Tom Heyer & Sachith Ranamukha, Austrak - Module 5: Turnout Prestressed Concrete Bearers: Engineering Behaviour and Design

RISSB National Turnout Workshop Page 1 of 21 Newcastle, May 2013

Turnout Prestressed Concrete Bearers:

Engineering Behaviour and Design

T. Heyer and S. Ranamukha

Austrak Pty Ltd. Brisbane, Qld 4105 Australia

SUMMARY

This paper is aimed at providing a technical understanding of the engineering behaviour and design of turnout prestressed concrete bearers.

The turnouts are generally used to divide a track into two or more tracks. As a result, a train, which has no transverse or turning device, can pass from a track onto another.

This paper addresses the current design, raw materials and manufacture of turnout bearers in the railway industry.

It begins with the conceptual design, parts and components, assembly method, analysis and design concept, experimental validation, manufacturing procedure, and quality control.

This material has been prepared with the objective to disseminate fundamental understanding of the design raw materials and manufacture of turnout bearers, which are a special type of railway sleepers. A number of illustrations have been used to demonstrate the concept and theory.

Notation and terminology used in this material have been complied with current Australian Standards and related track design manuals such as AREMA manual, UIC manual, and European Code.

1. INTRODUCTION

Conventional or ballasted railway track consists of rails attached to sleepers supported on ballast. Load is transferred from the axle to the wheel to the rail to the sleeper to the ballast and finally to the subgrade.

Turnouts are generally used to divide a railway track into two or more tracks. Turnout bearers are the main component of a turnout. They are used to support and guide the rails, switches, points, and crossings. Turnout bearers can be timber, prestressed concrete or steel.

Their main duty is to transfer and distribute the dynamic forces from the rail caused by train/track interaction to the substructure. The substructure consists of the geotechnical components, including the tamped ballast, subballast, and subgrade.

One of the key improvements in ballasted track over the last 50 years has been the widespread use of prestressed concrete sleepers and bearers to replace the traditional timber sleepers used. The use of prestressed concrete sleepers and bearers provides a low maintenance, longer life track with greater stability and higher load carrying capacity. Prestressed concrete (PC) sleepers and bearers are typically designed following either AS 1085.14:2003 (Australia), AREMA Chapter 30 (USA) or EN 13230 (Europe).

In Australia the analysis and design for the train/track forces are currently carried out using the Australian Standard AS1085.14-2003. The principle function of a PC bearer is to distribute the wheel loads from the rails to the ballast. The structural strengths of bearers must be able to bear both flexural and shear stresses induced in the bearers by the dynamic load actions. Bonding and bursting effects must be considered to provide the integrity to concrete bearers.

While the determination of the characteristics of a PC bearer is a simple process using structural sectional analysis the loads to which bearers are subject to is complex, due to the variables involved, resulting in significant uncertainty.

National Rail Turnouts Workshop 29-30 May 2013

T Heyer & S Ranamukha Turnout Prestressed Concrete Bearers: Engineering Behaviour and Design Austrak Pty Ltd.


The static loads applied are easily determined but the dynamic loads applied can vary greatly depending upon the speed and rolling stock characteristics. In addition, the load distribution between adjacent bearers depend upon the turnout structure, the ballast and sub-grade characteristics and maintenance standards. As such the applied bending moments can only be determined if the rail seat loads and ballast support conditions are known.

To account for these variables the different PC bearer methods adopted around the world each apply a dynamic coefficient to account the dynamic loads, a distribution factor to account for load sharing between adjacent bearers and a set of bending moment equations to account for the stress distribution in and under the bearers. The resultant applied bending moments are then used to select the PC bearer characteristics such as dimensions, concrete strength and prestressing force and distribution. The manufacture of prestressed concrete bearers requires a number of quality procedures to ensure that the bearers have achieved the required strengths and design specifications. Shortening of bearers over time must be considered and incorporated into the design to ensure acceptable tolerances during the manufacturing processes, installation period, and long-term services of bearers. This paper will discuss these key issues in details and be a guideline for further studies into the related topics.

Figure 1. Timber and Prestressed Concrete Railway Track



2. PRESTRESSED CONCRETE TURNOUT BEARERS

2.1 General

The production of PC turnout bearers is complicated by the variety of bearer types required.

The rail can be fixed in any position along the bearer and in varying angles to the bearer.

The overall length can also vary making each bearer unique and part of a ‘turnout set’.

