Decking and skimmer cracks

[tethys]

Hello TFP,

3 year old pool and I'm experiencing cracking in both of my skimmers, in the decking above the skimmer block, and in the coping stone that bridges the skimmer throat. And I can't for the life of me figure out why this is happening. Have had a few companies come out and take a look, and all of them seem to be very focused on just breaking the decking out and refilling. But as far as I'm concerned, if we don't understand why it is happening in the first place this is a wasted effort. Let me try and walk this through with some pictures:

Picture 1 - this is the pool shell as framed/formed, focusing on one of the skimmers (and you can see the auto-fill as well). Note that the skimmers are completely encased and poured as part of the pool shell itself.



Picture 2 - this shows the decking as framed/formed around the skimmer/auto-fill area. This is after the shell and skimmer block have been poured obviously.



Picture 3 - this is taken just 4 weeks after the initial decking was poured. You can see that there is a crack in the concrete already. This crack would continue to develop over time, to be joined by new ones. I have tried to shade where the skimmer block itself sits under the decking. Awfully suspicious that it seems to share a border with that crack!



Picture 4 - this is where we are at today. I have tried to annotate appropriately. I have taken a level out to the skimmer area on the deck, and can confirm that the decking over the skimmer block is actually heaving UP. It crests up right in the center of the section (right where you see the coping crack), and the peak runs from the pool to the edge of the decking. The rest of the decking around this area appears to be level as expected. If I rap on the deck concrete around the skimmer, it sounds very hollow underneath. So my assumption is that the deck pour has somehow lifted up from the skimmer concrete block in this area??


Why in the world would this happen? It is the same on both skimmers. I don't think the shell itself has moved because I see no cracking the plaster there, in the waterline tile, or in the surrounding coping, It is literally isolated to just above the skimmer block itself. Perhaps the mystery gets solved when I have that area demo'd - but I would dearly love to have some guesses in place before pulling the trigger.


Any thoughts from the TFP crew?

 

[kimkats]

Now that IS a puzzle! My guts says one of two things:

-somehow the dirt under then skimmer case got washed away allowing that area to settle Hummmmmmmmmmm a leak in the pipe allowing water to wash the dirt away over time?

-Something is growing in there causing that area to move up.

Which one we cannot say until it is open so you can see what is really going on.

 

[Bperry]

Would definitely check into the possibility of a leaked

 

[tethys]

Now that IS a puzzle! My guts says one of two things:

-somehow the dirt under then skimmer case got washed away allowing that area to settle Hummmmmmmmmmm a leak in the pipe allowing water to wash the dirt away over time?

-Something is growing in there causing that area to move up.

Which one we cannot say until it is open so you can see what is really going on.

That was my first thought - erosion under the deck or skimmer block, or a leak in that plumbing trench washing stuff away. But then I thought - how would that cause the deck to heave UP? I can see that causing sinking... but the deck on either side is level, and the skimmer area is raised!

 

[kimkats]

then I thought - how would that cause the deck to heave UP?

I have nothing!!! You are right on this thinking!

UP-something under it that is pushing it up.................roots?? How close is the nearest plant?

 

[Fouroaks347]

I have nothing!!! You are right on this thinking!

UP-something under it that is pushing it up.................roots?? How close is the nearest plant?

Is that area moving up or is the area around the cracked area actually moving down?

It could be that the concrete is sitting on the box looking like it is higher, but the rest of the concrete is sinking down around the box since the ground wasn’t prepared correctly or is being washed out.

Only way I know to see what’s under concrete is to ground radar it, or X-ray. GPR can go though concrete, show you the rebar location and voids in concrete or substrate.

 

[Bperry]

X2, The sinking is a much more common thing which causes the other side to raise up.

 

[tethys]

Is that area moving up or is the area around the cracked area actually moving down?

It could be that the concrete is sitting on the box looking like it is higher, but the rest of the concrete is sinking down around the box since the ground wasn’t prepared correctly or is being washed out.

