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Reinforcing Existing CMU Wall

Reinforcing Existing CMU Wall

Reinforcing Existing CMU Wall

(OP)
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?

RE: Reinforcing Existing CMU Wall

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.

RE: Reinforcing Existing CMU Wall

(OP)
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.

 

RE: Reinforcing Existing CMU Wall

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.

RE: Reinforcing Existing CMU Wall

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

RE: Reinforcing Existing CMU Wall

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.
 

RE: Reinforcing Existing CMU Wall

(OP)
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.

RE: Reinforcing Existing CMU Wall

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.

RE: Reinforcing Existing CMU Wall

(OP)
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?

RE: Reinforcing Existing CMU Wall

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.

RE: Reinforcing Existing CMU Wall

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.

RE: Reinforcing Existing CMU Wall

(OP)
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.

RE: Reinforcing Existing CMU Wall

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

RE: Reinforcing Existing CMU Wall

(OP)
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.

RE: Reinforcing Existing CMU Wall

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.

RE: Reinforcing Existing CMU Wall

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.

RE: Reinforcing Existing CMU Wall

(OP)
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.

RE: Reinforcing Existing CMU Wall

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.

RE: Reinforcing Existing CMU Wall

(OP)
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?

RE: Reinforcing Existing CMU Wall

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

RE: Reinforcing Existing CMU Wall

(OP)
It is already loaded.  I'm going to have them shore the joists near the ends.  I'm still trying to figure out how to shore the roof for the other two walls.  I'm also trying to figure out how to brace the wall at mid-height given that the wall is already significantly overstressed in tension.  It's not showing no signs of distress, though.  I don't even see hairline cracks in the mortar.  It's actually one of the better CMU foundation walls that I've ever seen.

RE: Reinforcing Existing CMU Wall

soil pressure killing it, or is this above grade?
 

RE: Reinforcing Existing CMU Wall

(OP)
It's almost completely below grade and it is the soil pressure that's killing it........ much more than I would have expected.

RE: Reinforcing Existing CMU Wall

you'd be surprised (or maybe not) how many completely unreinforced CMU basement foundations are all over the U.S. I know for there are many in my area.
I built my own home just a few years ago. I did no calc's on it (I think the only time I ever ran calcs for masonry was for exams/ Pe test). I had the mason reinforce my walls the way he does all of his basements which i think was #5's every 32"
For all I know, it doesn't check out

RE: Reinforcing Existing CMU Wall

(OP)
I know.  It's one of the reasons I hate residential construction and unreinforced CMU.  It's so far overstressed on paper that it shouldn't be standing, but it doesn't even show signs of distress.  I can only guess the soil has a lot of cohesion!

RE: Reinforcing Existing CMU Wall

When building, where I had gravity footing drains (no sump req'd), I used to backfill with #57 washed gravel (river stone) up to about 1.5 ft below grade. If the excavation was done properly and the over-dig was only 16" or so and was cut vertical, I had a hard time seeing any lateral pressure on the wall. It was expensive to back fill with gravel, but it virtually guarantees a bone dry basement and low lateral pressures on the wall (IMO).
Problem is, it invites water to the footing drain...

RE: Reinforcing Existing CMU Wall

Lion -

Now that more information is available, it seems like you are facing a typical basement built according to prescriptive codes requirements. Prescriptive codes are used because engineering a basement is not at all simple and many engineers have been driven crazy trying to find a better way for 20 or 30 years. I tried and played with the problem for 30 years before I gave up on the detailed analysis. After I was into the problem, I realized that it was much more difficult than what I had done earlier when analyzing and designing rocket test stands for both seismic and dynamic loads (gravity and cyclical) where the 1,500,00 upward thrust and 300,000# of fuel overhead subject to seismic and varying loads as the vertical/horizontal loads changed quickly that many factors that most assumptions do not accurately address the real situation.

You have determined that the problem is the lateral soil pressure, but there is no apparent distress. Usually, lack of control over the backfill show up with cracks, movement and other symptoms relatively early in the life of the structure, so this is not a very probable question now.

