Loose slabs on wet mix

[dlowry_uk]

Hi,
This is my first post, although I've browsed quite a lot as a guest for advice on a driveway (successfully completed), and a concrete slab patio (problems!).

I employed a local contractor, and laboured to reduce costs, so I was onsite the whole time. He's done good work previously (driveway, fencing), is open about costing etc. Basically, I trust him.

Patio is 450mm slabs, about 20sq m. He dug out to a depth of 3-4in, levelled for drainage etc, and laid on a wet concrete mix (shovelled in dry and watered in situ, doing a couple of slabs at a time). Joints were brush-filled after with 4:1 drymix mortar and watered in with a fine spray.

He'd recommended a wet mix for solidity - didn't mention any possible problem with "window-framing" but that doesn't seem to be a problem anyway.

Big problem is that several weeks later, a large proportion of the slabs were noticeably rocking, to the extent that the mortar joints are breaking and lifting out. When i noticed the first one, I thought it was a 1-off (as did he) and he replaced it. We're both worried about the apparent extent of the problem now - I don't need another week of hassle, and he doesn't need a week of unpaid work to fix it!

I decided to leave it for now so we can have the use of the patio over the summer, but eventually something needs to be done. Any hints on what may have gone wrong and how to fix it? I stress that i don't have a problem with the contractor over this (unless he later refuses to play ball of course!), and he seems genuinely concerned and at a loss to explain what's going on. He only works locally, has a very good reputation as a fair man and quality worker, so its not in his interests to upset a local customer who's given him a lot of work over the years!

He replaced the first slab by breaking out the concrete below and relaying in wet mix. That one seems solid a couple of weeks on. I really don't want to have to break up the whole thing and redo. If the bedding slab is structurally intact, is there a way relay the loose slabs on it with minimal height increase?

Thanks
Drew

[bobhughes]

He dug out to a depth of 3-4in, levelled for drainage etc, and laid on a wet concrete mix (shovelled in dry and watered in situ, doing a couple of slabs at a time)

Did he lay a proper base as described on the site and compact it?

[dlowry_uk]

Looking at the patios page, theres a prepared sub-grade, with the bedding layer directly on top. Depends what "prepared" means in terms of the sub-grade I suppose. Theres no sub-base material. The site was dug out and the soil compacted. Concrete bedding was laid directly on the subsoil.

His argument was that the concrete bedding would form a solid slab with no need for an additional subbase. Since its only taking foot traffic, that seemed to make sense.

[bobhughes]

I expect you looked here before you started. The sub base needed for a patio depends on what the soil is like. If it is soft then it will sink and become uneven. It may be that the whole lot will have to come up and a sub base laid and.

[Tony McC]

The bit that concerns me is how the bed was prepared. You state that the bedding was ...

laid on a wet concrete mix (shovelled in dry and watered in situ, doing a couple of slabs at a time)

... watering in situ would wash through any cement content and leave a weaker upper layer of bedding. If he prefers to work with a wet mix, why wasn't it mixed wet and placed wet?

[dlowry_uk]

The soil is firm. I think the problem is as described by Tony. When he replaced the first loose slab, the concrete underneath had a sandy surface. This was put down to a a poorly mixed barrowful of drymix at that point (of course, I was doing the mixing!). It seems more likely that Tony is right, and the watering left a weak layer all over the surface.

Question is - what's the remedy? Complete breakup and relaying, or can the loose ones be fixed without breaking up the bed?

Thanks
Drew

[andpartington]

hi
This might work depending on what your base is like
mix some cement with water in to a slurry and paint on the slab and the base pop the slab back in place if your base is ok it will work (or did for me any way)
let us know how you get on
Andy

[dlowry_uk]

Andy,

mix some cement with water in to a slurry and paint on the slab and the base

We'd considered that, but were worried about the strength of a cement slurry. Presumably its ok if it worked for you - i assume the idea is just to fill gaps to stop rocking without an appreciable rise in level.

Another thought was to use Ronafix, but spec sheets for this type of application say a minimum thickness of 6mm. Anyone have experience of Ronafix?

Drew

[andpartington]

hi

[quote]i assume the idea is just to fill gaps to stop rocking without an appreciable rise in level.

yes thats about it you may need to add some normal beding mix in any realy low or uneven areas.
andy