I’m Andy Parkin, Managing Director of the Multi Award Winning Speed Screed. I’m here to answer the question, “Why has my screed cracked?”
Why Has My Screed Cracked
It is quite a difficult question to answer, however I will give you an overview and hopefully may give you some ideas as to why this may have happened. As with any screed issues, we need to look at the full picture. There is no point in just repairing the symptom, we need to deal with the cause first. If we only treat the symptom then we are likely to have the same problem again.
To examine why has my screed has cracked, it should be noted that it is common to see cracking, and does not generally mean that the screeds integrity is affected.
Screed shrinks, and there could be an elemental of shrinkage cracking. Control of the shrinkage cracking, is whereabouts it cracks. Is it okay to crack there, is it okay to crack there? Anything with water in will shrink as it dries, and unless there are controls in the screed, it will crack by itself to release the stress.
When you experience cracking, it could be related to a general breaking down of the screed. You may start with a crack, but that might be a symptom of something else that is underlying and the screen may break down further.
You may have an innocent crack that is then trafficked, loaded, then starts to break down, which is a different issue in itself.
The screed will only be as good as the substrate that it is laid on. If the substrate is not sound, dry, contaminated, stable, insulation is rocking or you have gapping, there will be issues with the screed.
Preparation is always key when working with screed.
Has there been too much water, too little water? Too much water means that you’re gonna get excessive shrinkage and more cracking. If there’s too little water, then that means that the cement can’t hydrate or won’t hydrate all the cement particles so you’re gonna end up with a weaker screed. So, that may be an issue.
Staying on the subject of water, has the screed been cured? So, when we’re saying cured, it means retaining for seven days the water within the screed. So, it’s not dried out too quickly, there’s not been wind tunnels in the building, certainly kind of may through to October time is key times when screed should always be cured, as per the British standard. So, the screed may dry out, you may end up with dusty surfaces. In extreme cases, you may experience some cracking because of weakness.
It could be poor mixing practise on site. Now, that could be a multitude of things. It could be the way the screed has been mixed. Has it been mixed with just a shovel on a board? Has it been mixed in a free fall mixer so it’s not mixing properly? You’re ending up, you’re getting balling. Free fall mixer’s not suitable for a semi dry mix. Is it that there’s not enough cement in the mix? Has the cement ratio not been adhered to? So, that could be a reason why the screed starts to break down.
Is it the wrong grade and quality of the sand? Is that suitable? Does it meet the British standard? Is it starting to break down because it is the wrong grade? Is it too fine? It may look a nice smooth surface finish, but if there’s no sand at the top end of the grading, it’s going to lack some strength. So, maybe it’s the wrong grading of sand. Compaction is a major consideration for sand and cement. So, it needs to be compact in layers. So, layers of 50 ml. So, if you’ve got 100 ml screed, that’s two layers. If you’ve got 150 ml screed, that’s three layers you need to be compacting in. If you don’t compact in those layers, then you’re gonna end up with a nice, crusty surface but then nothing substantial underneath. It’s gonna crumble and you may see a breakdown of the screed and also some cracking.
Has the screed been trafficked too early? This is something that tends to happen on sites, busy sites. Contractors can’t wait before the next trades are getting in, so it’s early trafficking. So, that’s foot traffic, point loading, heavy loads are subjected to the screed. So, that can be, it can be impact as well so that you can see cracking related. So, all these things. The screed from the moment it’s laid to the time when the [inaudible 00:07:07] go on, British standard states that the screed should be protected at all times.
Has there been any stress relief joints? So, as we mentioned, this is probably the single biggest factor for cracking in screed. I would say probably 90% is gonna be down to shrinkage cracking. To try and manage shrinkage cracking, I don’t think you’d ever say that you’re gonna alleviate any shrinkage cracking, but to manage it, you would be looking at cutting it into [inaudible 00:07:39]. You’d look to put polypropylene fibres in there as well and both those two things will minimise shrinkage cracking. It’s making sure you’re putting the stress relief joints in the correct places to take into account the size of the bay, the shape of the room, and any restraints that are in the floor.
So, often if you’re seeing shrinkage cracking, it’s coming off corners, it’s coming off restraints like columns, it’s going across doorways. They’re classic shrinkage cracks. Movement joints. Has the screed got movement joints in them? When we’re saying movement joints, that’s to allow for movement within the building. So, within the substrate and in the structural element, you may well have movement joints. The screed should mirror those movement joints. If they don’t, then you may find if the building is moving and from that one ml, you may end up with a crack actually in your screed.
