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Steel Fibre Reinforced Concrete for PrecastSegmental Tunnel Lining
Navneet T. Narayan Technical Manager
Bekaert
Chandan Vaidya Sales Manager ‐ Hydro
Bekaert
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AbstractPrecast concrete segmental linings in metro tunnels have been traditionally perceived and designed using conventional steel rebars. While technically and practically proven for years with satisfactory results, there exist issues specific to segmental lining tunnels which perhaps could be still addressed in a better manner in such a system. In this paper, a new system of reinforcing precast segments with steel fibres as primary reinforcement is introduced. The advantages of using steel fibres vis-à-vis traditional reinforcement are discussed, along with guidelines to select the correct fibre types and dosages to ascertain the performance of fibres.
Keywords: Steel Fibres, SFRC, Precast Segmental Lining, Post Crack Flexural Strength
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INTRODUCTION
India has seen a spurt in the underground urban
infrastructure space over the last decade. A large portion
of this development has been contributed by the advent
of the tunnel boring machines and the associated
installation of precast concrete segmental lining tunnels.
Be it the Delhi Metro, which lead the way in this
revolution, or the following projects in Chennai and
Kolkata, it is clear that segmental precast concrete
tunnels are here to stay.
The scheme of reinforcements for these tunnel segments
has been with conventional steel reinforcement bars.
Most of the times, the governing tunnel design cases
have had to use steel rebars for safety during demolding,
stacking, erection and bursting compression effects due
to TBM ram forces or inline contact with adjacent
segments. The in-service loading parameters in such a
circular segmental lining are governed by compressive
forces with minimal bending moments. With these design
conditions, many engineers have found steel fibres as a
novel and economical way to reinforce precast
segments. It is possible to design for steel fibres to act as
primary reinforcement and may partially or wholly replace
conventional steel rebars in precast segments
depending on the nature and type of loading conditions.
Steel Fibre Reinforced Concrete (SFRC) is defined as a
concrete that has steel fibres that are a discontinuous
form of reinforcement, randomly oriented and discreet
within concrete and they are added in the matrix while the
concrete is being prepared. They lend several
advantages to concrete, enhancing its properties in more
than one way which ultimately makes them an attractive
alternative to conventional steel reinforcement. Some of
them are detailed subsequently.
Behaviour of SFRC
During the cracking phase of SFRC, the fibres present in
the concrete matrix bridge the cracks and transfer
tension across them during this process. This leads to
increased ductility, higher energy dissipation and crack
resistance of concrete and ensures a post crack load
carrying capacity. It must however be noted that steel
fibres causes no increase in the flexural strength of the
concrete, yet they achieve an increased ultimate load
capacity due to increased ductility and toughness of
concrete thereby helping in redistribution of load stresses
in highly indeterminate tunnel systems.
The performance of steel fibres is made clear with the
help of a four point beam bending test. Figure 1 illustrates
results from the experimental study conducted at IIT
Madras with 30 kg/m3 of Bekaert Dramix® steel fibres of
aspect ratio 80. It is observed that for plain concrete, a
sudden and brittle mode of failure occurs after the peak
load is reached. For SFRC, a strain softening
phenomenon is observed after the peak load in the beam
and steel fibres sustain large deflections at a load
reduced load level. Thus, with this kind of toughening
behaviour in the beam, post-crack flexural strength of
SFRC is guaranteed.
Figure 1: Behaviour of SFRC: Beam Tests at IIT
Madras
Criteria for selection of steel fibres
Steel fibres with different shapes and sizes have their
own effect on the behaviour and quality of SFRC. The
required steel fibre dosage to meet the design and
structural requirements has to be related to the steel fibre
performance and the most important aspects that control
the performance of steel fibres in SFRC are:-
· Aspect Ratio (Length/Diameter)
· Tensile Strength (> 1200 MPa)
· Geometrical Shape
· Fibre Network
· Homogenous mixing in concrete
·Compliance to manufacturing and performance
standards (CE certification based on steel fibre standard
EN 14889-1)
Higher the aspect ratio of steel fibres, the better the
performance of steel fibres in concrete. Higher aspect
ratios correspond to more number of fibres per kg steel
dosage, thereby providing more confinement to concrete
and improving its properties consequently.
The steel fibre length has to be in the range of at least 2
times the maximum aggregate size in order to bridge the
gap between two aggregates effectively where a crack
usually starts. Taking into account the fact that precast
concrete mixes may usually have coarse aggregates of
maximum 20 or marginally more, steel fibres need to be
50 to 60 mm long. The fibre length also has to be
sufficiently long to give enough anchorage to the matrix in
order to avoid an easy pull out. It is also preferable to
develop the required superior anchorage by providing
hooked ends to steel fibres. Furthermore, with smaller
diameter fibres, the number of fibres per unit weight
increases which densifies the fibre network, thereby
making it more efficient (Table 1).
