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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


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