Prior to applying any flooring layer onto a substrate, the preparation should be carefully taken. One of the evaluation criteria is moisture content. Below are some testing methods to measure the moisture value for concrete or cementitious screed floors.
The calcium carbide method uses the reactivity of calcium carbide with water. A typical sample is taken from the bottom layer of the screed or concrete and crushed. The crushed sample is weighed and placed in a pressure vessel. A glass ampoule filled with carbide and several steel balls are added. The vessel is then shaken and the glass ampoule is crushed and the contents mixed with the test material. Diffused water reacts with the calcium carbide to form calcium hydroxide and acetylene. This gas generates a pressure which is used as a measure of the reacting water quantity. The relevant moisture content can be read off from tables.
The carbide method is a very accurate way of measuring the moisture content of cementitious and gypsum based (anhydrite) floor substrates.
As the water content increases, so does the electrical conductivity of the screed and this can be utilised for its moisture content measurement. To do this, two holes are drilled, electrodes are inserted into them and the electrical resistance between them is measured. The relevant moisture content can be read off on a specific table.
The water content based dielectric properties of a material also can allow non destructive measurement with an electrode pressed onto the surface of the screed.
Measuring instruments such as “Tramex” are very useful to determine readiness for covering because they can indicate quickly and simply whether the moisture content measured means that a comprehensive CM measurement can be expected to be successful. Tramex is too inaccurate as a final measurement for cementitious screeds and is not suitable for anhydrite screeds because there are a great many different mixes available.
Plastic sheet test (ASTM D4263-83)
Information on the moisture content can also be obtained non destructively by the ‘mat test’. A rubber or PVC sheet / mat is laid on the screed surface and sealed at the edges. If condensation or discolouration of the screed (due to moisture) is found after 72 hours, it is not ready for covering.
Measuring of the relative humidity
An other method to verify whether the wood floor can be installed or not is the measuring of the relative humidity. This is conducted with different devices. The relative humidity should not be over 55% for unheated screeds or over 50% for heated screeds (Central European Climate).