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I have to cut a 100x500 mm hole in a concrete slab that is about 80-100 mm thick. I think that I have two options to do that:

1) 9" angle grinder with segmented diamond disc: cut the long sides, then chip away the short sides with chisel.

2) Hammer drill with with a large bit making holes close to each other and the just breaking it out with a chisel.

Option 1 is faster, but is more expensive (I also have to buy the disc besides the tool rent) and it looks kind of scary after I saw some kick back results.

Option 2 is cheaper (bits come with the tool rent), but seems to be more time consuming.

Both options will leave a rough finish that will demand for stucco or something.

PS: I can access the slab from the top and the bottom if it matters anything.

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  • Why are you making the cut out? What is it for and does the cutout have to be exact?
    – DMoore
    Oct 7 '13 at 15:33
  • This will be a shaft (passageway between floors) for electrical and water pumbling. It doesn't need to be exact, roughly 100mmx500mm.
    – Luiz Borges
    Oct 7 '13 at 15:44
  • Usually penetrations between floors have to be sealed in some way, to prevent the spread of fire due to the stack effect.
    – Tester101
    Oct 7 '13 at 16:24
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    I count my angle grinder as the second most dangerous tool I own. Second to my jointer/planer. High laceration hazard, and it produces a lot of dust. From a safety perspective, hammer drill.
    – Edwin
    Oct 7 '13 at 18:18
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The standard approach used by electricians routing large conduits through concrete floors is using a diamond edged hole saw cutting 3 or 4 inch holes. Several of these holes overlapping could do it. Or several holes near each other might suit your purpose. These drills need to be water cooled while drilling. You may be able to rent one.

Alternatively a smaller grinder could score the cutout on both sides, a drill could then break through the webbing left under the scoring and a chisel to finish it off.

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  • How small could that grinder be? Can a 4.5 angle grinder do the job (I'm thinking about buying one for some metal work)
    – Luiz Borges
    Oct 7 '13 at 16:31
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    Still not sure I totally understand what you want to do but if you just need holes for electric and plumbing then just drilling individual holes is the way to go. When running data lines we often used 1-2 inch diamond hole saws. It takes a while to get through. Add fire rated spray foam or rock wool in gaps to get the fire chief off your back too.
    – DMoore
    Oct 7 '13 at 16:53
  • I need to pass a venting pipe of 100 mm so I need it to be at least 100 mm wide, therefore I would rather make it larger than I need and fill the gaps with foam later. Easier to err to the larger side and have plenty of space.
    – Luiz Borges
    Oct 7 '13 at 17:23
  • @DMoore, you mean these, right? Two of those (75mm and 100mm) would be more expensive than the rental of all the other tools, I agree that the cuts will be perfect, but they will also need clean up (drill, break the drilled part with chisel, drill again, etc).
    – Luiz Borges
    Oct 7 '13 at 17:36
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    That is it. Hey I gave you the sledge hammer - that is my cheap contribution. This isn't as easy as you think. You can run into rebar and it is a pain in the ass.
    – DMoore
    Oct 7 '13 at 17:42
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I have broken up a lot of concrete and I would use a sledge hammer or jackhammer. Angle grinders are fine for making exact cuts but in the end you need the stuff broken up in chunks so you can get rid of it. The angle grinder doesn't help with that. Also the hammer drill might break things up into too small of pieces and take too long. I would be done breaking that up with a sledge hammer in 20 mins.

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    Wow, isn't that too much? I mean, sure it will do the job faster, but hitting the slab with a jackhammer or a sledge hammer won't risk causing some damage to the concrete nearby? Cracks or fissures? I really have no idea. Also the hole is quite small (4" x 20").
    – Luiz Borges
    Oct 7 '13 at 15:54
  • I have never damaged concrete more than a couple inches away using a sledge hammer. If you are worried about that then I would go with angle grinder rather than hammer drill where you might run into same issues. Also when using sledge hammer you don't have to swing it as hard as you can. There are ways not to damage surrounding areas.
    – DMoore
    Oct 7 '13 at 16:09

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