Concrete slab for garden shed

[Programmer]

Hi all. Yes, nothing better to do on a Friday than ask questions.
(Home in renovation + 3 young kids) = Fri night @ home...

I have 3 questions:

1. This one is for the brickies/concrete specialists et al. among our fold. Is there an equation that will convert Kilograms to Cubic Metres? We're talking cement here. I realise they're different beasts (Weight as opposed to Volume), but I'd like to think some smarty has the answer. I've tried many, many online calculators but this does not appear to be an FAQ. So maybe it's easy, tell me!

1a) If the above is impossible, here's one that some might have some experience with. Forgetting volume capacity of a specific wheelbarrow (assume it's your wheelbarrow to make it easy). How many WB loads would it take to move a Cubic Metre of cement?

2. Related to question 1. I'm doing a concrete slab that will be used as the base for a garden shed. It will only ever contain/house light boxes, a lawn mower, w/snipper etc.
a) Does it REALLY need to be reinforced?
b) Does it REQUIRE some base material (gravel etc) first? If we assume that not much water ever gathers in the vicinity, and the soil is fairly 'clayey', do I really need the gravel?

3. Again related. I reckon about 100mm thickness for above slab is about right. Any opinions? What about 75mm with a compacted gravel base?

OK, over to you....

[johnc]

As for weight have a go at this, its basically a hundredweight (114lb I think) to the cubic foot you can do the conversion as I've never bothered. There is some variation based on material but that's roughly the measurement for stone.

The thickness for the shed slab has more to do with the stability of the sub base than the compaction of gravel, and I would suggest that for all the mucking around I would go direct to 100mm with a thin layer of packing sand to smooth out the bumps. And yes rio is worth the effort, for the little bit it costs unless you are pouring a slab over compacted WA sand you will almost certainly end up with cracking if you leave it out, (if in doubt refer to Murphy's law). If there is some clay in the soil always, always, always use rio, you do not have a stable base as it will be very subject to moisture levels.

How many barrows, you should be able to shove .25 metres into an avareage barrow, just put plenty of air in the tyre and off you go. If inexperienced let go the handle if it tips, better to shovel back in the slops than break the handle in a futile attempt to drag back a tilting barrow. Its easier to get a mug (read mate) to come along with his own barrow and help to move the load into place, it will keep the truckie delivering a lot happier to.

In the end if chasing cost go for 75mm with mesh and a little packing sand, it will be stronger than 100mm without mesh and a compacted base. Even light mesh is better than none at all and a sheet off the roll of your local hardware store will do the job, and will cost less than the flat sheets normally used. Your local council will most likely give you the appropriate F rating for the area.

Good luck, John.

[bitingmidge]

Johnc is right, but it's Friday night and I've got nothing better to do than be a smarty pants!

1. This one is for the brickies/concrete specialists et al. among our fold. Is there an equation that will convert Kilograms to Cubic Metres?

Yep, as long as you know the density of the material you are using. One cubic metre = 1000 litres of water = 1000 kilograms of water (give or take a bit for distance above sea level and amount of mineral in the water)

We're talking cement here. I realise they're different beasts (Weight as opposed to Volume), but I'd like to think some smarty has the answer. I've tried many, many online calculators but this does not appear to be an FAQ. So maybe it's easy, tell me!

Very easy. You just have to know the density of all the materials in the mix, the amount of hydration, and the amount of air between the granules...all obtainable from your concrete supplier and given a couple of months of Friday nights you should be able to work it out! On the other hand the readymix guys will give you a rough number for a wet brew off the top of their heads.

It's heavier than water though...stones sink if you put them in a tub..duh!!

1a) If the above is impossible, here's one that some might have some experience with. Forgetting volume capacity of a specific wheelbarrow (assume it's your wheelbarrow to make it easy). How many WB loads would it take to move a Cubic Metre of cement?

How can you ask that question while forgetting the volume of a specific wheelbarrow? OK MY wheelbarrow is really big, 1/2 a cubic metre to be exact, so it will only take my wife 2 loads. Which is just as well, because wheeling heavy stuff eats into her mowing time!

