Lightweight concrete blocks and cracks

[Water]

I've read bad things about lightweight blocks on the internet especially around them cracking because of this im considering using the heavier blocks.

 

Is it the internet over reacting?

[Oz07]

Airated can be a pain for cracking. Lightweight concrete less so

[tonyshouse]

I have been saying it for 40 years, 35 with posts mostly on GBF 

 

as a builder I stopped using aircrete widening the cavity instead in the 80’s,  medium density blocks, no cracks or call backs better quality, only ones who moaned were the brickies 

[Russell griffiths]

Try and do a bit of research regarding the different type of blocks. 

Aerated or aircrete, or thermalites.   Are proper rubbish and I wouldn’t use them if they where free. 

 

Lightweight aggregate blocks are fine. 

 

Dont be confused with the load rating, even some of the shite aerated stuff can have a 7kn rating. 

[scottishjohn]

I was under the impression --wrongly maybe that most of the cracking was caused by expansion joints not in correct place 

or not enough of them  and founds not solid enough -builders cutting corners 

or rough handling  for the type of block they are by both delivery and builders using them 

 you could  level same ciritisms at ICF blocks if they are rough handled before errection and filling with any old mix of concrete or skimping on rebar around openings etc .either that or you are saying you don,t beleive in physics and the industry strength testing systems etc

my money is on poor usage of the product and corner cutting by users

 

 

 

 

Edited June 6, 2020 by scottishjohn

[Oz07]

I think most common reason aircrete cracks is too strong compo. Think you're talking like 6:1 for aircrete

[Oz07]

On another note most trench blocks are aircrete. I think they're supposed to perform better underground than something dense and brittle.

Think @PeterW knows a bit...?

[joe90]

I dislike all airated  blocks and have seen others have trouble with them. Like @tonyshouse I widened the cavity and used ordinary blocks (and can hang anything heavy off the wall if I want). 

[tonyshouse]

Reasons aircrete cracks - brittle. muck too strong, Slamming doors, lintels. Thermal movement, window openings, internal walls, structural movement, poorly laid, bolted into, stairs, door openings, 

[Water]

Appreciate the feedback chaps, sorry yes I did mean aerated blocks as I had been looking at the thermalite Aircrete Turbo Blocks

[Oz07]

Poses the question does it matter if an airated block cracks below ground once back filled?!

[Bored Shopper]

we've had to replace quite a few such blocks which cracked under incorrectly placed structural steels. Cost us two weeks delay and extra labour  was really scary 

 

[tonyshouse]

Cracks if you can see them, if not then they might well be letting in draughts 

[Oz07]

Underground?!

[Timedout]

I won’t use or specify aerated blocks. They are often abused by trades who think they are just concrete. They are not and nor are they homogenous. They don’t fulfil their compressive strength once cut and are tested and perform structurally only when oriented correctly. That does not include laid on their sides. I could bore you with a story of a technical inspection I did on a fire station a couple of years back but it’s too long. In summary, everyone from designer to tradesman got it wrong and the aerated block wall was going into failure and the roof was losing its support. 
I have built a few structures using lightweight blocks, Thomas Armstrong Insulite. Available in 3.5 and 7N. Lambda about 0.44 so way better than dense blocks, only a tad more expensive  than dense and cheaper than aerated. Cut easily, take a good fixing. Trades seem to like them. Why bother with aerated when these things exist. It‘s an up-north outfit but I am sure there will be manufacturers in other areas. 

[tonyshouse]

I use recycled aggregate Medium density blocks 

[Oz07]

5 hours ago, Timedout said:

I won’t use or specify aerated blocks. They are often abused by trades who think they are just concrete. They are not and nor are they homogenous. They don’t fulfil their compressive strength once cut and are tested and perform structurally only when oriented correctly. That does not include laid on their sides. I could bore you with a story of a technical inspection I did on a fire station a couple of years back but it’s too long. In summary, everyone from designer to tradesman got it wrong and the aerated block wall was going into failure and the roof was losing its support. 
I have built a few structures using lightweight blocks, Thomas Armstrong Insulite. Available in 3.5 and 7N. Lambda about 0.44 so way better than dense blocks, only a tad more expensive  than dense and cheaper than aerated. Cut easily, take a good fixing. Trades seem to like them. Why bother with aerated when these things exist. It‘s an up-north outfit but I am sure there will be manufacturers in other areas. 

What about celcons which are transverse rated. I've used them in a block and beam before now as was getting cheap. They're sold as a one size fits all product. 