The PC turnout bearers are usually produced without inclination and therefore transition sleepers with varying inclination are also required.

2.2 Bearer materials

Turnout bearers are commonly made of prestressed concrete or timber (see Figure 2). The engineering properties of concrete and timber are tabulated in Table 1.

Table 1 Engineering properties of materials

Material

Engineering Properties

Stiffness, E (GPa)

Compressive Strength (MPa)

Tensile Strength (MPa)

Density (kg/m3) Cost ($/m3)

Concrete 28 50 5 2400 200

Hardwood 18 50 50 1100 400

Concrete is a cementitously bonded composite that can be produced with a range of properties. One of the major advantages of concrete is that is strong in compression but it also has relatively low tensile strength. As such, concrete can be used more efficiently if it can be maintained in a state of compression. The tensile capacity of concrete can be significantly increased (to reduce the likelihood of cracking) by the addition of prestressed steel reinforcement. Prestressed Concrete (PC) is produced by the application of compressive forces from tensioned steel wires or strands. These forces act to reduce any applied tensile forces allowing PC to be used in situations where bending stresses occur. Turnout bearers are exposed to repetitive bending stresses in a railway track and as such the use of PC is an excellent example of combining a material like concrete, which is suitable for mass production, with high strength steel to provide the required tensile capacity. PC sleepers are produced in a precasting operation where rapid production rates combined with quality control processes can produce the large numbers of PC sleepers required to the specified tolerances needed for railway track construction. A precast PC sleeper is an “engineered” product where the structural and materials engineering processes are utilised to produce a sleeper that has the required strength and durability for the intended application. Quality management of the raw materials and processes along with in-process and final product testing ensure that each sleeper is produced to the tolerances required

This paper will address the design criteria and method for the prestressed concrete turnout bearers. However, for reference purposes, the related standards of other material types of turnout bearers has also been described.

2.3 Related Standards

Turnout bearers are classified as special sleepers according to AS1085.14-2003. As a result, the method of analysis and design of turnout bearers are generally complied with the standard requirements.

In case of concrete bearers, the shear capacity and other requirements must comply with AS3600-2001.

In case of timber bearers, the acceptance criteria must comply AS3818.2:1998 and AS1720-1997.



a) prestressed concrete bearers

b) timber bearers

Figure 2 Turnout bearers



2.4 Dimensional Evaluation

A turnout consists of a number of bearers of varying lengths. Rails are readily secured to the bearers at predetermined fastening locations to enable one track to be connected to another adjacent track.

A turnout normally has zero cant and typically consists of about 70 individual bearers each varying from its neighbouring bearers in length and in fastening locations. Transition sleepers with varying cants may be required to connect turnouts to canted track (see Figure 2).

Figure 3 a) flat; b) cant

Canted turnout bearers can be obtained by installing angled steel plates or using modified concrete profiles.

The depth and width of the turnout bearer may vary throughout its length if required. The minimum length of the turnout bearer is determined by the bond development requirements of the prestressing tendons specified in AS3600-2001. The base width shall be determined by the allowable bearing pressure.

Figure 3 shows the cross sections of normal sleepers and turnout bearers. Although the design concept is identical, the cross section is slightly different due to the duty and the orientation of turnouts.

a) typical turnout bearers b) standard sleeper

Figure 4 Concrete sleeper versus turnout bearers

The use of longer, wider, or stiffer bearers that increase the bearer-to-ballast bearing area has many of the same effects as reducing bearer spacing. Required right-of-way clearances and machinery limitations restrict bearer length.

Widening bearers introduces similar advantages to those resulting from increases in bearer length. However, widening sleepers beyond an optimal width is not effective. The optimum dimension is the point beyond which the ballast can no longer be effectively tamped.



2.5 Fastening System

Fastening systems include elastic resilient fasteners, cast-in nylon, high-density polyethylene, galvanized steel inserts, sleeves, or bolts.

If the fastenings are cast integrally with the turnout bearers, they must be installed in accurate positions within the specified tolerances at the locations shown in Table 2.

