EP2356292A1 - Method for producing a reinforced concrete part, and thus-produced part - Google Patents

Abstract

The invention relates to a method for producing a reinforced concrete part (1), having a tensioned portion subjected to pull stresses and tending to stretch under the load, and which comprises a reinforcing frame (2) with at least one tensioned longitudinal bar (21) rigidly connected to the concrete by an adhesive connection that determines a tangential adhesive stress along said bar that varies on the basis of applied pull stresses. According to the invention, each tensioned longitudinal bar (21) has, on at least one portion of the length thereof, a discontinuous series of spaced blocking areas (25) that each comprise a plurality of means (23) for anchoring into the concrete and which are separated from each other by a series of sliding areas (26), in each of which an increase in the adhesion stress above a limit value causes the bar to disengage, without disrupting the concrete, on at least a portion (27) of the length between the two blocking areas (25a), (25'a) with an extension of the bar corresponding to applied pull stresses, said extension being distributed over the entire length of the disengaged portion (27) of the bar (21). The invention also covers the thus-produced parts and the reinforcing bars for implementing the method, comprising an alternating series of blocking areas and sliding areas.

Description

 PROCESS FOR PRODUCING A REINFORCED CONCRETE PIECE AND A PART PRODUCED THEREBY

The subject of the invention is a method for producing a reinforced concrete part and also covers the reinforcements used for this purpose and the concrete parts thus produced. The invention particularly relates to the production of beams, slabs or floors subjected to bending forces but can also be applied to other pieces of reinforced concrete, for example thin shells or sails of various shapes.

In addition, the invention applies especially but not limited to the construction of structures that can undergo earthquakes or accidental actions. The reinforced concrete industry has grown significantly in the 20 ^th century but this technique while undergoing very extensive scientific studies, has changed little.

The properties of reinforced concrete result, as we know, from the combination of two materials with different properties, concrete which is essentially resistant to compressive forces and a reinforcement cage embedded in concrete and made of metal bars which resist pulling forces, at least if these are oriented in the direction of the rebar. The prestressed concrete, invented by Freyssinet, is based on the same principles of operation by simply giving the armature under tension, a prestressing role of the part in the opposite direction of the tensile forces due to the load, which allows increase the resistance to bending forces.

In general, as shown schematically in Figure 1, it is accepted that a reinforced concrete part on which a load is applied comprises, on either side of a neutral line, a compressed part and a part stretched under tensile stresses under the effect of the load and therefore tends to elongate. The reinforcement cage usually comprises two layers of longitudinal bars extending respectively in the compressed part and in the tensioned part and connected by a transverse reinforcement frame consisting of stirrups making it possible, on the one hand, to resist the forces cutting edges and / or vacuum thrusts and, secondly, to join together the two sheets so as to form a cage that can be made in advance and then introduced into the formwork.

When the part extends over a certain width, for example a slab, the reinforcement cage comprises several longitudinal sections connected by transverse reinforcement distribution.

The reinforcing bars are secured to the concrete by a binding bond determining, along each longitudinal bar, a tangential stress of adhesion which varies as a function of the tensile stresses applied.

The assembly thus forms a composite part having a stretched part in which the concrete and the reinforcement bars, joined by adhesion, elongate together until a limit value at which the tensile stresses exceed the tensile tensile stress of the concrete, resulting in the appearance of at least one crack in a part of the part, with an increase in stress and therefore elongation of the rebar on which the concrete discharges from the appearance of the crack. For example, if we study the flexural behavior of a normally reinforced reinforced concrete part, we can establish a constitutive law illustrated by Figure 2 which is a moment-arrow diagram indicating, on the abscissa, the elongation of the stretched part resulting from the deformation of the part under the effect of the bending moment indicated on the ordinate. In this model law, we can distinguish four successive zones. The part OA corresponds to the linear elastic behavior of the composite part with a simultaneous elongation of the concrete and the reinforcement.

Part AB corresponds to the establishment of cracking with an instantaneous increase in the deflection corresponding to the elongation of the stretched part with the steel-concrete adhesion being put into play. From point B, the tensile stresses are absorbed by the steels which are progressively loaded, over the BC range, up to their elastic limit, the mechanism of the adhesion leading to a relative sliding of the two materials , with a progressive increase of the deflection to the point C from which the steel reaches its elastic limit, with a progressive lamination of the two materials. It therefore appears that the deformation of the part as a function of the applied load depends on the tangential forces of adhesion between each reinforcing bar and the concrete which coats it, which balance the tensile stresses resulting from the tendency to elongation. the stretched part.

It has been known, particularly since the studies of Albert Caquot, that the forces which oppose the sliding of the reinforcing bars are bonding, friction and abutment forces, in the case of notched bars known as high-adhesion bars.

Bonding is a phenomenon of chemical adhesion between steel and concrete. The phenomenon of friction, which occurs after detachment of the bar, is due to the fact that an increase in the tensile force causes the appearance of cracks inclined relative to the axis of the bar and forming, in the concrete, trunks of cones that get stuck on the frame by working like ratchets or some kind of connecting rods.

