1 the science of shotcrete cib meeting june 5 th 2012
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 Materialso Acceleratorso Fibers
• Steel • Macro
Keys to Application 3
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
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 andaccelerator mix
Screen
Agitator
Compartment
Compressed air
Rotatingbarrel
Wear plate
Wear pad
Air line
Water lineWater controlvalve
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 too Early setting and
strength development
o Long term strengthso 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 Cemento 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 RetainersAdditions of steel and
high performance polymer fibers,
micro silica slurries
Alkali-free accelerators
for safety and durability
Low water - cement ratios0.32 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
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S 71 S 51 TCC766
SA140
SA145
SA160
SA161
SA170
Init
ial s
et
Fina
l set
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 7 144 10
Alkali-freeaccelerators
Modified sodium silicate accelerators
Conventionalaccelerators
pH Scale 0 4 107 14
Slump below 4in can prove difficulto Poor mixing efficiency of accelerator into stiff materialo Overdosing of accelerator due to poor pump piston
filling efficiencyo 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
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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 Fibreso High performance replacement for mesh and/or steel
fiberso Typical dose of ~ 10lbs per yardo Flexural toughness equal to steelo As cost effective or better than steelo Increased fire resistanceo 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.
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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.
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5
10
15
20
25
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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
hoseWrong
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
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
