STRAND FOR STRESSED CONCRETE STRUCTURE AND PROCESS FOR ITS PRODUCTION

Abstract: A strand for stressed concrete structures, formed as a twist with at least three wires provided with surface profiling. The essential feature of the invention is that the surface profiling of the wires (3) is formed as non-circular cross section profile continuously passing through the wire in longitudinal direction. Furthermore subject of the present invention is a process for the production of this type of strand. Here first the surface profiling of the wires is formed with cold deformation, then at least three wires are simultaneously twisted to a strand. The essential feature of the process is that the surface profiling of the wires is formed by drawing through drawplate of non-circular cross section, preferably of regular hexagon opening cross section with rounded corners.

Description: - * - STRAND FOR STRESSED CONCRETE STRUCTURE AND PROCESS FOR ITS PRODUCTION BACKGROUND OF THE INVENTION The invention relates to a strand for stressed concrete structures, and to a process for its pro¬ duction. As known, different strands are used for pre- stressed, or after-stressed (reinforced) concrete structures to take up the stretching foree. These are formed generally with three to seven pieces of 2 to 5 M thick cold drawn individual steel wires, so-called "elementary filaments" to cable-type strand with the use of twisting (stranding) machine. These strands transmit the stretching force to the concrete structure through adhesion, or anchorage " between the strand and the concrete. The experts have " been trying to find a solution for a long time, whereby adhesion or holding power of the strands to the concrete could " be improved, since this way the stretching foree could " be increased, which would " be particularly desiralsle especially in case of high load " bearing stresseά concrete structures, or long spans. PRIOR ART Such strands and cables are already known from the British patent specification No. 1 194- 758 and GPR patent specification No. 1 659 265, where evenly distributed (periodic) profiling of small depth is produced by rolling after cold drawing on the surface of the covering wires of the strand, just as in case of the simple concrete reinforcing rods. Although the adhesive capacity of-the strand in the concrete is improved by the periodic pro¬ filing formed with such indentation, i.e. "the an¬ choring length" will be shorter, however the in- creased risk of cracking or breakage must be reckoned with in the elementary filaments at the corners of the indentations upon the bending and torsional stres¬ ses arising during loading of the strand. Further¬ more, the arrangement of the periodic surface profiles within the strand is incidental, in other words, the "functional" cross section surfaces of the sections perpendicular to the longitudinal axis of the strand are different from each other, consequently the stress conditions arising during loading are also different. This implies that the possibly smallest cross section is to be- reckoned with for strength calculation of the strand, which however is unfavourable in respect of the steel utilization. Furthermore such process for strand production is recommended by the British patent specification No. 1 336 200, where the strand twisted from round wires is tightened by deformation with radial exter¬ nal pressure. In the course of this, mainly the covering wires pass through free de ormation, while their concentric cross section will be deformed to irregular shape. The purpose of tightening is to ob¬ tain the possibly smallest strand cross section mainly for the a ter-stressed structures. This tight¬ ening however inevitably entails reduction of the outer surface of the strand in contact with the con¬ crete, which in view of the foregoing is undesira¬ ble in respect of the adhesion. SUMMARY OF THE PRESENT INVENTION The invention is aimed at the elimination of above shortcomings, i.e. at the realization of such strand for stressed reinforced concrete structures, whereby the adhesion to concrete can be improved, and at the same time identical stress conditions can be assured in all strand cross sections. In order to solve the problem, initially such, strand was used, which has a twist formed ' with at least three wires provided with surface profiling. This wa.s further developed according to the invention in that the surface profiling of the wires is formed as non-circular cross section-profile continuously passing through the wire in longitudinal direction. The cross section profile of the wires contin¬ uously passing through in longitudinal direction may suitably be a polygonal profile, preferably a rounded regular hexagon. This enables a very simple pro¬ duction. According to a further characteristic feature of the invention such construction is also conceiva. ' ble, where the cross section profile of the wires con¬ tinuously parsing through in longitudinal direction is formed as such circular or polygonal cross section, which is provided with groove-like recesses and/or rib-like extensions evenly distributed along the cir¬ cumference. In the course of the development of the process for the production of the strand according to the in- vention the starting point wa,s such conventional pro¬ cess, where first the surface profiling of the wires is formed with cold deformation, then at least three wires are formed to strand with simultaneous twisting. The essence of the process according to the invention is that the surface profiling of the wires is formed by drawing through drawpla e of non-circular, prefera.- bly rounded hexagonal cross section. It is advisable to form the wires by turning around their longitudinal axis during or after for¬ mation of the profile, but before twisting. It is ex¬ pedient if the turning direction of the wires coinci¬ des with the subsequent twisting direction of the strand. Furthermore, it is preferable if the pitch of the twisting is selected to the multiple, suitably at least to tenfold of the pitch of wire-turning. PREFERRED EMBODIMENT OF THE INVENTION .The invention is described in detail with the aid of drawing, showing the cross section of the strand according to the invention given by way of example, and drawn to enlarged scale. As seen, the strand denoted with reference number 1, in this case consists of a round central wire 2, (i.e. "supporting core wire") and tangentially surrounding six covering wires 3. According to the invention the covering wires 3 in contact with the concrete in built-in state have non-circular cross section profile continuously pass¬ ing through in longitudinal direction for the purpose of increased adhesion to the concrete, and establish¬ ing indentical stress conditions in all strand cross sections. In the presented case the covering wires 3 are of regular hexagonal cross section, rounded on the corners. In the drawing the diameter of central wire 2 is marked with D , diameter of the circle drawable into the hexagon in case of the cover wire is marked with D-, diameter of the circle drawable around the hexagon with DA, and the embracing face- distance of strand 1 with L, , while its distance between centres is marked with L„. It is advisable to dimension the diameter D« of the central wire 2 greater by 3-4 % than the diameter Ωl of the covering wires 3 for better seat¬ ing of the covering wires 3. In case of the 1/2" strand - given by way of examples - the diameter D« of central wire 2 was selected to 4.26 mm, and the dia - eter DA of the covering wires 3 to 4.11 mm. It is noted that the covering wires 3 are shown in ideal state in the drawing, where the hexagonal faces of the adjacent wires 3 bear up on each other. In the reality this rarely occurs. Production of the strand 1 according to the invention is the following: The production technology of the central wire 2 and covering wires 3 is essentially conventional. In the production of the covering wires 3 according to "fckβ invention only the last step of the cold defor¬ mation is different, where special drawplate of hexa,- gonal opening cross section was used to obtain the required cross section. In the present case the covering wire 3 is turned around its longitudinal axis (naturally this can be dispensed with in given case) while passing through the drawplate. The pitch of wire-turning is determined by the pitch of the drawplate. During the experiments the drawplate was embedded as to be capable to turn around the advanced wire. After finishing the production of the covering wires 3 > the twist, i.e. the strand 1 is formed on a conventional twisting machine with the central wire 2 and six covering wires 3. In the course of the ex¬ periments the turning direction of the covering wires 3 was selected to be identical with the direction of twisting, furthermore the pitch of twisting was se¬ lected to about tenfold of the turning pitch. As a. result of this, a_fter the twisting favourable con- tact between the wires 2 and 3, and after building in the strand 1 the possibly most favourable anchorage were accomplished with the illustrated helical hexa.- gon profile. Naturally the strength properties of the strand can be improved by the conventional heat treatments widely used in the practice (e.g. tempering, stabi¬ lization), which however are obvious or the expert in the art, thus their description is unnecessary. The experiences of the experiments demonstrated that the illustrated strand 1 a.ccording to the inven¬ tion can be produced simply and productively at relatively low-additional cost, with traditional equip¬ ment. Owing to the hexagonal cross section of the covering wires continuous in longitudinal direction, any cross section of the strand is identical, con¬ sequently the arising stress conditions too are iden¬ tical. The surfa.ce area increased according to the periodic profiling results in improved anchoring capacity of the strand 1, consequently the load bear- ing capacity of the strand 1 and its safety factor are also increased. Or assuming for instance identical load bearing, compared to traditional strand of periodic profiling, the use of steel, i.e. material is less for the strand provided with covering wires of helical hexagon cross section according to the inven¬ tion. Naturally for example in the case of a. three- wire strand the central wire 2 is omitted. Moreover, in case of multi-wire strands even the central wire may be identical with the covering wires. Further¬ more, according to the invention in place of the above described hexagon profile any other non-cir- cular profile can also be used with similar result, e.g. as polygonal cross sections, thus regular penta.- gon, octogon, etc. In addition, such circular or poly¬ gonal profile too may come into question at least for the covering wires, which is provided with groove- like recesses passing through in longitudinal direction, and/or rib-like, or web-like extensions passing through in longitudinal direction. The cold deformation of the three latter ones may be accomplished in any other way, e.g. by rolling.