However, this simple friction may be insufficient and, to work together the concrete and steel to a higher level of stress, it is advantageous to use so-called high adhesion rebar. It has been proposed for a very long time to prevent the sliding of a reinforcing bar relative to the concrete which coats it while providing, along the bar, a plurality of spaced anchoring means forming abutments bearing on the concrete. US-A-843843, for example, discloses such a bar having spaced ribs and provides a corrugated profile to the smooth portions extending between the ribs, to increase the perimeter and, therefore, the connection between the ribs. adhesion.

Similar arrangements with spaced ribs are described, for example, in French Patent Nos. 420,102, 597,888 and 1,380,233.

Such spaced ribs are, however, only point stops.

Currently, the high-adhesion bars are thus provided, along their entire length, locking locks arranged obliquely to the longitudinal direction of the bar, so as to achieve a continuous blocking along the entire length thereof. Various known systems have been used for this purpose, the reinforcements may, for example, be twisted cold, or provided with oblique impressions made by cold rolling on the outer face of the bar.

The behavior, during the increase of the load, of such a high-adhesion bar is illustrated in FIG. 3 which is a so-called Tassios diagram representing the evolution of the tangential stress of adhesion τ as a function of the slip local S of the bar relative to the concrete that enrobe. We thus distinguish, essentially, three successive stages.

In the first stage OA of Figure 2, which corresponds to the normal loadings for which the part was calculated, the frame elongates slightly with the concrete which is still in its range of elastic behavior. The adhesion is then in a phase of resistance to the detachment of the reinforcement whose tendency to elongation is greater than that of the concrete which coats it. The tangential stress of adhesion τ _A at point A corresponds to the tensile tensile stress of concrete, from which, as indicated above, transverse microcracks appear. As a result, in the second stage AB, there can be a slight slippage of the reinforcement with respect to the coating concrete, the adhesion being ensured by shearing and then friction.

From the point B for which the steel-concrete mechanical connection reaches its shear strength limit, the sliding resistance of the reinforcement is ensured by the abutment of the notches or locking ribs formed on the surface of the bar . This results in a greater slippage of the bar to the point C from which the crushing of the concrete between the notches of grove promotes the development of compression cracking. Beyond the point C a residual friction develops in the most stressed parts, until complete rupture of the connection.

It appears that the smooth rebar behaves in the same way as the high-adhesion bars up to the point B from which the stop action of the locking notches comes into play. From the corresponding point B, for the smooth bars, at the steel-concrete bond failure, the tangential stress of adhesion decreases rapidly and the sliding increases, as shown by the part BE shown in dashed lines in FIG. In general, the cracking that occurs when the tensile strength of the concrete has been locally exceeded, occurs in the most stressed parts of the room.

The use of high-strength reinforcing bars, blocked throughout their length in concrete, thus makes it possible to increase the resistance to deformation and cracking of the structure whose reinforcement is determined for normal operating conditions. , with a certain safety factor.

However, during the expected life of a reinforced concrete construction, that is to say several tens of years, the cracks can expand gradually and cause corrosion of the reinforcements. In addition, it happens that a localized increase in the applied stresses causes a breakage of the most stressed bars and, consequently, the ruin of the structure.

Such an increase in stress can occur for example in regions subject to seismic shaking and it is known that, in these regions, particularly large shaking could produce the collapse of some constructions. In countries particularly subject to this risk, for example in Japan, special building construction techniques are used to avoid or at least considerably reduce this risk. However, these techniques are expensive and unfortunately can not be used in all risk areas. As a result, large-scale seismic shaking often results in extremely severe damage. On the other hand, these techniques are usually applied to buildings but, even in Japan, it appeared that important structures such as bridges could collapse.

The object of the invention is to solve such problems by means of a new technique for producing reinforced concrete parts.

The invention therefore relates generally to a method for producing a reinforced concrete part comprising, on either side of a neutral line, a compressed part and a tensioned part subjected to tensile stresses. and having a tendency to elongate under the effect of the load supported by the workpiece, and in which is embedded a reinforcement cage comprising, in the stretched portion, at least one stretched longitudinal bar secured to the concrete by a connection d determining adhesion, along said bar, a tangential stress of adhesion varying according to the tensile stresses applied, respectively on the bar and on the concrete of coating, an increase of the tensile stress in the concrete above a limit value resulting in the opening of at least one crack with a transfer of the tensile stress on the bar and a corresponding elongation thereof, in which process, at least in the portion When subjected to the workpiece, said tensioned bar is provided with a plurality of spaced anchoring means forming abutments bearing on the coating concrete.

In accordance with the invention, the means for anchoring the bar are distributed in a discontinuous series of spaced locking zones each comprising a plurality of means anchors (23) and separated from each other by sliding zones without anchoring means, in each of which a local increase in the traction differential between the bar and the concrete above a limit value, causes a detachment of the bar relative to the concrete which coats it, over at least a part of the length of said sliding zone between two blocking zones, said unhooked part being able to elongate without any disorder in the coating concrete under the effect of the tensile stresses applied to the tensioned bar.

