Having received excellent advice recently in another thread, I thought I would ask for your indulgence once again. Here's the problem first, the explanation + question (if it isn't obvious) follows:
Problem:
My bedding layer is crumbly (I can break it with my fingers) and has not adhered to the slab beneath, nor have the stones adhered to it particularly well.
I am in the process of removing it all to redo it. Thankfully it was only a small section of my overall project!
Explanation:
I am covering a patio with Solnhoffen crazy paving. I have been helped for most so far by a professional mason, who does everything by eye. Wanting to do a bit myself as he is unavailable for a few days, I reproduced his approach exactly, except for the bedding mix proportions, which I have been unable to determine as he does it without measuring anything. I therefore consulted this site and took the 8:1 proportions stipulated on the bedding mix page.
I already have a concrete slab upon which I am spreading my bedding layer.
I made the 8:1 mix, and I was careful to mix the sand and cement completely before adding the liquid. I used a plasticiser in the water (SikaLatex), and made the mix quite moist (not wet). It was immediately clear to me that my mortar was considerably sandier than that which my mason friend has been making...
I wet the slab and spread a bit of this mix into the water, and across the slab surface first, then I levelled my bedding appropriately. It was a hot day, and the work site is in direct sun.
I sprinkled the surface of the bed with cement (someone has already told me that this is not an ideal approach, but this is how my mason is working, and it has worked well for 90% of what we have done so far... The Solnhoffen stones are very flat, like tiles). Then I wet the individual stones before placing them and tapping them to level.
Working alone, inexperienced, this did not go super fast, and my bedding began to dry out. I remoistened it once with a fine spray of clean water.
And it's a disaster! As I said, it didn't adhere to the slab, and the individual stones, although not "loose", are very poorly held, and easy to unstick with the lightest lateral impact for the most part. Some of them come off clean, others have grabbed onto the cement layer, but this had pulled free from the bedding. I am removing what I did now, and the chunks of bedding that come out look sandy and I can crush them in my hand.
The rest of the project, done by my mason friend, is solid and well adhered.
Question
What did I do so wrong?? The mortar is clearly way too lean... Did I misunderstand the mortar instructions? (Standard Bedding Mortars)
how much water are you using, i always lay on a wet mix not like bricklaying mortar but wet enough to be paddled nicely by the mixer so it acts like a liquid in the mixer to get the chemical reaction going with the cement you need enough water.
then your adding further cement to the top which is pointless apart from further drying out your mix in the sun,
are you using course sand (grit,sharp,plastering etc) as opposed to the fine bricklaying sand?
you can inprove your mix by adding SBR into the mix or mixing a slurry with neat cement and sbr and painting it on the underside of each flag, i lust add a healthy glug to the gagging water maybe 0.5 litre to a 4 barrow large site mixer with 2 bags of cement in it.
by large mixer i mean like this
Giles
Groundworks and Equestrian specialists, prestige new builds and sports pitches. High Peak, Cheshire, South Yorkshire area.
Thanks for your reply. Let me answer your questions in order:
I am hand-mixing (wheelbarrow + shovel for this particular mix)
I add water until the consistency looks like that which we have been using up to now, which is quite moist. The mix that failed was initially moist enough to have a film of water rise to the surface when troweled.
Adding dry cement is pointless, you say... Hm. You are the second person on this site to tell me this. The problem is, the mason who is helping me is of the opinion that if he doesn't do that, the flags won't stick at all... I am unable to take sides in this as I have no experience. I am surprised by such diametrically opposed opinions, however... :0 Why would he think this, if, as you say, he is totally wrong???
I am using course (quarry) sand. I live in France, (Charente Maritime) and the standard concreting sand here is called Sable de Cadeuil, which is what I am using.
I am using a plasticiser called SikaLatex, which seems to be the same kind of beast as SBR. I am using the same approach as you suggest: a healthy slosh added into the mixing water.
(I have now finished cleaning off the failed section, and I could remove the bedding layer with a hand-held chisel. That's how much it didn't adhere!)
Thanks for your reply. Let me answer your questions in order:
I am hand-mixing (wheelbarrow + shovel for this particular mix)
I add water until the consistency looks like that which we have been using up to now, which is quite moist. The mix that failed was initially moist enough to have a film of water rise to the surface when troweled.
Adding dry cement is pointless, you say... Hm. You are the second person on this site to tell me this. The problem is, the mason who is helping me is of the opinion that if he doesn't do that, the flags won't stick at all... I am unable to take sides in this as I have no experience. I am surprised by such diametrically opposed opinions, however... :0 Why would he think this, if, as you say, he is totally wrong???
I am using course (quarry) sand. I live in France, (Charente Maritime) and the standard concreting sand here is called Sable de Cadeuil, which is what I am using.
I am using a plasticiser called SikaLatex, which seems to be the same kind of beast as SBR. I am using the same approach as you suggest: a healthy slosh added into the mixing water.
(I have now finished cleaning off the failed section, and I could remove the bedding layer with a hand-held chisel. That's how much it didn't adhere!)
charriso try my method when you have screeded out your sand cement mix mix some cement and water same consistency as batter mix for yorkshire puds pour on area where slab is going tap in down center only put your hand on outsides of slabs any splash marks wash of straight away with sponge then pick one back up you will see how much of the beding mix comes up stuck to paving cheers cookie :p ps am a better teacher than a writer :p
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> and although the site mentions 8:1 i never use a mix that weak
Yeh, neither does anyone else I have spoken to... Makes me wonder why the site mentions it.... The mix we are using (having finished off with the mason today, and paid closer attention) is 3:1... I'm not surprised 8:1 made a biscuit base! Good thing I didn't try to apply this advice to my whole project (on my own...) ???
It seems to me that the failure of my bedding was due to:
1) the proportions (8:1) being way too weak
2) the environmental conditions (hot, dry, direct sun) causing it to dry too fast (exacerbating 1)
3) my neat cement layer being too stingy (not enough), which is why many of the flags I had placed broke off clean.
About (3), a couple of folks here have indicated that this spreading neat cement across the levelled bedding is bad idea, or "pointless". I don't see why. There is ample moisture in the bedding to dampen the cement — and the flags are wet when positioned — making it into a pure cement bond bridge (and the water it is taking up from the bedding material has a bonding agent — like SBR — in it). The effect is not far from Cookie's suggestion (above) of a cement slurry painted on the back of the flag. Those who read my first post will know that I did have a couple of stones that didn't adhere at the beginning. Subsequently we made the bedding mix wetter, and that problem went away. All the flags now appear to be held on quite firmly, and the bedding is solid.
The mortar is clearly way too lean... Did I misunderstand the mortar instructions? (Standard Bedding Mortars)