SHOTCRETING - ResearchGate
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SHOTCRETING
K.V.SUBBA RAO
Shotcreting
• Shotcreting is a method in concrete work in which the
concrete mix is applied in layers under the pressure of
compressed air
• . Shotcreting is done with a special unit, which consists
of a mortar gun or concrete sprayer and a compressor
Shotcreting
• A dry mix of cement and aggregate, usually sand, is
prepared for shotcreting.
• The compressed air forces the mix through a hose to the
nozzle, moistens it with water supplied through another
hose, and then projects it at high velocity (130–170
m/sec) onto the surface being shotcreted.
Shotcreting
• A single shotcrete layer is 10–15 mm thick and has high
mechanical strength (40–70 meganewtons per sq m),
density, water tightness, and cold resistance
• Shotcreting is used in building thin-walled reinforcedconcrete structural components, such as shells, vaults,
and tanks, and in producing the lining in tunnels. It is
also used for hydraulic insulation, for finishing the joints
of reinforced-concrete structural components, and for
repair and strengthening of concrete and reinforcedconcrete components.
Shotcreting
• The shotcrete is placed layer-by-layer
• For shotcrete placing special equipment are used, where
by means of "dry" technology the mixture of cement,
sand and metal batched in specified proportions is
squeezed out with pressure air by means of flexible hose
to the nozzle atomizer, where the mixture is moisturized
with water. When the "wet" technology is used, prepared
concrete is delivered to the machines
Shotcreting
• Shotcreting has high strength properties; resistance of its
extension is 10 per cent more in comparison with
standard concrete. The cover of high strength, density
and waterproofing capacity is formed because of impact
laying
• Shotcrete application allows decreasing face thickness
and out-turns; excluding the necessity of formwork and
mechanizing the concrete work process totally.
Materials for shotcreting
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Cement
Sand
Coarse Aggregates
Water
Chemical additives
Compressed Air
Materials for shotcreting
• Chemical admixtures are added for acceleration of
setting and hardening.
• Sodium Carbonate = ½ Kg per 50 Kg bag of cement
•
Sodium Aluminate = ½ Kg per 50 Kg bag of cement
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Calcium Carbonate = 1 Kg per 50 Kg bag of cement
• Alternatively, following additives can also be used:
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Super plasticizer @ 1 % by weight of cement
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Accelerator (say sodium Silicate) @ 5 % by weight of
cement
Preparation of Surface
• The following precautions are necessary in preparation
of the surface before the shotcrete is applied:
• The surface should be free from lose materials. It should
be clean to receive the shotcrete layers.
• In case the surface is smooth it should be roughened
with sand blasting. Otherwise the surface could be
cleaned with air water jet.
.
Preparation of Surface
• The thickness of the shotcrete layer being only 76 mm
any undulations in the surface more than such thickness
will have to be backfilled with either masonry or lean
concrete depending on the site conditions and
specifications prescribed.
Mix of shotcrete
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Cement
Sand
Coarse aggregate
(5 mm – 10 mm size)
Proportion by weight
450 kg / m3
1100 kg / m3 (0.70 m3)
500 Kg/M3 (0.30 M3)
1:2.44:1.11
Application Methodology
• Spray pneumatically the first layer of shotcrete of 38 mm
thickness
• Welded wire mesh of 100 mm x 100 mm x 5 mm is
nailed with the help of nails. Binding wire is used to keep
the mesh in place.
• The second and final layer of 38 mm of shotcrete is
sprayed pneumatically over the first layer.
Precautions to be taken
• The finished shotcrete layer is to be kept wet
continuously for at least a week after completion.
• There would be rebound during the operations which
shall not be used for shotcrete.
• The time lag between the first layer and the second layer
is to be kept as minimum as possible.-say less than 8
hours.
Precautions to be taken
• After completing the shotcrete and after the mix is set,
the surface should be thoroughly sounded by a hammer
for drumming areas resulting from rebound pockets or
lack of bond etc., which should be cut out and replaced
with fresh shotcrete.
• Where dry process is adopted the jetting of water should
be synchronized with that of the concrete mix for a
homogeneous shotcrete.
• Where wet mix is adopted the specified water cement
ratio is to be strictly enforced.
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NSRC
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