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Publication numberUS2048253 A
Publication typeGrant
Publication dateJul 21, 1936
Filing dateFeb 5, 1934
Priority dateFeb 10, 1933
Publication numberUS 2048253 A, US 2048253A, US-A-2048253, US2048253 A, US2048253A
InventorsFreyssinet Eugene
Original AssigneeFreyssinet Eugene
Export CitationBiBTeX, EndNote, RefMan
External Links: USPTO, USPTO Assignment, Espacenet
Method and apparatus for the manufacture of hollow bodies of reenforced concrete
US 2048253 A
Abstract  available in
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Claims  available in
Description  (OCR text may contain errors)

E. FREYSSINET 2,048,253 METHOD AND APPARATUS FOR THE MANUFACTURE OF HOLLOW BODIES OF REENFORCED CONCRETE Filed Feb. 5, 1954 4 Sheets-Sheet 1 July 21, 1936.

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METHOD AND APPARATUS FOR THE MANUFACTURE OF HOLLOW BODIES OF REENFORCED CONCRETE Filed Feb. 5, 1934 4 Sheets-Sheet 2 was y 1, 1936. E. FREYSSINET 2,048,253

METHOD AND APPARATUS FOR THE MANUFACTURE OF HOLLOW BOPIES OF REENFORCED CONCRETE Flled Feb. 5, 1934 4 Sheets-Sheet 3 WWW lA/VE/V 707? July 21, 1936. FREYSSINET 2,048,253

METHOD AND ARATUS FOR THE MANUFACTURE OF HOLLOW BODIES OF REENFORCED CONCRETE Filed Feb. 5, 1934 4 Sheets-Sheet 4 A A A -7 43 an /1755;?- 57% 6;: 3% v 2% Patented July 21, 1936 UNITED STATES PATENT OFFICE Eugene Freyssinet, Ncuilly-sur-Seine, France Application February 5, 1934, Serial No. 709,879 In France February 10, 1933 15 Claims.

In the manufacture of reenforced concrete articles it has been proposed to subject the reenforcements to tension before hardening of the concrete and the resulting compression of the concrete make it possible to obtain either stronger articles for a given weight, or lighter for a given desired resistance. However, up to the present time it was not possible to directly subject to tension nonrectilinear reenforcements embedded in concrete, such for instance as the circular reeni'orcements that are provided in concrete pipes.

The object of the present invention is to provide a process of manufacturing hollow bodies of reenforced concrete in which nonrectilinear reenforcements embedded in concrete are subjected to preliminary tensioning, this process making it possible to obtain in a very short time a compact and resistant concrete article.

According to my process, I preferably make use of moist concrete, that is to say concrete sufiiciently rich in water to penetrate into all the interstices of the mould and between the reenforcements, however complicated their shape may be.

The process consists essentially in placing the moist concrete together with its reenforcements, (among which are non-rectilinear reenforcements, and in particular hoops), between a deformable mould and one or more mandrels which are also deformable and permit of transmitting compressive stresses to the concrete; then subjecting the concrete to a treatment intended to eliminate the excess of water, which may include a vibration or a moderate compression (involving no important deformation of the mould), or both simultaneously; after which the concrete, rendered compact by this treatment, is subjected to a strong compression under conditions such that the mould is deformed, producing an outward 40 deformation of the concrete so that the nonrectilinear reenforcements are subjected to tension.

I have found that the vibrating and compressing treatments performed in the first part 45 of the process give the concrete such a compactness that, although it has not yet begun to set, said concrete is given the mechanical properties of a solid body as long as it remains subjected to a sufiicient pressure. It follows that if suitably directed deformations are imparted to said concrete, these deformations will be transmitted to the reenforcements and will tension them. It is then only necessary to maintain the pressure exerted on the concrete until the latter acquires, due to its setting and hardening, a sufiicient resistance for maintaining the deformation previously produced and therefore the tension in the reenforcements. The compressing treatment is then stopped and the concrete piece is removed from the mould.

As the vibrating and compressing treatments considerably accelerate the setting of concrete, it is possible, with the process pursued according to my invention, to obtain the finished hollow concrete body, with its reenforcements subjected to tension, in an extremely short time. Besides, the setting and hardening of the concrete can be considerably accelerated by heating; the combined action of heat, vibration, and compression giving a highly resistant concrete in a minimum period of time.

