US2957240A

US2957240A - Method of making concrete reinforcing elements from ribbed steel bars

Info

Publication number: US2957240A
Application number: US604705A
Authority: US
Inventors: Robert A Brandes; Neumann Willy
Original assignee: Individual
Current assignee: Individual
Priority date: 1956-08-17
Filing date: 1956-08-17
Publication date: 1960-10-25
Anticipated expiration: 1977-10-25

Description

Oct. 25, 1960 Filed Aug. 17, 1956 R. A. BRANDES ETAL 2,957,240 METHOD OF MAKING CONCRETE REINFORCING ELEMENTS FROM RIBBED STEEL BARS 2 Sheets-Sheet 1 INVENTOR 8 ROBERT A. BRA/V055 W/L L Y NEUMA/V/V BY 741m," 72%;

ATTORNEYS Oct. 25, 1960 R. A. BRANDES ETAL 2,957,240

METHOD OF MAKING CONCRETE REINFORCING ELEMENTS FROM RIBBED STEEL BARS Filed Aug. 17, 1956 2 Sheets-Sheet 2 Fig.6!) H950 \\\/Z INVENTORS ROBERT A. BRA/V055 N/V L \\j 5 W151; Y NI'UMA I I BY Jaabum ATTORNEYS nite ice

Patented Oct. 25, 1960 NIETHOD OF MAKING (IONCRETE RElNFORClNG ELEMENTS FROM RIBBED STEEL BARS Robert A. Brandes, 32 Jaegerstrasse, Leverkusen, Germany, and Willy Neumann, 5-7 Von Sandtplatz, Koeln, Germany Filed Aug. 17, 1956, Ser. No. 604,705

1 Claim. (Cl. 29-5523) This invention relates to a method of making concrete reinforcing elements, and more particularly to such elements which are made from warm-rolled ribbed steel bars, and are resistant to aging and fractures after storage.

This is a continuation-in-part of our pending patent application Serial No. 485,284, filed on January 31, 1955, now abandoned.

It is an object of this invention to provide a method of making concrete reinforcing elements from ribbed steel bars, which elements can be subject to bending and unbending after prolonged storage without showing a tendency to fracture due to shortness of the material developed by aging.

It is also an object of this invention to provide a method of making concrete reinforcing elements made from steel bars of the aforesaid type made from steel produced with aid of a blast engine using pure oxygen or oxygenenriched air, Siemens Martin steel, which has been further deoxidized, and other kinds of steel having -a greater resistance to aging than normal commercial steel made by the Thomas Gilchrist process.

It is another object of this invention to provide a method of making concrete reinforcing elements from ribbed steel bars, which elements are to be used for reinforcing concrete and which are particularly free from shortness, when being bent or unbent for embedding in armored concrete before the latter is setting.

It is the general tendency in building with reinforced concrete to employ reinforcement bars, hooks and the like made from steel of high strength, in order to be able to reduce the diameter of the steel bars used.

Since alloyed steel, i.e. steel containing nickel, chromium, and other metals, is too expensive, it is common practice to use a steel which has been hardened by coldstretching, for instance, by cold-drawing through a matrix die. A smiliar hardening effect is also achieved by coldtwisting steel bars of Wires.

However, these methods of cold-stretching or coldtwisting iron or steel bars suffered from the drawback of damaging the outer or skin layers of the rods or bars, and it was recommended to chemically or mechanically roughening the bars by applying nicks, burs or notches therein before cold working them (Swiss Patent No. 161,760 to Ernst Schoch A.G.). It was also hoped to increase the adherence of the concrete to the bars by subjecting the latter to such treatment. For steel bars made of cold-stretched high quality steel having the aforesaid properties show considerable elongation when they are highly stressed. They show this elastic elongation also when embedded as reinforcement elements in reinforced concrete. However, due to its shortness, the surrounding concrete cannot follow the elongation of the steel, and cracks and fissures are formed in the concrete which are the wider, the more the reinforcement bars are subjected to stress. If the fissures become too wide, there is danger that humidity or corroding gases may come in contact with the steel and attack it. Therefore there is a limit to the degree of hardness of the steel used for reinforcement bars and the like objects, which limit is set by the necessity to avoid the occurrence of cracks of excessive width in the concrete. Cracks in the concrete are only admissible if they are kept so narrow that no detrimental influences can reach the embedded steel.

