Concrete linings

Concrete linings have been used for all sizes of artificial lakes and pools, from vast reservoirs of 20 hectares or more, down to small formal garden pools of a few square metres. An early use of concrete was in dewpond construction. Pugsley (1939) discovered that a number of pre-First World War dewponds were lined with rough concretes such as clay and lime mixed, flints in mortar, or alternate layers of cement and chalk or flints.

Advantages

1. Concrete linings are extremely durable if the original construction is sound. They can be dried out or exposed at the shoreline without harm, and are affected only slowly by erosive forces.
2. Concrete linings are vandal proof. This is an important consideration for ponds in school grounds and other urban areas.
3. Concrete lined ponds can be maintained without fear of damage to the lining. It is possible to wade into the pond and use tools such as shovels and cromes, which may damage a flexible or clay lining. Clumps of emergent vegetation can be pulled out without damage to the lining. Concrete linings are therefore useful for ponds which are intended to be maintained at a certain point in their succession.
4. If mixed properly, concrete has consistent quality, and it can be obtained at any time.

Disadvantages

1. More skill is needed to install concrete than other linings. Ponds bigger than about 8 x 5m will need reinforcement, and an engineer should be consulted on the design and construction. Concrete sets quickly, and either needs to be laid ‘all in one go’, or in sections with special waterproof joints.
2. The cement used in concrete contains chemicals that are harmful to aquatic life, and after construction a seasoning period is necessary during which the pond must be filled and emptied several times.
3. Concrete linings are difficult to repair if they are badly made, or if they crack due to settlement. Various products are available for the repair of non-structural concrete.
4. Although concrete linings can in theory be made to fit almost any shape, complex outlines and internal contours are much more difficult to construct than simple shapes. It may be easier to make a dish shaped lining, and then use stone and other materials to build marginal shelves and reefs which rest on the concrete lining.
5. The permanence of a concrete lining may be a problem, not an advantage, if for example, use of the site changes.

For most waterproof linings, from small garden ponds to large installations, flexible materials are now used rather than concrete. This is because they are much cheaper and easier to lay, and because technological development has produced materials which are durable for 20-30 years or more. Concrete, however, still has the great advantage of being vandal-proof, and allowing management without fear of damage to the lining. Where vandalism is a problem, the technique described below is recommended. In this type of structure, the concrete is not itself waterproof, but merely acts as a protective layer over the flexible liner. Concrete is not otherwise recommended. For small ponds a flexible liner is easier and cheaper, and for larger ponds, technical difficulties of laying concrete are such that the design and construction should only be done by specialists.

Ready-mix concrete

Ready-mix concrete supplied by lorry is the best option for the vandal-proof ponds described below. Ready-mix has the advantage that the hard work of mixing is done, and the product is of consistent quality. It avoids having to handle or store cement, sand and aggregates and often works out cheaper. However, vehicle access to the pond site must be considered, as well as sufficient labour to spread the concrete before it sets. For school and community areas, this labour requirement usually fits in well, as it is easier to organise a big voluntary effort on one day with the goal of completion, rather than a hard slog by a few people over a longer time. Suppliers of ready mix concrete are numerous in most areas, and you should be able to find one who can fit in with your requirements for mix, access and delivery time. Extended chutes and pumps are available for sites where access adjacent to the pond is not possible. Some firms will supply and mix materials on their lorry on site, while the customer barrows and lays the concrete, with about half an hour allowed to take delivery of a cubic metre.

The timing of the delivery is crucial. The pond excavation, plus any lining, must be finished, and the volunteers ready for action the moment the lorry arrives. You then need enough time to complete the laying of the concrete that day. On the other hand, the excavated surface or lining such as sand or polythene must not be ready too early, or it may get disturbed by weather, vandals or animals. If an excavation gets filled by an overnight storm before the lining is laid, you’ve probably chosen the wrong site!

Some suppliers will not deliver between mid-day Saturday and Monday morning, which may limit your options for a weekend project.

Vandal-proof ponds

A technique has been developed by Hampshire County Council, working with The Conservation Volunteers, to construct ponds which are completely vandal and leak proof. Concrete ponds are vandal proof, but difficult to construct and liable to leak. Flexible liners are waterproof and easy to lay, but prone to vandalism and damage during pond maintenance. The technique uses a flexible lining for holding water, and a concrete lining on top to protect it against deliberate or accidental damage. The concrete layer is not designed to be waterproof.

This is a ‘belt and braces’ technique, and uses various specialist materials which make the method seem complicated. An ‘environmental audit’ may question the wisdom of using such a quantity of materials to create a feature which in other substrates would occur naturally. However, the technique has so far proved totally reliable in creating valuable environmental and teaching resources within school grounds and other areas where ponds would not otherwise occur. Although complex, the bulk of the work can be done, and in fact has to be done, within one day, using voluntary labour with skilled supervision. Over 60 ponds have so far been created.

The following section gives detailed information on procedure and the organisation of volunteers, which may also be relevant to pond construction projects using other types of artificial linings.

The layers of the construction are as follows, listed from the top layer downwards, as shown in the table below.

Note on materials

Hampshire County Council recommend the Hi-Lastic polyethylene liner. Other good quality waterproof liners may also be suitable. For suppliers see here.

Armater is a honeycomb lattice material, supplied in sections 12 x 12.5m. The size required is 2m smaller in both dimensions than the polyethylene liner. Any extra can be sold to another school.

The concrete mix is 1:12 (1 cement:12 ballast), known as ‘kerb mix’. Only sufficient water to mix should be added, as too much water will cause the mix to slip under the bottom of the Armater cells, leaving a pool of concrete in the middle. Order from supplier of ready-mix.

Further information on the materials is given below.

