Magnesite Floor Failure

Property In London W7, with Magnesite Floor Failure
Property In London W7, with Magnesite Floor Failure

I’ve recently just completed a long Roadshow for the RICS, ironically, I was giving a talk on solid floor defects, and included a section on Magnesite floor failure, as a refresher. In the last talk I gave, I asked an audience of about 70 Chartered surveyors, how many had seen a magnesite floor recently; the answer was none. I myself have not seen for for quite a few years but only a week later, I attended a property in London to investigate an alleged problem with sulphate attack, and guess what I found?…

What is Magnesite Flooring?

Magnesite, or Magnesium Oxychloride, was a product widely used by the flooring industry between circa 1920 and 1960. It was especially used in industrial premises, as it was resistant to oil spillages; however, it was frequently used in residential premises.

It is a water based product, commonly reddish pink in colour, though can be pale yellow, or any other colour specified.

Magnesite Flooring
Magnesite Flooring

Magnesite flooring was made from a mixture of calcined magnesite and magnesium chloride solution with various fillers (e.g. wood flour, sawdust, asbestos).• It was typically laid between 10 and 25mm thick, but two coat applications could be up to 50mm thick. 

What’s wrong with Magnesite?

Firstly, Magnesite contains chlorides, so if there is any embedded steel reinforcement within the floor slab, then the concrete can be affected by Chloride attack, which will corrode the embedded steel. Corrosion is an expansive reaction, and cracking of the concrete is likely to occur, as the steel corrodes. You should also consider, that steel water pipes may be buried in the concrete, and these are equally at risk.

Many old concrete floors, do not have a damp proof membrane installed, DPM’s came into common use in the mid 60’s, but prior to this, many concrete floors had a waterproof oversite, a layer of bitumen was commonly used. It would be wrong to assume that Magnesite provides that same protection against damp, and in fact they are very vulnerable to dampness. Magnesite is water soluble, and will return to its previous state if exposed to enough water.

The Asbestos Risk

As discussed earlier, Magnesite can contain asbestos fibres, as a filler. Commonly, the way to deal with asbestos, once identified, is to remove it, using a licensed contractor, or to encapsulate it. However, you can’t encapsulate a Magnesite floor, as they are so vulnerable to deterioration when exposed to water. Obviously, if you tried to encapsulate by pouring a screed over the top, then you’d be introducing large amounts of construction moisture into the Magnesite. The underlying Magnesite, would then most likely turn to a Weetabix type consistency, and start to break up, leaving you with no suitable substrate support below the screed.

Magnesite Floor Case Study

In this particular property, a Chartered surveyor had recently attended, for a pre-purchase survey and noted heave, or an uneven concrete floor below the carpets; he then of course raised the alarm for a potential risk of sulphate attack.

I attended to sample the floor, but on pulling up the carpet, the cause of this uneven floor, was clearly Magnesite floor failure; the Magnesite having got saturated, subsequently expanding and causing large blisters in the floor.

It was still important to investigate the situation with the underlying concrete and I excavated a hole through the slab to sample both the concrete, and the underlying hardcore. However, on breaking through the 8″ thick concrete slab, we found that there was no hardcore, and the slab sat directly on wet clay, with no DPM installed. This of course means that the concrete is in direct contact with ground sulphates.

Concrete slab sitting directly on clay
Concrete slab sitting directly on clay

The concrete was also notably wet, and this moisture had transferred to the Magnesite, causing it to heave up, blister, and crumble. From the image below, you can see how the magnesite had delaminated from the concrete substrate, forming large blisters, which crumbled when you stepped on them.

Failed Magnesite Flooring
Failed Magnesite Flooring

We did take samples of the concrete for sulphate tests, but with a saturated slab and widespread failure of the Magnesite, my advice was to renew all the solid floors, with the only test required being for asbestos. Testing the magnesite for asbestos, was critical before any works to remove the flooring could proceed.

One final note worth mentioning for any surveyors looking to check for dampness in Magnesite. Magnesite is electrically conductive, so if using a hand held electronic moisture meter, it will always give a high reading for damp.

