Under normal conditions, the plaster (white) is exposed to the water (blue), which is bonded to the gunite substrate, or concrete (speckled), which in turn rests on dirt (tan).

Sometimes, an air cavity (a bond failure) can form between the gunite and the plaster, or between multiple layers of plaster. This is referred to as “delamination” and the cavity is referred to as a “void.” As long as the void is not connected to the surface of the plaster, the fact that the void even exists may not be known. Unless the plaster completely breaks free from the surrounding plaster, creating what is referred to as a “pop–off,” or unless the delamination is extensive, this is not considered by the plaster industry to be a defect.

In some cases, however, the void is connected to the surface by a small pinhole or hairline crack. Pinholes and cracks are often created from structural flexing of the delaminated plaster. 

Gradually, water from the pool penetrates the void via the hole or crack, and creates a localized chemistry environment completely separate from the water balance in the pool. As calcium hydroxide, from the plaster itself, bleeds into the void water, it creates a calcium–rich, high pH solution like a little “calcium/pH factory” beneath the plaster surface. The pinhole or crack is not of sufficient size to create a rinsing effect like that which occurs at a new plaster surface.

Calcium–rich void water and the pool waters natural alkalinity, come into contact with each other at the plaster surface, then react with each other, which results in the production of an insoluble calcium carbonate by-product. This insoluble calcium carbonate can build up a “slag pile” around the exit point, thus forming a nodule.

It is important to keep in mind that nodules are a symptom, rather than the disease itself. The disease is bond failure, cracking or some other imperfection in the plaster from the pour, so acid washing them does not fix the problem. I have removed the build up to eliminate the chance of someone cutting their foot on the sharp slag, but the spot where the the top layer of plaster delaminated remains.  

It is always a good idea to keep your yard clean when you have a pool. Dead leaves, flowers and loose trash [like styrofoam and plastic bags] lying on the ground will get into the pool when the wind kicks up. Debris like this will clog baskets and suction side cleaners, effectively stopping your pools ability to circulate water and keep itself clean. All debris when there is enough of it can cause problems, but flowers, like those from the Bogenvia bush, a lilac, or trumpet flowers for example, are like paper pulp when wet. Keeping Bogenvia bushes from getting too big will help a lot to control debris when it flowers. A small amount of these flowers can line a basket, clog the holes and effect water circulation. One plastic bag from a grocery store, or a sheet of newspaper can cause a loss of water circulation and shut a suction side cleaner down. 

Even if your yard is spotless, remember that your pools cleaning system can easily be effected by a neighbours tree, or a city tree adjacent to your property. You should always check the baskets after a storm if you are able, even if you pay for pool service. I visit my accounts once per week and a lot can happen in just a few hours when a wind storm comes through. Trimming trees like Palms for example when they have dead fronds hanging and seed pods dropping onto the ground should be trimmed back.  

A pump basket splitting open will allow debris to get past the basket and into the pumps impeller. When this happens, water circulation will be greatly effected and the system will lose filter pressure. Floor head systems will not have the pressure required for the pop ups to function and suction side cleaners, if not clogged themselves, will not have the water circulation required to keep them moving. Sometimes a pumps impeller can be cleaned once the basket has been removed, but, depending on the pump, the motor may have to be taken off to access the impeller.

Making the effort to keep your yard clean and check the baskets after a wind storm is important and will save you from a lot of unnecessary problems.