What is Rebound in Dry-Mix Process in Shotcreting?

If you were to research the American Concrete Institute or the American Shotcrete Association, you would find that both terms refer to the application process of good old fashion concrete. The concrete mixture travels from an applicator’s truck to the site via a large hose. Air pressure is applied and the mixture is “shot” into place at speeds approaching 200 mph. This is extremely important! Why? Because it is with the forced placement of the concrete that either process gains it’s merit. Any concrete placement should be “compacted” to remove voids, air bubbles that weaken the strength of the eventually hardened concrete. For concrete that is poured, applicators will use a variety of methods, tamping, vibration, etc, to achieve compaction, but no process by hand can compare to the compaction that can be achieved by the pneumatically applied shot-crete or gunite.

 Rebound Safety of  Shotcrete: Safety Equipment, Including Facing Shields or Goggles, Respiratory Protection and Waterproof Gloves.

Now what is “rebound”? Rebound refers to the aggregate, which bounces or deflects off of the receiving material and lands in areas other than intended. When the small piece of sand is in mixture, it is combined with the moisten cement and if properly placed will added strength to the structure. If on the other hand, bounces off something hard, such as the wooden form or a piece of the steel reinforcement, the cement is likely to adhere to the hardened surface, but the sand, now mostly stripped of the binding agent falls elsewhere. This rebound, must be gathered up and removed, not allowed to become part of the structure. To do so would be to create a weakened structure. It is known that the dry-mix process creates substantially more rebound than the wet-mix process. If the applicators are skilled in the process this deficiency can be overcome.