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Structurally Unsound Composite Floor Joists

Floor Fails to Take the Load in Bovis New Build


Typical composite I Beam

Typical composite I Beam

We carried out an inspection of a Bovis new build last week that was less than 2 years old and in buildings of this age you don’t expect to find structurally unsound composite floor joists in the floor but that is precisely what we did find.

Visual Defects

We noted a number of defects to the first floor all centred on the landing area between three bedroom access doors and the airing cupboard.

We noted a large run out to the base of a bedroom door but checking the door showed it to be square, whereas the floor to the landing area was sloped significantly.

Significant slope to floor

Significant slope to floor

Moreover, we noted large gaps to the base of the bedroom door frames, which were so large that we could see where the nails in the sole plate had pulled from the floor as the floor had sank.

We then found that the dividing wall between the bedroom and airing cupboard could actually be rocked from side to side, and of course this movement explained why a number of cracks had opened up to skirting boards and door architraves.


Floor defects point to simple conclusion

We’ve inspected properties on this site before so we are intimately familiar with the construction and poor build quality so we know that the floors are constructed using composite I beams, which are then overlaid with tongue and grooved chipboard.

Floor separating from stud wall

Floor separating from stud wall













Insecure stud walling

Insecure stud walling

Failure of Composite Floor Joists

In this particular case  the range of defects led to one very obvious and significant conclusion, the floor was failing under the weight of the Megaflo hot water cylinder in the airing cupboard. The cylinder was not sat on a plinth and we are of the opinion that the floor was not strengthened in any way to take the significant  weight of this live load, which could be in the region of 350Kg’s.


As expected we also noted that the kitchen ceiling below was showing evidence of bowing under the live load and we have to wonder how many more properties on this new development will suffer from the same problem?

Floor sinking under the load of the Megaflo cylinder

Floor sinking under the load of the Megaflo cylinder


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Dealing with sulphate attack in floor slabs

Remedial Works for Sulphate Attack in Solid Concrete Floor Slabs

New Kitchen Strip out Required to Facilitate Floor Slab Replacement

New Kitchen Strip out Required to Facilitate Floor Slab Replacement

Following on from our recent blog on investigating sulphate attack in solid concrete floor slabs; what we didn’t mention in that blog was that we got the lab results back, which proved what we suspected, that there were incredibly high levels of sulphates in the hardcore material and the failed concrete floor slab. What subsequently transpired was that the client asked us to tender and project manage the remedial works. Works were specified and put out for quotes to local building contractors; interestingly the spread in pricing was quite staggering, ranging from £15k to £36k. We got four quotes back before the client made a decision on who to award the work to.

Removing floor slab and other consequential works

Works are still ongoing and due for completion on August 19th but things are progressing incredibly well and the most complicated aspect is possibly the careful removal and refitting of a very expensive kitchen with Granite worktops. There is also a Bathroom to strip out and refit, a shower room and built in wardrobes in the bedroom.

Digging out sulphate contaminated hardcore material.

Digging out sulphate contaminated hardcore material.

We mentioned in the previous blog on this subject that we believed that leaking subfloor heating pipes had contributed towards accelerating the sulphate attack and indeed as the floors were excavated we noted a number of heavily corroded and leaking copper central heating pipes. These will all be cut out with new pipework runs being installed above the finished floor level.

Since the hardcore material was heavily contaminated with sulphates then it was critical that this material was excavated back to ground level and removed from site. We’d envisaged a slightly easier excavation process for the concrete since the original sampling area showed the concrete to be incredibly thin, however, the concrete proved to be circa three times thicker in some areas and showed a massive variance in thickness throughout the property.

Jablite insulation and upstands

Jablite insulation and upstands

The property is having to be done on a room by room basis since we have no external storage space for the kitchen and bathroom fitments, so the lounge was completed  first and  this has then provided the storage area for stripping out the kitchen and bathrooms.