So, the other thing that you need to look for, did it have edging strip? So, has it got somewhere for it to move. Edging strip, that can also be insulation as well around the edges. As the screed moves, if it pushes up against something that it can’t move, then that’s a restraint and you can see restraint cracking. So, it’s important to have the edging strip there just to allow, facilitate that little bit of movement. Generally these type of screeds are all non wearing screeds. There are a handful of products that are wearing screeds, but the majority aren’t. If it’s trafficked and not protected, then again you’re gonna see a kind of wearing down of the screed. You may end up seeing some cracking in extreme cases as well. So, it does need to be covered until the [inaudible 00:09:51] go on to ensure that you get the maximum performance from the screed.
Thermal cracking is something that we see as well. The screed only will move once by itself, so that’s during the shrinkage process. After the shrinkage process is completed, the product is classified as inert, so it won’t move. The only time it will move again is by external forces. One of those is heat. So, if it’s a heated screed and you’ve got thermal movement, it may potentially, if the joints aren’t sufficient, it may build up that stress and then look to lose the stress and it’ll look for the weakest point and crack along the weakest point.
With thermal cracking as well, you may get differential thermal cracking. Now that’s where you’ve got two areas that are heated at different temperatures and you’ve got that differential, so they’re moving differently and they crack effectively down the middle. It might be in the doorway between two rooms. One might be say a corridor that’s kept to relatively low temperature. The rooms are heated to a higher one and you can get cracking there. So, if there’s no joint, there may be the possibility of thermal cracking. But as we mentioned, the screed won’t move again by itself, so it could be that the building moves. So, where there’s movement joints, that might be a case where the building’s moving. It could be heave, so there’s something going on underneath, the substrate is pushing and it cracks. So, forces placed on the screed afterwards may cause cracking.
These are just some of the causes. It really needs investigating. It really needs looking where the cracks are running from and to. It needs to know the history, what is the substrate, what’s gone on to the substrate. It needs a lot of investigation. On the surface, shrinkage cracking often appears quite obvious and there are those tell tale signs. So, I think it’s probably good now just to really run through some of the repair procedures. It’s very rare that a screed needs to be replaced, so the repairs are generally fairly easy to do and I think we just probably classify them in probably three areas, and I’ll just run through them just to give you an idea.
So, for general shrinkage cracking, and we wouldn’t really look at any, repairing any cracks generally that are less than half a millimetre or less than a credit card width. So, the British standard states hairline cracks are generally not required to be repaired. Now, if you’ve got isolation of screed, so you’ve got a piece of screed that’s rocking and isolated, then that might be something that needs to be addressed. But generally when you’re repairing cracks, you need to be able to pour a low viscosity resin in and if you’ve just got a hairline crack, you can’t actually do that. So then what you have to do to get the resin in is you’d have to open that crack up to a two, three millimetre extent.
So, you can see that you’re starting off with a relatively stable screed that just has a hairline crack in and then you’re gonna have to drive a sort of two or three ml crack into the screed using a grinder. So, you can see that in certain instances, the answer is to leave well alone. If you are repairing the crack, you pour the low viscosity resin, the two part resin into the crack. If it’s quite a wide crack, you may bulk fill before you do this with a kiln dried sand. So, you bulk it in and then pour the resin over the top. You’d wait probably 15 minutes to allow it to settle and then you’d pour again and top off. You may do that again as well, just in case it runs.
Some times you may have a little bit of a gap under the screed and it starts to fill that up. So, some times it could run a little bit further than you anticipate. For a generally weak screed, so if it’s not sound and it’s starting to crumble and you think it’s because of lack of strength and other things, then a penetrating resin might be the answer. So, what that is is the resin actually sinks into the screed. So, it’s got to be open textured enough to allow that to happen. So, some times the surface needs to be mechanically abraded to allow the resin to sink in. But that sinks in, and then it hardens and strengthens the screed. That’s quite an expensive process and it is actually cheaper generally to remove the screed and to start again. So, you’d probably look at that if down time wasn’t an answer or wasn’t possible.
Some times removing the screed is as cost effective, as we say again, the penetrating resin. But if it’s kind of spot areas, then it’s probably a good idea to remove the screed in that particular area. So, it might be a classic area that’s damaged is the entrance to a building, so you’ve got all foot traffic. You’ve got wheelbarrows. You’ve got other loading on that particular area, and that can often get damaged. So, rather than trying to patch it up, repair mortars, and other product, it’s often better just to cut that area out and to relay. If you’re worried about getting floor coverings on and things like that, you may go with a fast drying screed so you’re not gonna be delayed any further.
But that’s an overview of cracking and repairs. As I say, probably 80, 90% of all cracks relate to shrinkage cracking and in themselves do not affect the integrity of the screed and it’s something which is relatively common with screeds and concretes. You try to minimise the shrinkage cracking with joints, with the fibres, but there are no guarantees that it won’t crack because of that. So, I hope that’s been helpful. If you need any further information, please get in touch and we’d be only too happy to help. Thank you.