GB1564816A - Reinforced concrete - Google Patents

Description

(54) REINFORCED CONCRETE (71) We, NETUREN COMPANY LIMITED, a Japanese company of No. 1621 Higashi-gotanda 2-chome, Shinagawaku, Tokyo, Japan, do hereby declare the invention for which we pray that a patent may be granted to us and the method by which it is to be performed, to be particularly described in and by the following statement:- The present invention relates to the reinforcement of concrete structures.

It is known to employ reinforcement embedded in a concrete structure to enhance the strength of the structure.

Usually the reinforcement consists of spaced parallel longitudinal principal bars and secondary reinforcing steel bars bent to form stirrups or hoops wound around the longitudinal principal bars. The secondary reinforcing steel bars serve to prevent rupture of the concrete and a buckling of the longitudinal principal bars due to the longitudinal principal bars being displaced in a direction normal to the axis by an axial compressive force.

The secondary reinforcing bars are wound once around the longitudinal principal bars to form a stirrup or hoop conventionally as follows: first a reinforcing bar is cut off to a length matching the configuration of the spaced longitudinal bars to be strengthened. The cut pieces are manually wound one after another around the longitudinal principal bars and they are fixed to the latter by binding. By repeating this process a number of stirrups of hoops are bound at specified intervals, and are finally anchored as a reinforcing member in the concrete structure.

This method of manually winding one piece after another around the longitudinal principal bars to form a reinforcing member is an extremely primitive and inefficient one. Besides, in a reinforced-concrete column for instance, when the column is under loading the longitudinal principal bars and the adjacent concrete tend to be displaced in a lateral direction by an axial compression, the lapped portion of a secondary hoop tends to slip out of the concrete, and the restraint for the longitudinal principal bars and concrete by the hoop is reduced and in consequence of buckling of the longitudinal principal bars leads to rupture of the concrete.

To avoid such disadvantages, adoption of a helical bar for secondary reinforcing purpose has been proposed. Thus a long bar of the same material quality as the conventional secondary reinforcing bar is bent to form a continuous spiral. Adoption of a helical bar for the secondary reinforcing purpose eliminates the step of manually winding one piece after another of a secondary reinforcing bar around the longitudinal principal bars and fixing them thereto: therefore the construction of a steel work becomes that much more efficient.

It is however difficult in field work to make sure that the secondary reinforcing steel helix is fixed with a specified pitch at specified position around the principal bars because the steel of the helix is of the same material quality as the conventional secondary reinforcement in a folded state: therefore when it is stretched out, it develops plastic deformation and it would require considerable time and labour to stretch it to a desired uniform pitch around the principal bars. For instance, to construct a steel cage 3 m high, 700 mm square, 200 mm pitch with a spiral square hoop of a 13 mm diameter, conventional secondary reinforcement steel bar was employed and a coiled spiral hoop was stretched to the desired pitch; a residual deformation was observed.This spiral hoop weighs as much as 44 kg it is a tremendous job to coil such a heavy bar around the principal bars on a high scaffolding, while making a pitch correction. In practice, such a spiral bar as high as 3 m is divided into several parts to decrease the weight and sometimes a special hanger is provided, whereby poor workability, low efficiency and poor economy are unavoidable.

Meanwhile, as mentioned above, in a concrete structure with a steel cage buried therein the axial compressive force can be resisted by the concrete, while the tensile force is resisted by the principal bars: and the main role of the secondary reinforcing helix is to strengthen the longitudinal principal bars. Due attention has not been paid to the tension force in a direction normal to the axis of the principal bars, or the shearing force, which is a problem for concrete structures forming part of an earthquake resistant building.

The invention provides in one aspect a reinforcement for a concrete structure, comprising a plurality of parallel, spaced, longitudinally extending steel bars, and a helical steel bar extending around said longitudinal bars, the helical bar having a yield strength such that, when it is released from a close wound helix with adjacent turns abutting, it extends over the substantially the entire length of said longitudinal bars, without undergoing plastic deformation.

In a further aspect, the invention provides a method of providing a reinforced concrete structure, the method comprising providing a plurality of parallel, spaced, longitudinally extending steel bars as a principal reinforcement, positioning a steel bar in a close wound helix with adjacent turns abutting around said longitudinal bars, said steel bar having a yield strength such that when it is released it extends over substantially the entire length of said longitudinal bars without undergoing plastic deformation to provide a secondary reinforcement releasing said helical bar, and pouring concrete in place about said helical and parallel bars.

