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7/27/2019 8. Shotcrete
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Shotcrete
Sprayed Concrete(Gunite = Sprayed Mortar)
CE 3420 Concrete Technology
Prof. Ravindra Gettu
IIT Madras
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Shotcrete: Definition
Concrete or mortar that is placed
and compacted by projecting itpneumatically at high velocity
onto a surface.
Wet-mix shotcrete: All major
ingredients are mixed before
introduction in the hose.
Compressed air is introduced to
the material at the nozzle. If an
accelerator is used, it is normallyadded at the nozzle.
Dry-mix shotcrete: mixing water is
added at the nozzle to dry pre-mix
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Shotcrete: Delivery Equipment
Akeleys invention, 1907
Present-day robot
for wet-mix shotcrete
Machine for dry-
mix shotcrete
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Shotcrete: Dry-Mix Process Binder and aggregates are dry mixed.
Mix is fed into delivery hose.
Mix is transported by compressed air to the nozzle, where water isintroduced under pressure.
Material is jetted at high velocity onto surface to be shotcreted.
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Shotcrete: Wet-Mix Process All ingredients (excluding the accelerator) are thoroughly mixed.
Mix is fed into delivery hose.
Mix is transported by compressed air to nozzle, where accelerator is added. Additional air is injected to increase velocity
Material is jetted at high velocity onto surface to be shotcreted.
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Shotcrete: Wet-Mix Process
Thomas
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Dry-Mix or Wet-Mix ?
Important factors
Equipment cost
Control of water
Productivity
Rebound
Interruptions
Work environmentTransport of mix
Cleaning
Wear of equipment
Dry-mix
Lower
Variable at nozzle
Lower
Higher
Not problematic
WorseNot problematic
Easier
Higher
Wet-mix
Higher
Fixed in mix
Better (in automatic
systems)
Lower
Problematic
Better (less dust)Limited (loss of fluidity)
Difficult
Lower
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Shotcrete: Components
Differences between shotcrete and a normal concrete mix
Shotcrete usually has:
Higher water/cement ratio
Higher sand content Smaller maximum aggregate size (10 or 12 mm)
Accelerator to speed up the hydration reactions
Plasticizers, stabilizers and mineral admixtures
High early age (e.g., 1 day) strength
Thomas
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Shotcrete: Components
Cement (350-400 kg/m3)
Aggregate
Silica fume (for cohesion)
Superplasticizer (in wet-mix process)
Viscosity-enhancing agent (in wet-mix process, to reduce
rebound)
Accelerator
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Shotcrete: Placing
Quality of placed shotcrete depends on the skill and
experience of nozzleman
Rebound occurs when aggregates and cement paste
ricochet off the hard surface on contact. Depends on:
Mix composition Surface regularity and inclination
Velocity and distance of nozzle from surface
Deviation of projection angle from 90
Sh Pl i
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Shotcrete: Placing
Sh t t Pl i P bl
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Shotcrete: Placing Problems
Shadowing = absence of concrete
behind reinforcement
Sloughing = partial loss of concrete
after placing due to own weight
Lamination = layers of concrete with
poor bond between layers
Thomas
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Shotcrete: Implications of placing
Placed concrete has different characteristics than the mix.
Properties of placed shotcrete are not necessarily homogenousor isotropic.
Compressive strength could be 10-25% higher in the plane
perpendicular to the direction of spraying; core strengths are
usually conservative (Thomas, 2009).
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Fibre Reinforced Shotcrete
Advantages:
Eliminates the need for mesh reinforcement Provides toughness and impact resistance
Improves crack control considerably
Fibres used: Steel
Polymeric
Glass (in spray-up process used for the manufacture oflightweight cladding panels)
Rebound is an important issue: In dry-mix shotcrete, rebound could be 30% for the fibre
concrete as a whole and an additional 10% of fibres.
In wet-mix shotcrete, rebound could be 10% for the fibre
concrete as a whole and an additional 10% of fibres.
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Applications: Scope
Shotcrete can lead to cost savings:
Where formwork is impractical, or can be reduced oreliminated
Where access to work area is difficult
Where thin layers or layers with variable thickness isrequired
Where normal casting procedures are impractical
A li ti S d
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Applications: Sprayed
Concrete Lined Tunnels
Soft Ground: soil or weak rock
A li ti S d
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Applications: Sprayed
Concrete Lined TunnelsSoft Ground
Thomas
Excavationme
thods
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Hard Rock (strength more than 50 MPa)
Applications: Sprayed
Concrete Lined Tunnels
Hard rock tunnel boring machine
A li i Sl bili i
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Applications: Slope stabilisation
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Applications: Swimming pools
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Applications: Water tanks
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Applications: Canals
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Applications: Shell roofs
City of Arts
and Sciences,
Valencia
(Spain)
White fiber-reinforced
shotcrete
5 cm thick
shell
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Applications: Repair
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Applications: RepairRepair of ship berth, Port of Saint John, Canada (1986-95)
Gilbride et al. 2002
Shotcreting from barge
View 10 years after repair
Deteriorated face of berth Prepared surface
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References
American Concrete Institute Committee 506 Reports Fiber-Reinforced Cement Composites, P.N.Balaguru &
S.P.Shah, McGraw Hill, New York, 1992
Sprayed Concrete Lined Tunnels, A. Thomas, Taylor &
Francis, London, 2009 ACI Materials Journal
Concrete International Journal, ACI