GB2197360A

GB2197360A - Prestressed concrete articles

Info

Publication number: GB2197360A
Application number: GB08627285A
Authority: GB
Inventors: Shoji Ikeda; Kiyoroku Fukayama; Takashi Hashiguchi; Akihiro Irie
Original assignee: PC Bridge Co Ltd
Current assignee: PC Bridge Co Ltd
Priority date: 1986-11-14
Filing date: 1986-11-14
Publication date: 1988-05-18
Also published as: GB8627285D0

Abstract

Prestressed concrete articles of the post-tensioning type comprise a concrete block 3, a tensioned steel member 1 prestressing the concrete block in compression, and spiral steel bars 15 embedded in the concrete block spiralling in the longitudinal direction around said steel member in the neighborhood of an anchored portion of the steel member. The spiral steel bars are bent from bars of high-tensile steel for prestressed concrete having a tensile strength of not less than 95 kgf/mm<2>, a yield point of not less than 80 kgf/mm<2>, an elongation at failure of not less than 5%, and relaxation of not more than 1.5%. <IMAGE>

Description

SPECIFICATION Prestressed concrete articles and method of producing the same BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to prestressed concrete articles or units and to methods of producing them. More particularly, it relates to prestressed concrete construction units and to methods of producing them, the units being reinforced by steel spirals at the anchoring portions of the prestressing steels.

2. Description of the Prior Art Prestressed concrete articles or construction units have high-tensile steel under tension therein to give a compressive prestress to the concrete portions thereof prior to the development of stresses due to the loads applied thereto. Thus, they are applicable to a wide range of structures, and are utilized both as in situ or on-the-site concrete construction units such as beams and girders of long-span structures, and as prefabricated units such as piles, sheet piles, panels, or railroad ties.

One of the methods of producing prestressed concrete articles is the post-tensioning method. According to this method, tensioned steel wires, strands, or bars which apply a prestress to the concrete (hereinafter referred to as steels for prestressed concrete or simply as PC steels) are placed in a form together with sheaths therefor and also with the necessary reinforcing steel bars for the concrete.

Fresh concrete is then poured into the form, and after the concrete has attained a predetermined strength, the PC steels are tensioned by the application of tensile forces to one or both ends of the article. Next, the PC steals are anchored to the concrete by anchoring members, and the PC steels apply a compressive prestress to the concrete.

According to this post-tensioning method, however, the portions of the concrete near the ends of the PC steel where it is anchored to the concrete are subject to localized bearing stresses due to the prestressing force.

Thus, in addition to compressive prestresses 10, localized tensile stresses, i.e., bursting stresses 11 and spalling stresses 12, develop in the concrete 3 in the direction perpendicular to the prestress in the area adjacent to the anchoring member 4 for the PC steel, as shown in Fig. 1. Thus, as shown in Fig. 2, bursting cracks 13 and spalling cracks 14 may develop in the sides of the concrete 3, which is very weak in tension. This greatly decreases the durability and mars the appearance of the concrete, and may ultimately cause the failure of the concrete 3 around the anchoring member 4 for the PC steel.

In order to prevent such failure of the concrete, spirals or lattices made of structural reinforcing steels are generally placed at the ends of the PC steels to reinforce the concrete in those portions. These spiral steel bars are effective for preventing the concrete from cracking due to the bursting and spalling stresses 11 and 12 shown in Fig. 1. Conventionally, the type of steel utilized for the reinforcing spiral bars at the ends of the PC steels has been a structural rebar (reinforcing bar) having a tensile strength of about 40 to 60 kgf/mm2.

Thus, the diameter of the bars which are bent into spirals to reinforce the ends of the prestressed concrete is of necessity relatively large to ensure adequate strength. The bending of the bars into spirals, therefore, is a difficult operation which takes a long time to accomplish. This is especially true in the case in which deformed bars having a non-circular cross section and an irregular outer surface are used. Although deformed bars are preferable as reinforcing bars to those having a circular cross section and a smooth outer surface because of a larger surface area which ensures strong bond with the concrete, deformed bars can hardly be used as the reinforcing spirals at the ends of the PC bars, for it is extremely difficult to bend them into spirals of predetermined dimensions and shapes.

The bending operation becomes more difficult in the case of spiral rebars utilized for large prestressed concrete structures, since the dimensions of the rebars must also be quite large to ensure the necessary strength.

Furthermore, the portions of the prestressed concrete near the ends of the PC steels have a complex arrangement of reinforcing bars such as stirrups and grids besides the abovementioned spirals, with the result that a narrow space is left therebetween before the casting of the concrete into the form. Thus, if the above-described reinforcing spiral bars have large cross sections, the space left in that portion of the form becomes still narrower, so that it is very difficult to fill the spaces with poured concrete. This may result in defective products with voids left therein.

SUMMARY OF THE INVENTION Thus, the principal object of the present invention is to provide prestressed concrete articles or construction units and a method of producing them, in which the reinforcing spiral bars at the anchored portions of the PC bars can easily be bent into predetermined spiral forms with less time and labor while ensuring adequate strength.

