Detection of setting time in cement hydration using patch antenna sensor

The dielectric constant of cement will decrease with an increase of moisture content, resulting in an increasing fundamental resonant frequency of a covered patch antenna. Based on this principle, the authors utilized a patch-antenna sensor to determine the initial and final setting time of cement during hydration by sensing the variation of moisture content. The theory of an equivalent dielectric constant model was adopted to exhibit the relationship between the fundamental resonant frequency of a buried antenna sensor and a dielectric constant in the covering cement paste. The phenomenon is then verified by both simulation in a high frequency structure simulator (HFSS) and experiments. The initial setting time of cement is regarded as the moment when the moisture content starts to increase rapidly and the final setting time is taken as the maximum moisture content acceleration. The setting times measured by the patch antenna sensor are compared with the results of the penetration resistance method using a Vicat apparatus with a testing error of less than 10%. A moisture content detection experiment was further carried out after the paste's hydration assuming the specimen was soaked in water. The results indicate that the resonant frequency of the patch antenna sensor is sensitive to moisture content change as fine as 4 parts per thousand with respect to the paste volume after cement hardening. The buried antenna sensor can detect the setting time during cement hydration and serve as a leakage detection sensor in service.