Shotcrete Sprayed Concrete (Gunite = Sprayed Mortar) CE 3420 Concrete Technology Prof. Ravindra Gettu IIT 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 Machine for dry-mix shotcrete Akeley’s invention, 1907 Present-day robot for wet-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 Thomas

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

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

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 Thomas

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: 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 rock

Applications: Sprayed Concrete Lined Tunnels Soft Ground Excavation methods Thomas

Applications: Sprayed Concrete Lined Tunnels Hard Rock (strength more than 50 MPa) 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) Deteriorated face of berth Prepared surface Shotcreting from barge View 10 years after repair Gilbride et al. 2002

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