Thermolite blocks not 'sticking' together - normal?

[piper64]

A small dwarf wall, part of a small porch, was built in August. Five courses high. I was in a rush and always doubted the exact ratio that I mixed the mortar.

I read that even at 6:1 it would be OK as a strong mix can split the blocks as it fries, so I thought fine, if I overdid the sand it's still OK.

The porch job is still ongoing regarding the windows, so the inner leaf of Thermolite blocks has not been touched. Today I noticed that one of the top blocks was wobbly, so I tried to rake out the mortar to re point. Then I noticed that all the blocks looked loose and as the mortar wasn't raking out as easy as I thought, I decide to pull down the entire inner wall. It came down easily.

The mortar was in fact quite hard, it just wasn't sticking the blocks together.

I have the job of rebuilding the inner wall arse backwards, but I can't do much about that now.

Speaking to my father in law, he said it would have been fine once it was rendered and plastered, so I've wasted my time pulling it down and have just given myself a load of work to put it back up.

Should thermolite blocks on such a small wall be not 'stuck' together? I would assume that any blocks should be firmly attached to eachother regardless of height?

 

[^woody^]

Aerated block are a bit crap with initial bonding as they suck the mortar dry too quickly. Damp them down before laying, and wipe any dust off the sides

6;1 mortar is OK

 

[seasickstevie]

I had an extension built twenty years ago.
Recently decided to make an alteration to one wall (altering pitch of roof etc.)

I was amazed how easily the Thermolite blocks just literally lifted off by hand, one-by-one - no problem!

As woody says, I think it's essential to wet each block as you build. And keep shielded from strong sunlight - Not often a problem I've found this year

 

[piper64]

Thanks chaps.

SNM
My father in law was mad when I told him that I'd taken the inner leaf down. He said the blocks were fine as they were, as you say. I wasn't happy about them though.

We chose the only hot dry days this year to build the wall, typical.

Taking the wall down has revealed another issue. Where the cavity tray, made from wide dpc, sits on the 100mm dpc on the outer leaf, it is not mortared at all as far as I can tell. The two layers are just resting on eachother, with mortar on the top of the cavity layer and bottom of the dpc layer, in contact with the dense blocks. Therefore, the wall, from dpc up is completely disconnected except by weight and it's only a 5 block high wall.

It feels sturdy enough, it's been climbed on and now has a heavy composite door and 3 windows connected to it.

Should I be concerned about this enough to take this opportunity to take the windows and door out and the wall down to start afresh?

 

[xdave]

Taking the wall down has revealed another issue. Where the cavity tray, made from wide dpc, sits on the 100mm dpc on the outer leaf, it is not mortared at all as far as I can tell. The two layers are just resting on eachother, with mortar on the top of the cavity layer and bottom of the dpc layer, in contact with the dense blocks. Therefore, the wall, from dpc up is completely disconnected except by weight and it's only a 5 block high wall.

A DPC will by its very nature cause the wall above to be completely disconnected from that below except by weight. Plastic DPCs are normally keyed slightly to help friction to stop the upper wall being knocked off, but the inner course of a cavity wall is often built off the DPM from the floor slab which will be nice and smooth. If the wall is sound then there is no need to remove it. What you do differently when you rebuild it?

 

[seasickstevie]

I'm not a brickie, but I've always assumed that a bed of mortar immediately both under and on top of the DPC is expected to create some adhesion.

 

[piper64]

xdave
Thanks, what I'd do differently is to put a layer of mortar between the dpc and the cavity tray, so all layers of material are sandwiched in mortar?

SNM
My thinking too.

The wall is rebuilt now, by father in law while I was at work, so I'll ask what he did.

 

[seasickstevie]

I need a father-in-law like that

 

[piper64]

 

[Emeraude119]

Ask many people and they will tell you that the mortar joints between bricks, blocks stone etc are to `glue' them together. However the real reason for using mortar between masonry units is to keep them apart, and seal the gaps between them.
Until relatively recently it was not possible to make bricks and blocks that were uniform and true, so mortars were used to make up for the irregularities in the masonry units.
Also until relatively recently strong binders in mortars were not available, so in general soft lime base mortars were used.
Because mortars are soft, does not make them not fit for purpose.
In the UK masonry buildings have been constructed for over 300 years in soft mortars where it is possible to lift the masonry units off by hand, and yet they have stood, without problem for centuries. What mainly keeps masonry units in place is the loads that are applied to them.
Over strength mortars (whilst often used with good intentions on sites)
lock masonry units hard together, so that when any drying shrinkage occurs in a wall it cannot be relieved in the perps, this increases the likelihood of shrinkage cracks occurring in a wet wall that is drying out.

 

[trowlerman]

thermolite are crap to lay in hot conditions mate I would rebuild , why do you have a tray in a porch ??

 

[piper64]

The job was done nearly 2 years ago and was rebuilt as I mentioned.

 

[Emeraude119]

thermolite are crap to lay in hot conditions mate I would rebuild , why do you have a tray in a porch ??

Aircrete blocks are manufactured in large cakes, which are then cut to the required block sizes using oscillating wires (a bit like the way wires are used to cut a block of cheese, or even those devices which are used to cut a boiled egg into slices)
As the wires cut their way through the aircrete, the vibration in the cutting wires ruptures the tiny air bubbles approximately 6 mm to -8 mm either side of each cutting wire. The material is still in its green state at this point.
Outside of this 6-8 mm zone the air cells are not affected, so they remain whole and in isolation to each other. This is why aircrete blocks provide high levels of insulation, have a very low air permeability rate compared to aggregate blocks, and why they float when placed on water.

The open cells on the surface of an aircrete block, can present high suction rates (try putting one of those prawn cracker things you get with a Chinese meal on the end of your tongue, as these have the same texture as the front of an aircrete block and you will understand the effect), but remember the open cells only go into the block to a depth of 6 - 8mm, so it is NOT the whole block which is sucking, just the first 6 - 8mm from the surface.
A LIGHT spray of water, or wipe down with a stock brush and bucket of water to just dampen the block, will knock back the suction and will normally do the job. Docking blocks of all types, should be completely avoided.

After cutting the blocks are `cooked' in autoclaves (very large pressure cookers) to complete the curing process. The blocks can stay warm for sometime after manufacture, (as being an insulating material it tends to hold onto the heat in the block pack). Manufacturers rotate stock so that blocks have fallen back to ambient temperature before they are taken out of the manufacturing base.

Blocks which are hot or wet should not be used on site. But once the blocks are on site it is the responsibility of the builder, to store and use them appropriately. In wet weather simple measures such as using ply hoarding on the top of block stockpiles can keep them reasonably dry, (shrink wrapping is also available for all blocks, but even this should be cut to provide a flap which allows the banding to be cut, and blocks taken out, but keeps the wrap in place until the last blocks are taken out) and in hot weather over sailing hoarding, helps to keep them shaded. When loading out, two blocks can be used to form a stand which keeps the rest of the blocks off a wet base / over site.
The tops of part built walls can be protected from heavy rain using a length of wide DPC held down with bricks / blocks, but there are now even large plastic clips available for this purpose. part built / standing walls take on relatively little water in heavy rain provided the tops are suitably protected.