The local authority inspector had his first inspection last week before the first concrete pour and was very pleased with what he found. You can see the Jablite polystyrene insulation and Jablite perimeter upstands, which of course are only installed to the external perimeter walls. The builders are fairly old school and are mixing the concrete on site as they go. The final image shows works well under way to complete the large section of flooring to the lounge.  In fact the lounge is now complete and is currently being used as a storage area for the kitchen and bathroom fitments and furniture. Works to this property are being completed under a CIOB Mini form of contract (general Use), which to my mind is far more suited to a project of this size than a JCT minor works.

Concrete being poured over 1200 gauge DPM

Concrete being poured over 1200 gauge DPM

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Investigating Sulphate Attack in Floor Slabs

1960's bungalow. Sulphate attack in floorslab?

1960’s bungalow. Sulphate attack in floorslab?

We were asked if we could carry out another investigation into potential sulphate attack in a floor slab this week. The scenario was that the property, a 1960’s bungalow in a Nottinghamshire village, had recently been subject to a homebuyers survey report, which raised concerns with regard to potential sulphate attack in the solid concrete floors. The Chartered Surveyors report contained the usual caveats, “Inspection of the floor was limited due to existing floor coverings” etc. However, the report contained the following statement… “It is noted that some floors to the property are uneven and require further investigation. This type of unevenness felt within the floor is an indication there may be a problem with sulphites in the base to the floors.” Unusual I thought, since sulphites are more usually found in food and wine rather than floor slabs. However, we got the crux of the problem and explained the potential implications to our client when we received the call.

Straight edge highlights degree of heave in floorslab.

Straight edge highlights degree of heave in floorslab.

Sulphate attack is caused when sulphates contained within the concrete aggregate contain sulphates or the hardcore sub-base below the concrete contains sulphates. These sulphates react with the Tricalcium Aluminate in ordinary portland cement (OPC) to form Ettringite, a crystal that expands as it grows, in the process often causing  substantial damage to the concrete floor slab. Moreover there can be consequential structural damage as heave within the floor can displace internal walls built off the floor or indeed displace brickwork in the outer perimeter walls.

In terms of the investigation process we are looking to establish a number of things.

  1. How significant is the heave or cracking in the floor slab?
  2. Is there consequential structural damage?
  3. Is there a damp proof membrane installed between the floorslab and the hardcore?
  4. Sampling of both concrete and hardcore for laboratory analysis.
  5. Measurements to aid providing the client with a budget figure for floor replacement should sulphate attack be confirmed.

Verdigris to copper radiator pipes and localised cracking

Verdigris to copper radiator pipes and localised cracking

On internal visual inspection it was immediately obviously that there was substantial heave to the floorslab in a number of rooms and we pulled up two carpets for a closer inspection. It was obvious to us that there was high probability that the floorslab had failed due to sulphate attack. Sulphate attack is expedited by the present of moisture and we also noted that there were copper central heating pipes running through the concrete floorslab and also the the central heating boiler pressure was low. This raised alarms that a central heating system leak may well be contributing to floor moisture levels. Verdigris on the copper pipes indicated copper corrosion caused by being in direct contact with the highly alkaline concrete.

Bitumin oversite removed and coring through concrete slab for sampling.

Bitumin oversite removed and coring through concrete slab for sampling.

We set about sampling both the concrete and the hardcore because in our experience, the hardcore is often the prime culprit and the source of sulphates. Obviously, if there was a DPM installed then this barrier between the hardcore and the concrete slab significantly reduces the risk of sulphates coming into contact with the concrete. However, the concrete itself could contain aggregate with a high sulphate content, hence why both the concrete and hardcore are sampled and analysed for sulphates. Additionally we have a cement content analysis carried out on the concrete because this helps us determine the ratio of sulphate to cement, an important factor in determining the severity of failure.

Hardcore material on left and concrete on right.

Hardcore material on left and concrete on right.

We cored through the floor, initially cutting through an oversite of poured bitumen, which of course acts as a surface applied damp proofing barrier, this in itself told us that we would not find a polyethylene DPM installed. DPM’s started to be installed in floors around 1965 so this property would have been at the very front end of installations had one been installed.

