US2869214A - Grouting of reinforcements in posttensioned concrete structures - Google Patents

Description

Jan. 20, 1959 CONCRETE STRUCTU Original Filed July 10. 1953 o in Mi VAN BUREN 2,869,214.- cnourmc OF REINFORCEMENTS IN POST-TENSIONED RES 2 Sheets-Sheet l 271: 4. Loosz YF-IREAD Vl A TEE IN VEN TOR.

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Jan. 20, 1959 M. VAN BUREN 2,869,214

GROUTING OF REINFORCEMENTS IN POST-TENSIONED CONCRETE-STRUCTURES Original Filed July 10. 1953 2 Sheets-Sheet 2 INVENTOR.

M ERS 144 N EURE/v MYW ATTORA/EVS.

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GRGUTIVG OF REINFORCEMENTS IN POST- TENSIONED CONCRETE STRUCTURES Myers Van Buren, Haworth, N. J., assignor to Raymond International Inc., New York, N. Y., a corporation of New Jersey Continuation of application Serial No. 367,332, July 10, 1953. This application May 16, 1957, Serial No. 660,736

1 Claim. (Cl. 25-154) This invention relates to the grouting of reenforcements such, for example, as tensioned wires or cables as used for post-tensioning concrete members or structures.

This application is a continuation of applicants copending application Serial No. 367,332, filed July 10, 1953, now abandoned.

One of the known methods for making so-called prestressed concrete bodies or members is to cast the concrete in a suitable mold provided with removable mandrels or other core means which, when removed after the concrete has set, will leave holes therethrough, through which wires or cables are threaded. Such wires or cables are then heavily tensioned by the use of jacks which apply tensioning forces to the wire ends, which forces react against the body of concrete causing the latter to be subjected to heavy compression. In accordance with one practice heretofore used, such wires are then permanently anchored at their ends to maintain the tension therein by the use of suitable end anchorage means which, after same is applied. permits the jacks to be removed while the concrete remains under permanent compression. It has been proposed tothen introduce cementmortar or the like in fluid condition along the tensioned wires in the holes in the concrete for the purpose principally of protecting the wires against corrosion. Other experts have advocated the injection of cement grout under pressure into the holes along the wires in such a manner as to securely bond the wires in place under tension so that the end anchorage means may thereafter be removed for reuse elsewhere with a considerable saving of expense. But the problem of securing an effective bonded relation between the tensioned wires and the concrete throughout their length has, under some circumstances, heretofore presented such difficulties that there has been a general hesitancy among many engineers to 'rely upon the grout bonding to the extent of permitting the end anchorage to be removed for reuse.

One serious difficulty in this connection arises from the fact that, in order to be able to place the grout bonding material in small cavities of considerable length, the grout must be in a quite fluid condition, particularly if the cross section of the cavity is relatively small or if such holes are quite long. In such cases the fluidity of the grout must be greater to insure complete filling of the holes. While such fluidity may readily be increased by increasing the water-cement ratio, yet such addition of water has three very harmful elfects, viz: it increases shrinkage of the grout before same becomes initially set, it decreases its bonding strength and also decreases its ultimate compressive strength.

Where the cored hole in the concrete which contains the post-tensioned steel wires is formed by a method in which the coring material is removed, thus leaving an unlined hole, and if the concrete is of a nature such as to be somewhat porous or such as to absorb water, then the groutbonding of the tensioned Wires by the use of grout which isinitiallyQin a highly .fluid' condition may be accomplished in the'fo'll'owin'g'wa'y: that is, the fluid grout is forced into the cavities between and around the tensioned wires until the spaces are completely filled from end to end of the concrete structure and the grouting material starts to run out of the end opposite the end of introduction. Then such opposite end of the hole may be plugged, whereupon the grouting pressure is increased and maintained at least for from 1 to 3 minutes. If such concrete is of a sufliciently porous or absorptive nature and if the pressure applied to the grout is continuously maintained despite the loss of Water from the grout into the concrete, this procedure will soon result in a low water content grout about the tensioned wire and of a nature such that upon setting thereof it will have a low shrinkage factor, a high bonding strength, and a high ultimate compressive strength.

However, the provision of suitable apparatus for introducing the grout and then maintaining same under the required high pressure until its water content is adequately reduced, involves certain serious difficulties, but certain features of the present invention effectively over come these difiiculties in a manner hereinafter explained.