Table 2 Permissible tolerance in AS1085.14-2003

Dimension/Part/Condition Tolerance

Length immediately after transfer 6 mm

Cross-sectional dimensions 3 mm

Location of centroid of tendons relative to the moulded surface of the sleeper 3 mm

Location of individual tendons subject to complying with clear cover requirements 5 mm

Lateral straightness 6 mm

Concavity of convexity of rail seat in any direction 0.5 mm

The inward cant of the rail seats 1 in 400

Differential tilt of the rail seats restraining face of the fastening + 0.75, 0.25

Range of track gauge tolerances to be specified by purchaser (e.g. +2, -4 = 6 mm) 6 mm maximum

Where plates are specified to hold the rails, the methods of attaching plates to sleepers and rails to plates shall be specified, for example, the use of clips and shoulders.

Insulation, where required, shall conform to the requirements of AS1085.19-2001.

Plates may be secured by two or four bolt systems. Allowance shall be made by the designer to accommodate both fasteners and prestressing tendons, while still providing adequate cover to the tendons.

Figure 5 Installed Fastening systems

shoulder

rail

e-clip

base plate



Figure 6 Fastening systems on bearers

The minimum clear concrete cover to tendons at the soffit of the bearers shall be 35mm. Elsewhere, the minimum clear concrete cover to tendons generally shall be 25mm with the exception that the tendon may be exposed at the end faces. The minimum clear tendon cover to an insert hole or fitting shall be 12mm. Dissimilar metals shall not be in contact with any steel tendon.

Nylon or high density polyethylene inserts shall be threaded both internally and externally so as to be replaceable as required in AS1085.18. The internal threading shall be designed to accept the diameter of coach screw or bolts specified. A locking device may be required to retain the coach screws in position.

The fastening insert pull-out test shall be performed on bearers incorporating either type of fastening as specified in Appendices G and H in AS1085.14-20003.

Figure 7 Nylon inserts and Screw spike



3. CURRENT ANALYSIS AND DESIGN FOR TURNOUT BEARERS

3.1 General

A turnout bearer is a member of a complex grillage system in which rails are connected by elastic fastenings to bearers that are supported on a non-rigid foundation. This affects the loadings for such bearers. The load actions on bearers shall be controlled by permissible stresses based on the current design standard.

In general, the track owners or railway organisations would draft the specifications and tenders for turnout bearers. The main information supplied is the wheel load, tonnage, shear force and bending moment envelopes. These are used to design the turnout bearers structural requirements.

However, sometimes, the clients do not specify the track responses but do specify the tonnage, typical layout of the turnouts, vehicle information, speeds and operation information, track parameters (rail size, bearer spacing, gauge length), and axle load. In this instance the shear forces and bending moments for bearer design are determined using the framework as follows:

3.2 Rail seat forces

Distribution of axle load. The distribution method is the same as used for standard sleepers. The distribution of axle load depends on the vehicle parameters. For a two axle bogie, the static wheel load (Q) is about half of the axle load.

Impact factor (j). In order to allow for dynamic loading, the bearer should be designed for a load not less than 2.5 times the static load.

Centrifugal factor. The effects of centrifugal force should be allowed for on the curved pairs of rails.

Other forces. Forces and moments from points motors and other requirement should be allowed for where appropriate and properly summed up with Q.

Load distribution. The method of distribution of the forces from rails and crossings is usually specified by the purchaser. Design of the sleeper by distribution of the load to the centroid of the bearer is not allowed.

In some cases that the bending moment and shear envelops have not been provided, they should be calculated by an appropriate structural analysis or finite element method.

3.3 Shear and bending moment

Support conditions. Generally, it may be assumed that the bearer will be supported over its whole length by tamped ballast. Exceptions may happen such as where a bearer extends beyond the ballast shoulder to carry points-operating equipment.

Moments and shear forces should be calculated using the previously calculated rail seat loads and assuming that the ballast and subgrade behave and can be modelled as an elastic foundation.

The foundation modulus may be calculated in accordance with the recommendations given by Australasian Railways Association.

In very poor ground or formation, consideration should be given to improving subgrade in the area of the turnout, to reduce the magnitude of the inducted bending moments. Field test measurement and inspection can be used to confirm the assumptions.

Shear forces and bending moments. Depending on the turnout layout and bearer pattern, the rail seat loads obtained from the above assumptions should be used to produce design bending moments and shear force envelopes for the bearer. However, in some cases, this analysis can yield small values of negative (or so-called ‘hogging’) bending moment.

It is important that adequate negative bending moment capacity be built into prestressed concrete bearers to cope with dynamic rebound effects, handling stresses and a degree of centre binding.