The reinforcing member for a concrete structure according to the invention includes a coil which can stretch by its own elasticity when released from a close-wound prestressed coil and does not suffer plastic deformation even if stretched by external force and accordingly can be stretched to a specified pitch with no irregularity in pitch and a desired pitch maintained over the entire length of the coil. The coil is employed as a secondary reinforcement for a steel cage in a concrete structure, which is strong enough to give ample resistance to compression in an axial direction and tension in a direction normal to axis as well as shearing force. The coil for the steel cage can have a stable contact with the principal bars, the coil being held after stretching in integral contact with the principal bars whereby the bonding with the concrete can be very good.

Since the steel bar of the coil is of highstrength, the bar can be made smaller in diameter than the conventional one and yet can develop the same strength in concrete structure as the latter; accordingly the coil is light and highly workable and easy to carry and place in position.

In a preferred embodiment, the helically coiled reinforcing bar has spiral grooves on the surface thereof and is of more than 55 kg/mm2 in yield strength and can stretch to a specified pitch by its own elasticity when released from a fully-compressed coiled state and does not suffer plastic deformation even if stretched by an external force. The coil is manufactured preferably by heating steel bar to a specified temperature in the furnace or by an induction heating coil; winding it in the heating state on a winding machine of specified configuration helically at a specified pitch; then cooling it in the air or, if necessary, using a cooling means. If thereby the bar has not attained the desired strength according to the present invention, said bar is preferably worked into a helical form after specified heat treatment.When a bar with a spiral groove thereon is of such a material that it is relatively easy to work, it will be desirable that helical winding be done by cold working, followed by heating to a specified temperature in a heating furnace to remove the stress developed in the bar when worked. The above described manufacturing method assures an accurate, and easy working high-strength helical reinforcing bar which can be exactly wound at a specified pitch.

Preferred embodiments of the invention will be described hereinafter with reference to the accompanying drawings, wherein: Figure I (a) is a plan view showing a reinforced concrete column manufactured by a known method; Figure l(b) is a longitudinal section view of Figure 1(a); Figure l(c) is a longitudinal section view showing a reinforced concrete column with a known helical reinforcing bar; Figure 2(a) is a front elevation view of a bar for use in the present invention; Figure 2(b) is a section view corresponding to Figure 2(a); Figure 3(a) is an elevation view illustrating the preferred method of manufacturing a coiled bar according to the present invention; ; Figure 3(b) is a plan view showing the relation between the winding machine and the cooling mechanism in Figure 3(a); Figures 4-5 are elevational views illustrating examples of a coil according to the present invention being wound around longitudinal principal bars arranged in a square form; Figure 6 is a plan view showing the anchor head formed at the end of a coil according to the present invention Figure 7(a) is an elevation view illustrating an experimental procedure conducted about the present invention; Figure 7(b) is a side view of the speciment in Figure 7(a); and Figure 8 is a diagram summarizing the results of an experiment conducted in the manner shown in Figure 7.

Referring to Figures I(at(c), a brief description of a conventional reinforcedconcrete column is to be given.

As shown in Figures 1(a) (b) a plurality of longitudinal principal bars 3 are buried along the periphery of a column: a plurality of secondary reinforcing bars 4 are also provided, each of which is cut off to a length matching the configuration of the longitudinal principal bars, i.e. the sum of the springs between adjacent bars 3, each cut piece of it being individually wound at specified intervals as a hoop reinforcement around the principal bars. The ends 4' of each reinforcing bar 4, after being wound around the configuration of longitudinal principal bars are bent toward the center of the column.A reinforcing member comprising longitudinal principal bars 3 and the surrounding secondary reinforcing bars 4 is placed in a formwork 11, concrete 2 is poured in place and when the concrete gains the required strength, the formwork is removed, thereby yielding a reinforcedconcrete column.

As pointed out above, this method of secondary reinforcement is very inefficient; besides, the restraint of the longitudinal principal bars 3 by the secondary reinforcing bars 4 is liable to be easily lost.

