How to patch the underside of a concrete cantilevered balcony?
June 25, 2010 9:24 AM Subscribe
cantilevered balcony How do I patch the underside of a concrete cantilevered balcony?
Hi,
We have a concrete cantilevered balcony not our balcony!, whose underside is starting to crumble, to the point where the steel rods (rebar?) that reinforce the concrete and attach the structure to the main frame of the house are exposed.
All the books that I read on concrete repair describe how to fix topside and lateral-side cracks, but I can't find any sources for patching/resurfacing the underside of a concrete slab, where gravity likely will make things difficult.
Can a repair be done by a novice or would I need to hire a professional? And if this is a repair that I can do, how so? Alternatively, are there any good books/references that you know of that describe such a procedure?
I assume that I will need to fabricate a concrete form of some sort. Can it be wood or will the concrete be too heavy? FYI, the slab is about 4 ft x 8 ft x 6 inches thick.
Thanks!
Hi,
We have a concrete cantilevered balcony not our balcony!, whose underside is starting to crumble, to the point where the steel rods (rebar?) that reinforce the concrete and attach the structure to the main frame of the house are exposed.
All the books that I read on concrete repair describe how to fix topside and lateral-side cracks, but I can't find any sources for patching/resurfacing the underside of a concrete slab, where gravity likely will make things difficult.
Can a repair be done by a novice or would I need to hire a professional? And if this is a repair that I can do, how so? Alternatively, are there any good books/references that you know of that describe such a procedure?
I assume that I will need to fabricate a concrete form of some sort. Can it be wood or will the concrete be too heavy? FYI, the slab is about 4 ft x 8 ft x 6 inches thick.
Thanks!
Definitely a job for a professional. It doesn't sound like a patch job - it sounds like a reconstruct job. Also assuming you are in a condo and this is for a neighbor's unit, you shouldn't be touching it at all. In the meantime, you probably don't want to be anywhere underneath this thing because it is obviously extremely unsafe.
posted by JJ86 at 9:51 AM on June 25, 2010
posted by JJ86 at 9:51 AM on June 25, 2010
Consulting an engineer is doubtless a good solution, but an expensive one. If you would like to do this more cheaply, what I pesonally would do is to get one or more tubes of silicone caulking compound of the type that is specifically formulated for repairs of cement, and caulk (it's probably available in your local hardware store for about $6/tube). This will definitely protect the exposed rebar from rusting, and will also help to generally glue things together. It doesn't add any significant structural strength, however. It might be good enough.
posted by grizzled at 9:51 AM on June 25, 2010
posted by grizzled at 9:51 AM on June 25, 2010
Best answer: I am a civil engineer, but I am not your civil engineer and this is not engineering or construction advice, but rather an outline of the procedure. I used to work for a structural restoration company that did a lot of this type of work. Definitely consult an actual structural engineer, because concrete falling from any height is highly dangerous.
That said, the basic procedure is to:
1. Sound the slab by lightly tapping on the concrete surface with a masons hammer. Bad concrete makes a dull thump. Outline bad areas with spraypaint.
2. Chip out the spalling concrete with a chipping hammer (we used pneumatic, but electric would work for a small job)
3. Sandblast the exposed area to clean the rebar and roughen the surface profile to improve adhesion of the repair mortar
4. Paint the rebar with a zinc-rich primer to stabilize and protect the rebar. Alternatively, some people like to use a bonding agent with corrosion inhibitors instead of the primer.
5. Soak concrete with a hose, then remove all surface moisture. Apply an overhead repair mortar to the chipped out area.
6. After the mortar has cured for 28 days, apply an epoxy or silicone coating to the concrete to reduce water intrusion.
posted by electroboy at 9:57 AM on June 25, 2010 [1 favorite]
That said, the basic procedure is to:
1. Sound the slab by lightly tapping on the concrete surface with a masons hammer. Bad concrete makes a dull thump. Outline bad areas with spraypaint.
2. Chip out the spalling concrete with a chipping hammer (we used pneumatic, but electric would work for a small job)
3. Sandblast the exposed area to clean the rebar and roughen the surface profile to improve adhesion of the repair mortar
4. Paint the rebar with a zinc-rich primer to stabilize and protect the rebar. Alternatively, some people like to use a bonding agent with corrosion inhibitors instead of the primer.
5. Soak concrete with a hose, then remove all surface moisture. Apply an overhead repair mortar to the chipped out area.
6. After the mortar has cured for 28 days, apply an epoxy or silicone coating to the concrete to reduce water intrusion.
posted by electroboy at 9:57 AM on June 25, 2010 [1 favorite]
Response by poster: Thanks for the feedback so far!