The value of the second compression depends on the nature of the concrete that is utilized, the shape of the reenforcements and the tension to which they are subjected. The pressure thus applied must be sufiiciently high for rendering concrete compact and resistant enough for preventing any relative displacement of the reenforcements with respect to said concrete. I have found that the resistance of concrete to penetration by the reenforcements is proportional to the pressure to which the concrete is subjected, and a simple calculation will indicate in each case the value of the pressure to be employed. My process makes it possible to obtain in the reenforcements very high degrees of tension, averaging kilogrammes per square millimeter and even more.

In vibrating moist concrete, there may arise a difliculty which results from the fact that, due to the accumulation of an excess of water in the upper part of the mould, the concrete present in this part of the mould tends to become excessively fluid, which would involve a lack of homogeneity in the piece. This drawback can be obviated by making use of complementary chambers which will be filled with concrete and subjected to vibration at the same time as the mould, so as to produce, between these chambers and the mould, an interchange of the concrete and the excess water. The water, that is driven out by the vibrations accumulates in these chambers, while concrete passes from said chambers into the mould.

Another object of my invention is to provide apparatus for carrying out this process of manufacture. The apparatus according to my invention essentially includes one or several mandrels of elastic structure into which a fluid under pressure can be injected, and a mould or shell capable of being deformed when a sumciently high pressure is applied to the concrete in the mould by the mandrel or mandrels. This deformation of the mould or shell may be: either an elastic deformation under the effect of the pressure applied to the concrete, or a positive deformation, resulting from an action on the organs through which the parts of the mould or shell are assembled. However, in this case, the artificial deformation of the mould must be so performed that the pressure to which the concrete is subjected does not drop below the minimum of pressure for which the concrete resists the stresses to which it is subjected by the reenforcements.

Besides the advantages above referred to, my process makes it possible to obtain concrete pieces of a very high industrial value due to their high resistance (very high Young's modulus) and to the fact that the concrete is not porous, these advantages making it possible to considerably reduce the thickness of pieces for a given strength.

' They amply compensate for the somewhat increased complication and cost of the apparatus. Besides the rapid setting of the concrete, obtained by vibrating and compressing, and eventually heating the concrete, the process renders the moulds and the apparatus available for a new operation after a relatively very short time so that the cost of the plant is quickly redeemed. The resistance of the concrete that is necessary for permitting the removal of the moulded pieces from the mould can be attained in a few hours or even less, which is a remarkable result.

Preferred embodiments of, my invention will be hereinafter described with reference to the accompanying drawings, given merely by way of example, and in which: I

Fig. 1 is a partial sectional view, in cross section of a mould for the manufacture of pipes;

Fig. 1a is a cross sectional view, similar to Fig. 1, of another embodiment of the invention;

Fig. 2 is a sectional view on the line 2--2 of Fig. 1; I

Fig. 2a shows a detail of the elastic mandrel;

Fig. 2b is a view similar to Fig. 2 showing another embodiment;

Fig. 3 diagrammatically shows the arrangement of a hopper at the upper part of an annular mould for pouring concrete thereinto;

Fig. 3a shows a detail of the hopper;

Figs. 4 and 5 are two elevational views, seen from directions at right angles to each other, showing the end of one of the longitudinal (rectilinear) reenforcements;

Fig. 6 is a sectional view on the line 6-6 of Fig. 5;

Fig. 7 shows this end of the reenforcement after twisting thereof;

Fig. 8 is a cross sectional view showing a detail of the elastic envelope of the mandrel;

Figs. 9 and 10.are sectional views of a mould for the manufacture of hollow concrete bodies for forming floors;

- Fig. 11 is an end view on a larger scale of the structure of Figs. 9 and 10 showing how the longitudinal reenforcements are subjected to tension;

Fig. 11a shows a detail of a modification;

Fig. 12 is a vertical sectional view showing several moulds juxtaposed inside a frame of reenforced concrete provided with jacks;

Fig. 13 is a sectional view on the line l3l3 of Fig. 12;

In the embodiment shown in Figs. 1 to 8, the apparatus essentially comprises an elastic cen tral mandrel consisting of a rubber envelope I disposed on the outer surface of a rigid core 2, and an outer shell 3. Between the mandrel and the shell there is left an annular space 4 in which the reenforcements are disposed and into which 5 the concrete is to be poured. In the example illustrated in the drawings the mandrel consists of several superposed parts, each comprising a core made of a highly resistant concrete ring 5 surrounded by a metal sheet 6 and a rubber envelope I. Each of these envelopes is preferably reenforced by longitudinal rigid rods la (Fig. 8) so as to maintain a cylindrical shape. At their ends these envelopes are connected in a fiuid tight manner to the corresponding ends of elements 6 by means of rubber rings I folded in U-shaped fashion (Fig. 2a) and glued on one side to these metal sheets 6 and on the other side to envelope I. These rings work in the same manner as U-shaped leathers. Pipes 8, extending through the concrete of rings 5 and also through sheets 6, make it possible to inject a fluid under pressure into the interval between these metal sheets and rubber envelopes I. The pile of rings 5 rests on a circular support 9 making it possible to reach the bottom of the apparatus.

