Leaks in concrete ponds are very often a sign of the beginning of the end

Concrete pond leaks. If the pool is cracked then it is either because the concrete and foundations have moved - therefore the design of construction had limitations - or the concrete has decayed because of age or bad mixing and materials.

If the problem seems to be caused by movement of the concrete, then you can either go with it or stop it.

Going with it can only be considered a temporary plan of action. Chase out the cracks as deep as they go to good solid concrete and to at least an inch or 25mm width at the top. Then fill them with a thick tar or bitumen mastic. This will give you the flexibility you will need if there are any more slight movements.

Alternatively, you fill them with a mix of mortar containing bonding solution like SBR Bonding (and nylon reinforcing fibres if can get them). Paint the chased out cracks with bonding solution as well before you fill it with the reinforced mortar.

Having said all that, this wont necessarily last five minutes if the overall structure of the pool is weak. If, for instance the pool is on a steep slope, there is a considerable propensity for the thing to slide down hill, albeit a fraction of an inch a year, as a result there is a tremendous strain on the front edge and base of the pool. In which case the pool must be strengthened so that it remains rigid and stabilized. Alternatively resort to lining it with a flexible liner. This would be the sound advice from most water garden product retailers.

In the former case the strengthening can come from a reinforced concrete collar poured around the outside. Inside the pool you will need to lay an additional fibre reinforced concrete base. The thickness of this would depend upon the state of the existing base, but it should be a minimum of 2ins and preferably a good 4ins deep.

If the pool is formal or has vertical sides, a self-sufficient strength can be gained from a blockwork framework, with the pool built just inside the old pool and backfilled behind the blocks with concrete. This too would need a concrete base poured after the blocks were laid.

When pouring a concrete base, it is important to cover the whole base in one go within 24hours, so that it can it can harden as a complete skin. It does not matter too much if the thickness required is not reached, as long as the next layer I it laid as soon as possible. If this is the case, leave the first layer level but roughened, and keep it from going off to quickly with damp sacking or plastic sheeting.

If this all seems like too much upheaval and the concrete in the pond seems to be fairly stable and perhaps only the surface is crazed, then a new 'high performance' surface render will give it the necessary extra life. A product called Fibrocem, which is a ready mixed sand and cement, also contains fibreglass fibres stabilised chemically so they will not be dissolved in the cement. If the instructions for its application are followed precisely and it is applied to clean stable concrete it can be as efficient and more durable from accidental damage than any flexible liner - but it works out twice as expensive as the most expensive liners.

Very fine hair-line cracks in concrete can be stabilised by various paints. Products designed for the building industry for waterproofing water containers or tanking buildings can be effective, but check for pollutants (chlorines and algicides as in exterior paints) and also excessive lime. Bitumen paint is a good old standby if you don't mind black, but will peel off virtually instantly if it goes onto anything but bone dry concrete or render.

The rubber paints generally available to seal concrete ponds need the luck of a lottery winner to work. They need perfect humidity and a stable temperature within a specified range whilst they are applied and go off. This makes them impractical in all but ideal circumstances.

One product that is fairly easy to obtain, which waterproofs any thing that absorbs water, is G4. It is an epoxy based paint or varnish, which, although it is a bit expensive, is most effective on wood, stone and concrete and is the answer to many insoluble problems.

The other alternative other than resorting to flexible liners; is to fibreglass the concrete. This is usually the sort of decision taken by those who are familiar with the idiosyncrasies of the medium and probably have access to large quantities at wholesale rates. Otherwise it is a fairly expensive and time-consuming option. However the results in proficient hands are of durability and easy maintenance out of virtual dereliction and decay. This is definitely an option to consider if you are contemplating contractors to sort out the problem rather than doing it yourself.