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