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?
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
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
RE: Reinforcing Existing CMU Wall
RE: Reinforcing Existing CMU Wall
RE: Reinforcing Existing CMU Wall
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
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
RE: Reinforcing Existing CMU Wall
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
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
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
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
RE: Reinforcing Existing CMU Wall
RE: Reinforcing Existing CMU Wall
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
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
RE: Reinforcing Existing CMU Wall
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
RE: Reinforcing Existing CMU Wall
The building is already "loaded" no?
RE: Reinforcing Existing CMU Wall
RE: Reinforcing Existing CMU Wall
RE: Reinforcing Existing CMU Wall
RE: Reinforcing Existing CMU Wall
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
RE: Reinforcing Existing CMU Wall
Problem is, it invites water to the footing drain...
RE: Reinforcing Existing CMU Wall
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
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
RE: Reinforcing Existing CMU Wall
RE: Reinforcing Existing CMU Wall
Dick
Engineer and international traveler interested in construction techniques, problems and proper design.
RE: Reinforcing Existing CMU Wall
RE: Reinforcing Existing CMU Wall
RE: Reinforcing Existing CMU Wall
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
RE: Reinforcing Existing CMU Wall
RE: Reinforcing Existing CMU Wall
RE: Reinforcing Existing CMU Wall
If the wall is really only deficient in flexure there should be no need to improve the shear capacity.
RE: Reinforcing Existing CMU Wall
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.