Figure I (c) illustrates an example of a helical bar 5' used for secondary reinforcement. The method illustrated in Figure l(c) is free from the drawbacks in the method illustrated in Figure l(a), (b), but it has a different drawback that it becomes difficult to wind the reinforcing bar around the longitudinal principal bars 3 over the whole length of the column with a specified uniform pitch maintained so long as the reinforcing bar 5' is of the same material as the principal bars.When the helical bar fails to be wound around the longitudinal principal bars with a specified uniform pitch maintained over the entire length, the restraint of the longitudinal principal bars and the concrete by the helical secondary reinforcing bar will be lost and accordingly the bonding is lost between the concrete and the longitudinal principal bars with the result that the longitudinal principal bars fail to perform their function. Suppose a helical bar of 700 mm square and 200 mm pitch is to be made with a bar of 13 mm in dia. In that case, so long as a bar of the conventional material quality is employed, residual deformation is unavoidably developed when the bar is worked to 200 mm pitch at the plant, folded, bound, carried to the field and there unbound.

According to the result of an experiment carried out by the inventor, for the purpose of avoiding development of residual deformation in a helical bar of 200 mm pitch used for reinforcement in the construction of a 600 mm diameter steel cage it was necessary to set the yield strength at 25.2 kg'mm2 for a bar of 9 mm diameter and at over 35.36 kg/mm2 for one of 13 mm diameter: in the construction of a 500 mm diameter steel cage using a helical bar of 200 mm pitch for the secondary reinforcement it was necessary to set yield strength at 34.8 kg/mm2 for a bar of 9 mm diameter, at 50 kglmm2 for one 13 mm diameter; and in the construction of 400 mm diameter steel cage the yield strength had to be 56 kglmm2 for a bar of 9 mm diameter and over 77 kglmm2 for one of 13 mm diameter, otherwise a deformation develops and pitch correction has to be made on account of a pitch error developed when stretching. To correct the pitch irregularity, a force of 43 kg was required in the construction of a steel cage for the concrete column of 3 m height and 500 mm diameter using a helical bar of about 13 mm diameter.Even if a force of 43 kg suffices for the correction, an exact correction of pitch is practically impossible; further the work of pitch correction on a high scaffolding would be dangerous.

The present invention firstly aims at eliminating the drawback mentioned above in the case of using a coil or helical bar for secondary reinforcement. Generally speaking to remove the residual stress in a helical reinforcing bar, it is an easy method by making the diameter of the bar as small as possible. However, for restraining the longitudinal bars and preventing them from being buckled as a result of being displaced in a direction normal to axis by an axial compressive force, the helical bar must have a strength high enough to attain this purpose. Thus when a smaller diameter of reinforcing bar is used its strength must be that much increased.

According to the present inventor's calculations, the yield strength required of the reinforcing bar to restrain the longitudinal principal bars is 132.7 mm2 24 kglmm2x =82.7 kWmm2 38.5 mm2 if a helical bar for concrete reinforcement (its yield strength being 24 kglmm2) with diameter 13 mm (section area; 132.7 mm2) is to be replaced with a structure steel bar of diameter 7 mm by this invention (whose chemical composition is shown in Table 1, section area; 38.5 mm2) which permits easy correction of pitch.When a 500 mm diameter coil of a helical reinforcing bar 9 mm diameter (section area; 63.6 mm2) a yield strength of 63.6 mm2 24 kg/mm2x =53.9 kg/mm2 kg'mm2x 28.3 mm2 = kg/mm2 was needed to replace it with one of 6 mm diameter with the same chemical composition as above (section area; 28.3 mm2) with the same chemical composition as above. Thus if a 500 mm diameter coil is to be wound using the bar 6 mm diameter, the bar to be used will be required to have a yield strength of more than 55 kg/mm2. In that case, however, it will not be all right if only the yield strength is made high.It would be poor economy to increase needlessly the yield strength and correspondingly decrease excessively the quantity of bar used, because in that case the bar would suffer too much strain and the concrete which cannot absorb such a strain would rupture.

TABLE 1 Chemical composition (%) C Mn P S - - less than less than 0.050 0.050 Note: Content of C and Mn are not specified.

From the two conditions to be satisfied that the restraint on the concrete by the reinforcing bar be maintained at a practically satisfactory level and that the material quality does not deteriorate up to 450"C of temperature rise in a structure on fire, the upper limit of yield strength of the helical reinforcing bar is preferably set at about 130 kg"mm2 for the present invention.

