Swimming Pool Control Joints

[JAE]

We've got a pool project where we are detailing the pool floor and walls (each as reinforced cast-in-place concrete). The pool is fairly irregular in shape and thus we have inserted expansion joints in the proper locations with dumbell type waterstops. In other cold joints we have specified a hydro-expansive waterstop.

We also placed crack control joints throughout the floor of the pool and used the hydro waterstops in them as well.

Question: we have now been asked if a shotcrete operation would be acceptable in lieu of cast-in-place. We agreed that if the geometry and reinforcing were the same, the procedure for placing the concrete would be OK as shotcrete. However, the plan samples that they sent us include very irregular pool types with NO control joints whatsoever. I know that shotcrete produces a denser concrete, but doesn't it need control joints also as it is still concrete and still will shrink?

 

[jheidt2543]

JAE, I'm with you in thinking that crack control joints would be required. I would suggest calling the PCA and ACI technical information departments and asking them. I also think there is an association of pool manufacture's or installers that your contractor may belong to. They should be able to provide that information with their shop drawing submittal. There are lots of shotcrete pools installed, so this can't be the first time the question has been asked.

 

[Ron]

JAE...down here in "swimming pool world" there are usually no joints in the pools. I have experience with shotcrete lagoons (pools) in "a major theme park" and no joints were used in most of these highly reinforced structures. The thinking is that the high quantity reinforcement keeps the cracks very close together and very tight, thus reducing or negating the need for specific jointing, provided the structures and/or their interior surfacing are resilient enough to preclude leaks.

 

[JAE]

Thanks, you two...will check out the PCA/ACI idea. Ron, no joints? Really? ....with the higher reinforcing I can see the cracks staying small...but don't the cracks, small as they are, still leak?

We've basically got quite a bit of reinforcing in the pools (two layers) The floor is 10" thick with two layers of #5@12"o.c. ea. way. Pretty hefty I know, but we've got a site with sand and some concern for wash out and voids.

The pools are irregular in that there are three main bodies of water - a diving area, an olympic area, and a water playground(zero depth) area. These we have separated with expansion joints as the areas connect with little necks.

So would we be foolish to leave our CJ's in? They are spaced at about 24 to 32 feet on center.

 

[DaveVikingPE]

Ron,

While I don't have immediate experience with theme parks, a grad school pal of mine did her thesis on corrosion of epoxy-coated rebar in the marine envronment. As it happened, our school was located pretty close to many major theme parks (in Florida) and I believe I overheard her and others discussing cracking in the concrete at Epcot Center's "Living Seas" which for all intents might as well be a swimming pool. Here's the question: how would one control cracks in such a structure if there are no joints?

 

[Ron]

JAE..Yeah, no joints in the moderate sized structures. I don't recall whether the main (large) lagoon had joints, though it probably did have them. The large lagoons were cast-in-place.

Joints in water-holding structures are notorious for leaking. I don't view this as a reason to preclude their use...just enforce better construction techniques. The configuration of the structure will likely dictate whether you really need joints in the classic structural sense (i.e., total length of structure, restraint conditions, control joints at regular spacing, expansion joints every 100 to 150 feet, etc.). Considering that the structure could be thought of like a continuously reinforced pavement with no joints (but a need to retain water!), then the reinforcement quantity needs to be sufficient to keep the cracks very tight. ACI 350 "Concrete Sanitary Engineering Structures" recommends joints, pretty much as you have specified. These are consistent with ACI 224. Tight cracks (0.004" or less) should not present a problem as this is consistent with ACI 224 as well. Further, the surfacing on the concrete (coatings, "Marcite", other) will help the minor cracks.

Quality of the concrete and construction control will be key.

DaveViking...the cracking in the Living Seas was minor. The corrosion issues in that one were related more so to a specific construction anomaly (tie wire tip cover!) than to cracking. I believe they ultimately went to an active (impressed current) corrosion protection system for the tank. That structure was cast in place with 2 layers of reinf.