FI72370B - FOERSPAENNINGSLINA FOER BETONGKONSTRUKTIONER FOERFARANDE FOER ATT TILLVERKA LINAN OCH SAODAN LINA INNEHAOLLANDE BETONGKONSTRUKTIONER - Google Patents

Abstract

In a prestressing strand for use in stressing a concrete structure, having one or more central core wires and a plurality of outer wires extending helically around the core wire the surface of at least the core wires is modified in order to increase the friction resistance to relative movement between the core wire and the outer wires. This strand is found to give more accurate stressing, where the conduit in the concrete for the tensioned wire is curved. The outer wires may also have modified surtaces. The modification may be mechanical indentations or surface conditioning treatment such as oxidation, etching, coating or deposition.

Description

1 72370

Prestressing rope for concrete structures, method of making the rope and concrete structures with such a rope

The invention relates to a prestressing rope for tensioning concrete structures, which is arranged in a manner known per se when tensioning a concrete structure and which comprises at least one central core wire and a plurality of outer metal wires tensioned helically around the core wire to wrap it. Such ropes are often used as prestressed reinforcement elements in prestressed concrete structures, in which case they can be placed, for example, in curved pipes in the concrete structure. The pipes are formed, for example, from pipes made of steel or other material and pre-cast into a concrete structure. The invention also relates to a process for the manufacture of such a rope and to a concrete structure having such a rope.

A commonly used model of a prestressing rope, e.g. disclosed in DE-A-1 659 265, comprises six outer metal wires of equal thickness and one core wire with a diameter 2-5% larger than the diameter of the outer metal wires. The larger size of the core wire is important to achieve good cohesiveness and for the outer metal wires to fit snugly against the core wire. Although this type of rope is the most common used for prestressing, the invention is not limited to this particular rope construction but also applies to other rope constructions which are the general design indicated in the first paragraph.

Figures 1 and 2 of the accompanying drawings show a prestressing rope with a single core wire b and six outer metal wires a in both longitudinal and cross-sectional order, respectively. Figure 1 also shows the pitch of the thread in which each of the outer metal wires is located. For the whole rope, this pitch S is referred to herein as "thread pitch length". Figure 2 shows the largest dimension of the 2 72370 cross-section, referred to herein as the largest rope diameter. It is common for the pitch of a thread to be expressed as a multiple of the diameter of the rope. For prestressing ropes, this length S is generally 12 to 18 times the diameter, although the value of S in this range is not essential to the present invention.

The present invention originates from the fact that studies on prestressing ropes do not appear to have provided the most suitable practical rope structure suitable for use in prestressing.

The extensibility properties of a prestressing rope are usually exploited by tensioning the rope under unrestricted direct conditions. Here, it has been found that the ratio of the main stress of the rope cross-section to the stress of the rope differs slightly from the elastic coefficient E (Young's coefficient) of the metal-wire material. Minor deviations may occur depending on the rope manufacturing method and its construction.

When prestressing ropes are used in curved pipes through concrete structures, different tensile forces occur in different cases after tensioning at the ends of the rope. By calculating, it is possible to find the relationship between the total elongation of the rope in the curved tube and the tension forces present at these ends. From this it is possible to learn the behavior of the tension forces in the rope along the length of the rope when a predetermined elongation is caused to the rope.

It has now been found that in such cases, when tensioning a prestressing rope, large deviations between the calculated elongation and the actual elongation can be observed, depending on the variations in the friction properties within the rope, the variations in the cable manufacturing methods and perhaps other factors. If the quotient of the main stress over the rope cross-section and the elongation per unit length of the rope is mentioned by the expression "deformation coefficient", it is found that when deforming the rope in curved tubes, this deformation coefficient that the coefficient of deformation in cases of substantial variation is, as a rule, lower than the coefficient of elasticity. This is more dangerous because when using the calculated elongation for a prestressing rope, it cannot be certain whether the desired tension exists along the entire length of the rope and whether the concrete structure enters the desired prestressing state.

The stress and deformation state of a prestressing rope in a curved appearance, in which the rope is subjected to transverse and frictional forces, is very complex and depends on many factors related to the properties of the material and the rope manufacturing methods.

