A REVIEW OF SHOTCRETE MATERIALS, MIX DESIGN AND APPLICATIONS

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A REVIEW OF SHOTCRETE MATERIALS, MIX DESIGN AND
APPLICATIONS
University of Cape Town
Concrete Materials and Structural Integrity
Research Group
M G Alexander and R Heiyantuduwa

• Department of Civil Engineering, University of
  Cape Town
• Shotcrete for Africa Conference
• 2 3 March 2009

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Overview

• Introduction
• Shotcrete processes
• Applications of shotcrete
• Shotcrete materials and mix design
• Properties of shotcrete
• Conclusions

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Introduction

• Shotcrete is a form of concrete commonly made
  with small
• sized aggregate, pneumatically projected at high
  velocity from
• a nozzle onto a suitable substrate to produce a
  dense
• homogeneous mass.
• Advantages of Shotcrete
• Economical
• Formwork not required
• Can achieve high compressive
• strengths
• Good bond with various substrates

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Shotcrete processes dry mix process

Transport pneumatically

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Shotcrete processes wet mix process
Aggregate
Aggregate
Aggregate
Batch
Aggregate
Batch
Aggregate
Batch
Aggregate
Batch
Aggregate
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Aggregate
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Aggregate
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Aggregate
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Aggregate
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Aggregate
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Aggregate
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Aggregate
Aggregate
Aggregate
Aggregate
Aggregate
Batch
Aggregate
Aggregate
Aggregate
Aggregate
Aggregate
Aggregate
Aggregate
Batch
Aggregate
Batch
Aggregate
Batch
Aggregate
Batch
Aggregate
Batch
Aggregate
Batch
Aggregate
Batch
Aggregate
Batch
Aggregate
Batch
Aggregate
Batch
Batch
Batch
Batch
Batch
Batch
Batch
Batch
Put into delivery equipment/pump
Put into delivery equipment/pump
Put into delivery equipment/pump
Put into delivery equipment/pump
Put into delivery equipment/pump
Put into delivery equipment/pump
Put into delivery equipment/pump
Put into delivery equipment/pump
Put into delivery equipment/pump
Put into delivery equipment/pump
Put into delivery equipment/pump
Put into delivery equipment/pump
Put into delivery equipment/pump
Put into delivery equipment/pump
Put into delivery equip. / pump
Mix
Mix
Mix
Mix
Mix
Mix
Mix
Pump
Mix
Pump
Mix
Pump
Mix
Mix
Mix
Pump
Mix
Pump
Mix
Pump
Pump
Pump
Pump
Pump
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Cementitious material
Batch
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Inject compressed air
Inject compressed air
Pass through nozzle
Inject compressed air
Pass through nozzle
Inject compressed air
Pass through nozzle
Inject compressed air
Pass through nozzle
Inject compressed air
Pass through nozzle
Inject compressed air
Inject compressed air
Pass through nozzle
Inject compressed air
Pass through nozzle
Inject compressed air

• Batch

Water
Water
Water
Water
Batch
Water
Batch
Water
Batch
Water
Batch
Water
Batch
Water
Batch
Water
Batch
Water
Batch
Water
Batch
Water
Water
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Water
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Water
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Water
Water
Water
Water
Water
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Water
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Water
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Water
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Water
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Water
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Water
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Water
Water
Water
Water
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Water
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Water
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Water
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Water
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Water
Batch
Water
Project onto surface
Project onto surface
Project onto surface
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Comparison of dry and wet-mix processes
Dry-mix process Wet-mix process
Instantaneous control over mixing water and consistence of mix at the nozzle. Better suited for mixes containing low-density aggregates, refractory materials and shotcrete needing early strength. Batches of mixed material are capable of being transported longer distances. Mixing water is controlled at the point of manufacture and can be measured. Special mixing and transporting equipment needed. Less dust generation and cement loss.
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Comparison of dry and wet-mix processes (contd)
Dry-mix process Wet-mix process
Minimal waste and greater placement flexibility. Low water/cement ratio results in higher strengths. Very operator sensitive. Better assurance that the mixing water is thoroughly mixed with other ingredients. Higher wastage. Lower rebound. Capable of greater production. Higher cost of placed material.
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Applications of shotcrete

