Green Building Forum - RAAC / thermolite

RAAC / thermolite

Rex — Sat, 02 Sep 2023 15:15:16 +0100

What ho one and all,

Been following the RAAC / schools closure topic with interest.

While not unduly concerned for myself as my house is timber framed but the external rendered walls are thermolite block.

What is the difference between this RAAC stuff and thermolite blocks? Obviously my walls are not load bearing but I would hate to think that they will crumble in time, albeit, probably long after I have gone.

Your thought are most welcome

Thanks and toodle pip

RAAC / thermolite

owlman — Sat, 02 Sep 2023 16:42:51 +0100

I was curious too, but from what I've discovered they are similar inasmuch as both are aerated. Apparently it's the addition of rebar upsets the product integrity. I tried to find out if the basic mixes were different but as yet nothing.
Apparently Aircrete blocks are safe in domestic properties so we shouldn't worry. The inner leaf of my property is Thermalite (1984 ), and when I've seen exposed bits when doing roof work there were no signs of any deterioration. I seem to remember mortar adhesion and strength ( needing a weaker mix, I think ?) could be a potential problem at the construction phase.

RAAC / thermolite

Peter_in_Hungary — Sat, 02 Sep 2023 19:15:06 +0100

My understanding at the moment is that the RAAC will suffer if it gets damp and it will as a consequence loose its compressive strength, it is also possible for it to loose its compressive strength for other reasons about which I have no knowledge.

Reinforced concrete relies on the rebar to give strength in tension and the concrete gives strength in compression - a happy relationship when in a suspended slab the rebar is at the bottom and the concrete is above, they work in conjunction with each other. If however the concrete looses its compressive strength then there is nothing to stop the slab folding up - and the ceiling falls down.

Thermloite blocks will suffer from wet but the stresses on them are nothing like the compressive forces on a reinforced suspended slab so I wouldn't expect the same type of catastrophic failures.

Health warning - I am not a SE and this is just my understanding at the moment

RAAC / thermolite

WillInAberdeen — Sat, 02 Sep 2023 23:05:23 +0100

Good explanation https://www.bbc.co.uk/news/education-66669239

The main problem is water getting through to the steel reinforcement, which corrodes unseen and can fail unexpectedly.

Aerated blocks in house walls don't have or need steel reinforcement, apart from wall ties.

RAAC / thermolite

Peter_in_Hungary — Sun, 03 Sep 2023 10:52:38 +0100

Apart from water getting to the rebar I also heard that any freeze thaw cycles in the RAAC due to water/ damp can cause the collapse of the concrete structure and thus loosing its compression strength.

Whilst still in the UK I knew a chap whose business used RAAC concrete as a bulk filling material. The process was to fill most of the volume with RAAC then pour a reinforced slab of conventual concrete on top. I don't know it such structures also are affected, I guess it depends upon how much the slab relies on support from the substrate RAAC.

RAAC / thermolite

Artiglio — Sun, 03 Sep 2023 16:33:29 +0100

Rather expect there are lots of councils up and down the country looking into the construction of the flat roofed blocks of flats that were popular for social housing in the 50â€™s -80â€™s. Though youâ€™d also hope that given that the issue is hardly a new thing , councils have already checked. Itâ€™ll utter chaos if there are anymore than a tiny percentage of homes affected.

RAAC / thermolite

owlman — Sun, 03 Sep 2023 16:57:57 +0100

Posted By: ArtiglioRather expect there are lots of councils up and down the country looking into the construction of the flat roofed blocks of flats that were popular for social housing in the 50â€™s -80â€™s. Though youâ€™d also hope that given that the issue is hardly a new thing , councils have already checked. Itâ€™ll utter chaos if there are anymore than a tiny percentage of homes affected.

Especially as the design life is generally accepted as 30years.

RAAC / thermolite

djh — Sun, 03 Sep 2023 17:17:45 +0100

Posted By: owlmanEspecially as the design life is generally accepted as 30years.

