1. Shotcrete Sprayed Concrete (Gunite = Sprayed Mortar)
CE 3420 Concrete Technology
Prof. Ravindra Gettu
IIT Madras

2. 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

3. Shotcrete: Delivery Equipment
Machine for dry-mix shotcrete
Akeley’s invention, 1907
Present-day robot
for wet-mix shotcrete

4. 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.

5. 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.

6. Shotcrete: Wet-Mix Process
Thomas

7. 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
Higher
Not problematic
Worse
Easier
Wet-mix
Fixed in mix
Better (in automatic systems)
Problematic
Better (less dust)
Limited (loss of fluidity)
Difficult

8. 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

9. Shotcrete: Components
Cement ( kg/m3)
Aggregate
Silica fume (for cohesion)
Superplasticizer (in wet-mix process)
Viscosity-enhancing agent (in wet-mix process, to reduce rebound)
Accelerator

10. 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°

11. Shotcrete: Placing

12. 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

13. 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).

14. 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.

15. Applications: Scope 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

16. Applications: Sprayed Concrete Lined Tunnels
Soft Ground: soil or weak rock

17. Applications: Sprayed Concrete Lined Tunnels
Soft Ground
Excavation methods
Thomas

18. Applications: Sprayed Concrete Lined Tunnels
Hard Rock (strength more than 50 MPa)
Hard rock tunnel boring machine

19. Applications: Slope stabilisation

20. Applications: Swimming pools

21. Applications: Water tanks

22. Applications: Canals

23. Applications: Shell roofs
City of Arts and Sciences, Valencia (Spain)
White fiber-reinforced shotcrete
5 cm thick shell

24. Applications: Repair

25. Applications: Repair
Repair of ship berth, Port of Saint John, Canada ( )
Deteriorated face of berth Prepared surface
Shotcreting from barge
View 10 years after repair
Gilbride et al. 2002

26. 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