In the above the yield strength required of the secondary reinforcing bar to restrain the longitudinal principal bars has been considered that the bar is to be replaced with one of smaller diameter for gaining better workability.

Next discussion is made on what yield strength the bar must possess in order that no residual deformation may develop therein.

The result of a calculation has revealed that when a 400 mm diameter 3 m length of helical bar is stretched to a 200 mm pitch from a fully compressed coil state, the yield strength will have to be 45 kg"mm2 if residual deformation is not to be permitted to develop. Accordingly by setting the lower limit of the yield strength of the helical reinforcing bar at about 55 kg/mm2 it is possible to secure the effect of preventing the buckling of the longitudinal principal bar and the effect of no residual deformation being developed when the secondary bar is stretched to a specified interval from a fully compressed state following its close-fit winding and accordingly a desired uniform pitch being maintained over the entire length of the longitudinal bars by merely stretching the secondary bars to a specified length, as well as the effect of being lighter and of better workability.

According to the results of various experiments conducted later on the helical reinforcing bar according to the present invention, when yield strength of the helical bar is set in the range of 55 kg/mm2--130 kg"mm2, the above three effects can be satisfactorily attained even with varied helical bar coil diameter, pitch or diameter of the bar.

On the other hand, even if the helical bar possesses sufficient yield strength in the range stage above, thereby being able to give ample restraint on the longitudinal principal bars; and it has workability Improved such that merely by stretching from full-compressed coil state, the bar can attain a generally uniform pitch as desired over the whole concrete member length, the restraint on the longitudinal principal bars may be lost and the longitudinal principal bars may be liable to buckle, unless a satisfactory bond is established between the helical reinforcing bar and the concrete.

According to a preferred feature of the present invention, the helical secondary reinforcing bar is externally provided with spiral grooves to attain such requirement.

Figures 2(a), (b) illustrates an example of a plurality, e.g. six, of spiral grooves 53 externally over the whole length of bar 5.

As means to improve the bondability to the concrete it is conceivable also to provide a rib on the surface of the bar, but in that case the helical reinforcing bar will contact the longitudinal bars through this rib, resulting in that the contact surface loses smoothness and stability and the restraint of the helical bar on the longitudinal principal bars is adversely affected.

To ensure the smoothness and stability of the contact surface, a round bar may be used, but an improved bondability of the bar to the concrete cannot be expected therefrom. By contrast, if the bar is deformed by the spiral grooves 53 thereon, the contact surface of it with the longitudinal principal bars will be stabilized; and the bondability to the concrete will be improved, thereby ensuring stable, firm restraint of the helical bar on the longitudinal principal bars.

Next the method of manufacturing the helical secondary reinforcing bar according to the present invention is to be described.

The helical reinforcing bar of the present invention, being of such a high strength, is hard to work with exact pitch by the conventional method of winding. Figures 3 (a), (b) illustrate a method of manufacturing a high strength helical bar for use in the present invention.

In Figures 3 (a), (b), 51 is a coil of steel bar preliminarily cold-drawn to form the spiral grooves thereon. The coil is uncoiled along a specified path by a feeding device, for instance, the pinch-rollers 12-19, as in the prior art.

Uncoiled steel bar 5, after being heated to an ordinary hardening temperature for a specified time by an induction-heating coil 6, is cooled and hardened in a cooling mechanism 7. Thereupon, the wire is heated in a further induction heating coil 8 for a specified time to an ordinary tempering temperature, say, 350"--400"C, the wire heated to the tempering temperature is wound on a winding machine 9.

In Figure 3 (b) is illustrated an arrangement of winding machine 9 for constructing a coil of square section.

Frames 91-94 are secured at one end to the plate 9' at specified positions so that their positions may be maintained. The winding machine 9 arranged thus can be turned in the direction 20 at a preset speed and at the same time can be displaced in the direction 21 by means of a rotation-drive mechanism and a reciprocating mechanism. Therefore when the turning in the direction 20 and the displacement in the direction 21 of said winding machine 9 have been adequately set, the bar 5 heated to a tempering temperature can follow the frames 91-94 of the machine 9, whereby it can be easily worked to a square helical coil. The part of the bar thus helically formed goes into the cooler 10 with the rotation and displacement of the winding machine 9, where it is cooled with a jet of the cooling liquid issuing from circumferential jet nozzles.With this the winding process as well as the tempering process of the bar is finished, yielding a high-strength helical bar worked to a square shape and strengthened through heat treatment and cooling.

