# Forum Home Renovation Concreting  Help me with my Garden Shed

## TazManiac

Hey Guys, 
I am in the process of putting down a slab for my garden shed at our new home.  I have not worked with concrete before and was hoping someone could help me with some suggestions. 
The slab will be 3.1m x 3.1m. 
I have levelled the site and dug 100mm down and put in 100mm of FCR.   What size timber should I use for my formwork?  A mate suggested 90 x 19mm decking?What size reo should I use?Do I need plastic under the concrete on clay if I have the FCR to pour on to? 
Did any of that make sense? 
Any help would be appreciated. 
Aaron.

----------


## rod1949

The 19mm decking is fine as long as you peg/brace it sufficiently. 
I'd use F62 mesh. 
Plastic under the slab, Yes I'd do it. 
Explain what "FCR" means

----------


## TazManiac

Sorry, FCR = Finely Crushed Rock

----------


## Terrian

batter the crushed rock away from the formwork, let the concrete hold the crushed rock in place, f53 or 562 reo, 1m3 of concrete is what you will need for the area you describe. The decking will be fine to use as long as you put in plenty of stakes to hold it all in place. 
p.s, normally just use 20mm crushed rock

----------


## Bloss

I assume you dug down 200mm and then put in 100mm of fill?   :Smilie:  
In any case  :Wat they said:   :Arrow Up:  re plastic (yes) and reo etc and the battering away of the fill (sloping up from the bottom edge of the formwork). Really ensure the formwork is square by measuring across the diagonals and level too. I have installed some with a slight fall to one corner with a small down-sloping drain hole left in situ as pouring the slab just in case water does get in at any stage. 
For a slab for installing a standard garden shed I have found that a rebate around the top edge of around 19 x 19mm really improves water resistance and is easy to do. The depth need only be 19mm, but can be up to 30mm and the width depends on the width of the bottom channel into which the walls slot - unless you have very accurately measured the slab to suit the floor plan of the shed you make the width of the rebate a wider to allow for some adjusting room. 
Simply cut the rebate mold timber (eg: 30 x 19mm treated pine) to fit as a frame for the top inside of the formwork and after the pour is done tap it down evenly around the edge into the wet concrete and re-work the finished surface with a trowel. Carefully remove when cement is set. 
The idea is that the channel and wall bottom sits 15-20mm below the inside floor level so storms etc can't force water into the shed. 
Just an extra that works - not a must have though.

----------


## TazManiac

Thanks for the opinions guys. 
I just have one more question: 
I need to make 1 cubic metre of concrete so
How many 20kg bags of Cement
How much Sand
How much Gravel 
How much water do I mix with it? 
How many barrow-loads would this be? 
Would it be a good idea to havetwo mixers going at the same time to make this much concrete? 
Thanks again for the wisdom. 
Aaron

----------


## Bloss

Plenty of estimators & calculators for this and it is written on every bag of cement and concrete mix too . . . as are instructions for mixing in various ratios for different uses - including water volumes (which is really - sufficient to get the right consistency - which is just wet enough to place and finish, but not runny (as too much water weakens and can cause cracks too). 
eg: http://www.cockburncement.com.au/tec.../estimator.asp 
and this  is one of the best little books for DIY concreting:  http://www.concrete.net.au/publicati...retebasics.pdf 
For just a metre it'd be worth looking at pre-mix in 20kg bags (not quick set). No doubt not cheap, but when you count in sand aggregate and mucking about etc it's probably worth it.

----------


## Terrian

> Thanks for the opinions guys. 
> I just have one more question: 
> I need to make 1 cubic metre of concrete so
> How many 20kg bags of Cement

  12   

> How much Sand

  .65 of a cubic meter of concrete sand   

> How much Gravel

  .75 of a cubic meter of stone, 14mm  :Smilie:    

> How much water do I mix with it?

  somewhere between 50lt & 10,000lt  :Smilie: 
an impossible question to answer   

> How many barrow-loads would this be?

  about 12 full builders barrows (good luck moving it though  :Smilie:    

> Would it be a good idea to have two mixers going at the same time to make this much concrete?

  na, what we did was add stone, sand cement & water to the mix and let it go for a few minutes while we barrowed the previous load to where the job was being done, and it is only 1m3 that you are making. Having 2 mixers going will lead to less mixing time, and that can be bad as it may not mix all the ingrediants together properly.

----------


## an3_bolt

Just did a slab for my shed at 3.1 x 2.4 x 0.1m  
Got a truck in with readymix concrete- worked perfectly. Just really make sure you properly brace your formwork - I thought I placed enough and it was starting to bow using 90 x 35. Brace both vertical posts and horizontally away from slab - especially if you are using 19mm decking. 
Get a friend to help you - can get a bit hard to screed across 3m by yourself.

