New concrete wall formwork failed an hour after pouring


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Old 01-04-11, 09:36 PM
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New concrete wall formwork failed an hour after pouring

I have started another thread on my slab repair and those are going pretty well so far, and along side that project I was also doing some wall repair. Most of the wall repair were minor and just involved patching holes, but there is one wall that is pretty major.

This was the initial condition. A large kitchen window and the sink drain stubout was too high. Not low enough to accommodate a garbage disposer and a deep sink. So I need to lower it to 12" off the floor. The vent is to the right, as indicated in red.



First the demolition to expose the cast iron vent pipe, and the two copper pipes that tied into that.





I end up having to remove 4 rows of hollow block and only keep the exterior side of it. I then built a 2x6 wood frame around the new pipe, and the wood frame sat on top of the bottom row of the block wall, I filled the cells of the bottom row with solid concrete.





After some more thinking, I decided to go with a solid concrete wall. So I tore down some of that wood frame, and installed some rebars to make a solid wall. The wall has a tie beam across the top of the window, so the portion of the wall below the window is not really taking any load.






After the rebars were done, we attached some 3/4" plywood form on both sides of the wall using Tapcons screwed into the remaining blocks - this is where I made the mistake. I did not realized the one side of the block wall, after all that demo work, pipe work, more drilling for rebars, more drilling for Tapcons, has gotten weak and may have already had some cracks in them.

We poured the concrete all the wall up to the top of the form. For the wall we used Sakrete 80 lb. Concrete Mix, and mixed it in a bucket with a drilled attached mixer. Everything looked nice and neat. I used a mallet to bang on the form from both sides as we poured to make sure the concrete will compact and minimize bubbles and voids. This may have contributed to the problem as well.

After it's all nicely done we started to clean up, and did some other minor patching here and there and then I heard this noise. The new concrete wall "slumped". As in the form work has been pushed out further, some of the existing concrete block face attached by Tapcons moved, now the wall appeared two inches wider, and the top is lower.

It has already partly hardened so it is no longer workable, but with the formwork removed on one side it still "stands" on it's own - sort of. We quickly without having much time to think, used a chisel to make the wall narrower. It cannot be "worked back" but I could use a chisel to sort of pry out about along one side so it is now back to within 8" thickness more or less but of course not smooth.

I then put the formwork back, only this time we didn't use Tapcons, but drilled a few holes on the plywood, put in two long threaded rods and put nuts on both ends to tighten the formwork back to the original shape.

Then we mixed four more bags of concrete and filled back up to the top and smooth.

Next day, we took out the formwork and the shape stayed, except for the face that we chiseled out while it was still a bit soft. We used stucco - in some areas an inch deep in some other half an inch or so - to smooth it off.

Results, inside and outside:





I don't know how much damage the formwork failure did, I now the wall is not as strong as I had originally intended, but I am not too keen on breaking it out again and do it over either especially now with the pipes and rebars buried in it.
 
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Old 01-05-11, 04:46 AM
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Did you bury your stud wall in concrete? One question, when the stud wall was supporting everything and all the plumbing was in place, basically it wasn't broken...why did you fix it? I would have left the stud wall, applied cbu, lath and a layer of concrete stucco or other coating to accomplish the finish you wanted. The pour may need to be torn down at a later date to service your drains, while left in the stud wall could have been accessed with minimal tear out.
Your forms may have had a better chance if you had tied the side walls to each other, rather than relying on fasteners into the block to hold each individually. Sometimes drilling small holes and running wires between the plywood will help keep them vertical. When you're thru, just cut the wires flush.
 
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Old 01-05-11, 05:45 AM
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You have learned quite a few lessons here!
Plywood without adequate bracing is not sufficient to stop a form from bowing.
Never allow system components, electrical, mechanical, plumbing etc., to be placed in such a manner as to make them easily accessed for repair.:NO NO NO:
Never, never, never remove the support under an existing window, door. or cased opening without a way to hold the opening true. Personally, I thing the original contractor screwed up by allowing the plumbing to encroach on the support for the window sill in the first place. I agree with Chandler, you had it right the first time. As long as the wall below is plumb and not bowed out enough to interfere with the cabinet installation I'd leave it alone...the damage has been done. One caution though, set the window before you set the cabinets, if the window does not operate properly you will need to start over, cause it ain't gonna get any better.

