Concrete is a versatile material that forms walls, columns, swimming pools and other structures. Gunite, which received its name from its gunlike applicator, is the dry-mixed form of sprayed concrete or shotcrete. Since 1967, the American Concrete Institute has used the term shotcrete to refer to both dry- and wet-mix sprayed concrete. After builders apply wet concrete or gunite to a form or surface, the material sets and hardens, and is ready for painting or finishing.
Wet ready-mix concrete pours from a truck through a chute, and spreads onto a surface or form. The spreader can be a square-edged, short-handled or long-handled shovel or hoelike tool. The Concrete Network recommends spreading the concrete as closely as possible to its final thickness to ensure proper straightedge finishing. On the other hand, gunite sprays directly onto lightweight, erected wall panels. Compressed air forces the dry mix through a nozzle onto the receiving surface, where it combines with water.
Gunite and concrete contain cement, which binds together fine aggregate particles, such as sand, and large aggregates, such as gravel. Concrete usually contains fine and large aggregate particles, but gunite usually contains only fine particles. Ready-mix concrete contains water, but a nozzleman must add water to dry-mix shotcrete as it emerges from the nozzle. Gunite and concrete may contain additional cementitious materials, such as fibers or silica fume, to improve application and surface adhesion.
According to the Portland Cement Association, both often contain recycled ingredients, such as fly ash. The materials create durable, pest-resistant, energy-efficient structures. Concrete is especially suitable for columns, foundations, beams and other support structures. Gunite dries and sets faster than concrete, and readily adheres to overhead structures, curves and arches. Since gunite has a lower water-to-cementitious materials ratio, it provides a structure with low shrinkage and low water permeability rates.
The American Shotcrete Association states that only a highly-skilled, certified nozzleman should apply gunite. The American Concrete Institute certifies nozzlemen, who must precisely control the combination of water and dry mix at the receiving surface. The nozzleman must prepare a fresh gunite mixture at the construction site, reducing the risk of substandard mixtures. Concrete is prepared away from the construction site and may have defects that are not apparent until application. For example, concrete which lacks the necessary retarding admixtures may dry too quickly and crack in hot weather.