Group 8 PRESTRESSED CONCRETE SLEEPERS SUBMITTED BY: Mangesh Sharma Shruti D Prabhu Parijat Mishra Nakul Biluve Sandeep G.P. Prestressed Concrete Sleepers |2 INTRODUCTION Prestressed concrete is basically concrete in which internal stresses of a suitable magnitude and distribution are introduced so that the stresses resulting from external loads are counteracted to a desired degree. In reinforced steel concrete members, the prestressing is commonly introduced by tensioning the steel reinforcement. The development of early cracks in reinforced concrete due to non compatibility in the strains of steel and concrete was perhaps the starting point for the development of a new material like “prestressed concrete”. The application of permanent compressive stress to a material like concrete, which is strong in compression but weak in tension, increases the apparent tensile strength of that material, because the subsequent application of tensile stress must first nullify the compressive prestress. In 1904 Freyssinet attempted to substitute permanently acting forces in concrete to resist the elastic forces developed under loads and this idea was later developed under the name of “prestressing”. A prestressed concrete structure is different from a conventional reinforced concrete structure due to the application of an initial load on the structure prior to its use. The initial load or ‘prestress’ is applied to enable the structure to counteract the stresses arising during its service period. The main difference between reinforced and prestressed concrete is the fact that reinforced concrete combines concrete and steel bars by simply putting them together and letting them act together as they wish. Prestressed concrete, on other hand, combines high strength concrete with high strength steel in an ‘active’ manner. This is achieved by tensioning the steel and holding it against the concrete, thus putting the concrete into compression. This active combination results in a much better behavior of the two materials. Steel is ductile and now is made to act in high tension by prestressing. Concrete is a brittle material with its tensile capacity now improved by being compressed, while its compressive capacity is not really harmed. Thus prestressed concrete is an ideal combination of two modern high strength materials. Prestressed Concrete Sleepers |3 BRIEF HISTORY Before the development of prestressed concrete, two significant developments of reinforced concrete are the invention of Portland cement and introduction of steel in concrete. These are also mentioned as the part of the history. The key developments are mentioned next to the corresponding year. 1857 Monier, J., (France) Introduced steel wires in concrete to make flower pots, pipes, arches and slabs. 1886 Jackson, P. H., (USA) Introduced the concept of tightening steel tie rods in artificial stone and concrete arches. 1888 Doehring, C. E. W., (Germany) Manufactured concrete slabs and small beams with embedded tensioned steel. 1908 Stainer, C. R., (USA) Recognized losses due to shrinkage and creep, and suggested retightening the rods to recover lost prestress. 1923 Emperger, F., (Austria) Developed a method of winding and pre- tensioning high tensile steel wires around concrete pipes. 1925 Dill, R. H., (USA) Used high strength unbonded steel rods. The rods were tensioned and anchored after hardening of the concrete. 1926 Eugene Freyssinet (France) Used high tensile steel wires, with ultimate strength as high as 1725 MPa and yield stress over 1240 MPa. In 1939, he developed conical wedges for end anchorages for post-tensioning and developed double-acting jacks. He is often referred to as the Father of Prestressed concrete. Figure 1. Eugene Freyssinet 1938 Hoyer, E., (Germany) Developed ‘long line’ pre-tensioning method. 1940 Magnel, G., (Belgium) developed an anchoring system for post-tensioning, using flat wedges. During the Second World War, applications of prestressed and precast concrete increased rapidly. The names of a few persons involved in developing prestressed concrete are mentioned. Guyon, Y., (France) built numerous Prestressed Concrete Sleepers |4 prestressed concrete bridges in western and central Europe. Abeles, P. W., (England) introduced the concept of partial prestressing. Leonhardt, F., (Germany), Mikhailor, V., (Russia) and Lin, T. Y., (USA) are famous in the field of prestressed concrete. Prestressed concrete was started to be used in building frames, parking structures, stadiums, railway sleepers, transmission line poles and other types of structures and elements. In India, the applications of prestressed concrete diversified over the years. The first prestressed concrete bridge was built in 1948 under the Assam Rail Link Project. Among bridges, the Pamban Road Bridge at Rameshwaram, Tamilnadu, remains a classic example of the use of prestressed concrete girders. Figure 2. Pamban Road Bridge