A complete understanding of this has not yet been reached, although the inventors of the present patent application may present systematic variations by an experiential method. The rope testing method is described below.

It is therefore an object of the present invention to enable the control of stress variations as well as to minimize the stress variations of the prestressing rope in curved tubes.

The inventors have realized that a much better consistency between the deformation coefficient and the elastic modulus is achieved when the core wire is able to be tensioned more evenly along its entire length and is better able to act as a load-bearing element. The prestressing rope must also effectively remain cohesive in order to prevent slipping between the core wire and the outer metal wires, as this slipping causes the core wire to no longer be under full load locally.

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It has been found that improved prestressing ropes of the above design are obtained if at least the core wire or all the core wires have a modified surface, which reduces the tendency for relative movement between the core wire and the outer metal wires (e.g. increased coefficient of friction).

More specifically, the prestressing rope according to the invention is mainly characterized by what is set out in claim 1. The surface of the outer metal wires can also be modified, which reduces the tendency to such movement. Thus, the core wire and optionally also the outer metal wires are subjected to treatment to modify the surfaces of the wires so that the resistance to relative longitudinal movement between the core wire and the outer metal wires is greater than if the treatment had not been performed.

One treatment method for the core wire and optionally the outer metal wires is to mechanically mold the notch into the surface of the metal wire.

Another treatment method is to modify the surface condition of the wire to increase the coefficient of friction between the core wire and the outer metal wires. Thus, instead of accepting undesired variations in frictional properties within the rope, these frictional properties are exploited by increasing friction to reduce or even prevent any longitudinal movement between the core wire and the outer metal wires.

Several alternative methods are available for modifying the surfaces of the core wire and the outer metal wires in order to increase the coefficient of friction. One possibility is that an oxide layer is formed on the surface of the metal wire by heating it in an oxidizing atmosphere. It should be clear that even a very thin oxide layer is able to effectively affect the friction properties of the metal wires without affecting the strength and durability of the metal wires or without affecting the prestressing rope as a whole. Anyone familiar with the art will know how to provide such a thin oxide layer for each type of core wire by selecting the oxidizing atmosphere, process temperature, and metal wire processing (I.

5 72370 time without affecting the relevant physical properties of the wire material. Such properties include, but are not limited to, mechanical properties and durability.

It has also been found that the surface condition of a metal wire can be sufficiently modified by subjecting it to a chemical etching treatment. Chemical etching treatments for steel products are well known. Thus, no further explanation is required as to how such an etching treatment should be performed so that the surface becomes slightly rough without unduly damaging the physical properties of the material.

It has also been found that the object of the present invention can be achieved without modifying the metal wire itself if a resin coating to which an abrasive powder, a tack powder such as carborundum is added to the metal wire. In a pre-rope constructed of such a coated metal rope, particles of tack powder prevent or reduce movement between different metal wires.

Another possibility for treating a metal wire is to deposit the metal wire with a friction-increasing agent by an electro-chemical or electrostatic process. Experts in this field have a wide choice to provide a suitable deposit.

Although several different options for modifying the surface of a metal wire have been discussed, combinations of two or more surface modification processes may be used in the present invention.

The invention also relates to a method of manufacturing a prestressing rope and a concrete structure with one or more tensioned ropes according to the invention, as described above. The method is mainly characterized by what is stated in claim 8, and the concrete structure is mainly characterized by what is stated in claim 6.

6 72370

In such a manufacturing process, it is assumed that the manufacturing steps begin with a metal wire whose surface condition is heretofore conventional in the art, e.g., as supplied by the metal wire manufacturer.

Embodiments of the invention will be described below by way of non-limiting example with reference to the accompanying drawings, in which Figures 1 and 2 show a prestressing rope and have been described above. Figure 3 shows a front view of a prestressing rope test device.

Figures 3 and 4 show a concrete slab 1 with a thickness of 22 cm. A tube 3 passes through this plate and is curved to a radius of curvature R = 100 cm above an angle of 5.07 radians. The length L2 of the curved part of the pipe is thus 507 cm.

