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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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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