Accurate strength prediction models of ordinary concrete using early-age complex permittivity

Early determination of correct cement dosage for concrete mixes and prediction of their compressive strengths are very valuable in construction industry. This article presents a few compressive-strength prediction models for ordinary concrete using early-age measurements of relative complex permittivity. Both the dielectric constant and loss factor of the first-seven-day concrete with four types of mix proportions were measured for building the mathematical models composed of exponential functions at 6 GHz, whereas the measured dielectric data at 28 d were used for model validations. The proposed models not only have high regression accuracy with R-2-values of greater than 0.97 but also the low prediction errors of within 10%, which significantly outperform the existing models of the same kind. Through Cole-Cole plot analysis, this article reveals an interesting finding that a minimum loss factor was located at 3 GHz regardless of the age of concrete, which would be useful for wireless network design and planning inside a building. The average loss tangents of various concrete mixes have also been verified to have the same value (similar to 0.33) regardless of concrete age, which explains the confidence in use of early-age dielectric properties. Moreover, this article also initiates a precise prediction method of using loss factor and dielectric constant simultaneously in a 3-dimensional plot, wherein no explicit mathematical model is required.