Shell 3 consists of a series of staves consisting for instance of sectional irons I 0, on each of which is fixed, on the side of the concrete, a coating II which is resistant to pressure, plastic, and capable of allowing water to leak out therethrough, this coating consisting for instance of plates of hard wood, partly impregnated with bakelite and finally coated with sheets of a polished metal on the side of the concrete, so as to prevent the latter from adhering to the wood. These staves form a shell having the shape of a vertical cylinder. Each of them is kept in position by a'series of screw jacks Ha; the smooth inner end of each of which is maintained in a sleeve I 2 rigidly connected to the corresponding stave. These jacks are screwed in nuts l3 supported by a very resistant outer frame made of reenforced concrete. This frame comprises for instance a series of annular parts M, in which are imbedded the nuts l3, and uprights l5 carrying these annular parts. The screw jacks can be actuated individually for the adjustment of the apparatus, or simultaneously by means of gears l6 and racks H. The annular space that constitutes the mould can be closed at the top and at the bottom by means of lids l8 articulated on staves l0 and rigidly maintained in their closed positions by connecting rods 20.

Before pouring'concrete into the annular space 4, the reenforcements are fixed in position, to wit: the longitudinal rectilinear reenforcements 22 and the circular reenforcements or hoops 2i wound around the first ones. Reenforcements 22 are themselves fixed, by means of buckles provided at their ends, to pins such as 23, securely fixed to the staves owing to their being inserted in holes provided in steel elements In and also on metallic members 24 welded to said elements on the inner side thereof. The longitudinal reenforcements can be subjected to tension before the pouring of the concrete. For this purpose, I provide, at the ends of these reenforcements, members that serve both for fixation for applying the tension and for the anchoring of the tensioned reenforcements in the concrete. These members can be made for instance as follows: The longitudinal reenforcements 22 are coupled in pairs (22a and 22!), Figs. 4 to 6). To the 7 end of each pair of bars there is fixed a stirrup 25 of weldable steel having a resistance at least equal to that of the reenforcements and carrying at its lower end two lugs 26 which act as stops .against reenforcements 22a. and 22b. The ends of reenforcements 22a, 22b are welded at 26 to stirrup 25. Stirrup 25 is then twisted together with the ends of reenforcements 22a and 22b so as to obtain an element such as shown at 21 (Fig. 7) which forms a prolonged part of the pair of reenforcements to be subjected to tension and is provided with a ring 28.

The buckles 28 of the lowerends of the reenforcements are then fixed to the corresponding pins 23. The upper ends of the reenforcements are then gripped below twisted portions 21 by means of a suitable tool 29 and the reenforcements are subjected to the required tension. The upper pins 23, suitably positioned in the mould, are then introduced into rings 28 and tools 29 are removed.

I then place above the mould an annular hopper 30 (Fig. 3), which acts as the supplementary chamber above referred to. The lower edges of this hopper are applied, in a fluidtight manner against the elements II of the staves and elastic envelope I. This hopper is provided with vertical tubes 3| permitting the escape of the air present in annular space 4. The moist concrete is poured into said hopper and into the mould and the whole is vibrated by shocks on the ends of sectional irons III in the direction of arrow 32. and, on the other hand, by producing, through tubes 8 a vibratory injection of non-compressible fluid into rubber envelopes I the upper edge of which is covered by the concrete poured into the mould. It is also possible to produce shocks on sectional irons III in a direction at right angles to the surface of the mould as indicated by arrows 33. The concrete is thus caused to settle in the mould and the excess water is evacuated either through the interstices between the staves or by ascending through the concrete. This treatment is continued until the mould isv full of compact and firm concrete, the concrete also filling the lower part of the hopper. Said hopper will be preferably provided with devices for striking oil the concrete at the lower end of the hopper, these devices consisting for instance of knives such as 34 provided at the lower end of the hopper and striking oil. the excess of concrete at said end of the hopper when the latter is caused to turn around the axis of the mould. The hopper is then removed and the upper lids I8 are closed and strongly fixed in this position by connecting rods 20, compressing the concrete. A fluid (for instance water) under pressure is then introduced into the space between the core and the rubber envelope, with an increasing pressure, until the concrete is sufliciently free from water, which condition is attained in a few minutes. A pressure of 20 kilogrammes per square centimeter has been found to give very good results. Under the effect of this pressure, the excess of water in the mould is driven out through the coating II of the staves.

