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?

 

[Lion06]

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.

 

[ToadJones]

soil pressure killing it, or is this above grade?

 

[Lion06]

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

 

[ToadJones]

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

 

[Lion06]

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!

 

[ToadJones]

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...

 

[concretemasonry]

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.

 

[hokie66]

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?

 

[ToadJones]

any way to practically relieve soil pressure on the outside?

 

[buzter]

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

 

[concretemasonry]

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.

 

[buzter]

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?

 

[buzter]

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.

 

[concretemasonry]

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.

 

[buzter]

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.

 

[jhenry]

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.

 

[STRBeth]

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

 

[apsix]

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.

 

[kipfoot]

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.