Aircrete blocks

Cracking up: The Trouble with Aircrete Blocks

If you have been drawn to this blog by the title alone I will save you wasting further time by saying from the outset that I am talking about buildings here and not a mental breakdown, though that can never be ruled out in the building game.

Certainly the owner of the house in this story needs an extraordinary degree of optimism and patience in order to preserve his sanity as his new house continues to crack before his eyes. There are of course many reasons why buildings may crack but this blog is talking about buildings made of Aircrete blocks which a few months after completion started showing a number of large cracks. As with so many of these problems, everyone involved is pointing a finger at someone else. Is it the blocks, the brickies, the plasterers or the architect who is to blame?

Aircrete blocks are lightweight and have a very high degree of insulation. When introduced they seemed like the wonder product of the age and to some extent they do a job that no other materials can do. I think it’s accurate to say that when they were first developed the intention was to use them on internal skins of cavity walls instead of breeze blocks which are heavier and not such good insulators. This is still where they are mostly used. There are however a growing number of buildings being built with external skins of Aircrete and there are even buildings being built with solid Aircrete. That is to say no cavity. The appeal of blocks over bricks is speed and cheapness. They are a good product in their way but the builder needs to understand their limitations.

The appeal of blocks over bricks is speed and cheapness. They are a good product in their way but the builder needs to understand their limitations.

There is a golden rule in the building industry that states ‘mortar should never be stronger than the material it is joining’. It is a golden rule that is often broken. Having worked as a brickie’s labourer in my teens I can tell you from my own experience two very good reasons that this rule is broken: one is laziness and the other is ignorance. Often the two go hand in hand. The general advice is that a cavity wall is brought up more or less equally on both sides rather than building the inner skin and then the outer. Again this isn’t always done but if it is done then it is highly likely that the mortar being used is sometimes the same strength inside and out. Labourers just can’t be bothered to chop and change mixes or throw stuff away. At best they may put it back in the mixer and add a bit more cement but even that is a hassle so they tend to mix a fairly strong mortar for the bricks and serve it up for the blocks as well.

Another bit of advice is that Aircrete blocks should ideally be laid with a sand/cement/lime mix of around 6.1.1 or even 8.1.1 for Solar blocks. If you look at how much lime a builders merchant sells compared to the number of Aircrete blocks he shifts you will see that very few people follow this recipe. It is far more likely that they will weaken the mortar with a plasticisers or (in some cases) washing up liquid. This gives them a lightweight mix with plenty of room for movement but the problem here is that the amount of cement is not sufficient to cover all that sand. I would argue that for this reason alone lime is always better than plasticiser because it mixes with the cement and spreads it further to form a more consistent mix.

The block manufacturers are painfully aware of all these problems and issue guidelines to avoid cracking. This involves the use of movement joints which must come all the way through the render. You only need to look around at rendered houses to see how rarely this is done.

That is the ideal scenario but, as I have said, the reality is that the labourer will often knock up a 4 or 5 to 1 mix of sand and cement with a squirt of plasticiser which is then used throughout the build. If the Aircrete blocks are used on the internal skin only and that is later dry-lined with plasterboard then the subsequent shrinkage cracks will never be seen and in any event will probably do no harm. If the blocks are used on the external skins then the cracks cannot be covered because they will almost invariably show through the render. Even if the build mortar is the right strength to allow for movement in the blocks, this good work can be undone by applying render that is too strong. Getting the render mix right is absolutely critical but once again there are plenty of plasterers out there who struggle to keep a good coat of render on an Aircrete wall and to make matters worse their answer is to use even more cement. The real answer is to apply a slurry coat to the blocks and then when this is dry apply the scratch coat.

The block manufacturers are painfully aware of all these problems and issue guidelines to avoid cracking. This involves the use of movement joints which must come all the way through the render. You only need to look around at rendered houses to see how rarely this is done. People just don’t like the look of them. The other measure to avoid cracking is to use bed joint reinforcement at vulnerable points. This is typically around and below windows. The fact that there is no load directly beneath a window means that the block work can simply pull apart in the middle. Again you only need to ask a builders merchant for bed joint reinforcement to see that it is rarely used. Very few stock it and some merchants have never heard of it.

What this means is that block manufacturers can simply point to these omissions or errors and wash their hands of any problems. “If you don’t follow the guidelines you only have yourself to blame.” they will say. I would say they could help a lot more by printing the guidelines on the packs but I suspect they don’t really like the word ‘cracking’ to appear too close to their brand name. There is another little point that can also help prevent cracking in rendered walls (this applies to brick as well) and that is the use of serpentine curves in the scratch coat. It seems like such a small and insignificant thing but it can make all the difference. If the first coat of render is lined through with horizontal lines then the top coat will grab it along these lines. As that top coat shrinks it will pull on those horizontal lines and hold the wall in tension as the render dries out and tries to shrink. The problem is that all the tension is in a vertical direction so the natural tendency is for the wall to move in the opposite direction which is horizontally. So, as strange as it may seem, a horizontal scratch coat will produce vertical cracks.

Again I see hundreds of jobs where the scratch coat is horizontally lined often with a notched tiling trowel. In fact the whole approach of plasterers to rendering Aircrete is often completely wrong. They assume that the wavy lines which are put on the blocks at the factory are a key for their plaster or render which is wrong. If you walk around many Aircrete buildings a few weeks after they have been rendered and tap the walls you will often here a hollow sound. Shortly after that come the cracks, and after that the solicitor’s letters with everyone pointing the finger at someone else. The best excuse of all…..the weather.

