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Plaster/marcite is a combination of cement, sand, and calcium carbonate (plaster dust). When applied it is troweled until the “cream” is brought to the surface.  In that cream is calcium carbonate and cement.  The cream hardens and becomes a protective layer.  It is approximately the thickness of two to three sheets of paper.  This is why an acid bath is damaging to plaster/marcite, because the protective layer is so thin.  In addition, you have two dissimilar surfaces, the protective layer and the sandy cement material underneath which will cause separation or popping if the pool is left drained.

Plaster is typically the preferred finish for indoor pools. It has a longer life-cycle than paint but doesn’t have the longevity or aesthetic appeal of tile.

For pneumatically applied concrete pools, plaster is much easier for the contractor to finish than paint. If the contractor intends on shooting the pool shell, plaster and paint may be very close in cost because the wet concrete will need to be troweled to a smooth finish after it comes out of the gun.

  • Based upon historical cases, the plaster will spall (flake off) from five to fifteen years (5-7 in high traffic areas) and there is no reliable fix. Plaster spalls because:
    • The original bond to the concrete wall was problematical.
    • Aggressive pool water can leach minerals from the coating, weakening the plaster.

When plaster spalls, old plaster sections, which have not spalled, are frequently weak. As a result, re-plastering creates a scenario of putting good plaster on top of bad plaster.  For this reason, plaster contractors seldom guarantee the re-plastering of a pool surface.  The only sure way to re-plaster a pool is to sand-blast or water-blast the walls and floor down to bare concrete, which is an expensive and time consuming process.

  • Plaster that has discolored in a pool shell may be the result of hydrating or water that has been entrapped within the cementitious layer. The National Plasterers Council states that the causes are unknown and currently under investigation.  Steps to remedy the issue are:
    • Acid Wash
    • Drain & Heat
    • Acid Start Up
      • Acid Start Up is the only method that does not involve the draining and refilling of the pool and the associated risks of plaster delamination and spalling as a result of draining the pool. Steps to prevent corrosion of steel materials within and around the pool should be taken as a result of increasing the acidic levels within the environment.

 

In outdoor pools the plaster coat may separate and create a hollow space between the plaster coat and the pneumatically applied concrete pool wall. This develop usually occurs after 10 – 15 years and is likely accelerated by freeze/thaw cycles.

Re-plastering after plaster fails is difficult with regard to a quality control. Because new plaster is being applied to older, weaker plaster, most experienced contractors will not warranty their work because re-plaster projects frequently fail in a short period.

Pool plaster delaminates or spalls due to improper installation not poor water chemistry.

Plaster is made up of cement, sand, and water.

Pool plaster is commonly troweled in 3 to 5 separate passes – the early passes to place the material and the later passes to create a smooth final finish.

After plaster is troweled, excess water will bleed to the surface. Bleed water then evaporates from the surface.

  • There are two (2) common mistakes made during troweling:
    • If troweling is completed when bleed water is present it will force water back into the plaster paste which causes excessively high water to cement ratio which weakens the finished surface.
    • If troweling is completed too late after the surface is too dry a crust will form with a wet paste underneath. This will create a weakened zone subsurface.  This typically happens on dry, hot days with low humidity and wind.  If this happens, the finished surface will look fine and even last awhile if the pool is full of water.  However, when the pool is emptied, a 16th to an 8th layer of plaster will flake off in small areas or spots.
    • Pool plaster delamination/spalling is typically a rare occurrence and most often happens in areas that are challenging including step areas, around main drains, and shallow areas.

The Division 13 Cementitious Finish Specification includes a 3 year warranty on all cementitious finishes, including plaster.

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