Fiberglass pools are made of fiberglass-reinforced plastic; molded in the shape of a bowl. for installation; a construction crew digs an appropriately sized hole; place the necessary bombs; fill it with some sand and lower the pre-built structure into the hole. then; they level the pool; hook all the bombs and close off the area around them. usually; is surrounded by a concrete structure. * the built-in vinyl lined pools are structurally very similar to the ones described above; but they look more like conventional built-in models. the construction crew digs a hole and assembles the metal frame; plastic or wood around the perimeter of the hole. like in an unbuilt-in swimming pool; the team puts sand in the bottom of the hole and fixes the vinyl line-up to the structural wall. these pools are much cheaper than other built-in models; but not so durable. generally; the liner needs to be replaced approximately every ten years. * gunite pools are the most popular model in most of the united states. to build; the team digs a hole; puts the pump in place and assembles a structural grid with ;07/0;2 m reinforcing steel rods (frame mounted). the rods are 2.5 m apart and wired together. when the grid is in place; the team throws heavy gunite coverage; a mixture of cement and sand; around the assembled structure. the spray combines dry gunite mixed with water before being sprayed: this produces liquid concrete. the team applies the gunite and lets it dry for a week before applying the finish to the surface. the most popular finish is called plaster (a mixture of cement and white sand); but many people finish it with a special concrete dye; tile; stones or even fiberglass. gunite pools (and the like; shotcrete pools) are highly durable and can be built in any size or model. * reinforced concrete pools are similar to gunite pools; but they are much harder to build. instead of spraying concrete material around the structure; concrete is poured into conventional wooden forms. with the emergence of gunite methods; the construction of reinforced concrete swimming pools was left aside. in masonry block pools; the walls are built with concrete blocks. although the models of these pools are quite different; they are based on the same pumping and filtering systems. in the next sections; we'll see how these components keep the pool running. floating pool a built-in pool can look like a solid, immobile structure; however it can be compared to a boat; as it can float in the water around it. when the pool is empty; the pressure of this water can push the structure upwards and thus unbuild it. how the pressure needs to be equal on each side of the pool; this is one of the reasons why swimming pools are kept filled with water at all times. most modern built-in pools have a hydrostatic valve near the main drain to prevent damage from ground water pressure. basically; if this water pressure is great enough; will cause a slight fluctuation; which will open the valve. when the valve opens; water will enter and equal the pressure.