Concrete

R

richard7761

I may need 0.24 cubic metres of concrete for footings.

What folks would do is buy 25Kg bags of something like Mastercrete Original Cement.

Assuming that is what people do (in contrast to buying sand, cement and aggregate) I'm not sure how many 25Kg bags I'd need.
 
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I have just bought a bag of ballast & 10 bags cement from Travis perkins
Cement £3.85
ballast £38.50
Delivered
 
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For seperates I also found this:

http://www.source4me.co.uk/calculate_concrete_mix.php

Be careful to enter Depth in millimeters, not meters. That got me until I figured it.

So, for the exact cu m which is 0.2352 (0.24) the calculator says the number of 25Kg bags I need is:

Cement:..........4
Sharp Sand:....6
Aggregate:......12

Total:..............22

That is for a ratio of 1:2:4. Which I think is the acceptable ratio for regular footings.

The total mass is 498Kg. Which is close to the 480Kg you get with 19.2 (20) bags of Mastercrete.

Will now try to get a price for seperates from AWL down the road.
 
Okay carrying on:

If I buy 4 bags of cement and 18 bags of all-in ballast (the 22 bags) it will cost me £47.55 including VAT.

Orders over £50 are free delivery, and I have a few more things to buy.

So, I've gone from £76 (not including delivery) buying Mastercrete Original from Wickes, to £47.55 buying seperates from AWL - including delivery.

Assuming the ratios are correct for footings and the ballast is suitable.
 
I think you might be getting your terminology confused. Mastercrete is just cement, not pre-mixed concrete.
I'm not sure if it's a regional thing but I don't know why all the quantity calculators split the aggregate and sharp sand quantities when everyone seems to buy it as all-in ballast (aggregate and sharp sand mixture).
Anyway, let's use your 22 bag example and Wickes pricing structure as an example.
22 bags of pre-mixed concrete mix = £102.96
whereas
18 bags of ballast and 4 bags of Blue Circle Cement = £42.92.
I know which way my hard-earned would go.
 
I presume that when I put in the all-in ballast to the cement, the ratio to cement, sand and aggregate will be 1:2:4, and that this is right for footings.
 
Some issues:

http://www.source4me.co.uk/calculate_concrete_mix.php

I think you can make concrete for footings at ratio of 1:2:4 by VOLUME.

But, this calculator gives 1:2:4 by MASS!

So, the calculator is misleading somewhat, because the concrete needs to have a ratiio of 1:2:4 by VOLUME, not MASS.

As to the number of bags: I guess you are NOT supposed to try to relate the bags to any ratio. The calculator is saying, just get this number of bags per item (cement, sand, aggregate) and use a bucket to make sure you get the ratio 1:2:4.
 
It should be by volume.
Don't forget, if you're using all-in ballast then you only have two parts to your ratio - ballast and cement. So 4:1, 5:1 and 6:1 are examples. The footings ratio may be dependent on what the footings are for - a retaining wall, an extension wall, a small garden wall, etc.
 
richard7761 said:
Some issues:

http://www.source4me.co.uk/calculate_concrete_mix.php

I think you can make concrete for footings at ratio of 1:2:4 by VOLUME.

But, this calculator gives 1:2:4 by MASS!

So, the calculator is misleading somewhat, because the concrete needs to have a ratio of 1:2:4 by VOLUME, not MASS.

As to the number of bags: I guess you are NOT supposed to try to relate the bags to any ratio. The calculator is saying, just get this number of bags per item (cement, sand, aggregate) and use a bucket to make sure you get the ratio 1:2:4.
Click to expand...

ERRATA: I got a bit wrong: When the mass of ballast and cement per unit (tonne) are very nearly the same, ratio by volume is carried through to ratio by mass. They are interchangeable.

It's only when the mass of ballast is significantly different from cement that you cannot interchange ratio of volume with ratio by mass. And all-in ballast is practically the same mass a cement per unit volume.

The number of bags therefore should also reflect the volume ratio. Which it does, but not always very accurately as evidenced by my figures of:

Cement:..........4
Sharp Sand:....6
Aggregate:......12

(Put in 560mm x 560mm x 750mm (deep) and you get the above bag figures.)

That's a "bag ratio" of 1:1.5:3, and not 1:2:4. But, ideally, you would ensure the correct ratio by using a container when gathering items tohether in a plie, to get 1:2:4.
 
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