Moisture Meters

The vital role of moisture meters when assessing sub-floors for tiling.

By Ronan Carrigy Marketing and Sales Manager Tramex Meters.

Why do we still hear about so many moisture related floor failures within the ceramic tiling and floor coverings industries?

Surely, by now, everybody in the industry has heard of the dangers of moisture related problems, such as adhesive degradation, grout discoloration, condensation, movement, mould, and overall deterioration of the floor-covering? Not to mention the downtime required to fix the job and the damage to the installer’s reputation.

Surely, also, everybody in the industry has by now heard of the range of moisture meters and test methods on the market, including the British Standards Institution’s Codes of Practice for the installation of resilient flooring?

Indeed, it is fair to say that all the tools and knowledge required are now readily available from moisture meter manufacturers, and the many dedicated trade bodies and associations who work tirelessly for their industry to ensure best practices are followed and to raise the level of professionalism which separates the qualified professional fitters from the amateurs. Therefore, we surely should, at this stage, no longer be experiencing any moisture related floor failures?

The devil is in the detail
I am going to suggest, in the following paragraphs, that the simple truth is that some fitters are ignoring the small print.

The rule of thumb states that concrete slabs, and sand / cement screeds, are expected to dry at a rate of 1mm per day (or an inch per month) and Anhydrite screeds the same up to 40mm, or 2 days per mm when poured deeper (i.e. a 60mm screed will take: 40mm @ 1 day = 40 days + 20mm @ 2 days = 40 days which = 80 days in total).

Nonetheless, this ‘rule’ is always followed by three important words: ‘in ideal conditions’. This caveat seems to be most often ignored.

‘Ideal conditions’ are the optimum conditions that allow the moisture within the slab or screed to evaporate from the surface. The rate of evaporation will depend on ambient conditions. Almost all screed manufacturers state, in their guidance literature, that ideal conditions are in the region of 20ºC and 40% to 60% RH. Warm, dry, flowing air will allow for faster evaporation.

These conditions, while ideal, are obviously not the normal state of a building site in the UK for most of the year, with the exception of the three short months of summer, at which time doors and windows should be thrown wide open to create a good flow of dry air, lifting moisture from the material and carrying it away. However, for the rest of the year, when temperatures are closer to 5ºC and humidity upwards of 70% to 80% RH, leaving doors and windows open will have the opposite effect, introducing more humidity into the environment and slowing the drying further.

Then, with the addition of wet trades applying plaster to the walls, humidity in the air is raised even higher. At most times of the year, heaters and dehumidifiers are needed to artificially create those ‘ideal conditions’. Building sites which are not artificially conditioned will maintain a high humidity level and, when temperatures drop (overnight for example), can easily reach dew point, resulting in condensation settling on the surface of the floor, thus wetting and re-wetting the screed.

An obvious solution in this instance, and one which seems to be the go-to quick fix in the UK today, is the use of a DPM (Damp Proof Membrane). This will slow the rate of drying of the floor to a level which is not harmful to the floor covering.

DPMs can be ideal for this scenario (a sort of get out of jail card) and also in the situation where an older floor which was installed many years before, and should be dry, is showing high levels of moisture.

Moisture in a slab or screed should continue to dry slowly over many years even with floor coverings installed. So an older floor should not be expected to read as high as a brand new floor which is emitting its construction moisture. If a reading which would be regarded as normal for a new floor, is found in an older floor, it could be an indication that there is a breach in the DPC (Damp Proof Course) or even indicate that one was never installed in the first place. Again, this is an ideal situation for a DPM which will ensure that moisture, intruding from below the slab is not going to cause a failure. Make sure, in this case, to select a DPM that is suitable for both residual construction moisture as well as ground water vapour.

Once a DPM is installed, however, it becomes even more important to monitor the ambient conditions on site leading up to the floor cover installation. This is because when the condensation point is reached, in normal circumstances as described above, most of the condensation is absorbed into the surface of the screed, whereas with a DPM in place, this condensation will sit on the surface with nowhere to go, meaning even a small trace of moisture can cause problems for the adhesive . This consequence of the use of a DPM is often overlooked.

A Datalogger, such as The Tramex Feedback, for example, is a suitable tool for monitoring these ambient conditions over the course of the drying stage of the floor and right up to and during the installation. Readings of ambient temperature, humidity and dew-point are recorded by the device and read from a smart phone or tablet using the Tramex Feedback App.

Anhydrite screeds are particularly sensitive to high ambient humidity conditions and readily absorb moisture from the air, slowing or blocking the drying completely. Removal of the laitance from the surface of the screed after the initial curing will allow the surface to release its moisture, whereas not sanding/abrading the surface will result in the laitance hardening and making it significantly more difficult to remove at a later stage.

While some anhydrite screed installers will return to site after the initial curing period and remove the laitance as part of their service – and will even hand a copy of the instructions over to the contractor, ensuring that everyone is aware what type of screed it is and how to treat it – these highly professional screeders are the exception and unfortunately not the rule. The more common scenario sees the contractor arriving to site with no idea that this is an anhydrite screed and, therefore, how to treat it.

Knowing that the screed is anhydrite will have important ramifications on a number of aspects, including choosing which type of DPM to use. DPMs designed for concrete and sand/cement screeds are usually not suitable for use with anhydrite screeds. Manufacturers are now producing products for use specifically with anhydrite. However most cannot be used when UFH (underfloor heating) systems are installed. Moisture testing of anhydrite screeds is another issue that causes confusion for contractors as these screeds do behave differently to concrete and sand/cement screeds.

