Patio Issue - Concrete not set and swelled

[dan_l]

Hi,

I had a patio laid last year and all seemed fine during the summer but in the winter I noticed a few of the flags could move and I could pick them up, then over the winter some of the flags became raised (see picture). I have taken some of them up now and added a picture showing what the concrete is like now (very crumbly), do you know what is most likely to have caused this?

Thanks,
Danny

 

[HERTS P&D]

What caused it?................................bad workmanship.

Call them back.

Andy

 

[dan_l]

Thanks for the reply, any chance of being a bit more specific as to any thoughts on why the concrete would have gone like that?

 

[John D v2.0]

The concrete is just a filler for laying, the problem is with the base. They should have removed all unstable ground and put in well whacked hardcore to at least 75-100mm. If the ground was very dry when it was laid, and has became saturated, it may have swelled, which would break up any hardcore base.
If the ground conditions are really bad or the area is large, you need a concrete base with steel reinforcement and laid in bays, but for a small patio that would be unreasonable.

 

[dan_l]

I did think that about the base but looking at the bottom of the flags its almost like the concrete never actually set. Also this is a new build and we already had a path laid by the developers, 3 of those flags had to come up for power to go underneath and then were laid back at the same time as the new patio and with the same concrete and the same thing has happened to those 3 flags but all around them on the original path no issues which made me think it must be something to do with the concrete.
The base for the patio was quite deep with MOT and wackered well.

 

[dan_l]

Just to clarify the reason for the post, Im obviously getting the people back who did the job but I'd like to be armed with as much info as possible as to what has likely caused this.

 

[John D v2.0]

Does seem strange, maybe the original base was deep enough, and the new bit they just chucked a load of rubbish back in and a thin layer of concrete.
Don't worry too much about the concrete layer, it is just a filler and the strength comes from what's underneath. The laying course could just be sharp sand, the small amount of cement is just to stop it washing out around the edges.

 

[Charlie George]

Looks like the sub base is not deep enough, if flags are pushing up it won't be the laying bed, it also needs pointing, why was that not done, and any hot tubs i've seen are on a concrete sub base, lots of weight in one spot.

 

[Charlie George]

Just had a thought, how do you empty the tub?

 

[dan_l]

Thanks for all the replies. I get what you mean about the sub base on the new patio but if you see the pic attached this is the original patio which is fine but 3 slabs were taken up to run power and then put down a the same time as the new patio and these 3 swelled up and never attached to the comcrete but on the same base as the originals which are all fine.
Didnt point to fit in with the rest of the path/patio already laid.

 

[noseall]

Crap slabs. Crap bedding material. Crap laying. Crap prep too probably.

That is what it looks like to me.

 

[dan_l]

The slabs are bradstone are these crap? Can you elaboarate on the crap bedding and laying?

 

[noseall]

The slabs are bradstone are these crap?

Low end, budget slabs, yes. They can still be bedded correctly though, with wetter mortar and joints - not butted.

 

[dan_l]

Thanks for the reply, I wouldnt have chosen those slabs, the only reason Ive gone for them is because it's a new build and there are already quite a lot of them around the house, round the front etc and I don't really want to rip them all up as Ive only been in here a year, I just wanted to expand what we already have a bit. I did think the mix perhaps wasn't wet enough and that had been suggested by somebody else, regarding the joints, the rest of the path, and smaller orginals patio is butted hence the new part being butted and the original is fine, if butting them is there any other tips as I would like to stick with that with the rest being like that and the new part will be joining on to the original.

Thanks again

 

[^woody^]

It was the wrong sand for bedding. That type of sand does not stick to the slabs.