Q. I am planning to have a swimming pool built and have been told that there is a difference in strength and durability between shotcrete and gunite. Which is preferable for a swimming pool?
A. Shotcrete is the generic term that is now in use for any method "used to shoot concrete at a sufficient velocity to achieve proper compaction," according to ACI. This includes both the wet-mix and dry-mix processes. Gunite is the original proprietary name for dry-mix shotcrete, and both the name and the process are still in common usage. With a dry-mix process, all of the dry ingredients (cement, admixtures, sand and aggregate) are forced through a hose to a nozzle where they are mixed with water as the shotcrete is sprayed onto the surface. In a wet-mix process, wet concrete is pumped through a hose to a nozzle where compressed air is added to increase the velocity of the material.
Rich Heitzmann, executive director of the American Shotcrete Association, says that either method, wet or dry, can give excellent results. "It depends on the skills of the nozzle operator. But the skills are more critical when applying dry-mix since the operator controls not only the flow rate, but also the rate of water addition and even some of the admixtures. But asking which is better is like asking whether a hammer or a screwdriver is better—it depends on the application and the operator."