[Oz07]

Is anyone going to specifically say if they are bad underground or not. Most trench blocks I've seem are aerated and the party line is that the air pockets absorb the freeze thaw cycle better

[tonyshouse]

If they freeze when wet they die.  It is not easy to lay wide blocks properly. I like a below dpc route for water to drain away by. 
 

They are widely used - not for me though 

[MortarThePoint]

I'm learning on this, but there is also a lot of marketing at play around blocks. I feel there is a clear distinction to be drawn between aerated blocks (AAC) and aggregate blocks.

 

AAC blocks

 

 

"Unlike most other concrete applications, AAC is produced using no aggregate larger than sand. Quartz sand, calcined gypsum, lime (mineral) and/or cement and water are used as a binding agent. Aluminum powder is used at a rate of 0.05%–0.08% by volume (depending on the pre-specified density). In some countries, like India and China, fly ash generated from coal fire power plants and having 50-65% silica content is used as an aggregate.

When AAC is mixed and cast in forms, several chemical reactions take place that gives AAC its light weight (20% of the weight of concrete) and thermal properties. Aluminum powder reacts with calcium hydroxide and water to form hydrogen. The hydrogen gas foams and doubles the volume of the raw mix creating gas bubbles up to 3mm (⅛ inch) in diameter. At the end of the foaming process, the hydrogen escapes into the atmosphere and is replaced by air." Wikipedia

 

Thermalite and Celcon etc are AAC blocks that have a density in the region of 450 - 800kg/m3 and lambda of around 0.11 - 0.20 W/mK. Whilst easy to handle and thermally performant, these are the blocks that people typically worry about cracking. The article linked above notes as a disadvantage: "Installation during rainy weather: AAC is known to crack after installation, which can be avoided by reducing the strength of the mortar and ensuring the blocks are dry during and after installation."

 

Aggregate blocks

 

 

It's a bit more obvious how these are made, consisting of cement and aggregate. Typically available as Dense (~1900kg/m3) , Medium, Lightweight (~1400kg/m3) and Ultra Lightweight (~1000kg/m3) this refers to the aggregate type used in their manufacture. The marketing department has done well to describe a 100mm block weighing over 10kg as Ultra Lightweight and this is twice the weight of the equivalent AAC block. Their reduced weight comes from the use of naturally occurring (e.g. pummice) or man made (expanded clay, blast-furnace slag) aerated aggregate. As well as reducing weight, this lowers the lambda values, with 0.28W/mK available in Ultra Lightweight aggregate blocks.

 

Comparison

 

Obviously both block types have their place, but personally I'm inclined to avoid AAC. They have exceptional lambda values and can be half the weight of even the lightest aggregate blocks, but they can be unforgiving and present future problems. Not as good thermally, Ultra Lightweight aggregate blocks do provide a useful improvement in wall U-values over using denser blocks. The aggregate itself in Ultra Lightweight aggregate blocks won't be as strong as in Dense aggregate blocks, but the mechanical properties of an aggregate block will be a function of the aggregate and how well the aggregate is held together. Aggregate blocks aren't immune to cracking.

 

It may be obvious, but cracking happens due to movement so eliminating the sources of that movement is a key part of avoiding them. Shrinkage is one of the key reasons cracking could occur and avoiding the blocks becoming excessively wet can reduce this risk. There are some scary picture here.

 

Another reference: Forterra Pocket guide to Aggregate Blocks

Edited June 26, 2020 by MortarThePoint

[Oz07]

Great post

[Declan52]

I used lightweight blocks on all my inside skin and have had little to no cracking. Once my roof was on I let the house sit for 5 months, Oct to march, to dry out. 

The main issue can have is not enough water is put on the blocks before they are plastered. I used a hose and soaked them through and then some more. It means they won't suck all the moisture out of the scratch coat and dry it out to fast which results in all the cracking. Before the skim goes on the wall gets another soaking again.

[MortarThePoint]

On 26/06/2020 at 10:58, Declan52 said:

I used lightweight blocks on all my inside skin and have had little to no cracking. Once my roof was on I let the house sit for 5 months, Oct to march, to dry out. 

The main issue can have is not enough water is put on the blocks before they are plastered. I used a hose and soaked them through and then some more. It means they won't suck all the moisture out of the scratch coat and dry it out to fast which results in all the cracking. Before the skim goes on the wall gets another soaking again.

 

So did the 5 months dry out the block throughout and then it was important to make the surface wet to help the plaster?

[Declan52]

Yeah it gives the house, blocks and timber in the roof, a chance to dry out. 

If you don't wet the wall it will cause the scratch/top coat to dry out too quick as the wall will suck the moisture out of the plaster before it goes off. Also gives it a bit longer working time for you to get it on and wait on it to dry out just enough to start flattening it out and rubbing it up.