The concrete itself was of very low quality and only circa two inches thick  so it didn’t take long to  cut through. We packed both a sample of concrete and retrieved a sample of hardcore material before making good the hole with a quick setting concrete mix; we did note that the hardcore material was extremely damp, adding to our concerns regarding a potential subfloor leak. Carpets were replaced and stretched with a knee kicker and we dropped the samples off at our laboratory. We’ve not actually obtained the results yet but we have warned of the very strong possibility that the heave found within these floor slabs is caused by sulphates. The reason for our pessimism, is the obvious visual damage seen to the floors and the poor quality of the hardcore material, which appeared to be crushed builders rubble rather than the usual fly ash or blast furnace waste that can cause this problem. Crushed builders rubble is a low quality hardcore fill material that often contains sulphates.

Cored hole made good prior to refitting carpets.

Cored hole made good prior to refitting carpets.

Our report will contain commentary on any structural implications found and of course will confirm or disprove whether the heave and cracking was caused by sulphates. We will of course also provide additional commentary relating to supplementary factors, such as the leaking central heating  system. A budget figure to replace internal floor slabs came in at around £15k, so this is a costly problem to remedy  when found.


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Dealing with Construction Moisture

Why Construction Moisture Can Severely Delay a Development Programme.

Multi-Million Pound RC Framed Residential Development

We dealt with two issues this week relating to problems caused by construction moisture. A plaster beetle infestation and an investigation into the reasons why anhydrite floor screeds were not drying out within a multi-million pound residential development block in London. More on the plaster beetle infestation in a separate blog to follow quite soon.

Our client, was understandably concerned that pumped anhydrite floor screeds, which on some floors had been pumped in 6 weeks earlier, had failed to dry. The installation process for this particular screed required that the laitance be sanded from the surface of the floor once dry, this usually occurs at 3-7 days after installation, however any attempt at sanding the floor immediately resulted in a clogged rotary sanding pad.

Laitance occurs on the surface during settlement/compaction. During this compaction process, bleed water migrates to the screed surface. This brings with it fine particulates within the screed. Laitance is subsequently formed as a result of the evaporation of the bleed water, and once hardened can impede the drying process .

The advantages of laying a pumped anhydrite screed, as opposed to laying concrete is that you can generate quite significant savings on labour and time but only if optimum environmental conditions for drying the screed are present.

Other benefits of using  pumped calcium sulfate screeds are that they are self compacting, have very little shrinkage and rarely require movement joints. They are particularly suited to underfloor heating systems because they dissipate heat far better than a concrete floor slab would.


Wall channel between anhydrite screeds to each room.

Pumped anhydrite screeds do not provide a wearing surface so are overlaid with a suitable wearing course; this may be a compatible smoothing compound, carpets, floor tiles or any other finished flooring. Many floor screed failures that we investigate concern buildings with underfloor heating systems and a latex wearing screed. There can be a number of complex issues involved relating to material compatibility and poor commissioning of the UFH system but this particular issue was far simpler to identify. The developer had long since overrun the usual 3-7 day drying and six weeks later some floors were still soft on the surface and therefore unsuitable for sanding to remove the surface laitance. In anhydrite screeds, the binder reacts with the water in order to produce gypsum crystals. Around 80% of the anhydrite is converted to gypsum and this reaction uses a large proportion of the mixing water but the remaining moisture is lost by evaporation.

The block, containing around 56 flats over 4 floors, had almost all windows and doors installed but no heating or MVHR system, which would be installed too late in the programme to be of any help. What we consistently found is incredibly high internal relative humidity of around 87% and therefore very little difference between ambient temperature (15 degrees centigrade)  and dew point temperature (13.6 degrees centigrade).

Floor temps below dew point temperature

Scanning for relative moisture content across slab.

Floor temperatures were consistently recorded as being below dew point temperature, therefore proving that condensation damp was constantly rewetting the anhydrite floor screeds. To rule out any other potential moisture sources we like to scan the whole floor for relative readings, which shows up any unusually high peaks in moisture that may suggest another source of moisture, but we found consistent levels of moisture across all floor slabs on each storey. We tested the floor slabs for total moisture content (TMC) over all levels using calcium carbide and found readings ranging from 0.2% TMC at first floor level to 2% TMC on other floors. You would ideally look for a TMC of 0.3%, but no more than 0.5%.

