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Dave K 04-18-2011 12:46 AM

Replacing a portion of a concrete slab?
I have a studio apartment that is on a slab that is a compilation of two poorly poored slabs. There is a 5'X5' area in the center of a room that I am planning to cut out and re poor.

I dont know much about concrete but I plan to make a 5'X5' square hole and drill holes to glue 3/8" rebar into the existing concrete and poor new concrete into the hole.

How deep do I need to drill the rebar into the existing conc. What kind of glue should I use?

Do I need to do something for a moisture barrier? If so what should I use?

Any other tips?



stadry 04-18-2011 07:36 AM

1st, can you do this work legally w/respect to condo restrictions ? as a genl rule, all slabs-on-grade should have vapor barriers installed,,, currently the spec is 15mil ( stego is oft spec'd ),,, whether or not you need #3 bar is another issue - i would core holes 2x deep as the slab is thick ( eg, 4" slab = 8" deep ),,, the cured elevation may differ from the existing meaning you'd have to grind or overlay for proper alignment.

epoxy will secure the bar to the existing conc ( properly clean the hole ) but deformed steel is always recommended,,, don't know if this is avail in #3 bar to you.

Knucklez 04-18-2011 06:00 PM

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don't "cut" the concrete. the nice eaven clean seam that a saw would make does not provide for good adhesion to the new concrete. use an electric rotary hammer and the edges will be rough, 3-dimensional. so there is lots of area and adhesion and no leaky seam.


stadry 04-19-2011 05:13 AM

what 'nuck's talking about is ' mechanical [ or aggregate ] interlock ',,, despite anyone's best intentions, fresh conc doesn't bond to existing however, if the edge is left jagged as is case when using a breaking gun, there will be some ' holding ' power from the irregular edgewall,,, any liquid conc will shrink upon curing therefore no bonding's gonna happen.

since its an interior slab, there'll be little change in floor dimension from changes in temp but you will see a slight crack between old & new :yes:

Knucklez 04-23-2011 09:53 PM

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concrete dude. .. i'm a bit confused by your post. i think you are saying that due to concrete shrinkage the new concrete will still leave a crack or seam despite leaving a jagged edge. if that's the case then how do you do this properly ( i don't know, never done it before) ? just cut with a saw a nice long seam and then seal it later with masonary caulk? cutting with a saw is a lot of dust, so maybe smashing is still better, but still will end up having to deal with a crack/seam after the concrete drys?


stadry 04-24-2011 06:50 AM

yes, said that & still mean it,,, there will be TWO pieces of conc in the final floor no matter how properly its done,,, sawing the conc ( we have dust shrouds on our saws, guns, & drills hooked up to vacs ) ^ sealing the sawcut still doesn't camouflage it :no: same w/sidewalk replacement or conc hgwy slabs - now go chase a bunny :laughing:

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