I don't understand this bit. When was that decided? If they knew it at the time, why would anybody build with something with that short a lifetime? And of course, why wouldn't any body (i.e. the government) plan to replace them after that time.

RAAC / thermolite

owlman — Sun, 03 Sep 2023 17:48:43 +0100

In fairness, I don't think it was known at the build stage. Plus, I understand, there was a rush to get stuff up quickly. Full filling political promises I'm guessing. The 30 year bit I also understand was discovered later, and with no phased programme of demolition and re-build. Possibly a case of; " act in haste, repent at leisure. "
I'm only paraphrasing what I've read and I'm certainly no expert.

RAAC / thermolite

WillInAberdeen — Sun, 03 Sep 2023 17:51:52 +0100

Decided sometime around 1996 apparently

https://www.cross-safety.org/sites/default/files/1999-01/scoss-review-1997-99.pdf

Section 3.5 describes all the problems which are in the news today and refers to a 1996 BRE report "Reinforced autoclaved aerated concrete planks designed before 1980. BRE Information Paper IP 10/96." which is paywalled but apparently sets out the 30 years figure. At that time the advice was "inspect them carefully" rather than "replace them".

History does tend to repeat itself, I wonder what modern materials we are embracing now which will turn out to be a liability after 30 years?

RAAC / thermolite

owlman — Sun, 03 Sep 2023 19:32:04 +0100

Sticky tapes used for airtightness

RAAC / thermolite

djh — Sun, 03 Sep 2023 20:28:49 +0100

Posted By: owlmanSticky tapes used for airtightness

I hope not

Apparently they were first used sometime in the eighties in Germany and I haven't heard anything about any problems yet. So fingers crossed. Pro Clima say they had their Tescon Vana tested at Kassel Uni using 'accelerated aging' techniques to prove a 100 year lifetime. They've given me a 25 year warranty on my roof, which includes some as well as some of their membranes. Hopefully I don't need to call on it

RAAC / thermolite

djh — Sun, 03 Sep 2023 20:49:29 +0100

Posted By: WillInAberdeenDecided sometime around 1996 apparently

https://www.cross-safety.org/sites/default/files/1999-01/scoss-review-1997-99.pdf

Section 3.5 describes all the problems which are in the news today

Thanks for that. The photo on p20 is interesting. It reminds me of a railway bridge (rail above road) where we used to live. After one-collision-too-many they got fed up of having to close the railway every time until it was surveyed, so they installed extra dirty great big* beams next to the bridge but separated from it by a foot or so. The idea, which did seem to work in practice, was that any vehicle striking the beams would do itself more damage than it would cause to the beams, and not affect the railway bridge at all. They also put up a warning system to try to prevent the strike before it happened.

I see the section on cladding is all about stone and glass cladding, although it does mention innovation.

* technical engineering term

RAAC / thermolite

owlman — Mon, 04 Sep 2023 11:45:36 +0100

Re:- Sicky tapes
They are just one item, but over reliance on modern building adhesives and foams in general. Many, ( unlike your
Pro Clima ) product I doubt have had such rigorous testing.
I also wonder if PassivhÃ¤user and other "eco" homes in general will survive the first change of ownership. That's nothing to do with excellent initial build quality but more with Human nature. Once the words re-furb. or update are mentioned then there's a strong possibility bang will go the Passiv bit.
I could be wrong and an enlightened future generation may gladly adhere to a lifestyle boldly envisioned 30 years previous. All still good and somewhat efficient, and maybe better than most; I guess.

I occasionally manage to watch lunchtime TV, and the missus likes, find me a home in the country, or something like that. I'm quite curious at the want lists and the house viewing comments. They are a good insight into human nature. Often it's knock a wall down here and add an extension or conservatory there. Rarely is energy efficiency mentioned, it could be a result of editing, but looks and "feel" are important, with the "quarter acre" kitchen/living/dining/sun room being the most important. Oh and umpteen spacious bedrooms, all "en suite".

