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This paper presents the profiles of relative humidity (RH) readings taken at various depths in concrete slabs during a time period of over 230 days using probes attached to a humidity-reading device. It also examines the effect ambient conditions have on the drying process. For the experimental work, two sets of slabs were tested, one in a controlled environment with an elevated temperature and the other in a laboratory at room temperatures. It was found, not surprisingly that the slabs in the controlled room appeared to dry out at a much faster rate than those held at room temperature. However, a residual of moisture remained within the slab and the average residual RH over the depth of the latter was as much as 85%, as compared with 80% for the former. This suggests that the industry's standard, specified in BS 8203: 1996, concerning the application of floor coverings when the surface reaches a RH of 75% needs to be treated with caution especially when drying is artificially accelerated. Due to this moisture residue remaining in the slab, any impermeable covering applied to the surface may result in a number of defects occurring, leading to expensive repair work later on. A scheme for modelling the process numerically is presented, employing the finite element method that will eventually account for changing ambient RH, non-linear diffusion coefficients and sealing of the surface at some point in time.
Holmes, N., West, R.: Moisture Migration in Concrete Slabs during Drying. Proceedings of the International Congress-Challenges of Concrete Construction, University of Dundee, pp. 173-180, September, 2002.