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A concrete floor that looks dry can still hold enough moisture to ruin a newly coated epoxy system. Substrate moisture is one of the most common causes of blistering and peeling after application, and the regrettable part is that it can be detected before coating if it is measured correctly. This article focuses on one task only: measuring concrete floor moisture before epoxy coating — what to measure, which method to use, and how to read the results.
The overview of concrete floor preparation before epoxy coating already mentioned moisture testing as one step in the process; this article looks in depth at that step alone.
Why substrate moisture can destroy an epoxy coating
Concrete is a porous material. Water exists in a floor in several forms: water remaining from pouring and curing the concrete, water rising from the ground when there is no moisture barrier under the slab or when that barrier has failed, and water from the service environment, such as wash-down areas or condensation in cold stores. When a sealed epoxy coating is applied on top, moisture vapour has no way out. Vapour pressure building up under the film can be large enough to push the coating up into blisters, and then peel it off in patches.
Signs of moisture damage to a floor usually appear after a few weeks to a few months of use, not immediately at handover. Visual inspection at the time of application is therefore not enough: the floor must be measured with instruments, at many locations, before the decision to coat is made.
What to measure: three forms of moisture to distinguish
There is no single number that describes every kind of substrate moisture. A surveyor is usually concerned with the following three forms:
- Surface moisture: water and moisture vapour in the top few millimetres of the surface, where the coating will bond. This is the form of moisture that most directly affects adhesion.
- Moisture within the concrete mass: water retained inside the body of the slab, which can gradually migrate to the surface over time. Newly poured floors usually fall into this group.
- Rising moisture from below: water from the ground or groundwater pushing up through the slab. This is the most dangerous form because it does not disappear on its own over time and needs its own moisture-barrier solution.
One measurement method correctly reflects only one form of moisture. Using the wrong method gives a false sense of safety — for example, a dry surface while the body of the slab is still wet.
Common measurement methods
The sealed plastic sheet method
A transparent plastic sheet is sealed at its edges onto the floor and left in place for a specified period, after which condensation under the sheet or darkening of the floor is observed. This method is simple and cheap, but it gives only a qualitative result — whether moisture is present or not — not a number to compare against a product limit. It is suitable for quick screening, not for the final decision on a large area.
Relative humidity in the concrete mass, measured with a probe
A probe is placed in a drilled hole in the slab at a specified depth, sealed, left to equilibrate, and then the relative humidity of the air in the hole is read. This method reflects moisture within the concrete mass, suits new or thick floors, and gives a number that can be compared with the limit published by the coating manufacturer for its product.
Moisture vapour emission rate measurement
A sealed container holding a desiccant is stuck to the floor for a specified period, then weighed to calculate the amount of water vapour leaving a unit area in a unit of time. This method directly measures the amount of vapour escaping from the slab, very close to the mechanism that causes blistering, but it is sensitive to temperature and air humidity and requires strict adherence to procedure so that results from different measurements can be compared with each other.
Surface moisture meters
Handheld meters give quick results using an electrical signal or waves, but the results are relative and are affected by reinforcement steel, salts in the concrete, and old coatings. Surface meters are best used to scan a large area and find abnormally damp zones, and then confirmed with one of the quantitative methods above at exactly the suspect location.
Where to measure, and how many points
Substrate moisture is not evenly distributed. Locations that should be prioritised for measurement include: areas that have suffered water ingress or ponding, wall bases and slab edges, areas near water sources, the lowest point of the factory floor, and newly poured floors that have not had enough time to dry. For large areas, measurements must be taken on a grid covering the whole floor, rather than at a single convenient point near the entrance.
Measurement results also need to be recorded together with the measurement conditions: floor temperature, air temperature, air humidity, and the time of measurement. The same floor, measured on a cool morning and at midday in the sun, can give different results; a measurement record without its accompanying conditions will be hard to use for comparison if there is an acceptance dispute.
Reading the results and deciding
- Dry floor, within the product limit: apply a standard coating system following the standard procedure.
- Slightly damp floor, above the limit for a standard coating but still within the capability of a moisture-tolerant system: choose a moisture-tolerant coating or primer strictly according to the technical data sheet of that product — do not assume on your own that any water-based coating can tolerate every moisture level.
- Heavily damp floor, or signs of continuous rising moisture: the cause of the moisture must be dealt with first, or a dedicated moisture-barrier solution used. Coating directly over a floor with continuous rising moisture is a certain way to have to repair it.
The permitted moisture threshold is not a single common number for every epoxy coating. Each product has its own limit stated in the manufacturer’s technical data sheet, and that number is the basis for acceptance. This article deliberately does not give a common threshold, to avoid applying one product’s threshold to a different product.
For a damp floor, a water-based coating system such as AquaBase is one of the directions to consider together with the consulting unit, because the general characteristic of water-based coatings is better moisture tolerance than solvent-based coatings under some conditions. The final choice still rests on the actual measurement results and the technical data sheet of the product.
Note: this article presents the general framework of epoxy coating materials, not Kadier-specific specifications. The specific parameters of each coating system are valid only as stated in the official technical data sheet of that product.
Request a consultation on the coating system suited to your floor condition.
Illustration: Pexels / Ksenia Chernaya.
