Spraying can perhaps be most accurately defined as the pneumatic application of a finely-graded cement mortar or concrete. The object is to force the mix onto a surface so that it will adhere firmly and harden to maximum density. This is achieved by literally shooting a jet of the mix through a nozzle at considerable force. A surface which is to be sprayed with concrete must be thoroughly cleaned of all dirt, oil or other foreign matter. When a surface is to be built up to a desired thickness by spraying, it is essential to wash each layer down thoroughly with water after it has hardened. For most repair jobs a mesh of reinforcement should be placed over the surface, both to increase the strength of the finished job and to provide the necessary degree of support for the mix. The size and extent of this reinforcement depends on the nature of the job, or more precisely on the thickness of cover needed. For repair work, the most commonly used material is a 4 inch by 4 inch mesh and 3 inch by 6 inch mesh of 8 gauge or 9 gauge wire. Galvanized, welded-wire fabric should be used when constructing a sprayed concrete wall. The wire in the fabric should be spaced not more than 3 inches apart in each direction. Alternatively, galvanized, expanded metal with openings not greater than 2 and one-fourth by 6 inches can be used. Experience has shown that the best technique for applying sprayed concrete is to hold the nozzle perpendicular to the surface to be sprayed and about 3 to 4 feet from the surface. To ensure that reinforcement is firmly embedded, however, it is advisable to tilt the nozzle slightly so that the concrete is forced behind the steel alternately from both sides. The concrete should be applied over large surfaces by moving the nozzle uniformly over quite a narrow range so that the spreading effect is limited to a small area. Moist curing of sprayed concrete is essential; the duration of the curing should be the same as for concrete placed conventionally.