Concrete slab question

[n74tg]

How comfortable would any of you (particularly John and Glenn) be with a partial basement room slab of only 3" thickness.  The slab will be roughly 12' x 22' in area and reinforced with #3 rebar (3/8 diameter) on a 12" grid.  The floor will not be subject to any heavy point loads.  I expect the heaviest thing in there will be wooden storage shelves and a wooden worktable.  I live out in the county so there will be no building code issues with whatever I choose to do.

I'd like to use a 4" slab, but am limited to 3 yard concrete truck capacity per load, and also to one load per day max.  I don't particularly want a cold joint in the concrete slab.   The concrete they typically mix is 3000 psi concrete, however I suppose I could ask for stronger if need be.

If there is something I am overlooking please speak up.

[Rover]

My 2 cents.
The key to the success would be the granular base.  If it doesn't have soft spots, then nothing should happen to the concrete.  
If it were me, I'd go with 4" thickness and the cold joint.  Cold joints are not a bad thing.  Create a key or step in the edge of the first slab.  For reinforcing, I'd use welded wire mesh.  30Mpa concrete will be strong enough.
Others on this forum would have more practical knowledge.

[Billy Bob]

I agree with Rover... a joint isn't such a bad idea.  An expansion joint would be good, anti-crack wise.

Aren't you, like, within a quarter yard of your three yard limit, anyway?  I'm tired enough I may have bollixed the math, but that's what I came up with.  Wait a minute, at one third of a foot thickness, that's about twenty square feet, (I think! )[smiley=lipsrsealed.gif]... I'm actually pretty good at this when I'm not brain fried... I'll look at it again after some sleep.

[n74tg]

Billybob:
Your math is correct; I am close to the 3 yard limit; however I didn't mention that part of the footer for the basement slab hasn't yet been poured and I was gonna pour it at same time as slab.  Fresh measurements today indicate the footer will take 3/4 yard so now a 4" slab and the footer can't be poured from the same load.  However, a 3" slab and footer will scare 3 yards to death; so my original question still stands.

I've already bought, cut and bent the rebar; so switching to wire mesh is out of the question.

John and Glenn - I'm interested in your opinions too.

[JRR]

You might consider pouring on two separate days ... should take several days for moist concrete to cure ... just keep moist by laying tarps, and spraying with water,  over first pour after intial setting.

The first pour should be over entire area ... just a bit thin.  Second day's pour will be on top of first pour  ... hence, no vertical joints.

Then relay the tarps ...or keep spraying water.... after intial setting of second pour.

[glenn kangiser]

Here is what I can tell you about this, n74tg.  I am assuming this floor is a fill floor only and has nothing to do with supporting the structure.

Maybe no one else will agree with me but I put a 21000 lb forklift on a slab that was 2" thick in a few places because the ground was so hard we couldn't dig it.  We didn't crack it.  Note that we added fibermesh- great stuff and only costs a little more-- it can many times take the place of rebar.  I have seen floors poured by professionals 6" thick -laser screeded --cut every 20 feet -- with cracks in them --  You know why --because concrete has a mind of it's own --It knows that you don't want a crack so it does it. :-/

I would never guarantee a customer that their floor wouldn't crack because I knew that the concrete would hear me and split itself right down the middle.

In my opinion 4" of concrete is way too much for a house floor - 3000 PSI compressive strength -- I don't weigh near that much.

Case in point - in India where there is not enough money to buy all the concrete the Concrete company wants to sell you they get by with 1/2 " thick floors with hessian under it over a non-compacted soil base.  They do this in England also.  They put a plunger post every 3 feet then fill it then put the cement over it.

My opinion is that even at 1" thick - done properly it will still hold - if it gets a crack - patch it or seal it - you'd have to do the same with a 4" floor if it cracked.  I have 2 of these experimental floors now - based on Ken Kerns information from The Owner Built Home.  Below the surface you can't tell how much concrete is there - Most sidewalks are a nominal 4" -many are only around 3"

Nothing against the guys who want to use 4" either - if you feel better about it that is the way to go.  I just don't see where a human floor has to be as heavy as a truck floor.



The above floor now has a washer and dryer on it and it hasn't broken through.  The creator of it in India rated it at 40 lbs per square foot but had about 450 concentrated load on it in places without breaking it.  I substituted aviary netting experimenting - hessian is probably better (burlap).

[Billy Bob]

n74tg what I think I was trying to figure is, with three yards to work with you'd be closer to four inches than three, anyway, ( that's before hearing about the footers).  If you want to lean towards a thicker slab, how about a pallet of small concrete pavers distributed inside your rebar grid?  Or like jrr says, two pours... you could score the surface of the first pour with a rake before it sets, to promote a good mechanical bond area.
I agree with Glenn, the heavier slabs are over-engineered for domestic purposes.  You can maybe get a decent floor with no pour at all by using soil cement... lightly till portland cement into the top few inches, moisten with a hose, rake even, then run a plate compactor over it.  I know this works for walkways and driveways,  and have read it makes a decent  floor.  Gotta say I haven't tried it for a floor, but now am intrigued enough to maybe experiment.

[John Raabe]

Yes, yes and yes. The base under the slab will likely determine the success of a thinner pour. Fiber mesh additives make concrete much more forgiving.

For residential floors the minimum thickness before structural problems might happen is not known. It would be an interesting test but not a test likely to be done in the U.S. (who would profit?) I expect an inch and 1/2 would be plenty over a good base and with control joints for shrinkage and temperature response.

Ken Kern documented a variation that was being done then in South Africa. "A 3" layer of 8 parts crushed stone to 1 part cement is spread over tamped earth. This no-fines surface bed is immediately covered with a 3/4" cement screed, merely for a smooth finish." (p. 282, Owner Built Home) This method uses less cement and no sand, is 3" thick overall and has a working surface 3/4" thick. I assume sand was expensive or not available.

[PEG688]

Ok maybe it could work  but why take the chance it will fail to save 1 yard of concrete or two , $150.00, If it goes south , you decide to puta wall here , etc etc . Not worth the effort to save a few dollars up front. For short term gain. Just MO , no offence inteneded nor implied

[n74tg]

Okay guys, here's what I think I'm gonna do.  Divide the 12x22 slab up into two 12x11 slabs.  Put an expansion joint in between (rebar grid will run thru the joint).  Pour the back half of the slab (and the remaining footers) as one pour of 4" thickness.  Then pour the front half (as 4" also).

I particularly like this for two reasons:
1.  I didn't mention that in the back half 12x11 that one corner (roughly 5'x7') is to be an underground tornado shelter.  Footers have already been dug and poured for it.  Vertical rebar has already been put in the footers, rebar which will penetrate thru the slab and run all the way thru the block walls and then bend and be connected into the concrete slab roof for the shelter.

2.  As I am pouring and finishing off this slab by myself, doing a smaller slab seems infinitely easier AND gives me a convenient place to stand when using the bull float.  

[glenn kangiser]

In that case then I would agree with the rest.  A bit more of a reason to have the heavier floor than just to have a smooth flat place.