Pool Crack Repair: Causes and Repair Methods

If the ground beneath the pool does not have the same bearing capacity at every point, one end of the pool shell settles more than the other. This risk grows for pools built on fill, on sloping plots where half the pool sits on excavated ground and half on fill, and on alluvial soils where the groundwater rises and falls with the seasons. In our article on ground and earthquake precautions in Izmir, we described the foundation and joint solutions for plots of this kind separately.

Concrete is a material that is strong in compression and weak in tension; the tension is carried by the reinforcement. Stress concentrates at the corner where wall and floor slab meet, and around the openings for stairs and skimmers. If no additional reinforcement has been placed at these points, or if concrete pouring was interrupted and a cold joint formed, the crack starts there. When no movement joint is left between the pool and the building or terrace slab, earthquake and thermal expansion loads are transferred directly to the pool.

If concrete poured in hot, windy weather is not watered sufficiently, the surface dries faster than the concrete beneath it and a network of fine cracks forms. Repair mortar applied too thickly or with too much cement cracks in the same way. These cracks are generally below 0.2 mm, appear as short lines that change direction, and do not leak water.

Although superficial cracks are considered harmless, they should not be left alone. In winter, water that gets into the crack freezes and thaws and hollows out the area beneath the finish; over time the tiles work loose. That is why hairline cracks are sealed when the finish is renewed or during annual pool maintenance.

In summer, an open-air pool loses 3-7 mm of water a day through evaporation; on a windy, hot day this rises further. In the bucket test, we fill the bucket up to the pool's water level and draw a level line both inside the bucket and on the pool wall. The test is carried out once with the pump running and once with the pump off. If the loss increases while the pump is running, the leak is usually in the pressurised return line; if it continues with the pump off, the shell, a crack or the suction line is suspected.

With a leaking crack, a few drops of food colouring released into the water are drawn towards the crack. The pool is not drained before the location of the leak has been identified; in an empty pool the crack may appear closed, and it becomes impossible to tell which crack is leaking.

An empty pool is a light vessel compared with the water around it. In the rainy season, or on a plot close to a stream bed, groundwater pushes the pool floor upwards; on site, we have seen pools rise by a few centimetres and floors crack down the middle. Before draining, the hydrostatic relief valve in the floor is checked and, if necessary, water is drawn off from the surrounding area with a well. In our article on what to know before draining a pool, we set out these checks in order.

There are also repairs that can be carried out underwater. Epoxy pastes that cure in wet conditions are used for small cracks and for leaks around fittings; these are temporary or limited solutions and require diver workmanship.

In injection, injection nozzles called packers are placed along the crack at intervals of 20-30 cm, and the resin is fed under pressure from the bottom upwards. When the resin emerges from the next packer, that section of the crack is full. Injecting rigid epoxy into a crack that is still moving achieves nothing more than shifting the crack a few centimetres to one side.

If there are many cracks and the shell is no longer moving, finishing the pool surface with a waterproof PVC membrane or a reinforced liner is more durable than repairing them one by one. This decision is usually taken as part of the pool renovation and refurbishment steps, together with the finish and the installations.

Ready-made filler kits seal the mouth of the crack; they do not fill its interior. Before application, the crack is opened into a V shape with a grinding disc, the dust is removed and the surface is brought to the moisture condition specified by the manufacturer. Filler applied without this preparation comes away in the first winter. Before the pool is refilled with water, the material's own curing time is observed; depending on the product, this ranges from a few hours to a few days.

A crack that reopens at the junction of wall and floor, at a corner or next to a previous repair is not sealed with filler. The movement that caused the crack is still continuing there, and the cause must be found first.

Across the whole process of having a pool built, the decisions that determine cracking are made in the first two weeks. Foremost among them are the geotechnical survey, compaction of the excavation base, the blinding concrete, two layers of reinforcement, pouring the walls and floor in a single operation where possible, and curing the concrete by watering it for at least 7 days after pouring. A movement joint is left between the pool and the building, retaining wall or terrace slab.

The second layer is waterproofing. A flexible cement-based waterproofing or membrane delays water from reaching the concrete and reinforcement even if a hairline crack forms; the reinforcement does not corrode and the crack does not grow. We explained the order of the waterproofing layers and the water-tightness test in our article on pool waterproofing.

In an existing pool, an annual check carried out when the pool is empty or the water level is low catches cracks while they are still small. This check is included in our pool maintenance contracts, together with the inspection of the finish, joints and fittings.