US 2080074 A
Description (OCR text may contain errors)
m L W mm mm 5 R mm .B m I P.
May 11, 1937.
Filed March 8,
Patented May 11, 1937 PATENT 'QF Fi'C-E 2,080,074 in enter: or 'REENFORCED CONCRETE Eugene Freyssinet, Neuilly-sur-Scine, and Jean Sailles, Paris, France Application March 8, 1934,
Serial No. 714,724
In France October 2, 1928 7 Claims.
' The present application is a continuation in part of the U. S. patent application Ser. No.
395,297, filed Sept. 26, 1929 in which we described 10 tained by means of thissprocess.
It has been proposed 'to subject the ree'nforce ments of reenforced concrete articles to preliminary tensions before pouring the concrete into the moulds and then to transfer this tension of the reenforcements to the concrete after setting and hardening of the latter, by releasing the devices by means of which the reenforcements were subjected to tension. However, if special precautions are not taken, no permanent improvement of reenforced concrete articles can be obtained in this way. This is due to the fact that, when the tensions of the reenforcements are transferred to the concrete, numerous causes intervene for reducing this tension:
First of all, in the course of the setting and Cal hardening of the concrete, the dimensions of.
the latter decrease, due to a certain phenomenon, which is called shrinkage.
Secondly, concrete compressed by reenforcements that have been subjected to tension is elastically deformed- Furthermore, a certain sliding displacement of the reenforcements with respect to concrete may occur in spite of anchoring members disposed on the reenforcements, until, due to the pressing together of the particles of concrete by the anchoring members, in the vicinity of these members, this sliding displacement ceases.
Finally, we discovered that, in concretes subjected to constant stresses, the concrete mass undergoes a progressive reduction of length analogous to a kind of shrinkage proportional to the stresses and which may be numerically more important than ordinary shrinkage, in the case of high tensions (see report of the session of September 22, 1926 of the Commission Technique de la Chambre 'Syndicale des Constructeurs en "Ciment Arm de France; Report of the Reenforced Concrete Convention, Liges 1930; Report of the Convention of the Association Internationale des Ponts et Charpentes, Paris, 1932).
The sum of these drops of tension can amount to 30 kilogrammes per square millimetre, in the case of very resistant concretes, and even much 5 more, in the case of ordinary concretes. Consequently, if the tensions to which the reenforcements are. subjected are lower.' than 30 or 40 kilogrammes per square millimetre, a time will come when these tensions will be reduced to zero by the total shrinkage .of the-concrete, so that the increase of resistance 'which was intended to be obtained by subjecting the reenforcements to tension is destroyed and constructions including such concrete pieces may be ruined.
, Now, the reenforcements that are employed at the present time are made of metals the elastic limit of which is, as a rule, much lower than 40 kilogrammes per square millimetre. In ordinary reenforced concrete, it is not known how to utilize the resistance of steels having higher elastic limits and the use of such steels is therefore unnecessary. The present regulations concerning concrete advise not to exceed stresses of 16,000 lbs. per sq. in. (which corresponds to 11.20 kilogrammes per square millimetre) for all kinds of steels (see H001 and Johnson Concrete Engineers Handbookf, page 37, section 56). Therefore, the use of steels of high elastic limit has not developed in the ordinary technic of reenforced concrete, the fact being that the reenforcements utilized in the ordinary technic are of relatively low elastic limit. Now, with such reenforcements, it is impossible to bring into play tensile stresses sufiiciently high for obtaining permanent improvements.
The object of the present invention is to provide reenforced concrete objects that are improved in a permanent manner, owing to the utilization of reenforcements having a sumcientlyhigh elastic limit in order that these reenforcements may be subjected to preliminary tensions which considerably exceed the sum of the possible drops of tension and that, account being taken of these drops of tension, there may remain in these reenforcements high permanent tensions which are an important portion of the initial tension imparted to the reenforcements.
