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Shotcrete is mortar or concrete pneumatically projected at high velocities
onto a surface. Sprayed concrete is also referred to as gunite if
using aggregates smaller than 10mm (Stevenson, 1999). Shotcrete linings
are applied by shooting mortar consisting of a thorough mixture of
cement, sand and water into place by means of compressed air. The
thickness of the linings varies from about 25-75mm and wire-mesh
reinforcing and fibre reinforcing is sometimes used.
The advantages of this method of construction are as follows:
- Equipment units are relatively small
and easily moved.
- Fine trimming of subgrades, precise alignment
and close conformity with design dimensions are not commonly
required.
- The lining is more easily placed over
rougher subgrade than concrete linings, and therefore it
is particularly
adaptable
for use in existing
rock cuts where trimming to exact line and grade would
be expensive.
For economy, shotcrete linings of 50mm or
less in thickness are frequently used on small channels or
in mild climates
where service requirements
are not severe. Hydrostatic pressures, settlement and expansion
or shrinkage of the subgrade readily damages thin shotcrete
linings.
Furthermore, the inherent difficulty in controlling the thickness
of the shotcrete application may result in a lining with areas
where the thickness is less than specified, creating
weak areas. In spite
of these problems, thin shotcrete channel linings have been successfully
functioning for over 50 years in some irrigation projects (Swihart
and Rutenbeck, 2001).
Shotcrete can be used almost anywhere where conventional concrete
or mortar would be used. Shotcrete can be reinforced with steel
or can be used without steel. It can use most admixtures appropriate
for concrete. However, since coarse aggregates are not included
in
the mix, this method requires greater proportions of cement than
concrete lining does. It can be applied in any thickness, and
to steep or even vertical slopes (Swihart and Rutenbeck, 2001).
Although conventional concretes can be sprayed (wet-mix process),
better results are generally obtained from the dry-mix process.
In the latter case, the dry ingredients are mixed and then propelled
by compressed air, the water being added at the spray nozzle.
This
requires skilled and experienced personnel, but produces dense
concrete with less shrinkage and better durability than the wet-mix
process
(Stevenson, 1999).
The application of sprayed concrete involves spraying the material
directly against the foundation soils without disrupting them,
and then building up the required thickness of lining by repeated
passes
of the nozzle in a regular pattern (Stevenson, 1999).
The final shape and thickness are determined by screed boards
and/or tensioned wires acting as guides. The concrete surface
can either
be left as sprayed or smoothed off by wooden floats. Joints can
be formed by the inclusion of suitable timber fillets or by tooling
grooves into the surface. (Stevenson, 1999).
Once in place, shotcrete is just concrete, with all the same
beneficial attributes and all the same shrinkage characteristics
that are discussed
in Concrete. Therefore shotcrete must be detailed with joints
at regular intervals to control cracking (Stevenson, 1999).
Shotcrete channel linings are generally used where they are more
economic than concrete. Long channels of large uniform cross-section
can justify the use of expensive, sometimes custom-designed concrete
channel lining machines. Shotcrete equipment is less expensive
than lining machines, but usually produces fewer cubic metres
of lining
per hour. The rate of placement is therefore slow in comparison
to slip-form concreting, and skilled labour is required to operate
equipment
and to control lining thickness. As a result shotcrete is not
generally economical for large lining projects. However, for
short channels,
for parts of channels with frequent sharp curves, for curve sections
of present concrete lined channels, for channel reaches in weathered
rock, and for the repair of channels, it has given excellent
results (Kraatz, 1977). Shotcrete is frequently economical for
small and
irregular cross-section channels or for short lengths of channel.
On some projects, omitting finishing operations (using the natural
gunned finish) saves on costs.
Construction inspection during shotcreting includes maintaining
the specified minimum thickness, testing for strength requirements,
and
maintaining proper temperature and moisture during curing. The
quality of a shotcrete lining depends greatly on the skill of
the nozzleman.
Pre-construction shotcrete panels are frequently required to
test the ability of the crew, the equipment, and the shotcrete
mixture
to produce shotcrete that meets specifications (Swihart and Rutenbeck,
2001).
Application of shotcrete as a full waterway liner is a possible
alternative to plastic sheeting or concrete. This material has
been used recently
for irrigation channels in the United States with reasonable
success in stable ground, although the climatic and installation
conditions
are much different to many areas of Australia. The long-term
suitability of the material as a liner is still under investigation
and no Australian
literature on the experience with shotcrete in channel lining
was located in the literature review.
Shotcrete lining has been used on trial sections recently in
the United States as part of the Deschutes Trials (Swihart et
al., 1994
Swihart and Haynes, 1999). There were a number of lining installations
using shotcrete either reinforced with steel fibres, poly fibres
or unreinforced. All sections were rated as being in excellent
condition after 7 years of service. The seepage reduction achieved
was 120L/m2/day,
which represented a 60% reduction in seepage.
The cost is estimated to be of the order of $30/m2 to line a
channel with 75mm of shotcrete over a geotextile (Sinclair Knight
Merz, 1998).
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Concrete
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