Unscientific, I know, but an interesting observation.

RAAC / thermolite

djh — Mon, 04 Sep 2023 12:28:13 +0100

Posted By: owlmanThey are just one item, but over reliance on modern building adhesives and foams in general. Many, ( unlike your Pro Clima ) product I doubt have had such rigorous testing.

Yes, I would hesitate to trust anything except Pro Clima and Siga tapes myself. There may be others that are good; I simply don't know.

I also wonder if PassivhÃ¤user and other "eco" homes in general will survive the first change of ownership. That's nothing to do with excellent initial build quality but more with Human nature. Once the words re-furb. or update are mentioned then there's a strong possibility bang will go the Passiv bit.

Yes, I agree and wonder if that might be another advantage of certification. At least it's clear what the standard is. Maybe our house will be lucky, in that it's such an eccentric build that a new owner is likely to be reasonably well-informed. And we built it so you can rip out all the internal walls if you fancy a redesign.

I'm quite curious at the want lists and the house viewing comments. They are a good insight into human nature. Often it's knock a wall down here and add an extension or conservatory there. Rarely is energy efficiency mentioned, it could be a result of editing, but looks and "feel" are important, with the "quarter acre" kitchen/living/dining/sun room being the most important. Oh and umpteen spacious bedrooms, all "en suite".

I don't think most people yet appreciate that "climate crisis" is somehow different to "business as usual"

RAAC / thermolite

owlman — Mon, 04 Sep 2023 12:51:32 +0100

Interesting fact, you may know already:
I occasionally catch a BBC 4 programme I think its called "more or less", all about fact checking the oft publicised numbers. On last night's edition they confirmed that China has emitted more CO2 since 2014 than the UK has since the start of the industrial Revolution. It kind of puts things in perspective, and with India on the rise and flexing industrial muscle the future doesn't look too bright.

RAAC / thermolite

djh — Mon, 04 Sep 2023 17:05:00 +0100

Could well be true, but OTOH China has a much larger population, as does India, so the per capita figures might be interesting. But the good ole' USA knocks them all into a cocked hat

https://www.carbonbrief.org/analysis-which-countries-are-historically-responsible-for-climate-change/

"In total, humans have pumped around 2,500bn tonnes of CO2 (GtCO2) into the atmosphere since 1850, leaving less than 500GtCO2 of remaining carbon budget to stay below 1.5C of warming.

"This means that, by the end of 2021, the world will collectively have burned through 86% of the carbon budget for a 50-50 probability of staying below 1.5C, or 89% of the budget for a two-thirds likelihood."

As you say, the future doesn't look too bright.

https://ourworldindata.org/grapher/cumulative-co-emissions
https://ourworldindata.org/contributed-most-global-co2

I see that the last link counts the Middle East, Africa and South America all as part of Asia! And note that all the figures are slightly out of date. Lots more links on the last page.

RAAC / thermolite

WillInAberdeen — Mon, 04 Sep 2023 17:36:45 +0100

The numbers are not that clearly presented, but here's a flavour

UK cumulative emissions since industrial revolution 75bn t Ã· 67million population now = 1100t per person now

US 1547t per person

China 190t per person

India 60t per person

Owlman this may be encouraging that Chinese and Indian emissions are still nowhere near as much as UK or US ! So we do have agency ourselves, it's not all in the hands of others.

Tim Harford's motto on R4 is brilliant: "is that a large number, and does it mean anything?"

RAAC / thermolite

djh — Mon, 04 Sep 2023 20:18:13 +0100

I wonder how the emissions per capita graphs year by year, especially for places with a long history of emissions.

Does it mean anything is a good question, since a lot of our emissions are greenwashed away as being somebody else's fault, for example.

