I thought it was crazy, then I decided it didn't sound so crazy. My parents' porch is concrete, with the top slab formerly resting on probably construction rubble with some concrete poured over it (I think) before the actual slab was poured on top.
There was a gutter/drain issue (now fixed) which washed a bunch of dirt/rubble away, resulting in some cracking and the front of the porch dropping 2-3" at the front; the rear seems to be ledgered on the house. It hasn't moved in a couple of years or so.
My dad's become enthusiastic about the idea of DIYing it by jacking it into about where it should be (and he has it most of the way there already) and filling the gap with Great Stuff or similar expanding foam. How crazy is this? I was worried about using insulating foam sheet under the slab for the upcoming garage, but SVRex corrected me that that's correct and normal. But Great Stuff != rigid foam insulation; or is it?
Given the large surface area, the test block's rigidity... It seems like it might well work, and wouldn't make the process of tearing the whole thing down and starting over much worse if it doesn't work out. Curious as to whether anybody's tried this sort of thing and/or knows more about the available expanding foams.
A quick search suggests concrete at around 150 lb/ft^3, which at 3" thick suggests that each square inch of foam would be supporting much less than a pound, though we'll certainly get uneven coverage and it'll end up shared in a probably uneven fashion between the forward edge and the ledger at the back. Still, those numbers seem promising.
I've never been happy with the water durability of that foam over the long term.
But I've got a void under my shop slab, so I'm watching this with interest. I know the fix is "mud jacking" but I don't know whats really involved...
I might be concerned with the material breaking down over time. Or whether or not it's suited to the proposed task. But I digress.
I had a problem with front and back porch slabs settling several inches not long after I had purchased this house from the builder. I contacted him and invited him to come out and look at it, and he was like, yeah, it should have supported by the (brick) porch and tied to the foundation of the house. Apparently, it was not tied to the house. I didn't realize it at the time, but he was just stalling me, since the clock was running on his period of obligation as the builder.
A year or so later, I contacted a business called A1 Concrete Leveling. They "pump it up" by injecting a cementitious material under the slab. This fills the void under the slab and lifts it to its proper level. Their local franchisee was very courteous and knowledgeable, and he did a great job. He said that the void under my slab was as much as 15" in some places. The concrete was apparently improperly supported and poured over loose fill.
I would suggest you contact A1 in your area. https://www.a1concrete.com/
They will be happy to quote your job.
ultraclyde (Forum Supporter) said:
Huh. Polyliftng : https://theconcretehero.com/2018/12/3-main-problems-with-mudjacking/
I wonder how different the foam is...
Far out! Looks like polylifting is the new and improved method. I wonder how you calculate the void volume? How do you not put too much in? Sounds like fun.
I've used the poly lift at work to stabilize a loose section of slab that had a 60 ton punch press banging away on top of it. 5+ years later and still working well - slab is stable and at same height as the surrounding floor.
Edit: This was not a DIY scenario. I watched a crew of technicians do this and from what I observed them doing it requires training and proper equipment.
My ex did this at her house but they did it a bit differently. They jacked it up with a large rubber air bladder, temporarily steadied it with some cut 4x4s and pavers, then poured in a 2 part poly foam. Hers was a groundhog that started the problem and then water continued washing it out.
That was 15 years ago and it is still rock solid with no cracks so I'm sure they have even better methods/materials now.
I know you can buy the 2 part expanding foam in quantity, I've seen it used on boats and cars. It's not cheap but I suspect it's a lot better for this than cans of Great Foam.
I think maybe you can get it in different "strengths"? I know that stuff can blow up a structure, but lifting a slab? Maybe it depends on how well it's vented?
Got a similar problem with my front porch, it's actually near the correct height at the end but sagging around 4 to 5 inches where it meets the house. It looks like it should have been tied in to the house but it's not. Contractor wants to pour more concrete to level out the top slab (it's a concrete slab on top of a brick foundation), which I think will just make the porch settle even more. Was about to make some inquiries to some foundation folks (I think A1 concrete leveling is one) about this very thing.
I had the front walk and a section of apron in front of the garage jacked up with foam 3 or 4 years ago. The apron area has a 5 foot tall, 8" thick brick wall sitting on it. Wall is about 6-7 feet in length. Both sections are still where they were jacked to and haven't shifted at all.
1988RedT2 said:
Far out! Looks like polylifting is the new and improved method. I wonder how you calculate the void volume? How do you not put too much in? Sounds like fun.
Not sure how they do it if there's a void under a slab that hasn't settled, but I've watched it being done to raise a settled interior slab in a split level house. They popped a chalk line on the walls and squirted the foam in through a few holes until the slab lifted back where it belonged.
In reply to newrider3 :
When mine was done they pumped foam in through 2 or 3 holes in each slab. They could pump in a bit of foam and watch the slab come up. Then they would repeat the process until the slabs were where we wanted them. It took 3 or 4 shots and they could tip the slabs by drilling a new hole on the low side and add a little foam.
