The Cement and Concrete Institute (CNCI) shows you how to construct a fish pond from concrete.
This method of pond construction is suitable for ponds with a diameter or length of up to about 5m.
The floor and sides of the pond are formed by a fairly thin continuous concrete shell, which is lightly
reinforced. No additional waterproofing membrane is used and the concrete shell serves as the water
container. The surrounding soil is relied on to support the shell when the pond is full of water.
For the system to work successfully it is important that the pond is constructed in undisturbed subsoil
and not in filled soil. The concrete in the shell should be thoroughly compacted to ensure that it is free
of voids. In recent years many of these ponds have been successfully constructed by professional landscapers and DIY enthusiasts.
Making provision for rainwater run-off
Rainwater that falls on the garden or paving surrounding the pond must not be allowed to run into the pond. The area immediately around the pond must therefore be sloped away from the pond as shown in picture 1.
It is advisable to create a nearly horizontal (but sloping down away from the pond) rim around the edge of the pond. This will serve to strengthen the shell. The rim can be left exposed, but will probably look better if it is covered with bricks or stones laid in mortar. See picture 1.
Excavate in undisturbed subsoil to the required size and shape, allowing for a total shell thickness of 40-50mm. The water depth should be at least 450mm, but if you intend to add fish to the pond, a depth of 700mm is preferred. The transition between side and bottom should be curved as sharp corners are difficult to build and are weaker than curves.
Lining and evacuation
This is done to ensure that the mesh stays clean and concrete used for the pond is not contaminated by soil. Mix one part cement and six parts sand. Use enough water in the mix to produce a workable, plastic consistency. Then line the excavation with a coat of cement plaster about 15mm thick.
The spacer blocks are placed onto the 15mm thick plaster lining after which the wire mesh will be placed against the spacer blocks. These spacer blocks should be 15mm thick and 30mm x 30mm in size. Make them from mortar and place the head of a 75mm nail in the centre of each block. The mixture for the spacer blocks is one part cement to one part coarse sand and just enough water to produce a plastic consistency.
For every square metre of pond, 16 spacer blocks are required. After casting the blocks, cover them with plastic sheeting for 24 hours to prevent drying out. Then cure them by keeping them under water for at least a week. Once the spacer blocks have fully cured, fix them about 250mm apart in both directions by pushing the nail through the plaster lining into the soil.
Place two layers of galvanised chicken mesh with 25mm openings against the spacer blocks. This will ensure that the mesh is 15mm clear of the plaster lining. The twisted wires of the two layers should, as far as possible, be at right angles to each other, as shown in picture 4. The edges of the mesh should be overlapped by at least 50mm, and laps staggered as far as possible. Ends of wires must be twisted under the mesh and not be allowed to stick out. The mesh must go all the way into the rim at the edge of the pond. Make hooks from 4mm-thick wire and use them to fix the mesh in position by pushing the hooks through the plaster. See picture 5.
Mixing the concrete for the shell
The concrete mixture for the pond’s shell should consist of one 50kg bag of cement complying with SABS ENV 197-1; type CEM I or CEM IIA, 60 litres coarse concrete sand, 20 litres fine pit (plaster) sand, 50 litres of 6mm stone, 4g potassium chromate or dichromate and enough water to give the mixture a plastic consistency similar to that of plaster. This one-bag batch mixture will be sufficient to cover three square metres of the shell.
The batching of concrete materials must be done accurately. Do not mix more than a one-bag batch at a time. The mixing can be done in a concrete mixer or by hand on a concrete slab or steel plate. The reason for the potassium chromate or dichromate is to prevent the zinc galvanising layer on the mesh from reacting with the cement and so impairing the bond between concrete and mesh.
Placing and curing the concrete shell
It is important to place the entire shell in one continuous operation to avoid creating joints that may become lines of weakness. Therefore, make sure you have sufficient material available.
Before you start laying the concrete, remove any foreign objects or debris, such as leaves, twigs or soil lumps from the excavation. The concrete placing should also be done in the shade if possible.
Using a plasterer’s trowel with the corners rounded off, press the concrete through the mesh, filling up to the mesh and removing the wire hooks as laying progresses. Once the mesh is filled up, apply another 15mm of concrete over the mesh so that the total thickness of the shell is 30mm. It is very important to press the concrete firmly into position to compact it thoroughly.
Next, wood float the concrete to give it a fairly rough texture. If a smooth texture is required, use a steel trowel, but only after the concrete has started to stiffen. Do not trowel if a layer of water is covering the surface of the concrete.
If the placing of the concrete cannot be done in the shade, keep the surface of the concrete moist by spraying it frequently lightly with water using a mist sprayer. Use only enough water to maintain a sheen on the surface. Cover the concrete with plastic sheeting as soon as possible to ensure that the concrete cures in a moist state. The plastic sheeting should preferably be light coloured (white or clear) as dark colours can cause heat to build up in the concrete.
The pond can slowly be filled two days after placing of the concrete, but the cover should remain in position until filling is complete. Parts of the concrete shell above the water level should be kept wet and covered for two weeks. If filling has to be delayed, keep the cover over the shell for at least two weeks during which time the concrete should be kept visibly wet. After two weeks, change the water in the pond before adding fish and plants.
Before attempting any repairs you first have to drain the pond. Cracks in the shell can be repaired as follows:
* Cut out a band of concrete 15mm deep x 150mm wide along the crack so that the crack is in the centre of the band, but do not cut through the mesh.
* Remove all loose material including dust from the area that has been cut out.
* Fix a 140mm-wide strip of galvanised mesh with preferably 13mm-wide openings to the existing mesh where the concrete has been cut out.
* Prime the back and sides of the groove with a mixture of cement and water made to the consistence of thin cream. The priming coat must be applied in a very thin layer. Do not pre-wet the concrete before priming.
* Immediately afterwards, before the priming coat can dry out, pack a fairly dry mixture of one part cement and two parts sand into the groove. The sand should be fairly coarse sand that has been passed through a 2.36mm sieve. Compact the patch using an orbital sander, fitted with a smooth steel plate on the surface, filling any low areas and scraping off high areas.
* Keep the patch damp until the pond is filled.