For prestressed concrete bearers, it is recommended that a value of negative bending moment be assumed that is at least two-thirds of the bearer’s calculated maximum positive bending moment. The bearers should be designed to have this capacity at all sections outside the transmission zones.

Prestressed concrete end zone stresses. Consideration should be focused on bursting stresses caused at the bearer ends by the tendons. Transverse reinforcement should be provided where required. Where substantial bending moments may occur at the end portions of bearers, bond stresses in tendons should be limited to prevent slip under the design moment.



3.4 Permissible stresses

The permissible stresses are the limit of stresses specified for construction materials in bearers under a design load, including concrete and prestressing tendons.

The flexural stress of a bearer can be calculated using the sectional analysis as follows:

Z

M

Z

Pe

A

P

where is the flexural stress; P is the prestressing force; e is the eccentricity; M is the bending moment (if any); A is the cross-sectional area; and Z is the sectional modulus of bearer.

3.4.1 Concrete stresses

At transfer:

The maximum permissible stress in the concrete immediately after transfer, and before deferred losses, shall be the applicable value given in Figure 8.

After losses:

The maximum permissible stress in the concrete under the working conditions assumed by the designer, and after allowing for all losses of prestress, shall be the applicable value given in Figure 9.

It should be noted that the minimum compressive stress at any cross-section through the rail seat area shall be 1.0 MPa after all losses without any applied load.

Figure 8 Concrete stresses at transfer (AS1085.14)

Figure 9 Concrete stresses at final (AS1085.14)

3.4.2 Tendon stresses

Under the working conditions assumed by the designer, the maximum permissible stress in the prestressing

tendons

sA

P (where As is the tendon area) shall:

- not exceed 0.8 fp for jacking stress in the prestressing tendons after losses due to jack friction only (the initial prestressing force applied); and

- not exceed 0.7 fp for the initial stress in a prestressing tendon immediately after transfer (the initial prestressing force after all losses).

For design purposes, the tensile strength (fp) for wire tendons shall be the minimum strength in the selected tensile strength range given in AS4672 for that tendon. For strand tendons, the minimum breaking force



given in AS4672 for the grade specified shall be used for design. However, it should be noted that the maximum diameter for the tendon should be less than 8mm for indented wires.

A challenge in bearer design is the wire pattern management. It should be noted that the bearing plate on the bearer requires two screw spikes each side. To accommodate two screw spike the wire pattern must have appropriate gap areas for pouring concrete as well as vibrating it.

3.4.3 Shear strength

It is common that the turnout will be subject to high shear forces. The shear strength check in accordance with AS3600-2001 is required to design of bearers. If the ultimate shear force V* is less than a half of factored shear capacity provided by concrete only (Vuc), no shear reinforcement is required for the bearers with the depth less than 750mm, or

V* < 0.5 Vuc

In general, the factor for flexural elements under shear is 0.7. In practice, it can be assumed that V* = 1.2 V for curved turnouts (slow speed), and V* = 1.7 V for straight turnouts (high speed), whereas V is the maximum shear force obtained from the shear envelope of bearer grillage analysis.

If V* > 0.5 Vuc or otherwise deeper bearers, the shear reinforcement must be provided in accordance with AS3600-2001.

3.5 Loss of prestress

The loss of prestress shall be determined by the methods specified in AS3600. For the preliminary design, a value of 25 percent may be assumed. In practice, a value of 22 percent has been proven and being adopted for the final design.

3.6 Development length and end zone

The development lengths of tendons shall comply with AS3600. Bursting and spalling forces shall be assessed and reinforcement provided if needed.

Figure 10 Minimum transmission length for tendon (AS3600-2001)

3.7 Bearing Pressure

In the design of bearers and sleepers, the allowable bearing pressure can be calculated by Clause 4.2.4 of AS1085.14-2003. The major requirement is that the bearing dimension must allow bearing pressure less than 750 kPa (AS1085.14-2003).

4. MANUFACTURE OF TURNOUT BEARERS

4.1 Manufacturing Methods

There are a number of different manufacturing methods that can be employed to produce PC bearers.