----------


## TazManiac

I am in the process of buying all the materials to do this slab. 
I have another question though. 
I was buying premixed sand/aggregate mix (which I believe I will need almost 1.5m3) and I was wondering how all that cement/sand/aggregate/water can add up to much more than 1m3 and only fill a 1m3 hole.  Surely the contents can't shrink... 
Also, when I was buying the bags of cement, on the back of the bag it says I need 20 of the 20kg bags for 1m3...  In the posts above and on the calculators I have seen it has always said between 12 and 14 bags... 
Any help?

----------


## Bloss

> I am in the process of buying all the materials to do this slab. 
> I have another question though. 
> I was buying premixed sand/aggregate mix (which I believe I will need almost 1.5m3) and I was wondering how all that cement/sand/aggregate/water can add up to much more than 1m3 and only fill a 1m3 hole.  Surely the contents can't shrink... 
> Also, when I was buying the bags of cement, on the back of the bag it says I need 20 of the 20kg bags for 1m3...  In the posts above and on the calculators I have seen it has always said between 12 and 14 bags... 
> Any help?

  Are you confusing bags of cement with cement mix? The dry mix is aerated with cement dust and sand etc - you will add water which will fill those air gaps - the wet volume is what matters. In any case _use what the pre-mix bags say_ - if they says it takes 20 bags to make 1m3 then that's what roughly it will take. You need an allowance too - say and extra bag (5%) in case you have misjudged. You can always use the extra - a bugger to find some when you are a shovel or two short!  :Smilie:

----------


## Terrian

> Also, when I was buying the bags of cement, on the back of the bag it says I need 20 of the 20kg bags for 1m3...  In the posts above and on the calculators I have seen it has always said between 12 and 14 bags... 
> Any help?

  about 20 bags of concrete mix per cubic meter
conmix is the pre mixed, each bag has stone, sand & cement 
12  20kg bags of *cement* per cubic meter.
these are just bags of cement, there is no stone or sand in the bags 
Now that you have bought the bags of conmix, you have no need to buy the stone or sand   

> I was buying premixed sand/aggregate mix (which I believe I will need almost 1.5m3) and I was wondering how all that cement/sand/aggregate/water can add up to much more than 1m3 and only fill a 1m3 hole. Surely the contents can't shrink...

  Nope, if you buy 1m3 of bagged cement mix, you will end up with 1m3 of very expensive concrete. 
ok, basically what happens is this.
you have stone, then you add sand, each grain of sand is smaller than the stone, the sand disappears and fills the gaps between the stone. 
Now add cement, each grain(?) of cement is smaller than a grain of sand, the cement disappears and fills the gaps between the grains of sand 
And finally you add water, the whole lot starts to look a little like dirty toothpaste  :Smilie:  but does not increase the volume much at all. 
You finally pour the concrete and it starts to dry out, the water evaporates and the concrete shrinks a tiny bit (usually the shrinkage is so small you wont see it with the naked eye)(how much it shrinks depends on how much water you add)

----------


## TazManiac

Thanks for the help guys. 
I didnt buy the pre-mix stuff.  I made sure of that.  I have taken photos of the bags that I bought. 
I don't mind too much if I have extras as we have a lot of concreting to do.  We just moved in so we will have to do paths and stuff too.  Can always use it for mortar too for the wood fired oven I want to build. 
If I buy the sand and aggregate pre-mixed from the landscape supplier how much do I need to get if the sand fills the gaps in the gravel and stuff.  Would 1 cubic metre be enough?  I was planning to get 1.5m3. 
Here's the pictures.  Maybe they just state that you need more than you do to make more sales.    
Thanks again for any help.  You guys are great.

----------


## TazManiac

I have managed to find that people call the pre-mixed sand/aggregate mix ballast and from this I have found numerous calculators to tell me how much ballast to buy.  Problem is I get different answers on how much of everything to buy, depending on which calculator I use.  I think 1m3 should be sufficient, and cheaper than 1.5m3.  Can someone please tell me if I am wrong? 
Thanks

----------


## Bloss

I'd get the little extra - as I said hard to find more when a few shovels short! Some will let you have it in 0.2m3 lots so do that if they'll let you otherwise get the extra. Even when hand mixing in a barrow or a small home mixer always a good idea to get a small 2nd job ready for any leftovers - a step or a bit of path or edge strip or even a frame mold to make a stepping stone or two with any leftover concrete. You pay for it anyway so better to use it than dump it.  :Smilie:  
This is a good site for info on concrete: http://www.concrete.net.au and on estimating volumes: http://www.concrete.net.au/concrete_...p?id=72&page=1

