ADVANCED PRESTRESSED CONCRETE.
Author: A.Pon Arul Yesu Raja, V.Manikandan.
3rd year Civil Engineering.
College Name Here
Abstract:
Pre-stressed concrete sleepers are the main components of railway track systems. To carry and transfer the dynamic wheel loads from the rails to the ground, their current design and construction are limited by allowable flexural stress constraints under service conditions. In current design practice for such a component, the dynamic load effects due to wheel/rail interactions are treated as a quasi-static load using a dynamic impact factor. Then, the allowable stresses eliminate a crack initiation. In reality, the impact events are frequently recorded because of the uncertainties of wheel or rail irregularities such as flat wheels and dipped rails. These effects cause cracking in the concrete sleepers, resulting in excessive maintenance. Limit states design philosophy for the pre-stressed concrete sleepers, containing ultimate and fatigue limit states, has been recently proposed based on structural reliability concept to rationalise the design method and minimise the maintenance.On the basis of probabilistic approach, the high-magnitude low-cycle fatigue limit states, which are more significant in terms of damage evolution, have been addressed in this article. Series of repeated impact tests for the in-situ pre-stressed concrete sleepers were carried out using the Australian largest high-capacity drop weight impact testing machine at the University of Wollongong. The impact forces have been simulated in relation to the probabilistic track force distribution obtained from a heavy haul rail network. This article focuses on the impact responses of the cumulatively damaged sleepers. The effects on such responses of the track environments including soft and hard track supports are also highlighted in this article. It is found that a concrete sleeper damaged by the impact fatigue cycles could possess significant reserve capacity for resisting the axle load in a similar manner as the undamaged sleeper.
INTRODUCTION:
Prestressed concrete is one in which it is capable of carrying high tension and compression. This technique is usually found in the concrete beams, columns, etc. Prestressed members are crack free under the working loads and it looks better and more watertight, providing better corrosion protection to steel.
PRESTRESSED CONCRETE:
Prestressed concrete is a method for overcoming the concrete's natural weakness in tension. It can be used to produce beams, floors or bridges with a longer span than is practical with ordinary reinforced concrete. Prestressing tendons (generally of high tensile steel cable or rods) are used to provide a clamping load which produces a compressive stress that offsets the tensile stress that the concrete compression member would otherwise experience due to a bending load. Traditional reinforced concrete is based on the use of steel reinforcement bars, rebar’s, inside poured concrete.
METHODS OF PRESTRESSING:
Prestressing can be accomplished in three ways:
Pre-tensioned concrete, and Bonded post tensioned Unbounded post-tensioned concrete.
PRETESNSIONED CONCRETE:
Pre-tensioned concrete is cast around already tensioned tendons. This method produces a good bond between the tendon and concrete, which both protects the tendon from corrosion and allows for direct transfer of tension. The cured concrete adheres and bonds to the bars and when the tension is released it is transferred to the concrete as compression by static friction. However, it requires stout anchoring points between which the tendon is to be stretched and the tendons are usually in a straight line. Thus, most pretensioned concrete elements are prefabricated in a factory and must be transported to the construction site, which limits their size. Pre-tensioned elements may be balcony elements, lintels, floor slabs, beams or foundation piles.
Sleeper:
In the railway track the sleeper which can transfer the load from rail to the ballast.To maintain the track at proper grade by allowing raising of the rail and tamping the required quantity of ballast below the rail.
Types of sleeper:
Stone sleeper Wooden sleeper Steel sleeper Concrete sleeper
Stone sleeper:
The type of sleeper used on the predecessors of the first true railway (Liverpool and Manchester Railway) consisted of a pair of stone blocks laid into the ground, with the chairs holding the rails fixed to those blocks. One advantage of this method of construction was that it allowed horses to tread the middle path without the risk of tripping. In railway use with ever heavier locomotives, it was found that it was hard to maintain the correct gauge. The stone blocks were in any case unsuitable on soft ground, where timber sleepers had to be used.
Wooden sleeper:
A variety of softwood and hardwoods timbers are used as ties, oak, jarrah and karri being popular hardwoods, although increasingly difficult to obtain, especially from sustainable sources. Some lines use softwoods, including Douglas fir; while they have the advantage of accepting treatment more readily, they are more susceptible to wear but are cheaper, lighter (and therefore easier to handle) and more readily available. Softwood is treated, historically using creosote, but nowadays with other less- toxic preservatives to improve resistance to insect infestation and rot. New boron based wood preserving technology is being employed by major US railroads in a dual treatment process in order to extend the life of wood ties in wet areas. Some timbers (such as sal, mora or azobé) are durable enough that they can be used untreated.
