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The Science of Shotcrete

CIB Meeting

June 5th 2012

A mixture of cement and sand

and water that is sprayed on a

surface under pneumatic

pressure

*Websters Dictionary

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What is Shotcrete?

Mix Design

o Key Factors

Water to cement ratio

Sand and Stone Gradation

Cement and Pozzolans

HRWR

Hydration Stabilizers

Other Materials

o Accelerators

o Fibers

Steel

Macro

Keys to Application

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Sprayed Concrete was invented in 1907, and is today widely

used for rock support world wide, both in mining and tunnelling.

For a long time dry mix application was the only way of

applying Sprayed Concrete, but in the seventies the wet mix

method was having its breakthrough in underground works

The development in Sprayed Concrete has gone a long way

since 1907, both in terms of equipment and concrete

technology. Especially since the wet mix method started to get

implemented, large technology steps has taken place

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Fines in cracks, fissures and joints

Stiffen and strengthen the rock mass

Transfer the rock load to adjacent stable rock (shear and adhesion)

Prevent relative movements No loosening over the time

Shear resistance to blocks

Rock must cut through to fall Sprayed concrete layer acts as a

shell taking bending forces and tension when bond is low

Weight

Comp

ressio

n TensionTe

nsio

nCom

pression

Shear strength ofshotcrete recess

Shear strength alongshotcrete-rock interface

Tangential stressesin shotcrete Tension rock

Adhesion

Thin layers (315 cm): Bridging effect

How Sprayed Concrete Works

In the dry mix method, a premix of sand and cement is

fed into the hopper of a machine that with the help of

compressed air convey the mix through the hose to the

nozzle where water is added.

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Dry cement, sand and

accelerator mix

Screen

Agitator

Compartment

Compressed air

Rotating

barrel

Wear plate

Wear pad

Air line

Water line

Water control

valve

Water ring Nozzle tip

For the wet mix method, sand, aggregate, cement, water and admixture are

premixed in a concrete plant

Application of wet mix Sprayed Concrete is mainly performed by the use of piston

pumps, that convey the concrete through the hosing system, and at the nozzle a

set accelerator and air is added.

The main benefit with the wet mix method vs. the dry is; improved quality, less

dust/improved working environment, less rebound, higher capacity and improved

safety

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Dense stream system

Sand/aggregate grading

Cement type and amount

W/C ratio

Type of Plastiscizer/Superplasticizer

Workability

Accelerator type

Hydration Control

Temperature

Accelerator dosage

Pulsation

Nozzle systems /set up

Nozzle distance

Nozzle angel

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High w/c ratio gives

slow setting and influences

end quality

Watch the moisture content in

the sand/aggregate

Moisture will vary

depending on where

the measurement are

taken

Keep

control

W/c ratio is critical to

o Early setting and strength development

o Long term strengths

o Long term durability - resistance to chemical attack

W/c ratio should be less than

0.45, and preferably closer to

a 0.4

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Mixing of different

fractions in the right

proportions is the key

Sand/aggregate grading curve influences:

Water demand

Workability

Reactivity with Accelerator

Rebound

Shrinkage

Durability

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Combined gradation of aggregates should fall within these limits

Usually a 2:1 sand to stone ratio

#8 stone or gravel is primarily used

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Type I, Type II or Type I/II Cement o Cementitious content ~ 800lbs

Fly Ash; Class F or C

Slag

Silica Fume

Proportion similar to

how pozzolans are

proportioned in concrete

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Superplasticisers for

very low w/c ratios and

high workability

Hydration control

admixtures for

maintaining

workability from 3 to

72 hours

Micro Silica

And Slump Retainers

Additions of steel and

high performance

polymer fibers,

micro silica slurries

Alkali-free

accelerators

for safety and

durability

Low water - cement ratios 0.32 to 0.45

Allows for higher slumps

High early and long term sprayed concrete strengths

Pumpable shotcrete mixes

Durability enhancement

Low dosage - cost effective

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Hydration stabilizers

Needed in almost every mix design

Control the hydration of cement

Maintain open time and pumpability for up to 72 hrs

Adjust dosage according to the needs

The addition of shotcrete

accelerators re-starts the hydration

process and causes immediate

setting

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Traditional

Sprayed

Concrete

New

Flexibility

with

Hydration

Stabilizer

Alkali-free offers setting

performance of

traditional accelerators

All alkali-free

accelerators promote

strength and durability

of sprayed concrete

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Blisadonna Railway Tunnel, Austria