at Rameshwaram, Tamilnadu PURPOSE OF PRESTRESSING For concrete, internal stresses are induced (usually, by means of tensioned steel) for the following reasons. To eliminate cracking in reinforced concrete To reduce deflections To increase shear strength The tensile strength of concrete is only about 8% to 14% of its compressive strength. Cracks tend to develop at early stages of loading in flexural members such as beams and slabs. To prevent such cracks, compressive force can be suitably applied in the perpendicular direction. Prestressing enhances the bending, shear and torsional capacities of the flexural members. In pipes and liquid storage tanks, the hoop tensile stresses can be effectively counteracted by circular prestressing. Prestressed Concrete Sleepers |5 TYPES OF PRESTRESSING SOURCE OF PRESTRESSING FORCE Hydraulic Prestressing: This is the simplest type of prestressing, producing large prestressing forces. The hydraulic jack used for the tensioning of tendons, comprises of calibrated pressure gauges which directly indicate the magnitude of force developed during the tensioning. Mechanical Prestressing: In this type of prestressing, the devices includes weights with or without lever transmission, geared transmission in conjunction with pulley blocks, screw jacks with or without gear drives and wire-winding machines. This type of prestressing is adopted for mass scale production. Electrical Prestressing: In this type of prestressing, the steel wires are electrically heated and anchored before placing concrete in the moulds. This type of prestressing is also known as thermo-electric prestressing. EXTERNAL OR INTERNAL PRESTRESSING External Prestressing: When the prestressing is achieved by elements located outside the concrete, it is called external prestressing. The tendons can lie outside the member (for example in I-girders or walls) or inside the hollow space of a box girder. This technique is adopted in bridges and strengthening of buildings. In the following figure, the box girder of a bridge is prestressed with tendons that lie outside the concrete. Internal Prestressing: When the prestressing is achieved by elements located inside the concrete member (commonly, by embedded tendons), it is called internal prestressing. Most of the applications of prestressing are internal prestressing. In the following figure, concrete will be cast around the ducts for placing the tendons. Figure 3. External Prestressing of Box Girder Figure 4. Internal Prestressing of Box Girder Prestressed Concrete Sleepers |6 PRE-TENSIONING OR POST-TENSIONING Pre-tensioning: The tension is applied to the tendons before casting of the concrete. The pre-compression is transmitted from steel to concrete through bond over the transmission length near the ends. The following figure shows manufactured pre-tensioned electric poles. Figure 5. Pretensioned Electric Pole Post-tensioning: The tension is applied to the tendons (located in a duct) after hardening of the concrete. The pre-compression is transmitted from steel to concrete by the anchorage device (at the end blocks). The following figure shows a post-tensioned box girder of a bridge. Figure 6. Post Tensioning of Box Girder LINEAR OR CIRCULAR PRESTRESSING Linear Prestressing: When the prestressed members are straight or flat, in the direction of prestressing, the prestressing is called linear prestressing. For example, prestressing of beams, piles, poles and slabs. The profile of the prestressing tendon may be curved. The figure shows linearly prestressed railway sleepers. Circular Prestressing: When the prestressed members are curved, in the direction of prestressing, the prestressing is called circular prestressing. For example, circumferential prestressing of tanks, silos, pipes and similar structures. The following figure shows the containment structure for a nuclear reactor which Figure 7. Linearly Prestressed Railway Sleepers Figure 8. Circularly Prestressed Structure Prestressed Concrete Sleepers |7 is circularly prestressed. FULL, LIMITED OR PARTIAL PRESTRESSING Full Prestressing: When the level of prestressing is such that no tensile stress is allowed in concrete under service loads, it is called Full Prestressing (Type 1, as per IS: 1343 - 1980). Limited Prestressing: When the level of prestressing is such that the tensile stress under service loads is within the cracking stress of concrete, it is called Limited Prestressing (Type 2). Partial Prestressing: When the level of prestressing is such that under tensile stresses due to service loads, the crack width is within the allowable limit, it is called Partial Prestressing (Type 3). USAGE AND ADVANTAGES OF PRESTRESSED CONCRETE Examples of PC members: sleepers, hollow beams, concrete piles of precast blocks, cylindrical water tanks The steels are mostly in the shape of wires or strands but sometimes as bars. The prestressed concrete increases the shear strength