After completion of the winding of one coiled helical bar by the winding machine 9, the rotation and displacement of said machine 9 are stopped; the power supply to the induction-heating coils 6,8 is cut off; the helical bar coiled on the winding machine 9 is taken off from the winding machine 9; cut a specified required number of turns, the turns fully closed up and then bound together.And though not shown in the drawing, if the motion of the winding machine 9 is restricted to rotation in the direction 20 and the winding frames 91-94 are inclined inwardly towards the axis of rotation of the winding machine with increased approach to the cooling mechanism, the helical wire taken up can be moved in the direction of the cooling mechanism and after being cooled, it can be cut into pieces at a desired number of turns, said pieces being sent on to the next stage of full closing up of the turns and binding.

The above example concerns a case of producing a helical bar of rectangular section, but a helical bar of circular, oval or polygonal section may be produced by adequately modifying the arrangement. The above example is a case of the bar being worked in a heated state of tempering and then being cooled in a cooling mechanism 10, but if necessary, the cooling may be done in the air instead of using the cooling mechanism 10. Further, when the bar with the helical grooves has the sufficient strength required by the present invention even without hardening, or when the bar has already been hardened, the stages of heating by the induction-heating coil 6 and cooling by the cooling mechanism 7 in Figure 3 (a) are omitted.Then the material has only to be submitted to the stage following the heating by the induction-heating coil 8 or it has only to be worked in the state of being heated by some method to 3000--4000C in a furnace or induction-heating coil. In this case, similarly to the above, the hot helical wire may be air-cooled or submitted to a cooling mechanism.

In the above method of production, the working of helical bar is done while the bar is in a very hot state of tempering, the winding can be accurate, smooth and easy; and with the heat-treatment ending with the cooling, a high-strength helical bar exactly worked can be produced. Meanwhile, through proper selection of the frame arrangement, rotational speed and displacement of the winding machine a helical bar of desired pitch and desired profile meeting the requirement of a steel cage can be produced.

Further, when the bar with the spiral grooves thereon is of such a material that is relatively easy to work it will be desirable to wind it into a helical form by cold working; and heat it to a specified temperature (300"C--400"C), thereby removing the residual stress developed in winding.

Next, an arrangement of the helical reinforcing bar of the present invention in a reinforced concrete column or a reinforced concrete beam is described referring to Figures 4-5, which illustrate an example of bar arrangement in a square steel cage.

The helical bar bound up in a fully closedup prestressed coil is delivered to the work site; as shown in Figure 4. The required number of longitudinal principal reinforcing bars 3 are erected on a foundation 22 parallel to each other to form a square cage and the helical secondary reinforcing bar 5 is arranged around the longitudinal principal bars 3.

With the lower end of the helical bar 5 secured to the lower end of one of the longitudinal principal bars 3, the helical bar 5 is unbound and stretched as indicated in Figure 5 and at a specified level of the longitudinal principal bars 3 the helical bar 5 is bound around the longitudinal principal bars 3 and fixed. Thereupon a formwork is erected around the steel cage thus constructed.Next longitudinal principal beam bars 3' are set at a specified level relative to the longitudinal principal bars 3 of the column and are bound with the longitudinal principal column bars 3 at right angles; in the same fashion a helical bar 5 in a fully compressed coil state is set around the longitudinal principal beam bars 3'; one end of the helical bar is secured to the junction of one of the longitudinal principal column bars 3 with one of the longitudinal principal beam bars 3'; thereafter the helical bar 5 is unbound and the other end of it is extended to a specified point and the extended end is fixed to the principal beam bar group 3'.

A formwork is erected around the column and beam. Then concrete is placed.

The securing of the longitudinal principal column bars, the joint of the intersection of the respective ends of the longitudinal principal column bars and the beam bars and the formation of the formwork, are known processes. The bar arrangement in the case of a circular, oval or polygonal secondary coil is done by the same method as in the case of the square coil To verify the effect of the present invention, a concrete structure using the helical bar according to the present invention was constructed; a comparison test with a conventional reinforcement was carried out to investigate the effect of the yield strength of the helical bar on the shear resistance of the structure.