Each manufacturing method involves: mould cleaning and assembly of fasteners; inserting prestressing tendons; placement, compaction and finishing of concrete; curing; concrete testing; de-moulding; transfer of prestress; separation of individual sleepers; sleeper inspection and quality control testing. The methods differ depending upon whether the sleepers are pre-tensioned or post-tensioned and the method of tendon placement and stressing (short line or long line). Each method has advantages and disadvantages. Each method has different capital investment requirements but also different labour requirements. Each method can be designed with varying levels of automation and labour requirements. To ensure that the appropriate choice of manufacturing method is chosen factors required such as: sleeper quality; production rate; sleeper types; supply period; amount of capital investment; availability and cost of skilled labour all need to be considered. For pre-tensioned PC sleepers the load in the tendons is applied prior to placing the concrete in the moulds. Once the concrete has reached the specified strength the prestressing force is released and is transmitted to the concrete through the bond between the tendon and concrete. Pre-tensioning is adopted in the single-mould, short-line and long-line systems. The PC sleepers must be maintained in the moulds until the concrete has cured to the specified release strength. With accelerated curing (low pressure steam) this results in approximately a 24 hour production turnaround for each mould. For post-tensioned PC sleepers the load in the tendons is applied after the concrete has reached the specified strength. The tendons are anchored at the end of the sleepers by mechanical anchorages and the concrete is placed under stress by the end bearing. Production rates can be limited (depending upon space and capital) and quality (surface finish, post-tensioning accuracy) can be a problem. PC sleepers produced by the post-tensioned method are not common. The short line methods use moulds that act as prestressing frames that react against the prestressing force of the tendons. The moulds generally consist of one to six individual sleepers cast side by side (single mould) and these can be arranged to have up to eight moulds placed end to end (short line). The frame anchors the prestressing tendons and production usually takes place in a carousel with the frame and moulds travelling from one operation area to the next. This reduces the need for large factory structures in comparison to long line methods. When using precisely manufactured moulds the required PC sleeper dimensional tolerances can be obtained. It is also possible to change sleeper moulds quickly providing a flexible process able to produce a wide variety of PC sleeper types. The short line methods achieve high utilisation of moulds (24 hour turnaround – using low pressure steam curing) with a relatively smaller factory area footprint. Running costs are generally higher than long line plants due to labour, material utilisation and the special anchorages used in some systems. With the long line method the production line consists of beds containing between 20 to 80 moulds arranged end on end. The moulds are fixed within the beds and all cleaning, preparation, casting and de-moulding equipment is mobile and travels along each bed. Prestressing tendons, common to all sleepers in a line, are stressed between two abutments or stress heads placed at either end of the bed. Application of tension to the tendons (stressing) occurs after mould cleaning and placement of fasteners. Both elongation length and prestress force can be used to check that the correct stress levels are achieved. Any minor errors in the elongation has minimal effect on the final prestressing force in the PC sleeper, which maintains consistent sleeper performance. Concrete is placed into the moulds via a casting machine which travels along the length of the bed discharging and vibrating a uniform volume of low slump high strength concrete. The outcome is a well compacted concrete with high density, uniformity and high strength. The strength gain in the concrete must be accelerated to obtain 24 hour turnaround which is generally achieved using low pressure steam. Once the correct transfer strength is achieved the bed is then de-moulded and de-stressed. This operation involves pushing the moulds off the concrete and supporting the sleepers. The bed is now effectively continuous lines of concrete either suspended or supported. The bed of sleepers is then cut at predefined intervals to form individual sleepers. The sleepers are now visually inspected, tested and stockpiled for despatch. While the required factory site is larger for long line than for the other methods the PC sleepers can be produced at much higher rates of production than for the other methods with consistent quality.



Figure 11 Turnout Bearers and Sleepers

4.2 Factory Design

The prestressed concrete bearer design and factory design need to be an integrated process. Systems management is used to provide consistency between all aspects of a factory start-up or new sleeper design and ensures an efficient, safe and dependable outcome. This includes documented procedures, which provide standardisation across all areas of Austraks operations: commissioning, training to factory operations to ensure consistent results. PC sleeper and bearer factories are built and operated in a range of environments from tropical to temperate to arid and in remote regions. Combined with a systems management approach this ensures that the PC sleepers and bearers are produced to schedule, economically and in accordance with the design specifications. In order to manufacture turnout bearers of varying lengths and fastening locations a special bed is normally used, which is one sleeper wide. Typically, the bed will be able to accommodate moulds of differing length. It will incorporate one common pattern of prestressing tendons that are stressed by a common jack.