Good Luck!
 
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Old 01-05-11, 09:04 AM
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I initially did the stud bracing because I wanted to have some support, but then there is a section of solid concrete cap above the blocked section, but below the window that was about 10" tall. I wasn't sure if the 2x6 were able to support such weight and that concrete cap was also damaged a bit during demo. I knew I had to remove that concrete cap and pour from the wood portion up anyways.

Then I thought with the 2x6, really they weren't 2x6, about 2.5 to 3" was drilled out to let the pipe through, I was also concerned about access to the pipe, but, realistically, if I did anything to that pipe, it would have to involve the connection to the vertical vent pipe, which was originally embedded in solid concrete and I wanted that to be repoured anyways. So I opted to repour the whole thing. I did leave two chambers of the stud brace alone, so if I need to tweak the stub out PVC pipe, I could.

This house was built in 1972, and as I remodeled it much of the plumbing was embedded in solid concrete in the walls. It was a pain to deal with them, but as you see, I couldn't leave that vent pipe sitting free standing like that, the question was do I repour partially or totally, and I opted to pack it all in.

Definitely learned a few lessons.

I guess my question now is - is the way I "fixed" the wall after the formwork bucked ok? or not?
 
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Old 01-05-11, 12:55 PM
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The short answer is that without seeing the wall I can't say! In my earlier reply I asked if the wall was plumb, is it? Loads are vertical and any deviation from the wall being plumb has an effect. The greater the deviation the greater the effect.
I would be concerned that the right side of the window jamb has a pipe running through a structural member. Think of framing a door or window in a stud wall. The header over the opening is supported by the studs, jack and king. The vertical pipe should not have been placed in this location in the first place. I understand that the concrete cap was intended to carry the load but when half of the cap is displaced by a hollow piece of what looks like PVC you are asking a lot of re-bar (which should of been epoxied into the existing concrete behind the vertical stack) and very little concrete. As I said in my original post, re-install the window, making sure to keep every thing plumb, level and square and see if the window operates without binding anywhere. Make sure that the wall does not interfere with the back of the cabinet. If every thing checks out you might be OK! I just cannot recommend that you cover something that I see as a structural defect! Hopefully Pecos and Concrete Masonry will chime in.
Good luck!
 
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Old 01-05-11, 03:44 PM
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I'm more of a flatwork guy. Walls aren't my thing, especially when they involve embedded plumbing and carpentry.
 
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Old 01-05-11, 06:42 PM
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brickyard blues, the 2" CI pipe was there originally in that position.

here is a picture of what it used to look like, the pipe was between the large window and a door. I guess they had to do that because the plumbing trap arm has a maximum distance to tie in and the door makes it impossible to do it anywhere else.



Above the window and old door is a solid concrete tie beam which the pipe was embedded in. Then I removed the door, blocked up the bottom and turned it into a small window. I actually wanted to extend the existing window horizontally but I could not figure out how to reconfigure the pipe to get it out of the way either and there is no way I would mess with the tie beam above.



Right now the wall is plumb. When the formwork bucked one side of the formwork stayed and the wall sort of "slumped", as in gotten wider and shorter. I then took out the extra width (with a chisel while the concrete was still semi-hard), but the formwork back, clamp/braced it further, and poured new concrete to top it off. When I took the forms off the next day, the good side is plumb, the bad side where I chiseled is rough so I smoothed in about an inch or so (unevenly) with stucco and now that side is smooth and plumb too.

I did want to drill rebars horizontally into the concrete below the window, but those are hollow blocks and it was really difficult to put two rebars around that pipe being there. So I opted for bigger rebars and bent them down at a 90. I did epoxy into existing solid concrete.
 
 

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