Scme of the experimental data are cited below.

1. Experimental Conditions 1) Specimen and its dimension: Concrete structure respectively holding the helical bar given in 2) as shear-reinforcing bar.

Length: 3300 mm Sectional area; 400 mmx 180 mm 2) Helical steel ratio* in Specimen and diameter and pitch of helical bar.

Steel ratio (%) 0.26 0.52 0.78 1.18 ~ Shear-reinforcing Diameter (mm) 66 9 9 bar (helical bar) Pitch (mm) 120 60 90 60 Yield strength of helical bar; 30 kg/mm2 60 kglmm2 130 kg/mm2 *Note: Helical steel ratio is the ratio of the sectional area of steel to the sectional area of concrete in a unit section of specimen.

3) Profile of Helical Bar: Spiral grooves are formed over its entire length.

Diameter (mm) 6 9 Number of grooves 3 6 Width of groove (mm) 2.8+0.3 3.3+0.3 Depth of groove (mm) 0.3 0.4+0.1 Pitch of groove (mm) 65-80 90--1 10 2. Experimental Procedure As shown in Figure 7 the beam 23 was supported at the points of 27, 28, 24, 24', loading was carried on 24, 24' by a 100 ton load cell, the points 24, 24' corresponding to the position of columns. In this experiment the ratio of shear span 25 to beam depth 26 is equal to 1.5, where shear span 25 is defined as the distance from the center of the beam to the inside section of a loading column. And the deformation caused thereby in the specimen are recorded; the ultimate shear stress is checked out from the maximum load on the points 24, 24': and therefrom the relation between the ultimate shear stress and the shear reinforced steel ratio is determined.

3. Experimental Results The results are summarized in Figure 8, in which the ordinate is the ultimate shear stress (kg"cm2) and the abscissa is the shear reinforced steel ratio PW. 29 represents the results using a helical bar of 130 kg/mm2 yield strength; 30 represents the results using a helical bar of 60 kg/mm2 yield strength: and 31 represents the results using a helical bar, of which the yield strength is in the range of 25 kg/mm2-30 kg/mm2.

Meanwhile the stress developed in the helical bar during the experiment was measured by an electric wire strain gauge; according to the measurement at a steel ratio 0.52%, a maximum strain of 3800 was developed. From this it has been revealed that the helical bar will have to be more 75 kg/mm2 in yield strength.

In a separate experiment from the above, as indicated in Figure 6 a button head 51 was formed at the end of a helical bar of the present invention said head was buried into a 15 cm square concrete of compressive strength 210 kg/cm2; and then it was submitted to a pull-out test using a conventional pull-out testing machine.

Thereby the concrete cracked for a buried depth of 15 cm from the concrete surface under a load 2.5 times as heavy as in the case of a bar with no such a head, while for a buried depth of 20 cm the concrete cracked when a load 4 times as heavy was applied.

Thus formation of a button head 51 makes it possible to manufacture high load-resisting concrete member. Thereby the end is preferably bent along the relevent principal bars such that the head 51 may come nearly at midpoint of the column or beam, for in that case the anchor will be reliable.

1. The secondary reinforcing coil according to the present invention can stretched to a specified pitch by its elasticity when released from the fully closed-up prestressed coil state and it does not suffer plastic deformation even when stretched after delivered to the work site in full compressed coil state. Therefore a desired uniform pitch can be approximately maintained over entire concrete member by fixing one end of the folded coil; then stretching the coil and thereafter fixing with the principal longitudinal bars at each specified interval. Thus it need no pitch correction mentioned above which has been required for conventional helical bar of say yield strength between 25-30 kg'mm2 of this kind.

2. In the present invention as shown in the above experimental data, by setting the yield strength of the bar of the coil in the range of 55 kglmm2-l30 kg/mm2, which is 2 H times as high as that of conventional reinforcing bar yield strength of 25-30 kg/mm2 the development of strain can be controlled within the practically tolerable limits and the ultimate shear stress can be carried extremely high.Thus the restraint of the longitudinal principal bars and concrete by the coil can be enhanced, thereby, together with the increased bondability to the concrete, preventing a buckling of the longitudinal principal bars and rupture of the concrete due to the axial compression in a reinforced concrete structure; but also an ample resistance to the tension in a direction normal to axis can be imparted to the longitudinal principal bars, thereby enabling the construction of an excellent earthquake resistant structure.

