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Concrete subfloors – which tests and what readings?

There’s been some confusion in the UK flooring industry regarding the different test methods for measuring moisture in sub floors and the different units of measurement associated with them.

To determine when a floor is dry one can be directed by floor covering manufacturers to look for readings such as ‘below 75% RH’, ‘below 2.5% moisture content’, or ‘below 4% moisture content’ depending on the type of test carried out.

This is a description of the main methods used in the UK, their respective units of measurement, and the interest of combining test methods for efficiency and a better understanding of the moisture conditions.

British Standard Relative Humidity Hood Test
A well-known test in the UK is the hood method for testing concrete base slabs or bonded screeds. This is a relative humidity test in which the insulated impermeable box is sealed to the sub floor for a period of 72 hours. The relative humidity readings are taken by inserting a probe into the box or from a hygrometer that is fixed into the box for the duration of the testing period. Further readings are then taken every 24 hours until two consecutive readings taken are the same. If the consecutive readings are 75% RH or below then the slab can be considered dry enough for a floorcovering.

This test is very thorough and when carried out correctly it can be argued it gives the most useful information of any test as it gives a clear indication of the amount of moisture actually being released from the surface of the subfloor as well as an indication of the rate at which the moisture is being forced out. The other obvious advantage is that it’s a British Standard test, which is very useful in the event of a dispute.

However, there is one significant challenging variable to the reliability of this test.

Temperature fluctuations
The possibility of skewed readings owing to temperature change can easily be missed by ignoring the British Standard advice for a follow up test, four hours or 24 hours later as mentioned.

It’s also often advised to carry out the tests in ‘ideal’ or ‘in-service’ conditions. It’s very important the person responsible for the testing understands why we need stable temperature and humidity when carrying out testing and how potentially serious errors can occur if not properly understood.

When the building isn’t in ‘in-service’ conditions there is a potential for dramatic temperature and humidity fluctuation from the outside environment. Even small changes in temperature can unsettle the equilibrium and lead to significant converse changes in the in-situ Relative Humidity values. It will then take several hours of stable temperature for conditions to return to equilibrium, before reliable and meaningful relative humidity readings can be achieved from the tests.

As an example, the graph in Figure 1 shows reading taken every 30 minutes when the ambient temperature dropped from 26.5deg C to 20deg C. Before the temperature changed, the slab was at a stable ERH of 72% and after the temperature changed the RH went upwards to 78%. It then took 6 hours to come back to a 72% and stability.

This was a single change in environmental conditions generated in a humidity chamber. In real world conditions it’s possible to have several changes over the same period owing to varying weather conditions.

A solution to this problem may be found by using a datalogging probe which allows for the readings to be monitored over time, and a stable period can then be identified. Testing using the Tramex FeedBack DataLoggers can be done for the Relative Humidity Hood or In-situ probe test.

British Standard Relative Humidity In Situ Test
Another humidity test method often used in the UK and specified by British Standards is the ‘in situ sleeve method’ which involves drilling into the concrete and inserting sleeves into the holes which are sealed for 72 hours. Readings are taken by inserting a relative humidity probe into the sleeve.

The probe is left to acclimatise until it reaches equilibrium, usually when there is no change in the readings over a 5-minute period. The moisture content of the concrete is indirectly measured, by measuring the relative humidity of the air space in the probe, which itself is in equilibrium with RH in the concrete. The results are therefore considered independent of any variables that may be present in the concrete.

Monitoring the RH Hood or In Situ test using the FeedBack DataLogger
The FeedBack DataLogger from Tramex is designed to make the job of moisture testing concrete much easier by allowing for storage of measurements of hood and in-situ ERH as well as ambient conditions simultaneously. This can be done in a couple of useful ways.

Having performed a non-destructive test with a concrete moisture meter (see below) and built up a picture of the overall condition of the slab in the fastest way possible, the user can locate the highest reading (wettest) spots in each area. These are the most suitable locations for performing the hood or in-situ tests.

Using the FeedBack DataLogger App, reporting tests couldn’t be simpler. The FeedBack DataLogger automatically records the in-situ RH & temperature as well as the ambient RH & temperature. The information can be readily viewed on the app so a stable period can be identified, and final test results acquired.

In addition, the location, time and date of the test will be stamped by the App in use with a cellular device. Reports can be given the title of the job site or room under test and readings recorded and added to the report page by in the app.

