Print Version: Why does plaster fail?

Why Does Plaster Fail?

Fail, like breaking away in patches, we mean. Common dissolving, thinning and roughening by the action of aggressive water we don’t mean. That’s your fault, unless you just got the job… But these days we see more and more of the former, a phenomenon called spawling, where small areas in the plaster seal coat - from the size of your hand to patches a few feet across - just let go and fall away, leaving unsightly and abrasive “holes” which often initiate budget searches for a new plaster job.

This problem shows up very, very often in “re-plaster” jobs – interior linings subsequent to the first (and usually the best) plaster application. Strong, detailed rehab specifications, demanding extremes in preparation, often do not exist; indeed some of you use the free specifications offered by the plaster contractors themselves. (They aren’t free…) If simple sandblasting or acid washing or both are used, the prepped surface is often still smooth. And some of that new plaster is coming off, falling out like crusty blisters to reveal the old plaster remains and concrete beneath - no matter how high the quality of the plaster or the work.

Here’s the deal with rehabs compared to successful new construction: Cementaceous construction materials – concrete, shotcrete, gunnite or, yes, plaster – must cure together to “cohesively” bond. (Have you ever tried, and failed, to patch old concrete – like a driveway – with new concrete?) In the life of your swimming pool, the pool contractor had only one shot at the ideal set of conditions; that’s when he troweled that creamy white plaster coating on top of the recently shot (or poured), brand-new concrete shell. This concrete structure began its curing process (chemically hardening) as soon as it was formed, and the subsequent plaster application did the same. They continued to cure together for months, even years. The greater part of this exponential process happened in the first few weeks, however - tapering slowly until, after a number of months, its structural strength and moisture content leveled off enough to call it “fully cured”. That plaster is not coming off in patches. It belongs to the shell.

A re-plaster, many years later, doesn’t have the privilege of curing with its sub-structure thereby bonding as one. For a re-plaster to stick, it must have mechanical tooth. The solid, near-plaster-free substrate revealed in the preparation effort usually needs to be distressed with a baby jackhammer - an electrical device called a bush hammer – in order to produce small, quarter-inch-deep holes the size of your fingernail in virtually every one to two square inches of the prepped surface. The plaster’s bond, then, does not have to depend on the impossible, cohesive bonding. It can hang on to those penetrations and stay attached for the life of its outer surface – hopefully ten to fifteen years.
Without descriptive preparation specifications, however, this costly mechanical distressing step simply will not happen. The contractor won’t contribute this to the work. Believe it.

What about bonding agents? Adhesives, concresives, prep-coats or bond coats? This consultant has no confidence in them, watching them fail over and over again for the last 30 years. And what about acid washing as part of the prep? SKIP it. Your contractor will never get it completely neutralized in that porous old surface you’re replacing.

The problems and solutions described above for rehabs should make some sense; but what about spawling failures on new pools? This is a situation that should never, never happen. The initial plaster should have been in place during the early part of this cycle to assure that the gray substrate and the white plaster cured together, literally becoming one cementaceous shell. Less than two or three weeks delay appears to be optimum, after which the two cure cycles are less and less coincidental.

When new jobs fail in the fashion described, fingers point at operators, the water, plaster contractors, the weather and the chemical choices; my finger points to the designer! This writer has a lot of friends in the designing world, yet has had only marginal success convincing them that plaster needs to be on there, curing, soon after the shell is set.

It seems that designers nowadays direct contractors to work on something called a “critical path”, a pre-planned, calculated time line which makes things run more efficiently, more cost-effectively. Do this first, then build that, interplaying the various trades to make the project go smoothly. In recent years many pool shells have been shot somewhat early in the project and, especially with indoor pools with complex natatoriums being built to contain them, the shell lays in waiting - empty. This planned delay lasts sometimes six months or even a year before the plaster subcontractor is called in, the lining is installed, and the fill water is finally turned on.

Plastering is delayed because needs to cure under water, so the pool must be full from plaster day on. With construction continuing for months, who wants to take care of a pool full of water, keep it free of construction debris and keep the workers safe for those months before opening day? Makes sense…

But the later in the pool shell’s curing period that the plaster is applied, the more likely a plaster coating may not bond. And you get to deal with it! Sometimes bonding agents are specified, which surely help a little. But history tells the story better than prophecy. The record is only fair for such intentionally delayed plaster work.

One California outdoor 50-meter pool was halted by a lawsuit. The huge, gray shell was done, the deck was in place, but there was no plaster and no water. A year later, when things were settled, a beautiful layer of plaster was trowelled in place and the turquoise fill soon began circulating for the first time. Within six months, however, the plaster started coming off in ugly ovals. The shell had thoroughly cured before the plaster was added. They had to drain, remove it all, prep it mechanically, and start again. The swim team was not happy…

So what can you do about your new, pocked pool? Check the construction history first. Make sure your water balance (Calcium Saturation Index) is dead on, then call your designer. Get an opinion from an independent consultant who is a specialist in plaster and cement and in SCUBA diving. Patching is not much fun nor very pretty, but it may come to that. Warrantee re-plasters are not much better than the belated, inferior first try unless the distressed prep is performed this time before plastering. In any case, another swim season is lost, and somebody besides the owner is responsible…

Your future aquatic center’s not finished yet? Get that plaster on the shell before three weeks are up, or cross your fingers.

~ kw