• New structures
• Linings and coatings
• Repair
• Strengthening and reinforcing
• Support of underground openings
• Refractory shotcrete
• Special shotcrete

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Example of shotcrete support and walls in a
tunnel project

• tunnelling

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Materials and mix design

• Cementitious materials
• SABS EN 197-1 common cements of Type I and Type
  II
• Type III cements should only be used in the 42.5
  strength
• class
• (Above cements often contain mineral additives
  in the blend)
• Condensed Silica Fume (CSF)
• Calcium Aluminate Cement
• Aggregates
• Aggregate used for shotcrete should comply with
  requirements for use in conventional concrete
  i.e. SANS 1083.
• Rounded aggregates are preferable (but are
  frequently crushed in SA, and therefore angular)
• Well graded aggregate is important with no
  fraction constituting more than 30 of the total

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Materials and mix design (contd)

• Mixing water
• The requirements for mixing water are the same as
  for
• conventional concrete i.e. clean and potable.
• Admixtures
• Accelerators, air-entraining admixtures,
  pozzolanic additives,
• plasticisers, retarders, activators
• Reinforcement
• Weldmesh, fibre reinforcement polymeric or steel

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Materials and mix design

• Typical shotcrete mix proportions

Mass (per m3)
Cementitious content - dry mix 350 450 kg
- wet mix 400 500 kg
Silica Fume (5 12 of cementitious mass) 30 50 kg
Water/cementitious ratio 0.35 0.5
Aggregate/cementitious ratio 3 5
Steel fibre (when present) 30 50 kg
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Typical properties of shotcrete vs. cast concrete
Property High quality shotcrete Cast in situ concrete
Compress. strength, 1 d (MPa) 20 6 (estim.)
Compress. strength, 28 d ( MPa) 30-60 30-50
Elastic modulus, 28 d (GPa) 34 31 (estim.)
Poissons ratio, 28 d 0.18 0.15 0.22
Tensile strength, 28 d (MPa) gt 2 (estim.) 3.8 (estim.)
Initial setting time (start-end) (min) 3 5 45 145 (estim.)
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Typical properties of shotcrete vs. cast concrete
(Contd)
Property High quality shotcrete Cast in situ concrete
Shrinkage after 100 d () 0.06 0.12 0.03 0.08
Specific creep - 60 d (/MPa) 0.01 0.06 0.008
Density (kg/m3) 2140 2235 2200 2450
Total porosity () 15 20 12 18
Permeability (m/s) 10-12 to 10-14 10-11 to 10-12
Microcracking, 28 d (cracks/m) 1300 -
Coefft. of thermal expan. (K-1) 8 15 x10-6 8 12 x10-6
Slump (estimate) (mm) 0 to 200 50 to 120
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Durability of shotcrete

• Particularly for linings in tunnels
• Carbonation
• Infiltration through the lining of pure waters or
  waters carrying aggressive agents, resulting in
  leaching and/or degradation
• Durability testing
• Can be done on mock-up panels
• Should also preferably be done on in-situ
  samples
• Boiled Absorption (BA)
• Volume of Permeable Voids
• Durability Index tests (SA tests currently being
  developed for conventional concrete)

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Conclusions

• The properties and performance of shotcrete are
  largely dependent on the conditions under which
  it is placed (i.e. curing, compaction) as well as
  on the characteristics of the particular
  equipment, and ultimately on the competence and
  experience of the application crew.
• Based on improvements in material quality and
  versatility, which provide enhanced physical
  properties, shotcrete techniques could further
  replace more conventional construction
  procedures.

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Thank you for your attention!