We have a screed that is weak, cracking or badly dusting. Is it necessary to replace the screed?

Hi, I’m Andy Parkin, managing director of the multi award-winning Speed Screed. I’m here today to answer a question. The question is, “We have a screed that is weak, cracking or badly dusting. Is it necessary to replace the screed?” This is a call that we probably get a couple of times a month. Somebody’s got an issue with the screed and the first reaction is, “Does it need to come out? Do we need to get rid of it?” And I think you’ve gotta examine the causes initially rather than just looking purely at the symptoms. But what caused the screed to do that? You could replace the screed and exactly the same thing happens. So it’s important just to look at those, the actual causes of it and not just the actual symptoms.

So if we look at the reasons why screed fails, why does it crack, why does it become dusty, why is it friable? You could be laying it in adverse conditions, so it could be freezing. So the water freezes so there’s not the access to placement, you’re getting bits of pockets of frost, of ice that might be within. It could then freeze and expand, ice expands at around about 9% so it could expand and you could get cracking, it could be friable on the surface and dusty. And then the water isn’t available to the cement to hydrate it. So that could be an issue. The other end of it is that it’s too hot. So it’s too hot and it’s actually taking the moisture out of it. So it’s drying too quickly. So that could make the screed in itself weak and dusty.

The wrong materials might have been specified. So the wrong screed. So if it’s the wrong screed, it’s not supposed to go in at that particular depth, or there’s some other reason behind why it’s been specified, it could be a certain strength that’s required. Then that could be the reason why you’re experiencing those kind of failures. The screed is only gonna be as good as the substrate that it’s on. So if the substrate is not sound, it’s not dry, it’s contaminated, it’s dusty, especially if you have bonding to it. Then you’re gonna have issues with your screed. It may be that you’re not going directly onto the, say it’s concrete substrate but you’re going onto insulation. If the insulation isn’t stable, if it’s rocking, if it’s not tightly buttered then you’re gonna have issues with that. So if the substrate is not stable, the screed’s not gan be stable and it’s gonna break down over time.

Too much or too little water. So we covered not having enough water in terms of it drying out, but it might be the mix initially, it’s not being mixed with enough water or it’s dried out too quickly, so it’s not gonna hydrate. Too much water in the mix is gonna mean that you’re gonna have more shrinkage cracking, you’re potentially gonna have issues of, where the screed is gonna be weak. If the screed isn’t cured so if you have got warm temperatures, so it’s recommended that you would put a curing membrane over the top of the screed to keep the moisture in. If that doesn’t happen and the screed dries out then you’re gonna suffer from lack of hydration, cement particles are not gonna be hydrated. It could be pour mixing practise. So, sometimes screed is mixed on-site with just a shovel onto a board and then just mixed. Sometimes a [free-four 00:04:16] mixer is used and what you end up with is you get the ball and you don’t get the mixing. So either of those two methods are not satisfactory.

It may be that somebody’s got the mix ratios incorrect and you’ve not got enough cement in the mix. So, that’s why it can fail through possibly a lack of strength. The grading and quality of the sand is very important. So it needs to be of a good grading. So with having enough of the sand on the higher end of the grading to give it its strength, but then having a good grading throughout to fill all the voids. And so imagine if you was to use say like a building sand. You’d end up with a lovely smooth finish to it but there’d be no body and no strength actually in the screed. So you’d end up with a weaker screed. The sand can be of silt and lignite in it as well which can cause issues there as well. So the quality of the sand is very important too.
Compaction of the screed, if it’s a sand and cement screed and generally it’s recommended to, by the British standard to compact in layers of 50 mil. So you must compact in those layers, otherwise you’ll end up with perhaps a nice surface but then underneath it’s then weak and starts to breakdown. Early trafficking of screeds is something that often causes a breakdown. So that can be foot traffic, it can be loading, it could be point loading, just step ladders etc onto screed, will ’cause damage to the matrix of the screed. Stress relief joints are very important within screed. The product will shrink, anything with water in will shrink as it dries and you need stress relief joints in there to minimise the risk of shrinkage cracking. Shrinkage cracking probably causes 80% of all cracking in screeds and so by actually placing stress relief joints in, you actually minimise the risk of shrinkage cracking.

