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 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.
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.
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.
I visited Berlin over the Christmas period, a City I’m very familiar with having been on numerous occasions and took the time to visit the outer provinces, including Potsdam and Mexikoplatz, in the process making some interesting observations, especially regarding concrete carbonation. I guess it’s an occupational hazard that I’m interested in seeing or detailing any interesting approaches taken to dealing with building defects in other countries and of course taking any lessons that can be learned and applied to the UK.
High Level Access
Roof Chimney Access
It was extremely interesting to note that a lot of buildings in Mexikoplatz have high level access built in to the roof structure to enable chimneystack maintenance. This often entailed that steps or ladder structure being accessed from a roof light, whereby, so long as a harness was worn, then chimney inspection and maintenance should be a relatively simple and inexpensive task.
Roof Chimney Access
Roof Chimney Access
A little foresight can help future proof the building
I was also struck by the importance given to roof ventilation, wherever mansard roofing or rooms in roof space were found, using the roof space for accommodation seemed a fairly common approach and clearly the phenomena of interstitial condensation with the roof structure had been well recognised and addressed. Most of the roofing was seen to have large clay bonnet type air vents installed in abundance, particularly interesting to me as I commonly find that roofing is poorly ventilated in the UK.
Roof ventilation at eaves level
1960’s large panel concrete block
For those of you thinking about teutonic efficiency in construction it was interesting to note that many of the defects that plague 1960’s large panel concrete construction in the UK also affect similar construction in Germany, in particular I’m referring to the incidence of carbonation in concrete due to poor levels of concrete cover on the rebar. The building in question was a fairly typical low rise block of flats in Potsdam and given its close proximity to the beautiful old Potsdam cathedral it looked rather out of place in its surroundings.
Large panel construction
The construction of this block will be familiar to anyone in the UK familiar with this type of non-traditional construction,which was mainly erected by local authorities in the 1960’s, in a drive to meet the high demand for housing at that time. This particular block, though fully occupied was in a sorry state due to extensive spalling of the concrete due to carbonation and it was clear that concrete cover was very poor, perhaps being in the region of 10mm thick in some locations. Extensive repairs will be required to this block though it was interesting to note that a similar 1960’s block adjacent to this one was currently undergoing demolition. For the short term there has to be a concern for spalling concrete falling from the block, though given the lack of any notably loose concrete, it would appear that at least this issue is in hand; possibly with the building owner at least regularly inspecting the block and removing loose concrete before it falls from the block.
Rebar corrosion due to concrete carbonation
Possible as little as 10mm of concrete cover to the rebar
Cracking due to expansive corrosion of embedded rebar
Grenfell Tower Disaster Highlights the Need for Careful Selection of External Cladding Products
Grenfell Tower Fire
I thought carefully about reprinting the article I wrote for the Construction Research and Innovation Journal in 2013 and you’ll understand that as an ex-firefighter of ten years, I have a special interest in fire protection generally, but particularly as it relates to high rise blocks, so the issues with Grenfell Tower Cladding have once again brought my concerns into sharp focus. It is also no surprise that we’ve now started to see the enquiries come in from concerned clients wanting us to review the safety of cladding installations on their high rise blocks.
I’ve been responsible for two multi-million pound refurbishments of high rise blocks and I know the problems that are encountered, particularly for poured in-situ and precast concrete blocks; which ironically are naturally endowed with a high level of inherent fire protection. Unfortunately, in the case of Grenfell, it is clear to me that negligent actions were taken that reduced that inherent level of fire protection.
Unloved 1960’s Tower Block
Fire risks generally relate to internal issues of poor compartmentation, in communal areas or service ducts; old salt glazed bin chutes can be a particular problem because they get smashed from residents dropping overly large and heavy objects down them. For both high rise blocks I was responsible for we changed the chutes for modern stainless steel installations. Furthermore, we focussed heavily on fire stopping in service ducts, compartmentation and generally mitigating the potential for internal fire spread. I am comfortable that both blocks are as safe as they can be. Interestingly, for Thomas King House in Coventry, we chose not to clad the block for a number of reasons, I still believe that this high rise serves as a model for how 60’s concrete high rise blocks should be refurbished. It was a high quality refurbishment that did not compromise that inherent fire resistance of the concrete and further proved that 1960’s Brutalist structures can be restored sympathetically. Interestingly, this refurbishment cost £3.5m and if we’d clad externally that may have put another £1m on the price, so at £8.7m I do not believe that Grenfell Tower refurbishment could ever be considered cheap, even at London prices.
