Print Posted on 08/05/2016 in Category 1

Concrete Pools – Shotcrete vs. Gunite

Concrete Pools – Shotcrete vs. Gunite

Which is better?

by Rex Richard

In concrete pools there are both poured-in place, and shot-in-place structures.  When referring to “shot-in-place pools” the terms “Shotcrete” and “Gunite” are frequently used.

I reality, the term “Gunite” is actually a brand name and not a material. (Often used when referring to “dry shotcrete”) Much the same as “Kleenex” has become a familiar term for the actual product “facial tissue”, so “Gunite” has become a familiar but mistaken term used for the actual material “shotcrete”.

There are two types of “shotcrete”, “wet shotcrete” and “dry shotcrete”.  They are both used in the construction of swimming pools and both can provide a strong and reliable structure when the mix and application is correct.  The difference is primarily when and how the water is added to the cement and aggregate and the type of equipment used to deliver the product to the structure.

Dry Shotcrete 

Dry shotcrete is delivered with the aggregate (sand) and the cement powder stored dry in separate “hoppers” in a “dry shotcrete” truck.  Basically the attendant at the truck meters the blend of aggregate, and cement, which are then pumped in their dry granular form through a hose to the site where the structure is to be “shot”.  At the end of the hose is a “nozzle” desgned to add water to the mix and apply highly compressed air to propel the now blended mix and apply it to the structure under pressure.

The benefit of this product is it is delivered and applied “fresh”, (never too old or “hot”), and assuming proper nozzle technique it is delivered in a “dryer” or “less wet” state than “wet shotcrete”.  As a dryer mix will have less shrinkage and has the potential of achieving a higher final compressive strength it is often claimed to be the superior material.

The weakness on this material is the human element.  This indeed is a theoretically superior material in final compressive strength, but there are factors of both human and machine error that can and does make this a material with potential for problems if it is applied incorrectly.

Incorrect application can include the improper mix of “sand to cement” ratio (either by human error or equipment failure), or the addition of too much water at the nozzle, (again either human error or equipment failure).

Due to the above factors we do not recommend it above a “wet shotcrete” application, nor do we find it necessarily problematic.

In both cases we recommend the structure be tested prior to advancing to the next phase. (See below)

Wet Shotcrete 

In contrast to “Dry Shotcrete”, “Wet Shotcrete” shotcrete is delivered with the aggregate (usually a sand and pea gravel mix) and the cement powder mixed at the concrte plant including the addition of water and strengthening additives and the delivered to the site fully mixed and ready to apply. The attendant pours the mix from the concrete truck into a hopper which then pumps the wet mix through a hose to the site where the structure is to be “shot”.  At the end of the hose is a “nozzle” designed to apply highly compressed air to propel the blended mix and apply it to the structure under pressure.

The benefit of this product is it is delivered and applied “computer batched” from the concrete plant  with scientific consideration given to air tempurature and humidity when blending the product. Though it is delivered in a  potentially “wetter” state than “dry shotcrete” the factors of precision in the product blend and use of additives to control hydration rate also yield a great product with low shrinkage and high final compressive strength yields.  By those who embrace this material it too is often claimed to be the superior material.

The weakness on this material can be the human element, though far less likely, but more likely it will be the time of delivery.  As the water is added to the mix at the factory, if too long a time passes the material becomes “hot” as it enters its chemical “cure” or “set-up” phase.  If the material is applied “hot”  the structure can be substantially weaker than specified by the engineering.

Due to the above factors we do not recommend it above a “dry shotcrete” application, nor do we find it necessarily problematic.

In both cases we recommend the structure be tested prior to advancing to the next phase. (See below)

Shotcrete Testing:

As both ‘Wet” and “Dry” shotcrete have potential for structural problems, but are both excellent materials, I recommend each installation be “site tested” to ensure the mix and application was proper.

The test is performed by shooting samples of each batch into a “cylinder” and delivered to an engineering “lab” to be tested for compressive strength.

This test is common and in fact required in some code jurisdictions and in not overly expensive when you consider the potential for property damage and expense in the case of a structural failure.


Rex Richard  is a leading swimming pool industry expert and consultant.  

Picture of pool construction courtesy of Carolina Construction Group.  Thank you!