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Building a Concrete Pond

By: Dr Gareth Evans - Updated: 10 Oct 2014 | comments*Discuss
 
Formal Koi Pond Shuttering Block-work

Although concrete is now less commonly used to build ponds than it once was and certainly makes more demands on the builder than the alternatives, it does have some definite advantages which still make it a material worth considering. Particularly when it comes to some kinds of more specialist water features – especially those needing vertical walls such as traditional formal or koi ponds – there is not much to match the strength and rigidity of concrete.

In the days when concrete was used extensively for pond construction, the standard method involved pouring the mix into a mould made with wooden shuttering. While this method allows for irregular shapes to be created, the concrete is prone to cracking and the rise of much cheaper liners and pre-formed ponds has almost entirely superseded this approach for “natural-look” water features. Most of today’s concrete ponds are built using breeze blocks, which are then skimmed with a sharp sand/cement/fibre glass mix.

Starting the Construction

The construction process for this type of pond is relatively straight-forward, if somewhat time-consuming. The first step is to dig out the hole to the required size; since most concrete ponds are square or rectangular, it is worth taking the time to mark out the outline making sure that the angles are true, the lines are straight and the walls as near upright as possible. Getting this right at the outset can save a lot of time and frustration in the long run since as the digging progresses, it can be surprisingly easy to drift off course. You should aim to go down around 5 - 6 inches (12 - 15cm) below the finished pond's intended depth, to allow for the base.

Once the excavation for the actual body of the pond is complete, you’ll need to dig a trench for the foundations – about 4 inches (10cm) deep and 8 inches across (20cm) – around the outside of the hole to support the block-work. Once this has been completed, it needs to be filled with concrete and then allowed to dry thoroughly.

Next, the soil at the base of the hole should be raked level, a layer of sand added and then tamped down firmly to compact it well, making sure that it finishes level with the foundations. Lay a 3inch (7.5cm) coat of concrete over the whole area and allow it to dry completely, before adding galvanised mesh for reinforcement and then finish off with a second 3inch (7.5cm) layer of concrete. Make sure it has dried out thoroughly before continuing. The whole base now needs to be skimmed with a half-inch (12mm) coat of cement containing reinforcing fibres. The purpose-made reinforcement fibres are designed to help prevent small cracks from appearing and makes the whole structure less likely to leak; they can be bought from most builders’ merchants and specialist water garden suppliers.

After 24 hours, once the cement base has had a chance to dry, the next step is to build the block-work walls, filling both the hollows in the blocks themselves and any gaps between them and the surrounding soil with concrete. A course or two of house bricks at the top of the block wall finishes the whole thing off neatly, providing a level edging for the pond.

Finishing Off Your Pond

The walls need to be left to dry for at least another 48 hours, after which time they can be dampened slightly and then skimmed with a half-inch (12mm) layer of fibre-reinforced cement to match the base. The final part of the real construction work calls for an edging layer of bricks or slabs to be mortared into place around the top of the wall, allowing an over-hang of a good 2 or 3 inches (5 or 7.5cm).

After a further 48 hours, the last step is to paint the inside of the structure with a good quality waterproof sealant – principally to stop the lime in the cement from leaching out into the water and causing problems for the plants and pond-life. Wait a few more days and then fill the pond. It is always a good idea to check the pH of the water in a concrete pond to make sure it is around neutral – pH7 – before you start planting up or introducing any fish; if the pH is much higher than this, the conditions will be too alkaline for pond-life to thrive. If this does occur, it doesn’t automatically mean that your attempts at sealing the concrete have failed, so it is worth waiting a few more days and testing again as the problem can sometimes be a short-lived one.

Building a concrete pond is undoubtedly a far larger undertaking than making one with any of the more common types of liner, but for some types of pond, it remains the best material for the job. Although they take longer to complete and require much more in the way of effort, if you take your time and proceed carefully, a very durable and good-looking pond can be yours.

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[Add a Comment]
@David. It's probably an insufficient enough amount that it won't even matter, but I'd use a neutraliser if you want to be 100% sure.
Anthony - 10-Oct-14 @ 2:06 PM
I am in the process of re-lining my pond with PVC liner. I have cemented house bricks around the edge and in doing so, pieces of cement fall into the base and stain the sides and floor. I will remove all the hard pieces and wash the base and walls, but this will always leave a smear of concrete on the sides and floor. Is there a risk to fish (Koi) or is some treatment required to neutralise any traces of the cement? Your respopnse would be appreciated Thank you David.
David - 9-Oct-14 @ 2:48 PM
Question i have what was originally a raised flower bed, solidly built and converted to a pond with a flexible liner to a pond. I would like to convert this to a pond with concrete base and render the walls and finally a painted surface. I was thinking putting re reinforcements in the base and running these up the walls approx 6"so once rendered it would be all one. don't really want to knock it all down and start again do you this this is feasible? or would it be better to stick with a liner??
Chris - 8-Jul-14 @ 12:56 PM
Hi, I really appreciate the info provided here. I just got a job to manage an integrated farm. I am an Agricultural and Environmental Engineer. A pond for catfish was part of the farm plan, in addition to poultry, vegetable garden, dog pens, brooding house etc, of all, I have never built a concrete pond for any one yet in the past. I consider the information here really helpful. Thanks. Engr. Ahiaba V.U
vicky - 9-Jun-12 @ 9:14 AM
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