I have been banging the drum for a number of years with regard to the choices and careful consideration of external claddings for two key reasons:
Structural Fire protection
Concrete Grenfell Still Standing Despite Fire
I’ve seen poorly considered and poorly specified cladding falling from buildings or breaching inherent fire protection… Grenfell Tower didn’t need to happen and should never have happened and the next disaster may well be someone getting killed when poorly designed, poorly specified or poorly installed cladding falls from a building onto the general public below. It is with these thoughts in mind that I decided to reprint my 2013 research paper (First published Dec 2013:Construction Research & Innovation Journal).
The Risky Business of Covering Up
Thomas King House Transformed
My interest in external wall insulation (EWI) stems primarily from my expertise in damp investigation. I had seen EWI systems inappropriately applied to a number a buildings that have walls which are meant to breathe. It was inevitable that these installations would cause long term issues with damp but this was compounded by the fact that I was seeing a number of failed EWI systems with cracks or failed building joints that would allow water ingress. My specialism in damp investigation and the fact that I was a senior manager responsible for managing millions of pounds worth of EWI work led me to research EWI failures in detail, and I was genuinely alarmed by what I found. Non-traditional housing stock normally presents three major problems for housing providers:
1) they are thermally inefficient,
2) they may have structural defects and
3) they can be unattractive and blight housing estates.
Whilst each problem presents unique challenges, none are insurmountable and life cycles can be cost effectively extended for a further 30 to 40 years so long as investment decisions are evidence-based. EWI systems have emerged in recent years as a popular solution to the first of the above problems, but the risks and drawbacks need to be understood. Furthermore, designed and installed without proper care, EWI systems can actually exacerbate the second and third problems – structural integrity and aesthetics. The most urgent message here is that most EWI systems are non-structural and therefore need to be fixed to load bearing fabric. There are times when a non-structural system is by no means appropriate for use, such as in Crosswall construction. A number of legal cases are currently pending in the courts or have already been settled following serious structural failures. It’s essential to understand EWI systems, and to specify and design them appropriately, but even that is not enough. Adequate site management is needed to avoid shoddy or inappropriate workarounds by operatives. But first, let’s look more closely at EWI systems, their composition and uses.
Poor SAP Ratings
The Standard Assessment Procedure (SAP) is the methodology used by the Department of Energy & Climate Change (DECC) to assess and compare the energy and environmental performance of dwellings. Since all non-traditionally constructed properties require an improvement in their SAP rating it makes sense to discuss external wall insulation. There are a number of EWI systems in use but unless you are looking for a structural system then there is little variation apart from the choice of insulation material. Phenolic boards (PF), PUR (Rigid polyurethane foam), PIR (Polyisocyanurate), EPS (Enhanced polystyrene board), or mineral wool are all frequently used as part of these EWI systems. Rigid phenolic insulation products offer best thermal performance when compared with rigid polyurethane or extruded polystyrene. Its low thermal conductivity allows minimal thickness of insulation, which allows for easier finishing around frame reveals, roof verges and soffits. For this reason it is one of the most widely used products. That said, phenolic board comes with a few known – and a few less well known – issues:
1. Demand for phenolic board has exceeded supply which has caused manufacturers to cut the 12 week curing period to six weeks in an attempt to keep the market supplied. There are some concerns with regard to the effect this decision will have on the quality of the product and there is some early anecdotal evidence regarding increased board shrinkage after system application. Phenolic boards were known to shrink, which can occasionally cause gaps to open up in the building envelope. Will we now see an increase in the severity of this problem? I believe we will.
2. Phenolic board has known acidic properties and should not be placed in direct contact with metal roof decks, wall cladding or stanchions. There are cases pending against manufacturers where phenolic boards have caused corrosion of steel roof decks.
3. Phenolic foam insulation has a significant environmental impact, exceeding that of other insulation materials. Significant amounts of petroleum and natural gas must be burnt during the manufacturing and refining processes, though the insulation industry has ceased to use chlorofluorocarbons (CFCs) in the manufacture of foam insulation products. Still it’s nasty stuff and you should consider whether continued use of phenolic board is a responsible business decision for you or your client.
