Do I / Should I "underpin" shed slab? and some newbie questions!

Hi all great forum, complete newbie to DIY but going to give everything a crack, learning heaps.

First job I need to get out of the way, had a 6 x 9 shed put up, on a 100mm concrete slab with reo, long story short, slab was done without footings, shed builder picked it up and underpinned the corners (cut out back corner and poured some footings with rebar/rods).

Anyway, when he finished the job he suggested if I wanted to I should dig away the sand/soil from the front edge of the slab (roller door / entrance) under the vapour barrier and concrete in some footings if I want to back the car in there to work on it etc, would reduce the likelyhood of it cracking the slab?

Ground was subjected to lots or rain over 2 months prior to the slab so I figure it's was good and compacted, perth sandy type topsoil, gets better further down obviously.

1. Just wanted some thoughts on it, what is the likelyhood of the it cracking from just driving a regular 4WD style car on it?
2. Would I have to under pin the length of the entrance? or could I just dig down and put in a could of extra footings say 1000mm apart and 300mm deepbetween the 2 corner footings?
3. Would the footing shrink away from the pad when it dries? ie causing it to be lose contact from the slab? so worse than leaving it as is?
4. And would it be worth it without connecting the footing with rebar
5. Also silly question, I got left a bunch of this already mixed sand/aggregate, just wondering if the concrete ratio would still be 1 part cement to 3 parts (already mixed sand/aggregate)

posted this in the concrete renovate forum as well but there doesnt seem to be a heap of activity in there judging by the other threads, hope you guys can help

Cheers

Hi Snowman,

Welcome aboard :)

I assume your shed is 6 metres X 9 metres ? I dont know anything about WA's soil conditions or normal shed practice over there but - Typically a slab that big for a building would have a strip footing or edge beam poured around the entire perimeter of the slab and also possibly at regular intervals in between. Either poured intergral with the slab (an edge beam) or poured first and then tied into the slab with 'Z' shaped bars (strip footing). Which would of course give you a nice solid section across the entrance. 9 metres seems like a along way for a 100 thick slab to go without some stiffening. None of this means your slab will necessarily fail BTW, I'm sure sheds in the old days were built with less than yours and are still standing !

All concrete cracks to some extent, hairlines are ok and something in the order of about 2mm is a concern.

How is your shed built ? Are there columns sitting on 100 thick slab alone ?
Why did the builder underpin the corners later ? Did the council/inspector him to direct to do it ? Did you have to get a building permit for the shed ?

As I said I'm not familiar with WA's practices so perhaps someone in the know will chime in but for a building permit shed on the east coast the slab would resemble a house slab.

Cheers,

Sam

Hi Sam

Yep building permit gotten and approved for shed.

Story goes original shed supplier gave me the impression that 100mm slab and reo was sufficient, (also from the preliminary plans) pulled the trigger based on that, concreter I hired said it was cool (didn't give me any reason to think it wasn't right).

Original shed was to be dyna bolted to slab, supplier the tried to switch plans on me to posts in ground, when queried, dogged them for final plans, noted the slab requirements were different to what was told me originally (fkn salesman) changed back to bolted but then switched the brand and price, said cya later at that point.

Found a shed builder that supplies locally, went back to bolt in, at the point of inspecting the slab he was all needs deeper footings for where the post bolt in, hence the under pinning.

Also mentioned the deeper skirting at that point, so he basically said the same thing about the thicker skirting around the edges, the 6m span is where the opening (single double roller door)

Any suggestions as to my questions?

I was amazed to discover that house footings only need to be 300mm deep on sandy soil in Queensland. The ground around my parent's place, which is near the beach on sandy soil, is low enough that you can see the underside of the footings most of the way around it.:o

It should be easy digging, so if your going to reinforce the lip of the entrance to the shed, I'd do it right the way along, with a bit of rio thrown in. I'm no concreter, but the concern you raise about the top of the added footing actually touching the underside of the slab seems legitimate. If you dug away along the lip and rammed roadbase in instead at least you could be assured that it was packed tight.

The building permit is to build the shed usually, not necessarily a structural certification. I struggle to believe the 100mm slab without any edge beam or footings of some sort would comply with design codes.

Quote:

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Originally Posted by 95snowman

1. Just wanted some thoughts on it, what is the likelyhood of the it cracking from just driving a regular 4WD style car on it? Probably
2. Would I have to under pin the length of the entrance? or could I just dig down and put in a could of extra footings say 1000mm apart and 300mm deepbetween the 2 corner footings? I would lay a continuous strip footing 300x300
3. Would the footing shrink away from the pad when it dries? ie causing it to be lose contact from the slab? so worse than leaving it as is? Ensure the footing extends say 50mm beyond the slab and 50mm higher to enable hydrostatic pressure above the slab bottom, then shouldn't be a problem
4. And would it be worth it without connecting the footing with rebar I would use some 3xY12 in the bottom of the footing, held off the ground by 75mm chairs
5. Also silly question, I got left a bunch of this already mixed sand/aggregate, just wondering if the concrete ratio would still be 1 part cement to 3 parts (already mixed sand/aggregate) 1:3 should give you a reasonable mix

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Dai sensi the slab does have footings 600mm deep, connected via reo

Thanks for the answers !

quick one is it ok to put rocks / old concrete at the base of the trench? I've got a bit that was left over from the slab / old curbing that just hasn't been taken to the tip, figure I can kill 2 birds with this job, the rocks are quartz that got dug up and save a little on materials

It is good practice to thicken any edge for traffic.

If the standard thickness is 100 a typical rule is to double the edge, say to 200. Tie is in with starter bars and reo.

There is no need for a full perimeter beam in non-reactive soils. A standard footing would be a 300x300x300 under slab where each of the portal columns are.