The concrete being thus rendered compact and hard, I may now proceed to a progressive increase of the outer diameter of the mould, either by further increasing the pressure in envelope I so as to produce, through the concrete poured in the mould, an elastic deformation of rings I4, which subjects the circular reenforcements 2| to the desired tension, or by unscrewing screws Ila by means of racks II, the space between the rubber element I and the core 2 being maintained, during this time, in communication with a hydraulic accumulator at a suitable pressure P. The elastic wooden cushion formed by elements I I is deformed, remaining entirely impervious to 5 concrete but allowing however the whole of the excess of water to flow out through it, as proved by experience. The external diameter of the pipe moulded in the intervening space 4 can therefore be increased by the necessary amount for giving the transverse reenforcements the required elongation.

In order to accelerate the setting of the concrete, I may heat it by means of pipes II b through which a heating fluid is circulated or through any other means such as hot air saturated with humidity present in a closed space surrounding the mould.

When the concrete is sufliciently hard, the fluid under pressure is discharged from envelope I and pins 23 are removed so that the tension of the longitudinal reenforcements 22 is transferred to the concrete. Screws IIa are then fully unscrewed. As the pipe has a diameter larger than that of the central mandrel and smaller than 2 that of the shell, it can be removed vertically from the mould. After cleaning it with water or compressed air, the mould is ready for a new treatment.

In the experiments I have conducted the pres- 30 sures exerted on the concrete ranged between 10 kilogrammes per square centimeter and several hundreds of kilogrammes per square centimeter, but obviously, higher or lower pressures could be employed according to the nature of the concrete 35 and to the result to be obtained.

I might also utilize a mould that is expansible both in the direction of its length and in the radial direction. This will permit of subjecting the longitudinal reenforcements to tension by compression of the concrete and therefore to do away with the preliminary stretching of these reenforcements.

In the embodiment shown in Fig. 2b, the action of screws Ila is replaced by that of wedges IId fixed on annular elements Ida and cooperating with other wedges IIe carried by sectional irons II). The two annular elements Ila can be moved in opposite directions by means of hydraulic jacks Mb, some of which are located between these annular elements while the others are interposed between the ends of these annular elements and stationary plates 0 rigidly fixed to frame I5. Sectional irons II) are supported by square-shaped members b bearing upon plates a through rollers c in such manner that these sectional irons and their supporting elements b can move radially with respect to stationary plates a.

It will be readily understood that if annular elements Ila are pushed away from each other 60 by means of jacks Mb, wedges I Id, which are displaced by these annular elements with respect to the other wedges I Ie, permit a radial displacement of sectional irons III and staves I I.

This embodiment is advantageous because it 65 is easy to simultaneously control all the hydraulic ac The apparatus might be further simplified as shown in cross section in Fig. la. In this case, core 2, its elastic envelope I and staves Illa are 70 completed by a second elastic envelope I50 placed on the outside of the staves, between said staves and a rigid annular element I5. A fluid under pressure can be injected between envelope I So and element I5 and also between core 2 and 75 elastic envelope I. The staves illa consist of sectional irons of a shape similar to that of arch-stones. The intervals between these staves can be-fllled with plugs 10b of a matter that is 5 elastic and permeable by water, such as wood,

eventually. coated with metal sheets which facilitate the assembling of the staves. Concrete is poured into space 4 as above explained.

Between [5 and [5a I inject fluid at pressure P which acts on the staves in an inward direction. Between I and 2 I inject fluid under pressure which compresses the concrete at a pressure P1.

As long as Pl is smaller than P the deformation of the wholeis very small.

