Advantages of Concrete Sleepers Over Wooden Sleepers in Railway Tracks

Posted on Aug 21, 2024 in Geology

SHOOTING IN WOOD SLEEPERS: DURATION AND WAYS TO INCREASE

Main Causes:

Mechanical Milking:

The rail is supported by the crossbar from an area of 182 cm2, which is subjected to a pressure of 90 Kp/cm2, equivalent to a load of 16 tn. On curves, the pressure increases as the sleeper is subjected to lateral forces and the stress distribution is not uniform under the edge of the dump. Besides the rail foot requested by the horizontal axis, it acts against the walls of the notch. This is widened, and the shoe acts on the screws that have oval holes.

Without a base plate, the rail foot deformities suffered weak amplitude, wearing the recess where the shoe will stay. For proper restraint, flanges will be eliminated for effective tightening lag, and in some cases, the sleeper will have to be re-installed, thus leading to a loss of thickness. So that in a few operations, it is out of service. To avoid this, we will bring metal plates or elastic seats between the shoe and the skiing.

Due to Cracking:

Mainly due to two causes:

  • A consequence of their own anisotropic structure of wood and tendency for hygroscopic equilibrium with the environment. The change in humidity can cause dimensional changes in wood and voltages that exceed the cohesive strength of the fibers and cause cracking.
  • Due to biological attack: Unprotected timber is attacked by biological agents (fungi). This attack is facilitated by the presence of cracks in the wood and can reach the total destruction of the sleeper. Particularly sensitive to the heart of oak if exposed to the outside.

Due to Chemical Attack:

It is manifested in the boxes of the sleepers due to the disintegration of the wood by steel stock.

Length of the Wooden Cross:

It stood at 5 years, but when subjected to impregnation treatment, its life is estimated at about 20 years.

CONCRETE SLEEPERS VS. WOOD SLEEPERS: BENEFITS

Concrete Sleepers:

  • High Lifetime: 2 to 3 times that of wood (50 years).
  • Exhibition of Elastic Characteristics: The homogeneity in the physical state of the material that composes it, over the entire period of use (homogeneity of the elastic characteristics of the support under the rail).
  • High Stability of the Track: Due to its weight (300 Ks against 80 Ks in wood).
    • Longitudinal Direction: It contributes to the use of long welded bars.
    • Transverse Direction: Decreasing the risk of buckling.
  • Adaptability of its Design: Its shape is most suitable to support service efforts.

Disadvantages of Concrete Sleepers:

  • More Expensive: They lead to better power; the problem of insulation of the wires to the rails.
  • More Weight: Inconvenient for handling.
  • Higher Modulus of Elasticity of Concrete: With respect to the wood, which is more stiffness of the track, although the consequences of excess material and its fluctuations do not appear significant.

FIXING PROBLEMS: DIFFICULTIES

No Concrete Durability Anchoring Fasteners:

Monoblock Solutions:

The first solution was to use the screws used in wood sleepers, bolted on wooden blocks embedded in concrete. This did not give good results because the union is not rigid or durable as a result of the influence of moisture on wood.

The solution to improve this was to thread the tang of wood in a metal coil, housed in concrete. A better solution is the loop-lag. Insert a metal loop where the lag screw.

It may also be using a pod cast, threaded inside and embedded in the concrete at the time of construction of the sleeper, but this is expensive and requires very precise manufacturing. This has the disadvantage that it takes an intermediate plastic characteristic.

The tar was used for this purpose, but with a mortar dust, it was very hard and could not tighten the screws easily (cold day) to correct the flattening of the elastic base plate. This drawback could be corrected by setting the shoe by a clamp applied to it, through a threaded nut to a bolt.

This solution has the advantage of allowing the use of a single cross to a different lane width of the flange (playing with dimensions of the cart). It is retightening, the nut now moving to correct the progressive crushing of the elastic base plate, not a final solution, given the need to keep these tightening periodically.

Mixed Solutions:

A solution adopted is to have a fireplace in the sides of the sleeper, which lead to the brace, which has some cavities in the form of an ellipse, with bolts that are inserted and then rotated a quarter turn, which cannot escape and are tightened with the nut provided in its head, so that the strut supports all efforts produced by rail.

FAILURE OF THE ARMED SIMPLY WICKED PIECE

As the shape of the cross in imitation of the wood, the passage of trains, due to peening step loads and aggravated by its stiffness, at the height of the rails under the crossbar, there is a dispersion of ballast and a concentration in the center of the cross. Over time, this produces a large negative bending moment, which means that the concrete works to pull in the center of the crossbar.

This Produces the Following Defects or Damage:

  • Cracking in the central part of the sleeper: Efforts, in turn, produce fatigue to which the concrete has little strength. First, slide the armature when the tensile stress reaches a certain limit and then crack. It grows with the density of traffic and came to the armor, getting rust. This corrosion weakens the bond with concrete, the result being the destruction of the sleeper at the point of maximum action of shear stresses. These ties produced low shear lanes and left to absorb the load, transmitting to the crossbar.
  • Another flaw that had these sleepers was: Lack of resistance to localized efforts under the influence of a suddenly applied load, due to their fragility. In the area of the fasteners, concrete is fractured and resulted in almost total disintegration.

Solutions Designed to Resist the Moments:

To avoid the appearance of cracks due to the tensions in the central part of the negative bending moment, exceeded the limit of the tensile strength of concrete, several solutions were developed:

  • Strengthening much armor: So they will not crack and disintegrate. Disadvantage: Overcharge.
  • Reducing the bending moment in the transverse cross-section:
    1. Acting on ballast: Groove the central part so that the sleeper does not rely on that area. Disadvantage: The groove is filled by the displacement of adjacent ballast from passing loads. Disposal at the cost of maintenance.
    2. Acting on the cross: The cross groove, thinner central part in her song. Disadvantages: The seat end runs out of space, and construction is difficult because of the complicated distribution of reinforcement.
    3. Slimming the central part of the sleeper in width: And the Mf in the central part decreases, which is the goal we pursue. This section shall be sufficient to resist this Mf, a way of thinning the central part, is to the triangular section, with the top down.

Other Solutions to Reach the Cross-Prestressed Concrete:

  • Sleepers of two bearings: The provision of tótulas to concrete sleepers was an indirect attempt to eliminate cracks in the central part to decrease the value base in this area of the bending moment. Cons: Inability to maintain track gauge, excessive degradation of ballast under her short life of bearings.
  • Supports independent or semitraviesas: The rails are supported by concrete cubes, arranged in pairs, one at the height of the other, but without a union between them. Its design is very simple because only resistant Mf (+). Disadvantages: Batting problems, cannot, etc.
  • Sleepers economists, two dice, or two blocks: The central part is replaced by a steel beam whose profile is sufficiently hard to ensure the maintenance of the road width and inclination of the rails and simultaneously sufficiently elastic to absorb the torsion or bending caused by inequality of batting under both heads or by the unevenness of one over the other. Disadvantages: Large steel consumption, poor maintenance of the track (low speed), corrosion of the brace, misbehavior in derailments, broken brace, etc.
  • Vagneux, steel and concrete rigid type, the center is a double-T girder, built by bonding two concrete blocks.
  • Prestressed concrete sleepers: Permits; Get a better resistance to alternating efforts, as the concrete compressive always work, reduces the thickness of the sleeper; Decreases need steel.