Proper way to extend concrete slab? - Team Camaro Tech
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post #1 of 18 (permalink) Old Apr 6th, 11, 05:46 AM Thread Starter
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Question Proper way to extend concrete slab?

I could look at DIY, but thought I'd ask here first...

I want to extend the footprint of my shed slab from approx. 10'x10' to 12'x12', basically just adding more slab on 2 sides.

It needs a new roof and I want to make it bigger while I'm tearing it apart.
What's the best way to tie the new concrete into the existing slab? I don't want the new floor to sink or shift. I thought about drilling horizontally into the existing slab and adding rebar, but that's too much work. I know a proper base is key, but that's the easy part.

Will just roughing up the 2 edges of the existing slab and wetting it before pouring the new concrete be enough to 'glue' them together?
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post #2 of 18 (permalink) Old Apr 6th, 11, 07:17 AM
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Re: Proper way to extend concrete slab?

I'm voting for the rebar. I'm not a concrete guy so have no professional experience. At my first house I had a slab that shifted outside the back door. I drilled holes and put Chevy head bolts in it and the poured a strip about three inches thick to support threshold. The rest of the slab still moves but at least the door doesn't stick shut.

Jeff
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post #3 of 18 (permalink) Old Apr 6th, 11, 07:33 AM
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Re: Proper way to extend concrete slab?

Oh No!!! Joe's gonna be diggin again.

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post #4 of 18 (permalink) Old Apr 6th, 11, 08:22 AM Thread Starter
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Re: Proper way to extend concrete slab?

Hmmmm... I AM replacing the head bolts soon. I might as well 'recycle' them and use them for something useful. Drilling an inch and a half deep hole is a lot easier than a 12 inch hole in concrete!
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post #5 of 18 (permalink) Old Apr 6th, 11, 08:28 AM
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Re: Proper way to extend concrete slab?

i am doing this right now, adding on to a existing slab, a general contracter advised me to drill into the existing slab with a spacing between holes every 32 inches and no less than 2 ft from a corner, insert a 5/8 dia rebar in 6 inches with 6 inches sticking out , this will anchor the new slab to the footer of the old slab transfering the load, on the new edges a footer would need to be poured with rebar also, 1 or 2, 5/8 rods in fl.
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post #6 of 18 (permalink) Old Apr 6th, 11, 09:20 AM
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Re: Proper way to extend concrete slab?

As Scott says just make sure you drill your holes so it requires beating the rebar in or alternatively (it will cost more) go get large concrete anchor bolts and drill and mount them so their half buried in the old slab. We run the nuts onto the end before pouring the fresh concrete, the new slab will not move. We tend to over engineer some but never had to go back and fix an issue. Give yourself a good base under the new slab as well.

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post #7 of 18 (permalink) Old Apr 6th, 11, 09:53 AM
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Re: Proper way to extend concrete slab?

I had to repair a contractor's butcher job and replaced a 2'x24' section. Pinned it with #5 rebar using concrete epoxy for this application. No adhesive is applied to the shear face. There are engineering specs to follow but I did not research it in great detail. Basic repair procedure is common sense and straightforward. This was said to be adequate for this particular situation by a different concrete contractor. I contacted him investigating the loser who did the work. 1-1/2" pin depth is not enough. If you don't have a good Hilte rotary, rent or borrow one. I'm sure the method will vary within the industry and unique construction requirements. Best to obtain information from a qualified, educated, responsible business owner. Or at least their competent employee.

Last edited by Sauron67MM; Apr 6th, 11 at 10:30 AM. Reason: punct.
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post #8 of 18 (permalink) Old Apr 6th, 11, 05:03 PM
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Re: Proper way to extend concrete slab?

As noted above, drill and place rebar. I would buy some of the automix epoxy to partially fill the holes then insert the rebar. When the epoxy sets, it ain't going anywhere. But know that the working time on the epoxy is on the short side, read the instructions carefully. Lowes sells or used to sell the automix cartridges that fit into a standard caulking gun. If you can have a buddy help you, you can use the caulking gun and he can insert the rebar, that way you can really move on the project. The problem with doing one or two holes is that the epoxy will set up in the automix tube rendering it useless.

Also, you have to blow out the holes after drilling them, everything should be a clean as possible for a good bond.

