Building a Gunite or Shotcrete Pond? Important Things You Need to Know About Gunite and Shotcrete

You’re probably planning a Shotcrete or Gunite pond, but I bet you do not know what Shotcrete and Gunite are. Come on. Take a guess. If you answered, a sprayed concrete, that would be incorrect. In reality, Shotcrete is an all-inclusive term used for describing the process of spraying concrete or mortar through either a wet or dry application technique.

Gunite, on the other hand refers only to the dry-mix process in which the dry cementitious mixture is blown through a hose to the nozzle, where the water is injected immediately prior to application. So in essence, what you end up with is still concrete. The only real difference is how it was just applied.

When people think of concrete, they think of concrete that is poured into forms that have steel rebar and mesh reinforcement, which is then allowed to set up and cure. The Shotcrete or Gunite process allows the concrete to be sprayed in place instead.

Shotcrete is a wet spray application, meaning that the concrete mixture is blended with water and pushed through the application hose pneumatically where it is applied to the job site. Gunite consists of a dry mixture that is pushed through the hose pneumatically until it gets to the gun nozzle, at which point water is injected into the mixture where it is mixed as it hits the job site.

In the end, one could say that besides a few minor differences, Shotcrete and Gunite work the same. Small differences such as how each material might be applied to a vertical or overhang area are about all you’ll find.

Now you may have also been thinking that Shotcrete or Gunite were actually waterproof after they have been sprayed and cured. This is a misconception. Remember when I said that you’re still spraying concrete? Concrete is not waterproof either. This is why, when building a pond, you need to seal the inside of the concrete shell.

The positive side of this is that because it is concrete, using the right waterproofing coating means you’ll not have to use a primer in order to get it to stick. Just prepare the surface as you would any concrete and you’ll be in business.

Keep in mind though, that because it is normal concrete, it will have to cure for a minimum of 28 days just like any other concrete. During the hydration process, the typical precautions of keeping the concrete moist are very important in order to minimize shrinkage.

One of the things you’ll find that is a plus for Shotcrete or Gunite is that because it is sprayed, a lot more of the air is removed from the over-all product. This means that the concrete is stronger than a typical poured type of concrete. Poured concrete might have a compressive strength of 2500 psi where Shotcrete or Gunite average out at near 4000+ psi.

The draw back with that is minor, only being air entrainment within the concrete. Air entrainment in the concrete is good for freeze/thaw cycles. During a freeze for instance, water is pushed into the concrete and can literally expand within the air pockets meaning the concrete will have a better chance at withstanding cracking.

A good spray contractor will know this and attempt to entrain at the proper percentage so as to accommodate for freeze/thaw when the scenario calls for it. This is referred to as the spacing factor.

One other thing you might ask your contractor about is whether or not your application could benefit from the Shotcrete or Gunite being partly composed of silica fume. Silica fume makes the Shotcrete or Gunite stickier and aids in the application process where a quicker build up is needed.

Finally, you should take care when deciding upon a contractor to apply your Shotcrete or Gunite. Here are some questions to ask both yourself and the contractor.

  • Does the contractor have a good track record of shooting ponds with the wet method? Dry method?
  • How many ponds have they completed with the wet method? Dry method?
  • Can you provide a list of past completed jobs?
  • How do they plan to incorporate the trimmed concrete into the shell? (The rebound and the trimmed concrete play a key role in the final quality of the pond shell.)
  • What concrete mix design do they plan to use?