Interior Surfaces of the Monolithic Dome

by David B. South

Shotcrete is applied to the inside of a Monolithic Dome when it is constructed leaving a sprayed finish. The sprayed finish will, of course, be rough from the impacting of aggregate. All of the following factors can change the interior surface characteristics of the dome

• The aggregate in the mix.
• The wetness of the mix.
• The type of aggregate, whether smooth or rough.
• The use of fibers.

There are various finished interior surfaces to choose from when building a Monolithic Dome.

Left as Sprayed

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In this case, the interior is left exactly as the spray operator finished it. We can break this into two general categories. One, spraying is done by heavier equipment for faster completion. This is usually done on the large, industrial storages. The surface is generally more pocked with deeper indentations because of the thicker pass made with the shotcrete machine. This thicker pass of soft concrete allows the rock to embed deeper into the larger crevasses and craters. This is what we would expect on large domes, especially for industrial use.

This surface can be smoothed and softened with a lighter application after it has set. The lighter application needs to be with a sand only, no big rock mixture. The lighter application will fill in the deeper craters and smooth it out.

Semi-Finished Surface

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For many applications we want the spray finish to exist, but we would like it to be, in general, a smoother surface. In this case the workman at the end of each of the last two or three passes of concrete will shave the interior surface with some type of device to cut off all of the points, all the "stalactites." This leaves a more even surface. And then the final shotcrete surface will be much smoother.

We see this more in offices and homes where we want a reasonably smooth surface, but do not want to spend the money to trowel it.

Troweled Surfaces

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Often one may choose to trowel a portion of a dome, say in the bedroom or the kitchen. Sometimes it may be below the 8 foot mark, and then leave it rough above. The troweling is generally done with something like plaster after the shotcrete is finished.

It is almost impossible to keep with a shotcrete crew and trowel right behind it. Everyone is in each other's way. The concrete sets too fast for the workers. There is just too much surface area which needs attention too quickly.

Troweling Using Drywall Mud

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There are several ways to trowel this next layer. One is to just simply use drywall mud and apply as you would any drywall mud. Generally it applies quite thick, and sufficient time must given between each coat to allow it cure, and allow the water to evaporate. The first coat or two will crack severely. After they have gone through their cracking then additional coats can be added. This is extremely easy to do and very fast. The mud can be purchased in premixed buckets or one can mix it on site.

Obviously, drywall mud is readily available. It can be purchased in quick-set as well as in normal-set. The quick-set has the advantage of requiring less time between coats. Quick-set costs a little more money. Normal-set needs more time to cure. Drywall mud is always more susceptible to water damage. This is not a problem for an occasional washing, but it is a problem if it is going to be continually wet.

Troweling Using Plaster

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Another technique is to use plaster. This is a technique which has been used for years. This plaster is mixed with Portland cement and lime and troweled. This is generally done by someone who has considerable skill in plastering. This method works very well.

The only caution -- if the plaster is to be in areas which are to be continually wet, such as shower stalls, then lime should not be used in the plaster. The plaster is generally applied in a two or three coat process. The first coat is called a "scratch coat." It is applied basically to size the wall, get it generally smooth. The second coat can either be the finish coat or the "brown coat." The "brown coat" is smoother than the scratch coat. It is applied usually a week or two later, up to three weeks later, to allow for the cracking in the scratch coat. The final coat can troweled extremely smooth, or it can be textured.

In today's world, most of the time we pay for rough surfaces, especially some of the heavy textures. After the surface is troweled smooth, then the texture is applied using a number of instruments to create an even, heavy texture.

Conventional plaster has been used for centuries. It is an extremely good choice. It is usually a little more money.

Troweling Using marcite

marcite could also be used. This is a vinyl plaster which is used on swimming pools. This would create a very slick surface which could handle a huge amount of water, such as might be utilized for a meat packing plant, or another type of clean room.

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