Most swimming pools use gunite, a form of concrete that has less water than cast-in-place concrete and shotcrete. Both shotcrete and gunite are applied to surfaces pneumatically, that is, with the use of high pressured spray guns.
By itself, concrete resists compressive forces well, which is why they’re often used in foundations and support structures. However, a filled pool exerts both compression and tension across its coverage. Concrete walls can buckle under tensile forces exerted by the wide base.
This is why cast-in-place concrete pool construction never starts without rebars. These steel bars’ weakness against compressive forces is compensated for by its strength against tensile forces. Reinforced concrete, as the combination is called, becomes strong against both push and pull forces. The rebars are arranged in a grid to double its tensile strength.
The relative dryness of gunite, on the other hand, makes it the ideal material for pool walls, as it provides strength to the aggregate in concrete. It also makes for a highly watertight structure, resulting in less pool seams, and thus a more durable construction. Because of the way they are applied, gunite and shortcrete are considered “free form” applications, which means that they can be used in various pool shapes, including curved ones.
Cast-in-place concrete still produces the smoothest finishes, which makes it ideal for pool floors. For the walls however, gunite, and to a lesser extent, shotcrete provide the necessary strength and flexibility.