4. Phenolic foam insulation will deteriorate if it is exposed to moisture or sunlight. It is important to store phenolic boards correctly and apply render to walls within 48 hours of fixing external wall boards. I have managed millions of pounds worth of EWI work and site management of this issue has been a consistent and ongoing problem. PUR board comes with similar known environmental problems but is also known to suffer a loss of U-value (thermal performance) with time. This is due to a combination of air infiltration and fluorocarbon gases diffusing out of the product (outgassing) over time. This rate of outgassing varies from product to product but in all likelihood a property that has a PUR insulation system installed will have a significantly reduced U Value 10 years on from the date of installation. The most cost effective and pragmatic choice of material from those under discussion is EPS, in particular, graphite enhanced polystyrene (GEPS), which will give a significantly improved U-value over standard EPS. Its long-term performance gives significantly less concern than that of phenolic or PUR and whilst it could never be considered a ‘green’ product it is in my opinion the more environmentally friendly and acceptable choice from among the main types of rigid insulation board in main use. Of course there are pros and cons with every material and whilst I would recommend GEPS for low rise stock, fire performance needs consideration in high rise stock.
Fire Safety Performance
A chemical called HBCD is often added to EPS or GEPS to improve fire performance and whilst you may not yet have heard about health concerns relating to the use of HBCD, it is on the verge of being banned or having its use restricted in Europe. In general terms EPS or GEPS has poor fire performance but can achieve a class E rating under BS EN13501-1 (‘Fire classification of construction products and building elements’) when enclosed with a laminated facing layer of the type seen in EWI systems. That being said, class E isn’t really acceptable for high rise applications and you would need to build in external fire breaks. Stone or glass wool products offer the best performance being rated at A1 due to their non-combustible nature. Whilst being the least environmentally friendly Phenolic foam can achieve a Euroclass B rating due to having a zero flashover reaction to fire. EWI System Failures There have been a number of EWI high rise system failures in Scotland, Birmingham, Wigan and the North West. One spectacular failure at Stanley House in Bootle, Merseyside, was captured on mobile phone can be viewed on YouTube. Several court cases are pending but in Birmingham the issue has turned into a dispute over whether the products or the standard of installation were the cause of these failures. What is clear is that insulation boards have moved or become partially detached from the external building façade. I have some concerns that systems have not been adequately wind tested for installation at height. Another known problem is that mechanical fixings into no-fines concrete have been very poor or completely inappropriate for the circumstances. There have been a significant number of hammer fixing failures into concrete, particularly no-fines concrete. There are three relatively recent high profile cases of EWI system failures: Stanley House in Bootle, Merseyside, owned by One Vision Housing; Glasgow Housing Association’s ‘Mini-Multi’ Blocks, Glasgow; and Derby House, Scholes Village, owned by Wigan and Leigh Housing.
The Youtube video of Stanley House clearly shows that the render had detached from the underlying insulation board when it failed. A poor adhesive bond, failed building joints, loose insulation boards, differential expansion – or a combination of all these factors – will cause failures such as this.
However, a spokesperson representing One Vision told me that they could not comment on the failure as a condition of the court settlement with the contractor.Glasgow Housing Association also refused to comment on the failures, but there were early reports of extensive ‘blistering’ in a number of multi-storey blocks shortly after installation1. This would tend to indicate that moisture was present under the render coat, meaning that materials were installed wet or that moisture found its way into the system due to failed building joints. A more recent failure occurred at Derby House in Scholes Village. The render coat detached from the underlying insulation boards in April this year. “Tenants demand action after slab near miss”, said the headline in local media after a piece of cladding reportedly fell 60ft to the ground just feet from the building entrance2. The accompanying photograph clearly shows that the insulation boards are still in place. This case was recent so it is likely to be still under investigation. In researching these cases I have found one instance where the EWI system was not certificated by the British Board of Agrément (BBA) – though it may have been sold as such – because the supplier had substituted one tested and approved system component for another untested system component. I have seen a number of occasions where installers are making these changes to system specifications, unaware that they have breached the BBA system approval, and clients need to be aware of this. Again, settlements are cloaked in confidentiality. This is understandable, but worrying. Clients and contractors are very reluctant to share the details of these failures due to a perceived risk of reputation damage. One client was even bound by a gagging order linked to their court settlement. This veil of secrecy does nothing to promote good practice or continuous improvement. I do believe that render failure will, more often than, not, be a sign that the underlying substrate has failed, and you should insist on opening up works extensively to seek more definitive proof of failure before accepting an over simplified explanation that moisture has crept past failed building joints. In my experience contractors prefer simpler explanations because they are cheaper to remedy.
