Re: Concrete removal - how much before being a safety consideration

Hi,

For aesthetic purposes, I wanted to relocate a few of my outlets and cables directly behind the TV instead of having it on the lower left corner.

As a result, I will have to chisel out the concrete then cover it up with another mix, then finish. The movement is not that far, I reckon 3ft horizontally to the rightand 4ft up so I can have the outlet and cable directly behind

Question is how much concrete can be removed before it can be considered a safety hazard?
Is chiseling out a beam or buhos a do or don't?
What if I were to cross the room along the flooring (buhos)?

Sorry for the late response.

No biggie chiselling out a masonry wall for anew conduit-run.
It's done all the time.

Generally speaking, and assuming simple loading...
As far as RC beams are concerned, as long as you restrict your
concrete removal to the lower half of the beam, and refill any
voids after conduit-laying (using 9:1:1 or better concrete, with
1/2" gravel), you'll be fine.

Under simple loading kasi... the bottom half of the beam is the
half dealing with tension: it's the steel that's doing the work,
not the concrete. You do not want to mess with the upper
half of the beam, which is under compression, and where the
concrete is doing a LOT of the work and really should not be
compromised.

Also, realize that a lot of RC structural members have about 1"
of cement plaster covering them. Since most adjustments to
your electrical circuit-runs will involve 1/2" conduits, you are not
necessarily going to be eating deeply into any beams/columns,
depending on where your junction and utility boxes will be.

Re: Concrete removal - how much before being a safety consideration

Very good question, Spyghost, and excellent response, Horge.

I'm in that similar predicament as I am relocating some speaker cable conduits as I remodel my Home Theater with better sound absorption.

@Horge
I understand the lower-half restriction for beams. How about for columns? As I relocate a speaker outlet horizontally, I will have to pass a column protruding from the wall along the way.

I plan to "tiktik" the wall to the column, then tiktik the sides and front of the column so that the conduit can be embedded on its surface, hopefully, not getting into the re-bar. The height of the conduit from the floor is between 3' to 4' while the column itself is a little more than 8' from floor to ceiling. Any restrictions doing this?

Re: Concrete removal - how much before being a safety consideration

@Horge
I understand the lower-half restriction for beams. How about for columns? As I relocate a speaker outlet horizontally, I will have to pass a column protruding from the wall along the way.

I plan to "tiktik" the wall to the column, then tiktik the sides and front of the column so that the conduit can be embedded on its surface, hopefully, not getting into the re-bar. The height of the conduit from the floor is between 3' to 4' while the column itself is a little more than 8' from floor to ceiling. Any restrictions doing this?

Hi, b. 🙂

Per building standards...
There should be a minimum 1" concrete cover around the column mainbars and ties,
although poor local workmanship can reduce this to 1/4". Added to that is roughly 1"
thickness of cement plaster, so you're not likely to run into trouble: I don't think you
will be laying a conduit thicker than 3/4" OD, and it will likely be less. Staying outside
the column core and mainbars as you are, it doesn't matter how high up the column
you decide to chisel for laying fresh conduit.

You're quite right to avoid messing with the column rebars (mainbars and ties)...
[COLOR="Red"]Strictly speaking, it is the vertical mainbars, and the concrete they hold WITHIN, that
supports vertical load. The ties are there to protect the mainbars from shear forces
as the building frame deforms/flexes, and handle non-vertical forces, most dramatically
during an earthquake, and to help retain the core section of concrete between the
mainbars and ties.

Any concrete outside of those main bars and ties is just bonus support: it's actually
there to help protect the steel and core-concrete from harmful elements (like, fire)
and then there's the cement plaster ont top of it, so you have leeway to 'tiktik' and
refill/replaster.

🙂

Re: Concrete removal - how much before being a safety consideration

for the sake of info, what if you accidentally 'broke in' to the main bars by let's say an inch? is there a way to 'fill' this in?

Re: Concrete removal - how much before being a safety consideration

for the sake of info, what if you accidentally 'broke in' to the main bars by let's say an inch? is there a way to 'fill' this in?

Harming the main bars and/or the core concrete is EXTREMELY bad juju.
Thanks to standard factors-of-safety in structural design, the column may still
be standing now, but is compromised, and come the next major earthquake,
you may have a disaster on your hands.

People who try to run a conduit through the core of an existing RC column
are asking for trouble, but they can reduce their risk by using RSC (rigid steel
conduit) for at least the portion inside the core: the steel has strength and
can help make up (a little bit) for the concrete it has displaced.

Assuming you harmed the main bars and/or ties...
You'd have to support the RC beams connected to the affected column with
multiple steel posts, then weld in new steel to replace what was lost. Welding
heat weakens the column's steel temporarily, which is why you need temporary
support/bracing. Then, you'd backfill any lost concrete as I described earlier: the
smaller, 1/2" crushed gravel is for better plasticity and void-filling.

Even then, your RC column is no longer as strong as it was before.

Bottom line, don't mess with the steel mainbars, steel ties or concrete core.
Stay outside of all that. "Exposing" the ties and mainbars is still 'staying outside',
as long as you don't damage the concrete core behind/within their embrace.

🙂

Addendum wrt RC beams:
The rule ("lower half only") is reversed for cantilever beams: the sort that
support, say, an overhang. With cantilever beams, you want to mess only
with the top half, which is mostly under tension, and not harm the lower
half which is under compression.

Obviously, this considers only simple loading. A structure is not a collection of
independent beams and columns, but an interconnected frame. Some beams
that are not cantilevered may actually be acting as girders and supporting an
upward load (like a cantilevered beam)... but this is usually an issue found in
larger structural frames, meaning larger structures, whereas most of what are
consideredin this forum are small structures (residences).

Re: Concrete removal - how much before being a safety consideration

Thanks, Horge. I can see you are an expert on this matter. You're a Structural Engineer?

We actually have a couple of SEs at work and their word is usually gospel. Nobody wants to cut corners on their advice. I could've asked them but I have this thing about not using work resources for personal benefit.

Salamat!

Re: Concrete removal - how much before being a safety consideration

Thanks also, how about blowing through concrete? What harm does that impose? For this purpose, I'm certain that what I intend to blow through is hollow blocks. Intention is to lay piping for LPG outside the out going in.

Re: Concrete removal - how much before being a safety consideration

Thanks, Horge. You're a Structural Engineer?

I'm just an architect 🙂

Re: Concrete removal - how much before being a safety consideration

Thanks also, how about blowing through concrete? What harm does that impose? For this purpose, I'm certain that what I intend to blow through is hollow blocks. Intention is to lay piping for LPG outside the out going in.

Punching a hole of so small a diameter through a CHB wall is no big deal,
save that you should consult your plumbing and electrical layouts, so as
to make sure you aren't going to hit a water-supply, soil or power line
lol...