Design and planning

To create a pond which is ecologically stable as well as diverse, and suitable for practical teaching purposes, Hampshire County Council recommend that school ponds should be a minimum of 60 square metres, and do not grant aid for anything smaller than this.Anything larger than about 140 square metres is difficult to construct in one day.

The preference is for a maximum finished depth of 750mm, with the deepest part off centre to give a variety of gently sloping profiles. Steps and planting ledges are avoided for safety reasons, and because they are difficult to construct usingArmaterfabric.Thedesigngivesthefullrangeofdepths up to 750mm for marginal plants and other organisms. As marginal vegetation can be removed as necessary by hand pulling, the shelf down to deeper water is not needed as a method of a control of marginal vegetation.

All weather access and dipping platforms to accommodate 6-8 children are included in the design.

Organisation

Order all materials in advance, and confirm the delivery date with the suppliers. Liners must be stored in a secure location, and away from sunlight which can damage them. Have a site meeting with the supplier of ready-mix concrete prior to delivery, in order to check the access. Timing of the delivery is crucial.

Construction of a 60 square metre pond needs at least 12 people for the day, and up to 20 can be usefully occupied. They will need to wear stout waterproof boots, and long sleeved tops to protect arms against concrete mix.

The following tools are needed:

Excavation

Mark the outline of the pool, checking the levels carefully.

If possible, arrange for the excavation to be made on the day before the lining is to be constructed, in order to lessen the chance of disturbance to the excavation, or of it collecting rainfall. JCBs or other excavators should only be driven by skilled operators, with attention being paid to the safety of other users of the site. It should take about 4-5 hours to excavate a 60 square metre pond. If possible, the project officer should stay on site throughout to supervise and sort out any problems. A trench around the excavation, normally made to secure a flexible lining, is not needed, as the concrete secures it. Retain sufficient subsoil for the rooting zone within the pond. The rest of the spoil should be removed for use elsewhere on the site, or tipped into skips for removal off the site.

Using  a  surveyor ’s  level,  check  that  the  rim  of  the excavation is level all around. Note where any minor adjustment will need to be made by hand, or by the placing of the liner. Remove protruding stones or other debris from the excavated surface.

Construction

1. Brief volunteers on procedure, tool use and safety considerations. Stress the need to avoid all unnecessary trampling on the layers as the construction proceeds.
2. Lay a single layer of old carpets, first removing any tacks. Do not use rubber-backed carpet as this may give off a gas. The carpet prevents stones from working their way up through the sand layer and damaging the lining. Alternatively, geotextiles such as Terram or Bidim can be used. Newspaper is not very successful as a substitute, as it is difficult to get even coverage on still days and impossible when it is windy!
3. Lay the sand, raking it to an even 75mm layer, and pressing it up against the edge of the excavation.
4. Open up the liner away from the pond and check for holes. This also dispels static electricity. Crease it up again with the outside edges accessible, carry it to the pond and lower it onto the sand.
5. Repeat for the geotextile and polythene sheet. Don’t cut off any excess until the end of the day. Aim to be at this stage by 11.30am for a day’s project.
6. Open up the Armater fabric and cut to size as necessary, using a craft knife or fabric shears. Lay it onto the polythene sheet.
7. The ready-mix is delivered and dumped on a large piece of building membrane or similar as near as possible to the pond. Form a bucket chain. Don’t overfill the buckets, but keep to a weight that the least strong person in the chain can safely manage.
8. The chain leads out to the middle of the pond, with the people at the end of the chain standing with each foot in a ‘cell’. Don’t get the toe of the boot under the cell wall, or the concrete will slide underneath and you will end up with a very shallow pond! Great care must be taken to avoid back injury when handing the buckets along the chain, as movement of the feet is restricted by the cells, and the polythene surface is slippery. Avoid any action which twists the back, and don’t take a loaded bucket until you’re perfectly balanced. Start filling the cells from the centre, then radiate out in ‘bulges’ as shown. This minimises trampling, whilst keeping the honeycomb fabric in shape. It should take 2-3 hours of hard work to shift 4.5 cubic metres of ready-mix.
   
9. Before you fill the outermost ring of cells, trim off the excess Armater, leaving complete cells. Trim butyl.
10. Fill outer cells, and finish with a collar of concrete to the edge of the excavation. Edge with turves to give a neat finish.

Filling

Water dissolves solutes from the new concrete, which raises the pH of the water, and will limit the range of organisms which the pond can support. Although ponds of a high pH, above 8.5, occur naturally in limestone areas, a pH below 8.5 is usually suitable for most ponds to encourage a wide range of organisms.

Given time, the water will stabilise, but most pond creation projects for schools and community areas require that planting is done as soon as possible after filling, in order to create a ‘finished’ pond. Don’t put any soil into the pond until the pH is correct, or the solutes will then become locked into the soil, and almost impossible to remove.

Various products are available to treat or waterproof concrete. Plasticisers can be added during mixing, or sealants painted on after laying. However, these are not always reliable, and the safest method, although rather tedious, is to fill and empty the pond several times until a suitable pH is reached.

Initially fill the pond, and leave it for about two weeks. Check the pH, which may be as high as 12, depending on the source of the aggregate. Pump the water out and refill. Leave for a week and check the pH again. If it’s below 8.5, the loss of solutes from the concrete is low enough to proceed. Pump the water out, put in the soil layer and any features, and then refill. If it remains above

8.5 at the second refill you can proceed as above, but a few weeks after planting pump out half the water and then top up, preferably with rain water. Water pumped from the pond should be allowed to soak away into the ground. It should not be discharged into the sewage system or into streams or other waterways.

For further notes on initial management of new ponds are given in Chapter 8 – The planting and management of new ponds.