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Old 09-30-2009, 11:14 AM   #1
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Small concrete slab that will be expanded later?


I want to pour a 4' x 6' 4" slab now, but plan to extend the slab sometime in the future.

What's the best way to join the slabs? Should I have rebar that comes out of the first slab now? Or should I drill holes and add rebar later when I pour the 2nd?

Any other tips?

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Old 09-30-2009, 01:05 PM   #2
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Small concrete slab that will be expanded later?


why rebar now OR in the future ? ? ? i don't have any in my d/w other'n the throat where vehicle weight ' shock loads ' the conc.

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Old 09-30-2009, 01:35 PM   #3
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Small concrete slab that will be expanded later?


Dowel the 2 together later with rebar.
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Old 09-30-2009, 01:44 PM   #4
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Small concrete slab that will be expanded later?


I agree with reallyconc. You are building a 4" thick slab, you will gain no structural advantage by installing rebar due to cover requirements of the steel. The only purpose for steel in the slab is as temperature steel, which will provide minimal crack control. The same crack control or better can be obtained by installing control joints, proper curing of the concrete, and use of additives if crack control is critical.

If the slab you are planning to construct is for structural support (i.e. a building or some other heavy load), you are in an entirely different situation, and a 4" thick slab cannot possibly work. You would need a structurally designed slab.

As for joining two slabs together, you could use steel rebars, but if the soil preparation is correct, i.e. you remove all incompetent soil and replace with compacted structural fill, you will not need to join the slabs, simply place a control joint there. If you are worried about frost heave, again soil preparation is critical, a small amount of steel will do nothing to prevent frost heave, all it will do is keep the out of level slab together.
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Old 09-30-2009, 01:51 PM   #5
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Small concrete slab that will be expanded later?


Kind of surprised no one has mentioned forming in a running "keyway". By far the easiest.
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Old 09-30-2009, 01:52 PM   #6
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Small concrete slab that will be expanded later?


Quote:
Originally Posted by Daniel Holzman View Post
I agree with reallyconc. You are building a 4" thick slab, you will gain no structural advantage by installing rebar due to cover requirements of the steel. The only purpose for steel in the slab is as temperature steel, which will provide minimal crack control. The same crack control or better can be obtained by installing control joints, proper curing of the concrete, and use of additives if crack control is critical.

If the slab you are planning to construct is for structural support (i.e. a building or some other heavy load), you are in an entirely different situation, and a 4" thick slab cannot possibly work. You would need a structurally designed slab.

As for joining two slabs together, you could use steel rebars, but if the soil preparation is correct, i.e. you remove all incompetent soil and replace with compacted structural fill, you will not need to join the slabs, simply place a control joint there. If you are worried about frost heave, again soil preparation is critical, a small amount of steel will do nothing to prevent frost heave, all it will do is keep the out of level slab together.

Ummmm, well, I think that's the point: for pennies on the dollar, put the steel in to hold the slab TOGETHER.
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Old 09-30-2009, 07:20 PM   #7
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Small concrete slab that will be expanded later?


since when does steel add compressive strength ? it DOES add flexural strength to resist moment loading ( bridge - hi-rise floors - driveway throats ),,, even in a slabs on grade, welded wire mesh adds strength to resist tension ( while the conc's ' green ' ) but, after that, it only holds the broken pcs together better you should pay attn to timing & placement of control jnts as danny post'd,,, keyway's usually done on airport aprons & some hgwys but, more often, longitudinal tie bars're left sticking our of the forms to cover ' w/the next pass,,, any steel or k'way's o'kill for this back OR front landing im-n-s-h-fo

then again, i am NOT an eigineer,,, come to think of it, may've said too much already
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Old 10-01-2009, 09:59 AM   #8
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Small concrete slab that will be expanded later?


I suggested a keyway for two reasons.

One, it's super simple and easy to do. Bevel a hunk of wood, and nail it on the form.
And two, not knowing how long it will be till this slab extension is added, rust could have gotten well established before the rebar is covered. I kind of doubt he'll be using epoxy coated.

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