GeneralQ What is the efficiency of dry-mix shotcrete? How much is over spray vs. how much sticks to the surface?
Dry-mix shotcrete may have more rebound (coarse aggregate that bounces off the surface) than wet-mix so it may be considered a little less efficient. However, predampening and the use of special nozzles can increase wetting and reduce rebound which makes the dry-mix efficiency approach wet-mix. Rebound may be estimated as 5 to 15%, with an average of 10% of the weight of the concrete materials. The experience and placing technique of the nozzleman can substantially affect the amount of rebound in either dry-mix or wet-mix. Overspray is much less and may depend on wind conditions and placing techniques. When looking at the overall efficiency of shotcrete placement in a given section, dry-mix materials can be tailored much closer to the actual need while wet-mix may have minimum concrete delivery volume and time constraints that would end up not using all the material delivered. Also, dry-mix has approximately ¼ the production rate of wet-mix, so in high-volume placements, wet-mix may have a natural advantage in productivity.
