WO1985004203A1

WO1985004203A1 - Method of conferring load and deformation stability on railway tracks by means of concrete sleepers

Info

Publication number: WO1985004203A1
Application number: PCT/SE1984/000086
Authority: WO
Inventors: Leif Berntsson
Original assignee: Leif Berntsson
Priority date: 1983-03-03
Filing date: 1984-03-09
Publication date: 1985-09-26
Also published as: SE8301181L; AU2699884A; EP0172810A1; SE8301181D0; SE449635B

Abstract

Method of conferring load and deformation stability on railway tracks by means of partially prestressed concrete structures, for instance concrete sleepers. The prestressing forces in the tendons during casting and curing may amount to one half of the value for full prestressing. In conjunction with both normal loads and sporadically occurring overloads, the magnitudes of the deformations in the sleepers will be regulated by the stiffness of the sleepers, the modulus of subgrade reaction of the ballast, and the ability of the rails to transmit force. Owing to enhanced deformability of the concrete sleepers in consequence of the reduced prestressing force in the tendons, considerable load energy can be absorbed without the occurrence of permanent damage. The structure as a whole is given such properties that the sleepers exhibit only elastic deformations, and that the cracks which occur in the concrete cross section in tension are completely closed after the load has ceased to act.

Description

METHOD OF CONFERRING LOAD AND DEFORMATION STABILITY ON RAILWAY TRACKS BY MEANS OF CONCRETE SLEEPERS

This invention relates to a method of producing, depending on structural design, choice of constituent materials, concrete and prestressing tendons, casting and curing processes and tensioning and release procedures, precast prestressed concrete units such as for instance sleepers for the load and deformation stabilisation of railway tracks. The principal object of the invention is to confer on the units such properties that, in conjunction with normal loading and occasional overloading, they exhibit only elastic deformations which are fully recovered, and that cracks which occur are fully closed after the load has ceased to act.

It is a characteristic of the ceramic material concrete that its tensile strength is considerably lower than its compressive strength. The ratio of tensile to compressive strength is in the order of 1/3 to 1/10 depending on the type of concrete. For this reason, a concrete structure acted upon by bending moment or tension is strengthened according to known technology by reinforcement provided in those parts where the tensile strength of the concrete is exceeded or where the tensile stresses are greater than a certain prescribed permissible value. Reinforcement is also provided in a concrete structure for purposes other than that of merely strengthening it. For instance, reinforcement is provided to distribute cracks which occur or to reduce the width of the cracks. It is fundamental to all concrete structures provided with non-stressed reinforcement that the reinforcement must on its own be capable of resisting all tensile forces in a cracked cross section or in a cross section where the tensile stresses exceed the permissible value. This implies that the entire tension zone is assumed to be ineffective, i.e. design is carried out for Stage II.

It was soon realised, however, that considerable advantages could be achieved if, by means of reinforcement, compressive forces were imposed in those zones where the tensile stresses due to load exceeded the ultimate tensile strength of the concrete. In this way, inter alia, the entire concrete cross section could be utilised, with increased loadbearing capacity and stiffness as a result. This principle gave rise to the nowadays familiar and well developed prestressed concrete technology. The pioneer of this technology was the Frenchman Eugene Freyssinet. He strongly advocated that the ability of high grade reinforcing steel to impose compressive forces on the concrete should be fully utilised. This technique was called by Freyssinet "a revolutionary method" of producing reinforced concrete structures.

However, in the initial stage of development of prestressed concrete technology, all the expectations which had been placed in a prestressed concrete structure were not realised. The properties which the units exhibited at an early age rapidly deteriorated as time went on. Due to losses, the prestressing force in the concrete gradually diminished to such an extent that the units finally behaved as though they were provided with non-stressed reinforcement. Freyssinet published his new theories in 1926, and developed them in a patent application in 1928. In this, he gave an explanation for the losses in prestress and presented a new theory for the shrinkage and creep of concrete. Even though Freyssinet's creep theory was later found to be relatively imperfect, his theory of shrinkage is still unsurpassed today. According to Freyssinet, shrinkage, the property of such significance for prestressed concrete, is caused by capillary forces in the partially water filled capillary system of the cement paste. Freyssinet based his theory on the Laplace Law concerning the state of equilibrium of liquid menisci in capillary channels, and on Lord Kelvin's thermodynamic considerations regarding evaporation and pressure above menisci. At equilibrium there is therefore a relationship between the partial pressure of the water vapour above the menisci and the diameter of the water filled capillaries. Water in narrow capillaries transmits large compressive forces to the surrounding material. The deformations in concrete will vary in step with the relative humidity of the ambient air. Freyssinet realised that, in order to accomplish "the new revolutionary concrete technology", it was necessary to utilise the highest possible grades of material in both concrete and reinforcement. The losses in prestressing force with time were established to be a result of time related deformations in both the concrete, viz. shrinkage, and in the reinforcement, viz. relaxation.

