New sandstone drive - slabs lifting, what’s wrong?

[Teafortwo]

Hello,
First may I say how immensely useful and informative this site is and how wonderful to find expert knowledge and experience generously shared.

We are in the process of having a new drive and path laid in sandstone flags. Flags went down last week and over the weekend we found more than thirty loose, i.e they can just be lifted by hand. It is obvious even to our untrained eyes that there is very uneven contact with the mortar.

Previously the drive was tarmac which has been removed and I give the guys top marks for hard graft as there was a double layer over an area 7 by 5 metres plus the path to the front door (two 8 yard skips worth). The existing sub base was deemed to be pretty solid so that was whacked down then another layer of finer type material added and again whacked down.

Contractor has then proceeded to bed the slabs using what I would describe as concrete, ie a mix of coarse pebbly material and cement laid to a depth of 50 mm approx, with a fresh batch mixed for each slab or two in the case of smaller sizes. Flags were laid straight on to this dry, no primer or wetting. Two days in we had rain all day which got torrential in the afternoon to the point where work had to stop and inevitably some of the slabs were walked on in the rush to tidy up. That is the reason the contractor has given for the loose ones though he admits that he doesn’t understand why some which were laid over the next two days have come loose. The weather since has varied –nights where temperatures have dipped enough for frost and warm dry weather in the days.

We are now very concerned about the future integrity of the drive and its ability to take the weight of a car. We had a long discussion with the contractor this morning and he has promised to put everything right – but if there is a fundamental error in the way the job has been undertaken I haven’t much faith.

Any advice would be greatly appreciated.

[lutonlagerlout]

for indian stone I prefer a slightly wetter mix with the addition of SBR , and if he has used concrete with 20 mm aggregate it should be at least 100mm think of bedding
picture may help but it sounds a rip up and relay job
LLL

[digerjones]

I would agree, doesn't sound too good, its not the ideal material for a drive either.
Like LLL says, some pictures would help

[seanandruby]

If the levels would of been low enough they maybe could of left tarmac as a base. As LLL says: will
be a complete "rip up".

[GB_Groundworks]

Is it 22-25mm stone not really suitable for a driveway and should be bedded on 100mm of concrete mix with a sbr cement primer applied to the back. Sounds like your guys need an afternoon with a brew and reading the wealth of knowledge on this site.

[Tony McC]

50mm depth of bed is not enough for a driveway, unless it's 50mm of mortar over a bound/rigid base or concrete or macadam. %0mm of mortar/concrete over an unbound/flexible sub-base will crack when trafficked, and as there is, reportedly, no adhesion of the flags to the bed, then the whole lot will start to move.