Rellie,

Do the slab, it is not hard just do some prep and make sure you have everything right before you pour.

Make it slightly oversize - say 20mm more than the outside dimension of your shed. set your level you want the top of slab to be by driving in a flat topped bit of timber in the centre of your slab area - should be slightly above ground level so you don't get water issues. All levels should then be worked off that top. As above a clear hose with water in it is a very good level - the egyptions used it on the pyramids!!

Dig down so you have min 75mm depth. Install timber formwork, making sure you have min 4 stakes on the outside of each holding in place - wet concrete is heavy and moves formwork easily. Make sure the top edges of your formwork are straight and dead level as this will govern how level your slab is.

You will have to get some reo mesh to hold the slab together. (concrete is crap in tension and all concrete cracks - yes all concrete cracks) F62 is pretty standard. (For those wondering on the labelling convention the 62 means 6mm bar at 200mm centres. Any overlaps of mesh should be at least 200mm. Reo should have a minimum of 50mm cover from any edge/surface of the slab. Plastic bar chairs to achieve this can be bought from hardware shops cheaply.

If it is a hot day or the soil is very dry you should wet the soil and reo before placing the concrete.

Concrete should be minimum 20mpa. And remember adding water is a no no. Even adding water to surface to help work a finish will reduce it's strength. If it seems too dry mix in a squirt of dishwashing detergent. This works wonders. Use a shovel or stick etc to poke and prod the concrete into place and ensure no voids in the concrete, especially along the edges.

Also make sure you have a screed and finishing floats as a minimum. The screed can be a piece of timber, so long as it is long and flat and can be run back and forth along the tops of your formwork to level the slab surface. If you haven't seen it done you start at one edge and whilst moving the screed back and forth sideways you pull it toward you pulling excess concrete as you come.

Finish with a trowel and work the surface to bring a smooth paste to the top. But don't work it too much. If you have an edging tool you should finish off by rounding the edges. Lightly running a broom over the finished surface will give a non slip surface if that is important to you. Don't touch the concrete once it starts to set, this will also weaken it.

Once set try to keep the slab wet for 7 days to gain strength. A dripping sprinker or hose can achieve this or cover it in black plastic. Watering a couple of times a day and letting it dry out in between is often worse than not watering it at all.

Leave the formwork for a few days and be carefull removing it - it can take chunks of concrete with it, especially if a bit rough.


EDIT: important tip for getting your slab square. Measure the diagonals. if they are Equal then your formwork is square.

Sailhack said...
I've seen worse concrete jobs. A polyethylene strip would've been better and you can use it as a level to screed off, I would've also put it against the brick wall. Also, cutting the control joints looks better than a tooled joint imo.

Don't let the colorbond/zinc sheet come into contact with the slab, it will rust in months. I built a 9m x 7.5m flat-packed shed a few years ago (not 3x3, but might be relative?), stood the shed up in the holes, poured footings & concreted the floor last, using the poly strips as levels, allow expansion/shrinkage and to keep the wall cladding off the concrete. I inserted metal control joint strips into the slab, which resulted in straight hairline cracks, but since then I've worked out that a cut joint is better if you want to seal the floor - sika the joint, then seal the floor.

Yes -never use the shed walls as formwork, they will rust out very quickly.

Agree that cut joints are generally better for finish - but only if done right, and they do cost more if you have to hire in a cutter yourself. If you have not cut them into the slab within 6-12 hours after set, then forget them, the slab has already cracked and cuts will be useless.

For a 3x3 shed slab, forget about crack control. It will be fine without them.

Prawnhead said...

you know the drill PR...... photos ....or a "limesone retainer" is really just a double gin with slice of lime in a fancy glass

Heres the retainer i did. Very pleased with myself. Although i guess some of the tradies will wonder what the fuss is about







The planned shed is going in the corner behind the black boy. The shed there is stuffed rusted throughout and the roof is 'gone'

Thanks everyone for your help

A couple of hours next Sat and you will be ready for the shed ?

Given the size of that area i would not be able to resist incorporating the 2 existing fences into a shed,rather thanbuy something to fit in the space. you only need 2 more walls ,one with a door, and some sheets for the roof. go high for storage and headroom.
if you were in kalgoorlie , you could come raid my offcut pile

Cassa said...
[I would be happy to help .Weekends only , no fee ,no charge at all , ohh ,,,maybe 2 coronas after work is done .

Cheers mate that's a mighty generous offer.

I'm a few weeks away yet, working and getting a few OT's in to pay for it will keep me busy for a bit. Plus all those pavers and general sh!t has to be gone before i start.

I'll certainly hold you to your kind offer Cassa thanks again

No worries, just need a bit of warning when
Meanwhile , fill your wheelbarow with sand and practice pushing it up and down the driveway a few times a day , this will build confidence on the day

If your going to do a concrete slab ,before you poor it make sure you set up some 25mm orange conduit with a bend so the power cable to the shed can come up from underground through the floor on the inside of the shed wall. Works out heaps neater than having conduit strapped to the fence

japie said...

We may have to stick a fair bit of retardant in it to get it to you and you pay the fuel!

You'll need a sh!tload of retardant from NSW to WA

So what does pre-mix cost you ? Say 25MPa for comparison

Had any quotes lately ?


$660 / m3 in Tom Price.

Maybe $270 / m3 in Perth at the moment ??

$ 180 in NSW

Just out of interest, a bit of friendly building professional advice when measuring anything, whether it is mesh, timber , wall cladding etc do a check measure ,if I had a dollar for the amount of times that either me personally or other tradespeople have cut material 100mm short I would be rich, it's always that amount 100 mm it's sort of the Bermuda triangle of measurements in the building industry other than that everything else is a breeze

Mark _australia said...

P.R

you wanted tips

My biggest one - seal the bugger. No matter how level the slab is, water will get in somewhere and it will piss you orf not being able to put stuff on the floor.

(1) afetr all is finished, lift it up and sit it on 10mm dowels, then squirt silicone sealant under it all round.
Then drop it, and seal again with a bead all the way around the base.

(2) seal the bottom U-channel in the corners with silicone

(3) use tin snips to cut a vee every 30cm or so in the outside edge of the bottom U-channel so it drains quicker

OOps I forgot to mention , this is a garden shed so being out of level a touch will not be a problem , but it will however allow water to run away from the floor, 15mm would do this .
Garden shed = garden tools , etc, if ya want to drink , socialize in here you wont notice the 15 mm

look around and get a concreter that works for one of the big builders. They have left over concrete from housing pours and will do you a deal. Last two I have had done they ask you to dig the hole and put down the form work. They do the pour its over in about 30 mins and is cheap as chips.

You blokes are all dead set legends. I cannot believe how much goodness is in this thread.

Thanks all. Looks like I'm a shifting stuff and digging LOL

Mark _australia said...

Strewth $300

I'd rent a cement mixer for half a day, you'd have it mixed and poured in 2hrs ish

Maybe $100 for rent and product

I can't tell if you are serious or not. If you are, I'd hate to be the poor sucker behind the shovel.