Concrete against existing slab inside

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Old 07-06-12, 08:34 PM
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Concrete against existing slab inside

Hello I have a home built in the late 40's with a concrete slab and a crawl space. This is going to be lengthy so I will try to keep it short. The floor above the crawl space has deteriorated to the point that it has to be replaced as well as all of the joists (2x4's nailed together and held up by hydraulic car jacks). In the process of doing this I am going to remodel my bathrooms. The house originally only had one bathroom and the previous owner converted the laundry room into an extremely small master bath. I would like to reverse this and go with a small guest bath and take the remaining space into the master bath. Since I am reducing the size of guest bath I am looking at reducing the size of the crawl space as well and would like to go with concrete in that room only.

What I am hoping you all can advise is what is required to pour concrete against an existing slab? Going back in with joists will be difficult as the area where this will be located is less than 6 inches above the dirt sub floor and would require a lot of dirt removal to create the space for the new joists, supports, clearance for drains and water lines etc. it makes more sense to me to just pour concrete up to the existing slab on 3 sides and frame it on the other. I am nuts? let me know..

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Old 07-06-12, 09:39 PM
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I'm having a little trouble following.It sounds like there is a non-slab portion(dirt floor) that has joists that are rotted. You want to rip out the joists and continue the slab in thaqt section. Is that correct?

How thick is the existing slab?

Can you get a hammer drill down there to drill a few parallel holes into the slab to insert the rebar?
 
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Old 07-07-12, 02:29 PM
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Bish80,

Yes that is correct, there is enough room for the hammer drill and the slab is 6-7 inches thick. There are some area's of the existing slab where rebar is already sticking out.
 
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Old 07-07-12, 08:44 PM
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How large is the new cement area? Lets say it is 6' X6'. I'm no expert, but if I was doing it, I would install several "cold joints" maybe 5 where the old concrete meet the new. A cold joint is the insertion of the rebar into the old concrete while leaving a tail for the new concrete to grab onto( at least 4"s in to the old concrete.. If you already have some rebar sticking out all the better. I think I would be more worried about the sides that are not reinforced. I would put a footer or 2 in on the non rebar side, make sure the ground is tamped down, followed by an inch of gravel,tamp again, then vapor barrier and finally, invite 3 of my strong friends over to help mix and move the cement.

If I'm confusing you, a diagram might help to give better advise.
 
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Old 07-07-12, 09:09 PM
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Bish80, thanks for the response. The room will be 9ft wide by 5ft deep. The existing slab covers 3 of the 5ft so the pour would be 2ft deep by 9ft wide. I plan on finishing the one side no touching the existing slab with a CMU wall that would essentially be 3ft tall by 9 ft wide. Any recommendations on how to tie the two together? Also would you recommend rebar for the whole pour or would welded wire mesh be sufficient? Thanks
 
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Old 07-08-12, 01:20 PM
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So the only problem you have is the cmu to the old slab over a nine foot span. I would put one rebar every 3rd block(obviously filling that block with cement).

Since the area is so narrow and long for the new cement. I think that a couple of cold joints along the side is all you will need. If you had a an extra piece rebar, sink it in lenghtwise just for fun( it is a pain in the butt to cut the wire mesh into a 2x9 shape and would be even more overengineered that currently advised.....Although there is nothing wrong withy a little overengineering

Any one else pout there have a better idea?
 
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