On the other hand, because the part has, in the concrete, randomly distributed zones of weakness, at which an increase in the tensile stresses applied above the tensile strength of the concrete causes, in the most stressed part of the part, the appearance of at least one crack localized at least in line with one of said weak areas, the opening of said crack determining, at this level, the cancellation of the stress of traction in the concrete and a local correlative increase of the tensile force applied to the reinforcing bar, with a corresponding increase in the tendency to elongation thereof under the effect of the constraints applied.

According to a particularly advantageous characteristic of the invention, the local increase of the tractive force on the bar, at a crack, determines a stall of the bar relative to the concrete, at least in the zone of sliding closest to said crack and over a length such that the stall force of the bar relative to the concrete at least partially offsets the traction differential between the two materials when this differential causes the stress to be exceeded. adhesion on the length considered.

In addition, the excess traction remaining applied to the bar can be absorbed, at least in part, by the neighboring blocking zone extending beyond the first sliding zone, the opposite side to the crack.

Therefore, according to another particularly advantageous feature of the invention, from the appearance of a first crack in a first zone of weakness, the reinforcing bar is detached from the concrete coating in at least a first slip zone, the closest to said crack and an increase in applied tensile stresses successively determines the opening of at least one secondary crack in another weakness zone of the concrete part and the stall of the bar in at least another sliding zone, the closest to said secondary crack, and so on as and when an increase in tensile stresses applied, the sum of the thicknesses of the first crack and secondary cracks open at a given instant , being a function of the increase in the elongation of the bar resulting from the increase in the stresses applied at this instant and this increase in the elongation spread over all of the slip zones, as the secondary cracks appear. However, as noted above, cracks can occur in areas of weak concrete that are randomly distributed.

In the case where a first crack is formed at a first sliding zone, the local increase of the tensile stress applied to the tensioned bar resulting from the opening of the crack causes a stall of the bar of share and else of said crack over a total length for which the stall work of the bar relative to the concrete compensates for at least a portion of the traction differential between the two materials.

On the other hand, in the case where a first crack is formed at a first blocking zone, causing a local increase in traction applied to the tensioned bar, at least a first part of this increase in traction is absorbed by the two parts of the first blocking zone extending on either side of the crack and the remaining part of the increase of traction on the bar is compensated by the stall force of the tension bar relative to the concrete on at least part of the nearest sliding area.

As a result, the number and distribution of the locking zones and the corresponding lengths of the sliding zones can be determined according to the distribution and the foreseeable values of the tensile stresses along each tensioned bar, taking into account the loads applied. so that the thickness of each crack does not exceed a given limit.

Advantageously, the relative lengths of the locking zones and sliding zones distributed along each tensioned bar are determined taking into account their position, so as to give the part the stiffness necessary to remain within a range of values allowed for the arrow of the room under a given load. According to another particularly advantageous characteristic of the invention, each locking zone extends over a length at least equal to a so-called sealing length of the reinforcing bar determining an adhesion stress at least equal to the maximum stress of allowable traction by said bar, and not more than twice that length of seal. The invention also covers the parts thus produced and the reinforcing bars used for the implementation of the method and comprising a discontinuous series of locking zones separated from each other by sliding zones.

Normally, each sliding zone of a stretched longitudinal bar has a smooth outer surface in the longitudinal direction. However, each stretched longitudinal bar having, in cross section, the area necessary for the desired tensile strength, the profile of said bar in each sliding zone can advantageously be adapted to give it the perimeter necessary for the contact area between the bar and the concrete provides a frictional connection to achieve the desired limit value of the tangential stress of adhesion in said sliding zone.

In particular, each stretched longitudinal bar may have, in cross section a flattened profile with a width greater than the thickness, so as to increase the perimeter relative to that of a circular bar having the same transverse area.

Particularly advantageously, each stretched longitudinal bar has, in cross-section, a corrugated profile with longitudinal, recessed and projecting portions, extending parallel to the axis of the bar, along the entire length of each sliding zone. . In another particularly advantageous embodiment, each sliding zone comprises a layer of particles releasably attached to the outer surface of the bar and protruding into the coating concrete so as to increase the bonding adhesion. with concrete and the limit value of the stress of adhesion from which an increase in tensile stresses causes the stall of the bar. Indeed, these particles are gradually detached one after the other, the bar remaining embedded in the concrete, as and when increasing tensile stresses, which allows to maintain the stress of adhesion to its limit value over a range of increase of said tensile stresses.

These particles may consist of grains of sand or gravel bonded to the outer surface of the bar or sprinkled and applied under pressure thereon at high temperature at the outlet of the rolling mill.

These particles may also consist of metal balls or filings fixed on the outer surface of the bar by electrofusion in contact.

Preferably, the particles thus fixed on the outer surface of each sliding zone of the bar, have various dimensions so as to become progressively detached according to the size of the fixed part, as and when the stresses of applied traction.

