If you want to start a fight, just ask a room full of spray concrete operators: What’s the best system for applying concrete?
Gunite- Dry Gunning
Gunite is a trade name for dry-gunned concrete, invented and patented by a North Carolina man. The term gunite has been used so much that, to most people, it means spray-applied concrete. People often talk about a gunite pool — meaning a concrete pool where the concrete is pneumatically applied or sprayed in place using air pressure. Many of us in the concrete spraying business have started using the term dry gun to delineate this process.
Dry gun means the cement and sand are injected into an air stream conveying it to the nozzle. The nozzle operator then adds the water at the nozzle and has total control of the water-cement ratio. The delivery hose of the mix is generally quite light, as the hose is mostly filled with air containing the mix of cement and sand.
Dry gunning allows concrete to be placed drier than most wet gunning, but that is not always a plus for dome building. Often the concrete is stacked at a very heavy thickness. This can be a disaster for an Airformed structure. Operator training is very critical and can mean the difference between success and failure of application. The danger of dry gunning the concrete on a Monolithic Dome increases as the size of the dome increases.
Shotcrete- Wet Gunning
Wet gun means the wet (already mixed) concrete is pumped to the nozzle. Air is added at the nozzle to carry the concrete mix to the target – again pneumatically applied. Here too operator training is important, but not quite as technical for wet gunners. In Monolithic Dome construction, application mistakes are usually far less critical than with dry gunning. Rebound, or the waste created by sprayed concrete falling to the floor, is usually half as much for shotcrete as compared to gunite.
Dry gun equipment is totally different from the wet gun. The air needed to transport the concrete is at least four to six times greater than that needed for the wet gun system. For wet gunning, a pea rock or grout pump is usually utilized. Monolithic has updated and improved the small peristaltic pump in an effort to provide more affordable equipment for wet gunning.
For house-size domes either system will work. But for the novice the wet gun is my recommendation.
The dry gun does not have the capacity for large domes, 50 feet in diameter and bigger. One Monolithic Dome contractor once had a five-man crew shotcrete 238 cubic yards of concrete in one day! They can routinely apply 100 plus cubic yards. This is well beyond the capacity of a dry gun system.