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In this study, a method for determining the setting time and the porosity of fly ash-based geopolymer binders (FAGPBs) has been established. This method permits a real-time monitoring of the geopolymerisation and hardening progressions. A parallel plate capacitor cell technique was used to determine the dielectric constant e r ' and dielectric loss factor e r ' ' of fresh and hardened mixes. Equivalent model-circuits of the measurement system were developed, and the accuracy of the system was verified. The dielectric properties were monitored at different ages until 56 days over a frequency range of 0.1–60 MHz. The alkaline solution to fly ash ratio, sodium silicate to alkaline solution ratio, and sodium hydroxide solution molarity were varied to study their effects on the dielectric properties. It was found that dielectric properties can provide valuable information when measured in the frequency range of 10–25 MHz. In this range, the dielectric values responded well to the variations with age and with mix properties. The relationships between the dielectric properties and the different physical states and constituent materials of FAGPBs could be used for quality control applications. The e r ' and e r ' ' time plots were effectively utilized to determine the setting time and the rate of geopolymerisation of FAGPBs. Measurements showed a strong correlation between e r ' and porosity; the e r ' values decreased 2.3-fold over a porosity range of 3–15%. The proposed regression models can be further used in determining the porosity of FAGPBs at a high accuracy level.

Introducing steel fiber to concrete materials enhances the performance of concrete structures in terms of their flexural strength, impact load resistance and limited crack propagation. The fiber concrete performance depends on the fiber content, dispersion and orientation. No standard method is currently available to evaluate these three parameters, especially in situ. This paper describes the feasibility of using a surface electromagnetic sensor as nondestructive radio wave test system to determine the concentration, dispersion and orientation of steel fibers in the concrete. The system needs to contact the concrete material and requires only one face of the concrete material for testing. The surface electromagnetic sensor can slide on the fiber concrete surface and measure the dielectric properties at various locations to assess the fiber distribution. In addition, the sensor can rotate to polarize the electric field to various angles and evaluate the fiber orientation. The mean and standard deviation of the measured dielectric properties increase with increasing fiber content and fiber dispersion. The results indicate that the dielectric properties are maximal when the fiber is oriented in the direction of the electric field and minimal when perpendicular. A simple linear model was established to determine the fiber content and flexural strength from the measured electromagnetic properties.