US2677957A

US2677957A - Prestressed concrete structure

Info

Publication number: US2677957A
Application number: US293113A
Authority: US
Inventors: Maxwell M Upson
Original assignee: Raymond Concrete Pile Co
Current assignee: Raymond Concrete Pile Co
Priority date: 1952-06-12
Filing date: 1952-06-12
Publication date: 1954-05-11
Anticipated expiration: 1971-05-11

Description

y 1954 M. M. UPSON PRESTRESSED CONCRETE STRUCTURE INVENTOR.

XWELL M UPSON.

- y g 1' Y AZ TZLZI Z-f Filed June 12, 1952 WWW MQ E 8 2 Q Patented May 11, 1954 UNITED srr FAN?! t FFIQ PRESTRESSED CONCRETE STRUCTURE Application June 12, 1952, Serial No. 293,113

1 8 Claims.

This invention relates to so-called rarestressed concrete structures and methods for constructing same.

This application comprises a continuation-d11- part of my co-pending application Serial No. 104,496, filed July 13, 1949, now abandoned in favor of this application.

In such prestressed structures, it is known practice to provide wires extending through the concrete and so arranged and put under heavy tension as to apply a prestress to the concrete by subjecting same to compression. The concrete is subjected to such compression after se+- ting and shrinking of the concrete has taken place and the compression strength thereof has been developed. I-leretofore, in order to install such wires or cables in place in such manner that the concrete in hardening will not adhere to the wires and prevent their being properly tensioned, one practice has been to so cast the concrete as to leave holes therein whereby the wires or cables may be threaded in after the mandrels constituting forms for the holes are removed. The wires after being thus put in place are subjected to heavy tension by forces reacting against the concrete to subject the same to compression and such tension is maintained by suitable permanent end anchorage means on the wires. In many cases, however, this procedure is impractical or prohibitively expensive, particularly where the structure is quite long, involving the troublesome problem of forming elongated holes in the concrete and particularly in the frequent cases where the holes would have to follow a path in order that the tensioned wires when placed therein will be so located in the structure as to provide the desired reinforcement at the proper locations.

Another prior proposal involved placing the wires a tube or sheath which was smooth interiorly and exteriorly except for longitudinal seams. After casting the concrete body about such tube, it was proposed to tension the wires therein by forces reacting against the concrete body to subject same to compression and then maintaf ing the tension by relying solely on pe manent anchorage means gripping the wires at t leir that is at or adjacent the end surfaces of the concrete. It was then proposed to fill the s aces in the sheath about the wires by injecting cement emulsion, but while such emulsion would protect the wires against deterioration, it would be inactive so far as any prestressing function is concerned and not subjected to stresses longitudinally of the wires. With this procedure, the construction and installation costs of the relatively complicated end anchorages required becomes excessive and the end anchorage means in practice has to remain permanently in place in the finished structure and thus same cannot be re-used to save expense. That is, if the end anchorages were removed, then the wires would not be safely held under the necessary heavy tension, because even if surrounded by cement emulsion, the wires together with the emulsion would be free to slip or creep longitudinally within the smooth interior of the sheath at least if the structure were subjected to heavy loads. Such permanent anchorage means also have the disadvantages that they unduly concentrate the stresses at or closely adjacent the ends of the concrete body and in many cases they are necessarily too bulky or numerous to be received in the available space.

In accordance with the present invention, the above noted difficulties re overcome in a relatively inexpensive way to provide a reliably prestressed structure in which permanent end anchorage means for the heavily tensioned wires are unnecessary, although same may be used in simplified form with some of the features of the invention, if insisted upon by inspecting officials or others.

In accordance with this invention, the heavily tensioned wires are fixed in relatively spaced positions within a sheath, each wire being preferably substantially completely surrounded by cement grout and thus firmly gripped thereby, and the sheath walls being so formed as to positively interlock with both the grout therein and with the concrete body cast thereabout. Over at least substantial distances inwardly from the ends of the concrete body, the wire tension acts to maintain successive portions of the grout and the sheath under heavy compression longitudinally of the sheath, whereby the concrete along outside the sheath is also maintained under a heavy compressive force, the compressed grout as bonded to the wires serving to maintain the wire tension while distributing the anchorage stresses over considerable distances along within the sheath. Thus any necessity for expensive, bl. lay permanent end anchorages and the excessive concentration of stress at same are avoided.

