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Author Topic: Cement Mix  (Read 45870 times)
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herbie
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« on: October 01, 2009, 06:48:57 AM »

I'm really confused as to what ratio I'm supposed to mix sand and cement for different jobs. Is it always 3 to 1? For example, I'm laying a patio with flat stones we're finding in the garden. I need to put a 'slop' in the joints. I'm concerned because it gets really cold here and I don't want it to crack. What should the ratio be for this?

Also, I made a slab of concrete with a hole in to place on top of the chimney stack so that the Inox tube can be fixed to it (the chapeau will fit on top of this). I made it 3 to 1 ratio and placed a wire grill inside. It's just over an inch deep but 3 days later, it still doesn't seem to have set hard. It looks like it's still soft in the middle as when you try and pick up the corner, it begins to 'flex' and would crack. What have I done wrong?

Does it matter which sand I'm using? It's a little rough, not the very fine stuff.
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Thumper
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« Reply #1 on: October 01, 2009, 08:53:07 AM »

Oh, that's a question and a half Herbie! Ratios vary depending upon the application, and nobody seems to agree exactly what they should be. Also, the specifications (usually shown as a "C" plus a number) have changed recently, so two systems are now being used side-by-side, both employing a C-rating, but each meaning something different. It's very confusing,

The amount of water you add also has an effect on final strength, and the rule of thumb is that the mix should have the consistency of warmed marzipan when ready. Too sloppy and the mortar/concrete will crack as it dries.

Here are some ratios for Concrete Mixes:

C7.5 (low strength) 1:3:6 or7 (Cement/Sand/Coarse Aggregate)
For general non-structural use – bedding in kerbs, posts, stabilising underground pipes etc.

C10 to C15 (medium strength) 1:4:6 to 1:4:5 (Cement/Sand/Medium Aggregate)
Used in typical house foundations, footings for garden walls, load-bearing areas etc.

C20 (strong) 1:2:4 (Cement/Sand/Medium Aggregate)
Used as a footing mix in house construction in softer ground. Also as the slab foundation to floors, bases for caravans and pathways, hard landscaping.

C25 (stronger) 1:1.5:3 (Cement/Sand/Medium Aggregate)
Can be used for foundations to larger houses and for creating floors. Can also take light traffic. Also suitable for lining pools and fosse septic.

C30 (very strong) 1:2:3 (Cement/Sand/Fine Aggregate)
A general-purpose, easy-to-remember mix for many hard-wearing applications.

C35 (industrial strength) 1:1.5:2.5 (Cement/Sand/Fine Aggregate)
Structural concrete for major construction work and roadways

C40/45 (atomic bunkers) Major civil engineering projects, bridges, skyscrapers and unlikely to be needed for domestic use!

Here are some ratios for Mortar:

General Purpose Mortar, for re-pointing a stone wall: Use a mix in the ratio of 1:3:0.25 (Cement/Sand/Hydrated Lime*)

General Purpose Mortar, for re-pointing a brick wall: Use a mix in the ratio of 1:4 (Cement/Sand)

Specialist Mortar for Soft Limestones, such as Tuffeau. 1:1:5 (Chaux Blanche (hydraulic, hard)/Aerienne Chaux (soft)/sand)

(* The addition of Lime makes the mortar more pliable and less likely to crack. Alternatively, add a "plasticiser", which is usually available in sachets or a plastic tub from builders' merchants.)

This is probably the best page I’ve found for explaining all this:

http://www.pavingexpert.com/mortars.htm
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Grampy John
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« Reply #2 on: October 01, 2009, 09:11:46 AM »

Yes, sound and comprehensive advice from Thumps, as you might expect; my only comment, you don't need to buy sand and stones seperately; in England, you buy "1/2 inch-to-dust", here "graviers" (have a good look first,see if it "looks right".
                                GJ
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Thumper
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« Reply #3 on: October 01, 2009, 09:23:04 AM »

Sorry, I didn't really answer your specific questions Herbie

Quote from: herbie
I'm really confused as to what ratio I'm supposed to mix sand and cement for different jobs. Is it always 3 to 1? For example, I'm laying a patio with flat stones we're finding in the garden. I need to put a 'slop' in the joints. I'm concerned because it gets really cold here and I don't want it to crack. What should the ratio be for this?

