Different concrete for different rooms?

I seem to remember hearing once that a possible way to float concrete floors is to simply set the concrete slabs of different rooms apart from each other. I guess it would be called cut-slab or something like that. I can't seem to find any info on the web about this kind of thing though. Can anyone point me in a better direction to look for this sort of thing, or give me a proper name so I can research it more? Thanks for any help, I'm just sort of investigating options.

Hello. I'm no expert but I don't think this is actually what would be considerd "floating" slabs. If I understand your question correctly. I think what you are talking about is decoupling existing slab connections for different rooms by sawing the existing concrete to the shape of these rooms, thereby creating a "gap". Then each room is constructed on these seperate slabs. Correct? I seem to remember seeing this done. However, once the room is build, I also remember seeing an isolated floated floor pad is added between walls too. This can be either a "rolled out" fiberglass insulation with neoprene or fiberglass "pucks embedded within the roll, and then layers of subfloor and finish floor added on top, or a shallow floated framework built with insulation added between the framing members. This is also floated between the walls if I remember correctly. I don't think complete floating rooms are built on these "seperated" pads though. At least to the extent that ceiling loads are placed on the new walls, as this additional weight may require the room to be supported on their own seperate footings incorporated into a slab as well, but I may be wrong. I have seen floating rooms built on floated concrete slabs, that were poured on an existing slab, however this support slab was probably engineered for the weight in the first place. My own experience with building a HEAVY wood floating floor on an existing "4 concrete slab ended up cracking it. Didn't even know it untill the studio had to be moved. Good thing though, as water had come up through the cracks under the floated floor. I think that might be a concern, even if the sawed joints were caulked, although I'm no expert. fitZ

Sword9 wrote:

I guess it would be called cut-slab or something like that. . .

Perhaps you read it in Chapter 3 of Acoustics101.com:

Auralex Acoustics wrote:

. . . If yours is concrete, consider (carefully) cutting a gap in the concrete between the two rooms first, then proceeding as shown. Cutting the slab is no minor undertaking, but you will be relieved to know that if you decide to do it, the gap does not need to be any wider than the width of the saw blade. N.B.: The cut must bisect the entire slab. If you are unfamiliar with the structural ramifications of doing this, please consult a local expert.

sharward wrote:

Sword9 wrote:

I guess it would be called cut-slab or something like that. . .

Perhaps you read it in Chapter 3 of Acoustics101.com:

Auralex Acoustics wrote:

. . . If yours is concrete, consider (carefully) cutting a gap in the concrete between the two rooms first, then proceeding as shown. Cutting the slab is no minor undertaking, but you will be relieved to know that if you decide to do it, the gap does not need to be any wider than the width of the saw blade. N.B.: The cut must bisect the entire slab. If you are unfamiliar with the structural ramifications of doing this, please consult a local expert.

You are correct that this is not a floating slab - just slab isolation - which is a great construction technique. I do have a few thoughts on the concept though. Other than the fact that the slab edge will not have an edge designed to support load - and may crak the slab if you construct a full room within a room loading up the edges, I wouldn't have any issue with this in an above grade slab. I would point out that unless you also cut to seperate the slab from the exterior concrete walls you can still transfer sound through the structure as a whole/ But I would not reccomend it in a full basement. One of the reasons for a slab in a full basement is to brace the foundation walls against the earth load that exists. Earth loading generally transfers at a 45 degree angle - thus a foundation with 7' of bury depth will have 24.5 cubic feet of earth loading up the wall at it's base. When wet (once again - depending on the earth - this load could be as much as 190' pcf) this could load 4655# per linear foot on the foundation wall at it's base. This could very well actually slide your slabs back together (which would make it as if you hadn't even cut it)- and could also cause stress cracks in the foundation itself. I don't believe it could actually cause a collapse - unless you were cutting the slab into fairly small pieces - but moving things around and causing stress cracks in the walls can also allow leaks to form in places where there was never a problem. IMHO this is the sort of thing you plan for in advance - not in retrospect. You can then properly thicken the slab edges to carry loads - and (in the case of a fuill basement) make other provisions to deal with the earth load on the wall base. SIncerely, Rod

Yeah, float was the wrong word there. Isolate would have been a better call. That's what I get for posting so early after waking up. So basically to sum it up, even if this is done properly, one would still have to build an actual floating floor on top of it to make it isolate properly? In considering doing this, basically, if you constructed the outer shell not connected to anything but itself, then poured individual "islands" of rooms, I can't see how there could a structural problem with the outer shell. If you have to double your flooring surface anyways though, I guess it's not a cost effective solution anyways, so I'll table the idea probably. I don't know if I read about this at acoustics101 or not. I'd hate to think I'm taking inspiration from a sales pitch.

Yeah, float was the wrong word there. Isolate would have been a better call. That's what I get for posting so early after waking up. So basically to sum it up, even if this is done properly, one would still have to build an actual floating floor on top of it to make it isolate properly?

Nope - if this is done right - one would not have to do anything else to isolate properly. Earth itself does a great job of isolation noise - it's the structure that transmits it........ so if the structure is isolated from adjoining rooms then you're all set. Understand something else - there is always a some amount of leakage between live rooms and the control room - which isn't an issue because it's not a problem for the engineer to hear the monitors slightly blended with the live sound. It isn't as if the 2 have to have true sound isolation.

In considering doing this, basically, if you constructed the outer shell not connected to anything but itself, then poured individual "islands" of rooms, I can't see how there could a structural problem with the outer shell. If you have to double your flooring surface anyways though, I guess it's not a cost effective solution anyways, so I'll table the idea probably.