Therefore, in a reenforced concrete article according to the present invention, the concrete of which has set and hardened and has undergone all the shrinkages and reductions of length above referred to, there remain permanent ten sions'of the reenforcements, which exert' on the concrete of the article permanent compressions. Owing to a suitable distribution of the tensioned reenforcements in the concrete, these compressive stresses areopposed to the stresses created in the concrete article by the external forces acting thereon, so as to compensate them.
80 kilogs. and even more.
Hie reenforcements utilized according to the present invention are made of metals having elastic limits which are considerably greater'than 40 kilo'gs. per square millimetre, so that they may be subjected to tensions at least equal to this value and generally much higher, say 60 kilogs., Of course the elastic limit can be increased by suitably treating the metal, for instance by previously drawing or tem-. pering' it. The, increase of the elastic limit may also result from the preliminary tensioning treatment itself, provided, of course, that the preliminary tension is-lower than the ultimate strength of the metal. The reenforcements that are in contact'with the concrete may be provided with anchoring members for preventing them from sliding with respect to the concrete. However, in most cases, the adhesion of reinforcement bars to concrete constitutes a sufilcient anchoring. I Figs. 1 to 4 of the accompanying drawing show, by way of example, two embodiments of devices serving to subject the reenforcements to tension. In Fig. l, the mould I, in which concrete is to be poured, comprises, at one end, a movable part 2. The ends of the reenforcements 3, which may be of unequal lengths and which are distributed in the concrete mass according to the diagram of the stresses to be created, are fixed, on the one hand to the mould through any suitable means such as gripping members 4 holding tightly the ends of the reenforcements (see the detail view of Fig. 2 which/is a section on the line 2-2 of Fig. 1') or any other equivalent organ, and, on the other hand to the movable part 2 by means of similar gripping members ia. This movable pa- 2 is connected to the piston of a powerful draulic-jack 5a the. cylinder oi which is stationary while the mould is fixed, at the opposite end, to a stationary point 5. The reenforcements, made of a metal of high elastic limit, are first subjected to the required preliminary tensions by admitting into the jack a liquid under suitable pressure, which displaces the movable part 2 and stretches the reenforcements. Concrete is then poured into the mould, the reenforcements being kept tensioned. When concrete has set and hardened, the pressure in jackfia is brought back to zero and consequently the tension of reenforcements 3 is transferred to the concrete. The movable part 2 is then removed, after the means 4 for fixing the reenforcements have been removed and the concrete piece can be taken off from the mould. Reenforcements of unequal lengths must be used when the cross section of the piece is not uniform in order that concrete may nevertheless be subjected to a uniform compression. There is for instance the case of a post of the shape of a frustum of a cone or of a frustum of a pyramid. This post will be provided with tensioned reenforcements 3 (Fig. 3) of unequal lengths. The number of these reenforcements in a cross section of the post is larger at the big end of said post than at'the small end thereof. With tensioned reenforcements all of the same length, it
would be impossible to subject the concrete to high compressive stresses without risk of crushing the concrete at the small end of the post. In the example of Fig. 4, the ends of the .reenforcements 3 are imbedded in blocks of concrete 8 by providing in said blocks suitable gripping means or means through which the blocks can be pulled away from each other, for instance holes such as I. Once the concrete 9; ill? blocks has set and hardened and is sufliciently strong, the reenforcements 3 are subjected to the required tensions by moving blocks 6 away from each other, for instance by means of powerful hydraulic jacks. The reinforcements being thus kept in the tensioned state, we dispose between blocks 6 walls 8 forming the mould and concrete is poured thereinto. When concrete has set'and hardened, walls 8 are removed and the pressure exerted by the hydraulic jacks is released. The reenforcements tend to shrink, and thus to bring blocks 6 toward each other, thus exerting a compression onthe concrete in the mould.
In the example of Figs. 1, 2, and 3, anchoring members prevent the tensioned reenforcements from sliding with respect to the concrete. These members will consist of hooks 9 provided at the ends of the reenforcements or of any other organ, either integral with or. fixed to said reenforcements, for instance local thickened portions thereof.
