Australian Society For Concrete Pavements · Australian Society for Concrete Pavements 4th Concrete...

Australian Society for Concrete Pavements

4th Concrete Pavements Conference

Macro Synthetic Fibres for Innovative Concrete Track Slab Design and

Construction

Todd Clarke BE (Hons) Dip Eng Prac GradIEAust

Lead Engineer

Elasto Plastic Concrete Pty Ltd

ABSTRACT

As the spending on rail infrastructure in Australia increases, the need for innovative solutions

for track design and construction intensifies. Traditionally, track has been laid on ballast

material that provides a relatively stiff bearing surface which transfers train loads to the

ground below. As train loads get larger, and speeds increase, maintaining the durability of

ballast material becomes more challenging. At high speeds, or with heavy loads, ballast can

be pulverised, causing track levels to shift, potentially jeopardising ride quality and safety.

Concrete track slabs have recently been used as an alternative to attempt to overcome

these issues, particularly for high speed rail. They have also become commonplace in metro

light rail construction, where the rail corridor is shared by road traffic, and the rail is

embedded into the concrete track slab. Concrete track slabs are used as they provide a

longer design life and require less maintenance than traditional ballast systems. Their

application is seen more regularly in situations, such as tunnels or bridges, where ballast is

difficult to place, or using it impacts on the alignment of the structure.

Traditional concrete track slabs have higher construction costs than ballasted systems,

making the option less attractive. The use of macro synthetic fibre reinforcement allows a

significant reduction in the initial cost of using concrete track slabs, by increasing

construction speeds, as there is no need to cut, place and tie conventional steel

reinforcement, while removing the durability concerns associated with stray current corrosion

of reinforcement.

This paper aims to initiate discussion between track slab designers, owners and operators

on the use of macro synthetic fibres in track slab construction and includes some

international case studies. The design and construction practices that were used for each of

these will be discussed. It will provide an outline of the methods used for design and analysis

of the macro synthetic fibre reinforced track slabs, as well as guidance on construction

practices that can be used to optimise the installation and performance of the track slab.

ASCP 4th Concrete Pavements Conference 2

Macro Synthetic Fibres for Innovative Concrete Track Slab Design and Construction [Todd Clarke]

Introduction

Concrete track slabs are increasingly gaining acceptance for new rail construction in

metropolitan areas, particularly in bridge and tunnel structures. They have also become

commonplace in metro light rail construction, where the rail corridor is shared by road traffic,

and the rail is embedded. Concrete track slabs are preferred for their longer design life and

reduced maintenance requirement than traditional ballast systems. They are also useful

where ballast is difficult to place, or it’s use impacts on the alignment of the structure.

A recent development in concrete track slabs has been the inclusion of macro synthetic

fibres, as a replacement for conventional steel reinforcement. Fibre reinforced concrete

technology began in the 1970’s with the introduction of steel fibres to the concrete industry.

Macro synthetic fibres were developed in the late 1990’s as a more cost-effective and

environmentally friendly alternative to steel fibres. The use of fibre reinforcement improves

concrete properties such as impact and spalling resistance, bursting resistance, increased

flexural capacity and reduced shrinkage cracking. Traditional concrete track slabs require a

higher initial financial investment than ballasted systems, making the option less attractive.

The use of macro synthetic fibre reinforcement allows a significant reduction in the initial cost

of using concrete track slabs, by reducing the time, and therefore the cost, of track slab

construction. Contractors typically note a 25% – 35% reduction in construction costs when

using macro synthetic fibres to replace steel in their concrete track slabs. Macro synthetic

fibres are also an excellent means of reinforcing sections of track where electrical isolation is

required.

Ballasted Track vs. Concrete Track Slab

Common railway construction practice is to use railway ballast, a quarried and crushed

stone, as a stiff bearing surface to lay track fastened on to wooden, or precast concrete,

sleepers. The use of railway ballast allows for efficient construction speeds to lay track over

long distances.

Concrete track slab has developed more recently, as a means of reducing the maintenance

requirements of the railways, and hence reducing disruptions to the rail networks. Thuer and

Vocks(1) suggest some typical advantages and disadvantages of each construction method

as outlined in Table 1.



