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Discussion Starter #1
I am designing a layout for some weightlifting platforms in a gym. This gym is a warehouse with concrete floors, and while building the wood platforms on top of the floor is common, it would be so much nicer to recess the platforms flush with the floor. What type of contractor can do this?
 

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You want an experienced Concrete/Masonry contractor - to look at the project.

Just realize that this is not a simple "lowering the floor" project.
 

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Assuming the concrete slab is poured on grade (this isn't a second floor, is it?) then you will be cutting through and removing the slab; digging down a bit, and pouring a new, lower slab.

Sounds easy, but the particular conditions of the building and soil may complicate things. For example, it may be a very thick slab; it may be a reinforced slab; and there may be drain tile, electrical conduit, or other utility lines running through or underneath the area where you want to excavate.

You may also find that removing the existing slab allows the gravel base to "run out" from under the slab edges that remain - your contractor will then have to form the edge of the existing slab and inject grout underneath to support the edges.

If you have the budget for it, I agree, this would be very nice! It would also be very easy to do in new construction. But unless you're financially prepared for "problems" that you probably won't know about until it's too late, I'd recommend the raised platform option!

Hope this helps,

Richard Taylor, AIA
 

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We've done similar work in chemical plants on their concrete floors, but it was for mounting weigh stations and scale decks flush with the concrete floor. You take out the part about the type of building and the thing you're installing in the recess, and it's basically the same job. Pits were recessed down 4 inches.

Watch out for utilities, rebar, grade beams, footings, etc.
 

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Discussion Starter #6
Thank you Richard.

Your outline of potential problems is pretty convincing. I will probably abandon the idea and build on top of the floor.

Anyway,

If the concrete slab was thick, is it possible that a cut-out ~five-inches deep would leave enough concrete to support the platform(s)? Probably, there is no way to make a "cut-out" without removing concrete through the entire depth of the slab. (?)

What are the chances of getting the specs of the existing floor (thickness, reinforcement, pipes and conduit) to identify problems in advance? (Its a warehouse amongst similar buildings in the area/district)

Jeff
 

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Discussion Starter #7
Aggie,

You answered my question about the pit as I was typing it. Would the original contractors have filed the specs on utilities, rebar, grade beams, footings, and related, somewhere for future access?
 

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Depending on how old the building is, there should be drawings at the town. I'm doing some stuff for a country club that Trump bought and converted into a "Trump National at fill-in-the-blank", and you'd be surprised what the towns keep on file. Even if you went there and get the company name off the title block of any drawing, you have a decent chance of back tracking to the original firm. Also, you can generally tell where grade beams and footings are located by the layout of the structure. Conduit is a different story.

Trying to hog out poured concrete to make a shallow pit in the slab is not too viable. I'd try to talk you out of that idea if I was working for you.
 

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Trying to hog out poured concrete to make a shallow pit in the slab is not too viable. I'd try to talk you out of that idea if I was working for you.
But you used to do it? : "Pits were recessed down 4 inches"

Is it much easier to break all the way down through the concrete, and then build back up to floor level? Do you pour new concrete to bring the pit back up to desired depth?
 

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With scale pits, we saw cut the slab into a checker board, pick the pieces out (no jack hammering required beyond the first couple pieces if you do it right), excavate down, and prep the base. I've never tried to gouge out concrete. Too much wear on the equipment, too many questions about weakening the slab, too labor intensive, I know I'd run into rebar, etc. If it was just a shallow drainage channel around a machine, sure I'd do it. But to gouge out 4 inches from a 6 ft by 6 ft square, I'd never do that. Doing it properly (saw cut, excavate, base, pour) will leave you with a properly reinforced cut out in your slab.

Also, with scale pits, there is a metal lip going all the way around the edge. That metal lip doesn't get bolted onto the cut edge of the original slab. We make the hole wider, drill and install rebar pins in the face of the cut, install the rebar grid for the new concrete, and lay the metal edging in. That metal edge becomes our pour stop.
 
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