Having established the ideal conditions for drying, and set up the monitoring of these conditions using the relative humidity and temperature data loggers, one can then rely on moisture meters to measure the moisture content and/or relative humidity within the subfloor. It is very important to assess the ambient humidity conditions before further testing. This will help identify, or rule out, the possibility of condensation issues on site, which can otherwise be difficult to distinguish for moisture problems originating from the slab.

The three main tests in use in the UK are non-destructive Electronic Moisture Meters, the British Standard Humidity Box or hygrometer probe, and the German (DIN) Standard Carbide Method (Bomb test or Speedy test). However, performing humidity tests without first carrying out capacitance type testing is also risky. Humidity tests take a long time to perform and only measure a small sample of the concrete slab, hence it is quite possible to miss a problem entirely.

1. Electronic moisture meter testing is non-destructive and provides instant readings. A purpose-built concrete moisture meter, such as the Tramex CME4 & CMEX2, provides more helpful readings (of moisture by mass in concrete and cementitious screeds) than a general purpose, comparative moisture meter.

This method of testing allows the user to map a whole area, very quickly making an assessment of the moisture condition and locating the highest reading points for further testing with more elaborate, time consuming methods (such as the Humidity box test described below) when such methods are required. The standard method for doing this requires five areas be tested for the first 90-100sq. metres and three areas for every additional 90-100 sq. metres. Locations of high moisture, as well as average and low readings, should be selected for further testing with RH methods to British Standards.

When testing anhydrite screeds with an electronic moisture meter it is essential that the laitance has been removed from the surface of the screed to gain a meaningful reading. The laitance acts as a barrier or skin, trapping moisture at the surface of the screed, therefore producing a false positive reading on the instrument which is designed to take a correct measurement based on the drying curve of the slab/screed in normal drying conditions.

For the same reason it is important that ambient humidity conditions are within the normal range of 40% to 60% to avoid condensation which can also lead to false positive readings.

2. The British Standard Humidity Box measures the ERH (equilibrium relative humidity) of the screed. This is performed by affixing a sealed box to the surface of the screed with butyl tape (which, unlike silicone, does not affect the RH readings) in a location of the floor which reads highest with a preliminary electronic concrete moisture meter test. Ensure the box is out of the way of direct sunlight or drafts from open doorways. Equilibrium is achieved when the trapped air inside the box is no longer receiving humidity from or giving it to the screed. At this point a measurement should be taken. 75% RH or below is normally an acceptable result.

The length of time required for the airspace inside the box to achieve equilibrium with the slab or screed depends upon its thickness and whether a screed is bonded to the subfloor or not.

In the case of anhydrite screeds, which are usually poured to a depth of 40mm to 60mm and are normally placed over insulation, British Standards recommend that a 1st reading is taken after four hours, and equilibrium may be assumed when two consecutive readings taken at four hour intervals show no change.

In practice this test method is often reported to be performed unsatisfactorily. For example, many testers will leave the box in place for too short a time and it is rare to hear of anyone checking readings twice at four hour intervals. The reason for taking subsequent readings four hours apart is to ensure that the recorded reading is taken during a period of equilibrium.

A small change in ambient temperature will have a dramatic effect on the readings, destabilizing the fine balance of equilibrium inside the box and sending the RH reading up or down depending on the temperature change. This fluctuation in temperature can result in an unpredictable spike in RH (e.g. from 72% to 82% as in figure 1.) as the equilibrium is upset.

Stability will only resume approximately three to four hours later. For this reason, users of this test will often take a reading in the morning which reads high and possibly another in the afternoon which reads low and wonder which one is correct, causing further uncertainty.

But this uncertainty can be overcome. Verification of the Humidity Box test results is easier when used together with a Datalogger such as the Tramex Feedback. The external probe is placed into the box and monitors the temperature and humidity readings for the entire duration of the test, thus showing clearly when equilibrium was achieved and what the correct reading was at the appropriate time.

3. The CM test (known as the Bomb test or Speedy test in the UK) is the German national standard test and is required as a final certification of moisture conditions of slabs/screeds in many European states. The CM test involves removing a sample of the slab/screed with a hammer and chisel and crushing it using a mortar and pestle, then weighing the required amount and placing into an airtight chamber together with Calcium Carbide which, when in contact with moisture, produces acetylene gas. The higher the concentration of moisture the more gas is produced, which is read as pressure from the devices gauge.

This test is ideal for certain proprietary and fast-drying screeds, which act by chemically binding the majority of construction moisture and, therefore, cannot be tested with relative humidity or electrical impedance devices which will give high results.

In theory the CM test is the most suitable for Anhydrite screeds due to the chemical nature of the test, showing only ‘free’ moisture which can cause floor failure. In practice, however, the test is easy to get wrong and requires a good deal of knowledge and skill to get exactly right.

Common user errors include:
a) Taking a sample using a drill (which produces heat and, therefore, removes moisture from the sample). Instead, a hammer and chisel should be used to remove a portion of the slab/screed and breaking the core open to access a representative sample.

b) Not allowing enough time for the reaction between the moisture and the calcium carbide.

The process of performing the CM test involves shaking and resting the container intermittently for a period of approximately ten minutes, whereas often users will allow less time which can result in a lower reading.

This test is not a British Standard test. However many flooring manufacturers can specify the maximum moisture content thresholds using its values. A non-destructive electronic moisture meter with CM test equivalent values for concrete and anhydrite will allow the user to get a very useful and rapid assessment of the subfloor before performing the standard relative humidity tests.

Using a combination of moisture and relative humidity testing equipment allows for the monitoring of the ambient conditions throughout the drying process, instant moisture content testing and rapid mapping of the entire subfloor, and relative humidity testing of specific areas of the subfloor in accordance with the national standard methods for testing.

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