Concrete Ponds and Cracks | How to Repair Leaking Concrete Ponds
QUESTION ... We have a concrete pond that leaks without running the waterfall or fountain. We have cleaned it out to find where it might be leaking, but apart from a number of very hairline cracks there does not seem to be any major crack or hole that would account for a drop of six inches a more in a week. We dont really want to resort to putting in a liner if we can repair the pond. It has been part of the garden ever since we moved here 15 years ago. Can the pool be made watertight again.
ANSWER ... If concrete pool was laid with too lean a mix or without a waterproofing compound in the mix or they were not given some form of waterproof coating, over time the concrete can become permeable; signs of this sort of decay might be indicated by a fine tracery of hairline cracks or a powdering of the surface.
If when you have cleaned and dried it out the concrete seems still fairly robust and there is still some strength and solidity there it may be repairable. Tap the surface over the crack. If it sounds slightly hollow, hack away the surface until you get to a hard solid surface. Do the same in the powdery places too. Any cracks that reveal themselves as weak areas of concrete revealing a potential canyon, chase these out to 2 or 3cm and as deep as the go. Clean up the surface of any debris.
Coat the edges of any large cracks with a waterproofing and bonding compound called SBR for exterior bonding.
Next, fill in these cracks with a strong mix of sand and cement (3 to 1) with added SBR (according to the manufacturers instructions). If you can, add polypropylene fibre reinforcement to the mix. You only need a tiny amount. I have a bag full of these fibres that have lasted me years. Literally a handful will reinforce a ton of concrete. Where you get them from, I cannot imagine, but a friend who owned a concrete factory gave them to me! The difference they make will only be apparent in the very long run.
This repair mix is only any good if the concrete it is repairing is sound. Areas that have been hacked away to sound concrete must be rendered with the same mix.
Once the mix has gone off the whole pool must be waterproofed with a tanking slurry like VANDEX BB75. This has been used for sealing up potable water reservoirs for years, but it does leach out a certain amount of lime affecting the pH. Rinsing out a couple of times over several gets the worst of it out. This coating also has the effect of locking all the separate surface together into one integral whole.