Simply said, shotcrete is sprayed concrete or mortar. There are two different types of shotcrete: wet mix and dry mix. With dry mix, the dry ingredients are mixed and blown in a stream of compressed air through the hose to the nozzle where water is added immediately before the material exits the nozzle; only enough water for hydration is required. Elimination of surplus water results in high-quality concrete and virtually eliminates shrinkage and cracking.
In the wet mix process all the ingredients, including water, are premixed. The mix is then pumped to the nozzle where compressed air blows it into the substrate. Surplus water is required because the mix must have sufficient plasticity to be pumped through the hose. Although this results in some sacrifice of quality, the composition is as good as any quality concrete.
Shotcrete is perhaps the most diverse method available for concrete construction. Procedures for its proper performance are well developed, and high-quality work is regularly obtained.