Concrete slab strong enough?

[unclebhuna]

Hi

I've just started to construct my WFO and intend to go for a 42 inch size with a half brick igloo surrounding the firebricks. I have poured my slab which is 4inch thick and poured onto clay sub soil. I didn't use rebar in the slab and am now wondering if it will be strong enough. Before I go any further I was just after any thoughts as to whether it will likely be fine or I should bite the bullet and just rip it out and start again. First post so go easy on me. Thanks in advance!

[NCMan]

Re: Concrete slab strong enough?

I personally would not build on it if I were building it. There are quite a few factors that go into designing a support slab for things such as your WFO. First of all, let's begin at the ground and work up from there. You have clay soil. Is it prone to freezing there? Pouring on clay where it freezes is not a good idea. You should have 3 or 4 inches of clean gravel under your slab. Hopefully, it is located in a well draining area, also. Is the base virgin/undisturbed soil or fill? Next is slab thickness. You can't just go by how thick it is. I can pour a 4 inch slab that would be superior to a 6 inch thick one. Alot goes into it that affects the suitability of the slab. The base, the mix design, reinforcement, slump concrete was poured, drying/curing conditions all affect the strength of the slab. Too much to get into on here. As I said, if I was asked to build on it, I would politely decline. But, that's just me. Hope I was easy enough.

[stonecutter]

Re: Concrete slab strong enough?

+1....directly on clay is a deal breaker.

[unclebhuna]

Re: Concrete slab strong enough?

Thanks for the replies. Looks like I will have to have a rethink. No heavy frosts here and the ground is undisturbed but better safe than sorry. Wouldn't want to get part way through the build and find any movement or worse.... Now to the tricky bit of getting the slab up as it was formed as part of the same pour as some wall footings. I need to plan better as this project moves on!!

[NCMan]

Re: Concrete slab strong enough?

Can it be sawed off w/a concrete saw? It being a part of your wall footings is another no- no. Both slabs should be able to move independent of each other, not tied together. The way it is, if the slab or the wall footing settled/shifted, it might have an effect on the other. Just generally not a good idea to do it like that. If you do cut it away, you might want to place a piece of expansion joint between them before you repour. Photos help. There are lots of good, knowledgeable, helpful guys and gals on here to help along the way.

[unclebhuna]

Re: Concrete slab strong enough?

That sorts it, it's coming up tonight. I'll post pics once I start again!

[jeeppiper]

Re: Concrete slab strong enough?

The clay soil probably moves more than one would think. You definitely want to reinforce the slab....that is what gives it strength.

I probably went overboard, but my township requires the slab to have footers that go down 36 inches! So I have a monolithic slab with a 4" inch reinforced top with footers (i.e. walls) that go down each side 36 inches deep. My foundation is a monster, but I do have piece of mind!!!!

[unclebhuna]

Re: Concrete slab strong enough?

Wow. That is quite a footing! I certainly won't be doing anything that deep but will definitely go down a bit deeper with a nice, thick, compacted, hardcore base with a 6 inch reinforced slab on top. I'll start a bit of a build thread once I start again as I have found others very useful for ideas and the pictures really help visualise the project. If my build (coming from a complete building novice) helps inspire fellow novices to give it a go then I will be happy. I know I will make more mistakes on the way, but at least I made this one early on; before I had gone too far down the build process to easily rectify.

[Novaslo]

Re: Concrete slab strong enough?

I am starting my build and FB calls for 5.5 inches depth for the slab....I also live in an area with heavy clay soil. I dug my area down to 9 inches. I will be using 3-4 inches of Sand and then 5-5.5 inches of slab with plenty of Rebar. This is typical in our area if you are doing it correctly. I have a large patio that took 30 yards of Cement and goes around the house with walkways etc. This patio was built with 4 inches of sand and 4 inches of concrete, it has been 7 years - virtually no cracks.

Sand is your friend when you have clay soil.

Good luck.

[unclebhuna]

Re: Concrete slab strong enough?

Hi Novaslo. I've bitten the bullet and started ripping my slab up tonight. Its 6 inches think in places and essentially laid directly onto the clay rather than clay soil. The house is on the same surface and no problems with that. I suspect it would have been absolutely fine but better safe than sorry. I was more concerned about it being connected to the wall footings, rather than being a separate slab, and could see any movement in either causing problems. New footings will be laid in due course on a nice compacted bed of hardcore on top of the clay, followed by a 6 inch, reinforced slab. It will be going nowhere after that!

[pikemonkey]

Re: Concrete slab strong enough?

I'm not a builder, but reckon you should get some local advice before you rip it all up.

From what i see you have removed all topsoil.claysoil and dropped a slab straight onto the clay.
Maybe because it's not reinforced, it's no use as a slab, but from a layman's point of view, isn't your existing slab already a layer of well compacted hardcore? If you don't mind the existing height, can you not run a whacker plate over the slab and then pour another reinforced slab on and around it? I could well be talking rubbish, but pulling up a slab is a lot of graft if you can get around it.

[Novaslo]

Re: Concrete slab strong enough?

Good luck....
You are getting some good advice here. After you break your back pulling it out you will be in better shape (Physically) at least....

[unclebhuna]

Re: Concrete slab strong enough?

Too late pikemonkey it's already 3/4 up. Certainly back breaking work but every whack with the sledgehammer is a lesson in preparing properly in future. I tend to learn the hard way...

[pikemonkey]

Re: Concrete slab strong enough?

How are you getting on?

[unclebhuna]

Re: Concrete slab strong enough?

Old slab up now and just need to dig it down a bit and get my rebar sorted and ready to pour. A bit behind as been building walls all weekend but hope you have it poured by this time next week. Any suggestions on my concrete mix? Got ballast that is 3 parts stone, 2 parts sand. Was just going to add one part cement and keep it simple.