Other particularly advantageous features of the invention will become apparent from the following description of certain particular embodiments given by way of example and illustrated by the accompanying drawings.

Figure 1 is a perspective diagram of a reinforced concrete part such as a beam.

FIG. 2 is a moment-strain diagram illustrating the constitutive law of a part subjected to bending stresses. FIG. 3 is a stress-strain diagram indicating, according to the type of reinforcement, the evolution of the tangential stress of adhesion as a function of the elongation of the reinforcement.

Figure 4 is a diagram of a bending machine on a beam. Figure 4a is a diagram showing, for such a beam, the variations of the applied tractions, respectively, on a tension bar and on the concrete coating.

Figure 5 is a schematic detail view, at a crack and in longitudinal section, of a beam armed with conventional high-adhesion bars. FIG. 5a is a diagram indicating, in the case of FIG. 5, the evolution, at the level of a crack, of the tractions applied on a tensioned bar and on the concrete.

Figure 6 is a detail view in longitudinal section, of an armed part of rebar according to the invention, in the case of the formation of a crack in the right of a sliding zone. FIG. 6a is a diagram showing, in the case of FIG. 6, the variations of the tractions applied on a tensioned bar and on the concrete.

Figure 7 shows two cross-sectional views of a test beam, left in the vertical midplane and right at a blocking area.

Figure 8 illustrates the process of crack formation on several beams subjected to a first series of bending tests.

FIG. 9 is a diagram showing, for the various beams, the arrows obtained during this first series of tests, during the progressive increase of the applied load.

Figure 10 is a diagram indicating, for the various beams, the number of open cracks according to the arrow.

FIG. 11 is a diagram indicating, for the various beams, the cumulative opening of the cracks as a function of the arrow.

Figure 12 is a detail view, in longitudinal section, of an armed part of reinforcing bars according to the invention, in the case of the formation of a crack at a blocking zone.

FIG. 12a is a diagram showing, in the case of FIG. 12, the variations of the pulls applied on a tensioned bar and on the concrete.

Figure 13 is a schematic view of a tear test device on a metal bar embedded in a concrete specimen. Figure 14 shows schematically, in longitudinal section and in cross section, a second type of test beam provided with reinforcing bars according to the invention.

FIG. 15 is a table showing the results of a second series of bending tests carried out on beams of the type of FIG. 14.

FIG. 16 is a table indicating, for a test beam, the order of appearance of the cracks, their location and their thicknesses as a function of the load applied.

Figure 17 shows, in cross section, a round bar and a flat bar, provided with directional imprints. FIG. 1 shows schematically, in perspective, the conventional arrangement of a molded concrete part 1, inside which a reinforcement cage 2 is embedded. In the example represented, the part 1 is a beam with rectangular cross section, extending between two supports spaced apart by a distance L and having two facing faces, respectively lower 11 and upper 1 1 ', and two vertical side faces, respectively 12, 12'.

As is known, when such a beam is subjected to a bending force under the effect of a vertical load, its lower part T placed below a neutral line 10, is subjected to tensile stresses, and its upper part C is compressed. To withstand the stresses, the reinforcement cage 2 comprises two layers of longitudinal bars, respectively a lower sheet of bars 21 called bending, and an upper layer of bars 22 said mounting respectively parallel to the two faces of facing 11, 11 of the beam 1 and extending at a minimum distance of coating thereof. To withstand the stresses of shear forces, the two layers of longitudinal bars are connected by transverse reinforcements forming rectangular stirrups spaced apart from each other and distributed over the length of the beam.

All these arrangements are well known, Figure 1 being a simple example. In particular, the number of rebars, their cross-sectional areas and their arrangement depend on the shape of the part and the loads applied.

FIG. 2 is a classical moment-strain diagram, illustrating the behavior of the part 1 when it is subjected to a progressively increasing bending moment, indicated on the ordinate and causing an arrow, indicated on the abscissa, which increases with the applied load , causing a corresponding elongation of the stretched portion T and the lower facing face 1 1.

In the first part OA of the diagram, which corresponds to a linear elastic behavior of the part, the tensioned bars 21 and the concrete which coats them are joined by adhesion and simultaneously elongate up to a curvature C1, corresponding to the point A, to from which the tensile stresses generated by the curvature of the part reach the tensile stress limit of the concrete. It then discharges on the tensioned bars 21 which only take up the tensile stresses. This results in an almost instantaneous increase in the curvature of C1 to C2, corresponding to the AB bearing, with an extension of the tensioned lower bars 21 and a beginning of cracking.

As the load and, consequently, the bending moment increase, other cracks appear and open gradually. The piece then follows a moment-deformation law corresponding to the section BC whose slope depends on the mechanism of the adhesion which imposes a relative sliding of the tensioned bars with respect to the concrete with, correlatively, a variation of the position of the neutral axis. The slope of the line OA corresponds to the flexional stiffness E _c l of the part, E ₀ being the modulus of elasticity of the non-cracked concrete and I its inertia. Similarly, the slope of the line OB corresponds to the stiffness E _c l _f , where _f is the inertia of the part after the first cracking.