Interestingly, we were able to prove that the simple action of opening windows made very little difference to this problem by recording internal relative humidity two hours after opening windows in the block. This did nothing to reduce relative humidity and of course there was no difference between external and external temperatures so even ambient temperature remained the same.

There may sometimes be a case for installing dehumidifiers and maybe even supplementary heating but in this particular case we do not think that was the right strategy, since no internal doors or partitions had been installed and doors were often wedged open for trade activities. You need a sealed environment to stand any chance of success with a dehumidifier otherwise you are attempting to dry the world. In this particular case we recommended both positive pressure and negative pressure ventilation. We calculated the cubic volume of the block and recommended fan that would give us 12 air changes in the building over a 24 hour period. Fans would set up to force air in at one end of the building and extract air from the opposite end of the building. As a general principal, we do not believe that you should expect these high levels of construction moisture to take take care of themselves and a supplementary airflow is often needed if you are to keep your development programme on target. We make precise recommendations based on construction progress, material testing and environmental conditions found on site.

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Floored Again – Flooring Defects

Substantial Flooring Defects Encountered Wh
en Assessing Potential NHBC Claim

I recently carried out a substantial investigation into a relatively new build property in County Durham. The property was 6 years old and still covered by the NHBC Buildmark warranty. The property was suffering from major problems with damp and as always I warned the client of the need for invasive survey work.

I can’t cover all the defects found in a short blog but the flooring defects found are interesting because the construction methods used are rather odd.

Essentially the property was suffering quite seriously from wall base damp. The walls were dry lined on dot and dabs and in places the plasterboard was so damp it was crumbling. The client had an expensive hardwood floor down and this was ruined with damp staining and swelling of the timber.


Ruined Hardwood Flooring


The only way to fully understand what was going on was to take up flooring and expose the substrate. Skirting boards were removed and the hardwood flooring came up. We found the floor to be laid on a standard chipboard oversite and I’m sure you know what happens to chipboard when it gets wet, particularly when it is not bathroom grade. The chipboard was also taken up and we found that the solid floor was covered with Kingspan insulation slabs, these too were saturated and we removed a section of insulation to see what was under this. Incredibly, what we eventually found was that there appeared to no concrete floor screed installed. The SAM was supposed to be laid on a 75mm reinforced sand and cement screed but all we found was a compacted hardcore base covered with a thick layer of black Visqueen SAM (self adhesive membrane) acting as a DPM.

There were numerous serious faults with the installation of the Visqueen SAM even had it been laid correctly on a 75mm screed…

  1. The membrane had not been overlapped anywhere near the required 150mm and in fact had already failed at joints. One failure was apparent enough for us to be able to use deep wall probes through the gap in the membrane to check whether the underlying masonry was damp. See next image.


    Deep wall probes indicate that the underlying wall is saturated

    The overlap on the membrane was around 10mm rather than the required 150mm.

  2.  On removing a large section of chipboard and Kingspan insulation from the floor we immediately noted that the Visqueen SAM was severely punctured due to underlying sharp objects. See image.


    Badly punctured Visqueen SAM

    3. The Visqueen SAM was not installed to manufacturers requirements nor did it meet the installation standards required to maintain its BBA accreditation.

    4. The Visqueen was simply turned up the concrete block wall and was not connected to a physical damp proof course in the wall because there was no physical damp proof course installed in the wall. This resulted in severe saturation of the concrete wall base with 6.5% total moisture content recorded above the level of the SAM membrane. See image.

    Saturated wall base above level of SAM

    No DPC to party wall and saturated wall base

    The choice of Visqueen SAM as a physical DPM was utterly bizarre for what should have been a straight forward 1200 gauge DPM sheet installation but we can only assume that due to the thickness of the material the rogue builder thought he’d get away with not installing the floor screed.  He was of course wrong and the SAM was badly punctured by the underlying aggregate in numerous locations. Defects in this property ran to a 28 page report excluding any appendix material and whilst I wish I was surprised to find work of this quality, sadly it is all too frequent.

