Cracking and Discolouration in Gunite Pools

Uneven plaster and pitting in swimming pool

A swimming pool owner in Durban had some concerns about his swimming pool:
1. Possible Cracking in the Corner of the pool
2. Marble plaster discoloured
3. Marble plaster pitting and surface uneven in places.

Jason Sanders from BLUPOOL Durban, a member of the National Spa and Pool Institute of South Africa, attended to the pool inspection and reported the following:

Cracking in the corner of the pool:

There was a crack in the tile in the corner of the pool, and based on a visual assessment, we were not certain whether this crack was right through the concrete shell of the pool. Cracks are usually caused by movement of some sort. In some cases, swimming pool levels are raised after the shell is cast or gunited. The tile could crack at the joint. It could also be that the pool or the paving around that corner is settling.
Although there is no sign of the pool or the paving settling, the crack could become a concern later, especially if it widens, or continues further down into the shell.
The proposed pool location was moved from its original place to a new area in the yard. The pool was designed with piling underneath it in its original position, however, we cannot be sure whether piling was done at the new position.
It is was recommended that the builder (or engineer that signed the pool off) confirms that the pool was properly underpinned and complies with the specifications, even though the pool location was moved.

Marble plaster discoloured


The marble plaster has grey stains. Staining on plastered pools is commonly caused by:

1. Drop Out (Precipitation of metals in the water.) Metals can get into the pool from a number of different sources, fill water, sanitising products, algaecides, soil etc. If the metal content in the pool water is high, a sudden change in the pH may cause the metals to come out of solution and attach to the pool surface.

2. Slagment Stains. Black cement (which is commonly used for guniting the pool shell) has about 38% slagment in it, and this can migrate to the surface of the marble plaster under certain conditions.
Guarantees against stains on marble plaster are usually never covered by the builder. The more reputable builders have this very clearly spelled out in their contracts.
There are measures that can be put into place to prevent stains from getting worse. Good water balance and a maintenance dose of a metal chelating agent, such as Pool Magnet by BioGuard, can help. We recommend that pool water fully analysed quarterly by a pool shop that is a member of the NSPI, and that has sufficient testing equipment.

Marble Plaster pitting and uneven in certain places.

There are noticeable pits in the marble plaster. This can either be caused by poor water balance (low Calcium Hardness), or by inferior quality white cement, or by negligent application.
Calcium Hardnes. In this case, the water balance was checked by a reputable BioGuard dealer and NSPI shop member. The Calcium Hardness was correct at 198ppm (required 200 - 275ppm)
Inferior white cement. There is speculation in the industry that an inferior quality white cement was brought into South Africa and that this resulted in the marble plaster deteriorating in some pools. However these claims haven’t been substantiated with any evidence, to our knowledge, and this was supposed to have happened 10 years ago. It is unlikely to still be a problem as long as a good quality cement is used.
Application. In our industry we have experienced some plastering specialist contractors, when troweling and smoothing out the plaster on the surface, sprinkle water onto the marble plaster to give it a smooth ‘polished’ look. This creates dilution of the cement and weak spots in the bond between the marble dust and the white cement. This is usually only evident after a year or two, and can be aggravated by corrosive water. (Poor water balance - Calcium Hardness below 200ppm.)
The uneven marble plaster in places look like the trowelled finish of the marble plaster could have had more attention paid to it. Most contractors guarantee their workmanship on plaster for only a year or two.
The pitting cannot be reversed. Keeping pool water well balanced and the saturation index in check will slow down further deterioration of the marble plaster. The pool will need to be re-plastered at some stage, it is recommended that an NSPI member company is used to re-plaster the pool, and that the correct high quality marble plaster mix is used.