Backfill settling under new concrete deck - advice needed

[delrayser]

I posted this in my pool build thread but I think my questions got lost there -- sorry for the duplicate posts but I'd really appreciate anyone's thoughts on the following.

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So I guess we were due for for a hiccup or two in what has so far been a problem-free install. Over the weekend we got a couple inches of rain and it caused the backfilled area next to the gunite shell where the Bobcat accessed the dig to settle considerably. Wouldn't be a problem except the edge of our decking extends out over the backfill, and now there's a 1-2" gap between the concrete and the ground that extends a foot or two back. (I've tried to capture in the photos below, not sure how well they turned out.) Clearly, this is an issue and I've let the PB know, but I don't know what I should ask him to do about it. Any ideas? Can we fill the gap somehow, or does the deck need to be re-poured?

 

[Jannylee53]

Let us know what the pool builder says.

 

[BigEinAZ]

Is that corrugated plastic pipe a French drain? If not, you could temporarily plug the open end and pour flowable fill, sometimes referred to as CLSM (controlled low strength material). If it is a French drain pipe, then you could open up under that end of the slab a bit and blow in pea gravel or damp sand with a blowpipe and 185 CFM compressor. Face shield and hearing protection assumed.

 

[jimmythegreek]

either dig it out some more and use flowable fill its avail from ur local concrete company or try to blow or wash in some material. they should have compacted better I hope thats now whats going on under your shell

 

[delrayser]

I finally got our PB lined up to come out on Monday to look at the decking, will see what he says to do. Over the phone, his opinion was that the issue was aesthetic, not structural, because the area where there's a gap is up against the gunite shell and that part of the decking is supported by the rebar tying into the pool rebar. He said you could dig out a huge gap under the concrete and it would stay attached to the pool shell and be just fine. If the settling had been up against the house where there isn't the same support, then we might have a problem, but as it is he isn't concerned.

I still think I'm going to try and get him to shoot something up under there to close the cavity. Will see what he says on Monday when he comes out. In the meantime, if anyone has any input I'd appreciate it.

 

[Dirk]

From my layman's point of view, I'm calling BS. I'd get a second opinion on that (maybe more than just this forum). He's basically saying that the slab is OK since it is hanging off the shell. I suppose concrete could do that, and be strong enough, (that's how freeway overpasses are built, right?) if, IF, it had been engineered to do that. But what is more likely is that that slab was "engineered" (intended) to lay on properly compacted dirt, and there's just enough rebar to keep the two components from moving around. Not necessarily built for one component to be hanging off the other.

Just a guess.

But if it were my slab, I'd want to hear it was OK like that from someone other than the guy who would have to pay to have it redone correctly.

I like the idea of filling in the gap, but what's to stop the dirt under that patch from washing out next rain? Looks like something wasn't done correctly (filling, tamping, compacting, drainage, whatever), and just stuffing in filler may or may not be the correct fix.

 

[blakeusa]

Rain or not that's a symptom of not having a correctly compacted base. At least in new England you would do a 6 to 12 inch base of gravel and run a compactor over the area until it was level and hard as asphalt. If the base was travel or processed gravel with stone dust that settling would never happen. There is a process to drill holes in the concrete and pump filler material under the slab. This is how they level uneven concrete slab foundations and sidewalks. You can Google search how they do this. You may also be able to get a quote from local bendorvwho provides this service. This way you can tell or show your pool builder what should be done.

 

[sktn77a]

his opinion was that the issue was aesthetic, not structural, because the area where there's a gap is up against the gunite shell and that part of the decking is supported by the rebar tying into the pool rebar. He said you could dig out a huge gap under the concrete and it would stay attached to the pool shell and be just fine.

I suspect it will crack eventually.......... hopefully not taking the pool wall with it! I don't think the decking should be tied in to the pool rebar(?!)

Get the void filled - they can pressure pump some fill in there and everyone will be happy (except the builder who will be out of pocket $300 to $500).

 

[Dirk]

I suspect it will crack eventually.......... hopefully not taking the pool wall with it! I don't think the decking should be tied in to the pool rebar(?!)

I would sure like to know more about that. I was discussing the very same thing in another thread. I had it in my head that the shell and the deck should be completely separate, separated by an expansion joint, so that they both could expand and contract on their own without tearing the other apart. But the pool in the other thread also had rebar running between the two. Is it just stone coping and concrete deck that need that expansion joint? And the deck and pool are always tied together with rebar? Or is this somehow area-specific (climate), and that determines how it's done?