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The Problem with Calcium Sulphate Floor Screeds – Damp Floor



New development with floor screed failures

Malone Associates Ltd, were recently instructed by a large National developer to help resolve a dispute relating to one of their London developments. Essentially we were asked to investigate issues relating to ongoing dampness within a newly laid floor slab within a flat in a new low rise block. The history surrounding this matter was that a client had purchased the flat then noticed air blisters under the Amtico flooring in her lounge. The Amtico was taken up and a square section of the floor screed was excavated to see what was going on. My understanding was that moisture was found sat on top of the plastic resilience layer, the floor was patch repaired and further drying was being carried out and monitored until such time as readings were obtained that were low enough to justify laying the finished flooring.

We were unsure what methods of testing and monitoring was being carried out but had early concerns that the wrong type of equipment was being used.

On initial enquiries as to the type of floor screed used we were informed that Calcium Sulphate floor screed has been laid, specifically a Supaflo pumped screed by Cemex.
 It is worth noting at this point that the terms applied to Calcium Sulphate floor screeds can get confusing because they can also be commonly referred to as ‘Calcium Sulphate screed’, ‘Gypsum’ screed or ‘Anhydrite’ screed. All three terms refer to the same generic product.

Site Observations

We attended site in November and after initial visual inspection assessed the flat for condensation risk. To that end the following readings were recorded. Relative humidity = 47%, Ambient air temperature 19.2OC, Dew point temperature = 7.2OC. The floor temperature was recorded at 17OC or roughly 10OC above dew point temperature. These results proved conclusively that there was no active condensation damp problem in the flat on the day of inspection.

It could be seen on initial visual inspection of the floor screed was stripped of finished Amtico flooring and attempts had been made to strip the screed of flooring Adhesive and primer. This is a difficult process and it was clear that there were still patchy areas of adhesive residue across the lounge floor. We also noted a large patch repair area to the floor and noted the dissimilarity between the existing screed and the repair material. It was initially assumed by site staff that the repair section was calcium sulphate and therefore compatible with the rest of the floor. When I asked for this to be confirmed the staff member who repaired the floor confirmed that it was in fact concrete and as such this repair section is incompatible with the existing floor screed and would need to be excavated and replaced. This is because the Tricalcium Aluminate present in ordinary Portland cement (OPC), contained within the concrete repair section, will react with the sulphate in the floor screed and cause ettringite to form. I’m sure you will all recognise this problem as Sulphate attack, which is an expansive reaction and could well lead to problems with the finished floor in the near future.

Test Methods

We noted the presence of a Tramex CME (Concrete Moisture Encounter) within the flat and it became apparent that floor moisture readings were being obtained by this means. This is standard industry practice because these meters are specifically calibrated for concrete and concrete floor screeds. Indeed, we use a Tramex CME ourselves but we are aware that the tool has some limitations when it comes to a reliable diagnostic process. We took electronic moisture readings across the whole of the lounge floor to obtain comparative readings and our own readings tended to confirm and support the previous statement made by the developer, to the effect that moisture levels were highest nearer the patio doors leading to the balcony. We received our highest Tramex reading of 7.5% at this point, which would tend to indicate that the floor was significantly damp. We also noted a very high Tramex reading of 6% to the concrete repair patch. We were looking to obtain readings of 0.5% total moisture content (TMC) or <=75% equilibrium relative humidity readings (ERH). Some of the more attentive readers may have already spotted the potential problem with the use of the Tramex on a Calcium Sulphate floor screed… Did you note that I said they were calibrated specifically for concrete? Calcium sulphate is not concrete and therefore any readings obtained with the Tramex can only be referred to as a relative reading. Tramex’s own product information states that ‘The Concrete Encounter utilises “state of the art” electronic technology to provide the flooring industry with an accurate and simple to use non-invasive handheld instrument for nondestructive testing (NDT) of Moisture Content (MC) in concrete and comparative moisture readings in, gypsum and other floor screeds.’