Did the screed have movement joints in? So if a movement joint is different to a stress relief joint, a movement joint is to factor in the movement of the building, so its external movement that you’re looking at. So if the substrate has a movement joint in there, that must be mirrored into the screed. Otherwise you may find that it will actually end up putting a movement joint in itself and cracking. Whilst the screed is moving, it’s important that you have edging strips in place. So anything that the screed touches vertically be it walls be it columns, anything that’s a restraint in the screed needs to have edging strip wrapped around. So that’s like [inaudible 00:07:40] foam or at the edges sometimes you’ll use insulation. But it just needs to have some give to it to allow it to move.
Levelling screeds are generally non-wearing surfaces. There are some that will be wearing surfaces but in general they’re not wearing surfaces and should always be protected from the point of laying the screed to the point of laying the floor coverings. Damage is often from site traffic etc, is often caused at this point when protection isn’t used. So it needs to be protected right through until those [inaudible 00:08:21] go on as you’re likely to see some form of damage to the screed. The other type of cracking that sometimes you’ll see is thermal cracking, so due to thermal movement. So the screed only will move twice, so it moves initially during shrinkage and then the product is deemed to be inert.

After shrinkage cracking, the only other time it will move is through external forces be it heat passed through the screed. So heated screed or the building moves it in some way heave. Or some other stresses that are placed on the screed. If it’s thermal cracking, so it could crack just when the heat is introduced because there aren’t sufficient joints in the screed alternatively, it could be that you’ve got two areas that are being heated and at different temperatures. And you can get differential thermal cracking because they’re obviously expanding at different rates, so you could get the cracking in between rooms that have been operated at different temperatures. These are just some of the causes so you can see that what you don’t want to do is just purely remedy the symptoms. And then endure having the same issue.
But depending on what the actual cause was, you may be able to repair. Now when we talk about cracks, the British standard states, generally hairline cracks don’t necessarily need to be repaired. So if you think, you’ve got a hairline crack, and it’s stable, the screed is stable, both sides of the crack, there’s nothing that’s wobbling, there’s no point where the screed has become isolated, then generally you can leave that hairline crack alone. The rule of thumb normally is half a mil. Or a credit card, if you’ve got a credit card into the crack, then it’s probably recommended, as a rule of thumb, that you should used some repair compound to remedy the actual crack.

If you’ve got a hairline crack, it’s quite extreme to then look at repairing it because what you’d have to do, is you’d have to open that crack which is gonna be a two or three mil opening, you’re gonna need to put in to allow the repair compound to go in. So you’re taking something that’s no more than a hairline and then making it a two or three mil. So as I say, hairline cracks may not be needed to be repaired. Certain shrinkage cracks you may also leave as well. If it’s a floating floor, it may not require … the time when you perhaps need to look at it is if it’s a heated screed and it’s closing and opening, so that might be a factor depending what the floor coverings are.

So if we’re gonna repair these cracks we would probably use a low-viscosity resin. There are other products on but I think this is the most widely accepted and easiest product to work with. So you’d look, if the crack is fairly wide, you’d look at bulk filling the crack with kiln-dried sand and then pour the resin in. The resin, leave it to settle for 10, 15 minutes and then top off again, wait again and if you need to top off again, repeat the process. For a general weak screed, so you see the screed breaking up you’ve got to look at it. Does it need to be replaced? Or can I use a penetrating resin?

So if you was using a penetrating resin, this again, is a low-viscosity resin that seeps into the matrix of the screed. It needs to be open textured to be able to do this, so it may need some mechanical preparation to the surface of the screed. The resin pours in and solidifies and makes the surface of the screed a lot stronger and increases the overall strength to it. So that’s one way of dealing with an issue. It tends to be more expensive than actually replacing the screed but depending on the circumstances it may be the preferred option. So sometimes removing the screed might be the solution. But what you might do is you might just look if it’s localised damage.

So if you was to look at the, probably an area that gets damaged the most, which might be an entrance to a building, so it’s taking the most foot traffic, it’s taking the most loads as people are coming into the building with supplies etc and point loading at that point. So what it might be is it might just be the case of actually putting out a metre square and then replacing that particular section, rather than taking the whole of the screed out. So you’d need to look whether it’s localised damage or whether there’s an inherent problem throughout all of the screed. But generally spot repairs are gonna be the most cost effective and get you sort of moving quicker through without having to replace the whole of the screed and go through the drain process yet again.

So I hope that’s answered the question. If you have any further questions and want any further advice then just please contact us, we’ll be happy to help. Thank you.

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