The crumbly concrete problem

John,

You are probably right to identify the different mechanisms by which the water ingress starts, but the ‘bursting’ mechanism is similar for both products … Rust has a bigger volume that the metal it replaces.

MichaelG.

I've had all my concrete fence posts replaced this year, they actually fell apart with just the rusty steel reinforced bars intact. These posts were around thirty years old, you can see the the air holes on that have bubbled to the surface as the concrete set. The contractor said he has seen many posts like mine, hopefully these new posts will see me out.

Bob

the US put out and alert for a roofing concrete used in public building of the eighty's........

Wythenshawe council did nothing for years until a child was hit or nearly when a big lump fell out of the roof of the local swimming pool......

they had just spent 100's of tousands upgrading it......

stayed empty for a few years before demolition......

Have heard in maj construction like bridges etc they are now using ST/Steel rebar.....????.

when u drive around look at the conc bridges etc.....most have at least a meter of Bitumen painted at the bottom...

To help protect them......

a good salt water soaking does them a lot of harm......

Mind it keeps the money flowing to those at the top of the gravey train.........

Not only aerated concrete. Seem to remember the stone work around St. Pauls dome having to be replaced due to the 'Girdle' rusting and splitting the stones.

In all the concrete brutelism imposed on Bradford in the sixties, can only think of a couple still standing. One is Wardley House, an office development that due to nobody wanting to se was eventually given away and became 'The National Museum of Film and Television' also an IMAX screen. T'other, 'The Richard Dunn Sports Centre', this WAS due to be demolished but some brave (????) soul has managed to get a grade two slapped on it. Derelict site which costs £150K a year in 'Security'

Oh what a brave new world we created.

Regards Ian.

Agreed, John,

Alkali aggregate reaction was my take, on the original use of that term. Recognised from about the early’70’s as I recall - particularly with cement manufactured by the ‘dry’ process (as opposed to the ‘wet’ process where the raw materials were fed to the kilns as a slurry). The dry process involves heat transfer within cyclones as the kiln exhaust gases exit the system and the powder feed descends the cyclone tower before entering the kin at near to (or even above) calcining temperature - depending on whether extra fuel is injected at the flue gas exit to the kiln. Alkali metal salts (mainly as chlorides and sulphates), sulphates were a result sulphur content in the coal and gypsum in the raw materials These mostly evaporate in the kiln burning zone and recirculate by condensation within the cyclone tower. This led to build-up, and possible blockage, in the final cyclone feed pipe - from the hottest cyclone to the kiln.

The alkali metals either eventually passed through to the cement clinker or, in some cases some exhaust gases were bled away, via a bypass system, where the high alkali metal salts were collected (along with a fraction of what was the kiln feed) and dumped back to the quarry. In the much less energy efficient wet process, the alkalis were generally lower, even though far more coal was consumed in the process (approx twice the amount!). They were collected in the dust collectors and dumped in the quarry.

As a addition to the discussion of RAAC, it may be noted that the ‘norm’ for removing frost protection from curing concrete/mortar was around the ‘5N’ level - 10N levels were typical for one-day strengths under standard condition, so less in cold weather - so any RAAC which might absorb water could suffer from frost damage. Not a good idea for flat roof structures - so just yet another risk with older buildings - and particularly roofs over (heated) swimming pools!

I read some where that this 'stuff' has a design life of 30 years, trying not to swear but who in the hell constructs a building which will need replacing in 30 odd years??? And what idiots would think it's a good idea?

Tony

If I was king for a day I would buy all the spare stainless rebar from China, negotiate a government quantity package please while their economy is struggling

Hmmmm. a friend of mine is a civil engineer, with an involvement in newbuild housing estates.
Design life came up in one of our conversations a couple of months ago; he reckoned that most new builds he's involved with have a design life of 40 years; 50 if you're lucky and going for a more expensive option.

Other stuff I'd read suggested that 60 years was the normal design life for UK housing stock.

I've just looked around for an article to back up his comments, and dropped on this one about timber frame construction
http://www.brand-newhomes.co.uk/timber-frame-new-homes.htm

"In 2010, one in four new homes is being constructed using timber frame or, to be more accurate timber panel, construction."

