The Science of Shotcrete
CIB Meeting
June 5th 2012
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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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
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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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How Sprayed Concrete Works
Thin layers (3–15 cm): Bridging effect
Shear strength along
shotcrete-rock interface
Shear strength of
shotcrete recess
Adhesion
Weight
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Tangential stresses
Fines in cracks, fissures
in shotcrete
Tension rock
and joints
 Stiffen and strengthen the
Shear resistance to blocks
rock mass
 Rock must cut through to fall
 Transfer the rock load to
 Sprayed concrete layer acts as a
adjacent stable rock (shear
shell taking bending forces and
and adhesion)
tension when bond is low
 Prevent relative movements
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 No loosening over the time

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.
Dry cement, sand and
accelerator mix
Screen
Agitator
Compartment
Water line
Water control
valve
Air line
Wear pad
Wear plate
Water ring
Rotating
barrel
Compressed air
Nozzle tip
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

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
Dense stream system
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


Sand/aggregate grading

Temperature

Cement type and amount

Accelerator dosage

W/C ratio

Pulsation

Type of Plastiscizer/Superplasticizer

Nozzle systems /set up



Workability

Nozzle distance

Nozzle angel
Accelerator type
Hydration Control
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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

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
Keep
control
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 Sand/aggregate grading curve influences:
 Water demand
 Workability
 Reactivity with Accelerator
 Rebound
 Shrinkage
 Durability
Mixing of different
fractions in the right
proportions is the key
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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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Hydration control
admixtures for
maintaining
workability from 3 to
72 hours
Superplasticisers for
very low w/c ratios and
high workability
Alkali-free
accelerators
for safety and
durability
Micro Silica
And Slump Retainers
Additions of steel and
high performance
polymer fibers,
micro silica slurries
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

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
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

20
15
10
5
S 71 S 51 TCC SA
SA
SA SA SA
766 140 145 160 161 170
Final set
0
Initial set

Alkali-free offers setting
performance of traditional
accelerators
All alkali-free accelerators
promote strength and
durability of sprayed
concrete
Setting time (mins)

Blisadonna Railway Tunnel, Austria
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pH Scale 0
pH
Scale 0
4
4
7
7
10
14
10
14
Safe to human body
ACIDIC
NEUTRAL
Alkali-free
accelerators






BASIC
Conventional
accelerators
Improved working safety
Modified sodium
silicate accelerators
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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
Slump below 4in can prove difficult
o
o
o
o

Poor mixing efficiency of accelerator into stiff material
Overdosing of accelerator due to poor pump piston filling efficiency
High pulsation - layering effect
AFA has a lower viscosity, and more efficiently mixed with the shotcrete at
a temperature around 70 ºF than at lower temperatures
Correct set-up with air and accelerator lines and correct nozzle type is
key
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3D, Discrete Reinforcement
2D, Wire Mesh Reinforcement
On a single
horizontal
plane only
Multi-dimensionally
throughout
entire concrete thickness
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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
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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
o
o
o
o
o
High performance replacement for mesh and/or steel fibers
Typical dose of ~ 10lbs per yard
Flexural toughness equal to steel
As cost effective or better than steel
Increased fire resistance
Reduced wear on concrete pumping equipment
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Flexural performance
Flexural toughness
(ASTM C 1609)
(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.
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Load, kN

15
10
5
0
0.0
0.5
1.0
1.5
2.0
2.5
Deflection, mm
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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.
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600
Applied Load
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Energy Absorbed
500
25
20
300
15
200
10
100
5
0
0
0
5
10
15
20
25
30
Deflection, mm
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35
40
45
Energy, J
Load, kN
400
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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
A good concrete
pump is not
necessary a good
pump for application
of sprayed concrete
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Layering
or
Lensing
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Nozzle system and set up
 Air and accelerator introduction
 Air volume
 Air pressure
Air and
accelerator
hose
Air
hose
Concrete
hose
Air and
accelerator
hose
Wrong
Accelerator
hose
Wrong
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Application Technique – Nozzle Angle


Nozzle should
always point 90°to
the receiving
surface
90
°
For spraying onto
steel arches/lattice
girders exceptions
are required
90
°
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Application Technique – Nozzle Distance

For applications the
distance should be
between 1 – 2 m
90
°

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
90
°
1-2 m
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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
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