Claims: CLAIMS : 1. Strand for stressed concrete structures formed a.s twist from at least three wires provided with 5 surface p'rofilings, characterized in that the surface profiling of the wires (3) is formed as non-circular cross section profile continuously passing through the wire in longitudinal direction. 10 2. Strand as claimed in claim 1., characterized in that the cross section profile of the wires (3) continuously passing through the wire in longitudinal direction is a polygonal profile, preferably a regu¬ lar hexagon with rounded corners. 15 3. Strand as claimed in claim 1., characterized in that the cross section profile of the wires (3) passing through continuously in longitudinal direction, is such circular and/or polygonal cross section which is provided preferably with evenly distributed groove¬ 20 like recesses and/or web-like extensions along the circum erence. 4. Process for the production of strand for stressed concrete structure, where first the surface ^5 profiling of the wires is formed with cold deformation, then at least three wires are simultaneously twisted to strand, characterized in that the surface profiling of the wires is formed by drawing through drawplate preferably of regular hexagonal opening cross section 3° with rounded corners. 5. process as claimed in claim 4., character¬ ized in that the wires are turned around their longitudinal axis during or after the profiling of the wires. 6. Process a,s claimed in claim 5., characterized in that the turning direction of the wires coincides with the subsequent twisting direction of the strand. 7. Process as claimed in claim 6., characterized in that the pitch of the strand's twisting is selected to the multiple, suitably to tenfold of the pitch of wire turning.

Inventors: LAKATOS OTTO, MADARASZ LAJOS, LAKATOS, OTTO, MADARASZ, LAJOS
Applicants: DECEMBER 4 DROTMUEVEK, "DECEMBER 4" DROTMUEVEK
Application year: 1986
Main classification: E04C5
IPC codes: D07B1, E04C5