In definition of shotcrete, we have provided that impact of jet exerts such amount of force that materials are compacted not to sloughing or sagging. The expectation or target must be this but actual situation is never the same as that of practical cases.
The high velocity for projection exerts impacts on surface to be shotcrete which results some materials to rebound. Now we know shotcrete may be concrete or mortar.
In case of mortar, the coarsest particle is fine aggregate but in concrete, we have to provide some coarse aggregate. Obviously the coarsest particles of the mix rebound more.
Thus concrete has more rebound than mortar as materials of shotcrete. The additional information is that due to lack of coarse material, the projected in-situ shotcrete is richer than expected mix proportion according to batch.
What do you think the richer mix produce shotcrete of greater strength?
Obviously strength is increased with cement content but with this advantage, this will lead to additional shrinkage (slight increase) and require additional care.
Dear reader in next part we will discuss about percentage of rebound for different types of section and different process of production shotcrete (wet mix and dry mix).