and reduces radically both the deflections and the tensile cracks at service loads in such structures and thereby enables these high-strength materials to be used effectively. Prestressed concrete is particularly suited to prefabrication on a massproduction basis Prestressing increases the ultimate moment of resistance of the concrete and the resistance to shear forces and in some cases reduces the amount of steel required to resist shear. Prestressed Concrete Sleepers |8 The concrete and the steel are strictly tested during the prestressing operation, and a lower factor of safety is justified. Section remains uncracked under service loads, Reduction of steel corrosion, Increase in durability. Full section is utilized, higher moment of inertia (higher stiffness), less deformations (improved serviceability). Increase in shear capacity. Suitable for use in pressure vessels, liquid retaining structures. Improved performance (resilience) under dynamic and fatigue loading. High span-to-depth ratios .Larger spans possible with prestressing (bridges, buildings with large column-free spaces). Suitable for precast construction .The advantages of precast construction are as follows. Rapid construction, Better quality control, reduced maintenance, Suitable for repetitive construction, multiple use of formwork. DISADVANTAGES OF PRESTRESSED CONCRETE Because of the large forces carried by the tendons, the concrete is vulnerable to crushing at points where tendons / wires are anchored or change direction. The high stresses in PC increase the axial and bending deformation caused by creep. Prestressing needs skilled technology. Hence, it is not as common as reinforced concrete. The use of high strength materials is costly. auxiliary equipments. There is additional cost in Prestressed Concrete Sleepers |9 RAILWAY SLEEPERS PRESTRESSED CONCRETE Sleepers are the track components resting on the ballast formation transversely. Railway sleepers were first made in timber, and then a limited number of steel sleepers were used, followed by the now popular concrete sleepers. The majority of modern railway sleepers used in Australia are the prestressed concrete sleepers. In general, the sleepers are Figure 9. Prestressed Concrete Railway required to: Sleepers Support and restrain the rail foot. Sustain and distribute loads from the rail foot to the underlying ballast. Maintain the rail gauge and shape, and preclude rail inclination and track instability. Withstand longitudinal, lateral and vertical rail movements. Provide insulation between parallel rails, and resist wearing and loading, and endure extreme weather conditions from cold to hot, and from rain to drought. Due to the rigidity of the cast-in portion of the rail fitting, a concrete sleeper maintains the rail gauge much better than a timber sleeper, where the increase of gauge over the years is an accepted phenomenon. This applies especially to curved track. P r e s t r e s s e d C o n c r e t e S l e e p e r s | 10 The classical railway track basically consists of a flat framework made up of rails and sleepers that is supported on ballast. The ballast bed rests on a subballast layer that forms the transition layer to the formation. Figure presents the construction principle of the classical track structure. The rails and sleepers are connected by fastenings. Figure 10. Component of Ballasted Track The main advantages of a ballasted track are: proven technology relatively low construction costs simple replacement of track components relatively simple correction of track geometry (maintenance) small adjustments of track lay-out (curves) good drainage properties good elasticity Good damping of noise. The type of structure chosen depends not only on the expected axle loads and speeds, but also required service life, the type and amount of maintenance, local conditions and availability of basic materials. This means that the choice of a track system is a technical and economic question which has to be answered according to each individual case. In a ballasted track, the rails rest on the sleepers and together form the built-up portion of the superstructure. Timber and concrete sleepers, and to a limited extent steel sleepers. The general functions and requirements of sleepers are: to provide support and fixing possibilities for the rail foot and fastenings to sustain rail forces and transfer them as uniformly as possible to the ballast bed P r e s t r e s s e d C o n c r e t e S l e e p e r s | 11 to preserve track gauge and rail inclination to provide adequate electrical insulation between both rails To be resistant to mechanical influences and weathering over a long time period. Specific advantages of concrete sleepers are: heavy weight (200-300 kg) long service life great freedom of design And they are relatively simple to manufacture. TYPES OF CONCRETE SLEEPERS There are two commercially available types of concrete