There are also circumstances and conditions under which further difliculties may be encountered. For example, if the concrete body containing the cored holes if of a nature such that it has a very low porosity and very low' water absorptive characteristics so that the desired reduction in the water content of the highly fluid grout is not fairly promptly achieved, then the resulting grout bond will not only have unsatisfactory bonding and compressive strengths, but these disadvantages will be in creased by reason of excessive shrinkage of the grout. That is, for example, if the tensioned wires are in a horizontal position, the grout will tend to shrink away from the upper surfaces of the wires and particularly from the upper. wiresand the resulting bonding effect will be non-uniform and unreliable. The same results will also occur in cases where the cored holes for the tension wires in the concrete are formed by a method in which the hole forming mandrel means is not removed or completely removed even though the surrounding concrete might otherwise be sufficiently porous or water absorptive. That is, according to some methods for prestressing concrete, either steel tubing, waterproof paper tubing, water repellent material, etc. are left in the cored hole to form a lining means therein, and then the concrete member will not be able to take water from the grout either by reason of absorption or porosity. But with the present invention means and methods are provided for fully overcoming these difficulties as well as the abovernentioned difliculty of maintaining the grout under pressure for the required interval.

Various further and more specific objects, features and advantages of the invention will clearly appear from the detailed. description given below taken in connection r with the accompanying drawings which form a part of this specification and illustrate, by way of example, preferred arrangements of apparatus for carrying out the invention The'invention consists in such novel methods and combinations of features as are-herein disclosed.

In the drawings:

Fig. l is a vertical sectional view through a portion of a body of concrete to be stressed and showing wires passing into a cored hole in such concrete, together with an end anchorage arrangement used in connection with the tensioning of the wires and the introduction of grout bonding, generally in accordance with the invention disshowing a pressure tank arrangement of a preferred form, for carrying out the grouting methods of the present invention;

Figs. 2 and 3 are sectional views taken substantially along lines 2-2 and 33 of Fig. 1;

Fig. 4 is an enlarged sectional view showing a portion of Fig. 1 as constructed and arranged to carry out the present invention in one of its forms;

Fig. 5 is a sectional view similar to Fig. l but showing an alternative embodiment of the invention;

Figs. 6 and 7 are perspective views showing-two alternative forms of certain elements adapted to be used with the arrangement of Fig. 5; i

Fig. 8 is a sectional view similar to Fig. 5 but showing the invention as used in connection with a body of concrete in which the cored hole containing the tensioned wires contains a sheath formed of a helically wound metal ribbon, of the form shown in the copending application of Maxwell M. Upson, Serial No. 293,113, filed June 12, 1952; and

Fig. 9 is a longitudinal sectional view showing a small portion of the sheath of Fig. 8 and its relationship to a portion of the tensioned wire therein grout bonded in place.

Referring to the drawings in further detail, a portion of a body of concrete which is to be prestressed is indicated at 10 formed with a hole, as at 11, which is intended to extend entirely through the body of the concrete to the opposite end thereof for permitting tensioned wires, as at 12, to pass therethrough. In case the body of concrete 10 is, for example, a section of concrete pipe or a section adapted to form part of a hollow concrete pile, then the holes, such as 11, may be formed therein with apparatus and in accordance with the methods disclosed in United State patent to Van Buren, 2,602,979, granted July 15, 1952.

As further shown in Fig. 1, apparatus may be provided (similar to that shown in the above-mentioned Parry patent) for use in connection with the tensioning of the wires and for providing a temporary end anchorage therefor while same are being grout bonded in place and until the grout has set. Such apparatus may comprise the metal jacking ring 13 placed against the end surface of the concrete. Suitable metal spacer or stool means 14, 15 are placed against the plate 13 and these in turn serve to support an assembly of male and female locking elements 16 and 17. That is, the member 16 may comprise a block or steel having a conical cavity 18 therein against the walls of which the wires 12 are spread out and are adapted to be clamped by the conical outer surfaces 19 of the locking member 17. As explained in said Parry patent, jacking means may be applied to the outer ends, of the members 16 and 17 and to the wire for first tensioning the wires and then forcing the conical plug 17 into place, after which the jack may be removed for reuse elsewhere. plained in said Parry patent, the space surrounding the wires and within the stool members 14 and 15 may be filled with a suitable readily removable material 20, such as plaster of Paris. But-the wires 12 are posjtioned in an annular arrangement leaving a cavityfas at 2 1 at the mid-portion, which cavity, as well as continuation thereof, as at 21a in the member 17, are left free of; plaster, and a coupling, as at 22, is provided through which the grout may be injected under pressure to fill the cavities all along and between the wires 12 throughout the length of the hole 11 in the concrete.

