Anhydrite screed: how do you determine the readiness for laying?

When laying and processing screeds, the readiness for laying is one of the most important things to watch out for. If the required residual moisture is not reached, this can lead to serious problems. How to determine the readiness for anhydrite screed read here.

residual moisture

Every screed which is installed wet must have the required residual moisture before further processing or covering. This residual moisture is achieved by a sufficiently long drying process.

Technically, some terms play a role. The screed is then "ready to go" when it has reached the "equilibrium moisture content". The equilibrium moisture content means that the water content in the screed is in equilibrium with the surrounding room air.

Other terms often used for equilibrium moisture are:

  • residual moisture
  • Household moisture or
  • equilibrium moisture content

They all refer to the same condition, namely sufficient drying out of the screed. The screed is then ready to go when the equilibrium moisture content is reached.

Problems with lack of drying

If the moisture content in the anhydrite screed is still too high, and floor coverings are laid on the screed, this can lead to massive consequential damage affecting both the screed and the floor covering. This risk is particularly great if waterproof (vapor-proof) coverings are laid or if a moisture-sensitive covering (such as wood or laminate) is laid.

Typical readiness for anhydrite screeds

The residual moisture may amount to a maximum of 0.5% after measurement with the CM method. For non-vapor-proof and non-steam-sensitive coverings, 1% may occasionally be the maximum value, but this is the exception.

If the anhydrite screed is installed as a heating screed, the values ​​are sometimes even stricter.

Measurement of the maturity

Although there are specifications in DIN 18560 for the minimum drying time for all types of screed, this does not mean that after this time (or after dry heating of a heating screed), the required level of covering has actually been reached. It must be determined by measurement in any case.

The only officially approved measurement method that is also legally recognized in litigation is CM measurement. It is the standard measuring method that has to be used. Small samples are mixed with calcium carbide. The reaction produces acetylene gas, the pressure of which in the reaction vessel can be measured. The pressure rise is directly proportional to the residual moisture and can thus be determined exactly.

Tips & Tricks

Determining the degree of maturity is not up to the screed leveler, but is the responsibility of the top floor layer. Only in the case of screed the screed layer must decide when further treatment of the screed is permitted.

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