OR you could go to Bunnings in the morning, and have a look at their range of barrows, and if the volume is not marked ask... if you aren't used to wheeling heavy loads, DON'T get the big macho builder's barrow...get the handyman version that looks the same, but is about two thirds the size. 50% more loads is easier than trying to shovel one spilled one off the grass.

2. Related to question 1. I'm doing a concrete slab that will be used as the base for a garden shed. It will only ever contain/house light boxes, a lawn mower, w/snipper etc.
a) Does it REALLY need to be reinforced?

No. Do you REALLY not mind cracks? You might get away with it, it's only a shed after all...does it matter?

b) Does it REQUIRE some base material (gravel etc) first? If we assume that not much water ever gathers in the vicinity, and the soil is fairly 'clayey', do I really need the gravel?

Yes. See answer above. "Clayey" soil means movement, and you are going to need all the help you can get to isolate the slab!

3. Again related. I reckon about 100mm thickness for above slab is about right. Any opinions? What about 75mm with a compacted gravel base?

You will get away with a 75 mm slab and light reinforcing placed on the top side of centre. In the good old days that was pretty much the standard for light slabs, particularly where some surface cracks could be tolerated. But there are plenty of instances where sheds have 100mm slabs and no reo as well.

Don't forget to bung a bit of plastic down before pouring. If nothing else it'll keep the moisture in the concrete till it goes off and give a bit better chance of preventing cracks.

It's all a bit academic really, if it were my shed, I'd go 75 and reo. (Only 1 1/2 barrow loads then!! )

Cheers,

P

[journeyman Mick]

1) Concrete weighs about 2.5 tonnes per M3 - set, I think it may be as high as 2.7 tonne per M3 for a pretty wet mix.

1a) Sorry, don't know. The only time I ever shift concrete by the barrow load I'm working for some other mug and I'm too busy wrestling with the bloody barrow to pay any attention to anything else (besides wondering how much longer it's going to take! ) On my own jobs I get some other poor mug(s) to drive the barrows, or more likely, I get a pump in.

2a) Only if you don't want it to crack.
2b)Just needs to be placed on a firm, level, well compacted surface, which may or may not mean some form of fill.

3) 100mm is fine, but you can cheat and go 90mm by using some 90x35 pine stud as formwork.

Mick

[vsquizz]

Its not the weight On the slab its the oft overlooked lateral loads that apply during storm/cyclone whatever. Two years ago we had what is affectionatley called a Cock-Eyed-Bob come up our street. Picked up the little garden shed in the place directly behind me, over the top of our house (which had a gum tree already parked on the roof ) across the street and smashed into the front of the house over the road. Big mess, lucky nobody was hurt.

Now it may only be a little garden shed but make sure its secured down well before you have an insurance investigator sniffing at you door. You can even dig a trench in the perimeter and turf in old bricks and some concrete just to give some foundation to where the shed is secured and cut costs elsewhere.

Theres probably a guzillion backyard garden sheds sitting on concrete pavers and next to nothing else but hope this helps.

BTW: whilst you are well intentioned regarding the shed size and its usage
it will never be big enough

Cheers

[Programmer]

Thanks to all for the advice and opinions. There really is a clear divide of those that vehemently support reinforcing and those that (for tiny jobs like this) think it's neither here nor there... I've spoken to others (including pro concreters) that laugh at the prospect of reinforcing such a slab. "It's not just that it's a shed, it's a (expletive) GARDEN shed!, spend the money on beer". (these guys wouldn't be seen dead laying a GARDEN SHED slab...) However, I do agree it's probably worth it for simple peace of mind. Again, thanks.

[Jacksin]

If you are pouring a slab for one of these prefab garden sheds (that come in made-up panels) to sit on, it may pay you to rebate the edges of the slab down about 25mm. This stops the rain from seeping under the walls and wetting everything sitting on the floor.
If you are doing the slab this way, make sure you get it laid out square.

Or you can just seal the wall/floor junction with silicone but it looks YUK!
Jack

[ozwinner]

Midge, Maaaaate

When youve done with your missus, send round here, Ive tons of jobs she could tackle.

Al

[Marc]

On the topic of concrete slabs...