Steel reaches its elastic limit at point C of the curve. This results in a progressive plasticization of the two materials and, consequently, a slight change in the bending moments and a low stiffness of the part which results in a lower slope of the section CD. The maximum bending moment M3, which corresponds to the saturation of the capacities of the weaker of the two materials, is reached at the point D from which the part has a stiffness of zero, the deformation being able to continue with an elongation of the stretched part until the breaking of the reinforcing bars 21 which alone resist the tensile stresses.

As indicated above, this behavior of the part under the effect of a growing bending moment results in a corresponding evolution of the tangential stresses of adhesion along the tensioned bars 21, as illustrated by the diagram of FIG. 3. It can be seen, in particular, that from a tangential stress τ _A corresponding to the tensile rupture limit of the concrete, there is a slight slip of the reinforcement with formation of microcracks, the tangential stress of adhesion progressively increasing to a value τ _B which corresponds to the steel-concrete adhesion failure and from which the notches or blocking ribs of the high-adhesion bars come into play while coming into abutment on the concrete of coating. There is then a widening of the first cracks and a transverse cracking of the concrete which develops at the ribs of the reinforcement until the adhesion stress reaches an ultimate value τ _u corresponding to the rupture of the steel connection. -concrete.

However, if this ultimate adhesion stress is greater than the maximum tensile stress allowed by the steel, it is the rebar that yields, causing the ruin of the construction.

The inventor has deduced from this that the phenomenon of armature failure, which sometimes occurs in the event of an excessive increase in stresses, for example due to seismic shocks, could be related to the mode of operation of the high-strength reinforcing bars that it is usually used to increase the tangential stress of adhesion.

To solve this problem, the inventor has therefore analyzed the behavior, in the case of bending under the effect of a load, of the stretched part of a reinforced concrete part such as a beam or a slab resting on two supports. , in which is embedded a reinforcement cage having a lower layer of high-adhesion bars which are provided, along their length, transverse locks oriented obliquely to the longitudinal axis of the bar, to ensure a solidarization continues with the concrete of coating. As indicated above, with reference to FIG. 2, when the bending moment applied to the part determines a curvature C1 for which the tensile stresses resulting from the elongation of the tensioned part correspond to the maximum tensile strength of the concrete, one or more cracks begin to open. When the load is located, a first crack appears, normally, at its point of application. On the other hand, when the load is applied in two separated points, the tensile stresses are substantially the same between the two points of application of the load and cause, in this part of the room, the appearance of a certain number of cracks. These cracks are relatively randomly located because, during the casting, the constitution of the concrete, in particular the distribution, the granulometry and the degree of cleanliness of the aggregates, as well as the quality of the cement, can vary slightly, so that the room may have certain zones of structural weakness inherent to the quality of the concrete, for example air bubbles or more fragile or less clean aggregates, which favor the appearance of microcracks tending to widen when the load applied and as a result, the curvature of the workpiece increases.

This is the case, in particular, when an excessive overload is applied to a concrete structure, for example the crossing of a truck exceeding the axle load limit, in the event of an accidental collision of a vehicle. on a bridge stack, or during an earthquake.

As indicated above, this sometimes results in the breaking of some reinforcing bars and the ruin of the structure.

The inventor has sought to solve such problems and has specifically studied the conditions under which reinforcement and concrete work together to resist the stresses applied.

For this purpose, he has carried out bending tests on armed beams in various ways, notably by observing the location of the cracks, their order of appearance and by measuring their thickness, according to the load applied.

FIG. 4 shows, for example, a frame-shaped bending test machine 4 having a cross member 41 fixed at its ends to two columns 42 between which is placed a test beam 5 resting on two spaced apart supports. 43 via ball joints 44, 44 '.

The beam 5 is subjected, in its central part, to a progressively increasing load by means of a jack 45 bearing in one direction in the center of the cross member 41 and, in the other direction, on the beam 5, by the intermediate of a spreader resting on two ball bearings 46, 46 'apart, for example, a distance of 1 m.

By means of the jack 45, it is thus possible to subject the beam 5 to a gradually increasing bending moment. As indicated above, under the effect of the applied load, the stretched portion T of the workpiece tends to lengthen, and in the OA portion of the diagram of Figure 2, the tension bars and the concrete lengthen. the same way. However, the resulting tensile stresses apply differently on tension bars and on concrete which are subjected to T1 and T2 pull-ups in a ratio of approximately 1 to 15, respectively.

It can be assumed that the tensile stresses remain constant in the most stressed part, between the two points 46, 46 'of application of the load.

In the diagram of FIG. 4a, which shows the evolution of the tensile stresses applied respectively to the concrete and to the reinforcing bars, the two curves T1, T2 thus each have a step between the application points 46, 46 .

FIG. 5 is a schematic detail view showing, in longitudinal section, the behavior, in its most stressed part, of a reinforced concrete beam 1 comprising, in its stretched part, a sheet of reinforcing bars 21 at high adhesion provided, therefore, ribs 23 along their entire length and in which a crack 3 opens. FIG. 5a is a diagram indicating on the ordinate the tensile stresses applied, respectively, on a tensioned bar 21 and on the concrete of FIG. 'coating.