Table 1: Comparison of different types of steel fibres
However, the higher the aspect ratio of the fibres, the
more difficult it becomes to mix, handle and place the
concrete. This problem is compounded with the use of
loose fibres. Loose steel fibres become difficult to add to
the concrete and fail to spread homogenously in the
mixture. They usually tend to get entangled together to
form lumps in concrete and hence, it is also referred to as
“balling” of steel fibres. This problem has been overcome
with the development of glued steel fibres which are
bundled together with a water soluble glue to facilitate the
mixing and handling (Figure 2). Unlike loose fibres, glued
fibres do not pose any major problems during concrete
mixing and as soon as the mixing process starts, the fibre
bundles spread immediately throughout the entire
concrete matrix. The bundles come in contact with water
during mixing and the individual fibres start separating
out from the bundles due to the dissolution of the glue as
well as the mechanical scouring effect of the aggregates.
Homogeneously mixed glued flbres
Loose fibres ball during mixing
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Figure 2: Homogenous mixing in glued fibres vs fibre
balling in loose fibres
Advantages of steel fibres
Mechanical Properties
Steel fibres measure up extremely well against
conventional steel reinforcement in precast segmental
linings. Not only do they improve upon the mechanical
properties of traditionally rebar reinforced concrete
sections, they also eliminate most of the practical
problems associated with the use of traditional steel.
Addition of steel fibres in concrete leads to an
improvement in the following mechanical properties of
concrete:-
· Increased toughness
· Post crack flexural strength
· Resistance to crack propagation
· Resistance to chipping and spalling
· Energy absorption
· Shear resistance
· Impact resistance
· Fatigue resistance
· Durability
In traditional rebar reinforced concrete sections, there is
always a cover of plain concrete (usually 30 mm or more)
from the free edge of the section upto the level of steel
reinforcement. It is in this region that most of the
problems associated with concrete sections surface.
Precast lining segments are subjected to heavy
concentrated ram forces from TBMs which use them as a
reaction frame to drill forward. These forces could be to
the tune of 300 tons or more at each ram location,
transmitted through small contact shoe areas to the
segments side walls (Figure 3).
Figure 3: TBM Ram forces on the segment side walls
These exert tremendous local split tensile stresses which
demand a lot of local steel reinforcement to be taken care
of. Furthermore, between adjacent segments already
placed along the length of the tunnel, the problem of non-
alignment often results in concentration of stresses
between the segments, often resulting in local chipping
and spalling (Figure 4).
Figure 4: Chipping and spalling between adjacent
segments
These are cases where in traditional steel reinforcement
cannot sufficiently provide the necessary resistance
needed against such local failures. This often entails for
costly repairs which not only add to the delay, but also
affect the overall durability of the tunnel as such repairs
are often cosmetic and non-structural (Figure 5). Steel
fibres act as a 3D reinforcement and do no leave any
portion of the concrete unreinforced. Thus, the localized
stresses are easily withstood by SFRC and all the
associated problems observed with traditional
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steel reinforced sections are avoided.
Figure 5: Repair of spalled RCC at joints with mortar
Practical Advantages
Steel fibres have distinct practical advantages which help
quicker completion of projects. Firstly, the whole activity
of bar bending, tying and welding along with the
associated investments for setting up reinforcement
cage yards can be avoided. Adding steel fibres to
concrete in a batching plant and then simply pouring the
SFRC in the segment moulds eliminates one more time
consuming activity of lowering the steel reinforcement
cages in the moulds and ensuring proper covers (which
are often difficult to maintain). Thus, steel fibres add to
the productivity of the project by economizing on the
manufacturing process of the precast segments itself.
Furthermore, there are times when the reinforcement
cages have to be transported to precast yards near
project locations. If the distances are large, then these
may lead to additional transportation schedules and
costs (Figure 6). However, with steel fibres, such costs
can also be eliminated, adding to the overall efficiency
and economy of the project.
Figure 6: Transportation of reinforcement cages to
precast yard
Conclusion
The use of SFRC in tunnel segments in appropriate
geological conditions offers many technical and
commercial advantages over traditional steel
reinforcement. Long term properties of SFRC associated
with durability have been demonstrated for over 30 years.
Steel fibres in precast elements have been shown to be
robust and highly ductile products that withstand the
rigours of production, transportation and erection. Over
the last few years, there has been a considerable
advancement in the use of SFRC and it has been
accepted by owners, contractors and engineers from
around the world as an unmatched solution for
underground support.
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