US2705360A

US2705360A - Method of prestressing units with serration

Info

Publication number: US2705360A
Application number: US282776A
Authority: US
Inventors: Leonhardt Fritz
Original assignee: Individual
Current assignee: Individual
Priority date: 1952-04-17
Filing date: 1952-04-17
Publication date: 1955-04-05
Anticipated expiration: 1972-04-05

Description

April 5, 1955 F. LEONHARDT METHOSQQF PRESTRESSING UNITS WITH SERRATION Filed April 17, 1952 INVENTOR Fritz Leonhard? I ATTORNEY United States Patent METHOD OF PRESTRESSING UNITS WITH SERRATION Fritz Leonhardt, Stuttgart, Germany Application April 17, 1952, Serial No. 282,776

6 Claims. (Cl. 25-154) 1. This invention relates generally to methods and means for applying prestressing units to concrete masses such as girders or the like for the purpose of prestressing such concrete masses.

2. Channels provided in a concrete mass to receive a prestressing unit are frequently curved or contain a number of curves or bends.

3. In particular, this invention relates to a method and means of providing the curved portions of the channels provided in the concrete for the prestressing units with surfaces which will make it possible to readily insert and prestress the prestressing units, reducing the friction which normally occurs at these curves or bends.

In prestressed concrete wherein the tension is applied subsequent to hardening the prestressing members (wires, strands, or bars) are usually enclosed in sheet metal sleeves in order to have them longitudinally movable in the concrete when the tension is applied. The channels are sometimes made with the aid of rubber hoses which are pulled out after setting of the concrete so as to allow the insertion of the prestressing units into the resulting tubular cavities or channels. The prestressing units are mostly curved and, due to the stresses produced by bending, they are forced during the prestressing operation against the walls of the cavities or channels so that the sliding movement of the prestressing unit is prevented by friction.

The use of sheet metal sleeves will result in relatively low friction value. However, it has the disadvantage of producing a low bonding strength of the cement slurry, during its subsequent serration with the smooth metal sheets, so that the desirable narrow spacing for limited prestressing cannot be secured. Although in channels made with the aid of rubber hoses, the bond of the cement slurry with the surrounding concrete is satisfactory and, as a result, the serration becomes strong, this is accompanied by the development of increased friction so that only moderate curvatures can be used.

The invention avoids these disadvantages by using conventional rubber hoses for forming the sliding channels. However, it provides at each of the curves, rings of hard steel or bent tubular sections pulled over the rubber hoses. The hoses are then inflated so that their outside diameter becomes greater than the inside diameter of the steel rings or tubular sections. Moreover, the rubber hose is shaped, or tightened with a wire spiral placed underneath the exterior rubber layer, in such a manner that its surface becomes uneven after being inflated and, as a result, a profiled surface is likewise produced in the hardened concrete. By evacuating the air or liquid from the rubber hose after hardening of the concrete, its diameter decreases to such an extent that it can be pulled out of the hardened concrete. The steel rings remain in the concrete and form suitable sliding surfaces. In curves having small radii it is possible to provide double tubular sheet metal pieces having an inter- Patented Apr. 5, 1955 mediate layer of lubricant in order to lessen the friction to an even greater extent. The longitudinal profile given the concrete improves the bonding action so that with adequately shaped and sufficiently thin prestressing steel sections the bonding action becomes so eflicient that a small spacing is obtained and, as a result, limited prestressing with a relatively high concrete tension, is made possible.

In the drawings Figure l is a view in section illustrating one embodiment of the invention using rings to reduce friction in a concrete channel.

Figure 2 is a view in section of another embodiment of the invention using a tubular metal section to reduce this friction.

Referring to the drawings, in Figure 1, the hose 1 is shown as inflated with rings 2 constricting the hose at the curved portion of the concrete channel. As shown in both Figures 1 and 2, the flexible member or rubber hose 1 is shaped or tightened with a wire spiral 4 placed beneath the exterior rubber layer in such manner that when the hose is inflated as shown, it is constricted at certain points and its surface becomes uneven and a complementary profiled surface is produced in the hardened concrete.

Figure 2 shows the use of a bent tubular section 3 in place of the rings 2 of Figure 1.

It is to be understood that various modifications can be made without departing from the spirit of the invention and that the invention is not limited to the specific embodiments herein described and claimed.

I claim:

1. In a method of prestressing concrete which comprises forming a channel for prestressing units in said concrete by placing therein prior to hardening a flexible tube provided with rings or tubular sections thereon, said rings or tubular sections being of somewhat less internal diameter than the external diameter of the flexible tube and subsequently removing said flexible tube to leave said rings or sections in said channel with their inner faces extending out of the inner face of the concrete.

2. A method as set out in claim 1 in which the channel has a curved portion and the rings or tubular sections are provided in said curved portion.

3. A method as set out in claim 1 in which the flexible tube is expanded or inflated before removal to provide enlarged channel portions adjacent each ring or tubular section.

4. A method as set out in claim 1 in which the flexible tube is provided with a wire spiral and the tube is subsequently expanded or inflated to provide enlarged channel portions adjacent the convolutions of the spiral.

5. A method as set out in claim 1 in which prestressing units are placed in said channel after removal of said flexible tube.

6. A method as set out in claim 1 in which prestressing units are placed in said channel after removal of said flexible tube and cement slurry is introduced into said channel and surrounding said prestressing units.

References Cited in the file of this patent UNITED STATES PATENTS 1,650,307 Temple Nov. 22 1927 1,949,650 Lindas Mar. 6, 1934 2,413,990 Muntz Jan. 7, 1947 2,648,439 Miller Aug. 11, 1953 FOREIGN PATENTS 239,375 Great Britain Sept. 10, 1925