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Contents
Introduction
History
Materials of shotcrete
Thickness of shotcrete
Advantages of shotcrete
Types of Shotcrete
Shotcrete Processes
Testing of shotcrete
Mechanisms of Shotcrete support behaviour
Shotcrete support loading mechanisms
Support capacity
Application of shotcrete
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Introduction
Shotcrete is a concrete conveyed through a hose and pneumatically projected at high velocity onto a
surface, as a construction technique.
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Shotcrete
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Introduction
Since the development of New Austrian
Tunneling Method (NATM), shotcrete in
tunnels has been widely applied.
Reasonable prices, quick and simple
installation and permanence are some features
that make this type of support most preferable
amongst all kinds of supports in the new
tunneling method.
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Shotcrete as initial support in NATM
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History
Late 1800s: Early use of manually applied mortar using compressed air for concrete repairs.
1907: Carl Akeley develops a method of spraying plaster onto metal lath using compressed air.
1911: The first patent for the pneumatic application of concrete, known as "gunite," is granted to
C.Gunite.
1930s: Gunite becomes a brand name for dry-process shotcrete developed by the Allentown Shotcrete
Company in the United States.
1940s-1950s: Shotcrete gains popularity for constructing underground bunkers and tunnels during World
War II.
1960s-1970s: Wet-mix shotcrete, where the concrete ingredients are premixed before spraying, is
developed and improves the shotcrete application process.
1980s-Present: Shotcrete technology advances with the introduction of additives and fibers to enhance
properties and performance.
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Materials of shotcrete
Materials (Same as Concrete)
Cement
Water
Aggregates
Admixtures or Fibers
The proportion of cement to aggregate in shotcrete may be normally 1:3 or 1:4, the aggregate being a mixture
of sand and about 20 percent aggregate varying from 5 to 20 mm.
The major distinction between concrete and shotcrete is merely the method of placement.
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Thickness of shotcrete
Thickness (Generally 50 to 150 mm )
Depends upon
Type of rock
Extent of stratification and/or joints.
Size of the tunnel
Some variation may occur as per the actual site
conditions in each case
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Advantages of shotcrete
Application to any elevations because sprayed concrete adheres immediately and bears its own weight.
Can be applied on uneven substrates.
Good adhesion to the substrate.
Totally flexible configuration of the layer thickness on site.
Reinforced shotcrete is also possible (mesh/fiber reinforcement).
Rapid load-bearing skin can be achieved without forms (shuttering) or long waiting times.
Act as a main support component due to continuum action as plate unlike Rock bolts and steel ribs which
are spaced at an interval
Can also be used as secondary lining.
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Types of Shotcrete
1. Fiber-Reinforced Shotcrete (FRS)
FRS involves the addition of fibers to the shotcrete
mixture to enhance its structural performance.
Fiber reinforcement improves the crack resistance,
ductility, toughness, and flexural strength of the
shotcrete.
FRS is commonly used in applications where
increased strength, durability, and resistance to
shrinkage cracking are required.
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Fiber-Reinforced Shotcrete (FRS)
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Types of Shotcrete
Fiber used in shotcrete are available in three general forms.
a) Steel fibers
b) Glass fibers
c) Other fibers like nylon, polypropylene, polyethylene, polyester, and rayon.
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Steel fibers Glass fibers Polypropylene fibers
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Types of Shotcrete
2. Mesh-Reinforced Shotcrete
Wire mesh shotcrete involves the use of wire mesh
reinforcement in conjunction with the application of
shotcrete to enhance the structural performance of
the lining.
It acts as a primary reinforcement, providing
additional strength, crack control, and impact
resistance to the shotcrete lining.
It improves the tensile and flexural strength of the
shotcrete, particularly in areas prone to high tensile
stress or dynamic loading.
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Mesh-Reinforced Shotcrete
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Types of Shotcrete
3. Polymer-Modified Shotcrete
Polymer-modified shotcrete involves the addition of polymer additives to the shotcrete mixture to enhance
its properties and performance.
Polymers, such as acrylics, latexes, or styrene-butadiene rubber (SBR), are commonly used as additives.
Polymer modification improves the adhesion, cohesion, durability, and flexibility of the shotcrete.
Polymer-modified shotcrete exhibits improved resistance to cracking, shrinkage, and chemical attack.
The selection of the appropriate polymer type and dosage should be based on the project requirements and
desired performance characteristics.
