# Forum Home Renovation Concreting  How to calculate the number of bags of premix concrete

## aussieglen

Please help! I am frustrated in not being able to calculate a simple task! :Frown:   
I am erecting a new deck and am using "High wind post anchors" (Pryda). 
The 9 post holes are to be 300 round and 650 deep - simple enough. But for the life of me I cannot work out the number of bags of General Purpose concrete for the job. Bunnings sell 20kg bags (heavy enough for an old codger like me) and my head is spinning trying to calculate cubic meters and volumes and decimal points and pi and watching cricket etc. My school boy arithmatic keeps giving me different results!:eek:  
Is there a good samaritan out there who can simplify things for me. One web site suggests 320 x 320 x 50 per bag but this only confuses me more.:confused:  
Hopefully there is someone (or web site) who knows how many 20 kg bags per hole - or am I the only one who get confused? 
Compliments of the season to you all :Smilie:  
Aussieglen

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## bitingmidge

I think the fine print on the bag reads "10 kg of this product spreads to 320 x 320 (0.10 square metres) to a depth of 50mm." 
I'd be guessing therefore that you could get a 20kg bag to stretch to 320 x 320 x 100mm deep, which means .10 sq metres x .1m = .01 cubic metres.
Your holes are 300 diameter, or .07 square metres by .65 m deep which is near as anything to .045 of a cubic metre, or nearly FIVE bags per hole. 
You have nine of them, so I reckon it'd be a LOT cheaper and easier to get a minimix truck in with .42 metres (42 bags), and you'll know that the brew is going to be of consistent strength at the end of the day. 
Don't load your barrow too much,and it'll be Lot less work than mixing anyway! 
Cheers,
\
P (who failed maths at school so be careful!)   :Biggrin:

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## Dion N

I'm not to sure about the figure you got from the Web either  
if a 20kg bag has a volume of 320x320x50 when mixed, then each bag produces a volume of 0.00512m3 
If you have 9 holes of 300 diameter and 650 depth, then the volume of concrete required is: 
9 x height x pi x radius squared = 9 x 0.65m x 3.14 x 0.15m x 0.15m  
= 0.41 m3 of concrete 
Now, to calculate the number of bags 
0.41m3  / 0.00512m3 per bag = 81 bags or 9 bags per hole   
In my mind's eye, I can't see 9  x 20 kg bags plus water fitting in the size holes you describe. 5 bags per hole sounds much closer to the mark

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## Iain

Like the Midge said, get a truck in, better quality and a fraction of the price, get a few mates with barrows to help, only take about half an hour and buy a slab with the savings.
Your mates will love you for it.
Send me the funds for a return airfare and I will come too :Biggrin:   :Biggrin:   :Biggrin:

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## bitingmidge

> if a 20kg bag has a volume of 320x320x50 when mixed, then each bag produces a volume of 0.00512m3
> snip....s or 9 bags per hole

  Dion, 
I may be wrong, but I my memory says that for some confusing reason, TEN (10) kg has a volume of .005. 
There's a strange anomoly in that the stuff comes in 20kg's but the description is for half that. 
But I'd be happy to be corrected! 
I've just confirmed the following fromhttp://www.riversands.com.au/easymix_concrete.htm   

> 1 x 20 kg bag = 0.01m3       100 bags = 1.0m3

  So I'm happy that my sums are correct! 
cheers, 
P

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## bigdazza

Red Alert. 
Dont buy premix in bags. It will cost a fortune, and its &&&&&....really boney mix! 
You need about half a cube for your job. If you have a trailer, then go to your local landscaping supplies and get half a cube of premix (sand/gravel) and a couple of bags of cement. 
......Otherwise order in a truck. But they usually have a minimum order and you only need half a cube for your job, which is stuff all.

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## aussieglen

Thanks to all you good guys - you have been a life saver. I now have the comments and calculations to mull over and make the big decision - whether to use 41 bags = $245 and do the job bag by bag or get a load of sand gravel delivered and hire a mixer or to check out the Minimix ready mixed to specification. My access to the site is a drawback and would require barrowing in a fair way (50 meters) as would the sand/ gravel mix from where dumped, plus mixing. 
One comment regarding the quality of the premix in bags is of concern - can you confirm this or is this your own assessment?. The posts are load bearing (albeit only a deck) but I dont want the thing to disappear down the hole one Sunday afternoon in a rainstorm for want of a few bob extra for a quality product. Perhaps you may care to expound on the quality aspect of these bagged products. 
Meantime I am encouraged to start setting out the profiles and make a start on the holes before the weather gets hot! 
Aussieglen

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## elphingirl

For my personal choice, I only mix my own concrete when strength is not of concern (ie filling blocks or where there is so much reo in it doesn't matter), and where the time taken to mix will be negligible. Even barrowing in 50m sounds preferable than mixing to me.
Cheers
Justine

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## bitingmidge

> One comment regarding the quality of the premix in bags is of concern - can you confirm this or is this your own assessment?. The posts are load bearing (albeit only a deck) but I dont want the thing to disappear down the hole one Sunday afternoon in a rainstorm for want of a few bob extra for a quality product. Perhaps you may care to expound on the quality aspect of these bagged products.

  The stuff is of consistent when it's in the bag, it's lack of proper mixing, inconsistent water quantity, and inconsistency of placing (ie one bag at a time, hot weather, concrete being put into footings in four lots of different strengths, that cause quality concerns. 
Despite all of the above, in reality, footing materials only need to be stronger than the soil they are in! 
I don't think it'll be extra to get a mini mix truck in though! 
Cheers, 
P  :Biggrin:

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## Theremin

You should be able to get .5m3 delived for less than $245.  Try Concrete Taxi (www.concretetaxi.com) *-* they specialise in small loads and are in Brisbane.  Alternatively, you could buy a GMC mixer from Bunnings for $289, plus cement, gravel and sand (say $60), and while it will cost you a bit more you will have a cement mixer at the end of it (even if it is only a GMC one).

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## Pulpo

I agree with the previous post about the strength of the concrete is not really a major issue for posts in the ground, within reason. 
Hopefully its not a really clayey soil. 
If you go with the concrete bags, you still have to transport those to the site 50m. 
Do you intend to mix the concrete or just place it dry? If mixing a mixer is a must in my mind. 
In my mind I would lean on a few mates and go with the mini-mixer delivered. 
Far quicker and easier, my next choice would be to go with mixing my own batch. 
I would not have an issue with the quality in using the concrete bags, just do not over water. 
And the 0.01 cubic metres per bag sounds right to me. 
And the amount of concrete needed to fill the hole is correct; however how big are the posts for they will take up space in the hole, therefore less concrete would be required. 
But always order more concrete if going with mini-mixer delivered option, have some other areas boxed up to take any extra concrete. 0.5m cubic metre is more than enough given the measurements are right.  
Good Luck  
Pulpo

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## Iain

Another point I discovered recently, don't use quickset in a mixer, it just goes lumpy and won't mix properly, and it costs more for the honour.

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