The shotcrete presents a problem known as rebound. The rebound is the escape of part of concrete coarse aggregate as a result of the impact on the support it receives. Obviously, the rebound for economy needs to be minimized. In concrete with coarse aggregate projected rebound becomes important to rely primarily on operator skill. The percentage rebound of shotcrete depends on many factors such as: Effectiveness of the hydration of the mixture leaving the nozzle Water / cement: water-cement ratio is usually somewhere in the nozzle between 0.33 and 0.50 depending of moisture with the sands. Lower values give more bounce but if they are too high cause sagging of the layer, besides giving rise to a porous concrete and very resistant. Particle size of the aggregates: the influence of the particle size is very large and should be carefully studied. It is very important the content of sand and gravel.
Speed and output volume of the mixture through the nozzle unit time of impact angle and distance: On the rebound is also influenced by the inclination of the walls and the distance from the nozzle to the surface on which the application is made, which should be 1.0 meters. Operator Skill thickness applied. The amount of rebound in correct applications varies from 5 to 15% on floors or slabs, 15 to 30% in vertical or inclined walls and from 25 to 50% on roofs. In the first layer of shotcrete concrete implementing the bounce rate is always excessive, but decreases as the mattress is formed of cement and sand, embedding the coarse aggregate in this matter. In the first layer the rebound may be as high as 15% depending on many factors. A 2 or 3% of additives for shotcrete can greatly improve the mix and reduce the bounce rate.