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I was removing old thinset and self-leveling cement in an area in preparation to tile it when the 6" chisel and demo hammer I was using sunk in to a specific spot. I found that this specific spot had been filled with debris and had about 1" of cement poured on top of it. Quite obviously, this wasn't sufficient, and the end result was a series of cracks in this area and a hollow sound here. I removed the debris and was left with about a 2' by 3' hole. It's obvious that someone made a (bad) patch here in the past. There was even a discarded bandaid wrapper and fibers from orange shag carpeting in the hole.

There's no plumbing that I can find below and the plastic 6mil vapor barrier is in good condition with a few minor tears that could be repaired with tuck tape. There are two roots running below; they are rotting and probably belong to something long dead. There is a crack running from the door to the right of the picture to the hole. I plan to repair this crack at the same time by broadening and deepening the crack slightly and using a cement patching compound.

What product should I use to fill the larger hole? Plain ol' Quickrete, or some other form of cement? Aggregate or not? Any specific PSI ratings I should look for, or other prep work that I should do besides washing the areas that I hope the new concrete will bond to and making sure that there's enough clearance under the wire for the cement to engage it?

Hole in the floor

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The red marked area is the hollow sounding area; I should note that the hole is now large enough to encompass the hollow sounding area. This picture was taken a few hours ago. –  Karl Katzke Oct 1 '12 at 21:55
    
Can you live with the floor raised a bit? If so, I'd suggest putting down regular concrete to patch, then a layer of cement board or hardibacker to give you a nice flat cohesive surface to tile on. –  DA01 Oct 1 '12 at 22:22
    
Actually, I'll be laying a layer of DITRA and then tiling with a medium-bed mortar and a lot of care. I don't think Cement board is rated for the kind of application you suggest by the NTCA; you need to affix cement board to the slab somehow (screws or another cement product) and you'll typically end up with cracks where the pieces of cement-board meet due to the differential expansion and contraction. –  Karl Katzke Oct 1 '12 at 22:29
    
When you say "washing the areas [so it will] bond" does that mean you intend to use a concrete bonding agent, or just water? –  BMitch Oct 1 '12 at 22:53
    
If I need one, I've got no problem with using a bonding agent... I'm just not sure if it's the right approach. I'm trying to figure out if I should fill the entire hole with a latex/portland cement product without aggregate (which I know I'll need for the crack repair); or use a bonding agent and quickrete, or something in between. –  Karl Katzke Oct 1 '12 at 22:59
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Ok, I went to the big blue box store, read every bag they had there, and answered my own question.

As Cory (properly) pointed out, the mesh in the bottom of the hole isn't doing much good. It seems that this is a 'peak' in the sand that forms the substrate for my foundation; the sand slopes downhill to a depth of about ten inches on either side. I'm not sure why it's built this way, but as I can't really fix it now without doing a lot more work that I don't want to do to the vapor barrier and other aspects, I levered up the wire so that I could fit stones underneath it and there's now about 1/2 to 3/4 inch clearance between the wire and the bottom of the Chasm.

I just finished scrubbing it down with water and will let it dry overnight, and then tomorrow when I'm ready to mix I'll apply a primer that should help the concrete adhere.

I purchased this bag of fiber-reinforced high strength quickrete because over an area of the slab the size I'm patching, it shouldn't need to interact with the wire mesh much to gain the strength that I'm looking for. The sand should keep it from sinking any, and even if it cracks around the edge, it won't have much of a place to go given what will be paved over it when the DITRA gets installed.

Quickrete fiber reinforced high-strength concrete

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I would be more concerned that your wire looks to be at the very bottom of the concrete, making it useless. Concrete is very strong under compression but very weak under flex. The steel is there to take the flex and should be about center of the concrete. Definitely use a concrete bonding agent and Ditra. The Ditra will provide enough of a barrier that the cracks should not come through. Personally I would not put a huge pile of money into tile considering it might end up cracking

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Cory, I definitely agree. One sec and I'll post the solution I came up with. –  Karl Katzke Oct 2 '12 at 0:16
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