Comparing a Gunite Pool
to shotcrete or concrete
Southern Poolscapes only uses the gunite installation method!
All concrete that is pneumatically applied is shotcrete. But there are two different types of shotcrete:
Wet Mix Shotcrete (Buyer Beware)
Wet mix (typically called shotcrete) is delivered by traditional rotating drum concrete trucks and shot in place by a $5000 portable pump.
Most local higher end pool builders stopped using wet mix years ago because of increased cracking. Since it goes on wet, there is much more likelihood of shrinkage.
ASTM C94, "Specification for Ready-Mixed Concrete," states time limits for the discharge of concrete. It states that discharge of the concrete shall be completed within 1-1/2 hours of when water was added (which happens at the concrete plant.) With an average 30 minute travel time to the job site, this only gives the builder 60 minutes to fully shoot an entire truck of concrete. That is virtually impossible. Wet mix shotcrete cannot be used on a pool and adhere to the ASTM standard. This can also contribute to the extra cracking. Many builders have a bad habit of adding extra water while shooting which only accelerates the curing process and leads to more shrinkage.
Wet mix shotcrete contains extra stone in place of more expensive cement (which is why it is cheaper.)
The American Society of Concrete Contractors states that a waiver should be signed by the customer understanding that the discharge of shotcrete will be over 90 minutes and does NOT adhere to ASTM standards.
Dry Mix Shotcrete (Gunite)
Dry mix shotcrete (typically called gunite) is shot by a $200,000+ pump truck. To shoot gunite, it takes a sizable investment as opposed to just a little pump. Because it is shot dry with water mixed in at the nozzle, you can get a much stronger shell with more cement.
- The gunite process isn't prone to shrinkage cracks like wet mix.
- The gunite process doesn't have to be shot within 1 hour.
- The gunite process doesn't get cold joints and potential crack spots.
- The gunite process can net a psi up to 20% higher than wet mix!
- Gunite is used on the space shuttle launch pad. When NASA used shotcrete, it melted and disintegrated (they've done it).
Any "worker with a $5,000 pump" can do shotcrete, but to do gunite, there is extensive equipment and expense. Big companies use gunite because of all the advantages and performance. Smaller companies (that try to be "jacks of all trades") typically use wet mix to save money.
Gunite Pool Shell and Why NOT to Use "Rebound"
Rebound is the material that ricochets off the application surface during the gunite operation. Rebound is typically made up of the larger and harder particles (which tend to ricochet) and does not contain adequate cement, water or density to develop significant strength. Rebound, therefore, should not be used in any application where strength, hardness or durability is needed (such as the pool bond beam). Unfortunately, it is common practice by many pool builders to use this rebound in critical areas of the pool. If you are watching your pool builder install the shell and the material falls to the floor, none of this material should be used in anything other than the steps.
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