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Mastering moisture: Methods for screed moisture measurements

In the world of construction, where precision and durability are paramount, the importance of moisture measurements in screeds cannot be overstated, especially in a country where climate variations can be significant. Therefore, understanding and controlling moisture levels in screeds is crucial for the longevity and performance of flooring installations.

The accurate measurement of the residual moisture content in a screed is essential for determination of suitability for overlayment with floor finishes and preventing flooring failures. If a screed, or any surface, is overlayed with a floor finish when the moisture content is too high for either the fifinish itself or the adhesives with which it is affixed, then there is a high probability of delamination of the floor finish or damage to it.

The boundary between the screed installer and the soft flooring contractor has at times been a difficult one. The exact degree of preparation needed to a screed surface, and who should do it, has been on occasions a point of disagreement.

The most obvious example of this is the question of moisture content and certainly there have been a great many issues with floor finishes where residual moisture in the screed has been either the cause or a factor in failures of floor finishes.

The recurring problems with moisture are, perhaps, surprising since the process by which screeds dry is a very simple one. Different types of screed dry at different rates and some types are more dependent on the site conditions than others.

However, all dry by the same process and there is no chemistry or magic to it. All screeds contain water in excess of what is needed to ‘harden’ the screed and this moisture must go somewhere. Since the only avenue available to it is upwards into the room, that is where it must go. It will only move into the air above it if the air is dryer than the screed so we must ensure that the air remains dry. Usually this just means opening doors and windows on dry days, just like we would with our laundry.

While the process of drying of screeds is a simple one, the measurement of moisture in screeds is a little more confusing and perhaps this is the reason for some of the issues experienced.

There are four basic ways to measure moisture in screeds:

Electronic meters
Electronic meters seem an initially attractive option as they are portable devices that measure the electrical conductivity of a screed, correlating it with moisture content. Simple and quick to use, these meters offer a quick and non-destructive method of assessment, but most are only of limited use in providing in depth specific information. However, they are valuable for onsite spot checks and preliminary assessments. It must be noted that no electronic meter complies with any British standard.

Meters that measure resistance at the surface will usually have two ‘prongs’ that are simply pressed to the surface of the screed to give an instant reading.

These may be useful to ascertain the wettest point on the surface of a screed in order that a more appropriate method can be used on this spot but are otherwise of limited value. The actual reading obtained will not correlate at all with the moisture content in the screed and so can give no indication of the screed’s suitability for overlayment.

More sophisticated meters emit a ‘signal’ to a defined depth in the screed and measure the signal returned. Results from these meters seem to be inconsistent in most cases and, based on testing carried out over several years with Cemfloor, most are of little or no more value than those from prong type meters.

British Standard Method
British Standards (including 8201 for timber floors and 8203 for vinyl floors) use a method that doesn’t measure the absolute moisture content in the floor directly, but rather measures the relative humidity (RH) of a volume of air trapped in contact with the screed and given time to ‘equilibrate’ with it.

While it’s consistent to use a defined RH value to ascertain when a screed is suitable for overlayment with a floor finish, this RH value will, in fact, be achieved at different absolute moisture contents for different materials. Anhydrite materials, for example, will require to be at a much lower absolute moisture content to produce the required ‘safe’ RH value than would cement based materials.

There are two variations on the British Standard Method. The first traps a volume of air in an insulated box sealed to the surface of the screed. This is the ‘Hygrometer Probe’ Method. It’s defined in the standard and generally accepted that a maximum of 75% by this method is acceptable for most floor finishes, though manufacturers guidelines should be followed.

The second measures the moisture in a ‘sleeve’ drilled into the screed. Whilst in principle this method avoids some of the difficulties of the surface method it is NOT appropriate to use this method for screeds for two reasons:

The method was intended for measuring moisture in concrete and the original designer of the test stated that it is not suitable for samples of less that 100mm. Screeds, and certainly liquid applied screeds, are seldom thick enough for the method to be appropriate.

There is no currently agreed upon maximum acceptable reading to indicate suitability for floor finishes. The method always produces a higher value than the surface method, so it’s certainly not appropriate to consider 75% as the acceptable number. Initial testing suggests that a sleeve test result of about 84% (for Cemfloor) would be acceptable for floor finishes and would be the equivalent of 75% by the surface method.

Before this can be relied on however there is a need for further testing which, until completed, renders the method as unsuitable for screeds.

Carbide bomb/CM test/Speedy test
The principle of this method is to place an accurately weighed sample, removed from the floor, into a pressure vessel also containing a vial of a chemical. The cap for the vessel includes a pressure gauge and once this cap is installed the vial is broken to release the chemical to react with any water present in the sample. The reaction produces a gas.

The amount of gas produced, and so the pressure within the vessel, will correlate precisely with the amount of water in the sample. The pressure can be read, and the moisture content of the sample can be derived from this.

This method is recognised in many countries and complies with European Standards but currently does not comply with UK standards. Nonetheless it’s a reliable and accurate method that is beginning to be used in UK.

Laboratory testing
The most accurate method is laboratory testing. A sample is removed from the floor, immediately tightly wrapped so that it cannot lose water, and then sent to a suitable laboratory. The sample is then accurately weighed, dried completely in a suitable drying cabinet, and then weighed again.

The difference in weight is the lost water and a simple calculation yields an accurate moisture % that was present in the sample.

Moisture measurements in screeds are integral to the success and longevity of flooring installations. Construction professionals must prioritise accurate and timely assessments to avoid costly flooring failures, ensure the durability of floorcoverings and comply with industry standards.

The choice of moisture measurement method depends on factors such as accuracy requirements, time constraints, and the specific characteristics of the screed in question (eg if it’s anhydrite or cement-based).

As the construction industry continues to evolve, embracing innovative technologies and methods for moisture measurement will play a crucial role in enhancing efficiency and reducing risks associated with flooring installations. By understanding the significance of moisture management in screeds, construction professionals can contribute to the creation of durable, safe, and compliant buildings for generations to come.

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