Various further and more specific objects, features and advantages of the invention will appear from the following description, taken in connection with the accompanying drawings lustre-ting by way of example the presently preferred embodiments of the invention.

1 is a vertical sectional view showing the iegi n at one end of a body of concrete as prestreseed in accordance with the invention;

Fig. 2 is an enlarged sectional View showing in further detail a preferred form of sheath with wires grout bonded therein in accordance with the invention;

Fig. 3 is an enlarged sectional view taken substantially along line 3-3 of Fig. 1;

Fig. 4 is a side view of a preferred form of thimble means used for retaining the wires in proper spaced positions within the sheath; and

Figs. 6 and 7 are side elevational views of alternative forms of sheaths which may be used with the invention.

Referring to the construction of Figs. 1 to 3 in further detail, portions of a body or mass of concrete prestressed in accordance with the invention are shown at It. Such concrete may be cast by pouring same into forms or molds of conventional type in which prestressing assemblies, one of Which is indicated generally at II, have first been placed at the desired locations depending upon the pattern of reinforcement required in the structure. For example, if the structure is to be a beam. one or more of the assemblies I I may be suspended, or otherwise mounted, in the mold, in a properly curved or sagging position as indicated in Fig. 1.

Each of the assemblies H comprises a sheath [2 containing a plurality of high tensile strength wires as at 13 (in a typical case twelve of such wires are used in each sheath) and these wires are preferably positively held in spaced-apart positions by the use of thimble means such as shown at 5, located at spaced points along within the sheath:

It is important that the sheath, which may be formed of relatively thin sheet metal, have walls which are so shaped as to present areas protruding both interiorly and exteriorly, and which areas are positioned generally transversely of the sheath axis so as to positively interlock a mass of grout it within the sheath against any longitudinal slippage or creeping with respect to the sheath walls, which in turn are similarly interlocked with the surrounding concrete of the body ill. Another important characteristic of the sheath is that it should be of such form as to have considerable flexibility in directions permittin it to be transversely bent, whereby it may be located alon whatever curved lines may be required to comply with the desired pattern of reinforcement of the structure and still retain its annular cross-scction without danger of cracking, collapsing or interfering with the desired uniform spacing of the wires therein. A preferred form of sheath meeting these requirements is shown in Figs. 1-3 inclusive, and comprises a ribbon of sheet metal Wound helically to form a tube, the edges of the ribbon being so curved in cross-section as to interlock and form a continuous helical joint as indicated at It along between successive turns of the ribbon. Such a sheath may be constructed in a manner similar to that well known for providing flexible conduit for electrical wiring, although for the purposes of this invention the sheath will generally be of a diameter considerably larger than that in common use for electrical conduit purposes and the sheet metal in relation to the diameter may be thinner. The wires I3 may be threaded through the machine which forms the sheath so that as the convolutions of the ribbon are being shaped and their edges joined, the completed portions of the sheath and the wires advance together. The thimbles 15 may be inserted at spaced intervals at the point where the wires are brought into assembled relation at the entrance of the machine.

