cutting a hole in my cast concrete wall...REBAR ?

k13wjd · 26 Jul 2010

hi guys quickie here.

my house is of the cast concrete type. I believe it to be approx 6 inches thick, with 2 layers of re-bar.

I need to cut a 5.5 inch hole in it - for a central heating flue.

I'd like to do the work myself, COULD hire a tool, but am told that i'd need some fairly specalist tools to core cut it....

Is it going to be a hammer and chisel job ?

k13wjd · 26 Jul 2010

anyone ?

k13wjd · 26 Jul 2010

anyone ?

freddiemercurystwin · 26 Jul 2010

Only joking!

k13wjd · 26 Jul 2010

Damn you, i got all exited.

DSA · 27 Jul 2010

Is this a concrete column you need to core through? Do you know the centres of the reinforcing bars/have drawings that you could refer to? Was the building cast in-situ or constructed using precast units?

marshman · 28 Jul 2010

Diamond core will do it.

Static · 28 Jul 2010

marshman said:
Diamond core will do it.

On the assumption that the wall is not a critical element.. Hole shouldnt be within about 300mm of a wall end and definitely not in an RC column..

Is this a concrete column you need to core through? Do you know the centres of the reinforcing bars/have drawings that you could refer to? Was the building cast in-situ or constructed using precast units?

Answer these will help get a better idea of what your proposing..

k13wjd · 28 Jul 2010

no, its the face of the house - a 125mm hole for a central heating flue.

Can't get any info on the dimensions of the rebar. Its jesperson concrete, precast. its further away that 300mm from a wall end, and within a set of 4 lintel type re-enforcements.

i'm told rebar bill kill a diamond core cutter......

RonnyRaygun · 28 Jul 2010

You could hire a rebar locator. Not sure on their accuracy as I used one once and it told me there was rebar practically everywhere...but that might be because laying an RC slab is not an exact science

As long as you've got a bit of leeway with your hole postion I'm sure you could ensure that you miss the rebar, as it's not likely the rebar is closer than 150mm centres.

If you cut through the rebar you would need to be very careful about water ingress.

Not sure if the rebar would ruin a diamond core cutter, but I do know that a diamond blade on a 9" grinder goes through it no problem!

k13wjd · 28 Jul 2010

perhaps i should drill a centre hole, then dry core from one end until ( if) i catch rebar. Then drycore from the other end, punch the cores out, and cut through the rebar ?

theoldun · 28 Jul 2010

Rebar will kill a dry core bit stone dead. You need a wet core bit for rebar and then you are into big money.
Hire a 5kg SDS drill and buy 4 number 6mm by 210mm long SDS bits. Mark your hole out 25mm bigger than your flue pipe. Stitch drill all round the hole, about 40 number holes. You will soon know when your drill hits a rebar. When it does pull out and move along another 15mm as rebar will only be T18.
When you have done, drill another say 6 holes up the centre both vertical and horizontal to weaken core as much as poss.
Hire a 10KG breaker with a impact energy of 25J and blow middle out. Cut your rebar out with agle grinder.
Word of warning, there were numerous styles of jesperson concrete, precast houses, and the position of where you are cutting this hole could weaken the panel.
oldun

marshman · 28 Jul 2010

If you want to core phone round the hire shops and see if they have one of these,
http://www.priorityplant.com/HUSQVARNA-DM230-110V-DIAMOND-CORE-DRILLING-MOTOR/47.htm
as theoldun says dry coring through rebar isn't good but there is a cheat, you drill your pilot right through and dry core about 10 mm in
then you get someone the other side with a hose and trickle water through the poilet hole back to the corer,
what kill the core bit is when you slice through a bar at the edge of the core more or less cutting the bar length wise,
the little piece of bar on the inner core breaks lose from the concrete and wedges the diamond segment forcing it outwards and sometimes
cracking the diamond segment off the core bit.

DSA · 28 Jul 2010

You need to be sure that if you damage some of the reinforcement, then the panel will not be significantly reduced in strength. Note that as the panel is precast, it may well be prestressed.

If you have any concerns about this, you would be best advised to seek some professional advice from a structural engineer before you core the hole.

Why do you think there are two layers of rebar - do you know any more about the design of the panels? There is also no guarantee as to the size of the rebar - however it is unlikely to be T18 as that is not a standard size; T16 or T20 are the next closest sizes.

k13wjd · 29 Jul 2010

hi guys.

As for worrying about pre-stressed etc...There are 249 houses round me exactly the same. i'd say 60 percent of them have a flue drilled in the outside facing wall.......

I HAVE cut the hole - managed to speak to someone who has done it in the same place, and after lots of faffing about with a tape measure, decided i;d found a 117mm circle that should be re-bar free. 10 mins with a dry core, and job done.

Thanks for all the advice guys. Now, on with the plumbing !

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cutting a hole in my cast concrete wall...REBAR ?

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