Basic Steps to Applying Shotcrete to a Monolithic Dome

January 22, 2004

by Kris Garrison

To keep the thickness correct throughout the application process, apply the concrete as evenly as possible at all times.

Layer One

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The shotcrete is started at the bottom of the dome. First, a thick, tapered layer of shotcrete should be applied around the entire circumferance of the dome, at the base, up to about one foot high. This ensures the concrete on the footing is "good" concrete and not rebound shotcrete.

A 1/2" to 1" layer is then sprayed on the surfaced from ground level up to about 6' high. From 6' high on up to the top third of the dome, a 1/2" layer is applied. The top third of the dome is covered with 1/4" to 1/2" of shotcrete.

Layer Two

The second layer is usually applied the second day. Begin again at the bottom. Up to a 1" layer is applied from ground level to approximately 8'. From 8' to the top, a 1/2" layer is then applied.

Layer Three

If the weather is warm and the concrete is setting up fast and layer two is applied early in the day, layer three can be applied in the afternoon of day two. The third layer is an exact duplicate of the second layer except that dome will support more weight and the layers can go on thicker and higher. If possible, apply at least 1/4" to the top of the dome or you wind up having to spray just the top on the last days. By the third day, the concrete around the base of the dome will be strong enough to support additional concrete if it is needed for extra thickness.

Layer Four

Layer four is a repeat of the third. The base should be worked for smoothness. Particular attention should be given to the depth gauges. This layer can be applied in the morning and final layer in the afternoon.

The Final Layer

The final layer should be relatively thin (about 1/4" to 1/2") to permit a smooth finish. Before spraying this last layer, do a final depth check. If adequate thickness is not reached by this time, it is necessary to spray additional layers as needed.

The finishing layer of concrete should be sprayed from the top down. It seems easier to make a nice finish if the final concrete layer start at the top.

Note: It is very difficult to judge the depth of sprayed concrete as it is being applied. To insure proper thickness, check the depth gauges. A 1" layer can look very much like a 1/8" layer. To be sure of a uniform build-up of thickness, a uniform spraying pattern should be followed. This pattern can vary according to the nozzleman, but it should be consistent.

Tips

If at anytime during the concrete application there is so much concrete being applied that the Airform sags, the concrete should be immediately removed and the Airform returned to its normal shape. A lesser amount of concrete should then be applied.

It is important to use good shotcrete spraying techniques when shooting around rebar, that is, shoot from close enough to the bar and with enough force that the concrete cannot build up on the face of the bar, but closes around it from the back.

After the final concrete is applied, the air pressure should be left at 2" for 24 hours.