Raised concrete screen porch

[Firerep]

We are in the process of repairing a 12 x 21 concrete block screen porch with a slab floor. It is appx 4' off the ground.
We were called because the slab had failed on the porch due to settling of the fill beneath it. The slab broke and settled, pushing out 2 of the block foundation walls.
Our plan was to brace up the roof, remove the slab, remove the foundation walls, remove the footings, and start over with the foundation system. Then pour the new slab with rebar, filling the wall cells as we went. We did all the demo, poured new footings with 2 pcs of #5 rebar, and brought up the block walls 4' high. The walls are reinforced with #5 rebar that is bent over to become the rebar for the slab. the next day we brought in new fill dirt in 1' lifts, packing it with a wacker packer as we went. We had braced the walls with plywood and 2x4 bracing to keep the walls from moving as the fill was packed, but when we removed the bracing we discovered that every wall had moved and nearly every mortar joint was broken. The walls are bowed and ugly, and the entire foundation walls will have to be replaced. Obviously, the customer is upset! What went wrong with the block? Any other glaring problems with the plan?

[doyle]

Shouldn't the cells have been filled with concrete and allowed time to cure before doing your fill work?

Sounds like a big mess to try and repair. Hopefully you won't lose all your profit having to start all over again.

[Rich]

The plan itself sounds good. Are there any problems with the house foundation? Are there any subsurface soil problems - expansive material below your fill? That's the only thing I can think of.

[Firerep]

I don't know about the soil below the porch, and yes, there are some problems with the house foundation. They are not part of what we are doing, & I assumed that those problems were the result of the same poor workmanship that allowed the porch to fail. As for filling the cells of the foundation first, we had planned to pour the cells and slab at the same time, creating a sort of monolithic pour, tying the footing, walls, and slab all together. Since then, however, I have read that the slab can shrink, causing the top course of block to tip back and breaking my stucco in a few months. I read that I should install a bond breaker between the top of the foundation wall and the slab, which will allow the wall and slab to move independently. Thoughts?

[Shanley]

Mortar (like concrete) takes 28 days to fully cure which could be part of the problem...Under a lateral load the wall relies on the bond strenght between the cmu and mortar until it is mechanically anchored to the slab (ie 90 degree rebar out of the wall into the slab as you stated)....Masonry has incredible compressive strenght but relatively low tensile strenghth...I think a grouted and reinforced bond beam on the top course would have helped...Above all I think you should have allowed the cmu wall more cure.

[Firerep]

What do you guys think about bringing up the new foundation walls and pouring them solid then? How long should we wait to backfill at that point? What are your thoughts about not tying the slab to the foundation wall at all, but rather using a bondbreaker at the top of the wall to prevent slab shrinkage from breaking our stucco on the foundation?

[Shanley]

If your slab is connected by rebar to the wall and you allow the slab pour to penetrate these grouted reinforced cells at the 90 degree connections, say down 3 or 4 inches from the top of the wall, this should make the assemblage monolithic. Any cracking between the wall and slab would then occur within the cmu wall...I would not install a bond brake because you don't want to create differential movement which would surely crack the stucco. Maybe a metal lath over the transition joint before your scratch coat, or a decorative control joint would help. I confess stucco is not my strength.