The problem how to use steel of sufiicient but not excessive hardness, and to prevent at the same time the formation of cracks of excessive width in the concrete has only been partially solved in the art.

Apart from the abovementioned mechanical or chemical surface treatment, it was proposed by E. Hoifmann (U.S. Patent 2,260,779) to cold roll a number of short indentations into a rod on opposite sides thereof, which cold rolling may or may not be accompanied by a simultaneous stretching of the rod to a greater length. The rod was then subjected to cold twisting. While app-arently some of these purely cold-treated rods withstand subsequent stretching during the use of the rods without fracture, they suffer from a serious drawback which will be explained below.

The same drawback is inherent in reinforcing elements produced by a method preferred in the more recent art of making reinforcing elements, which method is developed by ISTEG Steel Corporation (U.S. Patent 2,256,- 060; 2,324,651 and 2,405,274). This method provides for first rolling, for instance, by a conventional warm or hot rolling or other usual rod rolling method steel bars having integral therewith a plurality of longitudinal, helical, or transverse ribs, or a web of several of the aforesaid types of ribs.

The longitudinal rib spacing and the helical pitch of such ribs is, for instance, defined under ASTM norm A 305.

The. preferred elastic properties of concrete steel II are in the order of an elasticity limit of up to 36 kg./mm. those of concrete steel III up to 42 kg./mm. and those of concrete steel IV up to 50 kg./mm.

As a further step in the ISTEG process this extension of the elasticity limit of normal Thomas steel from the average 27 kg./mm. to the above-mentioned higher limit was achieved by cold twisting the same, thereby preferablymaintaining the overall length of the steel bar or rod while simultaneously lengthening the outer or skin layers of the bar and, in particular, the protruding n'bs through the twisting action.

Due to their ribbed surface such bars are intimately combined with the concrete, and the cracks being formed when the steel bars are elongated under stress, remain very narrow, a great number of such small or hair cracks being formed. Thereby each crack is widened only corresponding to a very small portion of the total elongation of the reinforcement bar caused by stress. The problem of preventing the formation of excessively wide cracks of fissures in the concrete was thus solved. However, a new one arose immediately therefrom.

For the ribbed and twisted bars in particular of the harder concrete steels III and IV show a considerable deterioration as to strength. Furthermore, these reinforcement bars are usually not embedded in the concrete in the form of straight bars, but are bent to form reinforcement elements of a shape to be discussed hereinafter. Uusally this bending does not take place directly following the rolling and twisting process, but often after a certain time of storage. It is then done with the aid of a bending machine, for instance, on the building site. Very often the bending step requires additional unbending when the reinforcement hooks are to fit properly into a predetermined concrete casing or lining. Bending such reinforcement elements so as to fit exactly into predetermined casings would be completely uneconornical. They are usually bent to fit by hand when being mounted in the casing before embedding them in the concrete.

It was soon found that bent as well as bent and twisted reinnforcement bars provided with the above-mentioned ribbing tend, after a relatively short time of storage, for instance, after only 3 hours, somewhat depending .onthe nature of the steel used, to develop weak-spots atthe bent portion, which lead to breaking of the bars. This breaking takes place in particular when such bent bars are slightly unbent or bent in a different direction from the one originally applied. This serious'drawback has so far been a most serious obstacle to the use of ribbed hardened steel bars especially of the harder concrete steel types III and IV as concrete reinforcing elements.

It was now to be expected that further cold deformation of ribbed steel bars would lead to an additional weakening of the material, since it is well known that cold-worked steel shows aging after a certain period. This aging is due to a recrystallization process in the steel which implies to the latter a high notch sensitivity, which leads easily to fractures, for instance, under the impact of a sudden shock or fall.

This notch sensitivity is the more pronounced, the lower the temperature at which the element is bent and/or rebent, and/or subjected to sudden stresses.

Cold-worked steel having a smooth surface shows this drawback to a considerably smaller degree than ribbed steel, as each rib in the steel surface represents a notch reducing the diameter and weakening the steel bar. When such steel bars are twisted together or bent, the high notch sensitivity of the steel causes the latter to develop fine cracks and eventually to break under the stress. It was therefore, a particularly dangerous drawback of ribbed and ribbed and cold-twisted reinforcement bars that they would be built in their weakened condition into ferro concrete structures, and then break after some time, under suddenly increased stress.