In using the arrangement of Fig. 1 as thus far de-- scribed and according to the methods of said Parry patent, the grout is pumped in through a coupling such as 22, to fill the cavities about the wires in the hole 11 and when such cavities became filled and the other end of the hole 11 is plugged, the grout pressure as applied by the pump tends to increase substantially with the result that water in the iluid grout will be forced to pass into the pores of the, concrete 10,01 be absorbed,

Then, as further exin the concrete and this tends promptly to result in a low water content grout which will have the desired low shrinkage and other characteristics. However, due to the passing of substantial quantities of water from the grout into the concrete there is a tendency for the pressure in the grout to decrease with a consequent tendency for some ofthe excess water to remain in the grout instead of passing into the concrete. That is, even though the grout as initially injected about the wires in the hole 11, is subjected to a high pressure, for example 50 to 250 lbs. p. s. i., such pressures cannot be maintained say by merely shutting off the connection to the grout pump by a valve, but the pump has to be so arranged as to continually force in additional grout to compensate for the water taken into the concrete 10. Otherwise the grout would retain its fluidity to a degree such that it would not have the desired characteristics. Consequently, pressure has to be continuously applied for a substantial interval by the pump, yet without subjecting the hole 11 in the concrete to a pressure great enough to cause bursting of the walls of the concrete member. While this desired maintenance of the pressure could be achieved by providing the pump with some form of constant torque drive means, such a means would be either too cumbersome or expensive for convenient and economical use for this purpose, considering the fact that the usual concrete job involves grouting a great many holes in the concrete. Also the pump motor might be intermittently stopped and started to maintain the pressure from the pump at the desired level but this expedient, in addition to involving careful attention by the workmen at critical short intervals, would also tend to burn out or injure the pump motor. These difiiculties are, however, with the present invention, overcome in a relatively simple way by the use of pressure tank apparatus, as shown in Fig. 1a. operated according to a method described below.

As shown in Fig. 1a, a tank 25 is provided of a size preferably ample to contain enough grout for filling the cavities about the wires in several of the holes such as at 11, this tank having a cover 26', suitably bolted in place and adapted to be removed for washing out the tank when desired. The tank is also provided with a pressure gauge 27, and two inlets 28 and 29, having suitable valves, as indicated, and adapted respectively for introducing water and air under pressure into the tank. A hopper 30 is pro vided for receiving a desired quantity of grout mixture to be charged into the tank, this hopper being connected to the tank by a conduit 31 having a valve 32. The lower end of the tank 25, which is preferably of conical form, is connected through a conduit 33 to detachable coupling means, such as indicated at 22 in Fig. l, a suitable valve as at 34 being provided in conduit 33.

After the wires, such as 12, have been tensioned and the parts arranged as shown in Fig. l, the tank apparatus of Fig. 1a may be connected and used in the following way. First with the tank empty of grout, water is introduced therein through the connection 28 and allowed to flow through the cavities 21a and 21 and through the ho e. 11in the. nc ete bou the w r s- Thi ill as ou y loos p rticles ticq c e Q o he n red m eri l rom the 101 .11. and. t h -same time ca s the concrete forming the walls of the hole to be wetted whereby when the grout is later introduced, it will be able to proceed into and through the hole while still fluid and without becoming dewatered so rapidly that the hole might not become properly filled. Then after the water is blown from the tank 25 and from the hole 11 by air pressure from pipe 29, the valves of conduits, 28, .9 nd 3. a e o ed, aild. de ed q n ty 0t grout is introduced to the tank from hopper 30 by opening valve .32. A considerable space for compressed air is left above the body of grout in the tank. The valve 32 may then be closed and valve 34 opened whereupon the fluid grout will flow into, the hole 11 about the wires 12. Air pres ur .2 o mo e the, pe q a achm y n w be established in the tank to promote this fiow'which is allowed to proceed until the grout appears at the opposite end of the hole 11 in such quantities as'to indicate that the hole has been substantially filled whereupon such opposite end is plugged, for example with a suitably shaped wooden plug. Then immediately the air pressure in the tank is so increased as to apply to the grout, for example a pressure which may vary from 50 to 250 lbs. per square inch. While this pressure should be substantial, it is not particularly critical except that it should, of course, be insufiicient to cause any danger that the body of concrete will burst open. This higher pressure is maintained for a short interval, for example, 1 to 3 minutes and until the grout within'the hole 11 has lost to the surrounding concrete enough of its water content so that it will become quite firm all the way from the anchorage means of Fig. 1 to the other end of the hole 11. Then the valve 34 may be closed and the tank 25 uncoupled at coupling 22 without the need of any further expedients to maintain the pressure of the grout within the hole 11. That is, the substantial loss of water from the grout into the concrete will have caused it to form into a firm plug like means of low water-content grout which will have the desired low shrinkage characteristic and provide when set, the desired high bonding and high compressive strengths.