In general terms, the majority of EWI system installers are satisfied with mechanical hammer fixings alone whilst a minority of installers believes this is a problem and adopt a belt and braces approach to installation by both gluing and mechanically fixing boards to the external façade. I agree with this approach for low rise stock but would still issue a note of caution with regard to specifying a standard non-structural system for medium- to highrise stock. If the thermal solution is not given adequate consideration and fails then I have experienced first hand how extremely difficult it is to get contractors to resolve these failures, especially when high rise access alone (mast climbers or scaffolding) can cost them somewhere in the region of £100,000 to £250,000 per block.
EWI structural vs. non-structural systems
The majority of EWI systems are non-structural and therefore need to be fixed to load bearing fabric. There are times when a non-structural system is inappropriate for use such as in crosswall construction. Crosswall construction takes all the building loads from floors and roof on the gable walls. The front and rear facades of these properties are non-load bearing and therefore unsuitable for fixing a standard EWI system. In these cases you would choose a structural EWI system. Moreover, a structural EWI system has several other potential applications when considered for use on non-traditional stock.
• Full cladding of defective buildings (reduces need for difficult remedial work);
• Fully designed structural cladding for non-traditional medium- to high-rise structures. Designed to account for wind loadings etc.;
• Full over-cladding of defective or inefficient systembuilt structures (improving structural safety and thermal continuity);
• Enclosing balconies and walkways (converting external space into usable internal floor area);
• Forming new or extending existing parapets (improving safety at roof level).
You should also note the failures of mechanical fixings into concrete (particularly no-fines) and ensure a system can bypass this issue. This is a note of warning that should apply to all system design but you should note that bad site management during any installation process will negate any effective design process. I have visited sites on many occasions and seen the wrong size hammer fixings used or, more commonly, operatives not using depth stops attached to their drills and often even punching straight through walls when drilling for hammer fixings. The length of hammer fixing is critical to the design process yet I am convinced that they often don’t account for the depth of existing render applied to some non-traditional properties and therefore hammer fixings can be fixed to insufficient depth in the structural panel. It is all too easy to make assumptions about the depth of existing render and I often insist on having patches chiselled out to expose the substrate. This allows us to make a more informed decision about the required length for hammer fixings.
Pre-existing concrete defects
I previously discussed the defects relating to carbonation and chloride attack of concrete non-trads. The problem is of particular concern when dealing with high-rise stock because the repair of cracks and spalling can significantly add to your refurbishment cost. Each small localised repair can cost £30 to £40 to repair. There can be hundreds of such repairs on each high rise and this doesn’t include access costs or the cost of an anticarbonation coating. Rusting is of course an expansive reaction and treating rusted rebar is a key part of the concrete repair process to prevent future spalling. There is an argument that says overcladding with an EWI system cuts off oxygen required for the rusting process and therefore prevents any further deterioration of the rebar. This could to a degree mitigate the requirement for concrete repairs but where structural engineers are involved in the design process then they are less likely to accept this argument.
Is EWI appropriate for all properties?
In short, no. There are concerns about the potential for EWI systems to cause damp and this stems from two issues: Traditional properties built on the ‘overcoat’ principle and using traditional stone or lime mortars are meant to breathe. Adding EWI would affect the walls’ breathability and so is completely inappropriate for use on these properties. An ex-colleague of mine has been trying to find a solution for improving SAP on single skin stone properties in Cumbria, and chose an internal wall insulation system. Whilst this raises less concern it will still affect the walls’ breathability. I’ve seen a number of EWI systems badly installed and bridging the DPC. The installers may well claim that their materials will not wick moisture but you’d be wise to judge for yourself.
Well designed EWI systems have the ability to transform our estates, but if design is not given adequate consideration the estates will remain as bland as ever, or even be damaged in their aesthetics, particularly by the choice of gaudy colour schemes. Housing providers who give residents too much choice in the design of their estates often end up with a clashing patchwork of pink, green, violet, etc.. Colour choice of external render systems is one consideration, but all EWI systems offer a choice of architectural detailing as well. An effective one can be bricks slips. On the last scheme I was involved with we gave a great deal of consideration to all aspects of design, even to the extent of having illustrations done by artists so we could make better choices. To many organisations, EWI is nothing more than a technical solution to deal with thermal efficiency but they are failing to protect the future of their estates and communities by not giving adequate consideration to the design process.
Remedial Works for Sulphate Attack in Solid Concrete Floor Slabs
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.
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
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.