PORTLAND CEMENT CONCRETE - Materials for Civil and Construction Engineers - page 291

FIGURE 7.41 Shotcrete used in lining a tunnel.

Shotcrete is applied by either the dry or wet process. In the dry process,
a premixed blend of cement and damp aggregate is propelled through a hose
by compressed air to a nozzle, while the water is added at the nozzle. In the
wet process, all ingredients are premixed and pumped through a hose to the
nozzle and forced to the surface using compressed air. In either case, the noz-
zle should be held perpendicular to the surface to reduce the rebound of
coarse aggregates off the surface. The nozzle is held about 0.5 to 1.5 m (1.5 to
5 ft) away from the surface.
Supplementary cementitious materials, such as fly ash and silica fume,
can be used in shotcrete to improve workability, chemical resistance, and
durability. Accelerating admixtures can also be used to reduce the time of
initial set and to allow buildup of thicker layers of shotcrete in a single pass.
Steel fibers may also be used to improve flexural strength, ductility, and
toughness (Kosmatka et al. 2002).

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7.6.4 Lightweight Concrete

Students competing in the annual ASCE concrete canoe competition fre-
quently produce concrete with a unit weight less than water. The ACI Guide