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Types of Shotcrete
4. Accelerated Shotcrete
Accelerated shotcrete refers to shotcrete mixtures that contain additives to speed up the setting and early
strength development of the material.
Common types of accelerators used in shotcrete include calcium chloride, non-chloride accelerators, and
alkali-free accelerators.
Accelerated shotcrete is particularly beneficial in time-sensitive construction projects where rapid strength
gain is required.
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Shotcrete Processes
Shotcrete can be applied by two distinct application techniques
1. Dry mix shotcrete
2. Wet mix shotcrete
Dry mix shotcrete is more widely used in mining because of inaccessibility for large transit mix trucks
and also because it generally uses smaller and more compact equipment.
Wet mix shotcrete is ideal for high production applications in mining and civil engineering where a deep
shaft or long tunnel is being driven and where access allows the application equipment and delivery
trucks to operate on a relatively continuous basis.
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Shotcrete Processes
1. Dry Mix Shotcrete
Dry shotcrete components –which may be
slightly pre-dampened to reduce dust –are fed
into a hopper with continuous agitation.
Compressed air is introduced through a rotating
barrel or feed bowl to convey the materials in a
continuous stream through the delivery hose.
Water is added to the mix at the nozzle.
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Shotcrete Processes
2. Wet Mix Shotcrete
Shotcrete components and water are mixed
(usually in a truck-mounted mixer) before
delivery into a positive displacement pumping
unit.
Pumping unit delivers the mix hydraulically to
the nozzle where air is added to project the
material onto the rock surface.
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Shotcrete Processes
Dry mix Process Wet mix Process
Mixing water instantaneously controlled at the nozzle by
operator to meet variable field conditions.
Mixing water controlled at plant and measured at time of
batching.
Longer hose lengths possible, if necessary. Normal pumping distances necessary.
Limited to accelerators as the only practical admixture.
Compatible with all ordinary admixtures. Special
dispensers for addition of accelerators are necessary.
Use of air-entraining admixture not beneficial. Resistance
to freezing and thawing is poor.
Air entrainment possible. Acceptable resistance to freezing
and thawing.
Intermittent use easily accommodated within prescribed
time limits.
Best suited for continuous application of shotcrete.
Exceptional strength performance possible. Lower strengths, similar to conventional concrete.
Lower production rates. Higher production rates.
Higher rebound. Lower rebound.
Equipment maintenance costs tend to be lower. Equipment maintenance costs tend to be higher.
Higher bond strengths.
Lower bond strengths, yet often higher than conventional
concrete.
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Testing of shotcrete
1. Needle penetration test – For very early strength
2. Bolt firing method – For early strength
3. Core compressive strength – Strength after 28 days
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Testing of shotcrete
1. Needle penetration test
This method measures the force required to
press a steel needle with defined dimensions
into the shotcrete.
The strength can be deduced from this
resistance.
Suitable for strength levels immediately after
application of up to 1 N/mm2.
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Needle penetration test
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Testing of shotcrete
2. Bolt firing method
In this test, standardized nails are fired into
the shotcrete with a Hilti DX 450L gun.
The depth of penetration and pull-out force
are determined to obtain the compressive
strength.
simplified by Dr. G. Bracher so that the
strength can be determined directly from the
depth of penetration.
Range of Application - 1 N/mm2 to 15 N/mm2
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Bolt firing method
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Testing of shotcrete
3. Core compressive strength
Compressive strength can be obtained by
taking cores directly under a
compression tester.
This method is used mainly to check the
required final strength after 28 days.
Range of application - > 10 N/mm2
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Compressive strength test
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Mechanisms of Shotcrete support behaviour
a) interlock that is promoted by the bonding of the
shotcrete to the rock, and the tensile strength of
the shotcrete, preventing shear on the interface
and restricting block rotation
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1. Promotion of Block interlock
b) development of shear strength on the interface
between the shotcrete and the rock as a result
of irregularity of the interface surface
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Mechanisms of Shotcrete support behaviour
c) Penetration of shotcrete material into joints and cracks which will inhibit movement of blocks,
which is particularly relevant in very high stress situations in which some loosening and stress
fracturing will have taken place
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1. Promotion of Block interlock
Penetration of shotcrete into joints Stress fracturing of shotcrete
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Mechanisms of Shotcrete support behaviour
d) Prevention of block displacements by shear strength of shotcrete and tensile strength of shotcrete
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1. Promotion of Block interlock
Shear strength of shotcrete Tensile strength of shotcrete
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Mechanisms of Shotcrete support behaviour
2. Air tightness
If the applied surface support is airtight, entry of
air will be prevented or limited, and hence
dilation will be restricted.
