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14 messages
17/08/2014 at 10:51

Hi Everyone

yester I laid down my patio slabs with a mixture of 3 parts building sand to 1 part concrete. I made the mixture not too sloppy and basically could make a snowball out of it without it falling apart.

today I went to see how they had set and they seem pretty solid however, I tried to move a couple of the slabs with my finger tips from underneath and they came up. So my question is why haven't they stuck and what have I done wrong?

cheers

17/08/2014 at 11:25

don't worry guys I will pay someone to do it

17/08/2014 at 11:47

Might have been a bit too dry Cd. It should be wet enough that you can bed the slab into it and move it a little without it all squidging out the sides, but solid enough that it stays in a reasonable shape when you put the 'blob' down. If it's too dry there's isn't enough moisture to transfer to the dry surface of the slab - if that makes sense.

It's one of those things that's trial and error initially.

17/08/2014 at 12:22

Spraying the underside of the slab just before laying it as any dust will prevent the slab from sticking to the blob of cement underneath.

17/08/2014 at 13:52

And of course, fingerpoken ist verboten, naughty!

17/08/2014 at 13:56

What size slabs are they Cake dude? Normally just the weight is enough to hold them down if they are large. Like summerwine says,try to make sure the base is clean and the slab isn't wobbling after you've laid it. I always used to use sharp sand when I was in the building game but it shouldn't matter too much.

17/08/2014 at 15:35

Cake dude you made it too dry! You won't get them to stick down now, so I'm sorry but for the best finish you'll need to start again. You want your mix the consistency of porridge, sloppy enough to stay on the trowel or spade, but not sloppy enough to run off. It's difficult to get right first time.

19/08/2014 at 10:28

Yeah I made it so I could make a wet snowball out of them but not too wet so it falls through my fingers. Oh well I have so much on now i have a man coming in to clean up my mistake ! £200 for a stress free life lol

19/08/2014 at 12:53

Could you try running a hose gently over the whole area to further wet the sand/cement mix rather than lifting them. They should  'stick' over time with normal rainfall.

20/08/2014 at 08:43

Hogweed, you can't wet cement that's already set.

 

20/08/2014 at 09:52

Dave's right. He said it in a much more scientific way than I managed! 

A bit of practice was all you needed cd. it's easier if you have someone to show you though, then you get a feel for when it's right.

You'll know next time....

20/08/2014 at 14:10

I thought that was the problem - the mix was too dry to set! Sorry!

20/08/2014 at 16:46

The setting process of cement is irreversible up to a point and is dependent on weather conditions plus water content of the finished mix. In hot summer weather we would reconstitute (or knock-up as we called it) mortar when it was setting too quickly,but only up to a point because it interferes with that setting process.There's comes a time when you have to chuck it and mix a new batch.

Conversely,in winter the setting process is generally slower and quantity of water should be regulated accordingly. More specific to slab laying,generally speaking the larger the slab the drier (or stiffer) the mix can be,as too wet a mix can result in the weight of the slab simply displacing the mix. So generally - larger slab,drier mix - smaller slab,wetter mix 

20/08/2014 at 17:32
Hi you needed sharpe sand not building sand.

It helps to wet the bottom of the slabs as you lay them so they don't pull all the moisture out of the cement. Also the sharp sand allows water to drain through building sand holds onto the water.
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