Each ground-support project is unique and will likely have its own design and performance re-quirements. Performance requirements for shotcrete can generally be divided into two groups: fresh state parameters and hardened state parameters. The following briefly elaborates on the variables included in each of these groups.
The water/cementitious materials ratio is one of the most important parameters controlling shotcrete quality and performance under long-term conditions. In cases where the shotcrete may be exposed to severe environments, the water/cementitious materials ratio would be limited to a specified maximum value, e.g., 0.40. Also, limiting the water/cementitious materials ratio helps reduce shotcrete shrinkage.
Having an adequate air content and air-void spacing factor in the as-placed shotcrete mixture has long been recognized as critical for frost resistance of wet-mix shotcrete. Usually, in wet-mix shotcrete, an as-batched air content of approxi-mately 8 to 10% is used to achieve an as-shot air content of 3 to 5%. However, even if frost resistance is not a concern, there is a definite advantage in using a high air content during batching of shotcrete in that the air entrainment enhances the workability (slump) of the shotcrete. Upon impact on the receiving surface, the air content is reduced, resulting in a reduction in slump in the in-place shotcrete. In projects where the use of accelerators is specified, this slump-killing effect helps reduce the amount of accelerator required to provide slump reduction and shotcrete adhesion.14
The required slump of wet-mix shotcrete for a particular application depends on the specific