1 Shotcrete Sprayed Concrete (Gunite = Sprayed Mortar) CE 3420 Concrete TechnologyProf. Ravindra GettuIIT 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
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 velocityMaterial is jetted at high velocity onto surface to be shotcreted.
7 Dry-Mix or Wet-Mix ? Important factors Equipment cost Control of water ProductivityReboundInterruptionsWork environmentTransport of mixCleaningWear of equipmentDry-mixLowerVariable – at nozzleHigherNot problematicWorseEasierWet-mixFixed in mixBetter (in automatic systems)ProblematicBetter (less dust)Limited (loss of fluidity)Difficult
8 Shotcrete: Components Differences between shotcrete and a normal concrete mixShotcrete usually has:Higher water/cement ratioHigher sand contentSmaller maximum aggregate size (10 or 12 mm)Accelerator to speed up the hydration reactionsPlasticizers, stabilizers and mineral admixturesHigh early age (e.g., 1 day) strengthThomas
9 Shotcrete: Components Cement ( kg/m3)AggregateSilica fume (for cohesion)Superplasticizer (in wet-mix process)Viscosity-enhancing agent (in wet-mix process, to reduce rebound)Accelerator
10 Shotcrete: PlacingQuality of placed shotcrete depends on the skill and experience of nozzlemanRebound occurs when aggregates and cement paste ricochet off the hard surface on contact. Depends on:Mix compositionSurface regularity and inclinationVelocity and distance of nozzle from surfaceDeviation of projection angle from 90°
12 Shotcrete: Placing Problems Shadowing = absence of concrete behind reinforcementSloughing = partial loss of concrete after placing due to own weightLamination = layers of concrete with poor bond between layersThomas
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 reinforcementProvides toughness and impact resistanceImproves crack control considerablyFibres used:SteelPolymericGlass (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 eliminatedWhere access to work area is difficultWhere thin layers or layers with variable thickness is requiredWhere normal casting procedures are impractical
16 Applications: Sprayed Concrete Lined Tunnels Soft Ground: soil or weak rock
25 Applications: RepairRepair of ship berth, Port of Saint John, Canada ( )Deteriorated face of berth Prepared surfaceShotcreting from bargeView 10 years after repairGilbride 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, 1992Sprayed Concrete Lined Tunnels, A. Thomas, Taylor & Francis, London, 2009ACI Materials JournalConcrete International Journal, ACI