Moreover, the Tramex is only reasonably reliable when used on virgin concrete that is clean and dust free. Once primers, adhesives and other coatings have been applied to the floor then the Tramex becomes even less reliable. The floor in question had too much surface contamination for the Tramex to ever be reliable even it was concrete as opposed to Calcium Sulphate flooring.


Because the floor repair section was to be excavated and repaired we decided to test the floor using the calcium carbide meter. For those of you not familiar with calcium carbide testing, a small amount of test material is weighed and placed into the pressure vessel. This is then mixed with calcium carbide powder and the chemical reaction within the vessel releases acetylene gas that pressurizes the vessel and causes a reading to register on the gauge. The amount of acetylene produced is directly proportional to the amount of moisture present in the sample and this is the most reliable site test in use for establishing the moisture content of concrete and masonry products. The concrete repair section was indeed still damp, though this was a null point due to its excavation. We simply tested this area to complete our understanding of what moisture contents were actually present right across the floor compared to readings obtained with the Tramex CME. In fact the calcium sulphate screed proved to be bone dry with readings no higher than 0% TMC obtained.

Floor screed testing

Floor screed testing: Relative readings indicate that floor is wet when in fact it is dry.

We believe that the Amtico flooring was initially laid too early but subsequently after removal of the Amtico the developer has dried the floor thoroughly but been chasing their own tail due to false positive readings obtained from the Tramex.

After the floor slab was repaired we provided the client with ongoing advice for monitoring the deep repair section that was replaced but there are only three reliable ways of obtaining reliable results for moisture content in floor slabs, these are:

1. Calcium carbide testing

2. Testing equilibrium relative humidity (ERH) using Hygro sleeves.

3. Testing equilibrium relative humidity (ERH) using a floor humidity box.

Since options 1 & 2 involve destructive testing, then the only option available for ongoing moisture tests in the flat is option number 3. A floor humidity box should be placed on the large repair section and sealed with double-sided Butyl tape. We recommend that the humidity level inside the box is then recorded after a 72 hour period. If the ERH reading is 75% or less then you are free to proceed with the installation of finished flooring. Manufacturers information on the calcium sulphate screed used for the repair section indicated that the screed could take up to 50 days to dry so clearly an initial period of drying would be required before monitoring was started.

Ongoing Tests

The developer chose to use a ‘Tramex Hygrohood’ floor humidity box sealed in place with double sided butyl tape for ongoing moisture measurement of the repaired floor section. However this proved what we already know, in that readings of <=75% ERH are extremely difficult to achieve in floor screeds. Four weeks later I was called back to reassess the flooring after the developer failed to get ERH readings below 77%. Also you must remember that that ERH readings are relative to temperature as well as moisture yet product information makes no reference to the temperature benchmark at which the ERH reading should be taken. We generally recommend that rooms are maintained at circa 20oC when screeds are being dried out and this is a general recommendation that is often found; it therefore makes sense to recommend that an ERH reading of <=75% should be obtained with a room temperature of 20oC. The Hygrohood shows a reading of 77% ERH at 21.5oC but if we drop the room temperature to 20oC then the ERH increased to circa 80%.

The client asked if we could test with calcium carbide again and it was simply a matter of understanding what depth the latex screed could be laid to. This allowed us to take a series of shallow drillings to obtain enough material for calcium carbide testing. The test again proved that the repaired floor section was now completely dry and the developer could move forward with installing the latex screed. The drilled areas will be made good by the latex screed.

All this proves that obtaining accurate moisture measurement in the field can be fraught with difficulty and pitfalls. Moreover, I think some of the guidance information is poor; particularly the guidance that an ERH of <=75% should be achieved, perhaps <=80% ERH would be more realistic because once that figure was achieved at a room temperature of 20oC the floor proved to be dry. You need to adopt a pragmatic approach in the field, understand the limitations of your test equipment and thoroughly review build specifications and manufacturers product information.

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