Poured concrete screed preparation

[J K Tiling Cornwall]

Hi everyone. I have a floor tiling job coming up that is a poured concrete screed over piped underfloor heating. The screed has been down for many months (completely cured and underfloor heating has been ticking over on low). My issue is that the surface is quite smooth with a light sheen. Would this need to be mechanically abraded prior to laying a decoupling membrane? Can't find anything on surface preparation for poured concrete screeds. Will this be the same as surface laitance on a anhydrate screed? Or is this just the surface finish (similar to levelling compound)? Any help or input would be greatly appreciated 👍

 

[Dave]

Sometimes its the curing agent that gives it that shine. I always get them sanded to be 100% sure of a good bond.

 

[J K Tiling Cornwall]

Sometimes its the curing agent that gives it that shine. I always get them sanded to be 100% sure of a good bond.

Thanks Dave, my gut instinct was to key it up. Just didn't want to make additional work if it wasn't needed. Cheers 👍

 

[Huddersfield Tiling]

Sometimes its the curing agent that gives it that shine. I always get them sanded to be 100% sure of a good bond.

It’s called laitance: a weak surface layer that results from a poorly mixed (weak and watery) cementituous or calcium sulphate subfloors.
It’ll be flaky and easily lifted/scraped into by a sharp blade or strong tape. This should not under any circumstances be tiled onto without mechanical abrasion to remove such a layer prior to priming and tiling.

 

[Huddersfield Tiling]

Sometimes its the curing agent that gives it that shine. I always get them sanded to be 100% sure of a good bond.

Hi Dave,
I’m fairly sure that laitance (fines found on the surface) is a deposit that forms when too much water is added to a screed mix. The screed companies want to save costs and make it as weak as possible. Water is cheaper than bags of product after all.
I’ve seen some total sh*T screeds in my time.

 

[Huddersfield Tiling]

Thanks Dave, my gut instinct was to key it up. Just didn't want to make additional work if it wasn't needed. Cheers 👍

You’ll have to grind down the entire floor to remove the weak surface layer. Big job!

 

[Dave]

Hi Dave,
I’m fairly sure that laitance (fines found on the surface) is a deposit that forms when too much water is added to a screed mix. The screed companies want to save costs and make it as weak as possible. Water is cheaper than bags of product after all.
I’ve seen some total sh*T screeds in my time.

On the Agila screeds ( cement based ) a curing agent is sprayed on. The newer ones have it inbuilt now. The buggers keeps changing bloody things each yr.

 

[MultiHire Power Tools]

Hi Dave, a light grind with 30 grit diamond grinding shoes will do no harm whatsoever and will give you the key on the surface that you need to enable the next product to bond. Which machine depends on the size of the area. If you're stuck with 110-v or 230-v there's only a couple of machines but for a big area 3-phase is best. Have a look at the range click Stand up Archives - https://www.multi-hire.co.uk/product-category/hire/diamond-floor-grinders/walk-behind-grinder-hire/ Good luck.