However, the procedure as thus far described may not cause adequate dewatering of the grout at the region just where the wires emerge from the concrete and furthermore, if the concrete body 10 is made with certain materials and methods, such as by casting in a spinning mold under intense vibration with a minimum water content, then the resulting mass may be such as to be relatively free of pores and not adapted to absorb water. In such .cases, in accordance with important aspects of the present invention, further steps are resorted to to insure that the grout in the hole 11 will be plugged in place and its high pressure maintained until it sets, so that under the heavy pressure, shrinkage difficulties and diminution of its strengthare avoided. To this end, according to one embodiment of the invention, the sealing means, such as indicated at in Fig. 1, may comprise a suitable porous or water absorptive material, such that the grout contained within the cavity 21 will rapidly lose enough of its excess water content by escape into or through the material 20 so that it will become quite firm, forming a plug in the cavity 21 and the cavity portions adjacent thereto between the wires, thereby plugging the hole 11 to a degree sufiicient to retain the remaining grout inside the hole under the desired high pressure even though the pressure tank 25 (or any pump means if such is used) may be uncoupled and removed. For example, the sealing means 20 may take the form of a precast piece of easily frangible concrete which is made porous by including therein a minimum of fines or otherwise. Various plastic or other types of materials embodying connected pores might also be used for forming the sealing means at 20.

Instead of using a porous or absorptive sealing means at 20, in accordance with another embodiment of the invention the coupling means, such as at 22, may be formed to engage in the aperture 21a in member 17 with a loose thread, as shown in Fig. 4, so as to permit leakage of water at this point, as indicated by the dotted arrows, the leakage, however, being preferably such that very little of the solids content of the grout escapes. This will result in the formation of -a firm plug of the dewatered grout in the cavity 21a so that the tank may be uncoupled within a few minutes for use elsewhere and before there is any danger of setting of the grout in the tank or its discharge conduit 33, or in valve 34. Yet the plug of solidified grout in the cavity 21a will be adequate to retain the desired high grout pressure within the hole 11 until the grout sets.

Instead of using a sealing element, such as at 20 in forming in effect a filter allowing the grout contained in the space 42 to become quite quickly dewatered to an extent such as to form a firm plug at the thickly dotted areas in Fig. 5. Fig. 6 shows the manner in which the layers of cloth and screen wire are wrapped about the wires 12.

If desired, in lieu of retaining the cloth in place by screen wire or the like, a perforated sheet metal cylinder may be used, as indicated at 43 in Fig. 7.

Fig. 8 shows the application of the method and means of Fig. 5 to cases where the concrete body, such as at Ma, contains a hole 11a in which a lining sheath 45, has been cast in place in the manner disclosed in the copending Upson application above referred to. That is, as shown in further detail in the sectional view of Fig. 9, this sheath may be formed of a helically wound ribbon of steel, the edges of which are interlocked, as at 46, so that the sheath will tend to prevent any escape or leakage of any substantial amount of water from the grout into the surrounding concrete even though the latter might be of a porous or absorptive character. Yet in this case as with the arrangement of Fig. 5, sufficient water will escape from the grouting cavity 42a through the cloth and screening 40a, 41a whereby a firm grout plug will be provided in cavity 42a for retaining the grout in the sheath 45 under the desired high pressure until it sets, and after the pressure tank has been removed.

It will be understood that the coupling leakage feature above described may, if desired, be used in conjunction with the use of porous material, as at 20, in Fig. l or in conjunction with any one or another of the embodiments of Figs. 5-8. 7

It should be noted that while in lieu of causing the formation of the dewatered grout plugs in any of the ways above described, a coupling with a valve might be provided at the end of each end anchorage so that such valve could be closed upon uncoupling the tank or grout pumping equipment and thereby retaining the grout under pressure in the holes 11. However, some of the concrete would set and harden in such valves and thus be quite difficult to remove in any way without going to substantial expense or destroying the valve. The expense of new valves for the grouting of each hole would be substantially prohibitive at least as compared with the dewatered grout plug expedients abovedescribed.