Does it make sense to mix your own concrete to avoid grumpy truck drivers that frown upon wheelbarrows?
Say the job is 2 m3 Is it too far feched to borrow a mixer and showel it slowly in?

[vsquizz]

Borrow or hire the mixer, its only a bit of sweat. Mind you the price of bagged cement aint going down, then you need your sand, and it makes a bit of a mess etc. Most of the mini mixers have a rubber pipe that'll pump the stuff down the side of your house and straight into the form work. Works for me, but I generally like to do things the hard way

Cheers

[soundman]

Now about that weight loss programe!!!!

It is entirely reasonable to mix enough cement to lay a slab for a garden shed but I hope you are feelin fit.
Remember you have to shovel it, mix it, shovel it again, screed it & smooth it all before it goes off.
For a seasoned outdoor type , no problem. If you are a wasted shiny bum,... It'll kill ya.

If you are determined to do it, park the mixer right next to the form work & get the gravel dumped close to that.
Mix It & tip it straight in the hole & get at least one good strong mate to help you.

Cheers

[Programmer]

If you are pouring a slab for one of these prefab garden sheds (that come in made-up panels) to sit on, it may pay you to rebate the edges of the slab down about 25mm. This stops the rain from seeping under the walls and wetting everything sitting on the floor.
If you are doing the slab this way, make sure you get it laid out square.

Or you can just seal the wall/floor junction with silicone but it looks YUK!
Jack

I'd forgotten about this good idea, thanks Jacksin.

[jackiew]

It is entirely reasonable to mix enough cement to lay a slab for a garden shed but I hope you are feelin fit.
Remember you have to shovel it, mix it, shovel it again, screed it & smooth it all before it goes off.
For a seasoned outdoor type , no problem. If you are a wasted shiny bum,... It'll kill ya.

Cheers

I have not very fond memories of barrowing the sand, gravel, cement from the front garden to the back garden and then shovelling it all into the cement mixer for a shed base that you could have built a block of flats on .... not the recommended fitness regime for a pregnant woman but we both survived it. Wouldn't do it again though.

Who cares whether the concrete delivery guy is grumpy if you barrow it in - you only have to put up with him for a short period of time and probably no-one else ( including his wife ) cares what he thinks anyway so why should you

... but if it works out cheaper than the chiropractor bills after doing it yourself then get the concrete pumped in.

[Sturdee]

.................to avoid grumpy truck drivers that frown upon wheelbarrows?

A couple of years age I got .8 metre of concrete in a mini truck and had to barrow it about 8 metres. The grumpy driver carried on about it.

I gently ( but forcefully :eek: ) told him to either shut up and take it away and I would complain direct to his head office about his attitude or to get on with it so it could be done within the free 15 minutes delivery time.

He got on with it and even helped with barrowing.


Peter.

[Pulpo]

Cement and concrete are not the same.
The weight of cement per cubic metre could be worked out by looking at the 40kgs bags in the hardware store and think how many would fit in a cubic metre at a guess maybe at least 15bags.

Anyway concrete weighs 2.4 tonnes per cubic metre, or so I've told by a couple of engineers. However this does vary depending on mpa.

Although my wheel barrow is builder barrow not sure of its size, at a guess I
would load no more than 0.1 of cubic metre. That's still too heavy.

Generally for a garden shed I would not be too fussed about reo, just depends on soil type for it has high clay content then movement is higher and I would consider it.

Probably would put a moisture barrier down, that is a piece of plastic.

I've helped lay many foot paths around houses that were only 50mm thick with no reo or compacted base, obviously the earth was level and firm.
All seemed ok no cracks.

The cost saving by buying the materials and hiring the mixer would be NOT much cheaper than having the concrete delivered, for anything under two cubic metres.
cement per cubic metre 6.5 bags $55 [too much weight for the boot]
aggregate and sand 2 tonnes $75 [plus delivery]
Cement mixer hire $65
Plus water 200 litres per cubic metre.

Just have a few mates with at least two wheelbarrows ready for off loading the concrete from the truck.

I would go with Micks idea of 90*35 studs for form work much easier and cheaper, also consider a slight rebate on the edges for the shed frame as mentioned before.

Good luck

Pulpo