Shotcrete presents a drawback known as rebound. The rebound is the escape of part of the coarse aggregate of concrete as a result of the impact that gets on the stand. Obviously, to obtain economy rebound should be minimal. In projected with coarse aggregate concretes the rebound becomes important primarily depending on the skill of the operator. The shotcrete bounce rate depends on many factors such as: effectiveness of the hydration of the mixture that comes out of the nozzle relationship water / cement: the water/cement ratio usually oscillates at the nozzle between 0.33 and 0.50 depending on humidity having the sands. Lower values give highest percentage rebound but if these are too high cause the lift of the layer, in addition to give rise to a concrete porous and not very resistant. Granulometry of aggregates: the influence of the particle size is very large and should be carefully studied.
Sand and gravel content is very important. Speed and output volume of the mixing nozzle by unit of time angle and distance of the impact: on the rebound also influences the slope of the walls and the distance of the nozzle to the surface on which the application, which must be of 1,0 meter is done. Thickness applied skill of the operator. The amount of rebound in correct applications varies from 5 to 15% on floors or slabs, from 15 to 30% in vertical or inclined walls and 25-50% on ceilings. On the first layer of concrete gunite that applies rebound percentage is always excessive, but decreases as the cement mattress is forming and sand, by embedding the coarse aggregate in this mattress. In the first layer rebound can become 15% depending on many factors. A 2 or 3% of gunite special additives can improve much to the mix and reduce the bounce rate.