Non-destructive assessment of the rate of hydration and strength gain of concrete

The hydration reaction between water and cement governs both the rate of strength gain in fresh concrete and also the final strength of hardened concrete. These strength properties are typically assessed by casting small samples (often cubes) taken from the fresh concrete and conducting destructive strength tests at selected days after casting. The hydration reaction causes a reduction in free water content in the concrete mix over time. The dielectric permittivity of a material is influenced by free water content, and thus ground penetrating radar (GPR) offers the potential to assess changes taking place during hydration. This paper reports a study where 45 concrete specimens, consisting of 3 different concrete mix designs, were cast and tested over a period of 92 days to establish both the changes in dielectric permittivity (determined from time domain GPR data) and the strength gain in the concrete (determined from compressive strength testing). Results showed a strong correlation between decreasing dielectric permittivity with increasing age (and increasing compressive strength) for each concrete mix design, and indicated the potential for GPR to be used as a rapid, nondestructive method for assessing hydration rate and strength gain as concrete changes from the freshly mixed to hardened state.