As shown in Fig. 2, to form the helical joint, one edge of the sheet metal ribbon is bent to form a channel lBa of semicircular cross-section, the other edge of the ribbon being curved inwardly as at 51) to engage in such channel. In this way a joint may readily be formed which will be sufiiciently tight to prevent the cement mortar or concrete before it sets from entering the sheath and yet the joint will permit successive turns of the ribbon to move with respect to each other enough to lend the desired transverse fiexibility to the sheath. It will be noted that externally the joint provides a helical depression or groove, the side wall areas of which are positioned generally transverse to the sheath axis so that when the concrete ll; fills this groove and sets, the sheath is effectively locked against longitudinal movement in the concrete body. Also the interior portion of the joint lfia protrudes inwardly of the sheath to a degree suiiiciently to provide helically extendin areas also positioned generally transversely of the sheath axis and thereby acting to lock the grout l4 against longitudinal movement within the sheath. Furthermore, the internal portions of the joint at its serve to provide means against which each of the wires 53 will contact at uniformly spaced points, thus leaving space for grout be-- tween each wire and the interior wall surfaces of the sheath at all points along each wire except the points of tangent-y of the wire with each succeeding turn of the interior portion of the helical joint. The space between the wires and the interior wall surfaces of the sheath in fact, extends continuously and helically so that the grout as injected into the sheath is free to flow readily into all parts of this helical space and thus to insure fillin of the same and firm bonding of the external surfaces of the wires to th internal surfaces of the sheath.

As best shown in Figs. 3 and 4, the thimbles [5 each preferably comprises a hollow cylindrical portion ll formed at one end with a plurality of outwardly projecting teeth 58, one between each pair of wires. The longitudinal bore through the cylindrical portion permits the grout M as injected into the sheath readily to pass through each thimble and the external surface of the cylindrical portion I? of each thimble insures that the wires will be held out away from the center of the sheath and in contact, or substantially in contact, with the sheath joint portion as at Ilia. The radial teeth 18 will insure that all of the wires are circumferentially spaced apart so that each wire individually may be surrounded by grout. Such spacing apart of the wires also insures that the grout as injected into the sheath will be free to pass freely between the wires into the space between the wires and the interior walls of the sheath. These teeth l8, as shown in Fig. 4, preferably are so limited in width (along the direction of the wires) that they will not objectionally interfere with the bonding action of the grout on the wires.

The spacing of the thimbles l5 may vary depending upon the diameter of the sheath assembly and the degree to which the same is to be curved in the concrete structure. If the sheath assembly has to be bent around relatively sharp curves, the thimbles should be more closely spaced than when the sheath is straight or nearly straight, in order to insure that each wire will be positively retained spaced apart preferably throughout, from contact with the adjacent wires. Heretofore it has been proposed to use ahelicoidal springlike core within a group of such wires, but if the sheath and wire assembly is located along a curved path, as is Very often the case, then the wires when heavily tensioned will collapse such a helicoidal member to varying degrees,

thereby permitting the tensioned wires to shift to one side of the sheath. This in turn will prevent each wire from being individually surrounded and properly bonded by grout and also result in variable tensioning of the wires and with some of the wires more heavily tensioned than others. These difficulties, however, are eliminated by the use of the rigid thimbles here described with radial teeth or serrations, such as to positively hold each wire spaced from the others and in position against the helical joint portions at iiia.

After the concrete body it has been cast about the sheath assembly or assemblies H, as hereinabove explained, the concrete is first allowed to set and develop its compressive strength, whereupon the wires l3 are heavily tensioned as by hydraulic jacks applied to the ends thereof and in such that the tcnsioning forces react against the body of concrete and subject same to heavy co apression. It is important that high tensile strength wire be used, capable of withstanding ior example tension up to about 220,000 to 240,000 lbs. per square inch, and in the usual case the tension applied will amount at least to the greater part of the ultimate tensile strength of the wire, for example 160,000 lbs. per square inch. Preferably the wires are temporarily retained in the tensioned condition by the use of removable and reusable end anchorage means accompanying the hydraulically operated jacks as disclosed in the co-pending application of Richard M. Parry, Serial No. 132,654, filed December 13, 1049 (Patent No. 2,609,586, granted September 9, 1952). After the wires have been tensioned, a mass of high strength Portland cemerit grout mixture in free flowing condition is injected under heavy pressure to completely fill the spaces between and surrounding the wires in the sheath i2. After this grout is allowed to set and assume its compression strength, in secure bonding relation to the wires and sheath, the removable end anchorage means may be taken off and if desired the wires it, as well as the ieath, n? y be cut off flush with the surface of the concrete body, indicated at Hi.

