Need a little more information here.  

Why are you doing this?
Is the CMU wall failing somehow?
Is this for a new building next to a lot line?
What is the foundation soil like?
Is the CMU wall part of an existing building?

Mike McCann
MMC Engineering

proposing to raise a parking/display area for a car dealership for enhanced visibility. i don't want to load the existing wall. there is a service area behind the existing CMU foundation wall. soils are dense till with low groundwater. i'm debating on fusing them together vs. independent systems. thanks.

Sounds like another reinforced CMU wall is your best bet.  If you use a concrete wall, there has to be formwork between the walls.

I agree.

Mike McCann
MMC Engineering

should the 2 CMU walls be tied together? also modifying the existing footing to accomodate the new wall.

You need a new footing for the new wall.  It will probably be a cantilevered wall, so the footing will be offset to the high side.  If the existing footing projects to that side, you may be able to cast the new footing above the existing.  I see no advantage in tying the two walls together.  The existing wall will likely require waterproofing.

Depending on where you are, there should be flashing at the top opf the wall(s)to prevent water from entering the joint and possible freeze-thaw action, expanding the joint and breaking down the block.

Mike McCann
MMC Engineering

One other option if concrete is the way to go and you footing can be worked out with such a close adjacent wall would be to use "stayform" or a like stay in place form and use a typical form panel for the exterior and a blind panel for the interior. The blind side would have an expanded mesh backed up by sacrificial rebar. Clearance between the walls would still be needed in the 3-6" range, but there would be no concern of flammable or organic materials from the forms being left behind.

I suppose another option would be to brace the CMU to resist the loads applied by a one sided concrete form, but the A-frames needed for the concrete side and the bracing needed for the CMU side would be challenging and expensive. Though there are likely some hungry material suppliers these days that might be willing to undersut there prices for years to come by going absurdly cheap.


Daniel

picking up on an old thread; the last response to my concept from DTGT2002 is where this is heading. some updates on the existing conditions: soil conditions are primarly ledge, a small area in which the ledge drops off. structural fill will have to placed. also expansion joints along the wall to deal w/ diff. settlement. the 'stayform' option is going to be used. no space is intended in addition to the form. any flammable concerns? also the 'add-on' footing will be 4-ft beyond exist. ftg with a a 1' build up & over to the existing weall. new wall/ftg. will have a chamfered interior with diagnol reinforcing (wall to ftg.) sorry for the long winded description. please comment. let me know if anyone would like to see a sketch. thanks.

Sketches alway help.  Otherwise, I am confused.

to hokie66: what is the best way to get a sketch to you or post on this site? thanks.

Civ66,
Not that I do it much myself, but I think the most common way is, in Step 3 of your post, click on the link to ENGINEERING.com, and upload your file there.

Are you working with a contractor familiar with working with stayform? Its faily straight forward, but not too common for a wall of much height.

I have used it for 20'+ wall heights in multiple risks.

If you comment on flammability is related to the stay in place form - "Stayform" is a product name referring to an expanded steel mesh product. When combined with sacrificial reinforcing steel, it becomes the stay-in-place form on the the backside or blindside of a wall system.

When approving susch a form system, consideration should be given to the environment in which it is placed. The sacrificial rebar and the form ties secured to them will likely be exposed to the elements on the backside of the wall. In an especially agressive environment, this steel could be a path to the interior wall reinforcement.


An internet search should get you info on Stayform or similar material. An experienced contractor may be harder to find, but it is not an overwhelming challenge to work with, so long as appropriate materials and concrete pour rates are used.

Keep posting questions and I hope it helps.


Daniel