If dilation occurs Rock mass will fail.
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Failure due to dilation
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Mechanisms of Shotcrete support behaviour
3. Structural arch
Deformation of the rock mass induces
stresses in the support, which then resists
further deformation of the rock mass.
Important in this structural mechanism is the
strength of the shotcrete and its flexural
rigidity.
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Stresses due to deformations in rock mass
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Mechanisms of Shotcrete support behaviour
a) Slab enhancement:
The application of shotcrete support effectively
decreases the slenderness of the slab and
increases its buckling resistance
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4. Basket mechanism
Buckling failure
b) Beam enhancement:
This is similar to slab enhancement – shotcrete support on
the underside of a roof beam may enhance the bending
performance, and hence stability, of a roof beam
c) Durability enhancement
Some rock types deteriorate on exposure and when
subjected to wetting and drying, and the mechanism of the
shotcrete support is to seal the rock to prevent exposure and
hence preserve the inherent strength of the rock.
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Mechanisms of Shotcrete support behaviour
d) Extended face plate
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4. Basket mechanism
Shotcrete support will extend the area of influence of
rock bolt and cable faceplates.
e) Mechanical protection
this is an extremely important mechanism, since
mechanical damage will quickly destroy the
effectiveness of shotcrete support.
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Shotcrete support loading mechanisms
1. Wedge and Block loading
When a block or wedge of rock is defined by
fracture or joint planes, it may displace and
load the liner locally. With “rigid” and
bonded liners, shear stresses will be induced
in the shotcrete along the perimeter of the
block.
If breakdown of the bond occurs, the
mechanism will tend towards a localized or
point load acting on a “basket”.
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Shear resistance of thicker
shotcrete membrane
Tension in membrane
and bond strength
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Shotcrete support loading mechanisms
2. Distributed surface loading
Shotcrete support subjects to distributed load that may cause due to several alternatives situations: failed
rock, under the action of gravity (static); squeezing rock conditions, due to high stresses or swelling
(static); rock burst loading - about a 1m thickness of fragmented rock is often ejected at high velocity
during rock burst events.
Distributed loading causes the shotcrete to provide support with a basket mechanism.
Localized deformation may occur at locations of fractures and rock joints, which will particularly be the
case when the shotcrete is well bonded to the rock surface, and when the roughness of the rock surface
prevents shear on the interface.
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Shotcrete support loading mechanisms
3. Stress induced loading
well bonded shotcrete will be subjected to the
same deformations as the rock.
It may be stiffer, or more brittle, than the
jointed, fractured rock mass, and therefore may
fail prematurely under the imposed
deformations.
Shear, bending , buckling or tension, or more
complicated failure mechanisms, such as
combinations of these, and possibly others,
may also occur. The result could be stress
induced spalling of the shotcrete.
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Shear failure
Bending failure
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Shotcrete support loading mechanisms
3. Stress induced loading
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Buckling failure Spalling of shotcrete
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Shotcrete support loading mechanisms
4. Water pressure loading
water pressures will be distributed pressures
which may be sufficient to fail undrained
shotcrete support.
5. Bending loading
Since the support is installed in the roof and
sidewalls only, the floor may deform freely. The
consequence could be greater convergence at
floor level than roof level, and hence bending
loading on the shotcrete, particularly in the
haunch areas
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Bending in shotcrete
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Support capacity
1. Support Capacity of Shotcrete
psc =
2 qsc tsc
Fsc B
Where,
qsc = shear strength of shotcrete (300 t/m2 in most cases)
tsc = thickness of shotcrete (m)
B= size of opening (m)
B Fsc = Horizontal distance between vertical planes of maximum shear stress in shotcrete (m)
Fsc = 0.6 ± 0.05
Psc = Support capacity of shotcrete lining (t/m2)
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Support capacity
1. Support Capacity of fiber reinforced Shotcrete
pfsc =
2 qfsc tfsc
Ffsc B
Where,
qfsc = shear strength of fiber reinforced shotcrete (550 t/m2)
tfsc = thickness of fiber reinforced shotcrete (m)
B= size of opening (m)
B Ffsc = Horizontal distance between vertical planes of maximum shear stress in SFRS(m)
Ffsc = 0.6 ± 0.05
Pfsc = Support capacity of fiber reinforced shotcrete lining (t/m2)
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