Using steel chains as reinforcement?? 1

[ibrahimzm]

Hi,
I'm working on a job in which the engineer is using a novel way to reinforce CMU walls. He's using steel chains instead of rebar, inserted from the top and then full grouted with 4000psi concrete. I guess he's been asked to justify the process and so he's come to us to do an evaluation. I was wondering if anyone had come across such a situation before and could advice us about the viability of such a process.
Just from first glance, I feel that using chains as concrete reinforcement would lead to local stress concentrations that could cause the chain to break or yield locally. Perhaps it might be ok to use these chains at a lower strength value than what they're rated for.
Any advice would be greatly appreciated.
Thanks

 

[concretemasonry]

In my opinion, that is certainly "novel" that could lead to a "zipper-type" failure because of a true lack of contuity in the reinforcement. Localized failure of the chain links and the grout can be expected.

What type of loads is he expecting the chain to resist? Uplift? Flexure? Shear?

Because of the metallurgy and different manufacturing methods of chain/link types, you will have little chance of finding any historical support. You can find some basis for barbed wire, but chain could be tough.

I hope he was planning on 4000 psi grout and not 4000 psi concrete - that gives you a clue to the technical basis. That strength level appears to be excessive and possibly detrimental.

What type of structure and where is this experiment located? That may help to explain the approach.

 

[civilperson]

A great approach that shows understanding of the role of reinforcement in grouted masonry. Similar to a series of lapped bars but hooked for development of tension stress. The steel strength of chain is very high.

 

[ibrahimzm]

I'm sorry, I did mean 4000psi grout and not concrete. I should recheck my messages before posting them.
The loads are mostly from backfill against the wall. He's using it for restoring basement walls that have moved and/or have developed cracks in them. The chains are easier to insert than rebar since they can be bent and dropped easily.

 

[concretemasonry]

If you are trying to reinforce an existing masonry wall with cracked joints, your main problem will be flexure of the strip spanning from footing to the first floor.

If you calculate stresses, you will not be able to use the block as part of the depth of the menber. You will be limited to the size of the grouted area and the reinforcement. This make the tension reinforcement more critical.

Since the wall moved and must be straightened, removal of the backfill and vertical loads would be necessary. In this situtaion, it is often better to reconstruct the wall to a proper standard and use the full thickness as a structural member.

 

[OlafTheBlack]

A chain carries loads in three ways - axial, shear, and bending of the links. Reinforcing rod carries loads through axial stress only. The rod is stiffer. The masonry is going to move and crack more to get the chain to fully stress, especially if the chain has any slack. Using a chain makes no sense.

 

[civilperson]

I disagree with Olaf. The tension in rods or in chain is the stress for reinforcement. If the chain has any slack, then the voids will fill with grout. The lapped splice of rebar is a similar loading mechanism as the "lap" of links in the chain.

 

[joshuaroark]

If I were in your shoes, I would go to a concrete supplier and ask to make a cylinder and to have it tested. This would be much cheaper and effective than trying it out in a wall.

 

[csd72]

civilperson,

you are forgetting that the grout filling the voids is much weaker than the chain. If there is any slackness in the chain then the concrete between would fail due to crushing a lot earlier than the bars would.

joshuaroark,

Good point, it cant cost that much to get a couple embeded into cylinders and tested.

csd

 

[concretemasonry]

You do not use cylinders to test grout.

See ASTM C1019. The sample is cast in a form made from masonry units with absorptive paper separationg the grout from the masonry units.

Cylinders give misleading results when testing grout because of the required slump and curing conditions.

Dick

 

[Lion06]

This whole idea seems absurd. I could almost see this from an architectural standpoint if you could "see through" the grout and actually view the chain as reinforcement (this would make a great conversation piece). But you can't - and I can't imagine the chain is saving money (any significant amount anyway).
Are the walls being demolished and rebuilt? It sounds like it or I am wondering how you are going to drop a chain into an already built wall (unless the wall is currently not grouted and that is just being added. If the wall is being demolished and rebuilt, you have to have rebar dowelling from the ftg into the wall for shear transfer - why not just extend it all the way up the wall?

 

[Unionmason]

Chains have been used for hundreds of years to reinforce masonry domes. I'm guessing the chains are partially tensioned to a fraction of the compressive strength of the unreinforced block after the dirt is removed from the exterior of the wall to allow the block to return to near plumb. Then grouted to bond the block back together. One could even use some sort of sleeve over the chain to allow further post stressing after the grout cures. I think the big savings would be eliminating special post tension fittings with the use of chain instead of cable. And of course fewer number of grouted cells saving time/money. As an aside someone is selling flat carbon fiber bands as broken basement wall repair method with propriety anchors. I love to see the chain #s compared to conventional grout/re bar system.

 

[Zambo]

why not use small diameter rebar in bundles or prestressing strand. Both could be threaded in as easily as chain.

 

[Unionmason]

Zambo, I think there is a building sitting on the block wall. You can't get a full length bar in the cell with out a large slit cut. Even if they aren't tensioning the chains I assume they are held by a wire in a 'stretched' condition from the top of each piece of chain. My earlier post assumed a 'clean out' hole at the bottom of the grouted cell to fasten the chain to the spread footing with an expansion anchor. More work but a lot fewer grouted cells needed.

 

[faromic]

I think the chains would help in compression but definitely not in flexure. Think about it, a rebar by itself is pretty stiff, especially No's 6 and higher. when placed in concrete it's resistance to bending is much higher. A chain doesn't have any stiffness. When bonded in the grout the chain still won't have any resistance to flexure. If the wall is subjected to a lateral load, the cracks that are created due to flexure at tension side will reach the chain. Once the cracks reach the chains (or reinforcment at a distance d from the compression face), the reinforcement (chains) must resist the flexure. I don't see the chains resisting this becuase a chain is not a continuous material, the concrete will continue to crack at the points where the chain links meet and will eventually fail if the load is high enough.

 

[Lion06]

faromic-
I disagree that the chain will be resisting flexure. The COMPOSITE SECTION of the block and reinforcement resists the flexure, the reinforcement (whatever it may be) only resists the tension portion of the flexure - the block is resisting the tension.
I do agree that the chain is a bad idea for all the reasons mentioned in previous posts.

 

[faromic]

I agree. That's what I meant; that the chains will not resist flexure. Isn't the section similar to a concrete wall, but the chain doesn't have any stiffness.

Cheers