This removal of the temporary end anchorages will have the effect of transferring the stress in the wires to the grout it by which the wires are bonded within the sheath. In the usual case the wires and surrounding parts will be sufficiently stretchable resilient so that the wire tension will cause compressive forces to be distributed in the grout longitudinally of the sheath for a distance up to eighteen inches or more, inwardly from the end of the concrete structure. Concentration of the stresses at the wire ends and at the end surfaces of the concrete body, is thus avoided. It will also be apparent that the ends of the structure will be free of obstructions such as are caused by some end anchorage devices heretoiore used, and consequently the structure may be placed in closely abutting relationship with other structures. If the structure is one having an end portion of relatively small diniensions or is a relatively thin concrete wall such as the wall of a building or the wall of a pipe, the end anchorages heretofore proposed and w ich were quite bulky, could not be accommowithin the available mass of concrete, whereas with the present invention, the concrete does not have to be any thicker at the end porions of the reinforcement than at other regions.

The total surface areas of the wires as bonded in place by the grout will normally be substantially greater than the area of the internal surface portions of the sheath, and with the compressive forces in the grout distributed as above explained, there will be no tendency for the wires to slip in the grout. Any tendency of the grout to slip with respect to the internal surfaces of the sheath will be prevented by the surface portions at its of the sheath joint which will react against the grout thus not only cause the grout to be maintained longitudinally under heavy compression, but also tend with a wedgelike efiect to compress the grout radially inwardly in more iirrn gripping relation with the tensioned wires. Thus the body of concrete It], the sheath, the grout it and the wires are effectively interlocked, the wire tension being maintained by reason of the that the grout M, at least for a considerable distance i wardly of the ends of the sheath, is maintained in state of compression, as are also the sheath portions surrounding such grout, and the entire body portion of the concrete from one end of the sheath to the other is maintained in the desired prestressed compressed condition.

The positively interlocked relationship of each successive portion of the sheath with the concrete about the same the grout and wires inside thereof, has the further important advantage that if the reinforcing means either at the ends of the structure or elsewhere should be come injured or if cracks should develop in the structure, the remaining parts will still be maintained in prestressed condition since the maintenance of the wire tension does not depend on the permanence of any end anchorage The entire prestressed concrete structure for example, be out in two transversely of the prestressing wires and sheath, and each resulting part will still remain prestressed substantially effectively as before. Thus structures prestressed in this way are well adopted for cases where they may from time to time have to be cut apart when making alterations, improvements, additions or other changes.

While for the usual case the sheath construction of the type shown in Figs. 1-3 is preferred, if desired the sheath be of the form shown at 2i; in Fig. 5, which comprises a sheet metal tube formed with a helical corrugation 2% having spaced apart turns or if preferred, the form shown at 22 in Fig. 6, which comprises a simple sheet metal tube with a series of spaced-apart circumferential grooves 23, may be used. lit will be noted that the corrugations or grooves as shown in "Figs. 5 and 6 are such as to provide relatively elevated and depressed portions which present areas pr truding both interiorly and e:: teriorly, and generally transversely of the sheath thereby providing for interlocking of the grout in the sheaths with respect to the concrete externally thereof.

The sheath of the form shown at 2 in Fig. 7

may comprise a helically wound ribbon of sheet metal, the edges of the succeeding turns of which are suitably folded over to form a helical joint 25. In the event either the external or the internal portions of this joint are such as not to present suitable or suiiiciently protruding areas to interlock with the grout and surrounding concrete, then the sheath may be formed with numerous indentations as at distributed over its surface to provide the desired relatively elevated and depressed portions. The joint 25 is preferably so formed that the edges of the convolutions of the sheet metal ribbon are free to move enough with respect to each other to lend the desired degree of transverse flexibility to the sheath permitting it to be placed in curved positions. If the corrugations as at 2! in Fig. 5, and the grooves as at 23 in Fig. 6, are made of substantial depth, then the forms of sheaths shown in these figures may readily be placed in curved positions.

