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

I’m Andy Parkin, Managing Director of the Multi Award-Winning Speed Screed. I’m here today to answer the question “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.  When someone has an issue with screed the first reaction is, “Does it need to come out? Do we need to get rid of it?” Firstly you have to 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 to look at the actual causes not just the symptoms.

Too Cold or Hot?

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. The water freezes, you start getting pockets of frost/ice. It could then freeze and expand, ice expands at around about 9% 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. That could be an issue.  At the other end of the scale it might be too hot.  If it is too hot it will be taking moisture out of the screed. It is drying too quickly, and this can leave the screed weak and dusty.

Wrong Specification?

The wrong materials might have been specified, and in effect the wrong screed.  It might not be suitable for a particular depth, or there is some other reason behind why it has been specified, it could be a certain strength that is required.  For those reasons alone that could be why you are experiencing failures.

Substrate

The screed is only going to be as good as the substrate it is being laid on.  If the substrate isn’t sound, not dry, is contaminated, it’s dusty (especially when bonding), then you are going to have issues with your screed.

If laying directly onto insulation rather than directly onto the concrete, and if the insulation isn’t stable, is rocking, is not tightly butted, the screed may breakdown in time.

Water

Too much or too little water? Earlier we covered not having enough water in terms of it drying out, but if initial mix doesn’t have enough water or has dried out too quickly,  some cement particles are not going to hydrate. Too much water in the mix will mean there is likely to be more shrinkage cracking, and weak screed.

The screed needs curing, and it is recommended that a curing membrane is laid over the top of the screed to keep the moisture in. If that doesn’t happen and the screed dries out then you will suffer from a lack of hydration, some cement particles are not going to hydrate.

Site Practice

It could be poor mixing practise. Sometimes screed is mixed on-site with just a shovel directly onto a board. Sometimes a  free fail mixer is used and what you end up with balling of the screed.  Neither methods are satisfactory.

Mix Ratio’s

Perhaps somebody got the mix ratios incorrect and there is not enough cement in the mix?  The screed could be failing due to a lack of strength.

The grading and quality of the sand is very important.  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. If you was to use  building sand, it would provide a lovely smooth finish to it but there would be no body and strength in the screed.  The sand can also contain high levels of silt and lignite, which can cause issues.

Compaction

Sand and cement screeds should follow the British Standard and be compact in layers of 50 mm. Compaction in layers is essential, otherwise you will end up with a nice surface but then underneath it is weak and starts to breakdown.

Early Trafficking

Early trafficking of screeds is something that often causes a breakdown. Foot traffic, or loading of the screed, point loading from things like ladders, will ’cause damage to the matrix of the screed.

Stress Relief Joints

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 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.

Movement Joints

Did the screed have movement joints in? A movement joint is different to a stress relief joint, and is to factor in the movement of the building. If the substrate has a movement joint, it must be mirrored in the screed, otherwise you may find that it will actually end up putting a movement joint in itself and cracking.

Edging Strips

Whilst the screed is moving (shrinking), it is important that you have edging strips in place.  Anything that the screed touches vertically be it walls, columns, anything that stops the screed from moving needs to have edging strip wrapped around.  The edging strips can be compressible foam or insulation, it just needs to have some give to allow it to move.

Non-wearing Screed

Levelling screeds are generally non-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, and is often caused at this point when protection isn’t used.

Thermal Cracking

Another type of cracking is thermal cracking, due to thermal movement. The screed only will move twice, it moves initially during shrinkage and then the product is deemed to be inert, not moving by itself only from external forces.

Heated screed or the building moves it in some way heave, or some other stresses that are placed on the screed, are the only ways in which the screed will move once the shrinkage process has completed.

If it’s thermal cracking, it could crack just when the heat is introduced because there isn’t sufficient joints in the screed alternatively, it could be that you have two areas that are being heated at different temperatures. This could cause differential thermal cracking because the two areas of screed are expanding at different rates.  Cracking can occur in between rooms that have been operated at different temperatures.

You can see that simply replacing the screed can purely remedy the symptoms, and not solve the cause.

Repairs

Depending on what the actual cause, you may be able to repair the screed. When talking about cracks, the British Standard states, generally hairline cracks don’t necessarily need to be repaired.  If you have a hairline crack, the screed is stable on both sides of the crack (no wobbling), there would be no point in repairing unless the screed has become isolated, and it can be left alone. The rule of thumb normally is half a mm (credit card width),  you might then look to use a repair compound to remedy the crack.

To repair a hairline crack is quite extreme,  you would have to open up the crack to around 2/3mm with a grinder.  This would then allow a epoxy resin to be poured into the crack.  Effectley you have to damage the screed to then repair it, hairline cracks in the main do not need to be repaired.

Certain shrinkage cracks may also left, when a floating floor covering is being laid for example.  If the screed is heated, the crack may be opening and closing due to thermal movement, and if the floor covering is bonded a repair may be required, however take into account if the screed is able to expand and contract due to thermal movement.  You could repair the crack in a heated screed, and then it cracks again as there isn’t a movement joint, and the screed cracks itself.

Which Product is Required to Repair Cracks?

To repair cracks a low-viscosity resin is normally used. There are other products on the market but this is the most widely accepted and easiest product to work with. If the crack is fairly wide, you would look at bulk filling the crack with kiln-dried sand and then pouring the resin in. Leave it to settle for 10-15 minutes and then top it up again, wait again and if you needed be top up again.

Penetrating Resin

What can be used for a weak screed that has started to break up?  Does it need to be replaced? Or can I use a penetrating resin?

If using a penetrating resin, it would be a low-viscosity resin that seeps into the matrix of the screed.  The screed needs to be open textured to be able to do this, if the surface is closed it may need some mechanical preparation to the surface of the screed to open it up. The resin pours in and solidifies and making the surface of the screed a lot stronger and increases the overall strength.

That is one way of dealing with the issue, however 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, however you should consider is it only localised damage, and therefore a only that section needs to be removed.

An area that gets damaged the most, is the entrance to a building, taking the most foot traffic, and other loads.  In this example it could just be the case of replacing a metre square than the whole of the screed.

Check if the the damage is localised or if there is an inherent problem throughout all of the screed. Generally spot repairs are going to be the most cost effective and gets the project moving quicker without having to replace the whole of the screed.

So I hope that answered the question.  The subject is very complex, and needs to answered on a project by project basis.  If you have any further questions and want any further advice then just please contact us, and we will be happy to help.

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