In the example of Fig. 4, the anchoring members consist of the two concrete blocks 6, in which the reenforcements will be, in turn anchored, by means of hooks or the equivalent.
For practical purposes, we generally use carbon steels in the form of wires ofdiameters ranging between 6 and 15 millimetres, cooled rather suddenly in the course of rolling, so that the ultimate strength of these steels ranges between and kilogs. per square millimetre and their elastic limit ranges between 45 and 50 kilogrammes per square millimetre. This elastic limit is increased up to 80-90 kilogrammes per square millimetre. have been subjected to'thermic treatments so that their ultimate strengths may be as high as kgs. per square millimetre, particularly in the case of special steels, such as silicon steels. The tensions that give the best results range between two-thirds and three-fourths of the elastic limit of the metal, so that after all the drops of tension above referred to, there remain permanent tensile stresses of at least 30 or 40 kilogrammes per square millimeter. In such products, the normal working fatigue should range between A; and 2/5 of the ultimate strength of the steels, which permits of subjecting the steels to stresses of 50 and 60 kilogrammes per square millimetre and even more without any risk of cracking of the concrete.
The products obtained according to the present invention differ from ordinary reenforced concrete products in that their strength is considerably higher than that of an ordinary reenforced 1. A piece of reenforced concrete which comprises set and hardened concrete having undergone shrinkage, and reenforcements of a steel having a high elastic limit in permanent tension anchored in said concrete and adhering thereto along their whole length, whereby said concrete is permanently subjected to compressive stresses.
We may also use steels that '1.
2. A piece of reenforced conlcrete which comprises set and hardened concrete having undergone shrinkage, and reenforcements of a steel having a high elastic limit in permanent tension anchored in said concrete and adhering thereto along their whole length, said reenforcements being tensioned and distributed in accordance with the diagram of the tensile stresses that are to be developed in the piece by determined ex thereto along their whole length, said reenforce-,
20 ments being in permanent tension.
4. A piece of reenforced concrete which comprises permanently compressed set and hardened concrete having undergone shrinkage, and reenforcements of a metal the elastic limit of which 25 is higher than 40 kilogrammes per square millimeter, anchored in said concrete and adhering thereto along their whole length, said reenforcements being in permanent tension at a rate of at least kilogrammes per square millimeter.
5. A piece of reenforced concrete which comprises permanently compressed set and hardened concrete having undergone shrinkage, and reenforcements of a metal the elastic limit of which is higher than 40 kilogrammes per suare milli- 3 meter, anchored in said concrete and adhering thereto along their whole length, said reenforcements being permanently tensioned, at a rate of at least 10 kilogrammes per square millimeter, and distributed in accordance with the diagram 40 of the tensile stresses that are to be developed in the piece by determined external forces acting thereon, so as to produce in the concrete permanent compressive stresses that balance these tensile stresses.
6. A piece of reenforced concrete which comprises permanently compressed set and hardened concrete having undergone shrinkage, and a plurality of rectilinear reenforcements made of a metal the elastic limit of which is higher than 40 kilogrammes per square millimeter, anchored in said concrete and adhering therewith along their whole length, said reenforcements being in permanent tension at a rate of at least 10 kilogrammes per square millimeter, the length, distribution and tension of these reenforcements being so chosen, in accordance with the system of tensile stresses to be developed in the piece by determined external forces acting thereon, as to produce in the concrete permanent compressive stresses that balance these tensile stresses.
7. A piece of reenforced concrete which comprises permanently compressed set and hardened concrete having undergone shrinkage, and a plurality of parallel rectilinear reenforcements made of a metal the elastic limit of which is higher than 40 kilogrammes per square millimeter, anchored in said concrete and adhering therewith along their whole length, said reenforcements being in permanent tension at arate of at least 10 kilogrammes per square millimeter, the respective lengths and tensions of these reinforcements being so chosen, in accordance with the system of tensile stresses that are to be developed in the piece by determined external forces acting thereon, as to produce in the concrete permanent compressive stresses that balance these tensile stresses.
' EUGENE FREYSSINET.