Why is my patio moving? - And how to repair / rectify it?

[Timbo]

Hello all

We have recently had a large extension built and at the end of the build we asked our builder to lay a patio for us. The patio is approximately 8.8 m x 6.4 m in size and laid with Marshalls Wildwood concrete paving slabs (600 mm x 450 mm) onto a full bed of mortar. The work was undertaken during the first few weeks of the year and so the weather was cold on certain days with morning frosts, although the site is not really exposed being between two other houses. The sub base was a mixture of type 1 and crushed concrete bits and of course other building rubble left over from the bigger build so bricks, roof tiles etc which they then compacted down with a whacker plate. The patio was laid in a half brick bond pattern and the pointing contains a dye to match the colour of the slabs. Everything appeared ok until the summer arrived and with it hot dry weather which has resulted in around 80-90% of the slabs starting to rock at the corners slightly when walked on, enough to both hear and see. Strangely when it rains the rocking and movement reduces considerably, although is still present in some of the worst affected slabs i.e. where it meets the bi-folds doors into the house. I wondered whether this had something to do with the rain causing the pointing to expand slightly and reduce the movement? There are small hairline cracks along the pointing at the junction with the slabs and the movement has caused the pointing to break up in some areas. Also when tapped with a rubber mallet some of them sound hollow maybe indicating a gap between the underside of the slab and the mortar bed?. We are in ongoing discussions with our builder who is being difficult about sorting it out arguing that it has been caused by us walking on it although we did not use it for at least 2 weeks after the work was completed and then only in a certain place between A and B whereas the moving slabs affects the entire area although some at the edge ones appear ok. I don't think any of it should move and to fix it properly I suspect will require the whole lot to be relaid unless someone on this forum knows better? the builder appears to think some kind of patch replacement of certain slabs will do the trick but I'm not convinced and I don't want them coming back every 2 months. The installation instructions from Marshalls suggest that the 'backs of the slabs should be primed using a proprietary priming product or fine mortar slurry prior to placement' which I know that they did not use with the slabs simply laid onto a full mortar bed. The builder believes this is recommended for all of Marshalls products and is advisory only and not a mandatory requirement for concrete slab laying - again is that correct?

If anyone has any ideas on what is going on here it would be much appreciated as to be fair to the builder he seems equally baffled saying he has never seen this before and I would not consider them to be cowboys even if patio laying is not the majority of their work.

Many thanks in advance.

Tim

Photos here.....

https://goo.gl/photos/jopFoichkkCLNUds6

[dig dug dan]

We hear this tale time and time again on this site.
I cannot load those photos, but if the product is a natural stone, they SHOULD have had a bond briďge applied to the underside of the slabs. This is a mixture of sbr and cement slurry, brushed on prior to laying. Without this, the slabs will not bond to the bedding layer and will come loose over time. However, this is not normally required with a concrete type slab.
Perhaps lift a slab so we can see the bedding layer underneath. Did he do a full bed, or spot bedding, and what sand did he use?

[Timbo]

Thanks dig dug dan

The blocks are concrete and the sand used was a mix of builders and sharp. The mortar bed was a full and not spot.

Could it just be the pointing has shrunk? Or should they not move irrespective of the pointing?

I'm having trouble posting photos to this forum as my IT skills could be better! But maybe try the link below.

Thanks

https://goo.gl/photos/jopFoichkkCLNUds6

[dig dug dan]

Did you actually see them lay a full bed, or did they say they did?
I cannot fathom why they should be moving. Its possible that the ground under the subbase is spongey and moving, but if the correct depth subbase and compaction was done, then not a problem.
cannot see photos still.
what pointing was done? What mix or what product was used?

[Timbo]

Yes I know it's on a full bed as one of the slabs on the single step has come loose and it's evident by looking underneath. The builder also confirmed they had used a full bed and not spots or dabs.

Not exactly sure on the pointing materials other than I think they may have used either builders sand or kiln dried sand? Together with around 7ml of liquid cementone dye I think although I couldn't guarantee without asking them. I know it was put in reasonably dry (although still trowelable) as they didn't want to stain the slabs. As previoulsy stated the weather was Jan to Feb so cold.

Am I right in assuming that now the slabs are moving they can't be simply fixed by replacing the pointing and would require the actual slabs to be relaid? The slabs should surely be fixed down enough to not rely on the pointing to hold them in place?

This is a weird one for sure and it's interesting that it has you scratching your head as well. I wondered if it was to do with the cold weather maybe freezing the sub base and therefore expanding it slightly, the dry weather has then shrunk everything resulting in a small gap between the sub base and the underside of the slabs? Do tell me if I'm talking rubbish as although it was in the middle of winter and therefore cold we didn't have snow or very long periods of permafrost etc.

Fundamentally I assume something has moved but not sure what?

I don't understand the photos as both myself and the wife can view them fine on an iPad via the link provided. Can anyone else see the photos?

Thanks for your continued interest.

[dig dug dan]

I can only assume the water has seeped under the slabs, frozen and caused them to lift. Hopefully someone elsemmay have some advice.

[rxbren]

Can you lift any other slabs up in the main area rather than from a step?

[lutonlagerlout]

the step almost certainly has a full bed
for slabs on the main area to move my money is on spot bedding and/or a weak mix with no SBR
in fairness the lay out pointing looks pretty good
but slabs should not be moving after 6 years let alone 6 months!
LLL

[Timbo]

We could in theory lift a slab in the middle of the patio to see what is going on and whether a full bed has been used but I'm pretty sure it will confirm a full mortar bed and not spots or dabs etc.

Is it usual to be using SBR on concrete slabs? I thought that was more for use on natural stone? Although marshalls do recommend its use on this wildwood paving type but which I know the builder has ignored saying it's not necessary.

I'm not sure that they compacted the sub base in layers and maybe just compacted a single layer in one go, would that make much of a difference? even if it felt solid before they started laying the slabs.

[Timbo]

Given all of the above can anyone advise on the following:

If a patio moves does it then require completely relaying? Or could you simply repoint?

Do concrete slabs require sbr to be applied to the backs? People appear to have conflicting views on this one and I've no idea why?

Why would you not follow the manufacturers own installation instructions if they state the use of sbr.

Thanks to all

[Tony McC]

If a patio moves does it then require completely relaying? Or could you simply repoint?

There's a FAQ covering this very subject.

Do concrete slabs require sbr to be applied to the backs?

They don't normally *require* SBR but it wouldn't do them any harm. We only "require" SBR or other bonding agents on paving materials with low absorption rates, low enough to prevent the formation of a good bond to the bedding mortar.

Why would you not follow the manufacturers own installation instructions if they state the use of sbr.

Because a shockingly high proportion of users don't ever bother reading or following manufacturer's guidance. They *assume* they know all there is to know. On an almost weekly basis I am confronted by contractors, landscapers, builders, and DIYers who swear blind they followed the guidance only for it to be then shown that they patently didn't, otherwise their paving wouldn't be in the mess it is.

[Timbo]

Thanks Tony, your comments are very helpful.

Given the recent rain my patio is now hardly moving at all as the pointing (and maybe the sub base?) must have expanded. Which makes it more difficult to show the builder that we have a major problem. I'm not sure I want to wait until next summer for it all to dry out sufficiently for the slabs to move enough to convince the builder that it all requires relaying!

As a side question, does adding dye to the pointing mortar make it shrink more? Or was it put in too dry? Or maybe both?