As shown in this diagram, during the opening of a crack 3, the tensile stress T1 applied to the concrete vanishes at the crack and the tensile stress T2 on the steel increases correlatively. This results in an increase in the elongation of the bar 21.

However, it is known that, in the case of a high-adhesion bar, the pull-out strength of this bar is greater than its tensile strength if the sealed length exceeds a certain length called "sealing length", at from which the bar is totally blocked in the concrete. In the case of a reinforced concrete part, the length of a longitudinal reinforcing bar greatly exceeds this length of sealing. Therefore, when a crack initiation 3 is created in the concrete, the two portions 21a, 21 'of the bar extending into the concrete, on either side of the crack 3, are completely blocked by the ribs 23 and it is therefore only the length of steel 24 corresponding to the thickness e of the crack, which is subjected to elongation. In case of increase in the stress, the crack width 3 can be enlarged, e.g., 1/10 ^th to 2/10 ^th and 3/10 ^th of a millimeter, which means that the free steel length 24 in the crack will be brought to double and then triple, the sealed portions 21a, 21 'remaining locked in the concrete. As no steel can withstand such an elongation, an excessive increase in the stresses causing a widening of the crack and, consequently, an excessive elongation of this short length of the bar will cause, by necking, the sudden rupture thereof with a risk of collapse of the structure.

The inventor therefore thought that it would be interesting to allow the concrete to drop in the vicinity of the crack, so that the bar could lengthen the length necessary under the effect of applied tractions, without causing disorder in the concrete coating or necking of the steel.

For this, he had the idea to include in the high-adhesion bar slip areas devoid of notches or anchoring ribs and thus allowing a detachment of the bar without disorder in the concrete.

However, as indicated above, if the cracks first form in the most stressed part of the room, their location remains random because it depends on the quality of the concrete which can not be absolutely homogeneous.

It is therefore advantageous, to take account of this random distribution of cracks, to provide, along a reinforcing bar, several smooth zones spaced apart from each other.

On the other hand, the cumulative length of these smooth zones must be limited, so that each tensioned bar remains of the high-adhesion type over most of its length, in order to keep the concrete part a stiffness allowing to limit its deformation under flexion. We have therefore developed a new type of rebar whose principle is shown schematically in Figure 6.

According to the invention, instead of being formed, as usual, over the entire length of the reinforcing bar 21 in order to achieve a continuous blocking, the notches or ribs 23 are arranged in spaced apart locking zones 25 each having a length I and separated from each other by a zone 26 having a smooth surface and extending over a distance d.

As previously, the forces applied on the reinforced concrete part 1, for example a bending moment, cause the extension of the stretched portion T of the part and, consequently, the placing in tension of each tension bar 21 and the concrete 16 which enrobe, with the appearance of at least one crack primer 3 when the tensile strength of concrete 16 is exceeded.

An increase in the tensile stresses applied determines, correlatively, an increase in the adhesion stresses on either side of the portion 24 of a tensioned bar 21 corresponding to the opening of the crack 3 which, in the case of the Figure 6, is formed at the smooth zone 26 between two locking zones 25. At the right of this crack 3, the traction differential between the tensile stress of the steel T2 and that of the concrete T1 is maximum. When the shear stress applied by this traction differential exceeds the peel strength of the steel which is lower in the smooth zone 26a of the bar, the bar will become detached from the concrete.

It thus forms, on both sides of the crack 3, two unhooked parts 27, 27 '(FIG. 6) extending over a total length 2 which is a function of the increase in tensile stresses applied, due to the opening of the crack.

Because they are unhooked concrete, these two parts 27, 27 'of the bar will be able to lie freely and the elongation corresponding to this increase in tensile stresses, will therefore be distributed over the length 2d 'of the unhooked part. For example, if the unhooked length 2d 'is 50 mm, the bar 21 can extend from 50 to 50.1 then 50.2 then 50.3 millimeters if the crack increases from 0.1 to 0.2 then to 0.3 millimeters. A steel bar can perfectly support such elongation distributed over a length of about 50 millimeters whereas, in the case of FIG. 4, this elongation was limited to the only free portion 24 of the bar, corresponding to the width of the crack.

In addition, as and when the increase in traction, the recess will be able to extend over the entire length d of the smooth zone 26a, along which the concrete coating is therefore not subject to any tensile stress. As shown in FIG. 6a, the tensile stress T1 then vanishes over the entire length of the smooth zone 26a and presents a bearing at this level, on either side of the crack 3, the tensile stress T1 on the steel increasing correspondingly on a bearing of the same length. As a result, the resulting elongation of the bar will be distributed over this entire length of time, without any mess in the concrete. With this possibility of extending the bar 21 at a crack, along a smooth zone 26, it will therefore be possible to avoid or, at least, considerably reduce the risk of breaking frames that can lead to the ruin and the brutal collapse of a work.