Using the camera function a photographic image of the device with all readings as well as location, time and date stamp in the photo can be recorded. This can be stored on the device or uploaded to a report. Watch this space for more articles and videos on how to make the most out of your FeedBack DataLoggers.

Non-destructive moisture meters
Instant and non-destructive. To any floor fitter or inspector with an eye on their watch and wallet, these are words that matter. The Tramex instant test for concrete is completely non-destructive and specified by many flooring manufacturers around the world because of its simplicity, precision and repeatability of results.

The meter is pressed onto the surface and sends a signal into the concrete screed or sub floor and the moisture content is given as a percentage of the dry weight. Non-destructive meters may also have Carbide Method (CM) equivalent scales for concrete and anhydrite.

Internationally recognized standard ASTM F2659 calls for three-to-five readings in the same location. If there are any variations in those readings, record the highest result. This is repeated in at least eight locations for the first 1,000sq ft and five for every 1,000sq ft thereafter. The ease of use and speed of the instrument allows for many tests to be carried out over a large area in a short amount of time. The non-destructive testing method is also extremely useful when the screed contains underfloor heating pipes.

The user instantly identifies the locations of high moisture content, building up a moisture map of the entire slab and identifying when and where to test further using the Carbide Method or the Relative Humidity in situ and hood tests, as outlined in BS 8201, 8203 & 5325. Non-destructive meters are ideal for reducing the amount of RH tests carried out, thus reducing the time taken for testing.

A commonly asked question is how the RH% Hood or In-situ measurements and the Non-destructive MC% relate.

In a laboratory situation where temperature and humidity are constant at say 80%RH and 20deg C, a sample of average quality concrete will eventually equalise at about 4% MC. (see figure 2.) In the field, however, conditions are usually anything, but stable and so temperature changes can cause large swings in RH test results and a high ambient RH of over 65% can result in condensation on the surface of the slab, causing higher MC% readings. However, other factors also affect the correlation between RH% and MC%, especially the water-cement (w/c) ratio.

A sorption Isotherm chart such as the one in figure 2 provides a helpful indication of the measurements that should be expected so when readings are far apart from each other and do not correlate as expected, it can be a good indication that one reading could be very wrong and that further investigation is needed. As such, we can see that performing two different tests is worth much more than the sum of their parts.

How do we know when it’s the right time to do the 72-hour RH Hood or in-situ Test?
By testing non-destructively and based on the approximations of the Sorption chart above, the inspector can get an instant and precise evaluation of the moisture conditions within 20mm below the surface of the slab using the non-destructive moisture meter. A moisture map of the entire floor can be built up, showing how ‘wet’ the slab is and where the wettest locations are.

This moisture map can be used as a guideline to help determine when and where it’s best to locate the installation of the hood or in situ relative humidity tests. If the non-destructive test reveals high moisture content, it may be preferable to allow more drying time before carrying out the 72-hour test.

The instant non-destructive test will allow for monitoring and mapping moisture conditions until it is dry enough to ‘sign off’ with the British Standard Hood or in-situ Rh test, or indeed a Calcium Carbide test if specified.

Calcium Carbide Test
The Calcium Carbide, or Carbide Method (CM), test is carried out using a ‘carbide bomb’ meter, which is a flask-like device with a pressure gauge. Widely used in Europe, it is common to find specifications related to this test. In most cases the readings from this meter should be less than 2.5 (according to DIN standards) for the concrete to be considered dry for flooring purposes.

To perform the test a small sample of concrete is chiseled and crushed into a powder, weighed and placed into the flask. Calcium carbide is added, and the vessel is sealed and shaken. The moisture within the sample reacts with the calcium carbide, producing a gas and the pressure from this gas is measured on the gauge. The calibration of the carbide method meter is based on free water in concrete as opposed to total water content. Therefore 2.5% on this CM test is equal to about 4% moisture using the oven method, which explains why we are sometimes directed to look for 2.5% and sometimes 4%. The Tramex non-destructive meters have scales for MC% moisture content as well as scales for the equivalent CM tests for concrete and anhydrite. These scales can be used for monitoring and mapping moisture conditions until it is dry enough to ‘sign off’ with the British Standard Hood or in-situ Rh test, or Calcium Carbide test if specified.

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