Only way I know to see what’s under concrete is to ground radar it, or X-ray. GPR can go though concrete, show you the rebar location and voids in concrete or substrate.

Definitely heaving up. The surrounding area is completely level from one end of the deck/pool to the other. And the decking area right where you see the coping crack is crowned up. At least 1/4", maybe a bit more. So that's one data point. The other data point is the hollow sound in that area. It sounds like there is a void between the deck concrete and the skimmer box concrete. Like they have separated, with the deck concrete pitching up. I hadn't thought about using GPR. But given the fact that this needs to be fixed regardless, I suppose tearing it out and visually seeing what is below is maybe where I'm at.

 

[tethys]

X2, The sinking is a much more common thing which causes the other side to raise up.

That is what makes this so bizarre. Definitely tenting up over the skimmer area: it's at a higher level than the coping, I can put a level on this area and it will crest right on top of the coping crack, etc. etc. The areas of the deck and coping to the left and right of the skimmer are true level.

 

[tethys]

I have nothing!!! You are right on this thinking!

UP-something under it that is pushing it up.................roots?? How close is the nearest plant?

I'd be inclined to lean in with the roots theory... there is a shrub just about 1.5' away. BUT. My other skimmer on the other side of the pool is doing the same Dang thing (not pictured), and it doesn't have *anything* next to it.

 

[Bperry]

That is what makes this so bizarre. Definitely tenting up over the skimmer area: it's at a higher level than the coping, I can put a level on this area and it will crest right on top of the coping crack, etc. etc. The areas of the deck and coping to the left and right of the skimmer are true level.

The deck should be sloped away from the water about 1/4” per foot.

 

[tethys]

The deck should be sloped away from the water about 1/4” per foot.

Decking is sloped away from the pool. But is level parallel to the pool. Except for the skimmer area, which is raised up.

 

[kimkats]

Well a diggin you will a go

 

[Bperry]

Decking is sloped away from the pool. But is level parallel to the pool. Except for the skimmer area, which is raised up.

Im stumped then. Maybe the big freeze a couple years ago caused it? Or maybe that didn’t affect Austin?

 

[ajw22]

You should have an expansion joint around the skimmer concrete block.

The deck is moving and putting pressure on the skimmer area. The expansion joint should be there to allow the deck to move independent of the pool structure that includes the skimmer block.

You need to remove the deck from around the skimmer and install the expansion joint.

Expansion Joints and Coping - Further Reading

www.troublefreepool.com

 

[tethys]

You should have an expansion joint around the skimmer concrete block.

The deck is moving and putting pressure on the skimmer area. The expansion joint should be there to allow the deck to move independent of the pool structure that includes the skimmer block.

You need to remove the deck from around the skimmer and install the expansion joint.

Expansion Joints and Coping - Further Reading

www.troublefreepool.com

I wondered about this! Do you happen to have any design diagrams or construction pics specific to this approach around skimmers? I didn't see any at the link you provided. I'm curious how this would work. The block of cement that entombs the skimmer appears to be monolithic with, and set to the same height as, the pool shell itself. So if this block doesn't extend up to deck level... just can't picture how that expansion joint would get set in?

 

[ajw22]

Let’s see if @AQUA~HOLICS has any pics of expansion joints around a skimmer.

You can do it a few different ways. One is to do a slip expansion joint on top of the concrete block like used with cantilevered coping.

Or box an expansion joint around the concrete block.

And put an expansion joint around the skimmer.

Key is to allow the deck some movement without putting pressure on the skimmer.

 

[AQUA~HOLICS]

Sorry no pictures of expansion joints around skimmers, not typically installed.

 

[rcerf]

Here’s the way mine was done. The patio and expansion joint goes around the skimmer box , and we put coping over it. I had them do it this way just for the problem you’re having now. The patio doesn’t come in contact with the skimmer box.

 

[rcerf]

Here’s a little better picture.