A reinforced concrete beam or slab will therefore better resist the passage of a load exceeding the limit for which it was calculated or localized overloads resulting from an earthquake.

Indeed, because the sliding zones can be distributed over the entire length of the reinforcements, we can benefit from the possibility of extending them in any place where can form cracks tending to widen. On the other hand, in the unhooked area of the bar, concrete is no longer driven by steel.

The crack is therefore less likely to widen and no other crack can appear over the length d of the unhooked zone 26a since the concrete is no longer taut.

So we had the idea that by spreading smooth parts along each stretcher bar, it would be possible to enlarge the area where can appear randomly cracks and increase the number of them by correspondingly decreasing their thickness Max.

In order to develop this new type of high-adhesion reinforcement, several series of beams provided with reinforcing bars according to the invention have been subjected to bending tests carried out under the same conditions on a test machine of the following type. 4 to allow comparisons, all these test beams were similarly armed by a reinforcement cage 2 shown schematically in cross-section in FIG. 7 and in longitudinal section on the various views of FIG. Figure 8. To take into account essentially the role of the tensioned bars, this reinforcement cage 2 has a triangular shape having only three longitudinal bars, respectively two lower bars 21 in the stretched portion of the beam 5 and an upper bar 22 in the compressed part, said bars being connected by triangular stirrups 20. FIG. 8 shows the results of FIG. a first series of bending tests carried out on six types of beams, respectively 51 to 56, in which the reinforcement cages are made in the same way and comprise, for simplicity, only three securing brackets 20 placed respectively in the central part and at both ends of each beam. Figure 7 schematically shows such an arrangement on its left side which is a cross-sectional view along the line AA of Figure 8, at the central yoke.

According to the invention, the lower longitudinal bars of the test beams are provided with locking zones whose number and distribution vary from one beam to another.

To facilitate the realization of the locking zones, these tensioned bars 21 consist of smooth metal strips with a flattened section, as indicated in FIG.

In fact, in this first series of tests, to simplify the construction of the beams, flat bars with a smooth surface were used with simple locking points made by small transverse irons 28 welded to the flat upper faces of the longitudinal bars. as shown on the right side of Figure 7 which is a cross-sectional view along line BB of Figure 8, the number and distribution of these transverse irons varying according to the type of test beam.

Thus, the first beam 51 diagrammatically represented in the upper part of FIG. 8, comprises a single central blockage ₀ formed by the lower part 20a of the central yoke 20, welded to the two bars 21 which, in a conventional manner, are simply provided with anchor bolts at both ends.

The second beam 52 is provided, on the other hand, with five blockages comprising the same central locking at ₀ and, on either side thereof, two pairs of transverse irons welded on the bars 21, and constituting four blockages, respectively a _u a ₂ on one side and a \, a ' ₂ on the other side. It is thus possible to vary the number and spacings of the blocking points constituted by the transverse irons placed on either side of the central blocking 20a and more or less spaced from each other, all test beams having the same scope. for example 1.5 m between the supports 44, 44 'for a distance of 0.30 m between the load application points 46, 46'. For example, in the case of the beam 52, the four transverse irons 27 constituting, with the central iron 20a, the five blocks a ^, a ₂ , a ₀ , a \, a ' ₂ are separated one of the another from a distance of about 25 cm for a span between supports 1.5 m. As shown in Figure 8, the beam 53 has four transverse irons, each side of the central iron 20a and, therefore, nine blockages respectively bi .... b ₄ , a ₀ , b'i b ' ₄ apart the one of the other about 14 cm. The beam 54 has seven transverse irons on each side of the central iron 20a, 15 blocks spaced apart by 9.4 cm. The beam 55 has 10 transverse irons on each side of the central iron 20a, 21 blocks spaced apart by 6.8 cm and the beam 56 comprises 30 transverse irons 31 blocks spaced 4.7 cm apart.

All these beams were subjected to bending tests under the same conditions and the appearance of microcracks was observed during the progressive increase in the load applied by the jack 45.

In the various diagrams of FIG. 8, the location of the cracks and the order in which they appeared have been indicated by vertical lines.

Thus, on the beam 51 comprising only a central blocking at ₀ , the successive appearance of two cracks, respectively a first U crack on the right and a second crack f ₂ on the other side, is observed on either side of it. left, these two cracks being located in the central part, the most stressed, of the beam, substantially equidistant from the vertical median plane.

The beam 52 comprises, on each side of the central locking at ₀ , two locking points spaced apart by a distance of approximately 25 cm for a span of 1.5 m between the two supports 44, respectively a ^, a ₂ , on the left and a \, a ' ₂ , on the right. During the increase of the bending moment applied, successively appears a first crack to the left of the central lock at ₀ , a second crack f ₂ to the right, a third crack f ₃ to the left of the first crack f 1 and a fourth crack f ₄ to the right of the second crack f ₂ .

Everything happens as if the sliding zones attracted new cracks, thus avoiding an enlargement of the first crack fi.

