Does anyone have knowledge of laying concrete slabs?

[SafetyThird]

I'm looking at turning a section of our barn into a garage space. I have an MX5 inherited from my brother which I intend to rebuild over the next couple of years. I can board off the side wall easily and I'm installing shelving along the opposite wall and can build a set of doors at the end. With some LED strip lights in there, it's a decent working environment.

The one issue is the floor which is hard packed clay and gets very dusty and isn't the greatest thing to by sitting/lying on when working on vehicles. I'm thinking of putting a concrete slab down instead. a 150mm slab should be more than enough and would need 5 cubes of concrete, probably fibre reinforced. The big question is can I just pour the slab on the solidly compacted clay, with a vapour barrier in between.

The barn has been there for roughly 40 years, the ground we're on is solid clay as far down as we've had to dig elsewhere on the farm (15' for a sewage treatment plant install). You'd need a pickaxe to make a dent in the barn floor and so I figured that's likely more solid that wacker plated gravel that normally goes down for a concrete slab.

Would I need 150mm or would 100mm be enough? I figured for the additional cost it's not worth skimping. Also, would I need to put steel reinforcing sheets in? I have a load of old bricks behind the barn and could use those to support the mesh off the floor.

Then it's a case of could I feasibly do the job on my own, or with another person to help. I'm not looking for a polished finish, just smooth enough for a garage floor. I've laid one small slab before, levelling with a board against the sides of the shuttering but found it wasn't the easiest thing and only after found out about concrete vibrators, which can be had on ebay for £60 https://www.ebay.co.uk/itm/203754351593 ... R7LMv-3WYA

Alternatively, I could hire a bigger concrete poker and a powerscreed which I could probably manage with a friend.

I can back a truck up to the entrance to the barn and hopefully they could run the chute a decent way into the area to be filled and then it's a case of rake it into place and vibrate it flat.

Does that sound remotely reasonable? Picture shows the area. I'd put a board between the main upright posts on the right and across between them at the front. Obviously I'd clear everything out, blow out all the dust and crap and lay in some vapour barrier.

[Galahad]

I had a new double garage built here a couple of years ago. Get the floor as flat as you possibly can. Mine was finished with a circular skimmer (what are they called?) but not actually polished smooth. Even if you don't want it polished flat you will want to paint it (otherwise you will be plagued with dust anyway) and starting out as flat as possible will help greatly. I spent about 5 days putting two coats of paint on mine.

[Oliver90owner]

It needs a hardcore base, for a start. Any ‘heave’ due to changing water levels/content can easily lead to cracked flooring. Fibre reinforcement is generally sufficient. Concrete is strong in compression but weak in tension. 100mm, on a well prepared base would easily accommodate a MX5 - if that is just a car. But what else might you be putting on it?

[Oldgreybeard]

The "circular skimmer" is a power float, gives a superb finish in the right hands and is definitely the way to go to get a smooth and flat surface. The key is to know when to start power floating. The concrete needs to be poured and trammelled to get it level, and will need to be vibrated if there is steel fabric included (depends on the size of the slab - the steel mesh will prevent cracking on large slabs). Once the concrete is down and levelled you then need to wait for it to start to cure - very temperature dependent, but usually more than a couple of hours. Once it's at the right stage the power float will polish the top surface and get a very even and smooth surface.

My garage floor was laid like like this, the build up being 150mm of compacted MOT 1, the DPM, then 150mm of non-reinforced concrete. I used 150mm because the slab was quite large (6.5m by 4.5m), 100mm would be OK on a smaller slab. This was power floated about 3 hours after being poured and trammelled and gave a surface that didn't need anything else doing to it, although I opted to give it a couple of coats of light grey epoxy floor paint, just to make it easier to clean and reduce the chance of staining. The painted floor does give a useful psychological effect, too. It almost encourages me to get on with things, and not put them off!

[John_S]

If 150mm is equivalent to 5 cu metres, then I would go for that. 10mm would be 3 cu metres and the concrete company would charge you for the empty space in the wagon, so there would not be much savings.

Lots of useful information here

https://www.pavingexpert.com/concrete

[CrofterMannie]

I'd be tempted to pour it directly onto the hard clay ground. A car isn't that heavy and it doesn't look like you'd be able to get anything too big into that she'd. I would, however, put some steel into it. It makes it a lot stronger.

For comparison the last shed floor I did is 125mm with 6mm steel mesh which is plenty strong enough for driving a 4 ton tractor over.

You should be able to get a good enough finish without using a power float. 2 people with an aluminium tamping bar got a good enough finish for the floor in the extension I'm building.

[Joeboy]

Last one I did with my son for the cabin to sit on. 4mtrx4mtr and about 150mm thick, scraped off the surface earth and dug down until firm, shuttered and the we went for it with a standard cement mixer. He loaded and mixed, I ran the wheelbarrow. We dressed the surface off with a edged board. Took about a week to dry and I did a fair bit of spraying water on it during the curing process. Turned out well.

I agree with crofter mannie, I'd likely scrape, shutter, lay in some steel mesh and pour it. There may be an XJS or Bentley in the future, better to have it really strong.

[Stinsy]

When I did this I did the following:

- Dug down and got it as flat as I considered reasonable.
- Tamped the earth down with my boots (considered hiring a whacker but couldn't be bothered).
- Made the form with 8x2.
- Backfilled earth around the outside of the 8x2.
- Laid chicken wire.
- Mixed: 2-bags ballast, 1 bag sand, ½ bag cement, with a capfull of "plasticiser" and a glug of PVA in a bucket of water. Made the mix pretty sloppy.
- Got into a rhythm of tipping a mix out of the mixer into the barrow, making a new mix and setting the mixer going, barrowing tipping and raking the concrete, and repeat.
- When the form was full I used a 4x2 to flatten it off, moving it constantly left and right while pulling it towards me. It took me a few passes to get this how I was happy.
- I then cleaned up all the mess and hosed down the mixing area.
- Over the next couple of days I sprayed it with a light mist of water from the hose and used a "float" that I'd made out of a piece of 20x95mm wood and a bit of 4x2.

The first and most important thing to realise is that this is exceptionally hard work! If you can possibly get someone to help you it'd make all the difference, I was on my own and up against concrete curing on me.

My next tip is to get the stuff delivered. I went to B&Q after work on Friday night, loaded 600kg of sand/cement onto a trolly, got it to the till, paid, got it to the car, loaded it into the car, drove home, and unloaded it. Ready to get going first thing Saturday. On Saturday morning I woke up knackered and sore before I even started the job.

You can use "fibres" in the mix instead of chicken wire. But B&Q had chicken wire, they didn't have "fibres".

That i how I did it. Not claiming it is the perfect method. There is more than one way to skin a cat. You'll find loads of people telling you "you have to do this" or "you have to do that", and you'll find conflicting advice all over the place. Just do it how you want to!

[nowty]

Whichever way you do it I recommend a concrete truck or at least get stuff delivered.

Reminds me of my own enforced concrete laying in difficult conditions.



It still annoys me that I got one of my concrete edging stones the wrong way around so the pattern does not match. Its the bottom left one at the waterline.


I might go down the hole later and take another picture to see how it looks 10 years later.

[Oldgreybeard]

Doesn't seem like that heroic saga was ten years ago. I still have a strong image in my head of plasterboard props holding back the shoring timbers . . .