These drawbacks are avoided and the objects stated hereinbefore are attained by our invention which provides a method of making concrete reinforcing elements from rolled, and preferably hot rolled ribbed steel bars, which method is based on our discovery that steel bars which have been ribbed in particular by hot rolling, are free from the above described aging phenomena, if these bars are subsequently cooled and then cold-stretched beyond the elasticity limit (or yield point) to such an extent that a permanent elongation in the order of 10 to 15% of the length of the hot-rolled ribbed bar is achieved. The preferred elongation of the bar is about 12.5%.

Ribbed steel bars which have thus been cold stretched, after cooling off following the hot rolling, to be permanently elongated by 10 to 15% and preferably about 12.5%,'can be safely bent to the conventional shape of reinforcement elements for ferro-concrete structures. None of the above described dangerous effects of aging will occur, even though the bars have been stored over a long period of time, and are then bent and rebent.

In addition to the step of cold-stretching a ribbed steel bar, the latter may, in a further step, be subjected to a moderate twisting in the cold. This will lead to a further increase in hardness of approximately 5 to over the increase achieved by. cold-stretching the ribbed steel. However, this additional step also causes an increase in notch-sensitivity due to the above explained detrimental aging effect. It may, therefore, be more desirable to omit drilling altogether and to forego the lastmentioned slight increase in hardness.

The strength of a reinforcement element according to our invention may further be increased by using steel which has been produced with aid of a blast engine using pure oxygen or air enriched with oxygen. Furthermore, steel may. be used which has been largely deoxidized. Steel produced by the Siemens Martin'process also shows a, greater. resistance against aging than normal commercial steel made by the Thomas Gilchrist process. This 4 property of Siemens Martin steel may be still improved by further deoxidation.

Although the making of the last-mentioned kinds of steel is somewhat more complicated and expensive than that of normal commercial Thomas steel, the result obtained when making ribbed reinforcement elements by the process according to the present invention from such kinds of'steel is even more satisfactory.

' Another feature of the invention is based on our discovery that'the cold-stretching step which leads to a straight elongation of the rolled and cooled ribbed steel bar by about 10 to 15% can be carried out at great speed. Surprisingly, quick stretching of the bars does not lead to an increase in notch sensitivity at the foot of the ribs, i.e. tendency to cracking and breaking of the bar when under bending stress.

On the contrary, quick. stretchinghas the remarkable efiect of further favorably increasing the yield point and the overall strength of the bar.

While, in normal stretching, the time used for effecting an elongation of 10-15 is '3 minutes, we now bring about this same elongation in 9 to about 30 seconds. This is approximately the top speed at which present stretching machines can be operated. 7

According to another feature of the invention, when, for instance, we use a steel of slightly increased carbon content, such as 0.08 to 0.10% C, instead of the normal soft Thomas steel containing 0.05 to 0.07% C, 0.3 0 to 0.40% Mn, and traces of silicon, we cold-stretch the rolled and cooled ribbed bar at high speed, within, for instance, about 10 seconds, to an elongation of 8 to 9% of the length of the cooled ribbed bar. Thereby we obtain a cold-stretched bar of maximum uniformity.

making the ribbed reinforcement elements, according to content and a correspondingly higher initial yield point before cold-stretching, the treatment according to the invention will further increase this yield point, up to about kg./mm. At the same time the elongation required can be further reduced to about 4 to 6% of the length of the cooled ribbed bar.

Thus, a ribbed bar made of steel containing 0.25 to 0.30% C, 1.0 to 1.30% Mn and 0.30 to 0.70% Si which has a yield point of 42.2 l-;g./mm. is cold-stretched to obtain a linear elongation of 4.3 to 6.5%, whereby a steel bar is obtained which has a yield point of 74.9 to 80.6 kg./mm. and a tensile strength of 75.4 to about 81 kg./mm. (breaking point). I

The ribbed and stretched reinforcement elements according to the invention can be stored for any desired time and remain free from the dangerous notch sensitivity due to aging, and the consequent tendency to crack and break when being bent or dropped and otherwise subjected to sudden shocks. In particular, they are free from that drawback, even when bent and/ or rebent at low temperatures, for instance, below the freezing point (+5 to 5 C. and lower). The bars can be bent to any desired angle up to a right angle, although the normal bending angle is an angle of 30 to 60 and preferably 45. Standard rules require that re-bending about 22.5 should be sustained. Actually the bars, manufactured according to the invention, which were first bent to form an angle of 45, sustain re-bending to a straight bar and further bending in opposite direction by 45 without cracking.

