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ChiefVOL 01-19-2012 05:11 PM

4" concrete slab, form, pour and finish
I plan on installing a 40' x 25' 1,000 sq ft 4" slab floor for a new home I am building in Dunlap, TN. I have a never used 75 year old cinder block foundation along the perimeter of this 40 x 25. Can I pour the slab over this foundation? Should I? Will the foundation under the slab add structual support for walls built on the slab over the area where the foundation is? Ball park figure, can I pour this slab for $1,200. I suppose I will pay someone to help me form it and finish it. Actually, I will help them since I do not know what I am doing! I am designing and building a passive heated, trombe wall solar home of 750 sq ft - the other 250 feet will be porch. Any suggestions would be greatly appreciated.

Daniel Holzman 01-19-2012 05:30 PM

First off, I don't know why you would extend the slab over the concrete block. At only 4 inches thick, the slab has little structural value, it is mostly there to provide a nice walking surface. Hopefully those blocks sit on a proper foundation, as they are going to hold up the walls of your house.

As to cost, well this is not a cost estimating forum, but I can offer you a bit of a clue. You are placing 1000 sf of concrete 4 inches thick, which is a bit over twelve cubic yards in place. On a commercial job, you would pay at least $300 per cubic yard in place, even in the lowest cost part of the country, which would put the slab at over $3600. Maybe in rural TN, if you do some of the work yourself, you could get by for less, but not by much. Unless there is free TVA concrete available, I don't see $1200, but call around, get some bids. You do want control joints in that slab, right? You do want a finish on that slab? And you might even want a little steel mesh. And as for helping out, if you have no experience, reality is you may be more hindrance than help, when placing a large slab the key is to work quickly, efficiently, and correctly the first time. Too big a pour to be learning on.

ChiefVOL 01-19-2012 07:47 PM

Shows how naive I am. I had forgotten about control joints. I suppose they can be hidden under interior walls? Wire mesh was always used on construction sites I have seen. The price I was suggesting was for the concrete only. I believe I can hire a local person to assist in forming and finishing - but in my heart I know you are right to utilize an expert. If something is wrong here at the bottom, no use in moving upward I suppose.

The reason I considered pouring the slab over the existing foundation is to keep from digging another one. Should the slab be poured just to the edge of the existing foundation - it is at ground level - then use the foundation to build the walls on? Should I pour the slab before I start the dry-stacked cinder block walls or lay a few rows of block then pour right up to the blocks so there is s snug fit?

joecaption 01-19-2012 08:00 PM

Make some calls and let local the pro's tell you how to do it. Do not be second guessing there ideas from people on this site that have never even seen you site, do not even know the local soil conditions.
I 100% agree the last thing I need is a homeowner that has never done this to be standing around telling me how to do my work.

ChiefVOL 01-19-2012 08:04 PM

Yea, you're right about that. I would only get in the way and P... them off. I will continue to educate myself. Thank you for taking the time and your wise input, I really appreciate it!

cleveman 01-19-2012 10:33 PM

I'm not sure about the frost depth in your area, it may be quite shallow. You may only require a reinforced perimeter, but let me explain a good way to do an on-grade slab in my area.

Our frost depth is 42". The footings need to be 42" deep. So what I do is get a guy to go around with his trencher and dig out a 8" wide trench about 42" deep. Then I go and head off the corners and put some 2" polystyrene on the inside of the trench, leaving a 6" wide trench. On the top of the trench, I form up, with the 48" of polystyrene on the top of my forms.

Then I fill this trench with concrete as soon as possible. I put a rebar down near the bottom, and one up near the top. I put in vertical rebar every 4 feet or so.

Next I come back and bend those rebar down into the field and pour the field. On the floor of the field, I can have a plastic vapor barrier or some more polystyrene. Obviously all the plbg is in place beforehand and, yes, lay your walls out where the control joints will be.

I would pour a 24" wide building in two pours 12" wide, so there would be a form down the middle which would be removed the next day. Rebar is set in the first pour, which ties it into the second pour.

And there you have it.

This is a diy forum, so I'll encourage everyone to diy. You won't know when you're in over your head until it happens.

To this day, I still feel nervouse when a concrete truck drives by.

Anyway, the 12' pour lets you float and trowel with a stick from both sides and ends, then you can hand trowel from both sides out about 4'. This leaves 4' in the middle. When you can get out on the concrete, you get out on it with your foam pads and run down it one time and you are finished. Remember, this is not your finished floor. You need to produce a floor which you can lay tile on top of, not play with a baby on.

ChiefVOL 01-20-2012 11:03 AM

Thanks Cleveman - You make me believe I CAN DIY. I will continue to study. I like the idea of two pours of 12' each. Much more manageable and two f us should be able to finish it. Should I build control joints into the pour or rent a concrete cutter and do it after pouring?

jomama45 01-20-2012 11:11 AM


Originally Posted by cleveman (Post 828509)

This is a diy forum, so I'll encourage everyone to diy. You won't know when you're in over your head until it happens.

Although I agree, one of my favorite sayings is "show me someone who's never lost a concrete pour & I'll show you someone who's never found their full potential"............ :laughing:


Originally Posted by ChiefVOL (Post 828868)
Thanks Cleveman - You make me believe I CAN DIY. I will continue to study. I like the idea of two pours of 12' each. Much more manageable and two f us should be able to finish it. Should I build control joints into the pour or rent a concrete cutter and do it after pouring?

Cutting is generally preffered inside, as well as alot easier to DIY.........

ChiefVOL 01-20-2012 11:32 AM

One of the few pours I have ever participated in - albeit as a peon, they said we had to cut it within 24 hrs - if it was going to crack, it usually does it quickly. Any thoughts?

jomama45 01-20-2012 01:37 PM

It varies depending on man factors, but generally speaking, 24 hrs. is a good rule.

cleveman 01-20-2012 04:56 PM

You may want to rent a screed as well. I use an alu screed with an electric vibrator on it.

The saw cut will probably be best.

Yes, cut it as soon as you can.

You should learn about curing as well. I like to have poly sheeting under the slab. You may also want to use xps instead. Anyway, you'll want to keep the concrete moist afterwards. You can also investigate spraying it with a sealer, which aids the curing process. Or just get it a bit damp and cover with tarps.

About any holes for plbg, etc., just make a wood box and attach it to a stake or two. Position this above where you will have a pipe, etc. coming through the concrete. Cover the tops of these with some plastic, and leave them 1/4-1/2" low. Then just pour your concrete over them and don't worry about finishing them. Go back later and put your pvc in the fitting down there, solder on your copper, whatever, and finish the 4x4 or 6x6 hole with some quickrete.

It would be nice to have two guys on the screed during the pour, one with a rake, and one running the chute. In the beginning, the two on the screed will run rakes and shovels and get you out quite a ways before they start screeding. You have to get back to the beginning in good time to start floating. I've done it with 2 guys on a 12x72 pour, so I think you can handle it.

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