If P1 becomes smaller than P, the diameter of the mould increases, the staves being pushed apart from one another, so that the concrete of the pipe expands, thus subjecting its reenforcements to tension. This embodiment, which is very simple and relatively inexpensive, has another very important advantage. It makes it possible to adjust the deformation of the pipe or any other body to any desired value which can be immediately checked up. For this purpose,

it sufiices to provide an auxiliary reservoir 85b the volume V of which is definitely known and which can be connected through a valve I50 with envelope l5a-. If this valve He is opened at a certain time, this causes the transfer into this reservoir, which is supposed to be empty, of known volume of fluid and a corresponding deformation of the pipe. If the fluid that is utilized is an incompressible fluid, the deformation of the pipe will correspond to a volume equal to the volume V of the reservoir.

Figs. 9 to 15 illustrate the manufacture of hollow bodies for making floors.

In this case, the mould includes two lateral walls 35, preferably grooved so as to facilitate '45 walls, which serve also for the fixation of the ends of elastic mandrels 38, which are disposed parallel with the rectilinear reenforce- 'ments 31. Transverse reenforcements 40, which may be of undulated shape, as shown in the drawings, are placed around the mandrels and.

between them. The rectilinear reenforcements 31, which are provided, for instance, at their ends with twisted elements 21, made for instance as above described with reference to Figs. 4 to 7.

are subjected to tension by means of gripping members such as ll and fixed to the end elements 36'. These reenforcements can also be subjected to tension by fixing their ends to nuts a in which are engaged screws Mb bearing against the wall 36 of the mould (Fig. 11a).

The concrete is then poured into the mould. Upon. the concrete, rendered compact by the vibrating treatment, I then place the upper plates 39 of the moulds. In a very strong frame 42 made of steel or of reenforced concrete, I then juxtapose several of these moulds A horizontally, as shown in Fig. 13 and I juxtapose several of these horizontal rows B (Fig. 12). Frame 43 is provided, on its inner faces, with jacks 43 which may be applied against the wall of the pile of moulds with a variable pressure. Once the moulds have been inserted into this frame, I close it by means of plates 42a and I block the system by means of jacks 43a. The concrete in the moulds is first subjected to compression by introducing a liquid under pressure into the elastic mandrels. This compression has for its effectto drive out the excess of water in the concrete. I then release the jacks that bear against the lateral walls of the moulds 35 and plates 39, 39a (all the jacks shown in Fig. 12), maintaining the water pressure in the elastic mandrels. or giving any desired value to this. pressure. Consequently, the concrete is deformed around the elastic mandrels, which subjects the transversal reinforcements to tension. Once the con.- crete is sufliciently hard, pressure is allowed to drop in the jacks and in the mandrels and the finished pieces can be removed from the moulds.

Of course the process according to the present invention could be applied to the manufacture of any other kinds of hollow bodies.

In the preceding description, it has been stated that concrete that has not yet set can acquire. under the effect of compression a resistance sufficiently high for transmittmg the necessary deformations to the reinforcements and therefore subjecting them-to tension. Of course, the principle of my invention is not modified if the deformation of the concrete is produced when the concrete has already begun to set, by making use, for this purpose, of a compression of said concrete suflicient for producing its deformation in all directions without breaking.

While Il'have described what I deem to be preferred enrbodiments of my invention, it should be well understood that I do not wish to be limited thereto as there'might be changes made in the arrangement, dispositionand form of the parts without departing from the spirit of my invention as comprehended within the scope of the appended claims.

What I claim is:

1. A process of manufacturing hollow reenforced concrete bodies, which comprises, placing in an annular mould having a deformable outer shell an annular reenforcment, pouring concrete into said annular mould, subjecting the concrete to a treatment for removing the water in excess that it contains, subjecting it to an outward radial compression so as to deform both the concrete in the mould and the deformable shell and thus to subject said reenforcements to tension, and maintaining this compression for a time sumcient for permitting the concrete to harden to such a degree that the tensioned reenforcements cannot penetrate into said concrete.

2. A process of manufacturing hollow reenforced concrete bodies, which comprises, placing in an annular mould having a deformable outer shell and a deformable mandrel annular reenforcements, pouring concrete into said annular mould, subjecting the concrete to a treatment for removing the water in excess that it contains, deforming the mandrel so as to subject the concrete in the mould to an outward radial compression sufiicient for deforming both the concrete and the deformable shell of the mould whereby said reenforcements are subjected to tension, and maintaining this compression for a time suflicient for permitting the concrete to harden to such a degree that the tensioned reenforcements cannot penetrate into said concrete.

