Using pre cast mini concrete slabs in a floating floor.

princeplanet

Member
Joined
May 9, 2017
Location
Melbourne
I'm on the 7th and top floor in an office building and the professional advice is I need a box in a box with around 100mm concrete for the suspended floor with a 100mm air gap. Proper engineered and calculated rubber blocks and the correct type and amount of insulation in the gap. The walls and ceiling will either be built on top of this floating floor, or separately floated (still unsure of the pros and cons there...). Isolation against neighbours on my own level is NOT a problem, I rent out the nearby rooms to others so I can choose tenants who are OK with sound leakage.

The existing slab is only 125mm thick, but the structural engineer sees that luckily the steel beam support is such that the floor can take 650 kg per m2 (including Live Load and Additional Dead Load). The floated floor will either be for the entire Live Area (34 m2) or just the drum room (12 m2). The structural engineer may not allow all 34 m2 to be floated (remains to be seen after I supply him with some plans).

Now, if we can just assume that the above is all well and good, here's my question: Because I cannot get a poured concrete slab on the 7th floor (for various reasons), I am left with options such as compressed fibre cement sheet layers, or a sand box. But a recent idea I have been floating (woops, no pun intended), is to use 50 mm thick reinforced concrete paving blocks, say 800mm x 400mm, and simply stack them tightly (staggered) in two layers to create a 100mm thick concrete slab. The cracks could be filled, although as only high frequencies would leak through these cracks, they will be attenuated through the existing slab, so perhaps I wouldn't bother.

Stability is an issue of course, how will these pavers be supported? Perhaps L shaped steel rails? Which would rest on the rubber isolation mounts. What about the stability around the edges that needs to support the walls and ceiling weight? Hmmm, good question! Perhaps a metal perimeter frame can be screwed or welded together to stop the pavers from moving? Or maybe the wall can be built right upon the steel rails ?

I understand these questions can only be answered by engineers, or someone with much experience, but I welcome all your thoughts on this. I also realise that untested ideas are not encouraged and fully expect a chorus of nay sayers, which is fine as well, I mean, I'm happy to be talked out f this, but only if a case against the idea is compelling.

So, any takers? ;)
 

kmetal

Kyle P. Gushue
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Joined
Jul 21, 2009
Location
Boston, Massachusetts
What are your isolation requirements?

Generally floating floors are a bad idea because proper ones are very very expensive and require highly detailed design, including a floor layout plan and weight distribution analysis ahead of time. Thats for one that will work... All the others are a waste of cash. Floating walls are also snake oil.

If your issue is impact noise, ie footsteps, there is a design for a floating floor in rod gervias' book, home studio build it like the pros.

If your issue is airborne noise, ie sound, then airtight mass is what you need. For the floor you could use a self leveling concrete like gypcrete. For walls and ceiling youd use a standard mass air mass double wall system, and and independently framed ceiling resting on your inner wall leaf.

I highly suggest you get rods book.

I also highly suggest you hire a professional studio designer like rod, as you could kill people with a faulty design and end up in prison. Or at minimum waste a ton of money for little added benefit.

The design of a floating slab is no light feat.

Things like comercial or residential zoning restrictions, and local building codes must be adhered to.

It is very likely that doing this properly will make no sense from a cost benefit perspective. Exceptions being extremely disposable income, or your a seasoned audio professional who has to work from home.

I hope this doesnt sound discouraging, rather i wish to point you in the right direction, hopefully preventing jail time, and bankruptcy, while you explore other options. Renting a suitable space or building a standalone building on your premises are viable options. So is building a mobile audio trailer like they use for live broadcasting and mobile recording.

Having done nearly a million dollars worth of studio builds in the last ten years, i know enough to know when to call in more experienced designers to accomplish the goal. Floating slabs are one of those times.

Cheers, and welcome to RO.
 

princeplanet

Member
Joined
May 9, 2017
Location
Melbourne
Thanks for the reply. I have had 2 other studios built for me , both on upper levels, in the last 25 years, so am not a total newbie :)

It's to be a music studio where rock drums need to be attenuated through the floor. I've had several acousticians help me with the calculations after doing numerous tests. Minimum floated floor would be 75 mm concrete over 75 mm air gap. Preferred would be 100mm over 100 mm air gap.