That's possibly a little too much cement for the sand, so you could add more sand, but don't make it too "sloppy". I know it's tempting to make it runny, so it flows into the gaps easier, but it will tend to crack more easily. If you're filling in the gaps between slabs that have already been laid, you could use a "dry" mix.

Quote from: herbie
Also, I made a slab of concrete with a hole in to place on top of the chimney stack so that the Inox tube can be fixed to it (the chapeau will fit on top of this). I made it 3 to 1 ratio and placed a wire grill inside. It's just over an inch deep but 3 days later, it still doesn't seem to have set hard. It looks like it's still soft in the middle as when you try and pick up the corner, it begins to 'flex' and would crack. What have I done wrong?

I think you've used a 'mortar' mix for this instead of a 'concrete' mix. For something as structural as the slab you're describing, you will need to include some 5-8mm grit/gravel to give it strength. The wire mesh is great, but probably won't be enough. I'm assuming you've  made a wooden mould? A sheet of old but clean melamine, perhaps lightly waxed with furniture polish, with a few battens nailed down to make the shape, size and depth/thickness you want. Put something round in the middle where you want the hole to be - plastic may be best, but anything that's the diameter you need - and secure it to the board. Then lay in a tranche of your concrete mix to a depth of about one centimetre or so and tamp it down. Position your metal grille around the thing that's creating your hole, and trimmed to fit to within a centimetre of the battens around the edge, and then fill in with the rest of your mix.

How thick is your slab? If it has to spread across the top of a wide chimney, it may need to be about 4 or 5 cms thick, in which case you could probably include a second metal mesh. Then, once you've filled the mould, use a length of wood resting on the tops of your battens to smooth off the top (or what will be the bottom) of your slab, tapping down to puddle out the water and achieve a smooth finish. You may need to leave it several days to set. Then break off the battens (easier if they were screwed to the melamine sheet!), remove the circular tin or whatever, and the finished slab should slide off the smooth melamine.

Quote from: herbie
Does it matter which sand I'm using? It's a little rough, not the very fine stuff.

Rough is fine, if you see what I mean!
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herbie
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« Reply #4 on: October 01, 2009, 11:19:49 AM »

Wow! This is comprehensive, thanks so much. I need to digest some of this.  So, I've probably used too much water when I poured my mix into the mold and this is why it's cracking. I think the sand/aggregate I'm using is good as it's actually gravier that I'm taking the larger stones out of. I'm still not too sure as to the mix I should use since the sand and gravel are mixed together.

I initially thought that using a dry mix would be better on the terrace but someone suggested using a 'slop' but seeing what's happened to the neighbour's terrace already, and it was done by a builder (and it's not even cold yet!), I'm thinking that a dry mix makes more sense. 
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herbie
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« Reply #5 on: October 02, 2009, 06:37:59 AM »

Going to have another go at this today. The last slab fell into pieces! lol I'll let you know how it goes.
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Thumper
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« Reply #6 on: October 02, 2009, 08:05:41 AM »

Just one more thing to remember. Your Inox tubing will expand lengthways - possibly by several centimetres - when the fire gets hot, so make sure you leave enough of a margin around your hole in the middle so that the tube can slide in and out as it heats and contracts. By the time it gets to your concrete collar at the top it won't be expanding in diameter, so you don't need to leave a lot of space around it, but just enough to stop it snagging. 
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herbie
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« Reply #7 on: October 02, 2009, 13:12:46 PM »

Thanks Thumper. I think we'll be OK because it's a flexible 'gaine' so when it expands it should just bend sideways. We'll leave plenty of margin at the top anyway though.
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herbie
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« Reply #8 on: October 05, 2009, 07:57:36 AM »

I've taken some pictures of our patio in progress. As you can see, it's a real 3D puzzle!

http://cathysavels.com/blog/viewpost/78.html
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Thumper
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« Reply #9 on: October 05, 2009, 08:19:06 AM »

Clearly a woman of many talents . . . but has the new concrete top to your chimney set OK this time?
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herbie
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« Reply #10 on: October 05, 2009, 12:14:24 PM »

Thanks Thumper. We were just about to put the mold together again and start mixing when our neighbour came round and said that what he had done was to simply buy a bag of mixed mortar.  We held off and think we may do the same. It may save a lot of wasted time! lol
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