Cure-state monitoring and water-to-cement ratio determination of fresh Portland cement-based materials using near-field microwave techniques

Quick and nondestructive determination of cure-state and water-to-cement (w/c) ratio in fresh Portland cement-based materials is an important issue in the construction industry since the compressive strength of these materials is significantly influenced by w/c ratio. In this paper, the results of a study demonstrating the potential for early determination of cure-state and w/c ratio of Portland cement-based materials, using a near-field microwave inspection technique, are presented. This technique utilizes the reflection properties of an open-ended rectangular waveguide probe radiating into Portland cement-based materials at 5 GHz (G-band) and 10 GHz (X-band). The results demonstrate the ability of near-field microwave sensing techniques to determine the state of hydration of cement paste and concrete with 0.50 and 0.60 w/c ratios and varying aggregate contents. An empirical formula relating the magnitude of reflection coefficient to the curing time is presented. Using this empirical relationship, the w/c ratio of cement paste and concrete can be unambiguously determined when daily monitoring of the reflection properties of the specimens is performed. The potential for utilizing this technique for on-site monitoring of cure-state and w/c ratio (and compressive strength) determination is also discussed.