About Roger Bisby

Roger Bisby
Roger Bisby is an English television presenter and journalist, known for his expertise in the British building industry.

9 comments

  1. Avatar

    Dear sir,
    Can you give me a render mix for block work .

    Many thank.

    • Roger Bisby

      Hi Barry

      If it is aircrete block I would suggest that you put a slurry coat on the blocks to form adhesion. This can be diluted SBR with a 1-1.6 lime cement and sharp sand. Make sure it is roughly brushed so the next coat 1-1-6 will stick to it again with a little SBR in the mixing water. Scratch this up deeply and then apply a thinner coat of the same mix as your finish coat. If you are worried about cracking then you can lay some fibre glass mesh from Teco at Shoreham into the scratch coat. It works really well and I haven’t had any problems with cracks since I did it.

      If your blocks are dense concrete you can go for a stronger mix of 1-1-5 or even cut the lime down to a half of the cement. It depends what you like.

  2. Avatar

    Hi Roger.
    I need to patch in some areas of pebbledash rendering on the back wall of our holiday cottage. In small areas ranging from 6 inches square to approach 18inches square, the weather facing wall had flaked off down to the ? Scratch coat covering the block work. We want to patch these in before repainting it in masonry paint
    We have plastering sand and a agog cement but don’t know what proportion of sand to cement plus water?

    We are very isolated so want to have a go at the job ourselves. Fingers crossed??!

    Please can you help with advice on proportions to use?

    • Roger Bisby

      It is very common for the top (butter) coat to part company. If you mix up a nice 4 to 1/2 1/2 sand cement – lime and add some SBR to the mix water it will stick really well. You have to trowel it on and throw the pebbles at it. For some reason it always looks like a patch even after it has been painted but it will stay there and the frost won’t get behind it.

      • Avatar

        Bless you. I just want to be clear on what you mean as I am a 75 year old great grandmother !!! Not a regular builder. Haha!
        Do you mean 4 and a half times sand to ? A mix if half cement and lime?

        Also, please can you tell me what SBR stands for ( I think Zi need to make a journey to the coast !!?

        Should I add just enough water to make a THICK consistency?

        Sorry I am a nuisance … I have tackled many jobs at our remote cottage …. But never patching in rendering ! My husband is 80 with mobility problems …. So it’s down to yours truly!

        Bless you. Jan

        • Avatar

          Oh Sorry.
          I meant 4 shovels of sand to 1/2 shovel of lime and half shovel of cement.
          You can use a plastic tub to measure it out so long as you keep that ratio of four tubs sand half tub of cement and half tub of lime.

          Come to think of it you can simply buy a bag of premium cement which has a plasticiser and just do a 4 to 1 mix sand to cement. Then you don’t have to buy a bag of lime. I just like the lime because it makes the mix nice and fatty like putting on butter.

          You still need the SBR which comes from Everbuild or Evostik.
          SBR is basically a latex that you add to the water to help the mortar stick to the wall. It is better than PVA.

  3. Avatar

    Hi there,

    Just came across your article, which is very interesting. I had been thinking of building a small house using aircrete blocks. Though not the ones built by H&H or the other companies. I wanted to use blocks like they do in America, which are much larger with holes in them. There are a couple of companies here which can custom make them cheaper than conventional bricks etc. However having read your article I am not sure what is the best way to go. I was planning Aircrete panels with a metal frame to carry the structural weight of the building. i was also considering using aircrete panels in the roofing. When I was planning this I had no idea of the cracking issue. Would it make a difference if rubber powder from recycled tyres was added to the aggregate of the panels when they were being manufactured. I have also read that steel wires can be placed inside them during the manufacturing process, would any of these techniques make a difference to the cracking issue.

    What in your opinion is the best low cost eco-ish way to build a house, that is if I have to go back to the drawing board. I have considered many options; rammed earth tyres & Cob – too labour intensive and time consuming, Straw bales – risk of dampness could cause the bales to rot thus destroying the house. Regular bricks – not very eco and quite expensive and labour intensive due to size and mortering. Regular concrete panels – cold and very heavy. Breeze block – unsure. Ideally I would build a house out of stones and motar, the way the old farmhouses were built, when done correctly these houses are warm and full of character. It is difficult to get the stones at reasonable cost though plus they are a bit time consuming. The only way I know round this is put them in metal cages and pour mortar on them. However this is time consuming and problematic regarding transportation etc. I was thinking of using this technique for foundations though

    By the way I live in central Scotland and it can be very cold here. i would really appreciate some advice. i am on a budget. I have been researching house building techniques online for ages but have only just found a plot within my budget and am awaiting to find out if my offer has been accepted.

    • Avatar

      Maree, I’ve also been looking into alternative building here in California. I’ve steered completely away from the rammed earth tires due to off gassing of the tires. I do like the rammed earth with forms, done like conctere but its a bit labor intensive. I think you have to decide first your finances and then decide which method will best fit the budget vs labor intensities. Ive decided the lowest cost is earth bag construction. A bit labor intesive but hired help shouldn’t require a lot of skills. Roger’s suggestions of lime rendering is perfect for this application. If you’ve already decided or cstarted tour project I’d like to hear what direction you went and how it’s going.

  4. Avatar

    lightweight block motar materials ratio tell me.

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