Que – Plaster Spalling

One of the arts or skills of the pool plastering trade is properly timing the troweling process. If troweling is performed when water is present on the surface of the plaster, forcing water back into the plaster paste causes excessively high water:cement ratios in the surface finish, weakening it.

Sometimes the surface dries too fast. If the surface becomes too dry, with a dry “crust” on the surface but a wet paste underneath, a weakened zone can be formed just subsurface. This can happen when plastering on dry hot days, during low humidity, in wind, etc. It can also include too much drying between trowel passes, excessively late hard troweling, and overworking (over-troweling) the finish.

If this happens, the top of the finished plaster will look fine.  However, after time this weakness is manifest by a thin 16th to 8th inch thick layer of the surface flaking off, generally in small areas or spots. Often, these weak areas do not flake off until the pool is drained of its water and the plaster dries out. 


Above are two pictures of flaked pool surfaces: You can see the round indented area with the loss of plaster. This could be described as a “moon-like crater” look, or a shallow and sunken pit look.  Note that the newly exposed surface (where the flaking occurred) is extremely rough and coarse, while the older existing surface surrounding the missing plaster area is much smoother. Also, the newly exposed plaster will be the original color, distinct from discolored surfaces that may surround it.

The cement/concrete industry has documented the cause of this problem as a defect caused by improper application and troweling. (see, for example, the section on “Delamination” on page 5 of: https://www.oboa.on.ca/events/2009/sessions/files/Slab%20Surface%20Prevention%20Repair.pdf)

It is referred to by many names, including surface flaking, peeling, buckles, blistering, delamination, and even the confusing term “scaling.”  

But in the pool plastering industry, the term “spalling” has become the commonly used term to describe this shallow surface loss. We in the pool industry use the term delamination or bond failure for deeper bond separation between new and old plaster or between plaster and the substrate.

Since the clear majority of pool plaster does not spall, blister, or flake off, we can see that most plasterers properly apply, cure, trowel, and finish the pools they plaster. Spalling is relatively rare, and when it does occur it is most commonly found on a step radius, on shallow end floors, and around the main drain – these being places where finishing can be an additional challenge for many reasons.  A step radius is occasionally over-worked to ensure smoothness and straightness, and the floors sometimes get too much troweling due to being walked on by the pool finishers while troweling.  Walls are rarely over-troweled. 

One thing that does not play a role in plaster spalling (or peeling) is water chemistry – aggressive water is not required to form spalls. In fact, as we have mentioned, the inherent weakness leading to spalling is created before the pool is ever filled with water.

If aggressive water was a factor in causing spalling or flaking, then rain water (especially acid rain) would be causing this problem consistently on all cement and concrete surfaces.  Rain water has zero alkalinity and hardness, with a pH normally below 7.0.  Instead, we only occasionally see this problem on small areas of concrete slabs, where nearby the concrete is relatively smooth with no visible defects.  This would confirm that something other than aggressive water caused this problem since it rains everywhere.

Also, acid-started and acid-washed pools would be preferentially prone to spalling – but they are not. And the Cal Poly/NPIRC/NPC test pools did not “spall” when subjected to aggressive water.

The NPC’s Technical Advisor has incorrectly blamed spalling on aggressive water chemistry in the past, and their Technical Manual also contains this inaccurate information.

Are we to believe that aggressive water somehow attacks and penetrates plaster in specific spots, and then dissolves plaster under the surface, which somehow undermines the plaster in this area… and then the plaster surface pops off, creating something that looks like a sunken pit or peeled area?  And then, that this aggressive water somehow does not attack the plaster surrounding the popped (flaked) off area, leaving it smooth and unaffected?  Sometimes only a few spots flake off, and sometimes there can be over a hundred.

Don’t try and make sense of this concept, it doesn’t make sense, and it is wrong.  Aggressive water molecules do not have a brain in which to plan out a specialized attack upon pool plaster and cause defects in isolated spots.