sleepers: twin-block and monoblock sleepers, as shown in Figure. Figure 11. Types of Sleepers The twin-block sleeper, which consists of two blocks of reinforced concrete connected by a coupling rod or pipe, and the mono-block sleeper, which is based on the shape of a beam and has roughly the same dimensions as a timber sleeper. The advantages of the twin-block sleeper over the mono-block sleeper are: well-defined bearing surfaces in the ballast bed and also high lateral resistance in the ballast bed. P r e s t r e s s e d C o n c r e t e S l e e p e r s | 12 The advantages of mono-block sleepers are their lower cost and the low susceptibility to cracking. The twin-block sleepers were originally developed in France and used in Europe, India, Brazil, and Mexico. The monoblock sleepers first came from the UK and have been adopted in countries such as Australia, Canada, China, Japan, the UK, the USA, and the former USSR. DESIGN CONSIDERATIONS FOR SLEEPERS The principle function of the rail road tie is to distribute the wheel loads carried by the rails to the ballast. Although the concrete sleeper is a simple determinate structural element, it is not possible to precisely evaluate the loads to which the sleeper is subjected to due to the various uncertainties and complexities in the variables involved. The factors influencing the design of concrete sleepers are: Static and dynamic loads imposed on the rail seats, depending upon the type of track (straight or curved), running characteristics, speed and maintenance of vehicles. The ballast reaction on the sleeper, which is influenced by the shape of the sleeper, its flexibility and characteristics of the ballast. STATIC AND DYNAMIC LOAD A static wheel load of 110 kN at the rail head with the heaviest locomotive was found to cause a vertical sleeper reaction of 60 kN on a straight track. Taking dynamic affects into consideration, the Indian railways have adopted a design load of 150 KN based on the static load of 60 KN which is amplified by a dynamic factor of 150%. On curved tracks, the maximum horizontal wheel load was observed to be 120 KN at the rail head. Considering the distribution of this load among several sleepers, it was estimated that 70 KN (60% of the horizontal reaction) is to be resisted by each sleeper. Due to lateral forces in curved tracks, the vertical force is increased on the outer rails and reduced on the inner rails. The force Q shown is a passive rail force exerted by the rail to prevent the overturning of the sleeper. Figure 12. Normal Condition P r e s t r e s s e d C o n c r e t e S l e e p e r s | 13 Figure 13. Curve Condition Figure 14. Bending Moment Diagram BALLAST REACTION The pressure distribution assumed on the sleeper is such that the bending moments computed from them should more or less conform to the bending moments actually developed at the critical sections, based on extensive field observations. Also, the pressure on the ballast should not exceed the permissible allowed pressure on the formation. Field observations by the German railways indicate that for sleepers designed for a bending resistance higher than 10 KN m P r e s t r e s s e d C o n c r e t e S l e e p e r s | 14 at the center, the percentage failures is of the order of 1 percent. The Indian sleeper is designed for a moment capacity exceeding 10 KN m at the center to safeguard against adverse formations due to black cotton soil. LAYING OF CONCRETE SLEEPERS Concrete sleepers are heavy and manual handling is not only difficult but may even cause damage to the sleepers. Mechanical handling of concrete sleepers, therefore, becomes necessary. For this purpose, the Mechanical Relaying Systems consisting of portal cranes are used. Normally a set will consist of two portal cranes. Figure 15. Railway Sleepers Being Laid OPERATIONS CONNECTED WITH RELAYING Preparatory work at site of relaying: It consists of following procedure. Since concrete sleepers are laid with LWR/CWR all preparatory works as listed in LWR Manual should be carried out before laying concrete sleepers. In addition, longitudinal section showing the existing rail levels should be plotted and proposed rail level determined taking into consideration the following points. 