Concrete is a porous material, and throughout its lifetime, the concrete absorbs and releases moisture to and from its environment, depending on the temperature and the humidity of the environment. Concrete consists of sand and pebbles held together by a paste of cement and water. Concrete hardens and acquires its strength when the cement hydrates – meaning that it reacts chemically with water – and in a process producing heat, it forms a binder that glues sand and pebbles together. This process begins a couple of hours after the components are combined and is largely complete after 1 month. However, that the concrete is fully hardened does not mean that the concrete is dry. On the contrary, there will be some water left, which must dry or be removed before further work can be done on the concrete layer.

Eliminating this water is a time-consuming process, which is highly dependent on the room temperature, air humidity, one or two-sided drying, the quality of the concrete, the thickness of the concrete layer and the substrate. A traditional concrete floor has a w/c ratio of 0.65 and is often cast in a 100 mm layer on a polystyrene base.

If the air humidity is kept at 50% RH, and the temperature at 20 °C, it will take 3-4 months to achieve a concrete moisture level of 85% RH. But often, the temperature is lower, and the air humidity considerably higher, which makes it necessary to extend the drying period by several months. Drying traditional concrete to a level of 85% RH should therefore be expected to be a very lengthy process that can easily last 4-6 months. And even that can only be achieved if the building has been sealed off quickly to initiate the dehumidification process.

When installing a Dinesen floor make sure that the residual humidity in the concrete does not exceed 85% RH when using an approved vapour barrier.

It is difficult to carry out an accurate measurement of concrete moisture, and surface hygrometers are not nearly accurate enough. You need to measure the moisture content in the middle of the concrete layer. This may be done by placing a sensor in a hole drilled into the concrete. After some time, the sensor achieves moisture equilibrium with the concrete, and the relative humidity can be determined.

A more accurate way of measuring is to carve out a piece of concrete from the middle of the concrete layer and carry out a humidity test in a laboratory. With a chisel hammer, a hole of approx. 50x50 mm is carved in approx. half of the construction’s thickness. The material (the concrete) is collected in an airtight container and brought home for analysis. The method is also used for concrete constructions with underfloor heating. Here, the construction is scanned before sampling with a floor scanner and/or a thermography device that can reveal the embedded underfloor heating pipes.

The finding of this test is documented in a humidity report, which can serve as part of the contractor’s quality assurance process. A humidity measurement is a very cost-efficient insurance that may save the contractor and the client substantial costs.

Floors adhered directly to concrete must, in addition to the requirements for moisture, cleanliness and flatness, meet certain strength requirements.

The strength requirement is referring to EN 14293 & EN 13813, minimum: CT-C30-F5

If a screed is applied on top of the concrete, this layer must have the same strength and be cast in a way that transfers the strength between the two layers. Typically, this is done by cleaning off the concrete and laying out a primer before casting.