Building a fish pond
Scope
This leaflet deals with the construction of a pond shell in concrete.
Aspects of pond design and maintenance such as water circulation and filtration
are not included and the
reader is advised to get specialist information on these at the design
stage.
Introduction
The method described in this leaflet is suitable for ponds with a diameter
or length of up to about 5 metres.
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 con-crete 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 is thoroughly compacted to ensure that it is free of voids.
Many ponds of this type have in recent years been
constructed successfully both by professional landscapers and do-it-yourself
enthusiasts.
Making provision for rainwater runoff
Rainwater which 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
Figure 1.
Edge detail
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 Figure 1.
Figure 1: Details of edge
of pond
Excavating
Excavate in undisturbed sub-soil to the required size and shape allowing
for a total shell thickness of 40 to 50 mm. The water depth should be
at least 450 mm but 700 mm is pre-ferred for fish. The transition between
side and bottom should be curved as shown in Figure 1 - sharp corners
are difficult to build and are weaker than curves.
Lining the excavation
This is done to ensure that the mesh stays clean and concrete used for
the pond is not contaminated by soil.
Line the excavation with a coat of cement plaster about 15 mm thick using
1 part of cement to 6 parts of sand, both materials measured in a loose
state. Use enough water in the mix to produce a workable, plastic consistence.
Spacer blocks
You will need at least 16 blocks for every square metre of pond shell
including the rim. Make spacer blocks of mortar and place the head of
a 75 mm nail in the centre of each
block. Block size should be 15 mm thick by 30 mm by 30 mm. Block details
are shown in Figure 2.
Figure 2: Details of spacer block
Mortar for spacer blocks should consist
of a mixture of 1 part of cement, 1 part of coarse sand and just enough
water to produce a plastic consistence. 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.
Fix spacer blocks about 250 mm apart in both directions and by pushing
the nail through the plaster lining into the soil as shown in Figure 3.
Figure 3: Plaster lining and spacer block
Mesh
Place two layers of galvanized chicken mesh (25 mm open-ings) against
the spacer blocks to that the mesh is 15 mm 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 Figure 4.
The edges of the mesh should be over-lapped by at least 50 mm and laps
staggered as far as pos-sible. Ends of wires must be twisted under the
mesh and not be allowed to stick out.
Figure 4: Orientation of
mesh layers
Fix the mesh in position with hooks pushed
through the plaster as shown in Figure 5. The hooks can be made of 4 mm
thick wire.
Figure 5: Fixing the mesh
The mesh must be continued into the rim
at the edge of the pond.
Mixing the concrete
Concrete for the shell is mixed in the proportions:
Cement complying with SANS 50197-1: 1 bag (50kg)
Coarse concrete sand: 60l
Fine pit (plaster) sand: 20l
6 mm stone: 50l
Potassium dichromate or potassium chromate: 4g
plus enough water to give a plastic consistence similar to plaster.
Notes:
- Batching of concrete materials must be done accurately.
- Do not mix more than a one-bag batch at a time.
- Mixing may be done in a concrete mixer or by hand on a concrete slab or steel plate.
- Chromate salt (potassium dichromate or potassium chromate) is included in the mix to prevent the zinc galvanizing layer on the mesh from reacting with the cement and so impairing the bond between concrete and mesh.
Placing and curing the concrete
For a waterproof durable pool it is important to follow the steps given
below carefully.
It is important to place the entire shell in one continuous operation
to avoid creating joints which may become lines of weakness. Make sure
therefore that sufficient materials and labour are available.
Before starting to place the concrete remove any foreign objects or debris
such as leaves, twigs, soil lumps, etc from the excavation.
Concrete placing should be done in the shade if at all 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 placing progresses.
Apply immediately afterwards another 15 mm of concrete over the mesh so
that the total concrete thickness is 30 mm. It is very important to press
the concrete firmly into position to compact it thoroughly.
Wood float the concrete to give 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 placing is not done
in the shade keep the surface of the concrete moist by spraying lightly
but frequently with water using a gardener’s mist sprayer. Use only
enough water to maintain a sheen on the surface.
Cover as soon as possible with plastic sheeting to ensure that the concrete
cures in a moist state. The plastic sheeting should preferably be light
coloured – white or clear – because dark colours can cause heat
to build up in the concrete. The pond can be filled slowly two days after
placing 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.
Change the water after it has been in the pond for a fortnight before
stocking with fish and plants. (Several water changes are better.)
Quantities of materials
A one-bag batch mixed to the proportions given above should be sufficient
to do about 3 m2 of shell.
Repairs
Drain the pond before attempting repairs. Cracks in the shell can be repaired
as follows:
- Cut out a band of concrete 15 mm deep by 150 mm wide along the crack so that the crack is in the centre of the band. Do not cut through the mesh.
- Remove all loose material including dust from the area which has been cut out. if possible use an industrial vacuum cleaner to do this.
- Fix a 140 mm wide strip of galvanised mesh (25 mm or preferably 13 mm 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 1 part cement and 2 parts sand into the groove. The sand should be fairly coarse sand which has been passed through a 2,36 mm 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.
Appendix
To obtain chemicals in small quantities contact:
Chemical Traders of South Africa(Pty) Ltd
Tel: 011 828 7800
Fax: 011 828 8881
Cement &
Concrete Institute
PO Box 168, Halfway House, 1685
Tel 011 315-0300 Fax 011 315-0584 e-mail info@cnci.org.za
website http://www.cnci.org.za
Published by the Cement & Concrete Institute, Midrand, 1997, reprinted
1999, 2001, 2003, 2007
©Cement & Concrete Institute