concreting

[gailando]

how many 20kg bags of concrete would I need for a concrete slab 6 metres x 2 metres. Thank you

[Jewelleryrescue]

Hi @gailando 

I am going to assume  a general purpose  cement  GP  mixture for slabs for a  shed floor or  path and you will doing 70 mm  -  100mm thickness ,

Cement 1   sand 2  Aggregate 3

Estimated 12 bags (70mm thick) you may need up to 3 bags  more to finish the job.  If you over order bunnings will refund fresh  un damaged  bags

Bastion 20kg General Purpose Cement I/N: 0760330    $8^.59

^{I hope this is some SOLID advice.}

[gailando]

Thank you so much for your advice. It has helped so much . I had no idea how to work it out. Once again thank you  

[JacobZ]

Hi @gailando,

A warm welcome to the Bunnings Workshop community and thank you for your question.

For a 6x2m concrete slab that is 100mm thick, you need 1.2 cubic metres of concrete. In 20kg bags such as the one @Jewelleryrescue has mentioned, you would need 126 bags.

You can use this Concrete Calculator from our supplier Dingo Cement to confirm this.

Let me know if you have any further questions.

Jacob

[gailando]

Wow , that is a big difference in the amount of bags needed from the previous answer. That is why I have been unable to work out the amount because everytime I tried to calculate I got different answers. Thank you so much for your assistance. 

[Jewelleryrescue]

Hi @JacobZ 

I think there is some thing wrong with those calculators I laid a  slab close to 5 meters by 2 meter by  120mm on average. I had bought 80 Bags of 20 kg cement I used near half  of them I had so many left over  but used them in other projects. 4 car trips with 20 bags. Plus I did some extra slabs out of that lot and morter for bricks. and paver cement pads just  tring to use it up.

My first random internet  calculator  also  was around 126 bags for my  job and this estimate for @gailando  We might have used the same one.

I used another calculator and its  saying 12x 20 kg bags so  ??? 

Well  I am still stumped at the estimate of 126 bags 

Just checking my  real usage.

For me is roughly 1 bag of cement per  builders wheel barrow (1/2 a bag per mixer load,  2 mixer loads = fills the wheel barrow.) 

So 126 bags makes 126 full builders wheel  barrows  thats alot of concrete,  You also know what volume  a builders wheel barrow can fill.  now  times 126 of them

A mental double check via material wieght.

So  my weight estimate is Cement 1/2 bag is  10kg. sand  20 kg  agg is 30kg  so total  60 kg per mixer load. Double that is 120kg per wheel barrow load. well that checks out.  120kg per  builders barrow load is a close  reality you know this when you have to push it many times.

So approx it took me 20 wheel barrows to fill 5mx2m x 0.12  or  1.2 m cubed  thats  20 to 30 bags of GP cement   that is  being generous

This  is doing my head in lol.  

My   conclusion is buy 40 bags of cement and buy more at need.  And by the  end  you will just need a few extras if al all.

[JacobZ]

Hi @Jewelleryrescue and @gailando,

The equation for calculating the volume of concrete is fairly simple as it is just a volume calculation. 

$$\text{Volume}=6\text{m}\times 2\text{m}\times 0.1\text{m}$$

Volume=1.2m3

I think the issue we're having, which I missed in my previous response, is that Jewelleryrescue is referring to cement as opposed to concrete.

Cement, such as this Bastion 20kg General Purpose Cement, is a powder made from heated and crushed limestone and clay. When mixed with water, it hardens and sticks to other materials, acting as a glue. It’s mostly used in construction as part of mixtures, like concrete or mortar.

Concrete, such as this Bastion 20kg Concrete Mix, which I was referring to in my previous response, is a mixture of cement, sand and aggregate that can be mixed with water and used with no other additives.

The difference is that concrete is already mixed, whereas cement needs to be mixed with sand and aggregate to make it into concrete.

If you were using cement, like Jewelleryrescue's calculations are based on, you would need to add the sand and cement to the mix. A standard mixing ratio for concrete is 1:2:3, 1 part cement, 2 parts sand and 3 parts aggregate.

As a bag of cement is roughly 0.01m3, when mixed with sand and aggregate, a single bag of cement will make 0.06m3 of concrete. This means you would need 20 bags of cement, 40 bags of sand and 60 bags of aggregate to make up the 1.2m3. The sum of these materials equals 120 bags total. With 5% extra for mishaps, 1 bag cement, 2 bags sand and 3 bags aggregate, this makes your 126 bags of concrete.

I hope this has clarified. If not, please let me know and I'll try to explain further.

Jacob

[Jewelleryrescue]

Hi @JacobZ 

That explains the calculation differences thanks for that I should  have seen it myself  but a am always in learning mode open to better ways and ideas that really work.

Thanks for longer descriptions of the dual sets of calculations.