Locating and rationalizing 4" concrete wall reinforcement recommendations?

[largehearted]

Hi all.
We're repairing a 4" concrete wall behind the tile face of a swimming pool. This 4" concrete wall is called a "protective concrete wall" in the original design drawings.

Behind it (imagine you're standing in an empty pool) is a 12" foundation wall, and behind that is an equipment space, so - as I understand it - the pool water pressure creates tension stress in the vertical reinforcement of that 12" foundation wall beyond. This arrangement has performed well for the 12" foundation wall, which did not observe significant cracking in. Yay! The protective concrete wall failed, though, with significant cracks forming a cheshire cat smile through the tile finish, across its 40ft length. The original 4" protective concrete wall had wiremesh called out thus: #66-1010. There are plenty of forensic possibilities for the wall failure including excessive stress from the water pressure or the wall's buckling under its selfweight (no tiebacks are shown in the original details). We did not probe many spots to identify if the wall might have had joints relieving temperature and shrinkage pressure.

My mind wants to move on from the forensics to replacing the failed wall with a better one, but they really are one and the same issue. How do you reinforce a concrete wall with anticipated bending behavior like a wearing slab, chlorinated water 1.25" away, and a 4" thickness?

I have a concrete textbook that points to the ACI 318-11 recommendations for non-load-bearing walls:
(1) thickness >= 4" or 1/30 of the least distance between lateral supports (ACI 318-11 14.6.1),
(2) minimum vertical reinf. ratio 0.0012 for WWF not larger than W31 or D31, (ACI 318-11 14.3.2) ...
(4) minimum horizontal reinf. ratio 0.0020 for WWF not larger than W31 or D31 (ACI 318-11 14.3.3) ...
most code provisions are irrelevant to a 4" concrete wall, or specific to rebar reinforcement (which I typically am specifying even in SOGs, beside the points).

I'm confused about their rationale for giving 5/3 as much reinforcement in the horizontal direction as in the vertical. Imagining our protective concrete wall as a wearing slab with 9ft by 43ft dimensions, if it were to deflect with the foundation wall behind, wouldn't it bend one-way with the vertical reinforcement being this primary bending direction? Worse than not understanding their rationale, I think these provisions have since fallen out of the ACI?

Any help understanding these minimum reinforcement provisions, their applicability, potential design considerations (at the level of the concrete, not waterproofing, finishing, etc.), or even potential forces at play in the original protective concrete wall failure would be massively appreciated, and probably help me far into my career with light concrete work like this.

Best,
Noah

 

[RobertHale]

What are the support conditions of the 12" structural wall as well as the 4" wall? Depending on the support conditions, the wall may be behaving like a deep beam.
I'm not an expert on the code provisions, but I think that the horizontal reinforcing ratio is larger because of shrinkage (walls typically being longer than they are high)

 

[Just Some Nerd]

The horizontal ratio being larger due to shrinkage seems like a good guess, I agree with Robert there.

I wouldn't know if you can strictly consider the wall to be non-loadbearing if the goal is to prevent cracking, the wall has to deflect in the process of transferring load to the second wall unless the second wall is incredibly rigid. Under constant load from the pool's water pressure, I would expect that second wall to creep over time, so the first wall will also deflect to match it.

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Why yes, I do in fact have no idea what I'm talking about

 

[largehearted]

Robert: the 12" foundation wall beyond terminates (with a haunch) into the floor slab above (which a composite slab of a 7" frame slab with #5@8, then a pool deck topping slab of 3-3/4" w/ WWF), and it shear keys and dowels into a mass concrete slab below. I believe the backing foundation wall is about 10ft clear-clear whereas the face wall is about 8ft clear-clear. The deep beam point is an intriguing one, and perhaps compression could crush that front face while the tension face with higher reinforcement ratio resolves the stress. None of the characteristic deep beam traits (depth-span, depth-width, point loads away from supports) are glaringly relevant though.

Just Some Nerd: Myself and the senior engineer I'm working with had the same sense that you do, the wall sees loads on one side. It has an accompanying expected shape of deflection, and so we both reflexively wanted to place more reinforcement in that direction. "It's more of a sideways wearing slab than the non-load-bearing wall case you were looking at" might have been exactly what I needed to read.

I had guessed the exact same rationale that y'all did about the horizontal ratio being higher because the wall span is anticipated to be greater than height. Am I off base in thinking, then, that a change in volume should result in directional stresses that are a function of the expanding length (in each restrained direction)?

The fact that the tallest wall failed, and the smile shaped failure across the face make me lean towards explanations that factor in (i.) the 9ft height and (ii.) the 43ft 'span' between restraints at each side. So I think we designers need to choose reinforcement for the repaired wall with the understanding that (1.) the wall needs expansion-contraction stress release or it'll see greater stresses in the span direction than along its height, and (2.) the wall is gonna bend and creep along its short direction. I think the volumetric horizontal reinforcement of 0.002 might just be reasonable for both directions.

Does anybody have a strong answer for what #6#6-1010 was calling out for the original wiremesh? If they were calling out American Wire Gauge (AWG) #6@10"oc, I'm seeing those have 0.162in dia, thus 0.0247in^2/ft ... The 0.002 volumetric minimum would suggest 0.002*4in*12in = 0.096in^2/ft, so nearly 4x as much steel diameter. Youch. That's what we'd be leaving in the - fine condition - other 3 side walls.

Also thanks everyone for not teasing my hysterical overwriting, lol.