Methods of semi-precise breaking of a cinder block in half lengthwise?
June 19, 2007 6:40 AM   Subscribe

I want to use cinderblock halves for the border of an outdoor project, but my question -- how to break them along intended lines without cracking/fracturing the rest of it?

I'd like to break a cinder block (two-hole) lengthwise, that is, down the center of the two holes, so that I'll have two wide "m" shapes. How can I break the cinderblock semi-precisely, without fracturing the rest of it? I'm going to be implanting them into the ground, so the exactness isn't necessarily perfect, as long as I have to flat surfaces on the edges of both halves (two flat tops on the "m" shape) to line up end to end to make a border. Any suggestions of how to accomplish this, without having to buy/rent a concrete chainsaw or drilling perforation holes? Solutions that are slow/tedious are just fine.
posted by vanoakenfold to Grab Bag (16 answers total)
Do you already have the cinderblock? If not, consider buying block that more closely aligns with your project. There are many styles and shapes of block available. If you already have the block, your best bet is going to be a concrete saw. You can rent one from a local rental agency like A-1. This is NOT a tool for the faint hearted. It's noisy, heavy, and loud. You will need eye and ear protection, and should wear gloves, long pants, and preferably steel toed boots. The concrete saw will make short work of a stack of blocks.
posted by cosmicbandito at 6:50 AM on June 19, 2007

In my experience it is not possible to do what you want without renting a special saw. You can score a brick and then use a sharp tool to make it break fairly evenly. I cannot imagine a cinderblock breaking smoothly as you want it.
posted by JayRwv at 6:51 AM on June 19, 2007

To get a clean break you need a score line. You could hand chisel it or use a Sawzall with a concrete blade. Then use a chisel to break the brick along the score line. I would start with the inside part first and then do the end parts.
posted by caddis at 6:57 AM on June 19, 2007

I would try scoring using a grinder with a masonry wheel.
posted by probablysteve at 7:10 AM on June 19, 2007

Having a hard time visualizing your project... if it's the bottom part of the "M" that you are going to implant into the ground, can't you just NOT cut them, and just bury the whole block a little deeper?
posted by hermitosis at 7:23 AM on June 19, 2007

I would try scoring using a grinder with a masonry wheel.
posted by probablysteve at 10:10 AM on June 19 [+] [!]

That was my first thought too, but do you think it would fit inside the cinder block to score the center wall? If it would it would probably be more accurate than a Sawzall.
posted by caddis at 7:28 AM on June 19, 2007

Even if you score either end of a block, unless you can also score the webbing in the middle, the ends will break off instead of giving you the M shape you're looking for. Why not just buy a 4" block instead?
posted by LionIndex at 7:40 AM on June 19, 2007

It is going to be more work than is worthwhile, especially trying to saw the middle web as LionIndex mentions. A cold chisel wouldn't work as well as a saw and you might end up throwing away every other block after it cracked unevenly. I'd suggest using patio block buried on it's side. They can be had cheaply and are virtually the same size.
posted by JJ86 at 7:45 AM on June 19, 2007

A concrete saw is really the right tool for this. Blocks aren't that strong in the corners, and you'll probably break quite a few if you apply too much force. Even if you manage to split some in this manner, you'll end up with a pretty weak block. If you're using 8" blocks, cutting some sections out of 4" blocks would likely work better.
posted by glibhamdreck at 7:47 AM on June 19, 2007

Cinderblock's cheap, and you already said that uneven is fine. Just use a small sledgehammer and a masonry chisel (get the one with the approx. 4" blade). This works fairly well for us breaking 120 lb. retaining wall blocks, and will be easier for your blocks.
posted by RikiTikiTavi at 7:49 AM on June 19, 2007

Cement is cheap. Build a mold w/ 2x4s and screws, and make your own cinder blocks in any shapes you like.
posted by popechunk at 8:31 AM on June 19, 2007

do you think it would fit inside the cinder block to score the center wall?

maybe, if you remove the handle and guard

if not, you could always use it to cut in about 1" on either end of the center web and then use the chisel
posted by probablysteve at 9:17 AM on June 19, 2007

I'd use a masonry blade, cut entirely through both end webs, then cut both sides of the center web an inch or two down. Into one of those cuts, drive a sharp wedge, not a blunt one. Using a long chisel and a 1 pound-2 pound hammer, apply blows into the middle of the intact portion of the web and see if you can crack it. My bet is that it'll not be that hard.

My experience is with stone, and those I use have a grain that assists in cleaving them. Concrete is different, but the trick is to focus stress and pre-stress so that things TEND to break the way you want them.

Wear safety glasses!
posted by FauxScot at 11:28 AM on June 19, 2007

Response by poster: I'm making the concrete chessboard from this question, and considering this as a border.

can't you just NOT cut them, and just bury the whole block a little deeper?

Just using one block would be twice as expensive as cutting one in half and using the halves, and I am super cheapo. One objective is to be as inexpensive as I dare =/

To the saw advisers, see last sentence of question =P I'm making the tiles of the chessboard in the manner popechunk describes already.

Is there perhaps some kind of cinderblock-material weaker solution that I could douse the necessary sections with? Hmm.. maybe I ought not go with this border then =P

Thanks ennyhoo =D
posted by vanoakenfold at 7:17 AM on June 20, 2007

Response by poster: Forgot to put the link in, duhr! This question =D
posted by vanoakenfold at 7:19 AM on June 20, 2007

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