Just to clarify - this is a balcony for the front entrance of a single-family house.
It's only about 4-5 feet off the ground - you need to climb a few steps to access it. So there's no risk of it falling on someone's head.
Sorry, the picture was probably misleading, but it was the only decent one of a cantilevered balcony that I could find!
posted by bitteroldman at 10:03 AM on June 25, 2010
Just to clarify - this is a balcony for the front entrance of a single-family house.
It's only about 4-5 feet off the ground - you need to climb a few steps to access it. So there's no risk of it falling on someone's head.
Sorry, the picture was probably misleading, but it was the only decent one of a cantilevered balcony that I could find!
posted by bitteroldman at 10:03 AM on June 25, 2010
Best answer: To answer your actual questions:
1. Yes, hire a professional. They can do it better and more easily than you can.
2. See above for the general procedure.
3. Concrete Repair Illustrated.
4. Most forms are wood, unless you're mass producing something or have modular forms. You can attach these directly to the concrete by using stainless all-thread or wedge anchors.
If you would like to do this more cheaply, what I pesonally would do is to get one or more tubes of silicone caulking compound...
Don't do this. It won't work. Routing and sealing cracks is a good preventative maintenance method, but once you're getting actual spalling, it's pretty useless.
Also, Step 6 above applies to the entire balcony, not just the repair area.
posted by electroboy at 10:07 AM on June 25, 2010
1. Yes, hire a professional. They can do it better and more easily than you can.
2. See above for the general procedure.
3. Concrete Repair Illustrated.
4. Most forms are wood, unless you're mass producing something or have modular forms. You can attach these directly to the concrete by using stainless all-thread or wedge anchors.
If you would like to do this more cheaply, what I pesonally would do is to get one or more tubes of silicone caulking compound...
Don't do this. It won't work. Routing and sealing cracks is a good preventative maintenance method, but once you're getting actual spalling, it's pretty useless.
Also, Step 6 above applies to the entire balcony, not just the repair area.
posted by electroboy at 10:07 AM on June 25, 2010
Electroboy has good advice.
Here is why this is important: concrete is very strong in compression. That's why it makes good roads. But it is very weak in tension. That's why there is rebar in it, to improve its tensile strength. It works because the ridges in the rebar lock into the concrete and neither thing can move.
If the rebar isn't surrounded by concrete, its effectiveness is reduced. Especially if it starts to rust through.
So, if you are confident that you can perform the steps he outlines, give it a go. The test would be that when the repair is done, do the tapping with the hammer thing and see if it sounds solid again. If it does, you are probably in good shape. If you aren't confident about it, get someone else to do it.
posted by gjc at 4:07 PM on June 25, 2010 [1 favorite]
Here is why this is important: concrete is very strong in compression. That's why it makes good roads. But it is very weak in tension. That's why there is rebar in it, to improve its tensile strength. It works because the ridges in the rebar lock into the concrete and neither thing can move.
If the rebar isn't surrounded by concrete, its effectiveness is reduced. Especially if it starts to rust through.
So, if you are confident that you can perform the steps he outlines, give it a go. The test would be that when the repair is done, do the tapping with the hammer thing and see if it sounds solid again. If it does, you are probably in good shape. If you aren't confident about it, get someone else to do it.
posted by gjc at 4:07 PM on June 25, 2010 [1 favorite]
Best answer: Ok, since you're not repairing a balcony slab that's overhanging a walkway or something, you can probably skip the sandblasting and use a wire wheel instead. Also, one really easy way to make overhead type repairs without lying on your back and troweling overhead is to:
1. Chip out all the bad concrete, prep and paint rebar.
2. Cut a full depth hole in the middle of the repair area so you can pour the repair mortar from the top of the slab.
3. Attach plywood form to the underside using tapcons or wedge anchors. Caulk any gaps.
4. Pour patch using the recommended repair mortar, use a powerful sander on the underside of the form to vibrate the concrete into place and remove air bubbles.
5. Strip forms according to mortar manufacturer's recs.
posted by electroboy at 8:13 AM on June 28, 2010
1. Chip out all the bad concrete, prep and paint rebar.
2. Cut a full depth hole in the middle of the repair area so you can pour the repair mortar from the top of the slab.
3. Attach plywood form to the underside using tapcons or wedge anchors. Caulk any gaps.
4. Pour patch using the recommended repair mortar, use a powerful sander on the underside of the form to vibrate the concrete into place and remove air bubbles.
5. Strip forms according to mortar manufacturer's recs.
posted by electroboy at 8:13 AM on June 28, 2010
This thread is closed to new comments.
posted by BrooksCooper at 9:39 AM on June 25, 2010