Concrete prefab, base size., Have I mucked he size up?

[Chris Bacon]

Asked this over at Cormaic's, but no joy (maybe (much!) laterrrr, but panic on, so here now). This is the slab I've just made with my Aldi concrete mixer).

Nominal 6000mm by 5000 mm prefab garage for use as garden shed (by my measurements it will be 6020 by 5035 actualsize).

Slab is 6050 by 5050.

Have I mucked it up and made it too small to use?

The "Plain ConcreteHardstanding" section of (pavingexpert) suggests at leasat 150mm between edge of slab and garage structure. I've got 15mm, give or take a couple of mil.

DATA (if required):

ASCII art attempt at floor slab section:

.-------------------//----------------, | // | 200mm | --------//-------- | |__________| // |_________|

600mm

The edges (keels) of the slab are 200mm deep, 600mm wide, all around, changing to 100+mm in the middle, with A142 mesh.

Corner post weight 100kg, side post weight 70kg, section weight 36kg each 4 sections per bay so 144kg/bay. 6 bays per side 5 bays for back.

Therefore weight of concrete structure each side is ~1414kg, back is

1160, front will be much less because mostly 4 x side-hinged doors, roof structure will be angle iron + Sterling board + corrugated iron.

[Jim GM4DHJ ...]

[Brian Gaff (Sofa]

So were you economising in concrete? Brian

[Chris Bacon]

No. There's probably rather more than 4.5 cu m there. I measured it carefully, and the base is spot on. I had previously read (somewhere) that the base should be just larger than the building. mCormaic's site says 150mm. Lidget Compton say 3" larger all around.

The question remains unanswered (apart from "sound"), unfortunately. If I need to do anything about it, I would rather know now, when I have some work to do welding up trusses, which will take time, during which more concrete could set.

What I am worried about is that the edge may give way, or the keel deflect and "rotate".

[Tricky Dicky]

Since you already have laid the base and the purpose of the extra width is there to prevent all the weight from being right on the edge thus prevent it crumbling. I would dig out a trench all round the base to the depth of the keels allowing an extra 150mm all round to be added. This could be reinforced with rebar and as a belt and braces to tie the extra concrete to the existing base I would drive some long concrete screws into the side of the existing base with about half the screw protruding.

Richard

[Tricky Dicky]

The manufacturer of my timber garage provided me with an exact base size, this was 100mm wider all round than the garage exterior measurements. Then again this was for a much lighter timber structure.

Richard

[Chris Bacon]

I was hoping not to have to do that. Would it really crumble? A quick calculation on moment just now (!) (assuming downward force exerted by wall/roof is 1750kg/side at 0.05m in from edge, force of keel alone centred 0.3m in 1440kg/side) in will easily balance, unless I have ocket that up too, so rotational force won't be an issue, Mix is C20, approx. However, if I am to do anything, perhaps I could go out further on one side/front, and leave the other sides as they are. 2/3 cubic metre of concrete, about. I blame the booze/.

[F Murtz]

Any reason why you can not make the shed smaller?

[John Rumm]

4.5 cu m by hand through a mixer is quite impressive! Was that all "one pour" so to speak?

I have seen plenty of slab foundations that were made exact size, and they have held up fine with brick garages on them...

[Andy Burns]

*If* it's required to comply with building control, then minimum 150mm either side of wall is what part A section 2E3 says ...

[Chris Bacon]

It's a prefab concrete building. While possible (I could replace bays with timber, for instance), I'd rather not.

[Nikki Smith]

It will be fine. You have effectively constructed a raft rather than strip footings, so any residual out-of-balance loads will be resisted by a small bending moment the central slab, which is fine assuming your mesh goes across everything. (and probably fine even if not)

The only issue would be if you have to make any fixings into the concrete near the edges as it might spall.

[Chris Bacon]

No, three lateral ~2m x 5m sections, each end taking the most concrete due to keeled structure. I nave named my Aldi mixer "Wonky Donky", because if you say that very s-l-o-w-l-y that's the noise it makes, the drum's drive is pressed into the drum itself, which is not very accurately made. Wonnnnkeeeee donnnnkeeee.

I am on the horns of a dilemma. Say I'll have 1700kg each side, spread over 6m length. 280kg to the metre, with most of the load being at the bases of the posts. The load won't be as evenly distributed as it is with brick.

OTOH my *other* "workshop" is similar, but is a single garage, and its base slab is only just big enough. The posts are within 1cm of the edge, the bays being set back ~3" due to being slotted into the posts. However, that slab, which has been there for decades, is only around

3-5" thick, on "hardcore" of old bricks, china, broken concrete, the odd copper pipe & all sorts. I'd guess the posts/bays are similar weight. The new "shed" has 8" thick solid concrete keels over a DPM so should be stronger, all else being equal.

The old:

The new:

(first bay, DPM in, awaiting fabric mesh on meshmen, shows 8" keel, 4" centre) (work in progress, middle bay) (last section, showing mesh fabric on meshmen, "front" of slab) ("The Diggings" with Wonky Donkey and a sunshade for the concrete, doubling later as a cover to stop it drying).

I'm tempted to try it. I'm also tempted to trench out one side another

200mm and concrete that, although there are disadvantages there too.

[Chris Bacon]

I have posted some pics on imgbb in another psot.

The only fixing I need to make is for the two side-by-side steel door flamess at the front, where I propose to weld 3 pieces of 50mm x 6mm flat bar to the bottom of the door frames which form almost the whole of the front of the building, bringing the bar back into the building by

150mm or so, and bolting that to the floor. More ASCII art:

/ Front Elevation \ <-roof slope exagg'd / \ ############################################### <- 6 x 2 "raiser" @%%%%%%%%%%%%%%%%%%%%%%#%%%%%%%%%%%%%%%%%%%%%%@ @% %#% %@ @ = concrete post @% %#% %@ # = timber post @% %#% %@ % = steel doorframe @% %#% %@ = = concrete raft @% %#% %@ @% %#% %@ @% %#% %@ @% x %#% x %@ @%x %#%x %@ =x=====================x=====================x= =============================================== ===============================================

x = steel flat bar retn'd into bldg and bolted to floor "3D effect"

[Nikki Smith]

I've done hundreds of pre-pour rebar inspections and that workmanship is better than most :)

Really, dont bother ... it's not needed and getting both side to act as a single lump is very hard.

[John Rumm]

Nice job - and you managed to avoid trampling the mesh down into the mix as well :-)

With the keel and the fabric reinforcement, the slab itself will do a fairly good job of spreading the load anyway. Any torsional load on the keels will be counterbalanced by that on the other side. I think you are over worrying!

[Chris Bacon]

Tank you sah. I decided to put some blocks off a pallet throuhj the mesh and put a scaffold board on, needed a bit of care tipping the concrete out of the barrow though. I have some meshmen left over...

I will have to drink about that overnight :)

[Chris Bacon]

Thank you too!

"Gissa job. Go on gissit. Gissa go, go on. I can do that". Never did it before though.

Are you a BC person, then? My base has not been approved.

I have looked at the thing again. The centre line (longitudinal) of the

4 slabs that make each bay is about 60mm in from the edge. The A142 mesh does not reach out that far. I take your point about joining concrete. There would be effectively a strip of concrete 200mm wide along one side, bearing the weight of one side and half the roof structure.