Gunite Versus Shot-crete

First we need to define both and identify common misunderstandings when it comes to gunite versus shot-crete.

Gunite for Swimming Pools

Gunite versus Shot-crete.  As described here is a dry mixture of sand and cement that is pneumatically projected for the purposes of building a gunite swimming pool.

Shot-crete for Swimming Pools

As described here is a wet “or” dry mixture of sand, aggregate, and cement (concrete) that is pneumatically projected for the purposes of building a shot-crete swimming pool.

Misconceptions

Many people believe that all dry mix applications are gunite.

Shot-crete can be applied from a wet mix or from a dry mix.

Which is better?

There are times when one is preferred over the other. Issues such as the clay content of the soil or cohesiveness of the soil should be a part of the decision process. Shot-crete and Gunite strength is measured in psi. Early on gunite was shot at 2500 psi and shot-crete was being designed for 3000 psi. Now however many gunite companies are achieving break tests of 6500 psi.

In the end it really just comes down to psi or the resistance to pressure over a square inch. Both Gunite and Shot-crete lack tensile strength (resistance from shear or side to side shift). Steel reinforcement is used to bolster tensile strength. A 3000 psi gunite has the exact compression resistance as a 3000 psi shotcrete. It is the psi rating to watch for not necessarily the product to achieve that resistance.

Shot-crete: Wet or Dry?

Each has its own advantages and disadvantages.

The wet mix requires less equipment to apply and a substantially less powerful compressor can be used. The wet mix is subject to the use of “hot loads”. Hot loads are concrete that has been sitting in the mixer too long. The cement begins to set so the operator adds more and more water to stop it from curing. The addition of water to a concrete mix severely weakens the concrete.

The dry mix water ratio can be tuned instantly at the nozzle. This allows the least possible amount of water to be added to the mix which increases the strength of the shot-crete. If the nozzle operator, nozzle man, applies the mix with too little water rebound is produced. Rebound is dry mix that didn’t get the water needed to bond. The rebound is thrown out of the swimming pool and discarded which can lead to additional materials being used.

What I should know?

Many people in the industry view gunite as a process rather than a type of shell.

There are situations when one is better than the other.

Make sure the psi of the product used is acceptable for the engineering used. If the engineering is calling for 2500 psi Gunite, make sure that the product whether Gunite or Shot-crete meets or exceeds that psi.