Making a Garden Pool From Concrete

A well-made concrete pool will last for years and is a constant source of pleasure. Unfortunately one sees many badly constructed concrete pools and these cause endless heartaches for their owners. What annoys me most about these pools is that they have one common and easily avoidable fault — haste in construction. For some inexplicable reason a man building a pool in concrete almost always loses his patience. He is so keen to get it finished and the plants and livestock introduced, that he skimps on essential tasks. But it is attention to detail that makes the difference between failure and success.

A design should be chosen that does not have walls that are too steep, nor curves and niches that are fussy, as these are difficult to build in concrete. Once a plan has been formulated the hole should be dug so that it is 15cm larger all round than the finished size. Any loose or crumbly soil must be firmed and the excavation lined with heavy gauge builders’ polythene in the same manner as one would use a pool liner.

When concreting a pool it is better to finish the job in one day so that all the concrete unites rather than over a period of several days, for leaks will often appear at the various unions. If it is impossible to complete the job in one day, be sure that the edge to be joined is roughed up so that the next day’s mix keys with it. No more than twenty-four hours should pass between the last mix and the first one of the next batch if leaks are to be avoided.

Mixing concrete is a wearisome task, but contrary to popular belief, there is nothing mysterious about it. A satisfactory mixture consists of one part cement, two parts sand, and four parts gravel, measured by volume either in a bucket or on a shovel. This is then mixed with a shovel in its dry condition until of a uniform grey colour. If one wants to play safe and introduce a waterproofing compound it is done at this stage, but stick to the manufacturers’ instructions.

Water is gradually added to the mixture which is constantly turned until of such a consistency that when a shovel is placed in the mixture and withdrawn with a series of jerks, the ridges formed by the movements retain their character. Concrete in this condition is ideal for laying. A mixture that is too wet often produces air bubbles and hair cracks in the surface, while a dryer agglomeration tends to crumble.

Well-mixed concrete is spread evenly over the pool floor to a depth of 10cm and as far up the sides as possible. Wire netting is then spread out over the surface of the concrete to act as reinforcing. The final 5cm of concrete is then laid over the top. The surface is finished with the deft sweep of a plasterer’s trowel.

When the sides of the pool are very steep or vertical some kind of formwork will have to be erected. This is usually made of rough timber and held in position to make a mould for the walls. Unfortunately concrete will stick to wood, so it is essential to limewash, grease or soak the boards with water before the concrete is poured in. This will ensure a neat clean finish when the formwork is removed. If the design is a little more irregular and the harsh straight lines of boards are not desired, plywood can be used. Being supple and bending to almost any shape it is extremely versatile, but remember that concrete can exert quite a pressure and any plywood or hardboard shuttering contemplated would have to be generously reinforced with stout timbers.

Coloured concrete can be made by adding pigments. These should be mixed with cement in any proportion up to 10 per cent by weight. Red iron oxide gives a red colouring, chromium oxide a rich deep green, cobalt blue a pale blue and manganese black a black, while the use of snowcrete cement and fine Derbyshire spar yields an excellent white finish.

Once the concrete has been laid and any remaining surface water soaked away, all exposed areas of the concrete should be covered with wet sacks. This is particularly important during hot weather, for if the concrete is allowed to dry out too quickly hair cracks will appear. If a large area of concrete is involved, frequent spraying with water from a watering can with a fine rose attachment will have much the same effect. This treatment is only necessary for a couple of days. Within a week the concrete will have set and we can then consider stocking the pool.

This is not possible straight away, for unlike liners and prefabricated pools, concrete structures contain a substantial amount of free-lime which can be harmful in varying degrees to both fish and plant life. One only needs to look at the milky water in a freshly constructed pool to see just how much of this harmful substance is present.

The patient gardener can leave the pool unstocked and open to the weather. After six months the concrete will be mature and relatively harmless. However, most of us having completed the hard physical work would like to see some progress made with planting. For us there are several alternatives.

Emptying and refilling the pool a number of times has the same effect upon the concrete as being exposed to the weather. But this is a laborious business and it is difficult to be certain when it is safe to introduce life. By far the best way of dealing with the free-lime problem is by using a neutralising agent.

I still like to fill the pool once and then empty it so that all the grit and similar debris is washed away. Once it has dried out the neutralising agent can be applied. It is usually sold as a white powder which, when mixed with water, turns into thick paste. Application is by brush, being sure to cover the entire surface. Apart from neutralising the lime, it reacts with the concrete and forms silica, thus sealing the concrete by internal glazing.

Another way of curing concrete is by using ordinary household vinegar. One part vinegar to 200 parts of water by volume. Allow this solution to remain in the pool for three days and then empty and rinse thoroughly. The pool will then be safe.

This can be done much quicker using 1 part vinegar to 10 parts water and scrubbing the concrete with a stiff brush. I am a bit hesitant about doing this as the concrete often bubbles and becomes rough with the chemical reaction. If you should decide to do it this way be sure to wash the pool out before introducing plants and fish.

Rubber and liquid plastic paints are often used to waterproof or safely alter the colour of a concrete pool. They are available in a number of pastel shades and once applied form a thick rubbery skin over the surface of the concrete. In doing this they not only prevent water from seeping away, but do not allow free-lime to escape. Most of the paints sold for this purpose react with the concrete and do not form a smooth waterproof skin unless it has been treated with a suitable primer.

Sometimes it may be found necessary to repair a concrete pool. This creates all kinds of problems, not least of all providing alternative accommodation for the plants and fish. It is, of course, necessary to remove them all, for if any freshly mixed concrete pollutes the water it can create all kinds of trouble.

When a leak is located, the cracked or crumbling area should be chiselled out until solid concrete surrounds it. The cavity can then be filled with the usual concrete mixture and allowed to set. A rubber or liquid plastic paint is then used to seal it completely. No guarantee can be given as to the permanency of such a repair, but under most circumstances it is satisfactory.

When either sealing a new concrete pool, or repairing an established one, most, if not all, of the water has to be removed. It was the fashion with pools which were built during our grandparents’ time to have a drainage system with a plug of some kind in the floor. This created a weakness and often resulted in the break-up of the pool at that point. As most pools were situated at the lowest level in the garden this made the task of constructing a satisfactory drain and soak-away quite difficult.

Certainly a drain cannot be recommended for a concrete pool and is impossible to incorporate in a pre-formed or lined construction. Apart from baling out the water with a bucket, which is perfectly acceptable when dealing with a very small pool, the ideal method is to attach a length of hose to the submersible pump outlet. Providing the pump is not required to lift the water any higher than the fountain or waterfall it normally operates, the hose may be of any length necessary to safely transport the water to a more distant part of the garden. Obviously, every last drop will not be removed, but the 15 or 20cm that remain can be baled out with a bucket and any residue removed with a mop.

When a raised formal pool needs to be emptied this can be quite simply done by siphoning the water off with a length of hose. Providing that the outlet is always lower than the level within the pool, the water will flow continuously.

30. July 2011 by Dave Pinkney
Categories: Water Features | Tags: , , | Comments Off on Making a Garden Pool From Concrete


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