# Suspended concrete floor above grade



## mravenzemla (Feb 9, 2011)

Hi, am looking for any information or tips anyone can give me on suspended concrete floors. The structure currently in place is 3/4" T&G ply subfloor on 12" wood I-joists. Am wondering in particular what kind of underlayment I should be using on top of the subfloor, am thinking of RedGard as a separation layer. Also, there will be PEX radiant heating pipe installed on the subfloor before the concrete is poured, and I'm wondering how best to protect that pipe when it comes to making saw cuts for relief joints. We are looking at probably a 2" thick slab with 6x6 wire mesh laid on top of the PEX and a polished surface, nothing too fancy but will have to be very smooth as this concrete will be a wear surface with a polyurethane sealer rolled over it.
Also, I've heard of some concrete floors having steel shavings put in the mix to increase the strength, does anyone have any experience with that process?
Lastly, there are 2 levels to the floor I'll be pouring on and a stair opening to the basement, what are some good ways to shore the concrete up at those openings without leaving an edge vulnerable to crumbling and cracking afterward?

Thanks,
Mark


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## Snobnd (Jul 1, 2008)

You need an Engineer to answer some of those questions, to protect you and the Owner. 
The questions I have for you are>>> Have you ever done a floor like this? What is your Trade?


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## CO762 (Feb 22, 2010)

mravenzemla said:


> Also, I've heard of some concrete floors having steel shavings put in the mix to increase the strength, does anyone have any experience with that process?


I've never heard of that, but there's a lot in this world I'm clueless about.

Where did you hear this?


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## speeder (Feb 6, 2011)

CO762 said:


> I've never heard of that, but there's a lot in this world I'm clueless about.
> 
> Where did you hear this?


I think it's because the OP asked about suspended concrete floor which is actually something different than what he is asking about.

A suspended concrete floor or suspended slab is a thick poured in place concrete floor in which the forms underneath the slab are removed leaving the suspended slab. This of course needs to be engineered.

What he is talking about is just an in floor concrete radiant heat. The engineering concerns are whether or not the floor was designed to take the extra weight of the concrete. Even at 2", it's heavy.

I've done lots of these types of floors and have never use any underlayment over the plywood subfloor. There isn't much concern of moisture as the concrete doesn't really hold it in. Of course you can always do something if budget isn't tight, but it really isn't necessary. I've had to do repairs to the pex pipes in these floors and the plywood is basically in perfect condition underneath the concrete.

Not sure how easy it is to make sure the wire mesh stays encased in only 2" of concrete with 1/2" of pipe underneath it. I've never done that before but we usually only use 1.5" of concrete. The concrete is always covered, so have never worried about cracks and such. Make sure you don't use fibers if your concrete is the final finish, cause those fibers stick out of the concrete. 

Don't worry about leaving an edge. I often do concrete stairs with 2" rounded nosing and have never had it chip . Concrete is tough! 

As for cutting control joints, just don't cut them that deep. 1/2" cut is more than enough, and that should give you 1" clear to the pipe.


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