Another object of the present invention is to make it more feasible to use deformed bars as the above-described reinforcing spirals, such bars being preferred because of their stronger bond to concrete.

A further object of the present invention is to reduce the amount of steel which is necessary for the above-described spiral bars to en sure the necessary strength, thereby reducing the weight of the concrete article.

A still further object of the present invention is to increase the space near the anchored ends of the PC steels which is left before the pouring of the concrete into a form, so that the concrete can easily fill the entire form without voids being left in the spaces between reinforcing members.

According to the present invention, a prestressed concrete article of the post-tensioning type comprising a concrete block, a tensioned steel member, and a steel bar bent into a spiral and embedded in the concrete block near the anchored portion(s) of the tensioned steel member is provided. The tensioned steel member may be steel wires, strands, or bars of high-tensile steel for prestressed concrete running longitudinally through the concrete.

The steel member is anchored to the concrete block at both ends thereof and applies a compressive prestress to the concrete. The spiral steel bar is bent from a steel bar of PC steel, i.e., the type of high-tensile steel utilized for prestressing the concrete.

Preferably, the steel bar which is bent into the spiral has an ultimate tensile strength of not less than 95 kgf/mm2, a yield point of not less than 80 kgf/mm2, an elongation at failure of not less than 5 %, and relaxation of not more than 1.5 %.

Furthermore, it is preferred that the spiral steel bar have grooves formed on the side surfaces thereof which have a lead not less than three times as large as the diameter of the steel bar.

According to another aspect of the present invention, a method of producing the abovedescribed prestressed concrete articles of the post-tensioning type is provided. According to the method of the present invention, a bar of high-tensile steel which is commonly used for tensioned steel bars in prestressed concrete is bent into a spiral of a predetermined shape and is used as the reinforcing spiral steel bar near the anchored portion of the tensioned steel member (such as steel wires, strands, or bars), applying a prestress to the concrete.

BRIEF DESCRIPTION OF THE DRAWINGS The details of the invention will become more apparent in the following detailed description of the preferred embodiments and in the accompanying drawings, in which: Fig. 1 is a schematic side view of the portion of a prestressed concrete member near an anchoring member of the tensioned steel member giving a prestress to the concrete, showing the state of stress in that portion of the concrete; Fig. 2 is a side view of the same portion of the prestressed concrete member, showing cracks appearing on the concrete surface thereof due to the spalling and bursting stresses therein; Fn rig. 5 is a longitudinal cross-sectional view of a prestressed concrete article according to the present invention;; Fig. 4 shows the preferred forms of the outer surface of the PC steel bars to be bent into reinforcing spirals in the article of Fig. 3, the two top views being transverse cross sections of the PC steel bars, and the bottom view being a side view of a short length thereof; Fig. 5 is perspective views of the shapes of the reinforcing spirals into which the PC steel bars are bent and embedded near the anchored portions of the tensioned steel member in the prestressed concrete article of Fig.

3; and Fig. 6 is a longitudinal cross-sectional view of a portion of another prestressed concrete article according to the present invention.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS Fig. 3 is a longitudinal cross-sectionai view of a column-shaped prestressed concrete article, in which reinforcing spiral steel bars according to the present invention are used.

In the construction of the concrete article (or construction unit) of Fig. 3, PC bars, or the type of high-tensile steel bars which are commonly used as the tensioned bars in prestressed concrete, are bent into reinforcing spiral bars 15. The PC bars to be bent into the spiral bars 15 are preferably made of a steel having an ultimate tensile strength of not less than 95 kgf/mm2, a yield point (the tensile stress corresponding to a permanent elongation of 0.2 %) of not less than 80 kgf/mm2, an elongation at failure of not less than 5 %, and relaxation of not more than 1.5 %. The PC bars to be bent into the spirals 15 may either be of perfectly circular cross section or they may be deformed, although deformed bars are preferred.The mechanical and other properties of the PC bars should meet the requirements of Japanese Industrial Standard G3109 (1977) "Steel Bar for Prestressed Concrete". According to this standard, the diameter of the PC bars ranges from 7.4 mm to 32.0 mm, from which a suitable diameter ensuring the necessary strength is selected.

The deformed bars to be bent into the spirals 15 preferably have a plurality of helical grooves formed in their surface. The transverse cross sections 15a and 15b in Fig.

4 illustrate bars having 3 and 6 grooves, respectively formed in their surface. The bottom illustration in Fig. 4 is a side view of a short length of such a PC bar 15c having a plurality of grooves. The lead of the helical grooves on the surface of the bar 15c is preferably not less than three times as large as the diameter of the bar 15c.

PC bars of the types described are produced by a Japanese company named Kshu- ha Netsuren Kabushiki Kaisha, for example, un der the trade name of URUBON, the diameters thereof ranging from 7.25 mm to 13.10 mm.

