Concrete Answers Needed
May 4, 2023 3:31 PM Subscribe
I need to fix the corner of this wall, which is in the entranceway to the basement of my house. I presume that I would need to use self-bonding cement mix, doing several applications to build up the layers until the corner looks whole and level again. Is this correct, and/or is there anything else I should know to do this repair?
It looks like cement render. You should be able to buy the render in a pre-mixed bag that just needs water added, making sure you don't mix it too thin or it will slump badly. An expert may be able to do the render in one coat, but I (a non-expert) would do an initial layer about half the thickness, then add another layer once it's stiff enough to stay in place. Before you start, follow unearthed's instructions on surface preparation, including the bonding agent.
It can be quite a task to get the render flat and I would use either a proper screed of sufficient length to cover the whole height or you could use a piece of timber that is straight and has a sharp edge - move the screed back and forth as you move it along the area. You can use the existing plaster as a guide for the height in your case, which makes it a lot easier. Then you need a foam float to get the surface texture something like the original. You may find it easier for the final coat to do each side separately, to make it easier to get the corner straight because you then have something to guide where the corner actually should be.
I don't think it's essential to add an expansion joint in the corner, but it is definitely a place where cracks might develop. You could add a foam strip right in the corner to avoid this, although I don't think I'd bother, as internal corners like that are less susceptible to cracking that joins in a straight area.
Getting a good finish with render is harder than it looks, but you have plenty of time to work with it and you could use the first coat to practice how the render responds to the foam float and how to get the flattest finish. While you have time to work with it, the more you paddle it around the more 'fluid' it becomes, so work carefully.
posted by dg at 10:23 PM on May 4, 2023
It can be quite a task to get the render flat and I would use either a proper screed of sufficient length to cover the whole height or you could use a piece of timber that is straight and has a sharp edge - move the screed back and forth as you move it along the area. You can use the existing plaster as a guide for the height in your case, which makes it a lot easier. Then you need a foam float to get the surface texture something like the original. You may find it easier for the final coat to do each side separately, to make it easier to get the corner straight because you then have something to guide where the corner actually should be.
I don't think it's essential to add an expansion joint in the corner, but it is definitely a place where cracks might develop. You could add a foam strip right in the corner to avoid this, although I don't think I'd bother, as internal corners like that are less susceptible to cracking that joins in a straight area.
Getting a good finish with render is harder than it looks, but you have plenty of time to work with it and you could use the first coat to practice how the render responds to the foam float and how to get the flattest finish. While you have time to work with it, the more you paddle it around the more 'fluid' it becomes, so work carefully.
posted by dg at 10:23 PM on May 4, 2023
How old is this building? That looks like a lime render/plaster to me, with reasonably severe rising damp the likely culprit, judging by the surrounding plaster.
Is it soft and crumbly, can you nibble away at the edges with your fingers? Identify the cause of the damp first, then proceed from there. Honestly, the gap is the least of the worry here.
Unless you become certain that this is a hard cement based render, and I'd be surprised by that, DO NOT repair with ANY cement. Plenty of experienced people who should know better will disagree with that, they are wrong, deal with people who understand old buildings.
This is a really good guide on dealing with the damp that also talks about why cement is such a bad idea. Then perhaps search for information on lime mortars and plasters and disappear down that rabbit hole, which is a really fun rabbit hole, I reckon anyway.
posted by deadwax at 5:20 PM on May 5, 2023
Is it soft and crumbly, can you nibble away at the edges with your fingers? Identify the cause of the damp first, then proceed from there. Honestly, the gap is the least of the worry here.
Unless you become certain that this is a hard cement based render, and I'd be surprised by that, DO NOT repair with ANY cement. Plenty of experienced people who should know better will disagree with that, they are wrong, deal with people who understand old buildings.
This is a really good guide on dealing with the damp that also talks about why cement is such a bad idea. Then perhaps search for information on lime mortars and plasters and disappear down that rabbit hole, which is a really fun rabbit hole, I reckon anyway.
posted by deadwax at 5:20 PM on May 5, 2023
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I did this for a living decades ago, it pays to clean the remaining concrete (e.g a wire brush and then dust off using a brush and then vacuum the surface - you want the best bonding surface possible). Also is there any loose plaster? - remove this while you're about it.
Then I used to paint the surface in a water-based concrete primer [ often described something like "a water based primer and bonding agent" 'for concrete or cement surfaces' ], I think this is sold in Canada through a company called Ardex - but a tile shop would advise on this (at least they would in my country), and then apply the plaster mix in two or three goes so it does not slump while it's setting.
It looks like the area is abutted by sheetrock (or plaster over plywood?) - it is likely the area of plaster damage may have been caused by small sideways movements of the sheetrock (as it's more flexible and responsible to humidity). I'use a soft/flexible joint maybe 0.5cm wide between that and your new surface to avoid this happening again.
posted by unearthed at 6:27 PM on May 4, 2023 [3 favorites]