Table 1 – Selected advantages and disadvantages of ballasted track compared to track slab (adapted from Thuer and Vocks(1))

The main disadvantage of using concrete track slab is the higher initial investment required,

as construction and material costs are higher than for traditional ballast track. If this initial

cost can be offset, then the option becomes more attractive. A means of reducing this initial

investment is through the use of macro synthetic fibres to replace or reduce the conventional

steel bar reinforcement in the concrete. This eliminates the need for steel fixing on site,

increasing the speed of construction, allowing longer sections of track to be laid in a shorter

time. As labour costs are a significant portion of any project budget, a reduction in labour

hours leads to decreased project costs.

Macro Synthetic Fibre Reinforced Concrete Track Slabs

Macro Synthetic Fibres

Macro synthetic fibres have been used in the construction industry since the 1990’s. They

are typically made of polypropylene, polyethylene, nylon, PVA or combinations of these(2).

The manufacture of macro synthetic fibres is governed in general by European Standard

EN14889-2:2006 which dictates that macro fibres are those which are greater than 0.3mm

Ballasted Track Concrete Track Slab

Advantages

Low initial investment Reduced track possessions as a result of lower maintenance requirements

Historical experience, state of the art construction and maintenance practices

Lower expenditure on maintenance, including reduced vegetation control measures

Easy modification of track geometry Superior stability and track alignment/geometry characteristics

Advanced machinery for construction and maintenance works

Ability to design and construct higher superelevations and tighter curves

Rapid construction, particularly over long distances

Reduced capital investment on rail related structures (tunnels, bridges, cuttings, etc.) attributable to beneficial track slab alignment (tighter curves, track alignment can follow existing infrastructure)

Ability for road vehicles (ie. Emergency services) to traffic the slab track if required.

Disadvantages

Inferior track stability High initial investment

Track alignment limits are stricter, leading to less route alignment possibilities

Tighter quality control required during track slab construction than ballasted track

Risk of ballast being ‘pulverised’ on high speed lines

Changes to track geometry / removal of track are more difficult and time consuming than ballasted track

Certain concrete track slab designs produce higher noise emissions than ballasted track in operating conditions

Construction time is longer, making construction during a track possession difficult



in diameter and are generally used where an increase in residual flexural strength is

desired(2). Typical macro synthetic fibres have lengths ranging from 30mm up to 65mm, and

equivalent diameters of 0.5mm to 0.8mm are common. They are generally added to the

concrete at rates of 0.5% - 1% of the total concrete volume. At these low volume additions,

the concrete mix design requires only minor modifications, such as the addition of a water

reducer, to account for the reduction in workability caused by the addition of fibres.

Macro synthetic fibres are used in concrete to obtain a residual flexural strength, which

allows for limit states design of fibre reinforced concrete to be undertaken. Residual flexural

strength can be determined using a number of internationally recognised standards that

have been developed. The most commonly used standard is the European standard

EN14651:2005, of which a typical load-CMOD (crack mouth opening displacement) output is

derived as per Figure 1(3). The values derived from this test method are used to determine

various design properties of the fibre reinforced concrete.

Figure 1 – Typical Load -CMOD diagram and Fj (j = 1, 2, 3, 4) (From EN14651:2005(3))

Design of Macro Synthetic Fibre Reinforced Concrete Track Slabs

The design of fibre reinforced concrete follows the same basic principles, regardless of fibre

type, based on the residual flexural strength values obtained from testing the specific fibre

reinforced concrete mix desired for use. For concrete track slabs where flexural capacity is

required, for example where track slab is laid on subgrade, then limit states design

approaches can be used to determine the required slab thickness and fibre quantity

required.

The majority of international design codes for fibre reinforced concrete base their

philosophies on the initial work conducted by the RILEM Technical Committee 162-TDF:

Test and Design Methods for Steel Fibre Reinforced Concrete. This work sets out design

calculations based on test data obtained using beam testing(4), which is also described in the

work and was later adopted as the EN14651 test method mentioned earlier.

Another guideline for design, is the recently released Technical Report 34 by the UK

Concrete Society, which provides guidance on the design of fibre reinforced concrete, based

on the work of RILEM. This guideline looks in particular at the design of concrete slab on



ground, and takes into account, to some extent, the interactions that occur between the

concrete and the ground when loaded in flexure(5). In fibre reinforced concretes, this

interaction can be substantial, and is not always accounted for in the design codes.