The form of the spirals 15 into which the PC bars are bent may be rectangular or circular, as shown in Fig. 5(A) and (B), respectively. The mechanical properties and the diameter of the PC bars, or the form and the dimensions of the spiral 15, are selected to ensure the necessary strength. When, for example, the concrete block 3 has the form of a rectangular column with a square base of 240 mm sides and a tensile force of 125 tons is applied, then a PC bar with a diameter of 11 mm having a ultimate tensile strength of not less than 145 kgf/mm2 and a yield point of not less than 130 kgf/mm2 may be selected and bent into circular spirals 15 having a diameter of 170 mm, a pitch of 55 mm, and an over-all length of about 500 mm (i.e., of about 9 pitches).The resulting reinforcing spirals 15 bent from the PC bars as specified above have been experimentally proven to give the same strength to the concrete 3 near the anchoring members 4 and 5 as the spirals of the same dimension bent from a bar of a structural rebar having a diameter of 16 mm.

Next, to construct the concrete article of Fig. 3, a PC steel member 1 in the form of a bar extending through a sheath 2 is placed in a form (not shown) together with the bent spirals 15 and the necessary grids, etc., (not shown), so that the reinforcing spirals 15 will spiral around the end portions of the sheath 2. Concrete is then cast into the form.

After the concrete 3 has set and has attained the necessary strength, the PC steel member 1 is tensioned from one or both ends thereof. The PC steel member 1 is anchored to the concrete 3 in that tensioned state by means of anchoring members comprising anchoring plates 4 and nuts 5 which screw onto threads 6 formed on the outer surface of the member 1 near the ends thereof.

Next, the tensioning forces applied to the member 1 at one or both ends of thereof are released, so that a prestress is applied to the concrete 3 by the tensioned steel member 1 through the anchoring members 4 and 5.

Fig. 6 is a longitudinal cross-sectional view of the end portion of another embodiment of the present invention, in which the tensioned steel member applying a prestress to the concrete 3 comprises PC steel wires or strands 18 instead of a PC steel bar. In the case of this embodiment, the PC wires or strands 18 enveloped in a sheath 2 are anchored to the concrete 3 by means of a female cone 16 and a male cone 17. Otherwise, the construction of the concrete article of Fig. 6 is the same as that of Fig. 3, the reinforcing spiral bar 15 spiralling around the portions of the sheath 2 near the ends thereof.

Claims

1. A prestressed concrete article of the post-tensioning type, comprising: a concrete block; a tensioned steel member running longitudinally through and anchored at the ends of the concrete block, said steel member prestressing the concrete block in compression; and a spiral steel bar embedded in the concrete block spiralling in the longitudinal direction around said steel member in the neighborhood of an anchored portion of the steel member, said spiral steel bar being bent from a bar of high-tensile steel for prestressed concrete having a tensile strength of not less than 95 kgf/mm2.

2. A prestressed concrete article of the post-tensioning type as claimed in claim 1, wherein said spiral steel bar is made of a high-tensile steel having a yield point of not less than 80 kgf/mm2, an elongation at failure of not less than 5 %, and relaxation of not more than 1.5 %.

3. A prestressed concrete article of the post-tensioning type as claimed in claim 1 or 2, wherein said spiral steel bar has helical grooves formed in the outer surface thereof with a lead not less than three times as large as the diameter of said steel bar.

4. A method of producing a prestressed concrete article of the post-tensioning type, comprising the steps of: bending a steel bar of high-tensile steel for prestressed concrete having a tensile strength of not less than 95 kgf/mm2 into a spiral of predetermined shape; placing a sheath longitudinally in a form together with the spirally bent steel bar, said steel bar spiralling around the sheath near an end thereof, and said sheath having a steel member extending therethrough; casting concrete into the form; applying tension to said steel member after the concrete has attained a predetermined strength; anchoring said steel member to the concrete; and releasing the tension applied to said steel member, thereby applying a compressive prestress to the concrete.

5. A method of producing a prestressed concrete article of the post-tensioning type as claimed in claim 4, wherein said steel bar is made of a high-tensile steel having a yield point of not less than 80 kgf/mm2, an elongation at failure of not less than 5 %, and relaxation of not more than 1.5 %.

6. A method of producing a prestressed concrete article of the post-tensioning type as claimed in claim 4 or 5, wherein said steel bar has helical grooves formed in the outer surface thereof with a lead not less than three times as large as the diameter of said steel bar.

7. A prestressed concrete article of the post tensioning type substantially as hereinbefore described with reference to, and as illus trated in Fig. 3 or Fig. 3 as modified by PC steel bars of Fig. 4; or Fig. 5A; or Fig. 5B; or Fig. 6 of the accompanying drawings.

8. A method of producing a prestressed concrete article of the post tensioning type substantially as hereinbefore described with reference to, and as illustrated in, Fig. 3 or Fig.3 as modified by PC steel bars of Fig. 4; or Fig. 5A; or Fig. 5B; or Fig. 6 of the accompanying drawings.