The aim of the research was to find an effective, reliable and cost-effective method for long-term monitoring of moisture in concrete structures. The slow diffusion rate of moisture through concrete requires that monitoring should be done over time scales of several years without periodic re-calibration. The solution arrived at was to use a quasi-coaxial transmission line, termed a cagecoaxial transmission line, as the sensing element. The transmission line, terminated in a short circuit, is encapsulated in a porous dielectric medium. It was found that the microstructure of the encapsulating medium had to be similar to the concrete in terms of capillary characteristics in order to track the moisture content of the material under test. The moisture in the encapsulating medium would change the electrical length of the transmission line by increasing the relative permittivity of the medium. The method used makes it possible to measure moisture levels to full saturation. Moisture content can be measured in terms of a percentage of saturation, which will be of considerable help as an early warning system of possible frost damage. A mathematical model was derived to calculate the relative permittivity in terms of moisture content in concrete. It was shown that to calculate the total permittivity of a solid porous medium with a dielectric mix formula, the formula must be expanded to include air, water and solid, before realistic values for the permittivity of the ingredients could be assigned. A dielectric mix formula was derived to account for the liquid to solid boundary effect on the permittivity of water in a solid porous material. The foundations were laid for the development of a reliable and cost-effective probe based on an oscillator, operating around 1 GHz, using the transmission line as a tuning element. The frequency of oscillation is a function of the apparent length, determined by the permittivity and therefore the moisture content, in the transmission line dielectric material. A method to convert this frequency to a format that can be monitored on a data logger system is described. The high oscillation frequency eliminates the effect of ionic conduction from dissolved substances.
| Identifer | oai:union.ndltd.org:bl.uk/oai:ethos.bl.uk:707603 |
| Date | January 2002 |
| Creators | Malan, Francois S. |
| Publisher | University of Bedfordshire |
| Source Sets | Ethos UK |
| Detected Language | English |
| Type | Electronic Thesis or Dissertation |
| Source | http://hdl.handle.net/10547/622073 |
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