And then this one;
http://www.brand-newhomes.co.uk/considerations-when-buying-a-timber-frame-new-home.htm

"The timber frame itself is normally "guaranteed" by the manufacturer for various periods ranging from 10 to 40 years. It is a commonly perceived opinion within the industry that 25 –30 years is a reasonably expected life span for a softwood timber framed building."

I've no idea about the veracity of either of those links; not my field of expertise.
I wonder if there is another problem looming.

Bill

Has everything in our society got to be a throw away item? Last time I looked a house isn't quite the same as white goods which last a short time and are then launched into some recycling scheme.

Latest update. 'Hunt to spend what it takes' to make schools safe.

Tony

Edited By Tony Pratt 1 on 03/09/2023 12:52:06

It appears that a "home" only has to last the period it takes to pay off the mortgage

Financially it can make sense to build something more cheaply with a lower design life than to build the expensive long life solution. On 30 years time it might be obsolete. However this means you have to firstly do the required maintainance, and secondly you have to make financial provision for replacement when the time comes. Neither of these are particularly good at getting votes. Lots of the road gullies around me are blocked, some of them even have grass growing. Consequence, when it rains the roads flood. They did find the money to replace all the footpath finger posts, because that's visible, basic stuff isn't. You can always put off maintainance, until something breaks

It reminds me of a recent contributor on here 'Drawings are for dum people and toymakers'

Buildings are built using drawings, do they burn / shred them in case they may be held liable later?

It never rains but it pours, they say:

[quote] Experts warn that the presence of RAAC also increases the danger of exposure to asbestos, which kills 5,000 people a year in the UK. Asbestos is safe while stable but if it is disturbed — for example, if a classroom roof made from crumbling concrete suddenly collapses — it could release fibres which can cause mesothelioma, asbestosis and lung cancer. Even removing or assessing RAAC panels could disturb asbestos, engineers said.[/quote]

From the Sunday Times

MichaelG.

In a sensible contract, drawings and specifications etc would be passed to the customer. Then sometime later an accountant will decide that the cost of storing them is a saving that can be made, so when you come to needing them, they've gone. Been there, done that. It is not usually the builder's job to store this information.

Wasn't that Hi Alumina Cement High Alumina Cement - Manufacturing, Pros & Cons (civiconcepts.com) the link imples this stuff is still great even though roof beams in a school collapsed back in 74.

What do you expect, some BBC morning news presenters don't what what a Blue Tit is so they dumb it down to the level they they understand.

If you are buying it from China I would recommend you check every single piece with a magnet, their certificates of conformity aren’t worth the paper they are written on.

..yes, we're so lucky that those kind of sharp business practices never happen in western democracies..

i live on a housing estate built by my onetime employer.That were built of block and clad with brick as was standard in the sixties .The estate was started in 64 and these houses will be standing for another 40plus years ,my house on the estate is 44 years old with no sign of any deteriation so, the only reason to go down the route of quick and cheap solutions is profit.I admire the modern rendering technique of housing but I don't think that it will out last as well as houses rendered in 1907 still with no spalling locally.If something works well why try to fix it.Building engineers used to work to a factor of ten so what whent wrong.Profit.

Regarding the question of the appropriate design life for a building I would say that a lot depends on its function. Our last Laboratory was constructed in the late 1960’s and by the time we moved out into our new building it was definitely past its ‘sell by date’. Corridors were crammed with freezers, shakers and incubators. Infrastructure like chilled water supplies were often fed from chiller units ‘parked’ outside or on roof spaces. The Lab as built had no networking as desktop computers did not exist. Much of the science and instrumentation had changed beyond recognition so power supply needs and the way spaces needed to be configured was very different. I could go on but you get the gist. Really we pushed the building at least 10 to 20 years beyond its usefull life as a biosciences Lab.

The new lab is very different and taylored to fit the way we work today. To my mind it’s false economy to build something that will last decades longer than it’s usefull life.
regards Martin