a review of shotcrete materials, mix design and applications department of civil engineering,...

A REVIEW OF SHOTCRETE MATERIALS, MIX DESIGN AND

APPLICATIONS

Department of Civil Engineering, University of Cape Town

Shotcrete for Africa Conference

2 – 3 March 2009

M G Alexander and R Heiyantuduwa

University of Cape Town Concrete Materials and Structural Integrity Research Group

Overview

• Introduction

• Shotcrete processes

• Applications of shotcrete

• Shotcrete materials and mix design

• Properties of shotcrete

• Conclusions

IntroductionShotcrete 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

Shotcrete processes – dry mix process

Transport pneumatically

Aggregate

Cementitious material

Water

Admixture: powder

Admixture: liquid

Fibres

Batch

Batch

Batch

Batch

Batch

Mix

Mix in nozzleGauge by eye

Put into mechanical

feeder

Project onto

surface

Gauge by flowmeter

Mix

Shotcrete processes – wet mix process

Aggregate


Water

Admixture: conventional

Admixture: special

Fibres

Batch

Batch

Batch

Batch

Batch

Pump

Project onto surface

Inject

Batch

MixAggregate


Water

Aggregate


Water

Aggregate


Batch

Water

Aggregate


Batch

Batch

Water

Aggregate


Batch

Batch

Batch

Water

Aggregate


Mix

Batch

Batch

Batch

Water

Aggregate


Mix

Batch

Batch

Batch

Water

Aggregate


Mix

Batch

Batch

Batch

Water

Aggregate


Put into delivery equipment/pump

Mix

Batch

Batch

Batch

Water

Aggregate


Inject compressed air


Mix

Batch

Batch

Batch

Water

Aggregate


Pump



Mix

Batch

Batch

Batch

Water

Aggregate


Pass through nozzle

Pump



Mix

Batch

Batch

Batch

Water

Aggregate



Pass through nozzle

Pump



Mix

Batch

Batch

Batch

Water

Aggregate


Aggregate


Water

Aggregate


BatchWater

Aggregate


Batch

BatchWater

Aggregate


Batch

Batch

BatchWater

Aggregate


Aggregate


Aggregate

Water


AggregateAggregate


Aggregate

Water


Aggregate Batch

Water


Aggregate

Batch

Batch

Water


Aggregate

Batch

Batch

Batch

Water


AggregateMix

Batch

Batch

Batch

Water


AggregatePut into delivery equipment/pump

Mix

Batch

Batch

Batch

Water


AggregatePumpPut into delivery

equipment/pumpMix

Batch

Batch

Batch

Water


Aggregate

Pass through nozzle

PumpPut into delivery equipment/pumpMix

Batch

Batch

Batch

Water


Aggregate


Pass through nozzle

PumpPut into delivery equipment/pump

Batch

Batch

Batch

Water


Aggregate



Pass through nozzle

PumpPut into delivery equipment/pump

Batch

Batch

Batch

Water


Aggregate

Water


Batch

Water


Batch

Batch

Water


Batch

Batch

Batch

Water



Batch

Batch

Batch

Water




Batch

Batch

Batch

Water


Pump



Batch

Batch

Batch

Water


Pass through nozzle

Pump



Batch

Batch

Batch

Water


Project onto

surface

Pass through nozzle

Pump


Put into delivery equip. / pump

Batch

Batch

Batch

Water


Comparison of dry and wet-mix processes

Dry-mix process Wet-mix processInstantaneous 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.

Comparison of dry and wet-mix processes (cont‘d)

Dry-mix process Wet-mix processMinimal 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.

Applications of shotcrete

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

Example of shotcrete support and walls in a tunnel project

• tunnelling

Materials and mix designCementitious materialso SABS EN 197-1 common cements of Type I and Type II o Type III cements should only be used in the 42.5 strength o class

(Above cements often contain mineral additives in the blend)o Condensed Silica Fume (CSF) o Calcium Aluminate Cement

Aggregateso Aggregate used for shotcrete should comply with

requirements for use in conventional concrete i.e. SANS 1083.

o Rounded aggregates are preferable (but are frequently crushed in SA, and therefore angular)

o Well graded aggregate is important with no fraction constituting more than 30% of the total

Materials and mix design (cont‘d)

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

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

Typical properties of shotcrete vs. cast concrete

Property High quality

shotcrete

Cast in situ

concreteCompress. strength, 1 d (MPa) 20 6 (estim.)

Compress. strength, 28 d ( MPa) 30-60 30-50

Elastic modulus, 28 d (GPa) 34 31 (estim.)

Poisson’s ratio, 28 d 0.18 0.15 – 0.22

Tensile strength, 28 d (MPa) > 2 (estim.) 3.8 (estim.)

Initial setting time (start-end) (min) 3 – 5 45 – 145 (estim.)

Typical properties of shotcrete vs. cast concrete (Cont’d)

Property High quality

shotcrete

Cast in situ

concreteShrinkage 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

Durability of shotcrete

Particularly for linings in tunnels:o Carbonation o 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 samplesBoiled Absorption (BA)Volume of Permeable VoidsDurability Index tests (SA tests currently being

developed for conventional concrete)

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.

Thank you for your attention!

a review of shotcrete materials, mix design and applications department of civil engineering,...

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