The invention will be better understood from the further description thereof in connection with the accompanying drawings in which;

Figure l is a lateral view of anembodiment, by way of example only, of a ribbed steel bar, according to the invention, having two longitudinal and a-plurality of obliquely transverse ribs; 7

Figure 2 illustrates the bar shown in Figure; l viewed from a direction at right angle to theformer view;

If, according to yet another feature, the steel used for the invention, has an even higher carbon and manganese Figure 3 is a partial cross-sectional view of the bar shown in Figure 1, through an obliquely transverse rib portion thereof;

Figure 4 is a partial cross-sectional view of the bar shown in Figure 1, through a longitudinal rib portion thereof;

Figures 5a to d illustrate the various steps in making a reinforcing element according to the invention;

Figures 6a to show schematically a number of differently bent reinforcement elements according to the invention;

Figures 7a to a show the various steps of a bending test carried out with reinforcing elements, made by the method according to the invention.

In the drawings, Figures 1 to 4 illustrate portions of a ribbed steel bar as it is obtained, for instance, by hot rolling a steel bar of normal Thomas steel, and subsequent treatment according to the invention.

The cylindrical bar or rod 1 is provided with two

longitudinal ribs

2 and 3, and a plurality of obliquely

transverse ribs

4 and 5, the latter being disposed on the reverse side of the bar. In the specific embodiment of a bar, shown in Figures 1 to 4, the transverse ribs 4 and have a distance of 0.7D from one another, D being the diameter of the bar, and form an angle of 66 with the

longitudinal ribs

2 and 3. Also, the

transverse ribs

4 or 5 have sloping side walls forming an angle a of, for instance, 45 with the surface of the basic cylindrical rod body, while the

longitudinal ribs

2 or 3 form a steeper angle 3 therewith (Figures 3 and 4). However, it will be understand, that these dimensions are only given by way of example, and could be varied within relatively wide limits, but correspond closely to oflioial standard requirements, for instance ASTM A 15-57 T and A 305-56 T in the United States and DIN 1045 and 4225 in Germany.

Also bars according to the invention could be provided exclusively with longitudinal, or transverse, or obliquely transverse ribs.

In ribbed bars of this type which have not been treated by the process according to our invention, but are directly twisted and then stored and later bent, notch sensitivity develops due to the aging process, usually during the first three hours of storage, at the foot of the ribs indicated by reference numeral 6.

The process for making the reinforcing elements according to the invention is illustrated in Figures 5a to 5d and comprises in combination the steps of hot rolling, for instance, a cylindrical steel bar as shown in Figure 5a to obtain a ribbed steel bar of the general type shown in Figure 5b and having a length L This bar is, of course, somewhat hot stretched by the rolling process itself. The ribbed bar (Figure 5b) is then allowed to cool and, as an important step according to the invention, cold-stretched to give that bar an elongation AL, i.e. a total length L which brings about the advantageous features of making the bar age-resistant or nonaging as described hereinbefore in detail.

The following optional step of twisting the cold-stretched bar (5c) leads to a bar having a helical longitudinal flange, as illustrated in Figure 5d.

The bars may then be stored and, for instance, after transportation to a building site, may be bent there to a desired shape and rebent in another, for instance, in opposite direcu'on, where this is required to fit the bars into the concrete casing.

Of course, the bending step may also be carried out before the bars are stored, so that only the rebending would have to be done at the building site.

Several conventional shapes of reinforcing bars are illustrated in Figures 6a to 0!.

Thus, Figure 6a, shows schematically a bar having two bends at A and B, forming an angle of deviation of 45 from the main direction in which the bar extends. The curvature of the bend is such that the radius R of the bending circle is smaller than 101), D being the diameter of the bar. In the example shown in Figure 6a, R is equal 5D, which is far smaller than minimum bending radius 10D conventionally required.