4.3 Mould Preparation

The bearer moulds are designed and manufactured to produce bearers within the required tolerances and for the large manufacturing quantities required. The mould is designed and constructed as to make sure that the bearers would be within tolerance after elastic and time-dependent shortenings. The moulds are fixed within the beds and all cleaning and mould preparation equipment is mobile and travels along each bed to service the moulds. Provision is made for the accurate location and firm support of the cast-in components during the placement and vibration of the concrete. The bearer mould consists of two pieces: a tub (fixed shape) and lid (variable, depending on fastening systems). It is also possible to change lids quickly providing a flexible process able to produce different PC bearer types from the one factory. The huge number of variations in length and positions of the constrained components require separate mould lids for each individual turnout bearer. Moulds are cleaned using manual or automatic brooms which effectively remove all residual concrete from the previous production cycle. In order to maximise casting performance mortar leakage are minimised. The internal mould surfaces must be clean and treated with a release agent to ensure the non-cohesion of the concrete to the mould. After the moulds have been cleaned a specially selected concrete formwork release agent is sprayed onto the mould surface. The use of the mould release ensures easy removal of the concrete sleepers from the mould while ensuring a surface finish to the required specification or standard. The required fastener components are then installed and locked into position using specifically designed fastener holding equipment. Special attention must be given to the mould where plates are used as part of it to create the indented marking in concrete. This is to prevent restraint type cracking due to shrinkage, especially in the long piece of bearers. The first set of bearers cast from new moulds or lids shall be examined whether the tolerance can be accepted by assembling the bearer with rails, plates, and fastenings. Moulds shall be adjusted as required and the subsequent casts measured until the bearers are within the tolerance.

Figure 12 Mould lids

Lids Ferrules



4.4 Wire Placement and Stressing

Prestressing tendons normally employed are high strength, low relaxation, indented wires. The wires consist of plain carbon steel with a carbon content of 0.70 to 0.85%. The wires are drawn from hot rolled rods to reduce the diameter and increase the tensile strength. The cold drawn wire is then subject to a stress relieving and tensioning process (low relaxation or Class 2 wire) in a continuous in-line manner. The stress relieving heat treatment is a continuous short term isothermal heat treatment that is designed to improve certain mechanical properties (e.g. ductility) and relaxation characteristics. Indents are applied after final cold drawing to improve bond properties between the wire and concrete. The prestressing tendon must have a high tensile strength (> 1500 MPa), must have a high bond with the concrete (significantly improved by indenting wire) and must have a low relaxation (to maintain the applied strain on the concrete during service). The strength of the tendon is critical to ensure that adequate prestress force can be applied to the concrete. Up to 80% of the tensile strength of the tendon is applied resulting in the requirement for strict quality control of wire used. Tendons of relatively small diameter are required to ensure a high level of bond so that the development length is reduced to suit the PC bearer length. The bond is also improved by indenting the wire. During the setup of tendons and wire pattern, any substance that impairs the bonding of the tendons to the concrete must be removed. The tendons are maintained in a clean condition until embedded in the concrete. The use of a large number of wires (24 – 44 wires per bearer) provides a more uniform stress distribution in the sleeper cross section. Placement of individual wires allows fine tuning of the design to provide the required capacity in an economic way.

Wire pattern control devices are used during concrete placement to accurately obtain the specified wire pattern and prestress force distribution in every bearer produced, providing uniform quality bearers. The tendon are stressed gradually to achieve the jacking force calculated.

Figure 13 Turnout bearer ‘bed’ ready for concrete placement

Tendons

Shear ligature

Tub (Mould)



4.5 Concrete Manufacture

The use of high strength concrete is necessary for the production of prestressed concrete due to the high compressive forces applied during transfer of the prestressing force from the tendons to the concrete. The use of high strength concrete also reduces prestress losses, improves resistance to cracking and ensures long term durability. The high strength concrete consists of Portland cement, supplementary cementitous materials (e.g. fly ash), water, aggregates and admixtures. Concrete is to be made by these materials in accordance with AS 1379. The characteristic compressive strength (28 days) should be more than 50 MPa, and the minimum compressive strength at transfer should not less than 30 MPa.

The selection and quality control of these materials is essential to ensure that the concrete produced meets the strength requirements and provides the long term durability required. Areas of concern for concrete durability include Alkali Silica Reaction (ASR), Delayed Ettringite Formation (DEF), Freeze-Thaw damage and corrosion damage.

The choice and quality control of the raw materials, along with the PC bearer design and manufacturing processes, need to be strictly controlled to ensure that the known causes of concrete failure do not occur during the required life of the PC bearer.