In order to minimise the effect of shrinkage and creep deformations, a reinforcing material was required which, on being prestressed, could be given large elastic strains without residual deformations. For this reason, special types or grades of alloy steel were developed for cold working, such as cold stretching, with ultimate strengths greater than 2000 MPa. In later years relaxation losses could be reduced by special treatment, for instance heat treatment designated thermalising. Development in concrete technology towards ever increasing concrete strengths created further opportunities for successful application of prestressed concrete technology in sophisticated concrete structures.

So-called full prestressing implies that the concrete, in all service states, shall remain uncracked and with the entire cross section effective. In the great majority of cases, this state of affairs results in optimum loadbearing capacity in relation to the self weight. Owing to the fact that the entire concrete cross section remains effective under load, stiffness will be a maximum. In certain fields of application for precast prestressed concrete units, excessive stiffness may cause some problems and even inconvenience. Examples of this are structures which are practically unloaded, or carry an insignificant external load, during most of the time. Under normal static loading these structures act as pre- stressed concrete structures without the occurrence of cracking.

Blows, impacts or other loads of overload character may cause cracking. Even though cracks have formed, the structure can work satisfactorily if the imposed load is of relatively short duration and the cracks close when load is due to self weight.

The object of this invention is to indicate, inter alia, a way in which it is possible to enhance the deformation capacity of prestressed concrete structures, particularly applicable to those structures which are exposed to full load for a short period and sometimes to overload. The enhanced deformation capacity is accom- pushed by virtue of the fact that deformations mainly occur by widening of cracks under load. A large proportion of the energy is absorbed by the reinforcement which holds the cracks together. The reduced stiffness of the unit may enable the load to be dispersed to adjacent loadbearing parts, which is particularly important in conjunction with blows and impacts. Enhanced deformation capacity and energy absorption in reinforced cracks is achieved in practice by the application of so-called partial prestressing, i.e. the prestressing forces are made lower than in full prestressing. Examples of structures in which the invention described can be applied are, inter alia, railway sleepers.

The prominent material for sleepers is wood. The reasons why wood has been chosen are many, such as economy and function. Experience is extensive, and stretches back as far as the introduction of rail- ways. Kinds of wood used in Sweden are pine, beech and oak. In contrast to pine, spruce cannot be used since spruce wood cannot be impregnated. Abroad, both softwood and hardwood is used depending on availability.

Wood as a material is characterised by anisotrαpy which is totally dependent on the structure of wood. The properties which are greatly directional in relation to the growth rings are determined both by the organic constituent material and the physical structure. The mechanical properties of wood are of such high quality that the material is generally well suited for use as constructional material. it is primarily the ability of wood to resist tensile and compressive stresses in the direction of the grain and to deform elastically which is utilised in this context. It is evident from this that wood can absorb and store mechanical energy, particularly over shorter periods. However, wood has a number of unfavourable properties, the chief of which that should be mentioned being moisture deformations such as shrinkage on drying and swelling on wetting, residual deformations in conjunction with long-term load, creep, and similarly permanent deformations in conjunction with blows and similar. Wooden sleepers are therefore an illustrative example of how well material properties can be utilised in the optimum manner for a definite purpose. The limiting factor for wooden sleepers, as in many other cases for structures, is economy.

As an alternative to wooden sleepers, experimental production of concrete sleepers was started in Sweden at the beginning of the fifties. Over the intervening years, many different types have been made both in Sweden and abroad as regards design and the principle of load resistance. Types of sleeper which may be mentioned here are monolithic block sleepers and twin block sleepers, and reinforced and prestressed designs. A number of different solutions have also been devised for fastening of the rails. The actual manufacturing process has been increasingly mechanised, so that production nowadays has the largest volume in precast concrete technology.

This invention relates to a method of accomplishing load and defor- mation stabilisation of structures such as railway tracks and similar by means of precast prestressed concrete units such as for instance concrete sleepers, by achieving deformability based on reduced prestressing force in the tendons. The technical effect of such a solution will influence production of units and the behaviour of the sleepers in the permanent way. The prestressing force which is imposed prior to placing of the concrete may, during the casting and curing process, for instance amount to one half of the value used for full prestressing, preferably 0.4 - 0.6 times the forces equivalent to full prestressing, at least over the range 0.3 - 0.7. Such a reduction of the prestressing force will confer several advantages such as simpler designs for formwork and accessories, reduced risk of fatigue in existing formwork, less movement in jacks during stressing, and less risk of rupture in strands and wires. When the prestress in the tendons is released in conjunction with demoulding, there takes place a movement which is particularly large in the long line method. Owing to the reduction in prestressing force, this movement will decrease in proportion to the magnitude of the prestressing force. The production system based on reduced prestressing forces will on the whole be safer, with few interruptions and repairs. It is advantageous to use prestressing wire of high grade steel for prestressed concrete structures and to use concrete made with hydraulic binders and aggregate of natural and synthetic origin, the approriate crushing strength for concrete at 28 days, determined on 15 cm cubes, is 30 - 90 MPa, preferably 50 - 70 MPa.