Figure 6b shows schematically a hook-shaped reinforcement element having four bends of 45 each and two bends of each.

Figure 6c shows schematically a reinforcement element having four bends of 30 each.

Figures 7a to 7d illustrate the various steps carried out in testing reinforcing elements made by the method according to the invention.

A number of such tests were carried out to determine the degree of resistance achieved in the reinforcing elements according to our invention. These tests comprised bending of straight ribbed and cold-stretched bars (Figure 7a) around a standard bending block having four times the diameter of the bar to be bent (R=4D). Officially prescribed resistance tests usually require'a standard diameter of the bending block for test purposes of six to seven times the diameter of the bar, while in actual building the reinforcement hooks are shaped around a bending block of ten times their own diameter. Naturally, the stress to which the material is subjected is the greater, the smaller the diameter ratio. Our tests were therefore carried out under extreme conditions of stress.

Aging of the material to be tested was brought about by submerging the bars for two hours in boiling water. Official tests usually require an aging of 30 minutes only.

Bending was then carried out to an angle of 45 (Figure 7b), and subsequent unbending was carried out amounting to 225 (Figure 7c). This latter unbending step always causes breaking of ribbed and/ or twisted reinforced hooks made by any of the known processes. Furthermore, the bars were then dropped on to a concrete floor from an altitude of 5 feet. Bars which have developed small cracks through the bending process usually break when dropped from the aforesaid altitude.

A number of tests were carried out under the aforesaid conditions, whose results are hereinafter described in detail with aid of accompanying tables.

Nine obliquely ribbed steel bars were used for testing having a nominal diameter of 16 millimeters. The ribs formed an angle of 60 to the longitudinal bar axis, they were evenly distributed over the whole length of the bar, the distance from one rib to another was 0.7 times the diameter, the flanges of each individual rib were sloped at an angle of 45. The height of each rib was of the bar diameter and the base of the ribs was 4 thereof. The cross section of the reinforcement bars to be tested was determined as being on an average equal to 1.98 square millimeters. The test bars were cut of an equal length of 70 centimeters and numbered from 1 to 9. On these bars having a length of 70 centimeters a 40 centimeter long section was marked, which served as the measuring distance for determining the elongation. The ribbed bars were then subjected to cold-stretching. The results of this step of the process according to the invention are shown in Table I:

Table I Len h (in cm. Elon ation Yield Maximal gt g Bar No. Point in Stress in Metric Metric before after in per- Tons Tons stretchstretchin em. cent ing ing 7 These elongated bars were stored for seven days in a tempered room and then divided in two lots. Five of them, No. 1, 3, 5,7 and 9, were used for determining the cross section of the elongated bars, the breaking limit in metric tons, the tensile strength in kilogrammes per square millimeters, and the ductile yield in percent. Results of these tests are shown in Table 11:

Table II Cross Breaking Tensile Ductile Bar No. Length Section Limit in Strength Yield a in cm. in mm? Metric in in Tons kg.lmm. percent 7 Bars No. 2, 4, 6 and 8 were also stored for seven days and then subjected to a bending test in the cold around a bending block having four times the diameter of the bar. Bending was carried out to an angle of 45 without causing any cracking or breaking at the bent portion of the bars. No such damage was caused either by dropping the bars from a height of 5 feet on to a concrete floor even when the bar hit the floor with the bent portion or the bar end.

Thereafter the second lot of bars was subjected to aging by submerging the bars for two hours in boiling water. Immediately thereafter the bars were bent backward by 22.5", without showing any cracks or breaking.

Bars No. 6 and 8 were unbent completely and then subjected to a new test of dropping from 6 feet height on to a concrete floor. No breaking or cracking occurred. The bars according to the invention, remain undamaged even if further bent by 45 in a direction opposite to the first bending (Figure 7d); still no cracking or breaking occurred in tests carried out to those extreme conditions of stress.

In the following Table III a number of further examples are given to illustrate the step of cold stretching in the method of making age-resistant ribbed steel, reinforcement elements according to the invention, in relation to the stretching time.

