Aircrete Blocks Cracking up – Unwanted Trouble

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 Aircrete 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 Aircrete 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’.

Aircrete Blocks Golden Rule

It’s a rule 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 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 topcoat will grab it along these lines.

As that topcoat 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 horizontal. 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.

More articles and info

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