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Posted: Tue Aug 14, 2012 7:09 pm
by SimonsPatio
All,
I was wondering if someone can help me.
My contractor installed my patio with a firm rubble base, sand and finally a concrete bed and laid the slabs on that base (the slabs are limestone)
We left the patio to settle for a few days and have been dismayed to notice a significant number of slabs are loose.
In fact I managed to pull a slab up with my own hands with absolutely no effort. It looks like the cement has not stuck to the slab (image attached).
The contractor says that they may not have cleaned the slabs on the cement side and this has led to the cement not sticking.
So, the question is, what do I do ?
Something deep down says that all of the slabs will have to be taken out and the job essentially started again. Is there any approach which would resolve the issue (which will last in the long term) without recourse to pulling the lot of them out
I've taken a photo of what was underneath the slab which i managed to lift by hand
Failed Paving
Posted: Tue Aug 14, 2012 7:35 pm
by r896neo
Hi first of all the sub-base makeup sounds unusual are you sure what went in and how much?
Rubble + sand = settlement
Whilst your loose slabs are not caused by this I would be nervous as to the long term stability of the base. Don't panic yet though as your mention of a concrete bedding course is also a little unusual and makes me think that perhaps you are merely describing it incorrectly and that may also be the case for the sub-base.
Anyway back to the slabs. To be honest initially the slab looks well laid with a good full bed and gritty looking sand. There are a few reasons as to why the slab would not bond to the bedding and dust is a possible cause. The other common one is too dry bedding mortar.
See this page
http://www.pavingexpert.com/bond_bridge_01.htm
As for remedies if the base is sound using an sbr cement slurry would stick them back down but be potentially very messy with what is a very easily stained stone.
I'm sure others may have ideas on how best to sort it.
Posted: Tue Aug 14, 2012 7:37 pm
by Carberry
Relay slabs on outside edge, use SBR in the mix to make sure they stick.
The slabs in middle will be held in place by the other slabs and pointing.
Posted: Tue Aug 14, 2012 7:39 pm
by msh paving
The link to pix dont work...,
was the slabs laid n a dry mix sand/cement if so the slabs wont stick, he needs to use a SBR additive to the mix and also mix SBR and cement powder and water then paint the backs of slabs to aid sticking them down tight,
as 1 slab has lifted there is a good chance more will follow,which will mean the whole lot will need relaying/bed taking up starting again,what has he pointed with?
MSH 
Posted: Tue Aug 14, 2012 8:25 pm
by SimonsPatio
All
I've tested the link a few times, and it appears to work for me. Hey ho, the vagaries of the internet.
Anways. he has pointed using a cement mix , and the pointing appears to have stuck well.
With respect to SBR is this what we need to get ...
Regards
Posted: Tue Aug 14, 2012 8:31 pm
by msh paving
The SBR is correct, same price as builders merchants,
got pix working , looks to dry to me, MSH
Posted: Tue Aug 14, 2012 9:18 pm
by local patios and driveway
Too dry, a wetter mix will work, ive not had any issues not using sbr etc.
Posted: Tue Aug 14, 2012 9:34 pm
by SimonsPatio
All
Thanks very much for the help.
Just one more question, will the old dry cement have to be dug out or would adding the SBR mix between the slabs and the base be sufficient?
Regards
Si
Posted: Tue Aug 14, 2012 10:12 pm
by local patios and driveway
Its got to go for a proper job
Posted: Wed Aug 15, 2012 7:30 am
by Tony McC
Where has this nonsense construction come from? Over the past couple of months I've looked at two jobs were the eejit contractor has laid sand over a sub-base, then laid a mortar bed over that.
Why would anyone lay mortar over sand? Has there been some methodology published in a brochure or other website that is showing this bollocks of an idea? It can't be coincidence that the same silliness has now cropped up three times in six weeks, can it?
Posted: Wed Aug 15, 2012 7:49 am
by SimonsPatio
I can only think that the sand was added to provide a level surface onto which the concrete is laid making it easier to lay the slabs
Not happy ....
i am about to tell the contractor it all needs digging out and relaying. .
Posted: Wed Aug 15, 2012 8:52 am
by London Stone Paving
Looks like it has been laid on a dryish screed to me, look how flat the bedding layer is.