A support beam 2 is placed against each end of the pipe 3, which has a wedge anchor 5 for a pre-tensioning rope on the left hand side and a similar wedge anchor 5 on the right hand side behind the hydraulic press 4 (schematically shown).

After the rope has been inserted through the pipe 3, the rope is fastened with wedge anchors 5, after which it is tensioned by a hydraulic press 4. The tensioned rope then consists of a straight part with a length = 175 cm, a curved part with a length L2 = 507 cm and another straight of a part with a length = 210 cm.

The tests were performed using the most common prestressing rope with a thickness of D = 1.27 cm, which included a core wire and six outer metal wires. Initially, the rope was subjected to nominal tension to stretch sufficiently, after which the tensile force was increased close to the full load value used in the tension technology. During the increase in tension force, the elongation of the rope and the tension force prevailing in it were continuously measured.

il 7,72370 Using the element method, the rope was shaved into elements and the stress and stress states of each element were calculated using the frictional force between the channel wall and the prestressing ropes. Experiments with separate small wrapping angles determined the coefficient of friction between the channel wall and the rope for many values of the rope tension forces. These coefficients of friction per element were included in the calculation so that it was possible to determine by calculating the tensile forces that should prevail in the rope based on the measured total elongation of the rope between the anchors 5. This value was compared with the actual prestressing forces prevailing in practice, from which the value for the deformation coefficient could be determined in each test performed.

This test was repeated on ropes with a modified core wire according to the invention, but otherwise with the same values. In each case, the value of the deformation coefficient was determined in a corresponding manner.

Thus, the values obtained for measuring and calculating the deformation coefficient showed that in all cases using a modified core wire (and optionally modified outer metal wires) according to the invention, a substantial increase in the deformation coefficient was observed, so that the difference between the deformation coefficient and the elastic coefficient also became insignificant.

Claims (12)

A biasing cable intended to be placed in a protective tube in a manner known per se under tension of the concrete structure and to which belongs at least one central core tree (b) and several outer metal trees (a), which are spirally extended around the core tree (b) for inserting the airona, characterized in that at least the core tree (b) has been treated for modification of its surfaces, so that as a result of the treatment, there is a greater friction to resist the relative movement between the core tree (b) and the outer trees (a) than if the said treatment has not been performed. 2. The bias line according to claim 1, characterized in that the outer metal trees (a) have also been treated for modification of their surfaces, so that as a result of the treatment there is a greater friction to resist the relative movement between the core tree (b) and the outer trees (a) than if the said treatment has not been performed, these outer metal trees (a). Biasing line according to claim 1 or 2, characterized in that the treatment of the core tree (b) and the outer metal trees (a), if performed, includes mechanically formed depressions on the metal tree. Biasing line according to claim 1 or 2, characterized in that the treatment of the core tree (b) and the outer metal trees (a), if performed, includes modification of the condition in the surface of the metal tree to achieve a greater coefficient of friction between the core tree ( b) and the outer metal trees (a). 5. A biasing line according to claim 4, characterized in that the treatment is at least one of the following: deposition of the surface of the metal tree with a friction increasing agent. 6. A concrete structure, characterized in that it contains at least one tensioned prestressing line according to claim 1. Concrete construction according to claim 6, characterized in that the tensioned surface extends through a curved pipe (3) within the concrete structure (1). A method of manufacturing a prestressing cable for tensioning a concrete structure, wherein the prestressing line is intended to be placed in a protective tube in a manner known per se under tension of the concrete structure and to at least one central core tree (b) and several outer metal trees (a), which are spirally wound around the core tree (b) for inserting the same, characterized in that at least the core tree (b) is treated before the winding period for modifying its surfaces so that as a result of the treatment a greater friction to resist the relative movement between the core tree (b) and the outer tree (a) than if the said treatment has not been performed. Method according to claim 8, characterized in that the outer metal treads (a) are treated prior to winding period for modification of their surface, so that as a result of the treatment there is a greater friction to resist the relative movement between the core tree (b). and the outer trees (a) than if said treatment has not been performed. Il ·