Alani and Beckett(6) have demonstrated through full scale testing, that the capacity of macro

synthetic fibre reinforced ground slabs is significantly greater than the values proposed by

the design codes. Roesler et al.(7) confirm this, and establish an increase in the flexural

capacity of the slab, generated by the macro synthetic fibre reinforcement.

Significant additional capacity can be seen in macro synthetic fibre reinforced slabs on

ground, as mentioned, and is often unaccounted for in design. This can be rectified through

the use of finite element analysis, to model the entire system, and capture the added

benefits of subgrade interaction. One such program that can support such analysis is

ATENA, which, when the material parameters are correctly defined, can provide accurate

analysis of macro synthetic fibre reinforced concrete track slabs. One such material model is

the modified fracture energy method, in which the fracture energy of the concrete and the

added fracture energy of the fibres are accounted for(8).

This method is used in conjunction with the material model proposed by RILEM TC 162

TDF. By accounting for the interactions that occur between the track slab and the subgrade,

optimal designs can be produced. Figure 2 presents an example of a finite element mesh for

analysis of a macro synthetic fibre reinforced concrete track slab, which includes both the

concrete and subgrade components to allow for analysis of the two in combination, and any

interactions which may occur between these.

Figure 2 – Finite Element Model of a Macro Synthetic Fibre Reinforced Concrete Track Slab(9)

Construction of Macro Synthetic Fibre Reinforced Concrete Track Slabs

The construction practices for macro synthetic fibre reinforced concrete track slabs, in

general, follows those of conventional concrete track slabs. The same methods of bottom-up

and top-down construction can be adopted, with the benefit of not having to cut, place and

tie traditional steel bar reinforcement. This enables significant improvements in construction

speeds, and hence a reduction in the construction costs of concrete track slabs.

Construction of the track slab can be undertaken by either manual or machine placement,

depending on whether the tracks are laid before or after slab placement. Joints should be

placed at regular intervals by either sawn joint or formed construction joint. Dowels,

preferably fibre reinforced polymer (to maintain a steel-free concrete section), should be

placed at all joints to provide load transfer, and minimise differential movement.



Case Studies

The following case studies illustrate the use of macro synthetic fibres as an alternative

reinforcement for concrete track slabs. For various reasons, and using a number of different

methods to verify the suitability of macro synthetic fibres, the following projects adopted their

use.

Docklands Light Rail, London

Macro synthetic fibres were incorporated into the track slabs for the Docklands Light Rail

extension in 2004 from Canning Town to London City Airport, and again in 2008 on the

London City Airport to Woolwich line. The track slab, varying in thickness between 150 and

200mm, was poured in sections of 2-3 linear metres. Extensive testing was undertaken to

determine the suitability of the macro synthetic fibre reinforcement, including a full-scale

mock up track slab pour.

These trials confirmed that the macro synthetic fibres were suitable for controlling the

thermal and drying shrinkage cracking that was

expected to occur in practice(10).

Figure 3 displays the full-scale trial section that

was conducted on site to determine the

suitability of the macro synthetic fibre

reinforcement.

The majority of the 6km section of the light rail

extension was constructed on bridge decks and

in tunnels, requiring macro synthetic fibre

reinforced concrete to be pumped long

distances. This was achieved with slight

adjustments to the regular concrete mix design

and minimal project disruption(10).

Given the stiff support structure provided by the

bridge decks and tunnels, the slab track on this

project was not required to provide significant

bending stiffness. Therefore, in this instance the

macro synthetic fibre reinforcement was

required to provide thermal and drying

shrinkage crack control. These sections of track

slab are approximately 10 years old now, and

current in service performance of the track slab

indicates that the fibres are performing their

function, as no deterioration of the track slab

concrete has been identified.

