cementitious materials ratio (w/cm) of concrete delivered to a project site, among other characteristics. Relating the measured electrical resistance to attributes of the mixture proportions, such as water-cement ratio by mass (w/c), is explored for 11 different concrete mixtures prepared in the laboratory. In these mixtures, variables included w/c, paste content, air content, fly ash content, high range water reducer dosage, and cement alkali content.

For a specific mixture, changing mixing water content while maintaining the same cement content did show a linear change in measured resistance. Even when the w/cm was maintained at the same level, changing cement and cement alkali content had an impact on the measured resistance. Considering the variability of repeated measurements, the measured resistance was not sensitive enough to distinguish between mixtures where the w/c varied by 0.10 or less. If the sensitivity of the resistance measurements can be improved, this device can be used to monitor changes in w/cm for a specific concrete mixture with a pre-established correlation at a ready mixed concrete plant.