Shot-Crete, Gunite what’s the Difference?

By Tim Long

There is a lot of confusion over the difference between Shot-Crete and Gunite and which process should be used for your swimming pool or spa. Many pool builders will argue why the process they use is better than their competitor’s without really understanding the differences themselves.

If you were to research the American Concrete Institute or the American Shotcrete Association, you would find that both terms refer to the application process of good old fashion concrete. The concrete mixture travels from an applicator’s truck to the site via a large hose. Air pressure is applied and the mixture is “shot” into place at speeds approaching 200 mph. This is extremely important! Why? Because it is with the forced placement of the concrete that either process gains it’s merit. Any concrete placement should be “compacted” to remove voids, air bubbles that weaken the strength of the eventually hardened concrete. For concrete that is poured, applicators will use a variety of methods, tamping, vibration, etc, to achieve compaction, but no process by hand can compare to the compaction that can be achieved by the pneumatically applied shot-crete or gunite.

Why the two different terms? Shot-crete was invented by Carl Akeley, who received a patent on the precess in 1911. Originally the concrete mixture, which is made up of cement, aggregate was pumped through the hose dry. At the nozzle water was added along with the air needed to project the mixture. Today this process is now known as “dry-mix”. In the 1950′s it became possible to mix the water first, then pump the “wet mix”, only adding air at the nozzle. This was important for two reasons. The amount of water could be controlled more closely and the aggregate size could be larger than just sand, i.e. pea-gravel. (Note: the amount of water in concrete is the single most important factor in determining the strength of concrete). Today shot-crete has become a generic term which refers to both processes. However some in the pool industry use the terms shot-crete to refer to “wet-mix” and gunite to refer to “dry-mix”.

Which is better? That’s a hard question to answer, because it depends upon several variables. In theory, dry-mix should be stronger. The key is adding only enough water for proper “hydration”, the process which causes the cement to bond to the surrounding aggregate, and not a bit more. If the nozzleman is “certified” by the American Shotcrete Association, he has been thoroughly trained and tested on how to properly control the amount of water being added in the dry-mix. However, if the nozzleman is less skilled, he may add either too much or too little water, either extreme greatly weakens the future concrete. In the “wet-mix”, the moisture content can be more closely controlled, eliminating this as a variable. It is true that the moisture content is greater than can be achieved with properly applied dry-mix, but the possibilities of too little or too much water, have been eliminated.

There is also the question of “rebound”. Rebound refers to the aggregate, which bounces or deflects off of the receiving material and lands in areas other than intended. When the small piece of sand is in mixture, it is combined with the moisten cement and if properly placed will added strength to the structure. If on the other hand, bounces off something hard, such as the wooden form or a piece of the steel reinforcement, the cement is likely to adhere to the hardened surface, but the sand, now mostly stripped of the binding agent falls elsewhere. This rebound, must be gathered up and removed, not allowed to become part of the structure. To do so would be to create a weakened structure. It is known that the dry-mix process creates substantially more rebound than the dry-mix process. If the applicators are skilled in the process this deficiency can be overcome.

In summary, gunite or the dry-mix should be stronger, but the margin for error is too great in my opinion. Having seen too many failed dry-mix applications in my years of experience, I much prefer to use a shot-crete or wet-mix process in the construction of my pools and spas.

Here are some pictues of the shotcrete process and a short video of shotcrete being applied.