As regards safety, did I mention that a structural engineer has been over this in detail and came up with a permissible load of 650 kg / m2. I have an acoustic engineer who will shortly be drawing up some options to run by the SE. So, the floated floor can and will be done, it's just a question of which material to use. Poured concrete can't happen, no access. Fibre cement sheets are expensive and not as dense as concrete, meaning I will lose precious ceiling height. A sandbox seems messy and prone to failure over time. My acoustic engineer seems uncertain about my idea of stacking concrete pavers on steel rails, so I'm asking around. It would be great if the discussion in this thread could be limited to this topic only : Could reinforced concrete pavers be used to replace a poured slab? What are the potential problems?

Thanks.
 

paulears

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Joined
Feb 7, 2014
Location
Lowestoft - UK
Watch this series of videos by the Spitfire folk who make the excellent sample stuff, when they installed a studio in an upper follow set of rooms.

 

paulears

Well-Known Member
Joined
Feb 7, 2014
Location
Lowestoft - UK
Surely your structural engineer is the best person to advise on the differences between poured floors and prefab concrete? You seem set on it, unless somebody leaps in and says "no, no" when no doubt you'd go back to the expert. With my own studio builds only ever using poured or sand filled floors, all that would worry me would be the practical things like still needing to screed the top surface for level, and any odd point loading differences when the things do not sit flat, and one support takes more load because the subfloor is higher at that point. Even screeding could be interesting if it finds gaps, and leaks through to ruin the subfloor to slab isolation. Even if not an audio expert, your structural guy would know the downside of prefab slab construction. I'd also hate to be the one to have to get umpteen tonnes of slabs up to the working floor!
 

kmetal

Kyle P. Gushue
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Joined
Jul 21, 2009
Location
Boston, Massachusetts
Why is it not possible to use bags of concrete brought up to the floor mixed on site.

The problem with pave stones is they are not a single sealed massive entity like a reinforced slab is. So besides a plethora of air gaps, you've got a bunch of individual peices vibrating at a different frequency and distribution than a single slab would.

The spring-like mounting units uses in a proper floating slab are embedded into the poured slab. Imposible to do with pre cast.

Stick with tried and true. If your acoustician doesnt know how to do a floating slab above grade, find someone experienced with that task and use them for the slab plans.

Its not just about the load bearing, but the distribution of the load as well.

No offense, but there is no reason to believe that the field of acoustics has missed something that you have discovered. If what your suggesting was a valid technique it would not take fishing for approval on threads. It would be a well documented method noted in several documents and builds.

So the short answer is no, it wont work. Use tried and true techiques, and professionals experienced with those techniques.

2x 12" rack toms doesnt equal 1x 24" kick.
 

princeplanet

Member
Joined
May 9, 2017
Location
Melbourne
Surely your structural engineer is the best person to advise on the differences between poured floors and prefab concrete? You seem set on it, unless somebody leaps in and says "no, no" when no doubt you'd go back to the expert. With my own studio builds only ever using poured or sand filled floors, all that would worry me would be the practical things like still needing to screed the top surface for level, and any odd point loading differences when the things do not sit flat, and one support takes more load because the subfloor is higher at that point. Even screeding could be interesting if it finds gaps, and leaks through to ruin the subfloor to slab isolation. Even if not an audio expert, your structural guy would know the downside of prefab slab construction. I'd also hate to be the one to have to get umpteen tonnes of slabs up to the working floor!

So if poured was not an option, would you definitely go a sand box? How well have they worked for you? Any moisture issues? What would you suggest as the best way to create the base to stop future sagging or leaking?
 

princeplanet

Member
Joined
May 9, 2017
Location
Melbourne
Why is it not possible to use bags of concrete brought up to the floor mixed on site.

The problem with pave stones is they are not a single sealed massive entity like a reinforced slab is. So besides a plethora of air gaps, you've got a bunch of individual peices vibrating at a different frequency and distribution than a single slab would.