3. In the coil according to the present invention, in which spiral grooves are formed thereon, when the bar is coiled around the longitudinal principal bars, it can contact the longitudinal principal bars in a stable manner the spiral grooves improves the bondability to the concrete, thereby assuring a reliable restraint of the longitudinal principal bars.

4. The conventionally used reinforcing bar has a diameter 9 mm or 13 mm and a yield strength in the range 25-30 kg/mm2 but the bar employed in the present invention has a yield strength more than 24 times the above value and accordingly the reinforcing bar to be used can be reduced much in diameter for the same coil. Thus a steel cage far lighter than the conventional one can be constructed. Namely, against 44 kg of above-mentioned 13 mm diameter conventional bar with a yield strength of 30 kg/mm2 with 200 mm pitch and 3 m in length for a 700 mm square helical hoop for a column, a bar with a yield strength of 130 kg/mm2 7 mm diameter according to the present invention which can perform the same function weighs merely 11 kg and is very convenient to transport and handle.

Meanwhile the earthquake resistant effect will be extremely great as compared with the conventional bar, because for the same weight one can coil around the longitudinal principal bars with four times as many turns as the conventional one over the same length.

5. Use of a coil according to the present invention ensures an extremely high value of ultimate shear stress as compared with use of conventional reinforcing bar of the same diameter. It has been experimentally confirmed that the ultimate shear stress in the case of the conventional helical bar of, say, 30 kg/mm2 yield strength at stirrup steel ratio of 1.18 / is equivalent to the ultimate shear stress in the case of using the coil of the present invention of 130 kg/mm2 yield strength at a stirrup steel ratio of 0.26 /".

Thus according to the present invention, the reinforced steel ratio can be reduced to 1/4 of the conventional value for the same ultimate shear stress.

6. If in this invention, a button head is provided at the end of the bar, a heavier load can be stood and a better anchoring effect will be gained than when no button head is provided.

The above examples show the coil of the present invention as a reinforcing bar in a steel cage for a reinforced concrete column and beam, but the coil of the present invention is applicable as a reinforcing bar in a steel cage for any prestressed or reinforced concrete structure.

WHAT WE CLAIM IS: 1. A reinforcement for a concrete structure, comprising a plurality of parallel, spaced, longitudinally extending steel bars, and a helical steel bar extending around said longitudinal bars, the helical bar having a yield strength such that, when it is released from a close wound helix with adjacent turns abutting, it extends over the substantially the entire length of said longitudinal bars, without undergoing plastic deformation.

2. A reinforcement as claimed in claim 1, wherein said helical bar has a yield strength of at least 55 kg/mm2.

3. A reinforcement as claimed in claim 2, wherein said helical bar has a yield strength of less than 130 kg/mm2.

4. A reinforcement as claimed in any preceding claim, wherein an enlarged head is formed at an end of said helical bar.

5. A reinforcement as claimed in any preceding claim, wherein said helical bar has spiral grooves formed in its surface.

6. A reinforcement as claimed in any preceding claim, wherein said helical bar is wound into a coil by a process including heating said bar, winding said bar while hot into a coil, and cooling said coil.

7. A reinforcement as claimed in any of claims 1 to 5, wherein the helical bar is wound into a coil by cold working, the coil subsequently being heated to a temperature of between 200"C and 400"C.

8. A concrete structure reinforced by a reinforcement as claimed in any preceding

Claims (1)

1. claim.

   9. A method of providing a reinforced concrete structure, the method comprising providing a plurality of parallel, spaced, longitudinally extending steel bars as a principal reinforcement, positioning a steel bar in a close wound helix with adjacent turns abutting around said longitudinal bars, said steel bar having a yield strength such that when it is released it extends over substantially the entire length of said longitudinal bars without undergoing plastic deformation to provide a secondary reinforcement, releasing said helical bar, and pouring concrete in place about said helical and parallel bars.

   10. A method of providing a reinforced concrete structure as claimed in claim 9 and substantially as described with reference to the accompanying drawings.

   11. Reinforcements for concrete structures as claimed in claim 1 and substantially as described with reference to the accompanying drawings.