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I need to enlarge a few holes from 40mm to 50mm in a cinder block wall and in a concrete beam.

Chiseling will do much damage and need patching, can I rasp it with a wood rasp?

  • Why not just drill a larger hole with a 50mm bit? – Steven Aug 14 '15 at 15:28
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The metal on a wood rasp would be way to soft to make any type of dent in the concrete. The only thing you will do is destroy the rasp.

You should use a roto hammer with the proper sized bit to widen the hole.

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Yes, you can and it will work. It will also dull your rasp.

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You could use a heavy duty drill and a diamond encrusted bit, such as this one

diamond bit

They can be found on-line and in some big box stores for about $20.

When grinding or cutting very hard materials like concrete, you should use a lubricant to reduce heat. Usually a small constant drip of water at the cut point helps.

 Images and links for illustration only, not an endorsement of goods or sources.
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This is the correct tool for rasping concrete but it won't fit in a 2" hole. I suppose you could ruin a file (and the better part of a day) to get it done; CMU's aren't that hard.

enter image description here

source

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I'm adding my own answer since I tried to use the rasp.

It didn't work. The rasp can ease hard corners on cinder block and even a little bit on concrete, but it won't bite enough to enlarge a hole.

The method I use was a hammer drill with a chisel bit. I hold the chisel against where I want the edge to be and pressed the trigger very slowly. I did this so that it dig about 1 cm inside the concrete, then I move to the side and did that again making a contour. After that I used more force to break inside the contour. That way I could make a pretty good edge that requires minimum patching.

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