Figure 3 – Docklands Light Rail track

slab using macro synthetic fibre

reinforcement(10)



The Szeged Tramway, Hungary

In 2010 and 2011 during construction of Line II, and reconstruction of Line I in Szeged, a

typical problem was encountered wherein particular sections of the track, in areas of the

electrical loops, the concrete slab could not contain any reinforcement as electrical isolation

of the slab was required. It was decided for these sections that a macro synthetic fibre

reinforced concrete could be used to replace the conventional track slab(11). To justify the

use of macro synthetic fibre reinforcement, significant testing was undertaken, in the form of

the JSCE-SF-4 technical specification.

The results from the testing were used as inputs into design calculations from the UK

Concrete Society’s TR34, as the embedded track slab resembles a flat plate structure

supported by subgrade. Finite element modelling was also used in this instance to

corroborate these results(11). A section of the Szeged Line I during construction is depicted in

Figure 4.

Figure 4 – Szeged city centre during reconstruction of Line I using fibre reinforced concrete(11)

Oita High Speed Rail, Japan

Japan was one of the first countries to introduce the use of concrete track slab to their rail

network, which came about due to their need for increasing high speed rail lines. The

Japanese National Railroad began using concrete track slab over 30 years ago on the

Shinkansen Line, after after maximum speeds were limited on the first line due to

construction with rail ballast(12). Concrete track slab is now used on over 2400km of

Japanese high speed rail.

Recently, the Japanese Railway Technical Research Institute undertook some R&D with the

aim of reducing the high capital expenditure required for concrete track slab. To do this, the

use of macro synthetic fibres was discussed, and after extensive research and trials, macro

synthetic fibres were in 2009, used in the concrete track slab on the Oita elevated railway,

which is part of the Shinkansen line.

One of these trials aimed to determine the suitability of macro synthetic fibres for resisting

the lateral forces that occur on high speed rail lines. A test setup was prepared as displayed

in Figure 5.



Figure 5 – Horizontal Load Test Setup(13)

Initial testing aimed at determining the adequate concrete thickness to resist this lateral load,

with the inclusion on macro synthetic fibres providing additional post-cracking resistance.

Figure 6 demonstrates the significant additional capacity provided by the macro synthetic

fibres. Two section thicknesses, 60mm and 100mm, were tested and the load applied was

plotted against the horizontal displacement(10).

Figure 6 – Horizontal Load vs. Displacement Results(10)

As displayed in Figure 6, an increase in section thickness also has an impact on the

performance of the fibre reinforced concrete under load. Section thickness, particularly in

thin sections, has a significant influence on the post-crack load capacity of the fibre

reinforced concrete section. This testing confirmed acceptance for use on the Oita Elevated

Railway. A section of the macro synthetic fibre reinforced concrete track slab can be seen in

Figure 7.



Figure 7 – A section of macro synthetic fibre reinforced track slab being placed on the Oita Elevated Railway in Japan

Further testing was undertaken, after completion of the Oita section of the Shinkansen Line,

to gain a deeper understanding of the shear properties of the fibre reinforced concrete. A

test program was derived, which investigated varying dose rates of the macro synthetic fibre,

as well as a range of shear reinforcement arrangements. It was seen in the results, shown in

Figure 8, that there was an optimal dose rate of fibres to achieve the desired performance.

Figure 8 – Horizontal load vs. fibre dosage

With the optimal dosage confirmed, testing was then conducted to determine the impact of

shear reinforcement configurations, and the impact they may have on the shear capacity of

the section.

Figure 9 depicts three such shear reinforcement configurations, and the tested results of

each. Specimen 1-3 is solely reinforced with macro synthetic fibres, specimen 2-1 is

reinforced with a longitudinal bar, two diagonal “shear” bars and macro synthetic fibres, and

specimen 2-3 is reinforced with a longitudinal bar and macro synthetic fibres. It can be seen

that specimen No. 2-1 showed the greatest shear capacity, but this was not significantly

greater than specimen No. 1-3, which contained only fibre reinforcement.



Figure 9 – Shear reinforcement vs. load capacity

This demonstrates the significant capacity that the fibre reinforcement provides to the

section, and that additional steel reinforcement provides minimal additional capacity in this

situation.