The reduced prestressing force has both a direct and an indirect effect on the properties of sleepers. The time dependent deformations in concrete, creep, and the losses in prestress, relaxation, are determined by the level of stress in each material. Creep in concrete is largely proportional to compressive stresses in the range where the permissible stresses are located. Losses of prestress due to relaxation are usually not proportional to the magnitude of stress, as stresses in the prestressing wire are at a relatively high level in full prestressing, 0.6 - 0.75 times the ultimate strength. Losses increase steeply as stress rises. Proof has been found that there exists a limiting value of relaxation which the final stress approaches at high stresses. Creep in the concrete can be indirectly influenced by changing the composition of concrete, which, in turn, may be a consequence of reduced prestress. Reduction of the quantity of cement, and the use of standard cement instead of rapid hardening cement, results in decreased shrinkage. A high quantity of cement and rapid hardening cement can be replaced in production by some suitable rapid curing process. Whichever alternative is chosen, it will increase the costs of the manufacturer.

The prestressing force can be made so large that cracking does not occur in the sleepers due to loads imposed by normal train traffic. In conjunction with sporadic overload, impacts due to deformed wheels etc, the concrete will crack. In sleepers in which the tendons are highly prestressed there is a great risk that the stresses will be so large that a residual strain will be imposed on the tendons. Owing to the fact that sleepers with full prestressing force have great stiffness, they are forced to absorb a large proportion of the energy due to the load on their own, with damage as a consequence. Sleepers with a reduced prestressing force will also crack. Owing to increased deformation, sinking into the ballast and transfer of energy to adjacent sleepers via the rails, the effect is confined to cracks in the concrete which close once the load has ceased. Sleepers with a reduced prestressing force adjust their deformations, to loading and the ballast.

It has been shown in tests that cracks of widths greater than 1 mm are closed after load has ceased if the force in the tendons is less than 0.65 - 0.75 times the full prestressing force. This complete closure of the cracks does not occur when the prestressing force exceeds the above value and reaches the yield stress. In cases where permanent cracks are formed,there is a potential risk of corrosion in the steel.

The mechanism of corrosion in tendons is very complex. In both the material and the environment there are factors which determine the rate of corrosion. The permeability of concrete to water, electrolyte containing chloride ions and gases, oxygen and carbon dioxide, are particularly important. An impervious concrete, i.e. a low degree of porosity, and a buffer of calcium hydroxide protect the steel against corrosive attack. It has been found that, if the concrete cover is sufficient and the quality of concrete is good, the rate of corrosion is very low if the crack widths are not greater than

0.15 - 0.2 mm. In concrete sleepers subject to stringent demands as to load absorption capacity, the quality of concrete will satisfy the requirements concerning low permeability. Durability of the tendons in conjunction with cracking is also secured owing to closing of the cracks or to small crack widths.

Fatigue in concrete sleepers has attracted some attention. Generally, prestressing tendons are sensitive to dynamic stresses, and the higher the mean stress, the more rapidly fatigue failure occurs. According to tests, fatigue in tendons does not occur if the upper stress is less than 0.55 σ_ult, and the lower stress is 0.5 σ_uit. In no case has fatigue failure been observed in concrete. By reducing the prestressing force in the tendons, a higher degree of safety against fatigue failure can be obtained. As pointed out before, concrete sleepers with a reduced prestressing force are more deformable and less sensitive to unequal settlement, for instance in conjunction with frost heave. Such sleepers shall also have the ability to function in tracks laid on a gravel ballast or a ballast material of inferior bearing strength. Reduced prestressing force decreases the instantaneous compressive strain in conjunction with release and subsequent creep, whereby a more reliable value of the gauge is obtained for sleepers made in accordance with this invention.

The following examples show some results from test loading of sleepers, both fully and partially prestressed ones. The tests were carried out by loading in flexure simply supported sleepers by means of point loads at the positions of the rails. The span was 0.60 m. Unless otherwise stated, the results are the means of two tests.

Example 1. Full prestress.

(initial stress = 0.72 σ_ult )

1.1 Age of concrete = 1 day

Crushing strength of concrete (15 cm cube) = 30-35 MPa Cracking moment = 27.8 kNm (P = 185 kN) Ultimate moment = 33.3 kNm (P = 222 kN)

(Bond failure and splitting failure along tendons at end).

