The ASA Technical Questions and Answers is a free service offered to all users, but primarily intended for engineers, architects, owners and anyone else who may be specifying the shotcrete process and/or has need for a possible answer to a technical question.
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PoolQ We will be shotcreting a pool and the designer has put an expansion joint in the pool going from the top of one wall through the floor to the top of the other wall. It also shows a 9 in. (229 mm) polyvinyl chloride (PVC) waterstop in this joint. I have seen this used with cast-in-place concrete, but not with shotcrete. I was wondering if there are any guidelines on shooting around a PVC waterstop.
This detail is normally only used on very large competition pools on the order of 164 ft (50 m) in length. It takes a lot of skill, technique, and care to properly encapsulate the waterstop and it should only be attempted by a shotcrete contractor with experience in this application. The successful encapsulation of the waterstop is more challenging with the dry-mix process than when using wet-mix shotcrete. The techniques for the proper encapsulation are generally developed by the individual shotcrete contractor and there is no specific “guideline” available for encasing waterstops.
PoolQ We will be tiling a pool. The pool’s shotcrete walls and floor were placed approximately 10 days ago. What is the earliest we can begin gauging the pool walls and floors?
It is generally good practice to let the shotcrete cure for the full 28 days before attempting to apply coatings or overlays. We would recommend you get a recommendation on the cure time from the manufacturer of the gauging product before doing the work.
PoolQ We’re building a pool 25 x 45 ft (7.6 x 14 m) and had a massive cave-in on our deep end. The builder wants to build temporary walls to shoot the shotcrete against, then remove the plywood walls and backfill with gravel. I’m wondering if this will work and if they will be able to remove the plywood without damaging the shotcrete walls. I know with typical forms you would prep the form with oil so the concrete doesn’t stick. Would that be necessary for shotcrete, too? Also, is there a recommended wait time for curing before we backfill?
Shooting shotcrete against a one-sided form (what you called a temporary wall) is a common way to build a shotcrete wall. Once the shotcrete sets and builds strength, the plywood form can easily be stripped off the back of the wall. Form release agents (not oil) can be applied to the plywood to make the stripping easier. Once the forms are removed and the concrete has gained adequate strength, the walls can be backfilled with compacted soil or gravel, depending on the drainage needs.
We recommend 7 days of curing. Continuous water curing is best, but if impractical, applying a curing compound on the exposed surface at twice the manufacturer’s recommended rate for a good seal is acceptable. If they remove the forms before 7 days, they should also water cure or apply curing compound to that newly exposed surface. The shotcrete needs to build up enough strength to resist the external force of the backfill, so check with the pool designer to see what they need for the required strength of the concrete before backfilling. With most good-quality shotcrete materials and placement techniques, you can expect about 4000 psi (28 MPa) compressive strength in 7 days.
PoolQ What is the minimum spacing between reinforcing bar recommended in a shotcrete swimming pool deep-end wall. The engineer is calling out for 12 in. (300 mm) walls with a double mat of No. 5 bars at 4 in. (100 mm) on center each way. In my opinion, this does not give enough space to properly encase the reinforcing bar with shotcrete without creating voids and trapping uncontrollable rebound.
The engineer designs the wall based on the loads anticipated on it throughout its service life. The minimum spacing for noncontact lap splices as indicated in ACI 506.2, “Specification for Shotcrete,” is:
“Clearance of at least three times the diameter of the largest reinforcing bar; three times the maximum size aggregate; or 2 in., whichever is least”
In your case with No. 5 bars, the 2 in. (50 mm) minimum probably controls and would then provide a 2.6 in. (64 mm) center-to-center spacing. No. 5 bars at 4 in. spacing can be shot properly with proper materials and technique. An ACI-certified shotcrete nozzleman will have had education that explains how this configuration or even closer spacing can be shot properly. Experienced shotcrete contractors doing structural concrete walls do this type of work routinely with excellent results. If you are concerned about encasement of the reinforcing steel in the back curtain of steel you may consider erecting only the back curtain of steel, shooting the wall out to the location of the outer curtain of reinforcement, erecting the outer curtain, and then shooting out to the final surface. Experienced shotcrete contractors have shot in thick sections (36 in. [900 mm] thick or more) with heavy reinforcement (No. 11 at 4 in.) using this technique.
PoolQ What is the recommended cure time for shotcrete pools and spas so that shrinkage cracks in finished tile work can be avoided?
Concrete, when applied using the shotcrete process, or cast-in-place, needs to cure for 7 days. Water is the best curing method (7 continuous days). Curing compound can be applied, but the membrane film that is formed will have to be removed by sand or water blasting (5000 psi [34.5 MPa] is recommended) before the plaster or tile can be set (it will create a bond breaker if not removed). There are curing compounds with a dissipating resin, which means after about 30 days in the sun, the material will break down. In either case, it is a good practice to pressure wash the concrete surface to remove the grit and dust out of the pours so that the plaster and tile will have a good bond. This is normal, everyday concrete curing practice that helps to prevent shrinkage cracks. The concrete being applied should have a water-cement ratio (w/c) of 0.35 to 0.45. Having the w/c at 0.40 at a 2 to 3 in. (50 to 75 mm) slump will keep the water demand low to help minimize the shrinkage. Wet-fogging freshly placed concrete before the curing process begins will also help prevent shrinkage cracks.
PoolQ What would be the right concrete mixture for a swimming pool? I found out that a few concrete plants have different mixes, so I wanted to know what would be the correct one.
There is no special requirement for shotcrete placement in pools. Basically, you are building a watertight structural concrete shell. Concrete should be a minimum 28-day compressive strength of 4000 psi (28 MPa). Most shotcrete uses a 3/8 in. (10 mm) maximum coarse aggregate size due to our small diameter delivery lines. The coarse aggregate (rock) being about half the weight of the sand content. We typically need a cement-rich paste so minimum cementitious content (cement, fly ash, slag) of 700 lb/yd3. We also need a low w/cm ratio for the ability to stack in the wall so maximum of 0.45, with most wet-mix concrete 0.42 or less. You may find our ASA Position Statements for our Pool and Recreational Committee helpful in providing more detail. Our current statements include:
- Compressive Strength Values of Pool Shotcrete
- Shotcrete Terminology
- Sustainability of Shotcrete in the Pool Industry
- Watertight Shotcrete for Swimming Pools
- Monolithic Shotcrete for Swimming Pools (No Cold Joints)
- Forming and Substrates in Pool Shotcrete
- Curing of Shotcrete for Swimming Pools
You can find the position statements freely available at shotcrete.org/products-services-information/resources/.