Concrete Garden Pools

Although concrete is a well tried and traditional method of pool construction, the more modern techniques, using flexible or pre-formed liners, require less skill and are likely to present fewer problems. However, concrete does offer a degree of permanence that is lacking with other construction methods. Moreover, such a structure may also blend readily with other more traditional landscape materials.

THE PROPERTIES OF CONCRETE

The successful use of concrete depends upon a very clear understanding of its properties, especially while under construction, and the limitations which may be placed upon it by the complexity of a pool’s design. If the pool is to have any vertical elevations or steeply sloping sides, shuttering will be required to support the concrete while it sets. Reinforcing rods or mesh will also be needed, and a knowledge of the various stresses and strains that the pond will be expected to endure will determine the type and quantity of reinforcing material to be used.

Wet concrete also remains workable for only a short time. It can go off within a few hours and become useless. When constructing a pool, it is essential that all the concreting is completed within a day. If you intend using ready-mixed concrete, it will arrive irrespective of the weather in one consignment and will have to be dealt with quickly. If the weather is very wet, a major problem can arise in trying to prevent the mixture from being diluted and washed out. It is extremely difficult to lay concrete and protect it at the same time. Mixing concrete yourself with a hired mixer will allow you to select a suitable day to safely undertake the task. However, it will be much more difficult to ensure a uniform mix, especially if waterproofing compound is being added.

WATERPROOFING

Concrete pools demand very careful waterproofing. Even the best laid concrete can be porous and permit water to escape, and one method of preventing this is to install a polythene membrane beneath the concrete. Alternatively, you can add a waterproofing compound when mixing the concrete. This will ensure that there is no porosity, although it cannot legislate for faulty workmanship and cracks.

Problems will arise from the escape of free-lime into the water, which is harmful to fish and can cause problems to some aquatic plants if present in excessive amounts. This will not be prevented by the inclusion of a waterproofing compound at the time of mixing However, the face of the concrete can be treated with a special product that not only traps the free-lime, but also seals the concrete by a process known as internal glazing.

Winter damage is another potential hazard for the concrete pool. It is vital to take precautions against this, for even the most professionally laid concrete can be damaged during a really severe winter. The problem is caused by the exertion of pressure by ice on the walls of the pool. If no relief is provided, this can cause fracturing of the concrete. To prevent this damage, float a rubber ball or piece of wood in the pool from the time of the first frosts, as this will absorb the pressure and protect the walls.

Constructing a Concrete Garden Pool

DIGGING THE HOLE

The hole for a concrete pool must be at least 15cm (6in) larger all round than its finished size to allow for the thickness of concrete required. It should be set out and dug in the same manner as an excavation for a flexible liner pool. If it is to have sides that are vertical or slope at an angle greater than 45 degrees, wooden shuttering or formwork must be created to support the concrete while it sets. This must be of sturdy construction to resist the considerable pressures produced by the weight of the concrete. Erecting this is a skill in itself, and if you have never had any experience of building formwork to create a suitable mould for concrete, it is essential to entrust this to a professional.

If you do not want to use shuttering and you intend lining the excavation with a polythene membrane, give the sides of the pool as shallow a slope as possible. This will make it less likely that the concrete will slump because of the smooth surface of the polythene.

LINING THE HOLE

Even though concrete can be laid directly onto the soil, it will be much better if laid over a polythene liner. Not only will this prevent water seeping from the pool, but it will also help to prevent the freshly laid concrete from drying out too quickly and developing hairline cracks. These often lead to unsightly surface flaking. The polythene itself need only be builders’ clear material.

When laying a polythene membrane, treat the excavation in the same manner as you would when installing a flexible pool liner, for it is equally at risk of being punctured. Remove all sharp stones and roots from the hole, and spread a layer of sand over the soil to serve as a cushion before laying the polythene. Although it is neither possible nor essential to remove all the creases when putting down the polythene is should be installed carefully, much as a pool liner.

LAYING THE CONCRETE

Working outwards from the centre of the pool, spread a 10cm (4in) layer of concrete over the entire excavation. Where appropriate, use a plasterer’s trowel to ensure evenness and a reasonably smooth finish. Make sure that the floor and marginal shelves are roughly level, using a short board and a spirit level. Then press a layer of wire mesh, such as that used for poultry runs, into the wet concrete as reinforcement. On top of this, spread a further layer of concrete, approximately 5cm (2in) deep. This is the final layer, so care should be taken to ensure that it is level where necessary, especially the floor and marginal shelves, and that it is polished to a neat smooth finish with a plasterer’s trowel.

Damp sacking or similar material should be laid over the wet concrete to ensure that it dries out slowly. If this is impractical, water the surface of the concrete gently several times a day until it has set, using a watering can fitted with a fine rose. This is less satisfactory, but it will prevent cracking.

SEALING

Once the concrete has set, it can be treated with a sealant to prevent free-lime from escaping into the water. Not only is this harmful to fish and plants, but it also creates white cloudiness in the water, which looks unattractive and eventually settles out, coating the foliage of submerged aquatics. The sealant is supplied in crystalline form for mixing with water and painting on to the concrete. It will also assist in waterproofing the concrete.

If bright colours are more to your taste than grey concrete, excellent waterproof plastic paints are available that not only provide a smooth coloured finish, but also assist in preventing free-lime from escaping. However, to ensure that they adhere properly to the concrete, a special primer must be applied to the surface first.

MIXING CONCRETE

It is quite possible to mix concrete yourself, but unless the pool is quite small, it is likely to be uneconomical and time consuming. It is essential that all the concrete required can be mixed and laid in the same day to avoid the development of weak areas where different mixes meet. Consistency is vital, a uniform mixture of materials, especially waterproofing compound, being essential to guarantee a high-quality finish.

If mixing will he done at home, use one part of cement to two of builders’ sand and four of gravel. Mix the dry materials together until they reach an even consistency and uniform greyish colour. Add water and waterproofing compound, carefully following the manufacturer’s instructions. Allow the concrete to mix for a few minutes, then turn out a little into a wheelbarrow and check its consistency. The mixture should be sufficiently wet that when a shovel is placed into it and drawn across it in a series of jerks, the ridges it creates should retain their shape without slumping.

Ready-mixed concrete is much better, but you have to calculate and order the quantity in advance, specifying the day you want it delivered. If the Weather is exceptionally wet, it will still arrive with all the attendant problems. However, its mix will he consistent and have the waterproofing compound evenly distributed throughout. You must be ready to use it immediately and have good access from the delivery point to the pool site.

CONSTRUCTING A CONCRETE POOL

1. Excavate a hole which is at least 15cm (6in) larger than the finished pool, to allow for the thickness of the concrete, and line with builders’ polythene to prevent rapid drying out.
2. Working outwards from the centre of the pool, spread a layer of concrete evenly over the polythene to about two thirds of its finished depth (that is about 10cm/4in) and smooth it off evenly.
3. Tap the concrete firmly and ensure that it is reasonably level before putting the layer of reinforcing wire mesh into place. This should be done while the concrete is still wet.
4. Once the reinforcing mesh has been pressed into the wet concrete, cover it with a further layer of concrete (about 5cm/2in thick) and produce a smooth finish with a plasterers’ trowel.
5. Cover the wet concrete with damp sacking or similar material and water the surface to prevent it drying out too quickly. Rapid drying out causes surface cracks to appear.
6. Once the concrete has dried out, apply a sealant to prevent free-lime from escaping into the water. If a plastic paint is to be used, the concrete should be treated with a special primer first.