The spring-like mounting units uses in a proper floating slab are embedded into the poured slab. Imposible to do with pre cast.

Stick with tried and true. If your acoustician doesnt know how to do a floating slab above grade, find someone experienced with that task and use them for the slab plans.

Its not just about the load bearing, but the distribution of the load as well.

No offense, but there is no reason to believe that the field of acoustics has missed something that you have discovered. If what your suggesting was a valid technique it would not take fishing for approval on threads. It would be a well documented method noted in several documents and builds.

So the short answer is no, it wont work. Use tried and true techiques, and professionals experienced with those techniques.

2x 12" rack toms doesnt equal 1x 24" kick.

Mix the concrete myself on site? I'd need a pretty big concrete mixer up there wouldn't I? Not knowing much about concrete (my other studios had light float floors), wouldn't the pour need to be all in one go? Surely I couldn't do a bit each day over 4 or 5 days? Also, the idea that each paver would vibrate independently and not as a contiguous mass is interesting, but I'm not entirely sure it's true. Does each brick in a wall vibrate separately? Even without mortar between them? Surely as they're touching each other then any common vibration would be passed through each? And if if not to the full extent of a single slab, remind me of the physics as to why this is not desirable? Cutting the shell in half of your kick drum and butting both halves together so that they are always touching will not technically perform exactly like the one piece version. But might be "close enough for Jazz" as they say? ;)
 

paulears

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Joined
Feb 7, 2014
Location
Lowestoft - UK
If you've ever walked on a prefab concrete floor before they screed it, you'll be amazed how they rock. That's what I meant by the suspension problems. Put one on a flat floor and they rarely sit flat. Some of the hardcore usually sinks too much and makes a small lump, and the moulds are often old and dented, creating small humps and bumps. Pumping concrete is a relatively recent building trick. Previously, smaller builds had to mix their own. Clearly manpower dictates the area you can fill in one go, so usually the slab is shuttered off into more manageable sections. Thinking about it a bit more - maybe you could do the prefab floor and then seal the gaps to prevent seepage ruining the isolation, then do the screen - or maybe do a sand top level, then flooring?
 

kmetal

Kyle P. Gushue
Well-Known Member
Joined
Jul 21, 2009
Location
Boston, Massachusetts
So if poured was not an option, would you definitely go a sand box? How well have they worked for you? Any moisture issues? What would you suggest as the best way to create the base to stop future sagging or leaking?

I have only used wood framed, sand filled decks, on slabs on grade. They have worked well and moisture wasnt a problem even in particularly damp studios. Its purpose was just to have a flat surface to build upon, as opposed to the uneven concrete. Isolation was not a reason.

I wouldnt use a sand box for isolation. Id use a gypcrete self leveling concrete floor poured onto the existing floor. If i were to go with some sort of floating design, id be looking into modifying the floating floor in rods book, were two layers of plywood are laid on top of rigid fiberglass batts. Or looking into concrete pouring methods. Perhaps using a crane/tubing/pump through a window, and using tbe stadard floating slab spring mounts. Id do this with the approval of an acoustician whos experienced your particular scenerario before.

Ive never had to do floating floors. The studio i did which wasnt on slab on grade, we used a sand filled deck for the drum booth, and the control room had 4ft of sand brought in underneath it. This was to dampen resonance, not as an isolation method/purpose.

There are plenty of mastering rooms built on floatimg slabs, on non ground level floors.

Also, the idea that each paver would vibrate independently and not as a contiguous mass is interesting, but I'm not entirely sure it's true. Does each brick in a wall vibrate separately? Even without mortar between them? Surely as they're touching each other then any common vibration would be passed through each?

It comes down to resonant frequency. Youve got several tiny peices vibrating independently or coupled/semi independently. These will vibrate at a higher resonant frequency than one large reinforced slab poured and cured as a single entity. While the mortar and coupling will lower the resonsant frequency it will still not behave as slab would. This effects the sound frequency it isolates, and how the spring mounts need to be tensioned, as well as how it bears loading.

You would have to calculate your design as is and wouldnt use the same calcs a slab would. The mortar and blocks would be potentailly different densities, and would not distribute vibration the same way one single mass would.

