Topping Mix As Slab

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Old 01-10-12, 02:33 PM
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Topping Mix As Slab

I have searched high and low for information on using something like a topping mix as a concrete slab. Why might I want to do this you ask? I have a lot of sand (coarse angular stuff) on my land that could easily be sifted. I have a slab I want to poor that will take no load other than some flooring, furniture, and people. It will be going inside of an existing structure with radiant tubing. I don't think 3,000 psi concrete is necessary for this application, and I want to get it nice and smooth anyway. I can save a lot of money buy doing portland cement mixed with sand in something like 5 to 1, or maybe even more sand if someone has the low down on the minimum mix I could get away with. The slab will be 4" thick. But 2" is the most I saw for topping application in my searches.
Questions:
What are the drawbacks of not using large aggregate in a slab? What is the largest ratio I could get away with?
Any other thoughts on doing this just to save about 75% of slab cost?
 
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Old 01-10-12, 09:40 PM
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You didn't give the length & the width of the slab. Portland & sand (3 sand to 1 portland) would be fine. However, it sounds like you'll me mixing for days if it's 4" thick.
 
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Old 01-11-12, 04:56 AM
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I agree with Pulpo. Topping "mix" is a mix. Use portland and sand (3:1), but with any size, your kids will be grown before you mix it all to a respectful depth. Sand is the cheapest part of the mix. Having it brought in via truck will make it so much easier and quicker. A 4" thick 12x12 patio will run about 2 yards, or 81 each 80# bags of quickcrete.
Easy reference: Concrete Calculator, Prices and Pouring Information - The Concrete Network
 
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Old 01-11-12, 08:53 AM
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Chandler, don't confuse him. It won't be quickcrete. That has stones in it.
 
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Old 01-11-12, 04:25 PM
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neorush,

You run a very real risk of winding up with a mortar bed that will develop lots of cracks in it. Especially if you've embedded heating conduits in it. Most people don't realize that coarse aggregate (rock) is what gives concrete its strength (not counting the Portland cement, of course). Repeated flexing from heating/cooling cycles of the floor will only increase the likelihood of excessive cracks developing. And batching 80 individual bags of anything is an open invitation for disaster, with respect to trying to get a smooth surface finish. The initial areas batched will start to set up before you are finished with the end areas, meaning you will have problems trying to float and screed everything smooth over the entire area (without blowing the water-cement ratio out of whack when you add water to soften the hard portions, leading to more durability problems). Don't ask me how I learned this lesson, early in my concrete finishing career.

Read my comments under the thread from a day or two ago, called "Patio slab, mudjack, or rip apart?" (found in Exterior Home Improvements). As discussed in that thread, a thin-bonded concrete overlay, instead of a mortar bed, is what you really should be doing. Unless you're prepared and willing to tear everything out and start over when the mortar starts crumbling apart in a few years.
 
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Old 01-16-12, 11:13 AM
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Sorry about the delay in getting back to everyone here. Thanks for the responses.
To clarify a little
I have ~7.5 cubic yards to mix (a little less than 30'x24' @ 4"). The real expense is that to get the cement inside the house I would need to rent a concrete pump. I got three quotes ranging from $800-$1,000 (+$10 per cubic yard pumped). My local concrete company is $114 / cubic yard.
Because of the nature of the pour I can do the house in about 25% increments because the slabs are separated room-to-room. With a couple of guys it should be reasonable to mix / pour ~2 cubic yards per pour.
Assuming 1 cubic foot mixed ever 10 mins, which I think is a little slow with 4 cubic foot mixer I have. It would take about 33 hours to mix every thing or around 4 days @ 8 hours/day.
I have a 50 horse tractor, so getting the sand to the site in quantity is not an issue.
This project really comes down to money for me, I don't mind the hard work, and friends work for beer. But I also do not want to do this again in 10 years.

The math:
- One 94lb bag of portland is $10 and one cubic foot.
- I need 7.5 cubic yards x 27 cubic feet/yard = 202.5 cubic feet.
- at a 1 to 4 mix this is ~50 bags of portland or $500.00 (free sand).
- Pump truck ~$900 + Concrete $855 + delivery $200 = $1955

This is a big difference in price. The whole reason I'm considering it.
BridgeMan45, your saying that topping mix will crack over time, this is what I was most concerned with, what if I only did a 2" pour? I'm just trying to find the most economical way to get tubing in some kind of thermal sink for efficiency.
 
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Old 01-16-12, 03:37 PM
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Your numbers are a bit fuzzy, starting with the thought that the beer your buddies guzzle doesn't cost anything. But more importantly, you're missing a critical part of my previous comments--rock is needed primarily to provide strength and prevent cracking. Does your sand have a uniform gradation, with a good blend of fines and larger particles? Meaning it's not super smooth and fine like mason's sand, but rather has a fair amount of varying-sized small stones (up to 1/4" dia.) in it? And is it clean (no organics or P-200 material)? If you can honestly answer "yes" to those questions, then you might get lucky and wind up with a minimal amount of cracking. Hope you're a good gambler.

And one last suggestion--no beer served until each day's work is completed. If you allow the help to get lubed up before you start batching, placing and finishing, you're likely to have a bigger mess on your hands. I've seen some concrete pours done by drunks, and they aren't pretty.

For what it's worth, I think a 2" overlay would perform as good as one that's twice as thick. Not yet approaching the lower limits (1-1/4" thick) of decent overlay performance.
 
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Old 01-16-12, 04:59 PM
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Just wondering Pros...but what about that self leveling stuff I have seen pumped in on several of the home improvement/repair shows? They just spread it out to the level required and let it sit for like 8 hrs before being walked on...done. No tooling. I assume it's something like the self leveling compound we used to use in the Navy to get steel decks ready for tile or rubber matting...only a lighter weight version?

To neorush...maybe I missed it...but what is there now? Seems like if you already have a regular slab and you just want enough to cover the radiant tubing, you may not even need 2" if you use the right material to cover the tubing (1/2"?) with an inch or so.
 
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Old 01-16-12, 08:06 PM
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The basic consensus here is you can not get away with topping mix as a slab without serious risk of cracking. This is exactly what my big concern was and what I needed to know.

To answer Gunguy45, there is nothing there right now, it's an old home that has been gutted, new roof, closed cell foam insulation in the walls and ceiling, the floor under the old planks is dirt, most of the old joists are dry rotten (100 years of sitting on dirt) so they needed to be replaced. We wanted to add radiant tubing, and poor a slab in the middle of existing rooms.

What I have basically settled on is not using any concrete, but add a vapor barrier to the existing dirt floor, rigid foam board on top of that, and add a new treated 2"x8" subfloor which I will fill with sand (for thermal mass) between the joists and run my tubing through the sand at 8" intervals. The final T&G floor will be nailed to the treated floor joists. From what I have read sand can be a great low weight thermal mass for radiant tubing. I'm always open to other suggestions / thoughts, thanks for all the great input!
 
 

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