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post #9 of 18 (permalink) Old Apr 6th, 11, 05:33 PM
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Re: Proper way to extend concrete slab?

I used the epoxy as I said and inserted the pins alone. There was no need for assistence. I got it from the mason supply yard, that's what they are there for.
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post #10 of 18 (permalink) Old Apr 6th, 11, 06:26 PM
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Re: Proper way to extend concrete slab?

I am a Plumber and have done and seen this all the time.I think 1/2" #4 rebar x 12" long five places on each side drilled on a angle without epoxy and four 11' pieces tied to the bar drilled in the slab will be suficient.The rebar will not pull out as the slab is being poured on all four sides.Even if not poured on all sides if you drill on an angle the rebar will not pull out.I have been doing construction for 35 years and this is what I would do to extend any slab....
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post #11 of 18 (permalink) Old Apr 6th, 11, 06:58 PM
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Re: Proper way to extend concrete slab?

Quote:
Originally Posted by Pro-Street69Camaro468 View Post
four 11' pieces tied to the bar drilled in the slab
I forgot I put the long #4 pieces in as well. I only used 5/8" pins because I grabbed the coated precuts off the truck. There are a lot of hacks out there as I found out.
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post #12 of 18 (permalink) Old Apr 6th, 11, 09:54 PM
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Re: Proper way to extend concrete slab?

We've drilled and pimmed concrete together and it has worked good. Get ya a good hammer drill and 1/2" masonary bit. Concrete isn't that hard to drill.

Experience...... It's what you get when you didn't get what you wanted.
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post #13 of 18 (permalink) Old Apr 7th, 11, 05:48 AM Thread Starter
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Re: Proper way to extend concrete slab?

Quote:
Originally Posted by Pro-Street69Camaro468 View Post
I am a Plumber and have done and seen this all the time.I think 1/2" #4 rebar x 12" long five places on each side drilled on a angle without epoxy and four 11' pieces tied to the bar drilled in the slab will be suficient.The rebar will not pull out as the slab is being poured on all four sides.Even if not poured on all sides if you drill on an angle the rebar will not pull out.I have been doing construction for 35 years and this is what I would do to extend any slab....
Drilling the shorts on a downward angle isn't something I would have thought of. Good idea! Thanks. No need to glue the pins in that way. It's only a shed anyway. But the one side that will be extended is the front where the door will be, so I'll take extra care in that area.
I have some chain-link fencing I can lay in the concrete as well. If I didn't live in such a harsh climate, I wouldn't worry about it.
I have a hammer drill... just need the masonry bit. Rent a mixer, calculate the amount of concrete and go to town!
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post #14 of 18 (permalink) Old Apr 7th, 11, 05:36 PM
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Re: Proper way to extend concrete slab?

Good luck Joe how about instead of a mixer get a U-cart...
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post #15 of 18 (permalink) Old Apr 7th, 11, 06:04 PM
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Re: Proper way to extend concrete slab?

Quote:
Originally Posted by HwyStarJoe View Post
I could look at DIY, but thought I'd ask here first...

I want to extend the footprint of my shed slab from approx. 10'x10' to 12'x12', basically just adding more slab on 2 sides.

It needs a new roof and I want to make it bigger while I'm tearing it apart.
What's the best way to tie the new concrete into the existing slab? I don't want the new floor to sink or shift. I thought about drilling horizontally into the existing slab and adding rebar, but that's too much work. I know a proper base is key, but that's the easy part.

Will just roughing up the 2 edges of the existing slab and wetting it before pouring the new concrete be enough to 'glue' them together?

Joe for such a small addition of your sheds foot print get some 2" x 2" square wire mesh in a roll its pretty cheap. lay it out covering the area you want to add the concrete. you can use a mason drill bit to drill into the edge of your exsiting slab. set up your wood forms with stakes to hold your edge flat and clean as the concrete flows into your designated areas of your added slab. make sure the rebar you use your able to bend them into a "L" shape. This will give you the ability to have the rebar come out of your existing slab and point down so you can have it hold the wire mesh in place. look into "short load" concrete outfits in your area. they will bring out the dry concrete and mix it on site as needed. its cheaper for a home owner than having a large truck bring pre-mix or hauling a cart.
if you have questions pm me.


Done what your talking about hundreds of times. this is a quick bunch of notes ...

good luck



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