| Summary: | Failure of surface coatings on concrete slabs can occur if there is excess moisture in the slab. There is a need for reliable standard measurement procedures to determine whether a concrete slab is dry enough to have a surface coating applied. Accordingly the Building Research Association of New Zealand commissioned this project to develop a top-surface mounted resistive technique for measuring moisture profiles in concrete flooring slabs. Geophysical vertical electric sounding (VES) techniques have been adapted to determine resistivity profiles in concrete slabs through mathematical inversion of apparent resistivity readings made at the surface. Relative-humidity profiles may then be extracted when the relationship between relative-humidity and resistivity has been determined.
The project has involved the development and testing of:
• 'wet' electrodes (ie. wooden electrodes wetted with a conducting solution) which are able to reduce and stabilise the otherwise high, variable and non-reproducible electrode-concrete interface resistance,
• a VES instrument comprising an array of electrodes multiplexed to a computer controlled resistivity meter and operated through a graphical user interface and software able to 'invert' the apparent resistivity curves determined,
• embedded electrode systems for independent measurement of resistivity profiles for use in evaluating the VES instrument and technique and determining the relationship between relative humidity and resistivity.
Resistivity ρ and relative-humidity ψ profiles have been measured using a range of concrete samples and the relationship between them, away from the dry surface region, has been found to be described by the equation ψ = -aln(ρ) + b where a and b are coefficients that are functions of depth and the age of the concrete. The ability of the VES instrument to determine resistivity profiles from non-reinforced slabs is demonstrated in this report. However reinforcing at shallow depths (30 mm below the surface) does not allow profile recovery and makes commercialisation of the instrument unlikely. It is suggested that the embedded electrode systems developed here, provide a convenient and inexpensive method of directly measuring resistivity profiles from which relative-humidity profiles may be extracted with a high degree of precision.
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