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ShotcreteSprayed Concrete
(Gunite = Sprayed Mortar)
CE 3420 Concrete TechnologyCE 3420 Concrete TechnologyProf. Ravindra GettuProf. Ravindra Gettu
IIT MadrasIIT Madras
Shotcrete: Definition
Concrete or mortar that is placed and compacted by projecting it pneumatically 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 normally added at the nozzle.
Dry-mix shotcrete: mixing water is added at the nozzle to dry pre-mix
Shotcrete: Delivery Equipment
Akeley’s invention, 1907
Present-day robotfor wet-mix shotcrete
Machine for dry-mix shotcrete
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 is
introduced under pressure.• Material is jetted at high velocity onto surface to be shotcreted.
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.
Shotcrete: Wet-Mix Process
ThomasThomas
Dry-Mix or Wet-Mix ?
Important factors
Equipment cost
Control of water
Productivity
Rebound
Interruptions
Work environment
Transport of mix
Cleaning
Wear of equipment
Dry-mix
Lower
Variable – at nozzle
Lower
Higher
Not problematic
Worse
Not problematic
Easier
Higher
Wet-mix
Higher
Fixed in mix
Better (in automatic systems)
Lower
Problematic
Better (less dust)
Limited (loss of fluidity)
Difficult
Lower
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
ThomasThomas
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
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°
Shotcrete: Placing
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
ThomasThomas
Shotcrete: Implications of placing• Placed concrete has different characteristics than the mix.• Properties of placed shotcrete are not necessarily homogenous
or isotropic.• Compressive strength could be 10-25% higher in the plane
perpendicular to the direction of spraying; core strengths are usually conservative (Thomas, 2009).
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 of
lightweight 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.
Applications: Scope Shotcrete can lead to cost savings:Shotcrete can lead to cost savings:
• Where formwork is impractical, or can be reduced or eliminated
• Where access to work area is difficult
• Where thin layers or layers with variable thickness is required
• Where normal casting procedures are impractical
Applications: Sprayed
Concrete Lined Tunnels Soft Ground: soil or weak rockSoft Ground: soil or weak rock
Applications: Sprayed
Concrete Lined Tunnels Soft GroundSoft Ground
ThomasThomas
Exc
avat
ion
met
hods
Hard Rock (strength more than 50 MPa)Hard Rock (strength more than 50 MPa)
Applications: Sprayed
Concrete Lined Tunnels
Hard rock tunnel boring machine
Applications: Slope stabilisation
Applications: Swimming pools
Applications: Water tanks
Applications: Canals
Applications: Shell roofs
City of Arts and Sciences, Valencia (Spain)
White fiber-reinforced shotcrete
5 cm thick shell
Applications: Repair
Applications: Repair Repair of ship berth, Port of Saint John, Canada (1986-95)Repair of ship berth, Port of Saint John, Canada (1986-95)
Gilbride et al. 2002Gilbride et al. 2002
Shotcreting from bargeShotcreting from barge
View 10 years after repairView 10 years after repair
Deteriorated face of berth Prepared surfaceDeteriorated face of berth Prepared surface
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