300mm. ballast cushion is available below the concrete sleepers. Clearances to structures are maintained within the accepted limits. The track and the road surface are suitably raised are approaches regarded. Where lifting of track is not possible at places like-below over line structures, on girder bridges and in yards, etc., suitable ramping out should be done. The proposed predetermined rail level should be indicated at suitable intervals along the tracks. Auxiliary track should be laid at 3.4 M. gauge keeping its centre line same as that of the existing track. P r e s t r e s s e d C o n c r e t e S l e e p e r s | 15 The existing welded rails should be converted into panels of suitable lengths such that the capacity of the portal crane is not exceeded by handling the old panel. Pre-assembly of Panels: Sleepers received from the Concrete Sleeper plant are unloaded and stacked at the base depot. Handling of Concrete Sleepers is done by portal cranes or separate cranes provided for the purpose. These sleepers are assembled into panels making use of service rails. While assembling the panels, elastic fastenings complete in all respects should be provided and correct uniform spacing’s of sleepers should be ensured. Formation of relaying train: The relaying train shall consists of two empty BFRs for loading released track panels, adequate number of BFRs loaded with pre assembled panels, BFRs loaded with portal cranes, one equipment and tool van, one crew rest van, one brake van and an engine. Actual Relaying: At speed restriction of 20 km. p. h. is imposed at the place of relaying and preliminary works such as loosening of fastenings, removal of ballast etc. are carried out in advance. On the day of relaying, traffic block is imposed and the relaying train enters the block section. After the relaying train is positioned, the portal cranes unload by themselves on the auxiliary track. The old track is dismantled and loaded by the portal cranes on the empty BFRs. The ballast is then leveled and the pre-assembled panels are laid in position. The new and existing tracks are joined by closure rails. After the last panel is laid, a ramp is made in two rail lengths between the existing track and the new track, to run out the difference in levels. The relaying train returns to the base depot where the old track panels are unloaded. Post relaying works: In subsequent blocks the service rails should be replaced by welded panels. MAINTENANCE OF RAILWAY SLEEPERS GENERAL Concrete sleeper track should be maintained by heavy on-duty track tampers. P r e s t r e s s e d C o n c r e t e S l e e p e r s | 16 An annual machine deployment programme shall be drawn by zonal railway and be circulated to the divisions before the beginning of the year. For spot attention/slack picking, multipurpose Tampers and Off-track Tampers shall be used as a regular measure on Concrete Sleeper Track, in the following areas of application: a. Picking up slacks in isolated stretches. b. Points and Crossing areas, c. Approaches to bridges and level crossings, d. Buffer rail joints/glued joints in LWR section, e. Block insulated joints/glued joints in track circuited stretches. Pre-tamping, during tamping and post-tamping attentions should be given to track. ANNUAL SYSTEMATIC ATTENTION ON PRC TRACK In addition to machine tamping, the gangs will carry out annual systematic attention to track from one end of gang beat to the other during work season. This will include; Examination of rails, sleepers and fastenings including measurement of toe load of ERCs. Inspection of and attention to insulated joints, switch expansion joints etc. Packing of approaches of bridges, level crossings, breathing lengths of LWRs and bad formation areas etc. Shallow screening of track. Through packing of track not maintained by machine, e.g. tracks not on PSC sleepers including all running lines of the yards and points and crossings in such running lines. Minor repairs to cess of bank. Boxing and dressing of ballast. Replacement of damage/missing rubber pads, liners and ERCs. TESTS FOR ACCEPTANCE OF SLEEPERS Pre-stressed sleepers are an important part of the railway construction. It is necessary to check their quality during the production and to ensure the sufficient reliability during their exploitation. Standardized three-point bending tests on the P r e s t r e s s e d C o n c r e t e S l e e p e r s | 17 hydraulic press were defined to ensure the quality control of sleeper production. Two types of tests are performed on sleepers; Check test for the profile under the rail. Check test for the profile in the middle of the sleeper. Figure 16. Testing configuration of the three-point bending check test of the sleeper in the profile in the middle of the sleeper The Indian railway standard specifications have laid down acceptance criteria by specifying the minimum cracking and ultimate loads of sleepers. Previous investigations on fatigue characteristics have indicated that as Figure 17. Testing configuration of the three-point bending check test of the sleeper in profile under rail. long as pulsating loads do not exceed 50% of the static ultimate strength of the member in compression or flexure, fatigue failure is rare. The axle load and train speeds have been gradually increasing over the past few years and so some advanced countries are already experimenting to develop a new form of permanent way consisting of continuous concrete slab with longitudinal and transverse prestressing without ballast, known as ballast less track, to cater to the needs of high speed heavy density traffic of the 21st Century.