Although certain particular embodiments of the invention are herein disclosed for purposes of explanation, various further modifications thereof, after study of this specification, will be apparent to those skilled in the art to which the invention pertains. Reference should accordingly be had to the appended claims in determining the scope of the invention.

What is claimed and desired to be secured by Letters Patent is:

l. A mass of concrete which has been allowed to set and shrink and which has embodied therein an elongated tubular sheet metal sheath formed with a helically extending joint, which joint forms relatively elevated and depressed portions on the exterior and interior surfaces of the sheath, the concrete being cast in adhering relation around said sheath, a group of high tensile strength wires extending along within said sheath and contacting said portions which are elevated on said interior surface, the spaces around said wires within said sheath containing a filling oi grout bonding the wires to the interior walls of the sheath, grout and the sheath being locked against longitudinal displacement with respect to each other and with respect to the surrounding concrete by relatively elevated and depressed sheath portions, successive portions of said grout along within said sheath being under heavy compression in a direction longitudinally of the sheath, and successive wire portions along within the sheath being under heavy longitudinal tension maintained by said compressed grout portions as bonded thereto, such wire tension as maintained by the compressed grout amounting at least to the greater part of the ultimate tensile strength of the wires and acting permanently to maintain successive portions of said grout, said sheath and the concrete adjacent the exterior of the sheath under heavy compressive forces distributed longitudinally inwardly from the ends of the sheath.

2. A mass of concrete which has been allowed to set and shrink and which has embodied therein a thin elongated tubular metal sheath which is corrugated generally transversely of its length to provide relatively elevated and depressed portions extending around on its interior and exterior surfaces, the concrete being cast in adhering relation around said sheath, high tensile strength wire extending along within said sheath and contacting said portions which are elevated on said interior surface, the space around such wire within said sheath containing a filling of grout bonding the wire of the interior walls of the sheath, said grout and the sheath being locked against longitudinal displacement with respect to each other and with respect to the surrounding concrete by said relatively elevated and depressed portions, successive portions of said grout along within said sheath being under heavy compression in a direction longitudinally of the sheath, and successive wire portions along within the sheath being under heavy longitudinal tension maintained by said compressed grout portions as bonded thereto, such wire tension as maintained by the compressed grout acting permanently to maintain successive portions of said grout, said sheath and the concrete adjacent the exterior of the sheath under heavy compressive forces distributed longitudinally inwardly from the ends of the sheath.

3. A mass of concrete which has been allowed to set and shrink and which has embodied therein an elongated tubular sheet metal sheath which is substantially curved and free of buckling and is formed with relatively elevated and depressed portions which present areas generally transverse to the length of the sheath and which protrude interiorly and exteriorly, the concrete being cast in adhering relation around said sheath, a group of high tensile strength wires extending along within said sheath, the spaces around said wires within said sheath and between the wires and the sheath containing a filling of grout bonding the wires to the interior walls of the sheath, said grout and the sheath being locked against longitudinal displacement with respect to each other and with respect to the surrounding concrete along the length of the sheath by said relatively elevated and depressed sheath portions, successive portions of said grout along within said sheath being under heavy compression in a direction longitudinally of the sheath, and successive wire portions along within the sheath being under heavy longitudinal tension maintained by said compressed grout portions as bonded thereto, such wire tension as maintained by the compressed grout amounting at least to the greater part of the ultimate tensile strength of the wires and acting permanently to maintain successive portions of said grout, said sheath and the concrete adjacent the exterior of the sheath under heavy compressive forces distributed longitudinally inwardly from the ends of the sheath.

4. The combination comprising a mass of concrete which has embodied therein a reinforcing assembly about which the concrete is cast in adhering relation, said assembly comprising a tubular sheet metal sheath formed of a helically wound ribbon, the edges of the convolutions of which are shaped to form an interlocked helical joint, such joint presenting a helical groove externally of the sheath and a helical ridge protruding inwardly of the sheath, a group of high tensile strength and highly tensioned wires extending along within said sheath, means at spaced points in the sheath for positively retaining each of said wires in spaced-apart condition and in contact with said helical ridge, and a mass of cement grout filling the spaces within said sheath and about said wires in bonding relation thereto, said helical groove and said helical ridge acting to interlock said grout with respect to said sheath and thereby to the surrounding concrete, against movement longitudinally of the sheath.