Problems with wood ties include rot, splitting, insect infestation, plate-cutting (known as chair shuffle in the UK)(abrasive damage to the tie caused by lateral motion of the tie plate) and spike-pull (where the spike is gradually worked out and loosened from the tie). For more information on wood ties the Railway Tie Association maintains a comprehensive website devoted to wood tie research and statistics.
Steel sleeper:
Steel sleepers are formed from pressed steel and are trough-shaped in section. The ends of the sleeper are shaped to form a "spade" which increases the lateral resistance of the sleeper. Housings to accommodate the fastening system are welded to the upper surface of the sleeper. Steel sleepers are now in widespread use on secondary or lower-speed lines in the UK where they have been found to be economical to install due their ability to be installed on the existing ballast bed. Steel sleepers are lighter in weight than concrete and able to stack in compact bundles unlike timber. Steel sleepers can be installed onto the existing ballast, unlike concrete sleepers which require a full depth of new ballast. Steel ties are 100% recyclable and require up to 60% less ballast than concrete ties and up to 45% less than wood ties.
Historically, steel ties (sleepers) have suffered from poor design and increased traffic loads over their normally long service life. These aged and often obsolete designs limited load and speed capacity but can still be found in many locations globally and performing adequately despite decades of service. There are great numbers of steel ties with over 50 years of service and in some cases they can and have been rehabilitated and continue to perform well. Steel ties were also used in specialty situations, such as the Hejaz Railway in the Arabian Peninsula, which had an ongoing problem with Bedouins who would steal wooden ties for campfires.
Modern steel ties handle heavy loads, have a proven record of performance in signalized track, and handle adverse track conditions. Of high importance to railroad companies is the fact that steel ties are more economical to install in new construction than creosote-treated wood ties and concrete ties. Steel ties are utilized in nearly all sectors of the worldwide railroad systems including heavy-haul, class 1’s, regional, shortlines, mining, electrified passenger lines (OHLE) and all manner of industries. Notably, steel ties (bearers) have proven themselves over the last few decades to be advantageous in turnouts (switches) and provide the solution to the ever-growing problem of long timber ties for such use.
Concrete sleeper:
Interest in concrete railroad ties increased after World War II following advances in the design, quality and production of pre-stressed concrete. Concrete ties were cheaper and easier to obtain than timber and better able to carry higher axle-weights and sustain higher speeds. Their greater weight ensures improved retention of track geometry especially when installed with continuous-welded rail. Concrete sleepers have a longer service life and require less maintenance than timber due to their greater weight which helps them remain in the correct position for longer. Concrete sleepers need to be installed on a well-prepared subgrade with an adequate depth on free- draining ballast to perform fully.
In 1877, M. Monnier, a French gardener, suggested that concrete could be used for making ties for railway track. Monnier designed a tie and obtained a patent for it, but it was not successful. Designs were further developed and the railways of Austria and Italy used the first concrete ties around the turn of the 20th century. This was closely followed by other European railways.
Major progress could not be achieved until World War II, when the timbers used for ties were extremely scarce due competition from other uses such as in mines. Following research carried out on French and other European railways, the modern pre-stressed concrete tie was developed. Heavier rail sections and long welded rails were also being installed, requiring higher-quality ties. These conditions spurred the development of concrete ties in France, Germany and Britain, where the technology was perfected. On the highest categories of line in the UK (those with the highest speeds and tonnages) pre-stressed concrete sleepers are the only ones permitted by Network Rail standards. Most European railways also now use concrete bearers in switches and crossing layouts due to the longer life and lower cost of concrete bearers compared to timber, which is increasingly difficult and expensive to source in sufficient quantities and quality.
On November 8, 2011, the U.S. Federal Railroad Administration (FRA) put into effect new regulations on concrete ties, with notices published by the FRA in the April 1 and September 9, 2011 U. S. Federal Register. The FRA notices say that the need for the new rules was shown by the derailment of an Amtrak train near Home Valley, Washington on April 3, 2005, which according to the U.S. National Transportation Safety Board was caused in part by excessive concrete tie abrasion. To be counted as a good tie under FRA regulation 213.109(d) (4), a concrete ties shall not be deteriorated or abraded under the rail to a depth of one-half inch or more. Limits on other types of concrete tie deterioration are also given.
