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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