Reinforcing Existing CMU Wall 1

[Lion06]

I've been doing some research on reinforcing existing unreinforced CMU walls. The general consensus seems to be that you pop out the face shells, drill and dowel into the footing and into the bond beam at the top, place the reinforcement and then grout the cells.

A couple thoughts came to mind when thinking about this that I didn't see addressed in any of the articles I've read so far.

First, is the grout to block bond good enough (without the one face shell) to make it act like a unit like a typical reinforced CMU wall would?

Second, it makes sense that you couldn't just pop all the face shells, drill and dowel, and then pour grout all in one clip. A typical CMU wall is retaining soil, and you'll be popping face shells on the inside of the basement where the CMU is in tension. Is there a good rule of thumb as to how many cells within some given distance is ok to pop at a time? Maybe do two at a time? Start with the center and one end, then work your way toward the other end so that you always have a distance of 1/2 the wall length between cells that your removing the face shells from. Does that sound reasonable?

 

[Ron]

Lion06...yes, the bond is good enough.

If you are concerned with wall integrity during the additional reinforcement, the have them use lateral shoring....keep in mind that the wall is standing and if done properly, the cell cuts do not have to be full height...better, but not necessary.

 

[Lion06]

Thanks, Ron. That makes me feel better about it. I was thinking about what you had said, too. I was imagining they would only HAVE to pop out the entire face shells on the upper and lower blocks to get a drill in there, but that the majority of cells in the middle could have either a thin cut through them or some wouldn't even need to be tampered with at all - as long as they can get thread the rebar through what is popped out.

 

[Ron]

Lion06...in some cases, you don't even need to slot the cell...you can thread the rebar from the bottom or top if you have room. It certainly does not have to be a full slot.

 

[ToadJones]

man, seems like a ridiculous amount of work....got to be a better way...like building another wall inside??

 

[abusementpark]

Lion06,

Have you considered using externally bonded FRP sheets to reinforce the wall instead?

Or wow about removing the top bond beam, placing the reinforcement and grout from above (assuming no obstructions), and repouring a new bond beam? You would only have friction to rely on for shear load transfer at the bottom of the wall, but it may be enough depending on your forces.

 

[Lion06]

I would like to do it from the top only, but there's an existing stud wall above the bond beam.

I suggested other alternatives, but the owner is convinced that he can get cheap labor in this market. As a result, he wants to use as much labor and little material as possible. I stressed the importance of making sure he gets competent labor, not just cheap labor.

 

[ash060]

Have reinforced existing walls as Ron mentioned on multiple projects. I have never had an issue doing it, and you would be surprised by how fast the contractor can do it. It is probably a better alternative than building a new wall.

 

[Lion06]

ash-

Do you have them brace the wall at mid-height during construction since you're removing section on the tension side of a wall and you can't relieve the retained soil?

 

[ash060]

Apologize Lion

I did not read your entire post, my experience has only been with above ground existing walls that needed reinforcing.

But I would recommend bracing if it is fesible, otherwise probably cut as little of the wall as possible to install the bar and they don't need much of a hole for grouting.

 

[concretemasonry]

Lion -

Just to get basic into perspective, it this real situation or hypothetical?

If real, why has the wall failed or have the loads changed? Lateral load that cause a flexural failure would require reinforcement and a reduction in the lateral loads.

Cutting out the face shells can be an amateur attempt if you do not know the configuration of the CMUs in the wall(core, locations and alignment from course to course).

Normally, the core spaces can be filled properly if a face shell is removed and a proper masonry gout (8"-11" slump) is pumped into the wall, topped off after consolidation and then mortar to packed into the top course. Unfortunately, many adjacent cores will be filled since normally, the cores are not totally sealed when common units are used. - Standard practice when filling walls in high security prisons between previously poured flat slabs.

Just filling all the cores only has a minimal effect on creating more usable strength of a CMU wall, since for flexural purposes, the outer stresses are determined by the outer materials and the interior grout is not as effective. Usually, the actual block strengths are far beyond the minimal ASTM requirements since it is too costly to make a minimum strength block.

