Australian Society for Concrete Pavements 4 th 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.
12
Embed
Australian Society For Concrete Pavements · Australian Society for Concrete Pavements 4th Concrete Pavements Conference Macro Synthetic Fibres for Innovative Concrete Track Slab
This document is posted to help you gain knowledge. Please leave a comment to let me know what you think about it! Share it to your friends and learn new things together.
Transcript
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.
ASCP 4th Concrete Pavements Conference 3
Macro Synthetic Fibres for Innovative Concrete Track Slab Design and Construction [Todd Clarke]
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
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
ASCP 4th Concrete Pavements Conference 4
Macro Synthetic Fibres for Innovative Concrete Track Slab Design and Construction [Todd Clarke]
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.