Sydney Metro Northwest, Australia

The Sydney Metro Northwest is a new 19km rail line stretching from Rouse Hill in Sydney’s

north west, to Epping in the inner west, which consists of 4km of a raised viaduct and 15km

of twin tunnels between Bella Vista and Epping. Designed by Mott MacDonald and being

delivered by a John Holland/CPB joint venture, the tunnels incorporate a macro synthetic

fibre reinforced concrete track slab. Being constructed using a top down methodology, to

ensure the accuracy of the finished rail levels, this track slab sits atop a slipformed concrete

invert slab and satisfies the required 100-year design life.

As occurred with the docklands light rail, this project required concrete to be pumped

significant distances, upwards of 300m. To ensure this was achievable, a full-scale trial was

conducted prior to commencement of the project. This trial is depicted in Figure 10. Upon

successful completion of the trial section, construction commenced on the macro synthetic

fibre reinforced track slab throughout the 15km of twin tunnels.



(b) Figure 10 – Pumping (a) and finishing (b) of track slab concrete during pre-construction trial (a)

Conclusion Concrete track slabs are gaining acceptance in the industry, and in some applications, have

become the convention for railway infrastructure. One of the major drawbacks of concrete

track slabs is the significant increase in upfront capital expenditure required for their

construction. A means of reducing these upfront costs and increasing the attractiveness of

concrete track slab is the use of macro synthetic fibres to replace the conventional

reinforcement in the concrete slab.

Standards exist that govern the production of macro synthetic fibres, as well as their design

in concrete structures. These can be used to create cost effective solutions for track slab

construction into the future. Numerous organisations internationally have initiated the uptake

of macro synthetic fibre reinforced track slabs, through extensive on-site and laboratory

testing which has proven their effectiveness as a replacement for conventional steel

reinforcement.

Chief Engineers of Rail Agencies, designers and contractors should consider macro

synthetic fibre reinforced track slabs as an option for new concrete track slab construction.

This will enable new rail infrastructure to be built with reduced construction expenditure.



References:

1. Thuer G, Vocks H. Track slab system technology in australia - innovation challenges

based on european long term experiences. AusRAIL 2012; 27-28 November; Canberra201

2. European Committee for Standardisation (CEN). EN 14889-2:2006 - Fibres for

concrete - Part 2: Polymer fibres - Definitions, specifications and conformity. Brussels,

Belgium: European Committee for Standardisations; 2006.

3. European Committee for Standardisation (CEN). EN 14651:2005 - Test method for

metallic fibre concrete - Measuring the flexural tensile strength (limit of proportionality (LOP),

reisudal). Brussels, Belgium: European Committee for Standardisation (CEN); 2005.

4. RILEM TC 162-TDF. Test and design methods for steel fibre reinforced concrete:

sigma-epsilon-design method. Materials and Structures. 2003;36(262):560-7.

5. The Concrete Society. Technical Report No. 34: Concrete Industrial Ground Floors -

A guide to design and construction. Surrey, United Kingdom: The Concrete Society, 2013.

6. Alani AM, Beckett D. Mechanical properties of a large scale synthetic fibre reinforced

concrete ground slab. Construction and Building Materials. 2013;41:335-44.

7. Roesler JR, Altoubat SA, Lange DA, Rieder K-A, Ulreich GR. Effect of synthetic

fibers on structural behavior of concrete slabs-on-ground. Materials Journal.103(1).

8. Juhasz KP. Modified fracture energy method for fibre reinforced concrete. Fibre

Concrete 2013; Prague, Czech Republic: Czech Technical University in Prague; 2013.

9. JKP Static Ltd. Numerical modelling of a macro synthetic fibre reinforced tramline in

Belgium [PDF]. 2015 [Available from: http://jkpstatic.com/references/.

10. Ridout A. Slab track goes synthetic. International Railway Journal. 2011:51-6.

11. Nagy es Tarsai Limited Partnership, JKP Static Ltd, Fiberguru Ltd. The beginning:

The tramway in Szeged. Innoteka. 2014:24-5.

12. Dieleman L, Fumey M, Robinet A, Ramondenc P, Martin D. Experimentation of a

track section without ballast on the new line of East Eurpoean TGV.

13. Ikeno S, Tsukishima D, Sugano T. A study on reinforcing fibre and shear

reinforcement in height adjusting concrete for elastic ballast track. Japan: Japan Railway

West, 2011.

http://jkpstatic.com/references/

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