Pitting and Discolouration in a gunite pool.
Discolouration in gunite pools is worrying. This caused concern for a swimming pool owner in Durban. He also had concerns about his swimming pool which had a crack, uneven paster and staining.
1. Possible Cracking in the Corner of the pool
2. Marble plaster discoloured
3. Marble plaster pitting and surface uneven in places.
Jason Sanders owner of BLUPOOL Durban, and a member of the National Spa and Pool Institute of South Africa, attended to the pool inspection and reported the following, amongst which was the discolouration in his gunite pool:
Cracking in the corner of the pool:
There was a crack in the tile in the corner of the pool. Based on a visual assessment, we were not certain whether this crack was right through the concrete shell of the pool.
Ground movement can cause cracks. In some cases, raising the swimming pool levels after the shell is cast or united can cause the pool to crack. The tile genearly cracks at the joint.
It could also be that the pool or the paving around that corner is settling. Even if 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.
In this specific case, the proposed pool location was changed 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.
We 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 has moved.
Related Article: Why does a Swimming Pool Crack?
Discolouration in Gunite Pools
The marble plaster has grey stains. Causes of discolouration in gunite pools:
1. Drop Out (Precipitation of metals in the water.)
Metals can get into the pool from several 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. The metals then attach to the pool surface.
2. Slag Stains (Discolouration in gunite pools.)
Black cement (which is commonly used for guniting the pool shell) has about 38% slag in it, and this can migrate to the surface of the marble plaster under certain conditions.
Guarantees against discolouration in gunite pools with marble plaster finishes are usually never covered by the builder. The more reputable builders have this very clearly spelt out in their contracts.
There are measures one can put in 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. Caused by:
- poor water balance (low Calcium Hardness), or by
- inferior quality white cement, or by a
- negligent applicator.
Calcium Hardness.
A reputable BioGuard dealer and NSPI retail member tested this. Calcium Hardness is correct at 198ppm (Ideal range 200 – 275ppm)
Inferior white cement.
If the white cement is of inferior quality or has expired, then the pool marble plaster can pit prematurely.
Application.
In our industry, we have experienced some specialist plastering contractors, sprinkle water onto the marble plaster when trowelling 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.
After a year or two, this becomes evident. Corrosive water (poor water balance – Calcium Hardness below 200ppm.) aggravates this.
The uneven marble plaster in places look like the trowelled finish of the marble plaster was below standard.
Most contractors guarantee their workmanship on plaster for only a year or two.
One can’t reverse pitting. Keep pool water well balanced and the saturation index in check. This will slow down the further deterioration of the marble plaster.
The pool will need re-plastering at some stage. Use a registered NSPI member company to re-plaster the pool. Use at high-quality marble plaster mix.