0

5

10

15

20

S 71 S 51 TCC

766

SA

140

SA

145

SA

160

SA

161

SA

170

Initia

l set

Fina

l set

Settin

g tim

e (

min

s)

Improved working safety

Less strength difference to base mix

Less dust and rebound

Lowered risk of ASR

Improved sulphate resistance when using standard OPC

Reduced environmental impact in hardened concrete

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ACIDIC NEUTRAL BASIC

Safe to human body

pH Scale 0 7 14 4 10

Alkali-free

accelerators

Modified sodium

silicate accelerators

Conventional

accelerators

pH Scale 0 4 10 7 14

Slump below 4in can prove difficult

o Poor mixing efficiency of accelerator into stiff material

o Overdosing of accelerator due to poor pump piston filling

efficiency

o High pulsation - layering effect

o AFA has a lower viscosity, and more efficiently mixed with the

shotcrete at a temperature around 70 F than at lower

temperatures

Correct set-up with air and accelerator lines and correct nozzle

type is key 21

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3D, Discrete Reinforcement

2D, Wire Mesh Reinforcement

Multi-dimensionally throughout

entire concrete thickness

On a single horizontal plane only

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For reinforcement, direct cost of fiber is 50 60 % of wire mesh

Shotcrete can be sprayed

in one layer

Reduces shotcrete

volume due to following

of irregular substrate

Better Logistics

Steel fibres Typically 1.25 in length

0.02 inches in diameter

Type I high tensile strength (ASTM 820)

Provides uniform reinforcement

Prevents brittle failure of sprayed concrete linings

Promotes durability

Faster reinforcement method than mesh

50 to 60 % cost saving over mesh

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Macro Synthetic Fibres

o High performance replacement for mesh and/or steel fibers

o Typical dose of ~ 10lbs per yard

o Flexural toughness equal to steel

o As cost effective or better than steel

o Increased fire resistance

o Reduced wear on concrete pumping equipment

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Flexural performance

(ASTM C 1609)

Flexural toughness

(ASTM C 1550)

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This test is used to determine flexural performance

characteristics of fiber-reinforced concrete; e.g., first-peak

strength, residual loads and strengths, toughness (energy

absorbed), and Re,x.

0

5

10

15

20

25

0.0 0.5 1.0 1.5 2.0 2.5

Deflection, mm

Load

, kN

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This test is used to determine the flexural toughness of fiber-reinforced

concrete (i.e., energy absorbed); this test is specified mostly for

underground (UGC) applications.

0

5

10

15

20

25

30

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0 5 10 15 20 25 30 35 40 45

Deflection, mm

Lo

ad

, k

N

0

100

200

300

400

500

600

En

erg

y, J

Applied Load

Energy Absorbed

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Pulsation

Type of equipment may influence pulsation

Low workability is low filling ratio, high

pulsation, reduced quality and higher cost

Integration with accelerator pump

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A good concrete

pump is not

necessary a good

pump for application

of sprayed concrete

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Layering

or

Lensing

Nozzle system and set up

Air and accelerator introduction

Air volume

Air pressure

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Concrete

hose Accelerator

hose

Air

hose

Air and

accelerator

hose

Air and

accelerator

hose Wrong

Wrong

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Nozzle should always point 90to the receiving surface

For spraying onto steel arches/lattice girders exceptions are required

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90

Application Technique Nozzle Angle

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For applications the distance should be between 1 2 m

Influence of nozzle distance

Incorrect nozzle angle and distance have a significantly negative influence on concrete quality, such as poor compaction, strength, etc., and will dramatically increase rebound

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90

1-2 m

Application Technique Nozzle Distance

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Same concrete mix from one truck, sprayed 10 minutes

apart!

Sprayed by two different nozzlemen during training

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Questions? For More Info: Wes Morrison wesley.morrison@basf.com 571.344.3286 www.meyco.basf.com