Shotcrete is both a material and a method. The material is concrete, mortar or grout. The method is the application of this material by pneumatic pressure through a specially adapted hose. There are two basic types of shotcrete: in one, the materials are combined in a central mixer and then transported through a hose, out the nozzle and into place; in the other, the dry ingredients are combined, transported through a hose and mixed with water at the nozzle. The former is referred to as the wet-mix process; the latter, the dry-mix process. In the dry-mix process, there is a wide range of control over the amount of mixing water used to adjust mix consistency as the job progresses. It also allows transportation of the mix through a longer length of hose than is possible with wet-mix. The dry-mix process is better suited for concrete containing lightweight porous aggregates than it is for a mixture with normal weight aggregate. It also requires the operator of the nozzle be exceptionally skilled in controlling water content and, therefore, consistency, strength and watertightness as the placement of the concrete progresses. To determine the mix proportions that will best suit the job at hand, it is advisable to test about three different mixtures falling within the range of 1 part cement to 3 to 4 and one-half parts sand. The sand pumped in the dry-mix process should be damp but its water content should be kept reasonably uniform. If it is allowed to vary widely, the nozzle man will have a difficult time maintaining acceptable slump and uniformity of the shotcrete in place. The wet-mix permits better control and more versatile mix designs than the dry-mix process. Any type of admixture such as an air-entraining, water-reducing and/or set-controlling agent can be used. Thorough mixing of the ingredients is easier to achieve. More accurate control of the mix proportions is possible. The mix proportions for the wet-mix process are about the same as for the dry-mix process. The entire mix is premixed to a slump suitable for the work. The slump is usually in the range from 0 to 4 inches.