# Forum Home Renovation Concreting  concrete blend..

## breakerboy2000

Hello, I need to mix concrete for a 1800x1000 slab. 
my local landscape place sells a 'concrete blend' of 10mm aggregate and sand, so you just add cement and water to it, 
but would this be an ok option for my slab? and what ratio of cement to the 'blend' would be best as im not sure of the premixed ratio of sand and agg in this blend. 
thank you for any replies.

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## ringtail

Yep its a good option if you have a mixer. I generally use 3 buckets of blend to 1 bucket of cement and add water to suit ( slowly) to make a stiff mix. Its really important to use a constant measuring device like a bucket. Do not use a shovel. Still hard yakka but miles cheaper and better than bags.

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## Belair_Boy

> Its really important to use a constant measuring device like a bucket. Do not use a shovel.

  I totally agree with Ringtail here, you need to be precise with your measurements to get consistent concrete.
I have found that the premixed  'concrete blend' is rather variable in the sand/aggregate ratio (the local depot just mix a scoop of each with the front end loader) and you can end up with a sandy or stony mix depending on the day.
It is ok if you are just bulk filling holes for posts etc but for a slab I would go individual quantities of each.  The price per tonne for the blend is much the same as the washed sand and aggregate individually so it is no more expensive to buy exactly the quantities you need. 
Also 10mm aggregate is on the small size for a slab (great for core fill though), 20mm is the norm and usually cheaper. 
The water cement ratio is also critical for concrete strength. Less water makes for stronger concrete and if you want 32MPa aim for 1:2 ratio water to cement (ie 20kg of cement use no more than 10 litres of water). 
Good luck and don't forget the reinforcing :Wink:

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## breakerboy2000

Thanks for those detailed answers guys, i m8ght just order the 20mm gravel and sand separetly, is there any good ways to work out how much in terms of m3 i should order? Cheers

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## Belair_Boy

> Is there any good ways to work out how much in terms of m3 i should order?

   It will depend on how thick your slab (1.8 m x  1 m) is going to be. 
If, for example it is 100mm thick:
1.8 m x 1 m x 0.1 = 0.18 m3 
So you will need about 0.2 m3 of concrete for each 100mm of slab thickness. 
You will buy your aggregate and sand by the tonne (or part there of) .... so ..... concrete is about 2.4 tonne per m3 
0.2 x 2.4 = 0.48 tonne  
So the combined mass of the aggregate sand and cement required would be about half a tonne. 
Hope this helps

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## breakerboy2000

thank you, 
but im wondering why I need to know the concrete weight per m3? as I will be doing the mix by volume not weight? and I can order sand and agg by volume? 
So, if total concrete vol is approx 0.2m3 with a 1:2:4 (cement:sand:agg) ratio for said slab,  
volume of cement= 0.2 / (1+2+4) x 1 =  *0.03m3* approx  
volume of sand= 0.2 / (1+2+4) x 2 = *0.06m3* approx. 
volume of aggregate= 0.2 / (1+2+4) x 4 = *0.12m3* approx.  
so could I just order those quantities per volume? i might have to pickup the agg myself from as it might be too small for them to deliver? 
and I could probably hit up bunnings for a few bags of sand and cement, as again, the sand may not be worth it for them to deliver.  
cheers.

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## Belair_Boy

> I can order sand and agg by volume?

  Generally you buy sand, aggregate and cement by weight not volume.
The volume will change depending on compaction (the amount of air trapped in with the solid) and is especially true for cement.  That is why you buy 20kg bags of cement and not 20 litres.  I have not found any suppliers that sell aggregate or sand by the m3, all have been by the tonne (or bag by weight).
You do have to watch for moisture content though.  Sand if wet will be heavier per m3 than if dry so beside paying for water it will need to be taken into account when adding water to your mix.
You will soon get a feel for the correct mix consistency in the concrete mixer but a small amount of extra water can have a big effect, so add water slowly especially if the sand is damp.
I cover my sand with plastic to keep it dry and I can achieve quite consistent results.
I do measure all my quantities by bucket (volume), but they have been calibrated by weight initially (using scales) so I know how many kg each one holds.  I have a separate bucket (of differing volumes) for sand, aggregate and cement, so mixing is easy, one bucket of each. 
It can get rather in depth when you start to take aggregate size and material density into account but for the general DIY project it is not so critical.
So long as you don't skimp on the amount of cement added, don't add too much water and don't overwork the screeding you are unlikely to go wrong. 
Good luck and happy mixing.

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