Concrete Shortcrete and Gunnite Pool Construction, What's the Difference?

by Pool Builders on 11-16-2014 in Articles

Concrete as a building material has been around for centuries and is made by combining small pieces of natural rock and a bonding material. In ancient times the bonding materials where things like clay or gypsum but in 1824 Joseph Aspdin invented what is known as Portland cement. Modern day concrete manufactures use slightly adjusted versions of Portland cement today and it has been proven to stand the test of time for many years.

In swimming pool design there are two main performance categories that whichever version of concrete is used need to be met, Strength and Water Tightness.

"Cast-In-Place" Concrete

Cast-In-Place concrete is what most people know as traditional concrete, and is installed by building a form / frame and pouring the wet concrete mixture into the form before smoothing out the surface using tools such as trowels.

As the concrete dries it hardens into a structure, as it does some of the water evaporates from the concrete. This act of evaporation make the concrete decrease in overall volume and can result in cracks in the concrete. To minimise the risk of these cracks when designing a concrete pool engineers put joins in the concrete which absorb the pressure shifts that occur during the process.

This method of construction however will result in the most consistent / smoothest finish of the concrete which is why It is favored by many designers, but it is also the most expensive and labor intensive method.

Shotcrete

Shotcrete is a free form method of applying concrete, which allows for more complex shapes more watertight structures. It is applied by forcing concrete mix at high pressure through a hose which is sprayed onto the surface. This method allows you to use a less water dense mixture of concrete whilst still maintaining the strength and water proof nature of the structure.

The mixture is dry enough to be sprayed directly on to pool walls without the need for form work to hold it in place. This results in a stronger wall with fewer joints than the cast-in-place method and decrease's the chance of crack and leaks forming in the future. Shotcrete does however generally leave the roughest surface out of the three concrete pool construction methods.

By far there are a lot less qualified Shotcrete operators that standard cast-in-place concreters and as such you must choose the correct pool builder for the job if using this method of construction. Failure to do so can have disastrous consequences.

Gunite

Gunite concrete application is very similar to Shotcrete in that it is applied via high pressure hose and is free form, but it differs by not adding water to the concrete mix until the very last minute. Whilst Shotcrete concrete is pre-mixed before it's pumped into the hose, Gunite is dry whilst traveling through the hose and is only mixed with water once it reaches the nozzle.

Due to this last minute mixture of water and concrete Gunite has a lower volume of water in the mixture than either of the other methods and as such is far less likely to crack and needs less joins. This is highly desirable to pool builders and because of this it is becoming an increasingly popular choice.

Similarly to Shotcrete there are very few experienced Gunite application specialists and by choosing an inexperienced contractor you run the risk of cracks or leaks forming later in the life of the pool.

None of these methods are perfect, and all of them are suited to different situations, by far the most common method for residential concrete pools is Cast-In-Place concrete but there has been an increasing trend of using a combination of cast-in-place for the floor and either Gunite or Shotcrete for the walls to get the best of both worlds.

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