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.