As I said - it's probably the MOST cost effective isolation you can get for the floor if it's new construction. As far as the outer shell goes - if it's new construction then it becomes all part of the original design - and there won't be a problem. Good luck, Rod

So would there need to be any sort of caulk or spacer installed between the slabs? Anybody know of any examples of this kind of construction detailed anywhere on the web?

Sword9 wrote:

So would there need to be any sort of caulk or spacer installed between the slabs? Anybody know of any examples of this kind of construction detailed anywhere on the web?

I don't know of any on the web - but i suppose I can work up a detail next week and get it posted. I'm pretty busy right now - so just be patient a bit. Yes there will be caulk and expansion joint materials. Rod

So I've talked with someone in the construction industry who says that a common practice for this sort of thing is to divide and separate the concrete with celotex as the spacer. I'm still unclear if this is then pulled out or left there or what, still researching this particular method. If I was to use a cut and unconnected slab method like this, I would basically be decoupling my rooms. Then, to make up for the mass that would probably in a floated floor, I could just increase the mass of my floor with an additional layer of gypsum or some sort of wood like mdf or plywood to make up for the additional low freq TL. correct?

Sword9 wrote:

So I've talked with someone in the construction industry who says that a common practice for this sort of thing is to divide and separate the concrete with celotex as the spacer. I'm still unclear if this is then pulled out or left there or what, still researching this particular method.

Actually - I prefer to use foam insulation - and I remove this after the concrete is finished (it comes out easier than the celotex).

If I was to use a cut and unconnected slab method like this, I would basically be decoupling my rooms. Then, to make up for the mass that would probably in a floated floor, I could just increase the mass of my floor with an additional layer of gypsum or some sort of wood like mdf or plywood to make up for the additional low freq TL. correct?

Nope - if you pour a slab with thickened haunches at the slab edges to carry the wall loads - all you need to do is standard floor finishes above - the earth itself gives you more than enough mass to handle the rest. You could never pour a thick enough elevated floor to equal the mass that this concrete is sitting on - and the concrete (without an air space below - like an elevated slab) will not act independent of the earth it's poured on. Rod

A few more questions on this method. When you pull the styrofoam/filler material from the gaps in the concrete, do you have dirt in the spaces? Or do you fill it in with some kind of filler, like tar or caulk or what? I've posted a basic, basic, basic picture of the open dirt idea, but also to make sure that I"m clear on the separation too. I'm pretty sure if nothing else, there'd need to be a vapor barrier somewhere, so I'm pretty sure that the dirt isn't right. Also, what's a good isolation spacing for this sort of method? Should the air gap between my room walls equal the gap between concrete slabs? Thanks for any help on clarifying this for me.

I'm pretty sure that you'd fill the gaps with acoustic caulk, and that the gaps need only be as wide as the concrete cutter saw blade. I'll leave it to the experts to say for sure though.

Any ideas where to turn for more info about pouring new isolated slabs within the same new building -- designs, methods and techniques, added costs, mistakes to watch out for, etc? Anyone on the board ever do this?

sharward wrote:

I'm pretty sure that you'd fill the gaps with acoustic caulk, and that the gaps need only be as wide as the concrete cutter saw blade. I'll leave it to the experts to say for sure though.

Perhaps, but there is no saw involved in this process. If one puts celotex or foam or anything like that in the crack, that'd take a lot of caulk.

Hello Sword and Rod. Here is an illustration of what I think Rod is talking about. Each slab would have to be poured at different times though I "think". The only thing I wonder about is if the slabs have to be "keyed" to keep individual slabs from sinking or misaligning height wise over time maybe. I don't have a clue how you would do this though, and keep them isolated. Maybe there is an industrial "keying" isolator in use already. I don't know though. And now that I think about it. Why would anyone need to poor a "floating" slab above this if they are isolated. Wouldn't this solve all the LF TL -airgap resonance problems that are being discussed other places? Seems like it would. Anyway, here it is. Not much more than what Rod said, but its good drawing practice anyway :) fitZ[/quote]

So I've found some things like this http://www.epoxysystems.com/11.htm http://www.epoxysystems.com/63.htm http://www.epoxysystems.com/632.htm http://www.epoxysystems.com/901.htm I also found this site which has cork-based expansion joint filling materials http://www.apscork.com/expansion.html And then I've also seen mechanical things like this http://www.larsenproducts.com/interspan.htm Are any of these products the sort of thing I should be investigating? Or am I way off course?

I'm still having trouble gathering more research on this construction technique. Does anyone know of any sites or books that might point me in a better direction? Thanks for the help.

Slabs that need to properly function structurally, and provide isolation, should be designed to do so before the slab is placed. If you cut all the way thru structural concrete, you will compromise the structural integrety when the reinforcement is interupted. Compressible fill for structural expansions can be: fiber board-asphalt impregnated fibers polyiso- closed cell rigid insulation cork closed cell neoprene spray foam insulation Expansion joint covers can replace the sealant for larger joints. I suspect the thickness of fill will be governed by acoustical requirements, rather than structural. Steel dowels between the foundations may be necessary to maintain elevation, and may transmit too!

So basically I could leave the styrofoam or celotex that's used to create the boundry of the concrete in and simply caulk over it to keep moisture from coming up from the ground? Or would I need something like backer rod instead of the board?

Keep it comin', guys -- please! :) At least two of us are glued to this thread, as we are seriously contemplating and/or planning to do this soon, and we want your brains. :mrgreen: