Knowledge Base

Moisture Measurement in Industrial Floors

 

(Article by Bill Gledhill, Contract Floors 05.02)

Industrial floors are invariably based on poured and/or power floated concrete slabs that, whether newly laid or old screeds, require proper treatment to make then suitable for finishing and the application of floor coverings.

The condition of the sub floor can, more often than not be blamed for floor failure. And quite rightly too. Despite the wide range of techniques, equipment and machinery available to engineer an ideal subfloor, there remains a need to improve preparation standards. Post installation problems such as adhesive failure delamination due to laitance layer, and of course water vapour and moisture being given off and causing movement in wood flooring are not uncommon.

Of course this prompts the question, why is it that proper sub floor prep is so often overlooked, skimped over or carried out with little regard to the consequences of creating a potential failed floor? Sub-standard all too quickly comes to mind when discussing subfloor preparation.

The problems are undoubtedly often due to difficult and unexpected site conditions. Coupled with a desire to get the job done as quickly as possible (a case of fast track work running off the rails), projects’ that inevitably actually take longer and overrun contingency funds. Yet, with modern preparation equipment there is little excuse for not doing the job properly.

However, before the concrete can be properly prepared for a coating or covering, its condition must be inspected and tested which involves taking meaningful moisture measurements. To do this, great care has to be taken to ensure the readings are valid and give an acceptable result that indicates the dryness of the whole area and thickness of the slab in question.

Slow to dry

Concrete is notoriously slow to dry out. Even British Standard (BS8203) emphasises the issue, stating that under ideal drying conditions a 150mm thick slab may take as long as 12 months to dry from the area in contact with the ground to the exposed top surface. Clearly accurate and thorough moisture testing is a prerequisite to producing a suitable subfloor for a floor covering or any surface concrete treatment process.

Moisture measurement

The ‘official’ method (as referred to in BS8203/8201 and 5325) for measuring moisture content, involved measuring the humidity of the air over a small enclosed area of the slab. It has drawbacks.

It usually takes over 24 hours for the air in the enclosure to reach equilibrium. The box or enclosure may also be accidentally knocked or kicked out of its original position and of course as with all moisture measurements they should be repeated at different locations to give a good ‘picture’ of dryness over the whole subfloor area.

Care must be taken when placing the enclosure to ensure it samples an exposed area of the concrete to obtain useful readings. Placing it over any coatings etc would give misleading readings. The method is not always satisfactory when used over power-floated slabs as the moisture gradient below the laitance increases significantly.

However, it is the industry standard recommended method and it is considered by many to give the most accurate assessment of subfloor moisture levels. But there are more refined techniques that are rather less prone to be affected by site activity and allow sampling over relatively large areas.

One company that specialises in moisture measurement solutions is Protimeter. This company has developed a number of measurement techniques that allow the user to gain a rapid initial assessment of floor moisture condition and to then make accurate and reliable ERH tests.

The company’s MMS instrument for instance, combines a moisture meter and a hygrometer, and is used to assess the overall moisture condition of the floor and quickly locate damp spots. The handheld portable device uses a radio frequency transmitter and measures the impedance of the material to a depth of about 15-20mm. Idea for identifying the worst areas, and for deciding where to place more accurate ERH measuring instrumentation.

For that purpose the company offers a system that uses sleeves inserted at the most appropriate locations for measurement to be taken. A hole is drilled (16mm diameter, small enough to be easily plugged after use) at each measuring station and then sealed humidity sleeves inserted. Being flush with the floor they are not a hazard or susceptible to damage from site activity and because they penetrate the substrate, the condition of the surface (laitance etc) does not affect readings.

The handheld MMS is plugged into each sleeve sensor for monitoring the rh level as the pocket of air in the sensor reaches equilibrium.

Alternatively using the same handheld instrument, a surface measurement (as per BS8203 etc), typically for screeds, can be taken by connecting the device to an enclosure placed on top of the subfloor For a quick and convenient assessment of overall subfloor condition. As with all moisture measurement techniques it is advisable to test at several points around the subfloor to gain an overall impression of floor moisture levels.

Be aware that old floors having coverings, coatings or contamination may prevent meaningful moisture measurement to be taken. The coverings whether vinyl, adhesive, paint resin, bitumen from old wood blocks or oil and grease contamination should be removed to allow access to the subfloor slab surface properly. As well as allowing practical measurements to be taken it will also encourage drying.

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