Using Concrete to Make Your Pond
Mark out with pegs the area to be covered by the pond. Check the levels with a spirit level to make sure that the whole structure will be set into the ground without any section projecting above the surface; if this is likely to happen, some additional soil should be used to build up the space to cover the concrete. The hole should be dug out allowing for the thickness of the walls and base, which depends on the shape, depth and size of the pond. Unless you are constructing the pond in solid rock you should allow at least 10cm (4in) thickness for a small pond and 15cm (6in) for a larger one.
Where possible incorporate some reinforcing material: chicken wire is suitable for small ponds, but larger ones need reinforcing with something more substantial, and the fairly complicated large ponds with several levels really require the services and expertise of a fully qualified structural engineer to work out the stresses. This is very important, for even a small pond can hold several tons of water.
Having excavated the hole, check it again for the levels. Prepare the reinforcing and spread the concrete over the base, adding the reinforcing where necessary. The whole is tamped down, to squeeze out any air bubbles, and then allowed to mature for a day or so. The shuttering for the walls is placed into position; this is substantial boarding to hold the wet and plastic concrete in place until it is dry. The concrete is poured into this mould in one action, vibrated to allow air bubbles to escape, and left to set. The shuttering can then be removed, and the base and walls covered with a slurry of sharp sand and cement rubbed well into the surface and allowed to dry thoroughly.
Fill the pond with water, and leave it for a day or two to absorb the chemicals from the cement, then pump it out and refill, and repeat this several times. An alternative is to treat the surface with a proprietary sealing product that will save all the filling and unfilling; water is then poured in and left to mature. Allow the natural and added chemicals to disperse before you introduce any plants or fishes. This will prevent problems later on.