Curing and Moisture Control Before a Moisture-Sensitive Floor Finish
A mud floor slab, 120 mm thick, was placed five days ago. The contractor removed the polythene curing sheet after 48 hours to allow following trades access, and the surface has been exposed since. The ambient conditions have been warm and dry. The client now wants to apply a moisture-sensitive resin coating. A handheld moisture meter reads 4.5% on the surface, and the contractor proposes to proceed because 'it's below 5%'. There is no recorded datum for the meter, no calibration certificate, and no measurement of ambient conditions. The slab has hairline cracks at 1.5 m spacing.
1. Evaluate the contractor's proposal to proceed based on the 4.5% meter reading.
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The proposal is not supportable. Problems:
- Uncalibrated meter with no datum - a handheld meter reading is a relative indication, not an absolute moisture content, unless calibrated for the specific concrete and corrected for temperature and cover. 4.5% means nothing without a calibration relationship.
- Surface reading only - a 120 mm slab at five days will have a steep moisture gradient; the surface can read low while the core is much wetter. Coatings fail from moisture migrating from below.
- Curing was cut short - removing the sheet at 48 hours in warm, dry conditions means the slab has not been adequately cured; this also explains the hairline cracks. Early moisture loss increases shrinkage and leaves a weak, porous surface.
- Cracks at 1.5 m spacing are consistent with restrained shrinkage from poor curing; they must be assessed before coating, as they will telegraph through a resin finish.
The correct approach is a documented, calibrated moisture test (e.g., in-situ relative humidity probe in drilled holes, or the standard sheet method), with ambient conditions recorded, and a defined acceptance criterion for the coating system.
2. Describe the testing and remediation sequence required before the coating can be applied.
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Testing sequence:
- Allow the slab to dry under controlled conditions; record ambient temperature and humidity.
- Use a calibrated in-situ relative humidity probe at the specified depth (typically 20% of thickness) at multiple locations, or the specified sheet method, with a calibration certificate and documented datum.
- Compare results against the coating manufacturer's stated moisture limit - not a generic 5%.
- Assess the cracks: width, depth, and whether they are live. Determine if they are shrinkage cracks needing routing and filling.
Remediation before coating:
- Continue drying (ventilation, dehumidification if needed) until readings meet the limit.
- Prepare the surface: remove laitance and any weak/dusting layer by mechanical means; fill and profile cracks per the coating system.
- Re-test moisture after preparation and immediately before coating.
- Apply a compatible primer/vapour-control layer if the system requires it.
Do not coat until the moisture criterion is met and documented; coating over a wet slab traps moisture and causes blistering and debonding.