Spalling of garden wall (frost damaged bricks)

[echase]

A builder built me a low garden wall (600mm max) 4 years ago. This was backfilled with earth to within 20-50 mm of the top. The top was normal bricks on their edge. After a couple of years the tops of 50% of them were badly spalled by frost with 5mm or more flaked off. I tried a silicone waterproofer but did not make any noticeable difference, possibly because the main water ingress is from the earth and not from rain.

I then in 2009 capped them with mortar and narrow B&Q sandstone paving slabs 50mm thick and wide enough to well overhang the wall. After the hard 09/10 winter 50% of these slabs have come loose and I see that there is further spalling of the bricks underneath, both of bricks that were previously and not previously spalled.

I intended to use a lime mortar as I read it’s more frost resistant, but think I made a mistake and only used it for part of the wall. Maybe the 50% that failed is the bit that used just sand cement mortar. A clue is that the mortar did not stick to the slabs at all on these loose ones, so maybe the problem was that mainly that I got the mortar all wrong. It was premixed Hanson stuff.

Some of the wall has small quarry tiles on the top and this failed in the same way although on these the tile adhesive was stuck to the tiles still. None of the faces have spalled below the top row.

Any ideas what to do next? I am thinking of applying a stabiliser solution (like dilute varnish) to penetrate the top of wall. Then apply a tanking type water based bitumen emulsion. Then blinding this with sand and lime mortar the slabs back again. The idea is to provide a little bit of flexibility between the mortar and the wall to stop minor spalling loosening the paving and seal it well against water penetrating at the mortar. But if the predominant problem is water penetration via the earth then I am not sure my solution will really help as it does not stop water seeping through the wall to the brick/ bitumen interface and spalling the brick there. Although the paving makes it harder for the temperature at that interface to get below freezing.

I suspect too that the builder’s choice of bricks was not great, although he used the same for a garage wall and that is fine.

A desperate solution might be to take off the top row and use a proper frost proof brick instead for that row, but it’s a lot of work and can I be sure that it still won’t spall along the top of the second row as my paving slabs don’t seem to protect that interface well?

 

[echase]

The wall is a cavity wall, apart from the top course, with maybe concrete blocks against the earth for the inner face. This maybe explains why the lower bricks are not spalling as they are kept drier by the cavity, whereas the top brick can directly pick up water though its end in contct with the earth.

If I removed the earth down a couple of bricks and tanked the inner brick/concrete block face would it stop enough water penetration from the earth to protect the top brick? It’s too much effort to remove all the earth down to the wall base and tank the lot.

For tanking like this is it enough to just use bitumen emulsion sealer or do I HAVE to add a mortar coat on top of the bitumen? As it’s a water based bitumen will it redissolve in water if I don’t mortar over it?

It’s single skin where the quarry tiles are so maybe the only solution there is to use an engineering brick for the top course. Fortunately that bit is a small length of wall.

 

[2929twin]

I reckon all your effort is aimed in the wrong place.
It may be a lot of hard work but dig out rear of wall, let it dry to a great extent, 2/3 good coats of bitumen paint( not water based), when this is dried, back fill with gravel or at least crushed bricks. Make sure the wall is drained at base.
Coping should be a frost proof brick or splayed concrete coping.
When sorted, then deal with the facing issues.

 

[^woody^]

If the bricks are spalling after such a brief time, then they were either not suitable (no frost rating) or seconds or otherwise unsuitable for this use

In which case no amount of remedial work will help

 

[ajrobb]

I wonder why a lot of garden walls are capped with engineering bricks on top of tiles?

The capping bricks face the cold sky and suffer much more severe frost than the vertical faces. You mention a "brick/ bitumen interface", so I assume that the bricks lower down are protected from moisture from the soil.

I'm no bricky, but I'd be tempted to remove the top layer(s) down to the bitumen protection and insert a DPC wrapped down over the bitumen and rebuild the top with strong bricks and strong mortar.

 

[echase]

Many thanks for input. To clarify, the faces have not yet spalled except on the top brick which is the wettest.

What about these 3 new solutions for the 3 different areas of this wall?

1) Main bit where the stone capping has recently loosened: Remove the top brick layer which is the only one spalled. Reduce level of flowerbed behind by 4”. Mortar the sandstone slabs over the 2 walls making the cavity. I.e. now the sandstone is capping the cavity and not a porous brick. As sandstone is much less water pervious there will be little water seepage into the capping and into the front face so they will no spalling at all (or it will take a long time). But is the mortar joint at the back between the concrete blocks and the capping at risk from frost? If the earth is built up again to the capping, to some extent it insulates the joint from low temperature. I am in Hampshire so temperatures not usually that low. Or would I need to tank the concrete blocks as 2929twin says to guarantee a dry mortar joint? Any point in adding a DPC strip below the mortar at back wall? Trouble is that with only the capping weight on top of the DPC won’t the DPC effectively just provide a weakness in that joint? Blinded bitumen paint might be stronger. There are no other DPCs in the wall as far as I know.

2) Tiled part: This has a concrete backfill for a shed base, which is also tiled up to the shed door, so I can’t lower it or get behind the wall without removing a lot of concrete. So instead remove top layer of bricks and replace with frost proof bricks. (Can you get mid red/yellow colour ones as most engineering bricks I have seen seem to be maroon or blue?) Tile over bricks.

3) Garden steps: Can’t lower the tops of these walls without also lowering the steps. If the capping fails again maybe use engineering bricks like 2).

The same builder at same time used these bricks and blocks to build a single skin garage wall partially underground. Block underground and brick above. He used a pea gravel fill and bitumen coat. But he also used sheets of polycarbonate triwall roofing on the outside to provide an extra water impermeable layer on top of the bitumen. Never seen that used before like that. But it does leak a little in heavy rainfall. I have reduced it by bitumening the inside face too, but after comment above maybe I should not have used a water based bitumen. I don’t think a single skin is enough and/or it gets through gaps around the edges of the poly sheets.