| 1. | Prestressing cable for precast concrete units, which said cable consists of a core part and of outer wires (2) twisted around the core part, whereat the core part consists of two wires (1) twisted around each other, the section of the said wires being based on a circle, or of one wire (3) twisted around its own axis and having a flattened section, c h a r a c t e r i z e d in that the direction of twisting of the core part (1 or 3) is opposite to the direction of twisting of the outer wires (2) . |
| 2. | Prestressing cable as claimed in claim 1, c h a r a c t e r i z e d in that the ratio of the pitch of the spiral of the core part (1 or 3) to the pitch of the spiral of the outer wires is within the range between 2 and 0.3. |
| 3. | Prestressing cable as claimed in claim 1 or 2, c h a r a c t e r i z e d in that the faces of the outer wires (2) are smooth or profiled in a way known in prior art. |
| 4. | Prestressing cable as claimed in any of the claims 1 to 3, c h a r a c t e r i z e d in that the diameter of the outer wires (2) is substantially equal to the diameter of the two core wires (1 ) , in which case the number of the outer wires is eight. |
The present invention concerns a prestressing cable for precast concrete units, which said cable con- sists of a core part and of outer wires twisted around the core part, whereat the core part consists of two wires twisted around each other, the section of the said wires being based on a circle, or of one wire twisted around its own axis and having a flattened section. In concrete structures, prestressing cables of steel are used, which are subjected to a tensile strain before casting of the concrete structure. Most commonly the cable consists of six individual wires twisted around a core wire. The object of the present invention is to im¬ prove the adherence of the cable to the concrete. The prestressing cable in accordance with the invention is characterized in that the direction of twisting of the core part is opposite to the direction of twisting of "che outer wires.
On the face of the prestressing cable formed in this way, a network formed by projecting ridges is produced, wherein the screw lines of the outer wires run crosswise relative the screw lines formed by the sides of the core part.
The invention and its details will be described in more detail in the following with reference to the accompanying drawing, wherein
Figure 1 shows a cross-section of the core part used in a cable of one embodiment.
Figure 2 is a side view of the same core part.
Figure 3 shows a cross section of a cable of one embodiment.
Figure 4 is a side view of the same cable, Figure 5 shows a cross-section of the core part used in a cable of a second embodiment.
Figure 6 is a side view of the same core part, and
Figure 7 is an axonometric view of a cable of a further embodiment, as cut into three parts.
The core part in the cable in accordance with the invention may consist of two wires 1 , whose section is circular (Figures 1 to 4) . These two wires are twisted around each other. According to Fig. 2, the twisting of the core part is right-handed, i.e. S-spiral.
Around the core part, six outer wires 2 have been twisted, whose section is also circular and whose diameter is about 1 _ times the diameter of the wires 1 in the core part (Figures 3 and 4) . The direction of twisting of the outer wires is opposite to the direction of twisting of the wires in the core part. According to Fig. 4, the direction of twisting of the outer wires is left-handed, i.e. Z-spiral. Moreover, the pitch of the spiral of the outer wires is- considerably higher than the pitch of the spiral of the core part. This means that, within a certain length of cable, the number of windings of the core part is clearly higher than the number of windings of the outer wires.
Figures 5 and 6 show an alternative embodi¬ ment of a core part. The core part consists of a flattened wire 3, which is twisted around its own axis. The direction and the pitch of the spiral are similar to those in the core part shown in Figures 1 and 2. The dimensioning of the section of the core wire is most appropriately such that its largest diameter is equal to the diameter of the outer wires 2, whereas the smallest diameter, perpendicular to the largest diameter, is one half of the largest diameter.
The invention is not confined to the embodiment described above only, but it may show variation in differ¬ ent ways within the scope of the patent claims. The number of outer wires may also be different from six, for example, four wires is enough, depending on the ratio of the diameter of the outer wires to the section of the core part. In Fig. 7 an embodiment is shown in
which the diameter of the outer wires is equal to the diameter of the two wires in the core part, and the number of the outer wires is eight. The figure shows three adjoining sections at intervals of 1/8 of the pitch. The pitch of the spirals of the core wires and of the outer wires is the same. The core wires are denoted with letter symbols a, b, and some of the outer wires are denoted with letter symbols e to h, whereby the effect of opposite twisting directions is seen clearly.
The outer face of the wires may be smooth, or the wire faces may be provided with projections or recesses to imorove the adherence.