3. A process of manufacturing hollow reenforced concrete bodies, which comprises, placing between a deformable wall and a deformable mandrel a non-rectilinear reenforcement, pouring concrete into said mould, subjecting said concrete to a treatment for removing the water in .exoess that it contains, deforming the mandrel so as to subject the concrete to a compression sufiicient for deforming both said concrete and the deformable mould and to give the desired tension to the reenforcement, and maintaining this compression for a time suflicient for permitting the concrete to harden to such a degree that the tensioned reinforcements can no longer penetrate into the concrete.

4. A process according to claim 3 in which the compression of the concrete which subjects the reenforcement to tension takes place before the concrete has set.

5. A process according to claim 3 in which the treatment for removing the water in excess in the concrete consists in vibrating the mould.

6. A process according to claim 3 in which the treatment for removing the water in excess in the concrete consists in subjecting said concrete to a moderate pressure.

'7. A process according to claim 3 in which the treatment for removing the water in excess in the concrete consists in simultaneously subjecting said concrete to a moderate pressure and vibrating the concrete in the mould.

8. A process according to claim 3 which further comprises first placing rectilinear reenforcements in the mould and subjecting them to tension before pouring the concrete into the mould.

9. An apparatus for the manufacture of hollow reenforced concrete articles, which comprises in combination, at least one expansible mandrel, means for expanding said mandrel, a deformable shell surrounding said mandrel consisting of a plurality of juxtaposed staves, a frame movably supporting said staves, and means interposed between said frame and said staves for controlling the relative position of said staves with respect to said frame.

10. An apparatus for the manufacture of hollow reenforced concrete articles according to claim 9 in which the last mentioned means consist of screw jacks.

11. An apparatus for the manufacture of hollow reenforced concrete articles according to claim 9 in which the last mentioned means com- 5 prise annular elements longitudinally movable in said frame, coacting wedges carried by said annular elements and said staves, and means for moving said annular elements in a longitudinal direction with respect to said staves.

12. An apparatus for the manufacture of hollow reenforced concrete articles according to claim 9 in which the last mentioned means comprise a tubular element surrotmding said staves, an elastic envelope interposed between said tubular element and said staves, and means for injecting a fluid under pressure between said tubular element and said envelope. 7

13. An apparatus for the manufacture of hollow reenforced concrete articles according to 20 claim 9 further comprising means for heating the concrete.

14. An apparatus for the manufacture of hollow reenforced concrete articles according to claim 9 in which the expansible mandrel comprises a rigid core and a fluidtight elastic envelope surrounding said core, the means for expanding said mandrel including conduits for feeding a fluid under pressure between said core and said envelope.

15. An apparatus for the manufacture of hollow reenforced concrete articles according to claim 9 which further comprises a hopper located above said mandrel and said shell, means for connecting the lower end of said hopper with 35 the upper ends of said mandrel and said shell. and means for vibrating both the hopper and the concrete located between said mandrel and said shell.

EUGENE FREYSSINET. 4o

Referenced by
Citing PatentFiling datePublication dateApplicantTitle
US2474660 *Jul 19, 1947Jun 28, 1949Rocla LtdManufacture of prestressed concrete pipe and the like
US2483175 *Oct 10, 1947Sep 27, 1949Vacuum Concrete IncMethod of molding prestressed structures
US2513527 *Jun 5, 1946Jul 4, 1950Viktor Sjodin OlofApparatus and method for molding hollow structures of concrete and the like
US2569612 *Oct 27, 1945Oct 2, 1951Pont A Mousson FondManufacture of reinforced concrete pipes
US2579801 *Feb 10, 1949Dec 25, 1951Curzon DobellConcrete pipe mold
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US4840200 *Oct 19, 1987Jun 20, 1989General AtomicsPrestressed tube
US4936006 *Mar 1, 1989Jun 26, 1990General AtomicsMethod of making prestressed concrete articles
US5065795 *Mar 5, 1990Nov 19, 1991General AtomicsPrestressed concrete articles
US7748972 *Jun 1, 2005Jul 6, 2010Carlos Fradera PellicerTensioning installation for the frameworks of pre-tensioned architectural elements
WO1988007934A1 *Apr 7, 1988Oct 20, 1988Gen AtomicsPrestressed concrete articles and methods for manufacturing such articles
Classifications
U.S. Classification264/71, 425/DIG.110, 52/223.3, 264/314, 425/128, 425/111, 264/228, 425/432
International ClassificationB28B23/12
Cooperative ClassificationB28B23/12, Y10S425/11
European ClassificationB28B23/12