A number of “raw material specifications” are required to evaluate local raw materials to produce the required high performance durable concrete. Regular sampling and testing of raw materials ensures delivered materials meet the agreed specifications.

Purpose-built batching plants are required to deliver consistent concrete efficiently and economically. The plants are designed around what materials are available locally, including flyash if required.

Due to the tendency of cement to harden with moisture the batch plants incorporate bulk silos which maintain the quality of the cement delivered. The silos utilise bin level sensors which help avoid overfilling while ensuring continuous supply.

Concrete mix designs and batch control are produced based on technical expertise and experience. Accurate production of concrete mix designs ensure a product produced to the strength specifications as economically as possible.

Figure 14. Concrete Batch Plant



4.5 Concrete Placement and Curing Concrete is placed into the moulds via a specially designed casting machine. The casting machine travels along the length of the bed discharging and vibrating a uniform volume of low slump concrete, and finished off with a vibrating screed.

The casting machine employs both internal and external vibration for greater concrete compaction. The continuous mould design allows for continuous vibration along the entire sleeper length through the channels of the pre-stress wires designed into the sleeper wire pattern.

The outcome is a low slump, highly compacted concrete which results in a high density finished product of uniform consistency.

This is essential for a) economy: low slump concrete is more economic to produce and b) durability: highly compacted concrete protects against potential corrosion and wear damage.

Tarpaulins are placed over the bed after casting creating an enclosed space under the moulds. Curing of concrete requires the use of water or steam to concrete, or the retention of the water in the concrete. Curing of the concrete shall start immediately after finishing and shall be conducted continuously.

Low pressure steam can then be introduced into this chamber under strict conditions using micro-processor controlled valves accelerating the concrete strength gain. Temperatures in the chamber are continuously logged as a permanent record for that bed. The rate of temperature gain and maximum temperature is strictly controlled to avoid any delayed ettringite formation.

The required concrete transfer strength is typically greater than half of the 28 day characteristic strength of the concrete when fully cured. As such, after steam curing is complete the concrete has reached the minimum strength typically specified to allow removal of forms from concrete. The final curing occurs while the PC bearers are stored in the outdoor stockpile removing the need for further wet curing.

Figure 15 Turnout bearer ‘bed’ after concrete casting

4.6 Demoulding, Destressing and Cutting



The PC bearers can be demoulded when the minimum compressive strength of concrete at transfer is achieved (minimum of 30 MPa in 12 hours by steam curing) or otherwise in accordance with AS 3600.

When removed from the moulds, the bearers shall be free from cracks, chipped edges, honeycombing and surface defects, which lead to inadequate cover.

The finishing surfaces must meet the requirements:

Rail seat surface shall be Class 1 finish

Other surface shall be Class 3 fished

Soffit surface shall be rough screed finish to achieve the covering and the uniform surface texture

The stress shall be released gradually and without interruption. Shock release of tendons is not allowed. After destressing of tendons and transfer of prestress to the bearer, the ends of bearers shall be sawn and trimmed mechanically, without damaging the concrete. The cut ends of the tendon shall not protrude more than 3mm beyond the surrounding face of concrete.

4.7 Testing, Inspection and Loadout

Each bearer shall be indelibly marked on the upper surface as installed. It should include the following information:

The mark required by purchaser

Year of manufacture

Mark of manufacture

Number of code of the mould in which the bearer is cast

The bearer should be clearly branded to indicate its position and orientation in the set as each bearer or sleeper forms part of a set in turnouts.

The manufacturer shall carry out measurements of dimensions and investigations of defects in each bearer.

Full records of the manufacturing process shall be maintained including

Concrete test result (Date of casting, batch number, water/cement ratio, curing condition, date of testing and compressive strength indicated by the test).

Prestressing tendon certificates or test results (identification of coils of wires, manufacturer’s name/reference, identification number of gauges/pumps/jacks, force applied, total elongation measured and calculated, elongation remaining after anchoring).

Sleeper load tests (date of casting, bearer identification number, date of testing, the loads at first crack, failure load, mode of failure, batch of bearer, type of test)

A detailed and comprehensive in-process and final product testing regime is incorporated into the manufacturing systems. In-process testing monitors the concrete quality while final product testing verifies the sleepers made. Concrete samples are taken from every bearer bed made to determine when the release strength is achieved and validate the characteristic strength required by the sleeper design. Every bearer is visually inspected with a predetermined number of bearers set aside for full dimensional checks and static beam load testing. Each bearer is also date stamped and through the use of the individual mould number each sleeper has full traceability for quality assurance. Once all the testing has been completed and the bearers certified for service they are ready to be delivered to the project

The bearers must be stored on a hard standing area. The storage area shall be capable of sustaining the loading imposed by bearer stacks without deformation or settlement that may be detrimental to the bearers.