Table III Duration Average Maxi- Length Elongamum Example No. of Bar Stretchtion Stretch- Charge (Meter) ing (Percent) ing Load (Seconds) (Tons) 5. 20 9. 12. 6 13. 8 5. 2O 18 12. 7 13. 8 A 5. 20 31. 6 12. 7 12. 5 5. 20 18 12. 1 12. 4 B 4. 80 30. 8 12. 5 12. 2 4. 80 10. 0 12. 8 14. 2 5. 20 18. 0 12. 6 13. 2 C 5. 2O 31. 2 12. 4 12. 7 5. 20 10. 0 12. 5 l3. 6 5. 20 18. 0 12. 8 12. 7 D 5. 20 31. 2 12. 9 12. 4 5. 20 9. 4 12. 9 5. 4 4. 80 17. 5 12. 7 4. 4 F 5. 20 30. 0 12. 5 4. 1 5. 20 10. 5 11. 5 26. 5 5. 20 17. 8 11. 8 26. 2 G 5. 2O 43. 0 11. 7 25. 5 5. 20 15. 0 11. 5 24. 0 5. 20 31. 5 11. 5 23. 8 H 5. 20 16. 5 13. 2 24. 5 5. 20 10. 5 11. 0 27. 0 5. 20 21.0 11.2 25.5 I V 5. 20 22. 0 13. 2 26. 4 4. 8O 9. 0 11. 2 25. 0 5. 20 11. 0 11. 2 25. 0

' in Table III, were tested and their yield point and maximum tensile strength, at break, under varying conditions was determined.

Table IV Yield Point Tensile Strength Bar Diameter kg/min. (at break), Charge k mj (a) Hot rolled. (5) Cold stretched. (c) Aged by imersion for thirty minutes in boiling water.

Furthermore, a comparativetest was made between difierent charges of ribbed steel bars which were subsequently treated in a conventional manner by cold-twisting, and other bars of the same charge that were treated by the method according to the invention.

All test bars thus obtained were subject to a first bending by 45 in a conventional bending machine having a bending roll of millimeters diameter, an aging treatment of thirty minutes immersion in boiling water, and subsequently to a re'bending in opposite direction to 45.

It will be understood that this invention is susceptible to modification in order to adopt it to different usages and conditions and accordingly, it is desired to comprehend such modifications within this invention as may fall within the scope of the appended claim.

We claim:

A method for making concrete reinforcing elements from ribbed steel bars, which reinforcing elements are in conformity with the oificial ASTM standard requirements for such elements and can be subjected to bending and unbending after prolonged storage without showing a tendency to fracture due to shortness of the material developed by aging, comprising the steps, in combination, of

(1) hot rolling a single strand steel bar consisting of a steel consisting of 0.250.30% of C, l.0-1.30% of Mn and 0.300.70% of Si, having a substantially circular cross section and consequently cylindrical surface, between profiled rollers so as to shape on the bar surface ribs protruding from the bar surface at least part of which ribs extend transversely about the bar surface, the transverse ribs forming an angle of at least about 60 with the longitudinal axis of the bar and having sloping sidewalls inclined at an angle of about 45 with said bar surface, a height of and a base width of A of the bar diameter;

(2) cold-stretching the resulting ribbed steel bar beyond the yield point of the steel so as to impart to the bar a permanent linear elongation of from about 4.36.5% and in such manner that. the resulting bar is free from any twisting exceeding an increase in hardnessof the steel by 10%;

(3) storage-aging the steel bar and (4) bending the ribbed elongated bar about a standard bending block having a diameter of about 4 to 10 times the bar diameter transversely to form a desired angle of up to 90 with the longitudinal axis of the steel bar, the resulting angularly bent elongated ribbed steel bar being resistant to fracture when rebent toward a straight bar by an angle of at least 225 and more.

2,216,758 Schmidt Oct. 8, 1940 10 Hoffman Oct. 28, 1941 Bradbury May 8, 1951 Frokjaer-Jensen Sept. 10, 1957 FOREIGN PATENTS Australia Ian. 6, 1953 Canada Jan. 20, 1953 France Jan. 4, 1955 OTHER REFERENCES Metals Handbook, 1948 Edition, published by The American Society for Metals (pp. 320-322 relied on).