I've got a decent 2 year old sidewalk next to my foundation wall that's sinking or just settling into place. It's opened up a deep gap that's less than 1/2" wide where it meets the foundation wall. Water usually flows away from the gap, but it can get in there + with freeze+thaw I think the gap will only get bigger.
How can/should I fill the gap ?
Sand mix concrete delivered through a grout bag ?
hydraulic cement ?
that liquid rubber concrete patch ?
I would suggest you use a portland cement/gravel mix to fill the gap. If your sidewalk continues to sink I see no other option than to rip it out and dump compaction sand into the area, rent a jumping jack and compact the sand. Then you will have a hard surface to set your forms on. Oh just another 2 cents, if you have to rip out your sidewalk, use a hammer drill to bore holes into the foundation, tie rebar into the foundation wall, then use cement to hold the rebar in into it. If you don't want to take any risk, weld the rebar that protrudes from the foundation to the rebar cage that holds your sidewalk together.
Do a Google search on "Kenseal" construction products. They offer a number of items for sealing joints/cracks. I've used a polyurethane product by "Sika"flex with success. It's flexible enough to survive movement from heave and remain sealed. You may have to "fill" the joint/crack first with some sand so that the sealant doesn't just run down beneath the sidewalk. Cement based products are rigid and any seal established will not survive movement.
After you've solved the water issue consider using a hammer drill to drill some holes in the sidewalk large enough to pour grout mixture through. Sometimes you can detect hollow areas under the slab by tapping on it with a hammer and observing a change in sound. Grout is best placed in hollow areas.
You might also check the yellow pages or internet for "mudjacking". This is a process by which grout is forced into voids beneath a slab by use of a pressure pump. Slabs can frequently be stabilized, or even raised to their original heights, with this process.
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