Wet-mix spraying has been most commonly used as this method gives a better working environment (high standard of health and safety), a higher and more consistent quality, and much higher production rates, with output rates as high as 20 – 30 m3 per hour, and low rebound (less than 10 %). With dry-mix spraying, much higher rebound levels (approx. 30 to 50 %) and lower performance should be expected.
The total cost of the sprayed concrete process is probably the most disputed issue during the decision process. Some contractors focus on material and equipment costs and put less emphasis on differences in labor, while others are willing to consider all imaginable influences, including savings on total construction time. Where sprayed concrete volumes are in the order of more than 50,000 m3, the difference in construction time can easily reach several months in a pure calculation of the time needed to place a certain volume of sprayed concrete onto the tunnel wall. The wet-mix process demands higher material costs, which is compensated by the multiplying effects of low rebound and high output.
The wet-mix method requires a rather simple supply system, using technology readily available in most countries, such as normal batching plants, ordinary Portland cements, truck mixers etc. The wet-mix design focuses on pumpability, low water/cement ratio and open time. The use of accelerating admixtures at the nozzle enables adaptability to given spraying conditions, e.g. position of application and water ingress. The wear costs of the whole wet-mix process are by far lower than those of dry-mix processes.