5. The combination comprising a mass of concrete which has embodied therein a reinforcing assembly about which the concrete is cast in adhering relation, said assembly comprising a tubular sheet metal sheath formed of a helically Wound ribbon, the edges of the convelutions of which are shaped to form an interlocked helical joint of a form permitting the sheath to assume curved positions free of buckling, such joint providing relatively elevated. and depressed portions on the external and internal surfaces of the sheath, a group of high tensile strength and highly tensioned wires extending along within said sheath, means at spaced points the sheath for positively retaining wires in a formation of annular cross-section, each in spaced-apart condition and contacting at spaced points with said portions which are elevated on said interior surface, and a mass of cement grout filling the spaces within said sheath and about said wires in bonding relation thereto, said ele vated and depressed por-i ons acting to interlock said grout with respect to said sheath and thereby to the surrounding concrete, against movement longitudinally of the sheath.

6. The combination comprising a mass of concrete which has embodied therein a curved reinforcing assembly about which the concrete is cast in adhering relation, said assembly C0111 prising a tubular sheet metal sheath formed of a helically wound ribbon, the edges or the con volutions of which are shaped to form an interlocked helical joint of a form permitting sheath to assume its curved position free of buckling, such joint presenting a helical groove externally of the sheath and a helical ridge protruding inwardly of the sheath, a group of high tensile strength wires extending along within. said sheath, thimbles with longitudinal holes therethrough at spaced points in the sheath having teeth protruding therefrom for positively retaining each of said wires in spaced-apart condition and in a formation of annular crosssection, and a mass of cement grout filling said thimble holes and the spaces within said sheath and about said wires in bonding relation thereto and. to the interior surface of the sheath, said helical groove and said helical ridge acting to interlock said grout with respect to said sheath and thereby to the surrounding concrete, against movement longitudinally of the sheath.

7. A mass of concrete which has embodied therein a tubular sheet metal sheath formed with relatively elevated and depressed portions on its surface, which present areas protruding interiorly and exteriorly and generally transversely of the sheath axis, the concrete being cast in adhering relation around said sheath, a plurality of high tensile strength and highly tensioned wires extending along within said sheath and arranged in a formation of annular crosssection, thimbles at spaced points in the sheath, said thimbles having cylindrical portions with apertures therethrough and having portions protruding outwardly therefrom between each pair of adjacent wires for positively retaining said wires in formation, and a mass of grout filling the thirnhie apertures and the spaces within said sheath and about said wires in bonding relation thereto, said elevated and depressed portions acting to interlock said grout with respect to said sheath and thereby to the surrounding concrete, against movement longitudinally of the sheath.

8. An assembly adapted for prestressing concrete constructions, comprising a tubular sheet metal. sheath formed of a helically wound ribbon, the edges of the convolutions of which are shaped to form an interlocking helical joint which provides relatively elevated and depressed portions on the external and internal surfaces of the sheath, such portions being adapted for locking the sheath against longitudinal movement with respect to concrete when cast about the assei bly also for locking engagement with grout when injected into the sheath, a group of high tensile strength Wires extending through such sheath and arranged in spaced apart relation and in formation of substantially annular crosssection, and thiznbles positioned at spaced points along ithin sheath, said thimbles having there-through permitting grout to be v injected along within the sheath to surround the wires therein, and such thimbles having portions protruding outwardly therefrom between each pair of adjacent wires for acting in conjunction with said portions which are elevated on the interior surface of the sheath for retaining said wires in said formation.

References Cited in the file Of this patent UNITED STATES PATENTS OTHER REFERENCES The Engineering Journal, Mar. 1947, pages 110-112.