When analyzing a CMU wall is composite structural element and not a collection of individual materials (CMUs, mortar and grout) working together. The steel reinforcement is just like adding steel to poured wall from a structural standpoint. As an example, the vertical capacity of wall CANNOT be determined by the lowest strength on one component. A block prism of 4800 psi can be made with 2500 psi mortar.

Good luck on your research and determine if it has not been done many times before by many professional organizations.

Dick



Engineer and international traveler interested in construction techniques, problems and proper design.

 

[Lion06]

Dick-

This is a real situation. I figure I need to completely bust out the face shells for several blocks from the top and bottom so the guys have enough room to get a drill in there to dowel into the existing footing and the existing bond beam at the top.

The wall hasn't failed, but it doesn't check out on paper (not even close). The cells are lined up pretty good.

I don't expect to grout all the cells, just at the spacing I determine the reinforcing needs to be at.

 

[ToadJones]

can you build some pilasters inside every so often to get he wall to work?

 

[Lion06]

I can, but that means excavating outside and underpinning to ensure the pilaster has a footing below it's entire footprint, no? Do you think it's ok to scab onto the existing footing for this? It also means cutting out the slab locally to provide footings/underpinning and a lot of doweling into the existing footing.

 

[ToadJones]

Pilasters on inside is what i was thinking
Usually the footing is wide enough physically to accommodate a 4" pilaster. Whether or not the footing will "work" design wise or if a 4" pilaster every so often will work is another question.
A mason I used back in the day (not long ago) to build residential foundations/ block basements used to sometimes suggest a 3' deep wing-wall(for lack of a real or better term) of sorts the stuck into the basement on long straight runs of basement walls. On finished basements, if we knew the partition layout, we'd plan for it and hide the little wing wall in the partition later.

 

[concretemasonry]

I suggest you look a little closer at your calculation methods, standards and assumptions relating to loads and continuity and especially the specific codes you are using. If there was no problem/failure or increase in loads, there must be a reason for the need for reinforcement.

Since most allowable stresses are based on the f'm of the masonry wall assembly and not the assumed code minimums. You can sample masonry units and use the conservative tables for the the f'm or go to cutting out a masonry assembly for testing to determine the existing f'm.

The real cost of properly grouting and reinforcing the is very high and can actually create new problems. Analyzing masonry walls takes real engineering and not pumping just numbers into program assuming minimal material properties.

Unfortunately, most U.S. engineers never are educated about the concepts of masonry design and analysis, especially when there is no problem/failure to hone in on.

I also go back to meeting with a fellow engineer in South America that just had designed a series of about 10 - 12 to 20 story 6" partially reinforced loadbearing masonry apartment buildings being built concurrently. I asked him what codes and standards he used used and he said "We use your codes and standards, but we use them better. I learned that working with Jim Amrhein in California in the 1970's." - No clean-outs for grout inspection since they had a random inspection of grouted cores by video cameras and all masonry units were approved and marked for strength at the production facility for the buildings that required 4 different strengths, depending on the floor level.

Take a second look to concentrate on the situation, especially if there is no problem indicated.

Dick

Engineer and international traveler interested in construction techniques, problems and proper design.

 

[Lion06]

The tension stress on the unreinforced masonry is over 300 psi - way to high. Adding a second story will actually help, but not get me close to the allowable.

 

[hokie66]

I think CM and Lion are talking apples and oranges. Lion's problem is flexural capacity, and CM's advice is for compression. Please correct me if I am wrong.

Back to your original post, Lion, I think your sequencing makes sense. And you don't have to knock out the whole face, just slots long enough to get bars in and lap them.

It doesn't go very fast, but with a deficient existing building, there is sometimes no choice. I once was involved with a ten storey building where many of the block backup walls were unreinforced or unfilled, and deficient in resisting wind loads. In that case, the brick veneer was also poorly supported, so had to come off and be replaced, so the reinforcing was done from the exterior scaffolding.

 

[Lion06]

You're correct hokie - my problem is flexural. The axial capacity is way more than it needs to be, but it's getting killed in flexure. I think the slot is the right thing to do, I agree. Do you agree that multiple face shells would need to be removed to allow room for a drill to dowel the rebar into the existing footing and the bond beam at the top?

 

[ToadJones]

How do you avoid problems while doing the repair work?
The building is already "loaded" no?