The Importance of Moisture Testing Floor Screed

The correct methods of moisture testing should be carried out, recorded and presented to the client PRIOR to the application of final floor finishes to avoid future failure.

Traditional cement / sand screeds have become increasingly dominant as the first choice for many screeding projects and standard practice would be to install the screed as early in the program as physically possible to allow for the 3 months plus drying time. The recognised benchmark for screed dry times are 1mm per day up to 40mm and 0.5mm per day thereafter. This means that a 75mm thick traditional screed will take 110 days to dry to an acceptable level for applying sensitive floor finishes. The drying time is also very reliant on a good airflow around the screeded area plus a controlled temperature of 20 deg C & 50% RH.

Once the required dry time has elapsed the screed would then be tested by the floor finishing contractor prior to applying sensitive finishes, such as vinyl, carpet, wood flooring, etc. Testing methods for the moisture content of floor screed vary in both time taken to achieve results and their reliability.

A hair hygrometer is the testing method recommended by The British Standard to record the Relative Humidity (RH) in screed. When using a hood to get results from above the surface of the screed the hood has to be left in place for 72 hours to allow the chamber under it to reach equilibrium with the humidity and the temperature of the floor slab. This is a timely testing method requiring a return visit to site and it is often impractical to ensure the hood is not disturbed by traffic on a busy site.

An RH Probe can be used to test the relative humidity in the screed below the surface in situ and requires a number of holes to be drilled in the screed. A plastic (sleeve) hole liner is placed into the drilled holes and these are then sealed left in place for 72 hours to allow the humidity in the sleeve to reach equilibrium with the humidity in the screed before making the humidity measurement with the RH Probe. The probe is inserted into the sleeve to record the RH and for each recording it needs to be left in place in the sleeve for approximately 40 minutes to reach the temperature and humidity equilibrium of that within the concrete floor slab. Readings should then be taken over a period of 20 minutes to allow for any drift in the meter readings. Once again, this is a timely method requiring a second visit to site.

For an accurate speedy result, the Moisture Content (MC) test is recommended. This is the testing method preferred by most product manufacturers particularly when testing modified screeds which are not currently covered by the latest revision of The British Standard. The Moisture Content test can be carried out either by a destructive method, a calcium carbide test, or non-destructive method using a Tramex Meter.

When carrying out a destructive calcium carbide test a sample of the screed is removed to approximately 50% of the depth of the screed and the weight of the sample carefully measured with scales. The sample is the entered into a vacuum flask together with a reagent and the flask is rotated in an orbital fashion and, following a timed process, the result is displayed in a pressure gauge as a CM%. This measurement is very accurate and is fairly quick to carry out each test (c. 30 minutes) but being destructive some remedial repairs to the screed are necessary.

Using a Tramex Meter to measure the moisture content of the screed is non destructive and gives instant results for the moisture content of the screed. It uses the principle of electrical impedance measurement using electrodes with special spring mounted probes on the base of the instrument with a low frequency signal being transmitted into the screed to obtain the measurement. Multiple MC readings can be taken in a very short time with the moisture content being instantly displayed and recorded. These are approximated equivalent reading to the carbide test method.

Before applying final floor finishes the final flooring contractor has a responsibility to test the substrate with the results being presented to the client for agreement. If the screed is above the maximum required moisture level specified by the floor covering manufacturer a decision has to be made on verification. There are three options: wait longer; employ an independent third party to carry out further testing or apply a surface DPM. The latter is expensive and ranges from £8 - £13 a sm which is sometimes more expensive than the screed itself. The lack of time usually forces a decision to proceed with surface DPM under duress.

The Solution

Technology has moved on to where screeding has become a science. There are many companies in the market place today such as FlexiDry offering fast drying screed products; both additives and cement replacements. When using a fast drying accelerant it is important to review the published data as most manufacturers rely on the benchmark of 20 deg C and 50% RH so it is best practice to allow some extra time in the program to ensure a positive result.

FlexiDry is a proven product in the real environment where dry times have not been dependant on temperature bench marked at 20 deg C. FlexiDry F0 was tested by DMC Rail Ltd and Balfour Beatty prior to installation at London's Blackfriars Station and FlexiDry F1 was tested with CSC Screeding Ltd at The Lancasters residential project overlooking Hyde Park. These projects had a total screed area of over 22,000sm. A small project was benchmarked recently with an Essex based main contractor where temperatures were recorded three times daily and moisture tested after the 21 day expiration (F3) and even though temperatures did not exceed 12C at any time, the test confirmed the screed was dry enough to receive final finishes.

A recent example of why a floor finish fails - Uxbridge College Case Study March 2011

A screeding contractor was employed to install FlexiDry F2 14 day dry screed to the new sports hall and associated changing areas at Uxbridge College on behalf of the main contractor.

In the main sports hall the screed was tested by the screed contractor at the request of the main contractor after 13 days. All areas had reached the required moisture content with the exception of the last area which was effectively tested prematurely. The floor finishing contractor argued that his test equipment should supersede the testing method recommended by the manufacturer. Increased pressure had been applied to the main contractor and it was decided to install a surface DPM to the 650sm at considerable unnecessary cost.

In the changing areas a moisture problem seemed to have occurred as the vinyl floor finish had bubbled and lifted. The vinyl was removed to expose the substrate and found that the adhesive had turned milky and wet. A layer of latex had been removed to directly expose the screed for testing. In the presence of the main contractor, floor screeding contractor and screed manufacturer a Tramex moisture test was carried out. The screed was a recorded as an average of 1.6CM%, the latex recorded at 3.0CM% with the adhesive being well and truly wet.

The results determined that the screed was dry but the floor finishing contractor had failed to test the screed to the manufacturer's instructions prior to applying a latex finish to receive the vinyl. As a result of the moisture test not being completed by the floor finishing contractor, it was not possible to determine the route cause of the problem and whether it was due to applying the vinyl whilst the latex compound was still wet; the failure of the adhesive; water penetration of the vinyl or that water had been reintroduced to the screed during construction.