Cutting 12" Diam hole in blockwork wall

[Andrew Mawson]

OK chaps - suggestions please.

I need to cut a 12" diameter hole in a workshop wall construction of which is a single skin of 9" x 18" x 4.5" 7 newton concrete blocks. Now I've all sorts of drills, angle grinders, hammers and even demolition breakers, so the problem isn't removing the material, the problem is doing it without the wall collapsing !!!

Current plan is drilling a circle of (say) 1/2" holes with a masonry drill and wiggling it from both sides to hopefully join them up without disturbing the rest of the structure. (I'll probably select a spot where the hole is fully bridged by an 18" block - position within a few inches doesn't matter.

Any other constructive suggestions very welcome.

(this is for a huge 3900 cuM/Hour extractor for a CNC plasma table down draught exhaust)

Andrew

[Roger Hayter]

Should be fine unless the 'lintel' block cracks or moves. Even if it does, it should be possible to re-mortar and replace as necesary the blociks above the hole without actually needing to support the wall. Unless there is a huge load on it, which would be a bit odd for a single block wall.

[dennis]

Biggest core drill you have and overlap the holes. Use a guide hole in a piece of ply to stop the core wandering and you can get a good finish with a close overlap. Shouldn't take long with blocks like that.

[Adrian Caspersz]

The drill used for Hatton Garden isn't doing much if you need it.

(what happens to evidence material such as that? Is it bagged up and placed forever in storage, destroyed or perhaps sold off to some ghoulish collector of crime memorabilia?)

use the plasma cutter?

As a computer programmer I like recursive solutions ...

[Robin]

Does it _need_ to be a circular hole in the _wall_ or would it work with circular holes cut with (say) a jigsaw in 2 pieces of (say) plywood which were screwed to battens (or "angle iron") in a _rectangular_ hole in the blocks? You could make that rectangular hole with your angle grinders, hammers, Chieftain tank or whatever other toys we can envy.

And if seriously worried about support, cut "slots" and fit uprights before removing the central part. (Or should that be a slot and fit a "lintel"?)

All capale of being tidied up with eg render to match the block?

[John Rumm]

12" is really not a large hole - old boiler flues were frequently as large as this.

Stitch drill and break out the middle with a SDS would be one approach.

If you want a neater round hole, overlapping core drilling also works.

You may be able to hire core kit large enough to do it as a single core.

Alternatively just chisel out the mortar and remove hole blocks, then fill in round your exist pipe with new mortar and bricks (or bits of)

There are also firms that specialise in this kind of thing. A local one here for example:

(they claim they can do a 1.5m hole!)

Now this sounds like fun ;-)

[tabbypurr]

I suspect it'd be as cheap to rebuild the wall as hire the tool.

NT

[The Natural Philosopher]

I think this is actually a plan. Remove a whole block from the wall, and pout the pipe through and make good with fresh concrete.

[GB]

I suspect the extracted air will go through any shape hole without complaining. Does it even need to be 12" just because the fan is that diameter?

[GB]

Hmm, just re-read the OP. The fan is moving 1m3 per second, so that would need a decent hole, I guess.

[newshound]

My first thoughts are that either stitch drilling or removing one block and making good afterwards would be fine.

But also agree with John about diamond drilling. One of my lads just converted a shop unit in a modern complex into a bar, which meant putting in loos on two floors. He was convinced the only solution for one loo was a macerator because of the building geometry. I pointed out that a six inch hole in the concrete floor (about a foot thick) would give an easy run for conventional soil pipes. A local firm did it for him for £300 in a couple of hours (that includes all the humping of kit and cleaning up afterwards). I think the price would have covered half a day's work, which was their upper bound estimate if there was much rebar.

[Roger Hayter]

Presumably it includes an inlet as well as an exhaust. Otherwise asphyxiation and the walls falling in would be a risk.

[Andrew Mawson]

The fan arrived today - by heck it shifts some air - no way of measuring if it's the claimed 3900 cu/M / Hour but it's certainly impressive :)

Andrew

[harry]

That is without air flow restriction. The actual flow with depend on the length of ducting, diameter, number of bends and the friction of the duct wall..

The power consumed will vary on the air shifted and the pressure caused by the restrictions.