The important aspect of tbe slab besides being massive, is having it resonate below 20hz, ideally 8hz, more than an octave below audible low freqs.

Physically speaking its akin to trying to couple 2x 12" heads into resonating like a 24" drum. Not cutting a 24" shell into two 24" shell. Your trying to peice together snaller masses to operate as one larger mass.

Mix the concrete myself on site? I'd need a pretty big concrete mixer up there wouldn't I?

Or a bunch of smaller ones, or crane. This isnt a diy part of the job.

Like i said, i know enough to know my own limits and scope of knowledge in the field, and floating slabs are not my area of expertise. I know the basics. Beyond that, an acousticuan experienced in elevated slabs is absolutely necesssary.

Close enough, is never my mantra for studio builds. Up until all other options have been exhausted, and its a matter of living with compromise. Close enough is however good enough for rock and roll, or jazz improvisation.

You can always have an acoustician experienced in floating slabs run the numbers for your idea or propose there own solutions should you want to pursue your idea further. I can only make comments based on tried and true methods ive been made aware of over the years if building and researching. I have had satisfied clients throughout, and one studio acheived a couple top 10 songs from 2014-2016. Ive been forunate to have some very experienced people help me when i was unsure.
 

paulears

Well-Known Member
Joined
Feb 7, 2014
Location
Lowestoft - UK
In my own very amateurish builds - which is 8 to date, I've only had a couple of 'rules'. The first is that when the cost of any element of the construction is more than the contingency fund I build in for unforeseen events, I take professional, paid for advice. Only two of my builds was upstairs - one in a 4 storey 1960s concrete floor construction, and the other in a first floor wooden subfloor 'cottage' style add on to a bigger building. The advice I asked for in the 60's building was about floor loading, and the answer came back that my weight data was perfectly safe - by a very large degree as it happened. However, despite the neoprene isolation between the structure and the building, the kick drums perfectly coupled and downstairs was very unpleasant. The cause was one I could have probably found out WITHOUT the acoustics people getting involved, if I had asked the correct question. In order for the sound of feet, chairs etc not being transmitted through to the floor below, the concrete floor was already floating, and my addition didn't impact on the safe weight loading but it apparently was enough to wreck the existing isolation and create a better path than I probably would have had leaving the floor as the original floor!
 

princeplanet

Member
Joined
May 9, 2017
Location
Melbourne
I've sought professional advice, but no one in Australia has experience building a studio on an upper level. It obviously gets done in places like Manhattan and I've written to the great Francis Manzella who thinks I can do 100 mm concrete with a 100mm air gap and be OK. There is one mob that floats floors for big gymns, nightclubs, theatres, bowling alleys etc but they quoted over $100k for floated floor around 68 m2. But I'm hoping to get a 34m2 floor done for under $20k. I think it can be done with either sand or fibre cement sheets, but there are problems with both of those options.

Getting 3 or 4 portable cement mixers with a whole crew of labourers is a new idea, along with the pavers idea. How large and heavy is a portable cement mixer? How much concrete will each hold?
 

paulears

Well-Known Member
Joined
Feb 7, 2014
Location
Lowestoft - UK
Wow! Those figures seem pretty high for what we'd expect in the UK, and I think I'd want some professional advice you can then claim against if it's a poor performer. Back in 2002 we had a 10 x 8m slab put down at ground level at from memory less than four grand!
 

kmetal

Kyle P. Gushue
Well-Known Member
Joined
Jul 21, 2009
Location
Boston, Massachusetts
Rod gervias, would know the correct answer and method.

I believe john sayers is down in Aus, no? Hes a well respected acoustician.

Its important to remember that both mass-air-mass and the harmonic motion (resonance) of the slab and its supports, come into play. The harmonic motion of tbe slab depends on its size and shape. So while i wouldnt dismiss the advice given from FM design, i would submit there is alot more to it than just slab thickness and air gap. I wouldn't expect such comprehensive advice for free, but, i also would be careful with vague one sentence advice on an otherwise highly detailed subject.

As far as pricing i had expected 50k USD for something like that here in the greater Boston area.
 
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