Concrete swimming pool construction: shotcrete or formwork?

Wikipedia defines shotcrete as concrete or mortar conveyed through a hose and pneumatically projected at high velocity onto a surface; the technique was developed by Carl Akeley and first used in 1907. It has two sub-methods: dry mix (gunite) and wet mix. In the dry mix the dry components are conveyed through the hose and the water is added at the nozzle by the operator; in the wet mix ready-mixed concrete is pumped and compressed air is supplied at the nozzle. The source states that the wet mix produces less rebound, waste and dust than the dry mix and places a larger volume in a shorter time.

The source states that shotcrete shows high compressive strength, good durability, watertightness and frost resistance, and lists swimming pool construction among its applications. The advantage of formed casting is measurability: the concrete is sampled as it arrives from the batching plant, and the vibration time goes into the record. In commercial pools and square-cornered geometry we prefer formed casting; in free-form villa pools we prefer wet-mix shotcrete.

The concrete class is written in EN 206 notation: C30/37 is concrete whose 28-day characteristic compressive strength is 30 N/mm² on a cylinder sample and 37 N/mm² on a cube sample. The source states that 28-day compressive strength is the standard specification measure of concrete quality and that, against its high compressive strength, the tensile strength of concrete is markedly low; that is why the shell carries steel reinforcement in the tension zones. For a pool shell C30/37 is the lower limit; on plots in contact with salt water or with a high water table it goes up to C35/45.

The water/cement ratio is the mass of water in the mix divided by the mass of cement. The source writes that full hydration needs about 0.35 mass of water per cement, that typical ratios lie between 0.40 and 0.60, and that mixes above 0.60 give porous and weak concrete. Excess water increases porosity and permeability, accelerates carbonation and reinforcement corrosion, and multiplies shrinkage cracks. Because watertightness is more decisive than strength in a pool shell, our specification keeps the water/cement ratio below 0.45; workability is obtained with a plasticiser admixture, not with water.

The source puts it in one plain sentence: all concrete structures crack to some extent, due to shrinkage and tension. The task of pool technique is to keep the crack thin and closed enough not to pass water, rather than to eliminate it. The tool for this is crack control reinforcement added on top of the load-bearing reinforcement set by calculation: closely spaced bars in both faces and both directions produce many hairline cracks instead of one wide crack, and a hairline crack closes itself with the crystalline admixture.

Double-layer reinforcement is standard in a pool shell: the floor and walls are reinforced with separate meshes on the inner and outer faces, and lapped corner bars are used at corners and at the floor-wall junction. Extra reinforcement is placed at changes of section such as steps, benches and depth transitions; cracks start most often where the section narrows. The surround of parts that pierce the shell, such as skimmers, inlets and the main drain, is wrapped with diagonal bars running out from the corners of the opening.

In a concrete pool shell, water escapes at three places: the cold joint, the form tie hole and the pipe penetration through the shell. The cold joint is the line of junction between the floor concrete and the wall concrete; when the two pours are made on different days, no monolithic bond forms between them. The remedy is a waterstop embedded in the floor concrete and running into the wall concrete; PVC waterstop or water-swelling bentonite strip is used. In shotcrete, making the floor and the wall on the same day removes this joint.

Form ties are the rods that connect the two faces of double-sided formwork; when removed, they leave holes in the shell. In pool formwork, ties with waterstop plugs are used, or the holes are sealed from inside out with epoxy mortar. At pipe penetrations a sleeve and a waterstop flange are used; the pipe is not embedded directly in the concrete. On top of these three details come a waterproofing admixture inside the shell and a cement-based flexible waterproofing layer on top of the shell; the layers are described separately in the waterproofing article on this site.

Curing keeps the moisture of the concrete during the first days so that hydration continues. Concrete poured at a 0.45 ratio without excess water cannot complete hydration if its surface dries out, and shrinkage cracks open on the surface; on a pool site the shell is kept wet for seven days and covered with wet sheeting in heat above 30 °C. The water-holding test is done after curing and before finishing: the pool is filled and the level loss is monitored for 48 hours with evaporation deducted.

Shotcrete was developed by Carl Akeley in 1907; the dry-mix method was registered under the name gunite, and by 1951 shotcrete had become the official general term covering both methods. The applications Wikipedia lists are tunnel and mine lining, underground car parks and excavation support, slope stabilisation and swimming pool construction. The reason it entered pool technique is plain: it can produce a free-form, watertight and frost-resistant shell with single-sided formwork.

Formed casting is the direct adaptation of reinforced concrete building technique to the pool; the knowledge of waterstops, form tie plugs and crystalline admixtures from water tanks and treatment structures was transferred to the pool shell by this route. The rule common to both methods is this: the concrete pool shell is a structure, the concrete class and the water/cement ratio are written in the specification, the joint and penetration details are drawn before the pour, and on site curing water is used, not mixer water.