Reinforcing concrete wall of outdoor hot tub?

Question

I just bought a home with a hot tub/pool in the backyard, and the hot tub is slightly elevated over the pool, and designed so that water will flow over the side of the tub and into the pool below (kind of like an infinity pool). I’ve noticed fairly large cracks in the top of the concrete wall of the hot tub and some of the water is flowing down into those cracks, rather than over the top and into the pool. My concern is that the water is causing erosion inside the wall, and if left unchecked, it will eventually collapse. The previous owners said it’s been in this state for a couple years (offer was as-is so I’m on my own for the cost, unfortunately).

I’ve had three contractors out to examine it and all three have said that it can’t be simply repaired, and will need to be torn down and rebuilt. Quotes range from $30-50k so I’m not keen to do the work in the short term. One of the contractors said it can probably be reinforced (via injecting epoxy or some other adhesive into the crack) and may last for years with this fix, but wouldn’t be able to guarantee anything.

Since the absolute worst case here is the wall collapsing (at which point I’d need to hire someone to rebuild it anyway), I’m inclined to go with the “reinforce” route in the short term. My plan is as follows:

1. Inject Loctite PL Premium as deep as possible into the cracks to strengthen the wall

2. Fill the gaps on top where cement has chipped away with 3M Bondo Putty to create a smooth surface

3. Coat the top with Gorilla Waterproof Rubberized Sealant Paste to prevent additional water from leaking in

Overall, does this seem like a solid plan?

One of my main concerns is that the Loctite will expand as it dries/cures (expanding inside the crack and actually making the problem worse), but I haven’t been able to nail down clear info on whether it will expand or not (seems like this may be the case for polyurethane adhesives?). Loctite PL Max seems to be a good alternative if this is the case. Anything else I’m missing or should do instead?

Here are photos of what I'm working with:

Side profile of tub, wall, and pool:

Looking down at wall of tub:

Closeup of crack:

Closeup of additional tiles that are separating but haven't yet broken off:

UPDATE 11/7:

I've included two videos of the pumps running which causes the water to flow over the wall, which in turn gives a better view of exactly where the leaks are taking place. The large crack (where the tiles are missing completely) causes the water to flow all the way to the bottom of the wall and out where a couple more tiles are missing, while the smaller crack where tiles are being pushed upwards causes the water to flow out mid-wall, between bricks in the facade.

I also contacted Loctite regarding the possibility of expansion upon curing and they responded that their Concrete Non-Sag product would actually be a better fit for my purposes. They didn't answer the expansion question, however, so I've followed up and will update this post when they do.

Front view of water running (one leak at bottom and one leak mid-wall). This view also shows that this isn't the first time water has leaked through, and the previous owners clearly did some (rather messy) repairs: https://imgur.com/a/zCl4LhY

Top view of water running with closeups of the wide-open crack in the middle and smaller crack on the far side: https://imgur.com/a/XrXvBa2

UPDATE 11/8:

Three additional photos (forgive the dirty pool - it was windy today!):

Close-up of missing tiles at bottom of facade:

Side profile of missing tiles at bottom of facade:

Top-down view of spa showing shape of wall:

Answer 1

Your hot tub surcharge on the pool's retaining wall may be moving the wall. In this scenario the sinking hot tub has been caused by soil settlement, and that soil settlement has been caused by the deflection of the retaining wall. A pool wall is an interesting retaining wall because it has the hydrostatic pressure of the water pushing back against the soil pressure from the other side. If the retaining wall has been deflecting, draining the water would exacerbate the problem.

To test the theory that the retaining wall is deflecting you would monitor the wall. A professional would setup two fixed points somewhere around your pool so that he can consistently place and orient an optical instrument with these points. Using somewhat regular time intervals, he would return, set up his instrument, and check your wall's position to see how it moves over time. If non-trivial movement was detected, then the theory is proved.

Tie backs come to mind as a non-invasive solution to brace such a wall. Once the retaining wall was braced, that would be the appropriate time to repair your cracking.

Alternatively, the soil below the hot tub could have been poorly compacted by the installer. Contractors rightly worry about damaging retaining structures by over-compacting the soil behind them. In this scenario if you don't have a clayey soil, then the soil has probably already reached compaction beneath the hot tub and nothing is moving anymore. In this scenario it's already the appropriate time to repair your cracking.

To repair your cracking, the main priority should be on getting a good foundation below your brick wall. The runniness of anchor cement makes it my favorite option. It expands by about 1% as it hardens, making it perfect for under the brick wall. That same characteristic makes it a bit risky for filling the vertical fissure, though. I would work to plug the exit points at the bottom of the fissure. Pouring water down the fissure, I would plug each exit point until the whole exit was effectively plugged (for plugging I would initially try using duct sealing compound and move on to caulks if that proved insufficient).

With the exit blocked, I would pour the anchor cement to fill the fissure in two pours. For the first pour I would keep it small. The purpose here is to block off the exits without a bunch of pressure that could potentially dislodge the exit plugs. For the second pour I would top off the fissure. I imagine each pour leaving a residue on fissure surfaces that make less of the lower fissure accessible to the anchor cement, so additional pours beyond two are suboptimal. Just buy too much of the anchor cement and return what you didn't need.

At this point you have a decision to make if you haven't done any deflection monitoring. If you leave the system in this state for a year, then any additional deflections are going to be very noticeable. If you level the surface, waterproof, and retile immediately, then you won't be able to notice new deflection until cracking shows through the new tile.

I see an anchoring epoxy from Home Depot that's 100 USD for 3/4 gallon. This is a nice runny filler also that would accomplish your intentions with the products that you cited. For any epoxy you should research its expansion/shrinkage characteristics.

Responses:

0. I saw your video. As you move from the uncracked area to the cracked area along the base of the masonry, is there any evidence that the pool's wall has been creeping out from below the masonry above? I think that I see in the video where the masonry's overhang shifts from significant at the uncracked areas to near zero at the area where the crack has grown largest. Assuming perfect workmanship on the masonry installation, does it seem like the wall of the pool is bulging out?

   In your second picture from the bottom there's an overhang of concrete clinging to the outer wall that sticks out over the other side of the crack. Looking at the crater on the other side that corresponds to this overhang of concrete, what are the relative vertical and radial displacements between the overhang and its crater? Is there any evidence of movement tangent to the wall's curve?

1. Yes the point of the plugs is to prevent the runny filler material from pouring out of the fissure.

2. If there's rebar in there, then 1% expansion is no big deal. If there's no rebar, then

   • modeling the outer concrete like a thin walled pressure vessel (not very accurate, but good for +/-100%),
   • assuming a 150" diameter for the outer wall,
   • assuming a 1" fissure width, and
   • assuming 4000psi concrete,

   I approximate the hoop stress at 480psi. ACI 318's rupture strength for 4000psi concrete is 470psi. The 1% therefore sounds too close for comfort.

3. I like the anchor cement because you can over water it for a very runny mixture that reaches the bottom of the wall and still get good enough bearing strength under the wall. I also like it because I expect that it would allow the retaining wall to continue moving independent of the masonry if the wall has been moving.

   I feel that something bonding the two halves together will mask a continuing deflection problem if it exists. The epoxy probably isn't as runny, and there's a higher likelihood with an epoxy filler that settlement from retaining wall deflection would drag both parts together. With both parts moving together, a continuing deflection problem would register as cracking masonry and cracking mortar. These are ambiguous signals and nothing as obvious as the crack reappearing and growing.