Concrete block retaining wall

I'm currently building a ~2ft high concrete block retaining wall, using 7N concrete blocks. The plan is to coat both sides in a good layer of bitumen, and then render the outside face (2 coats), and finish with paint. I'm installing regular drainage channels through the base of the wall.

A few questions for anyone who's attempted something similar:

1) Will the bitumen paint provide a sufficient level of water proofing? 2) Is there any problem rendering onto a bitumen painted surface? 3) What type of paint should I use on the rendered surface - a weatherproof masonry paint, or a porus paint?

Thanks for any advice.

Paul.

Reply to
PaulB
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PaulB wibbled on Tuesday 20 April 2010 08:33

Yes, if it's applied thickly - it has been used as a DPM under floor screed.

Not done it - but I believe the trick is to "blind" it with sand. If it were me, I'd coat the wall twice and blind the second coat whilst still sticky. The sand particles will embed themselves into the bitumen and the cemenet in the render will stick to the sand.

The other option could be to use a waterproof render which can be made by using SBR as an admix. Apply 4:1 Water:SBR to wet the blocks and allow to dry, then apply cement+SBR (1:1) as a slurry to the surface and whilst that is wet, render with an SBR render (it helps to keep a pot of slurry working with the pile of render and slurry just in advance of rendering a section.

This datasheet describes in more detail:

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advantage is that the SBR will add strength and some resiliance to the render. Disadvantage, it costs more - but 25l can be had on the internet for around 70 quid which is way cheaper than the builders merchants.

Having used SBR for floor screeding and for firming up poor substrates, I'm a convert.

I guess, if going for the SBR route, anything you like that's rated as exterior. If not, I'd be inclined to go for a true whitewash as that is permeable.

Reply to
Tim Watts

Bitumen on the soil side sounds a good plan. But bitumen on the front is all wrong. Nothing sticks to it, it'll prevent the wall drying out, causing freeze thaw damage, and its a awset of time & money.

Best to use a porous paint on the front of a retaining wall rather than regular masonry paint, it will enable the wall to evaporate damp, avoiding damage. Just basic lime paint works well - just mix builder's lime and waster to a paste, then dilute that 50/50 and apply.

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Reply to
NT

PaulB used his keyboard to write :

Don't do both sides, only the soil side good and thick. Keep the water out, but do not keep it in the wall. You also would not be able to render on bitumen. Use a porus paint, again to allow the moisture to escape the wall.

Reply to
Harry Bloomfield

Thanks for the good advice - the consensus seems to be bitumen on the back face, and then render + porus paint on the exterior.

Is there a danger with this solution, particularly in the winter, that moisture will be absorbed through the front face/render and then freeze/crack off the render in sub-zero temperatures?

Paul.

Reply to
PaulB

The OP doesn't explain why he thinks this is necessary; if it's just a

2 ft wall then I'm thinking this is just a garden project and can't see the point of the waterproofing.

I laboured for a brickie who built a not dissimilar wall for me - up to 3' high -10 yrs ago and there was no question of bitumen paint, etc. But then I wasn't rendering - didn't see the point as the guy laid the blocks sufficiently well that they look good without it. In

10 years there's been no seepage through the wall.

One thing that is required is to make sure the drainage behind the wall is comprehensive to the through drainage channels (just 1" plastic pipe laid at 3 or 4' intervals) - I used agricultural drainage pipe (stuff with holes in it) at the base and a lot of pea gravel.

Rob

Reply to
robgraham

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