Weak Screed Cracking And Dusting. Is it necessary to replace the screed?

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Weak Screed Cracking And Dusting?  What Should You Do Next?

I'm Andy Parkin, Managing Director of the Multi Award-Winning Speed Screed. I'm here today to answer the question "We have a weak screed cracking and dusting badly.  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? Can This Cause Weak Screed Cracking And Dusting?

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. During the summer months a large number of weak screed cracking and dusting are caused by not curing.

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.Screed Knowledge Centre

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.

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