Notwithstanding the use of the above-described expedients there may be cases where plugging formations of grout within or near the end anchorages will still be inadequate to maintain the desired high grout pressure within the holes 11 until the grout has set. That is, even though the concrete body may be of a largely non-porous and non-absorptive nature, some portions thereof or some portions of a sheath as at 45, if same is used, may permit loss of enough Water from the grout to reduce its pressure too far before it has set. In such cases the grout may be so formed that it will contain, or produce within itself numerous small bubbles .and thus when the body of grout in the holes 11 is once put under a substantial pressure even though considerable water may escape therefrom, such bubbles 'being under pressure will serve to continue to maintain the grout under sufficient pressure for the desired results. One way of forming such bubbles is to mechanically agitate the grout so intensely when it is being mixed that it will contain a considerable volume of small bubbles. Another expedient is to mix with the grout when it is formed a small amount of aluminum in finely divided particles form or particles of some other material which will, in due course, interact with the cement mixture to produce numerous fine bubbles scattered through the grout. Various saponifiable resins as well as other forms of resins and also materials which essentially comprise sulfonated hydrocarbons, may be mixed with the grout to cause formation of small bubbles therein after a time delay sufiicient to allow the grout to be injected into place.

The use of grout in which small bubbles will form or are initially embodied is in fact a desirable expedient to use in connection with all of the above-described procedures to insure that the grout is maintained under pressure around the wires until it has set, thus efiectively preventing the grout upon setting from shrinking away from any portions of the Wire surfaces.

In order to insure that the grout will have the desired fluidity when first introduced, to completely fill all the crevices between and around the wires, its water content should vary depending upon whether the grout is formed entirely of cement or with various percentages of sand. For example, if the grout is all formed of Portland cement, the water-cement ratio may vary from a maximum of about 0.50 to a minimum of about 0.30. In case one part of sand is used for one part of cement, such ratio preferably may vary from a maximum of about 0.70 to a minimum of 0.45. With 2 parts of sand to 1 part of cement, the ratio may vary from a maximum of about 0.85 to a minimum of about 0.60.

It will be understood that after the grout has become firmly set, securely to bond the wires in the hole 11, the sealing means 20 of Fig. 1 may be broken away, thereby exposing the wires at the region within the stool means so that they may be readily severed with a cutting torch or otherwise, whereupon the metal parts of the anchorage means will be free to drop oif. Similarly, with the constructions shown in Figs. 5 and 8, the wires 12 may be cut off after being securely bonded by the grout in the hole in the concrete, by the use of a cutting flame or other means applied to and then through the wrappings 40, 41, 40a and 41a at the region between the stool portions.

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 claim in determining the scope of the invention.

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

In the stressing of elongated porous concrete members by the use of a group of wires and the like extending through a cavity of small diameter and considerable length through the concrete member, said wires being so positioned in said cavity as to provide free space in the cavity along the wires and throughout their length, a method for grout bonding the wires while same are held in place under tension in said cavity, which comprises first forcing grout in an initially fluid condition having a high water-cement ratio of from about .85 to about .30 into the free space along said wires and between and around the wires until the entire cavity, including said free space along the wires, is filled with grout from end to end of'the elongated concrete member with each of said wires along its peripheral extent substantially surrounded by and in contact with said grout, preventing the escape of the grout from the ends of said cavity and applying sufficiently high pressure of at least about p. s. i. to said grout to force water from the grout deeply into the porous walls of the cavity along the length thereof into the concrete while retaining in the cavity the cement content of the grout, and then maintaining the grout throughout the cavity under high pressure for at least from about 1 to about 3 minutes while forcing additional grout into the cavity to compensate for the water passed off into the concrete, thereby lowering the water-cement ratio of the grout in the cavity to make the grout firm throughout the cavity from one end of the concrete member to the other and thus providing a firm plug-like means of grout throughout the cavity with the peripheral surfaces of the wires along their lengths being embedded in and bonded to said grout, said plug-like means having a lower watercement ratio, and having a low shrinkage factor during setting and, when set around the wires, a high bonding strength and a 'high compressive strength.

References Cited in the file of this patent UNITED STATES PATENTS

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