1.2 Age of concrete = 2 years

Crushing strength of concrete (15 cm cube) ≃ 70-75 MPa

Cracking moment = 27 kNm (P = 180 kN) Ultimate moment = 42 kNm (P = 280 kN)

(Wire fracture).

Example 2. 0.5 times full prestress.

(initial stress ≃ 0.36 σ_uit)

2.1 Age of concrete = 1 day Crushing strength of concrete (15 cm cube) = 30-35 MPa

Cracking moment = 21.8 kNm (P = 145 kN)⁺ Ultimate moment = 36 kNm (P = 240 kN) (Wire fracture) (⁺ Mean value of four tests, 141-151 kN) 2.2 Age of concrete = 2B days

Crushing strength of concrete (15 cm cube) = 63-68 MPa Cracking moment = 26.7 kNm (P = 178 kN) Ultimate moment = 41.2 kNm (P = 275 kN) (Wire fracture)

The crushing strengths quoted in the above examples were determined, at the age of 1 day, on cubes stored under the same conditions as the sleepers, and at the age of 28 days, on cubes stored under standard conditions. The crushing strength of concrete of about 2 years' age is estimated on the basis of the known development of strength in a similar type of concrete. It is well known that the short-term strength of concrete is greatly dependent on the prevailing curing conditions, particularly the temperature, and therefore the strength of concrete cast at different times may vary. Tests shown under 1.1 and 2.1 were made on sleepers produced at different times.

The magnitudes of the cracking moment in short-term tests are supported by considerably more tests than those reported here, whereby the cracking load for sleepers with full prestress is 180 kN, and that for sleepers with one half of the prestressing force is 140 kN.

The cause of failure in 1.1 is different from that in 2.1, namely splitting failure in the anchorage zone of the tendons at the end, and bond failure as a consequence of this. In 2.1 normal wire frac- ture failure occurred.

At 28 days and older, the differences in cracking moment and ultimate moment for sleepers with full prestress and one half prestress respectively are insignificant. The compressive strength and tensile strength in bending for the 2 year old concrete should be greater than for concrete of 28 days' age, and therefore the difference may be expected to be greater than that given in the results. The losses of stress in the tendons due to creep of concrete and relaxation of the steel at the different ages must also be borne in mind in comparing the results. The results of tests show that the properties of the sleepers appear to be equivalent at full and partial prestressing. The reduction in cracking moment by about 20% at the age of one day in sleepers with one half prestress, as compared with full prestress, may reasonably be supposed to have no practical significance, as sleepers at such an early age are never subjected to load. The real difference in properties for sleepers with one half and full prestress, with decisive advantages in favour of the former, is evident from the technical effect of the invention as follows.

In partial prestressing, stress in the tendons is less than in full prestressing, and the risk of fatigue failure is therefore reduced. Variations in load give rise to dynamic stresses which are superimposed on the static stresses. Tests have shown that fatigue failure which can be induced under appropriate dynamic conditions occurs only in the prestressing steel. Both Swedish and foreign investigations indicate that there exists a practical fatigue limit at σ_max/σ_ult = 0.55, provided that the minimum stress is at 0.5 σ_ult. In conjunction with cracking in the tension zone of the sleepers, caused by overload or the character of the ballast etc, the sleepers with partial prestress will not have less favourable properties, but the reverse. The lower prestressing forces in the tendons namely produce sleepers of greater elastic resilience. For one and the same moment, the internal lever arm is increased, with lower stresses in the tendons as a consequence. This has the result that the stresses in the tendons will not be so high that residual strains are set up. The cracks which will have occurred can then close once loading has ceased. By virtue of the above properties, the sleepers will be self regulating as regards their stiffness. If there are opportunities for load sharing by means of stiff sleepers, they will remain uncracked and will retain their maximum stiffness. If, on the other hand, the sleepers have cracked, they will distribute load owing to their resilience. Absorption and distribution of energy in tracks under load are adjusted in such a way that the magnitude of the stresses will be a minimum in the different loadbearing portions. To this technical effect must be added considerable advantages during the production stage of partially prestressed sleepers, which have been described earlier.

Claims

PATENT CLAIMS

1. Method of conferring load and deformation stability on structures such as railway tracks and similar by means of precast prestressed concrete units, preferably precast prestressed concrete sleepers, characterised by the fact that the prestressing forces in the tendons, during casting and curing of the concrete, are made to amount to 0.3 - 0.7 times the forces corresponding to full prestressing, preferably 0.4 - 0.6 times.

2. Method in accordance with Patent Claim No 1, characterised by the fact that prestressing wire of high grade steel is used for prestressed concrete structures.

3. Method in accordance with Patent Claim No 1, characterised by the fact that concrete with hydraulic binder and aggregate of natural and synthetic origin is used.