Shotcrete rebound varies for many different reasons, many of which you mention in your question. The water seepage must be controlled or the shotcrete will likely not adhere to the surface and will slough off as the water saturates the fresh shotcrete. Accelerator will help, but it is difficult, if not impossible, to achieve good results against a seeping surface. ACI 506R-05, “Guide to Shotcrete,” estimates approximate range of shotcrete losses from 10 to 30%. Some other factors affecting the percentage of rebound are:

Mixture design

  • Shotcrete process (wet- or dry-mix)
  • Concrete mixture design and materials (for example, microsilica will tend to create less rebound; more than 30% coarse aggregate can cause more rebound)
  • Plastic concrete properties (air content, slump)
  • Nozzleman competence
  • Vertical placement generally has less rebound than overhead
  • Thickness of buildup per layer

Reinforcing grid

  • Size and spacing of reinforcing
  • Stability of reinforcing grid