Patio slabs not sticking

onionsa · 20 Apr 2019

Got quite a few days through laying a patio and have found that some of the slabs aren’t sticking. A couple are wobbly which we can’t see why as they’re on a full bed. 6:1 sharp sand damp mix.

I’ve got some on a step with a 4:1 builder sand and they are stuck solid. Is it worth lifting the entire patio, skimming the bed and lying on a different mix? It feels like the slabs should bite the mix but they don’t seem to be. Have I done something wrong or missed a step?

Thanks.

23vc · 20 Apr 2019

Had the same issue years ago. Never sure exactly what I did wrong, other than possibly not soaking the back of the slabs beforehand. Indian sandstone flags, solid sharp sand cement bedding mix, not too wet.
I fixed it by sticking down the few that were loose with a cement and pva slurry.
SBR is probably a safer bet, but this was 5 years ago and they’re solid as a rock.

onionsa · 20 Apr 2019

How did you do the cement and pva slurry? Would you mind giving me the directions for this and it could save me quite a bit of time if I can ‘lift and stick’ the slabs without the need for digging out the bed and relaying.
Thanks.

23vc · 20 Apr 2019

mixed neat cement and PVA [/SBR], painted it/smeared with trowel all over the back of the slab or the bed, put slab back down. Just be careful not to get any of the mix on the face of the slab. The ones stuck like that are more solid than the ones done without.
also - I reckon walking on them too soon may have caused it too. Good luck

23vc · 20 Apr 2019

+ wet the back of the slab too

onionsa · 20 Apr 2019

Thanks very much. I tried to keep people off for 24hrs. Have just been and got some PVA so will give this ago. Fingers crossed! Thanks for your help.

noseall · 20 Apr 2019

onionsa said:
Thanks very much. I tried to keep people off for 24hrs. Have just been and got some PVA so will give this ago. Fingers crossed! Thanks for your help.

PVA is no good. You need an SBR slurry. We always coat the back of copings with SBR/cement slurry.

23vc · 20 Apr 2019

Agree, sbr better (as mentioned to the OP). But in my case, pva worked. As in rock solid 5 years later. Not sure why, but I’m not complaining.

noseall · 20 Apr 2019

Generally splashing with water works with concrete products. SBR is for smoother impervious stone products.

My guess is the slabs have been disturbed or not locked In.

onionsa · 21 Apr 2019

What does 'not locked in' mean?

I know for sure that I wasn't wetting the slabs - this was not something I'd taken note of from the many how-to's I've read in preparation for this project. What I've done now is to shave the bed. So taking off around 15-20mm of the previous mix and then making sure the bed and slabs are wet when they're laid. I got a few down like this at the end of yesterday but guess time will tell how they're adhering.

noseall · 21 Apr 2019

onionsa said:
What does 'not locked in' mean?

If they are butted with zero joint, then any gaps will mean they are vulnerable until you fill with say kiln dried sand. Full joints increases the stability strength of the slabs.

onionsa · 21 Apr 2019

Ah ok that makes sense, thanks for clearing it up.

John D v2.0 · 21 Apr 2019

You don't need to stick paving down, cement it's not like glue. Gravity holds them down but you do need to make sure they are fully evenly supported and not rocking or sliding about.

23vc · 21 Apr 2019

John D v2.0 said:
You don't need to stick paving down, cement it's not like glue. Gravity holds them down but you do need to make sure they are fully evenly supported and not rocking or sliding about.

For big concrete flags maybe, but for the mixed size/smaller/uncalibrated natural stone products there’s gotta be a degree of adhesion, or they’ll effectively be loose. Hence recommendations to wet or SBR the backs.

onionsa · 21 Apr 2019

These are 600x400 slabs just for reference. I think I'd rather have the adhesion and not as they're not yet pointed with kiln dried sand.

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Patio slabs not sticking

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