----------


## Terrian

> I have managed to find that people call the pre-mixed sand/aggregate mix ballast and from this I have found numerous calculators to tell me how much ballast to buy.  Problem is I get different answers on how much of everything to buy, depending on which calculator I use.  I think 1m3 should be sufficient, and cheaper than 1.5m3.  Can someone please tell me if I am wrong? 
> Thanks

  If the stone and sand are already mixed together, you need 1m3 to make 1m3 of concrete
If you are going to mix everything yourself, you need .75m3 of stone (14mm) & .65m3 of sand (concrete sand, not brick sand!) and a dozen 20kg bags of cement. 
Always order a bit more than you think you need though, 1.25m3 should be sufficient to make sure you get the job done, and maybe an extra couple of bags of cement. 
Do not use a shovel to measure than stone/sand/cement mix ! (I use 20lt plastic buckets at home) 
I see you have purchased GP cement, the other type is 'builders' either is fine (one ends up a tiny bit darker than the other, I think builders cement is a little darker, been too long to remember though)

----------


## Terrian

> and on estimating volumes: http://www.concrete.net.au/concrete_...p?id=72&page=1

  the problem I have with the mix amounts in that link is you end up with a 'boney' mix, fine for foundations, but very hard to get a nice finish on a pathway or similar.

----------


## Bloss

> the problem I have with the mix amounts in that link is you end up with a 'boney' mix, fine for foundations, but very hard to get a nice finish on a pathway or similar.

  In his case he is getting a sand aggregate mix so he'll get what he gets. 
A hard call for DIYers as concrete is an art based on the actual ingredients you have available and less on the mix proportions IMO. I like using washed gravel aggregates and grew up and learnt about concrete using graded rounded gravels and washed river sands. Then I went to where the usual mix was crushed grey/black rock (mostly basalt) with sharp edges and a not very pretty look if exposed on the surface - talk about 'bony' and hard to screed and lay off! 
Pre-mixed sand/aggregate is so common now that it is hard to use slight changes to the mix to balance out on the run while feeding separate sand and aggregate and cement into a mixer. I have never used anything but a shovel (and a bucket for water) and yet all the manufacturers and guides read as though the industry uses measuring tubs for all mixing. The mechanical mixers used in ready-mix trucks use standard measures, but the fact is that we use our judgement and in my case actually count the shovel fulls for the ratio I want (ratios of 1:2:5 or more commonly 1:3:5 for slabs, but in my mixer double that number of shovel loads). For those who do it often you can tell by look and feel whether the mix is OK - for a DIYer doing the first and often only job - it's get the best advice and good luck!

----------


## Terrian

> In his case he is getting a sand aggregate mix so he'll get what he gets.

  and hope that the guy getting paid $15ph cares about getting the mix near right  :Smilie:    

> A hard call for DIYers as concrete is an art based on the actual ingredients you have available and less on the mix proportions IMO. I like using washed gravel aggregates and grew up and learnt about concrete using graded rounded gravels and washed river sands. Then I went to where the usual mix was crushed grey/black rock (mostly basalt) with sharp edges and a not very pretty look if exposed on the surface - talk about 'bony' and hard to screed and lay off!

  yuck, sound like fun (not)
14/10 agg (thats 14mm & 10mm mixed together) and concrete sand FTW  :Smilie:    

> I have never used anything but a shovel (and a bucket for water) 
> and yet all the manufacturers and guides read as though the industry uses measuring tubs for all mixing. The mechanical mixers used in ready-mix trucks use standard measures, but the fact is that we use our judgement and in my case actually count the shovel fulls for the ratio I want (ratios of 1:2:5 or more commonly 1:3:5 for slabs, but in my mixer double that number of shovel loads). For those who do it often you can tell by look and feel whether the mix is OK - for a DIYer doing the first and often only job - it's get the best advice and good luck!

  true enough, but sometimes a site will say 'x+y+z = what you need' and some will stick to that no matter what, and wonder why things went tits up  :Frown: 
it is a bit different when you can look into the mixer and know wether it is right or wrong, the average DIY handyman wouldn't know, personally,  the sound is enough to know if there is too much stone or sand, the colour of the mix to know if it needs more cement. 
FWIW, I ran a garden supplies with mini mix for about 5 years, I currently have 1.5m5 of concrete sand, 1.5m3 of 14/10 pebbles & 1/2 doz bags of cement, and a sheet of f62 at home, I might even do something with it all some time soon  :Smilie:  
TazManiac, ask all the questions you can think of.

----------


## Bloss

With demanding clients I give them what they _meant_ to ask for. . .  :Wink:

----------


## Terrian

> With demanding clients I give them what they _meant_ to ask for. . .

   :Rotfl:  , I know exactly what you mean  :Smilie:

----------

