Stepping Stones, Stones are Moving, Wrong cement mixture?

[Lordbenny]

So, I cleared an area at the side of my house (originally concrete) smashed up the concrete to make a hardcore base and covered in 2 inches of sharp sand. Then a covering of Mypex (weed protective material). I need to raise the stone slabs that I have bought so that they are level with the gravel/small boulders when I lay them. I cut a square in the Mypex an inch smaller than the slabs and put down some stones. Then I laid some sand/cement mix and placed the slabs on top.

Problem is that now they're dry the slab is just rocking on the dried sand/cement. I've obviously done some thing very basic wrong. Why didnt the slab stick to the sand/cement.

 

[r896neo]

It hard to figure out what exactly your trying to do from your post? You mention a 2 inch sharp sand layer which would suggest you are block paving it or using concrete slabs 450x450 or smaller.

Then you say you have cut out a hole in the membrane and put stones in?? what are the stones for?

Then you say you layed the mortar on this lot and then the slab?

Slab not bonding to the mortar could be due to the sand you used? Shapr sand mixes are good for large areas of contained paving but for steps or other free floating areas where you need the slabs to stick you should use building sand.

Lastly the other reason for not bonding would be the mortar drying out too fast which is what would have happened if you layed mortar on clean stone?

 

[Lordbenny]

It hard to figure out what exactly your trying to do from your post? You mention a 2 inch sharp sand layer which would suggest you are block paving it or using concrete slabs 450x450 or smaller.

Then you say you have cut out a hole in the membrane and put stones in?? what are the stones for?

Then you say you layed the mortar on this lot and then the slab?

Slab not bonding to the mortar could be due to the sand you used? Shapr sand mixes are good for large areas of contained paving but for steps or other free floating areas where you need the slabs to stick you should use building sand.

Lastly the other reason for not bonding would be the mortar drying out too fast which is what would have happened if you layed mortar on clean stone?

Ok, lets answer some questions!

I'm not block paving, just stepping stones

600 x 600 slabs

The stones are to raise the height so they are not under the height of the gravel/pebbles that will surround the slabs

I used sand/cement mix (just add water) on clean stone, maybe to dry?

S
Thanks for the reply

 

[LooPrEvil]

Benny did you compact the sharp sand down? If not, when you put weight on the paving stones it will compact the sand into the gaps between the hardcore until it has sufficiently settled, thus giving movement of the paving stone.

 

[Lordbenny]

Benny did you compact the sharp sand down? If not, when you put weight on the paving stones it will compact the sand into the gaps between the hardcore until it has sufficiently settled, thus giving movement of the paving stone.

My problem is that the sand/cement mix didnt adhere to the bottom of the slab!

 

[r896neo]

Like i said if you put mortar onto clean stone it will suck all the moisture out of it and dry out too quick which is probably why they didn't bond.

I would damp the slabs a bit before laying or for a certain grab brush a bit of sbr onto the back of them. You should try to make sure it's building sand in your mix though as that will give the best grip.

 

[LooPrEvil]

Benny although the mortar has not adhered to the paving stone, this will not really be the issue. If you have tamped the paving stones down, then this will remove any undulations. The stone should therefore be ok - something is moving below the stone, either your hard core / sharp sand, or the stone was not completely bedded down, and rocks when walked on.

 

[r896neo]

Whilst that is true for bedding slabs in a patio etc that are pointed it is not the case for steeping stones as they have free edges all around and can be displaced by the lateral movement of kicking into them and or people rocking their weight off the side of them.

However looprevil may well be right in what he has said because clean stone is not a good material to raise the level in this case as it is not self binding in any way. It will simply sprawl unless it is confined. You should really use sub-base (MOT) or concrete.