For some reason, you are required to do an analysis. I would suggest you look closely at the parameters regarding the lateral loads and classic distributions and the real restraint (fixed, pinned, propped cantilever and diaphragm action)and supports if it is a typical home or apartment situation. This will also involve some realistic support from a geo-tech to determine the accurate response why it does not fit into the "canned" analysis result accurately. - For many years it was claimed that a honey bee could not fly, but somehow they still do with great results.

Good luck!!

Dick

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

RE: Reinforcing Existing CMU Wall

Lion,
Operating space would depend on the configuration of the blocks, but with a typical two core block, you should be able to use a rotary hammer within a single core.  The slot where you are drilling would have to be 3 blocks high to allow for the hammer and bit.  Do you need the dowels?  Can you rely on the slab at the bottom and maybe something else at the top?   

RE: Reinforcing Existing CMU Wall

any way to practically relieve soil pressure on the outside?

RE: Reinforcing Existing CMU Wall

What about using structural fibers Euclid or Grace)in your grout and not using the rebar?

RE: Reinforcing Existing CMU Wall

The use of fibers in grout is not a good direction to go because they are generally used for micro crack control and not for structural purposes. They could also interfere with the ability of the grout to fully fill the voids during the consolidation process.

Dick

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

RE: Reinforcing Existing CMU Wall

True basic fibermesh is used for micro cracks, but true structural fibers made by Euclid, Grace, or equivilant where they are used in foundation walls and slabs to replace rebar would they possibly work?

RE: Reinforcing Existing CMU Wall

Also we have used polyfoam on the exterior of some walls to releave pressure on tall concrete walls, but it was not a cheap process, but it does insulate as well if it is in a basement condition.  

RE: Reinforcing Existing CMU Wall

buzter -

Is there any documented reports and acceptance of the fibers in grout since grout is not like concrete.

A lower (bottom course) clean-out and an upper level opening in the core to be filled and accept the reinforcement before grouting with 8" to 11" slump grout has been the traditional method when it is desired to just reinforce (with the smallest rebar necessary) the existing masonry wall for lateral loads.

Popping out the face shells is not necessary and destroys the continuity is the wall.

Very little information on the existing walls, details and loading conditions is provided, so the only thing available is guesses.

dick

 

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

RE: Reinforcing Existing CMU Wall

I agree..and no I also have not seen any documentaion on using the structural fibers in grout, main reason also for me asking, throwing it out there waiting for a bite to see if someone knows or has used it in that way. And yes we use the traditional way as well for our walls.

RE: Reinforcing Existing CMU Wall

From what I have read, using fibrer reinforced concrete in any applicstion outside of secondary reinforcement or cladding panels is not code accepted practice in the US, So, you are stuck with using rebar in grouted filled cells as primary reinf. If you are concerned with the retaining wall moving during construction, then install temporary shoring whaler at 1/3 points and get it done.

RE: Reinforcing Existing CMU Wall

Abusementpark - Any info on FRP you can share?  Sounds like a great technique.

RE: Reinforcing Existing CMU Wall

As per hokie's comment I would look at the requirement for the dowels top and bottom.
If the wall is really only deficient in flexure there should be no need to improve the shear capacity.

RE: Reinforcing Existing CMU Wall

If your calculations determine that the wall is way overstressed and you don't see any sign of it then there is some reason why the wall is still in good shape.  

It might be that jogs and corners in the wall that act as buttresses or create two-way action, but I think you'd have mentioned this.  

More likely the soil just hasn't had time to impart a lateral load.  If you're dealing with clay backfill, then the lateral load will come eventually as long as the wall can resist it.

I'd also consider whether you really need dowels into the grade beam.  Maybe the slab or grouted lower courses could help with that.

Another idea might be to cut the slots as you've proposed but to construct formed pilasters inside the wall where you can place the reinforcing.  Pour the pilaster integrally with the block cells you've exposed.  This could get you a better d on the reinforcement and more control over its placement.  The additional capacity from the added depth might allow you to space them out a little more.

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