 

[sktn77a]

Don't know if its region specific, but I wouldn't want a deck locked rigidly to the gunite pool shell with rebar. I'd be more concerned about settling than expansion (although the latter is a concern, also)

 

[Dirk]

Exactly. I just don't get how they're supposed to move together, tied together, two completely different types of concrete structures...

 

[riley00dog]

Not sure whether it’s mine you’re taking about Dirk. My deck and pool are one structure. Same rebar and same concrete pour. Being one structure I would assume they would move together. Movement isn’t a concern of mine though since we build on/into hard shale. Nothing is moving anywhere. That’s how many pbs pour here unless Po elects to do their own decking.

 

[Winnie-the-Pool]

See my post in your build thread about this issue.

 

[duraleigh]

I had it in my head that the shell and the deck should be completely separate, separated by an expansion joint, so that they both could expand and contract on their own without tearing the other apart

That is essentially correct. They should be separated (I used roofing felt) so they do not bond and can move independently

 

[MinerJason]

Exactly. I just don't get how they're supposed to move together, tied together, two completely different types of concrete structures...

In most cases they should be able to move separately, and have something (roofing felt, plastic, etc) installed between them during construction to prevent them from bonding. Having pieces of rebar connecting them doesn't necessarily prevent them from moving separately though. It's quite common for rebar to span expansion joints in all sorts of concrete slab applications. My pool has cantilever decking that's connected to the shell periodically with rebar. It was built over 30 years ago, and the decking has experienced a little settling in some areas over time. Rebar is very flexible, and hasn't prevented the decking from lifting up off of the bond beam as much as 3/4" in one location.

 

[Dirk]

So is that rebar exposed, or buried in dirt? What about rusting?

 

[Rich D]

In most cases they should be able to move separately, and have something (roofing felt, plastic, etc) installed between them during construction to prevent them from bonding. Having pieces of rebar connecting them doesn't necessarily prevent them from moving separately though. It's quite common for rebar to span expansion joints in all sorts of concrete slab applications. My pool has cantilever decking that's connected to the shell periodically with rebar. It was built over 30 years ago, and the decking has experienced a little settling in some areas over time. Rebar is very flexible, and hasn't prevented the decking from lifting up off of the bond beam as much as 3/4" in one location.

Agreed.

I would not expect there to be any issues with having them joined if there is no frost and there was proper compaction.

This is a clear case of improper compaction and it should be corrected by one of the above methods.

I had a terrible experience with my builders lazy excavation crew. I had asked the Foreman how he was going handle the compaction and he told me they don't do compaction and the patio installers would take care of it. There was a 4 foot fill where they ramped out and it is impossible to properly compact 4ft of material from the top without expensive injection methods Wich are even questionable. I knew my soil was very Rocky so I had a mountain of sand there just for the backfill. I had told the Foreman not to do any backfilling as I did not trust him to do any of it properly, and I would do the backfilling my self that night. He ignored this request and back filled the 4' fill with the site material, slag gunite and wood from there forms as shown in the pic. I had to hand dig all that out then do the proper backfill and compaction.

 

[Dirk]

Wood, too? Unbelievable...

 

[MinerJason]

So is that rebar exposed, or buried in dirt? What about rusting?

Depends on the construction of the joint, but typically it's encased in whatever expansion joint material is used. In my case whatever they'd used was long since gone, and there was a 1/16"-1/8" gap where the expansion joint material had been originally. There were 4 pieces of rebar I encountered when I cut out a section of my deck to add a water feature wall, and they all were rusted about 1/3 of the way through at the joint. The rust didn't seem to penetrate very deeply inside the decking, and I didn't mess with the bond beam to see how far the rust had penetrated there. Best I can tell, the pool was constructed between 1979 and 1984, and the rebar did its job admirably. It kept the decking from moving laterally into or away from the pool, but allowed it to tilt without anything cracking when the soil settled over the years.

 

[MinerJason]

Agreed.

I would not expect there to be any issues with having them joined if there is no frost and there was proper compaction.

This is a clear case of improper compaction and it should be corrected by one of the above methods.

I had a terrible experience with my builders lazy excavation crew. I had asked the Foreman how he was going handle the compaction and he told me they don't do compaction and the patio installers would take care of it. There was a 4 foot fill where they ramped out and it is impossible to properly compact 4ft of material from the top without expensive injection methods Wich are even questionable. I knew my soil was very Rocky so I had a mountain of sand there just for the backfill. I had told the Foreman not to do any backfilling as I did not trust him to do any of it properly, and I would do the backfilling my self that night. He ignored this request and back filled the 4' fill with the site material, slag gunite and wood from there forms as shown in the pic. I had to hand dig all that out then do the proper backfill and compaction.

Yikes! That's a lot of work he created for you that could have been easily avoided.