RAAC / thermolite

Jeff B — Mon, 04 Sep 2023 20:49:58 +0100

Posted By: owlmanRe:- Sicky tapes

I occasionally manage to watch lunchtime TV, and the missus likes, find me a home in the country, or something like that. I'm quite curious at the want lists and the house viewing comments. They are a good insight into human nature. Often it's knock a wall down here and add an extension or conservatory there. Rarely is energy efficiency mentioned, it could be a result of editing, but looks and "feel" are important, with the "quarter acre" kitchen/living/dining/sun room being the most important. Oh and umpteen spacious bedrooms, all "en suite".

Unscientific, I know, but an interesting observation.

Not only is energy efficiency not mentioned but how the place is heated is often not mentioned at all except when there is a wood burning stove to be had. Oil fired C/H is never mentioned - at least not in any of the episodes I have watched.

RAAC / thermolite

chrisinbrighton — Mon, 04 Sep 2023 21:06:39 +0100

As owlman mentions above,alot of hitec materials that are helping with energy efficiancy have only been tested for a short period.
Ive been told that foam insulation seen in 1980s buildings has turned to dust and ive personally seen polythene thats become brittle like crisps,even though its not in sunlight.

RAAC / thermolite

owlman — Tue, 05 Sep 2023 15:53:53 +0100

Posted By: chrisinbrightonAs owlman mentions above,alot of hitec materials that are helping with energy efficiancy have only been tested for a short period.
Ive been told that foam insulation seen in 1980s buildings has turned to dust and ive personally seen polythene thats become brittle like crisps,even though its not in sunlight.

+1 in both cases, likewise mesh reinforced roof felt, gun applied tube adhesive. I also have doubts that breathable and vapour membranes will last the course, at least in a good enough form to 100% fulfil their original purpose.

RAAC / thermolite

jon — Sat, 09 Sep 2023 18:42:47 +0100

Raac contains reinforcement: the main big difference (other than what it was typically used for). Because it's aerated, it also carbonates quickly. Thermalite is similar in some ways. Carbonation isn't usually a strength problem but it does change the composition from alkaline to acidic. This provides ideal conditions, with moisture, for corrosion. The corrosion might not be obvious because it might expand into the aerated concrete. On top of that, the way the material works leads to deformation if it gets waterlogged. This leads to more deformation if ponding occurs.

After that it becomes more complex because of the failure mechanism (especially at bearings).

Hope this helps a bit

RAAC / thermolite

Cerisy — Sun, 10 Sep 2023 10:06:22 +0100

What jumps out from the technical comments is that the material would be lasting longer if properly inspected and maintained. Weâ€™re back to the nonsense called austerity that cut back on school maintenance budgets and let the schools handle it. Some used properly qualified staff, others used a mate of one of the governors! One school in Derbyshire stripped out old windows without checking that they had asbestos linings. It cost 750K to clean the whole school out before the kids could get back in. Thereâ€™s no way these guys will be inspecting roof coverings to check for leaks. Sad situation.

RAAC / thermolite

Rex — Mon, 11 Sep 2023 16:15:45 +0100

Ive been told that foam insulation seen in 1980s buildings has turned to dust and I've personally seen polythene that's become brittle like crisps,even though its not in sunlight.

So the insulation below the UFH will break down and the screed will become unstable?

RAAC / thermolite

Peter_in_Hungary — Mon, 11 Sep 2023 17:55:15 +0100

Posted By: Rex

Ive been told that foam insulation seen in 1980s buildings has turned to dust and I've personally seen polythene that's become brittle like crisps,even though its not in sunlight.

So the insulation below the UFH will break down and the screed will become unstable?

Are we talking about the CWI from the 80s which was Urea formaldehyde type foam insulation whereas today it is usually EPS or XPS used with UFH. So different materials. The Urea formaldehyde insulation from the 80 has known problems but I've not yet seen reports of issues with EPS (except shrinkage when subjected to heat under roof tiles - reference recent thread here). This is not to say in another 30 - 40 years problems won't arise but it has been used since the 60s (?) without falling apart yet.

RAAC / thermolite

Rex — Tue, 12 Sep 2023 10:09:26 +0100

I do realise that by the time there is a likely problem, I will probably not be too concerned.