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Chapter 9: Plaster

Plaster Material: Good plaster depends on clean sand and water. If either contains reactive chemicals, plaster quality is reduced. Use sand that experienced plasterers know produces a quality product. Refer to page 17 for mixing proportions.

Roof: Start at the roof top center and work outward. One person stands inside the tank to watch progress for those working on top. No plaster can be applied from below at this time. Excess water flows and drips down and will cause fresh plaster to fall. Wait until the ceiling has become stiff before attempting to plaster it. Brush roof top with a stiff broom to remove shrinkage cracks. Apply water to the roof as a fine spray. DonŐt let the roof become dry in the sun. Concrete pumps make roofs easier.

Wall: Begin the wall plaster on the shade side. Work both directions around the tank. Hand application is from both inside and outside the tank. Quality control at this point is a wall without voids. Do not apply a thin layer of plaster which does not penetrate and surround all the steel. Experienced plasterers are more likely to work too fast and leave voids than less experienced people who pack the plaster in with their hands. Have extra gloves available so that workers will use there hands for packing. Hand pack means use the hands to pack the plaster into the steel armature. Discover the full extent of voids by tapping the wall with a hammer and observing how the plaster settles and makes the void visible for hand pack. Do not finish trowel the wall too much at first, excessive vibration causes water to work to the surface, this, in turn, causes the plaster to slide off the wall.

Plaster Finish: Finish the inside ceiling when the plaster is firm enough to sponge float irregularities and accept more plaster. For wall finish, wait until the plaster has become firm, then use a wet sponge float to finish and smooth the surface. Start where the wall was first plastered. Use a plain sponge to brush off larger grains of sand and small pieces of gravel. The sponge float and sponge work best if they are rinsed and cleaned frequently. This technique is for both inside and outside. Apply more plaster in areas where steel shows through the surface. Use sponge float and sponge technique on any shrink cracks which may appear.

If the plaster is finished well, the tank should be water tight. A good routine is to seal the inside with a cement based sealer before the tank is filled. Use a push broom and a mixing tray to apply the sealer coats. These materials require one day to cure. Apply in the early evening and start on the shady side of the tank. Small leaks will eventually be sealed by minerals in the water as the water evaporates and leaves the minerals behind.

Cure: There are many ways to keep the tank wet until it cures (28 days = 100%). Old blankets and a hose work well if someone is present. A battery powered water timer can be used to turn a sprinkler on and off, place anagricultural sprinkler on top of a ladder inside, center. Black plastic over wet blankets holds moisture well, it also brings the temperature up and increases the rate of cure. The idea is to maintain moisture in the plaster. Once the plaster has become dry, the state of the material changes and it will not absorb water. Typical concrete used for a floor or foundation has a compression strength of about 210 kgf/cm2. Ferrocement cured well ranges from a low of 450 kgf/cm2 upward to as much as 840 kgf/cm2. These strength figures are a good indicator of how long the structure will last. Good concrete protects the steel from the elements and the structure lasts a long time with no maintenance. The effort put into a good concrete cure will be appreciated for generations.

Mechanical Application: Pumped ferrocement is a mixture of 505 kg of cement per cubic meter of sand. The mixture is pumped through a rubber nozzle. Compressed air throws the plaster against the steel armature. This is the tried and true mechanical technique used in the trades, though there are smaller plaster sand pumping systems available. A strong, active crew on the inside holds sheets of plywood up to the steel as a backing so the plaster doesnŐt simply pass on through. A second crew is required to clean and rotate backing plywood as it grows heavy with deposited plaster. The nozzle operators must also be fairly strong. Concrete sand has some grains that are almost gravel size. Some of these bounce back and accumulate as waste.

Supervision is a more intense part of the job as application speed increases. Clean up is also a bigger job. The quantity of voids decreases with nozzle operator skill and attention. Mechanical application becomes more practical with project size and where trained labor is either too costly or simply unavailable. The pros and cons of mechanical application are subjects of debate. Pumped plaster yields a fine tank. Huge jobs can be done by manually, with enough people.

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