Methods of pretensioning:
The various stages of the pre-tensioning operation are summarised as follows.
. 1) Anchoring of tendons against the end abutments
. 2) Placing of jacks
. 3) Applying tension to the tendons
. 4) Casting of concrete
. 5) Cutting of the tendons.
Various process involved in prestressed concrete sleeper:
o Preparation of mould. o Apply oil and grease to the surface of mould. o Placing tendons. o Applying tension to the tendons. o Casting of concrete. o Cutting of the tendons. o Steam curing. o Removal of mould. o Water curing.
THE SLEEPERS MANUFACTURED BY THE COMPANY ARE SUITABLE FOR THE FOLLOWING:
Normal Broad Gauge: This sleeper has a trapezoidal cross section having a width of 154 mm at the top and 250 mm at the bottom and a height of 210 mm at rail seat. Points & Crossing: These specialized sleepers are used to hold switches, CMS crossings and lead rails for main line and turnouts. High speed trains can run on these PSC layouts with utmost safety. Guard Rail: These are used at the approaches to girder bridges to prevent a derailed train from capsizing. Switch Expansion Joints: These are PSC sleepers for switch expansion joints (with 120 mm maximum gap) for long welded rails for 52 kg & 60 kg rails using corresponding chairs. Check Rail on Curves: Check rails are absolutely essential to offer an inner side for sharper curves, which are more than 50 to prevent derailment. Level Crossings: This is formed at various points where a road crosses a railway track at the same level and sleepers used here are made with 60 kg UIC or 52 kg check rail. Dual Gauge: The unique pre-stressed concrete dual gauge sleepers have been designed to cater to handle meter and broad gauge trains so that both trains can run on the same track. All the sleepers are manufactured under stress bench system with very strict quality control measures.
Prestressed concrete has the following merits:
1. Since the technique of prestressing eliminates cracking of concrete under all stage of loading, the entire section of the structures takes part in resisting the external load. In contrast to this, in the reinforced concrete, only portion of the concrete above neutral axis is effective. 2. Since concrete does not crack, the possibility of steel to rust and concrete to deteriorate is minimized. 3. Absence of cracks results in higher capacity of the structure to bear reversal of stresses, impact, vibration and shock. 4. In prestressed concrete beams, dead loads are practically neutralized. The reactions required are therefore much smaller than required in reinforced concrete. The reduced dead load weight of the structure results in saving in the cost of foundations. The neutralization of dead load is of importance in large bridges. 5. The use of curved tendons and the pre-compression of concrete help to resist shear. 6. The quantity of steel required for prestressing about 1/3 of that required for reinforced concrete, though the steel for the former should have high tensile strength. 7. In prestressed concrete, precast blocks and elements can be assumed and used as one unit. This saves in the cost of shuttering and centering for large structures. 8. With the advent of prestressed concrete, it has been possible now to construct large size liquid retaining structures not economical to build otherwise. Such structures have low cost and are preferably safe against cracking and consequent leakage. 9. Prestressed concrete can be used with advantage in all those structures where tension develops, such as tie and suspender of a bow string girder, railway sleepers, electric poles, upstream face of gravity dam etc. 10. Prestressed concrete beams have usually low deflection.
Prestressed concrete construction has following demerits:
1. It requires high quality dense concrete of high strength. Perfect quality concrete in production, placement and compaction is required. 2. It requires high tensile steel, which is 2.5 to 3.5 times costlier than mild steel. 3. It requires complicated tensioning equipment and anchoring devices, which are usually covered under patented rights. 4. Construction requires perfect supervision at all stages of construction.
CONCLUSION:
Prestressed concrete is a relatively young science in the context of the history of engineering solutions and will doubtless continue to develop. Throughout this period constant innovation and improvement has been made worldwide in design and durability of prestressing, much of it recently led by fib which is published in its Bulletins and this has led to new standards. At the same time new techniques and ideas continue to be proposed. One fundamental aspect of Engineering is that it always seeks to advance knowledge through experience and this is certainly true of the story so far for prestressed concrete. Further innovation of materials and uses will continue and should be welcomed.