shotcrete basf presentation
TRANSCRIPT
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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
Compression Tension
Tens
ion
Com
pression
Shear strength ofshotcrete recess
Shear strength alongshotcrete-rock interface
Tangential stressesin shotcrete Tension rock
Adhesion
Thin layers (3–15 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 ºF 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
35
0 5 10 15 20 25 30 35 40 45
Deflection, mm
Lo
ad
, kN
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 90°to the receiving surface
For spraying onto steel arches/lattice girders exceptions are required
90
°
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
90
°
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 [email protected] 571.344.3286 www.meyco.basf.com