What depth concrete base for a workshop extension?

Guys who did mine insisted on using well whacked type 1 and polystyrene insulation sheet underneath the concrete slab saying that, even with reinforcement mesh, concrete on simple ballast base was almost certain to crack. Presumably the polystyrene and type I gives a touch of flexibility so things can shuffle if need be. They also reckoned that going for a super smooth finish made surface cracking more likely.

Had to go down quite a way to get all the organic material out so we really had to put a decent amount of fill in before concreting.

Finished by tamping with a hefty batten so the surface wasn't quite smooth then green waterproof tongue and groove underfloor boards for the actual floor surface. Looks good and has lasted well. If concrete has cracked underneath I neither know nor care!

Clive.

Many years ago I worked in a civil engineering laboratory, at that time I did a number of drawings for house extensions and workshops. One thing that has stayed in my mind is the willingness of the concrete delivery guys to add far too much water to the mix, so they can get it "delivered" quicker. As John McN pointed out, not good to add too much water, weaker, more crack prone concrete will be the result.

To stop this happening I would borrow from the laboratory 6" cube moulds. The look of annoyance on the delivery guys face was plain to see, but it would ensure good concrete. If I was present when the delivery was made and thought the pour didn't look right I would get the cubes tested. pleased I did on a couple of occasions.

The point of my ramblings is to suggest that if at all possible the OP could get hold of a couple of cube moulds, their presence may well ensure that whatever thickness/reinforcement is used, it'll be a goodun'.

John

HI Speedy,

concrete is 1/2 the price in Crete than in the UK.......

my 2 post car lift just sorta balances on 4x20mm studs about 1/2 meter apart with a couple of long'ish solid legs for FORE and AFT support........don't fancy that coming loose with a Landy on it.....!!!!!!!

plus the building legs (6m high)..earthquake and wind forces......

the only regs are, that I'm not quite 100% about, is around 1 ton of rebar per 15tons of concrete.......

yes it's also seismic zone........keep on rocking....hahaha.....

it's near the edge of the Euro-Asia fault line......loads'a fun.......hahaha

Bob, I hope they don't float away either but given the local geology and our annual rainfall of less than 18", I don't expect a problem - and the builder has installed several others elsewhere too that are still firmly underground.

Watch this space though!

Thanks for the explanations John. I was planning on using a well-established local concrete firm who have been going strong for 80+ years. I just noted on their website that:

...supported with full product conformity and quality assurance certification, covered by BSI Kitemarks, ISO 9001 & 14001 (EMS) and CE Marking as relevant.​

I also forgot to ask: owing to difficult access, I was planning to also use a pipeline concrete pump. Labour saving aside, when compared to spreading a huge mound, is it much less likely disturb the hardcore + mesh by pouring as the pipeline outlet is slowly dragged across the plot?

They float in the surrounding concrete when it's being poured. I'm sure your builders know the score; we braced my fibreglass moulded car pit off the roof rafters to keep it in place during the pour, which was in several stages.

I hope you're considering forced ventilation, at least for the one with the paint and solvents in. Ideally for the wine cellar as well in case any welding gasses etc accumulate there.

Good luck with the build; Here's mine

Bill

p.s. Note the concrete delivery wagon; it comes dry and is mixed on site, thus you only need to buy as much as is needed, and the mix can be adjusted according to temperature and required curing time.

Edited By peak4 on 17/07/2018 17:36:19

As the workshop is free standing and given the size, you won't need any expansion / contraction joints , not on that floor size. If it is a brick / block construction then you will need foundations [ not the floor slab ] and if it is brick / block walls then best to use 25mm insulation around the perimeter of the slab, not only for cold bridging but it also takes up any movement at the interface between the solid walls and the floor slab [ which because they are two different entities, are prone to moving in relation to each other ] Normal concrete strength for a domestic floor slab [ up here ] is 30 newton. and the standard slump is 50. Too much water in the mix has an adverse effect on concrete. The only type that can be watered down to be free flowing is the fibre concrete ie Foundation Flow.

If it were me, I would go with 30 newton at 150mm depth and the A142 mesh which should be 40- 50mm up from the bottom of the floor slab.

The little castles spacers are cute but it isn't clear how high they are. You want the steel 1in from the bottom. It becomes less effective the higher it gets. There are cheaper sources of rebar than TP.

Michael Gilligan - I used your link item for our driveway, infilled with pea gravel - very effective in stopping the gravel spreading. Additionally, I used the left-over complete pieces, on their sides, to make a very light, easily supported fence to make the wheely-bins less of an eyesore. So far most pleased and the fence has withstood some distinctly unfriendly North Sea 'attacks'!

rgds

Bill

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Thanks for the useful review, Bill

MichaelG.

Thanks from me too, you might have just saved me laying a load of slabs behind the new garage.
I've already sunk some sleepers to act as an edging, so infilling with this and some gravel might be just the job.
When I priced it up elsewhere, it was a lot dearer than this, and they're only just down the road from me a s well.

Regards

Another Bill

A properly designed slab that small won't crack...

Ask the suppliers for a retarded load, not a watered load.