The bearers shall be stacked on timber with timber packing between layers in such a manner that unacceptable stresses do not occur in the bearers.

The recommended stacking method is to place the largest and longest bearers at the bottom of the stack and graduating to smaller bearers at the top. The packing height should be sufficient to maintain vertical clearance between layers and allow forklift truck to enter and withdraw. The packing width should be enough to prevent damage to the top surface of the bearers.

Figure 16 Turnout bearer stacked ready for dispatch

Figure 17 Turnout bearer stacked ready for dispatch



5. TYPE TESTING FOR DESIGN VALIDATION

Type Testing shall be carried out in a similar manners to sleepers. However, the location of the maximum design bending moment may not be at the rail seats as it is with sleepers.

The tests to be carried out at the rail seat shall be carried out at those cross-sections nearest the ends of the bearer that are designed to carry the maximum bending moment. The purchaser may specify more testing at the rail seat to determine the tensile strength and bond capacity.

To ensure the quality of bearers, apart from tolerance measurements, the type test may be carried out including

Concrete testing

Cylinder

NDT

Fastening system testing (if new system is adopted)

Pull-out test

Torque test

Bearer testing (if required)

Positive bending load test

Negative bending load test

Repeated load test (positive bending)

Figure 17 Beam testing



6. DELIVERY

The marked bearers shall be handled appropriately in order to arrange them onto the transport method without damage. To prevent any cracks or defects inspection of the bearers shall be done before and after transporting the bearers to the installation area.

The good must be delivered in full and on time to the clients specified delivery deadline and expected capacity of bearers must be met the specifications of the purchasers. Given the size of the application, and organisation required, it is critical that the specified delivery requirements are met. The product certificate must be accompanied with the bearers to ensure accuracy and quality.

7. INSTALLATION

Installation of turnout bearers requires careful planning to ensure that all of the equipment and required space is available at the clients assembly location for the installation process. The bearers shall be assembled in accordance with the turnout layouts designed by the track owner.

The gauge and tolerance measurements must be performed before moving the completed bearer assembly into track.

Figure 18 Turnout bearer set assembly



8. SUMMARY

This paper addresses the introduction to typical turnout layouts, components, and materials used. However, emphasis in this paper is placed on the turnout bearers manufactured using prestressed concrete.

Turnout bearers are employed in the turnouts as to provide support to rails and distribute track forces to substructure.

The current analysis and design of the turnout bearers have been discussed based on the permissible stress principle.

It should be noted that this paper is aimed for the training purposes. Technical terms and content information are based to the reference sources. The design and manufacture guidelines are laid out in the appropriate format to enhance the readers for further readings.

9. FUTURE OF TURNOUT BEARERS

A number of new product innovations are being developed, including:

Low profile prestressed concrete turnout bearers

Light bearer for applications in existing rail bridges

New fastener systems

New materials such as ‘Green’ Concrete

These developments will help the railway industry to have better and more environmental-friendly products for serving publics.

10. REFERENCES

Additional information sources for further investigating this topic can be found in these following references. For training purpose, most of the content in this paper is based on the sources in:

Esveld, C (2001) Modern railway track, MRT-Production, The Netherlands

Indraratna B and Salim W (2005) Mechanics of railway track, a geotechnical perspective, Taylors and Franscis, London, UK.

Jeff T and Tew GP (1991) A review of track design procedures: sleepers and ballast, BHP Research Laboratory, Melbourne, Australia.

Rail Corp (2007) Installation and maintenance manual. RailCorp, Sydney, Australia.

Standards Australia (2003) AS1085.14 Prestressed concrete sleepers, Sydney, Australia

Standards Australia (2001) AS3600 Concrete structures, Sydney, Australia

Concrete Railway Sleepers, FIB State of the Art Report, Thomas Telford, 1987.

Precast Concrete Railway Track System, FIB State of the Art Report, Bulletin 37, 2006.

Neville, A.M. and Brooks, J. J. Concrete Technology. Pearson Education, England, 1987.