The Brew Cabin

Last year I laid 230 18" concrete slabs on a 9" deep reinforced concrete slab around a new swimming pool I've built.
The slabs are laid edge to edge with "no gaps" (1-2mm on average to be precise).
Following much and contradictory advice I used a lean mix mortar (5 blobs) to bed all the slabs into place.
All was well until the first frost.
Most slabs are now loose as the mortar simply crumbled.

Then I found this site and hope was rekindled.
Do I remove all the mortar and use sharp sand (2-3"), do I use a stronger waterproof mortar mix.
What do I do to stop this happening again?

What is NOT an option is to change the slab spacing as over 100 had to be cut to fit just right.

Please help.

If you use spot bedding, then it is only a matter of time before problems start to appear. I get really bloody angry at so-called professionals, and some of the manufacturers, who continue to promote this crappy, slip-shod, amateurish method of flag-laying.

You've been the victim in this case - some eejit told you spot-bedding was acceptable, probably because they had seen some arse on TV doing it, or they'd seen a picture in the brochures from manufacturers (who really ought to know better - are you listening Marshalls, Bradstone, et al??). If they spot-bedded flags on a building site or commercial project, they'd be led to the site entrance and given directions to the local Job Centre.

You only option is to lift the lot and relay on a full bed of a decent mortar. Normally, for patio work, I suggest a 10:1 mix, but as this is a pool surround, you need to use a slightly stronger mix. I'd suggest a 6:1 mix laid no thicker than 50mm. You can add a bonding agent or waterproofer if you wish, but, as all this is laid over a concrete slab, it isn't going to make a great deal of difference.

Butt jointing is not recommended for pool surrounds, or any other paved areas where there's lots of water, as it allows water to get to the bedding layer without hindrance. However, as this will be re-laid on a full mortar bed, there's no risk of bedding migration or degradation due to saturation. In an ideal world, I'd still rather use a mortar joint, but you're insistent that such an option is not up for consideration.

Even if you do re-lay with butt-jointing, there's a significant possibility that all the pieces you've cut will not re-fit exactly. If any of the joints are a bit tighter or a bit looser than the original joints, then the cumulative effect is to re-position the end-of-course cuts. You'll have to reconcile yourself to possibly re-cutting those edge pieces.

Of course, you could be lucky and find that everything slots back into place exactly, but if I was to let you think this was bound to happen regardless, I'd be leading you astray, and, from the sound of it, you've already had enough of that.

Good luck!

Thanks for such a prompt and informative reply!
Pitty I hadn't found this excellent site earlier.

I will relay the lot as suggested, cursing quietly throughout!
Settlement isn't an issue, only frost.
Consequently I am keen to make the bedding mixture impermeable to water - as any water getting in can't drain down through the concrete.

Consequently, I am thinking of making a strong sharp sand & cement mix (with additive - pva?)and making it wetter than usual (stiff mortar consistency) - as "dry" mixes set fine, but can stay porous.

Does this make sense - or am I in spot bedding territory?!

Any advice gratefully received.

PVA doesn't add any real advantage to your bed. If you want to lay on a wet mortar (which I personally find awkward and unsatisfactory) then a simple plasticiser is all that's needed. Also, making a stronger mix doesn't improve water or frost resistance.

I'd say that if you were to use a 6:1 mix, with added plasticiser, you'll get a decent 'set' that will bond to both the concrete base and the underside of the flags, thereby minimising the risk of water penetrating down to the bed/base interface.