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Old 06-12-2012, 01:52 PM   #1
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Size Rebar Needed to Reinforce Bowed Block Wall



Looking for advice on rebar size and filling technique for reinforcing a block wall:

I have a 100 year old brick home (brick over stick frame) which sits on an early concrete block foundation - see image above for the block design. It appears that the foundation walls bowed in within a few years of construction. The vertical bow is significant: 4" at 5 feet above the floor. I have had several people look at the wall and they have said that the wall does not look like it has moved for many years so there is not an urgency to fix it. However, I have completely stripped the first floor down to the joists and have superb access to the top of the foundation and would like to slip in some rebar and concrete in while I have the opportunity.

PLEASE NOTE THE UNIQUE NATURE OF THE BLOCK: It has a front face and a back face that are joined by four steel pins. So the blocks have an open space that extends from floor to ceiling and from front of the house to the back, like a concrete form - there is no concrete webbing that is in the way.

I have bored a 2" hole in the sill plate and was able to slip 8 feet of 1/2" rebar vertically into the wall, all the way to the footer from what I can tell. I had to bend the end slightly since when I insert it from the inside of the house, I am pushing against the bow in the foundation. I think that I can get rebar in about every foot. In addition, I am going to try to drill a small hole into the foundation 5 feet from the floor to grab the rebar with a wire hook and pull it to the inside of the wall during the pour so that I can have it on the side of the cavity with the most tension.

I estimate needing about 2 yards of concrete. I can probably cut up to a 3-1/2" hole in the sole plate to fit a funnel (traffic cone) in there. Since the block sets up its own form, the concrete will flow sideways, I just need to be sure that the mix will flow through the 1" gap at the narrowest point. I will be ordering mixed concrete from a local concrete company.

QUESTIONS:
1) Will 1/2" rebar be adequate. I might be able get 5/8" in but it will be tough with the bow.
2)Advice on getting the concrete into the wall and how well it will flow sideways. Any advice on mix to request (something with high tensile strength and good flow through narrow spaces). I am hoping that I only need to pour every 8 feet; is that realistic?

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Old 06-13-2012, 06:35 AM   #2
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Size Rebar Needed to Reinforce Bowed Block Wall


My first thought (probably wrong) is the metal pins between the inside and the outside faces have failed.

If I understand your diagram and the situation (probably wrong again) if the outside face of the foundation is unbowed and the inside face has then that would support my theory that the pins have failed.

Since the pins are acting like Snap Ties in your soon to be "form" you could blow out the inside face of the foundation.

Either way you should get an Engineer on board.

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Old 06-13-2012, 09:11 AM   #3
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Size Rebar Needed to Reinforce Bowed Block Wall


First, a question: Which side of the block faces the interior, and which side faces the soil?

Second, why re-inforce the wall with such a large bow, and why not straighten the wall back to it's original location?
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Old 06-14-2012, 06:37 PM   #4
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Size Rebar Needed to Reinforce Bowed Block Wall


Sorry that I was not more clear. In answer to the questions, the thick side is on the outside to support the bricks. The pins have not failed; I was able to slide the rebar all the way down to the first course (Plus I could see in well through the 2" hole in the sill and they have not pulled apart). The reason that I do not want to rebuild the wall is that jacking the two stories of brick would be costly. As I mentioned the wall seems stable, but since the floor is open I want to add some insurance by filling and reinforcing the block. If the base kicks in the future I could take the additional steps of reinforcing the footer. The top is held in place by the doubled up 2x10 floor joists perpendicular to the wall.
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Old 06-14-2012, 07:39 PM   #5
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Size Rebar Needed to Reinforce Bowed Block Wall


The problem you have is actually quite a challenging, difficult structural issue. The block is effectively acting as two pinned blocks, and it is not so obvious how much rotation the pins allow. In my experience, filling block with concrete and rebar is of course normally done before backfilling the wall, and is very effective in strengthening the wall. However, adding concrete and steel to an existing, bowed wall, without straightening the wall, does not add much strength.

The reason is that the steel reinforcing is essentially forced to the center of the two blocks by the geometry. The center is the neutral axis, so the steel would add effectively zero additional moment capacity (strength) to the assembly. You would want the steel on the tension side (the inside in your case), but that is not going to happen given the geometry. The concrete will not help too much either, because it is going to be difficult to get it to bond to the existing block, and it is going to be close to the centroid of the assembly, where again it adds little strength to the assembly. Further, the concrete is really only good in compression, which is probably not the problem side.

I have seen block walls bow many times, and almost always the reason is lateral earth pressure, often due to moist or wet soil pressing against the foundation. Correcting the problem often requires addition of steel straps, steel soil anchors, or even steel I beams added on the inside of the basement. This type of problem is usually addressed by an engineer of course, but I sense that you don't care to spend the money to have a structural engineer advise you. In any case, adding the steel and concrete are not likely to hurt anything, but for the reasons I mentioned, they are not likely to cure the problem either. Best luck.
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Old 06-16-2012, 07:53 PM   #6
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Size Rebar Needed to Reinforce Bowed Block Wall


Thanks for the reply Daniel. Yes, I understand about the need to keep the rebar as far from the neutral axis as possible, hence my plan to drill a small hole at the maximum bow point and pull the rebar to the inner (tension) side of the block with some wire before thenpour. As the block wall bows and the block hinges on the outer edge, there may also be some argument that the neutral axis has shifted toward the outer face (althought this may not be happening if the pins are bending. I understand that the concrete would not provide any tensile strength and any bond to the original block would come from from the surface irregularities. The purpose of the concrete would mostly be to keep the rebar in place (under tension) if the wall starts to bend more. To a lesser extent it would prevent collapse of the outer face into the inner face and to reduce bending the pins might experience under the shear force created at the center of the wall.

I do not regard this as the final solution, merely a step in a process. I look at it as something that can be done easily while the floor is open, and which will be better than doing nothing right now. There is a paved driveway on the other side that may eventually be redone. When that happens we will upgrade the drainage and fill. I understand that clay soil on the outside wall may be providing lateral support to the wall, so this will need to be addressed before just trenching it out.
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Old 06-17-2012, 09:34 PM   #7
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Size Rebar Needed to Reinforce Bowed Block Wall


Honestly I think you could re-enforce this wall with a plan similar to what you're thinking, I've done it successfully for 20 years. BUT, I would never recommend re-enforcing the wall when it's in a bowed state, especially as bad as 5". If you do tear-out the driveway on the outside down the road, and dig up the exterior surcharge load and replace with better draining material, you'll be left with an equally bowed wall that will be extremely difficult to straighten due to the re-enforcement you added. IMO, you're best bet long term is to dig it out now & fix it the right way OR re-enforce as you're suggesting and plan on a total wall underpin in the future.......
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Old 06-18-2012, 12:28 PM   #8
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Size Rebar Needed to Reinforce Bowed Block Wall


jomama45, just wanted to be sure that you understood that there was 2 1/2 stories of brick above it (yellow brick with very thin mortar lines - in good tight condition). None of the foundation people have wanted to straighten the wall. They have all said that they would reinforce the wall in its current shape and add additional width to the wall in various forms (block wall, buttress, stainless reinforced parging, depending on the contractor). All seem to regard straightening as cost prohibitive and unnecessary. Even people that have experience jacking stone buildings have not recommended rebuilding the wall - perhaps because the cost would be more than the value of the building or perhaps because it would disturb the tight brickwork and cause rework where none is needed now.

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Last edited by frtools; 06-18-2012 at 01:24 PM.
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