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Cement question


Angie
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We recently laid a strip of concrete, it looks fine and is nice and smooth.  However, after it gets wet and when we sweep it there seems to be a thin layer of sand on top as though its finding it's way to the surface or is drying out on top.  As we are now going to tackle a larger area can anyone advise on this as we don't want it happening again?   Do we need to use more cement. Thanks in advance for any suggestions.
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4:1 I would have said and I would put some mesh in as well if I couldn't find some decent hardcore to put down first especially if your doing a bigger area. I would like to add I am no expert but thats how my father told me to do it and it's never cracked or exhibited the problem the OP has.
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Too much sand, possibly too fine. Should be 4:1 ratio and if you laid it recently, probably dried too fast with the heat and sun. Am afraid will continue to powder and eventually break up. You should also include reinforcement meshing too with a slab, this helps with weight and strengthens it.
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An 8 inch thick slab of mortar ( no large aggregate ) is, IMHO, a total waste of expensive cement. If you need load bearing you need concrete, with or without reinforcement. Mortar does not have the strength.

Any concrete laid in hot weather should be kept damp for a number of days.

Concrete cures ( hardens) from the internal chemical reaction. Too rapid drying out will weaken it.

A structural slab will reach 40% of full strength in 7 days and 100% strength after 28 days.
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8" slab - are you planning on parking a tank?

And yes, just sand and cement is no good. If you cannot get any hardcore then put some ballast in the base, run over it with a whacker plate and then lay the concrete on top - normally 4" is fine for a car.

Still, look on the bright side, you have only done one small piece and learnt the problems before laying the large area.

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Really appreciate all these answers.  We did put hardcore underneath the strip of cementing that we did and are going to lay some more in the larger area than we plan to do; we have also got the metal mesh to go in.  From what you've all said we used too much sand in the mix.  As its not such a big area we will probably lay another thin layer of the 4:1 mix of ballast and cement over the top and hope this cures the current problem. 

So pleased I asked all you fonts of knowlege before tackling the bigger project.  Going to start in 4 weeks time so it should be cooler and so hopefuly the concrete won't dry out too quickly.

Thanks again as always

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[quote user="Mary W"]

Really appreciate all these answers.  We did put hardcore underneath the strip of cementing that we did and are going to lay some more in the larger area than we plan to do; we have also got the metal mesh to go in.  From what you've all said we used too much sand in the mix. Possibly too much water to, if it's dusting  As its not such a big area we will probably lay another thin layer of the 4:1 mix of ballast and cement over the top and hope this cures the current problem. No it won't unless you get it back to firm non dusting concrete and use an admix like SBR/latex and even then it's doubtful if the remaining concrete is strong enough. don't bodge or you'll do it all over again. 

So pleased I asked all you fonts of knowlege before tackling the bigger project.  Going to start in 4 weeks time so it should be cooler and so hopefuly the concrete won't dry out too quickly. The wind/breeze can dry the top too quickly so cover with a tarp/bache for a day or two.

Thanks again as always

[/quote]
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  • 1 month later...
Making concrete is not difficult but you need to get the mix right, otherwise it will often age badly or not work all together. 1 m3 of concrete for a floor for example needs to contain 350kg of cement, thus around 10 to 12 bags per m3 of concrete. This is what i have used in new builds for suspended floors as well as concrete poured on the ground. As a general rule, when you mix put 30 - 35 spades of gravel/sand mix for one bag of 35Kg cement into the mixer. Dont over cut with water, as the more water you add, the more you dilute the mix and render it fragile. I usually aim for an homogeneous mix that sits nicely on the spade and does'nt fall off. Try to make concrete when its not too hot, and not raining....I often lay large surfaces in the evening during summer....and last thing, never mix up and less than 5C - the concrete will not 'cure'!!!....
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I'll answer that.

For foundations at 350kg/M3:

One 35kg bag of cement

9 buckets (90 litres) of sand 0-5mm

10 buckets (100 litres) of 20mm aggregate

17.5 litres of water (depending on how wet materials are)

Gives a mix of 140 litres

For reinforced concrete at 350kf/M3, walls, lintels, columbs and slabs:

One 35kg bag of cement

5 buckets (50 litres) of sand 0-5mm

7 buckets (70 litres) of 20mm aggregate

17.5 litres of water

Gives a mix of 100 litres

 

Note how much material is "lost" in the mixing!

155 litres of dry material mixed with another 17.5 litres of water ends up with 100 litres of concrete, I for one learnt this the hard way many years back when I bought 1M3 of ballast and expected the volume of concrete is not more when the cement was included, lessons learnt like that are never forgotten.

The quantities of cement  in red were omitted in the original posting!

 

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to be honest, when you need a large amount of concrete, i.e...more than 4m3, get it delivered by a local "centrale de béton"....Lafarge etc. They mix it up using computers and garantee the quality of the mix...you just have to pour it right. For smaller works, i'd go with a 3 for 1 mix...3 spades aggregate/sand for 1 cement...and as i said, easy on the water....all that said, that's how i mix on site with the cement mixer, if I want concrete for a suspended floor, pool etc....i'd get it delivered whenever possible...the price difference is around 10euros between self mixing and getting it delivered....its not cost effective to do large quantities yourself.
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[quote user="monsieur macon"]to be honest, when you need a large amount of concrete, i.e...more than 4m3, get it delivered by a local "centrale de béton"....Lafarge etc. They mix it up using computers and garantee the quality of the mix...you just have to pour it right. For smaller works, i'd go with a 3 for 1 mix...3 spades aggregate/sand for 1 cement...and as i said, easy on the water....all that said, that's how i mix on site with the cement mixer, if I want concrete for a suspended floor, pool etc....i'd get it delivered whenever possible...the price difference is around 10euros between self mixing and getting it delivered....its not cost effective to do large quantities yourself.[/quote]

MM, do you know the cost of readymix per/m3, It's been a while since I used Unibeton and that was ok ish but with the pump arm it's wasn't.

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me thinks that the price is around 120euros per cube delivered...which is a good price compaired to mixing it yourself....however, if you need the pump then they shaft you on the price...around 400euros plus a tariff for every cube....its a bit of a ripoff, but i havent found anyone doing it cheap......
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  • 2 months later...

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