Concrete ponds are usually built to hold larger volumes of water than most liner based ponds, and are commonly built up to and beyond 2 meters deep.

They can be raised from the ground, be built with sheer sides and have viewing windows installed.Concrete ponds are much longer-lasting and durable than regular liner ponds and allow you the option to install a more complex and efficient filter system.They are however, a substantial engineering project. They can be much more expensive and complex to construct. They usually require the assistance of qualified professionals.

The planning, location and design principles for a concrete pond are the same as those of a liner pond. There are many varying designs and building methods which can be employed in a concrete pond’s construction and these can alter from builder to builder, but the more common ideas and practices are as follows;

Digging the Pond

Concrete ponds are usually quite large and deep and therefore a large hole has to be dug and a lot of earth needs to be excavated. The easiest method is to hire a small mechanical digger or JCB, although access into the garden can sometimes be an issue.As the pond walls will probably be quite sheer there is sometimes the problem of collapsing. If the soil is quite dry or loose the walls may need to be temporarily held in place with sheets of plywood, pinned in place with long stakes bedded into the pond base.There is also the removal of the earth to consider. Skips can be quite expensive, but there tends to be only a slight cost increase in the price for larger skips.

The Pond Foundations

Once the hole has been dug, the soil under the base needs to be well compressed. In most cases hardcore is laid and then compacted with a mechanical vibrator.Before the concrete base is poured in, any pipes which connect bottom drains to the filter if a gravity fed filtration system is being installed, and the steel mesh used for reinforcing are laid into position.

The steel reinforcing mesh will substantially increase the strength of the base and vastly reduce the chances of cracking. The concrete base is normally around 30cm thick, therefore the reinforcing mesh should be suspended about 15cm from the ground so that it runs through the middle of the base. The easiest method is to lay the mesh on widely dispersed bricks.

When the concrete is poured it should be spread evenly across the base, but shouldn’t be tamped down too hard as this will bring liquid to the surface of the mix and cause it to weaken when it sets.

Building the Walls

The block-work walls can be built directly onto the new concrete base once it has hardened. The most popular materials used when constructing the walls are concrete blocks.Dense concrete blocks and hollow concrete blocks are heavy, give a good thick wall with plenty of strength and are relatively easy to build with. In a larger concrete pond steel rods can be inserted into the hollow concrete block’s cavities and backfilled with concrete, this gives immense structural strength to the pond walls. Any pipe-work required for returns, or items such as waterfalls, viewing windows and in-built skimmers should be considered when building.When the construction is complete any gaps between the sunken outer side of the pond wall and the ground should be backfilled with hardcore or concrete.The visible outer side of the wall on a raised pond can be rendered, clad or pebble dashed. Alternatively, an exterior decorative wall can be built to conceal the concrete blocks. Brick-work, slate or stone walls can be erected to give the pond its desired finish, and the cavity between the outer wall and the block wall can be insulated.

Benching the Base

To improve the efficiency of the ponds bottom drain the pond’s base can be benched and corners rounded. This ensures that there are no areas of the pond where waste and debris may accumulate. It also aids the flow of the pond’s filtered water.Corners are rounded by cementing bricks up the ponds depth and filling the edges with concrete.To bench the base, or angle it towards the bottom drain, angled slats are screwed onto the base and back filled with concrete and allowed to set. The walls and floor are now ready to be rendered.

Rendering

A render is the final layers of mortar which are skimmed onto the walls to give a smooth, professional finish. There are usually two coats of render used, the first layer is a normal mortar mix of three parts sand to one part cement, this is applied to the walls and base in the same manner as plaster to an indoor wall, the surface is then roughed so that the second coat will adhere.

The second layer of mortar should contain reinforcement fibres. These are 6mm long polypropylene strands which are mixed into the mortar to add strength and dramatically reduce cracking. Approximately 20g of fibres should be added to every Kg of mortar. Once the final render has dried, any protruding fibres should be melted away with a propane torch or sanded off to give a smooth inner wall.

Waterproofing

Concrete is porous, it absorbs water and allows it to seep through and away from the pond. It also contains lime which, if allowed to leach into the pond water, will adversely affect the pond’s pH.There are a few options available when choosing how to waterproof a concrete pond. The easiest and usually cheapest option is to fit a flexible pond liner. The pond liner can be fitted in the same way as it would if you were building a sunken liner pond. This can be fairly problematic if there are bottom drains, return hoses, etc built into the pond base or walls as the pond liner will have to be cut and securely sealed around any fixtures. Also, as most concrete ponds tend to be fairly square the liner may have to be pleated, this could create hard-to-clean folds in the liner which may build up with debris or harbour parasites. If the walls have not been rendered or are not entirely smooth, it is usually advised that a more heavy duty underlay is used to protect the liner from the rough brickwork. The liner itself may have to be fairly heavy duty in accordance with the volume of water that the pond will hold.Another method is to fibreglass the pond interior. Fibreglass gives an extremely tough finish and it is probably the strongest and most durable option available when building a pond. It is, however, very expensive and fibreglass should only be applied by professionals. You should use a company that already has experience in fibreglassing ponds. This is important as varying resin solutions are used by different companies who fibreglass cars and boats and the solutions they use could release toxins into the finished pond.

A more popular method of waterproofing a concrete pond is to paint the final layer of render with G4 concrete pond sealer. G4 seals the concrete, drying to form a resin-like barrier which waterproofs the pond and stops the concrete’s lime from leaching into the water once the pond has been filled. It can also be painted onto concrete flag stones, waterfalls and statues to make them safe for use in the pond. The general instructions for applying G4 are;

• Paint the G4 directly onto fully cured concrete using a standard paint brush (the concrete should be allowed to harden for 21 to 28 days), and you don’t have to apply a primer. The concrete can be weather-damp, but ideally it should be dry.

• The whole pond will need to be painted in a day. Try to pick a day with a good forecast. Once the first coat has been applied and starts to feel tacky, usually after 60 to 90 minutes, the second coat should be applied and then the same with the third and possibly fourth. You should not allow longer than 4 hours to pass between painting coats as the G4 will then dry in separate layers and be much less durable.

• Once you have painted the final coat, the G4 should be left to fully cure for 72 hours.

G4 is available in two finishes; black and clear. For a clear finish the pond will need 3 coats of Clear G4, a total of 550g per square metre of surface area to be painted. For a black finish the pond will need 2 coats of Clear G4 and a further 2 coats of Black G4, a total of 410g of clear per square metre and 400g of black per square metre of surface area to be painted.To calculate the internal area of your pond and the quantity of G4 required click here.