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15.4.4.2 Electrodeless Resistivity Measurement

The electrical resistivity change with time is regarded as a fingerprint of the hydration process of cement-based materials. Through monitoring the electrical resistivity of fresh cement and concrete, different periods in the hydration process can be identified. Thus, the measurement of electrical resistivity can be used to guide the construction process. Moreover, it can be used as a quality index for cement-based materials at an early age.

Previous researches on the electrical property of cement-based materials can be divided into two stages. In the first stage, two electrodes that are connected to a cement-based specimen for electrical resistivity measurement. Since any application of direct current to a cement and concrete specimen produces a polarization effect, this method cannot provide accurate results for the electric resistivity of cement and concrete.

In second stage, to overcome the disadvantage of direct current, high-frequency (1000 Hz) alternating current is applied for measuring the electrical resistivity. In this case, the polarization effect is eliminated. But the alternating current cannot solve the contact problem and gas releasing effect. To solve the contact problem, sometimes the researchers use an external force to fasten the electrodes and concrete specimen from two ends [29].

For these above reasons, the electrodeless resistivity apparatus is invented and used to investigate the hydration process of cement. The present invention overcomes the problems with the prior procedure by providing an apparatus and method for the electrical resistivity of cementitious materials that does not require the use of electrodes in contact with a specimen, but instead uses a contactless transformer-based method in which a specimen serves as the secondary coil of the transformer. As there are no electrodes, the problems involved with electrodes such as polarization, gas releasing, and contact are eliminated. The method is reliable, repeatable, and reproducible for fulfilling the basic requirements on an objective test method. It is a challenge to solve the technical problems encountered during the development of the new testing device.

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