Rust spots that grow in a cement pool surface is a sign of one of two problems: An oxidate object (like a rusty nail) was inadvertantly mixed into the plaster or Steel Rebar was placed too close to the concrete surface during construction.
Photo Left: A single brown dot grew into these three bleeding rust spots. A related problem is thin lines of rust that grow, from a few inches to eight or more feet long.
To uncover the problem, simply chip away plaster and, when necessary, a small amount of concrete.
You won't have to go far. The fact that you're getting bleed-thru means the object is less than an inch under the surface.
Note the small rock in the wall cement (photo right). This identifies the pool as being made of Shotcrete. Gunite, the other common pool shell cement, would be a consistent gray. (Gunite sand sometimes contains a much smaller rock but gunite hose clogs easily and crews are vigilant about what goes into the mix.) Larger rock (1" -2" limerock) in a pool is rare, identifying the shell as having been poured right out of a concrete truck and hand-packed.
What we find are three steel tie ends that were left close enough to the surface to draw in moisture. Rust finally worked its way into the rebar.
Every overlap in the 20' long steel bars and every other intersection of rod is secured with wire ties.
Typically tightened with a 'twister', it's not uncommon for the ends of some of these ties to be left sticking up.
If no one happens to knock these ends down, steel wire is left periously close to the surface. In time, water will get to the tie, rust it away and seep its way down to steel.
Use a 4" diamond blade fitted to a small angle grinder (rentable or available any hardware, Home Supply store) to cut and remove what you can of the steel bars.
I've used spray paint, acrylic caulking, pool putty, ground clamp potting mix and in a pinch, even gray and blue PVC cement to coat any steel or steel ends that remain. The idea is to provide a moisture proof barrier.
If the surface is to be patched, fill the repair in gray cement to within a quarter inch of the plaster surface.
Patch Marcite (white plaster) with sanded tile grout (available any tile, hardware or Home Supply store) color matched to the true color of the surface, usually a 'Bone', 'Eggshell' or other slightly off-white color.
If the entire surface is to be replastered, simply fill the void with gray cement, allow a few days to dry and plaster as usual.
(Left) Gunite cement is shot to form a pool shell. Note the small water hose 'T'd into the top of the nozzle. Gunite sand and cement is mixed at the pump and forced through the hose 'dry' where it's finally mixed with water at the nozzle. This gives the gunite gunman control over 'slump' or softness of the material. Shotcrete is completely mixed in a truck mixer and pumped 'wet'. The shotcrete gunman has no control over slump in the material.
Typically, the shell will be 3"- 5" thick and this steel rebar support will finish with at least 1" of gray cement covering it. In theory, the plaster coat (Marcite, Quartz or other pool plaster) should form a waterproof barrier and rust would never be a problem. In reality, cement is porous enough to demand a minimum 1" buffer between steel and water.
Note also the white main drain fixture at the workers foot. This serves as the only visual reference for floor slope. An experienced gunman will judge cement depth with his feet. Vertical pipe stubs are part of a floor cleaner system.
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