It has thus been observed that, thanks to this particular constitution of the tensioned reinforcing bars, from the appearance of a first crack and the stall of the bar in a first sliding zone between two blockages, an increase in the stresses applied traction successively determined the appearance of other cracks and the stall of the bar firstly in sliding zones close to this first cracked zone and then, depending on the value of the stresses, in other more remote sliding zones and so on, deviating on either side of the first sliding zone as the tensile stresses applied are increased.

However, it can be observed that the cracks are not located perfectly symmetrically on either side of the median plane of the beam because, as indicated above, the risk of opening a crack depends on the quality of the crack. concrete that is not absolutely homogeneous. For example, in the case of the beam 52 with five blockages, the four cracks observed are located in the central part of the beam, between the blocks a ^ and a'i, on either side of the central block at ₀ .

In the case of the beam 53 provided with nine locking points, the appearance of the first crack fi is observed on the sliding zone hi, to the left of the central locking at ₀ , then successively, the appearance, to the right of the blocking central to ₀ , a second crack f ₂ on the sliding zone h'i, then a third crack f ₃ on the sliding zone h ' ₂ , to the right of the blocking b'1, a fourth crack f ₄ in the sliding area h ₂ and a fifth fissure ₅ f in the sliding area h _'3 on the right of the locking b' _2. Figure 8 also shows the location and order of appearance of the cracks on the beams 54 (15 blocks), 55 (21 blocks) and 56 (31 blocks). It can be seen that, apart from particular cases that may be due to the constitution of the concrete, such as the crack f ₈ for the beam 54 and the crack f ₇ for the beam 56, these cracks are first located in the central part of the concrete. beam and then move away more and more from the median plane during the increase in the load applied by the jack 45.

The use of reinforcing bars comprising an alternating series of spaced sliding zones, separated from one another by separated blocking points, will therefore make it possible to distribute the cracking over a certain length of the beam, the appearance of a crack in a sliding zone causing the detent of the bar in this sliding zone by canceling the tensile stress on all the cut-in concrete, so that the widening of the crack is limited in this unhooked zone and that no other crack will therefore tend to form there, this part of the room being, so to speak, "vaccinated".

Thus, from the appearance of a first crack and the stall of a longitudinal bar tensioned in a first sliding zone, the gradual increase in tensile stresses applied will successively determine the appearance of cracks and the stall of the bar, firstly in or in the vicinity of the sliding zones located on either side of this first cracked zone and then, depending on the value of the stresses, in other more distant sliding zones on either side the first unhooked zone and so on, deviating on either side of it as and when the tensile stresses applied.

It has therefore been suggested that by judiciously distributing blocking zones and sliding zones along the tensioned reinforcing bars, it would be possible to increase the number of secondary cracks appearing successively on either side of the zone comprising the first crack and, correspondingly, reduce the thickness thereof, each secondary crack having a thickness such that the sum of the thicknesses of the first crack and secondary cracks open at a given instant, is a function of the overall elongation of the bar that results from the constraints applied at this moment. This lengthening is distributed therefore on all the unhooked parts reinforcements that correspond to these secondary cracks, as and when they appear.

For the implementation of the invention, it will thus be possible to determine the number and distribution of the locking zones, their respective lengths and those of the sliding zones, depending on the distribution and the predictable values of the tensile stresses. along each stretched bar, so that the thickness of each crack does not exceed a given limit.

This is illustrated by the diagrams of FIGS. 9, 10 and 11 which summarize the results of the tests carried out on the various beams. FIG. 9 is a diagram indicating, for each beam, the deflection measured during the loading tests and corresponding to the load indicated on the ordinate. We see that each beam has a limit from which the curve tends to an asymptote, the beam no longer opposing resistance to deformation. As expected, this limit is the lowest for the curve 1 corresponding to the beam 51 of FIG. 8, the sudden drop in the resistance corresponding to the stall of the tensioned reinforcements which have a smooth surface, on both sides. other central locking 20a.

The curve 2 which corresponds to the beam 52 with five blockages, has an upper limit and it is noted that the increase in the number of blockages gives the beam a higher resistance but only up to a certain limit. Indeed, the beam 56 having thirty-one blockages and corresponding to the curve 6 has a resistance a little lower than that of the beams 53, 54, 55.

It can be seen that the maximum strength of these three beams is practically the same while the number of blocking zones varies by more than 40%. The addition of blockages thus makes it possible to increase the resistance of the beam but only up to a threshold from which this resistance decreases. The best result is obtained for the beams

54 and 55 respectively corresponding to the curves 4 and 5 and comprising respectively fifteen and twenty-one blockages.

FIG. 10 is a diagram indicating, on the ordinate, the number of cracks that appear during the increase of the arrow indicated on the abscissa. As indicated above, for the beam 51 comprising only a central blocking, only two fi f ₂ cracks appear, the width of which therefore increases progressively during the increase of the deflection.

The greatest number of cracks is obtained for the truss 54 (curve 4) with nine cracks and for the truss 55 (curve 5) with eight cracks. It should be noted that these cracks appear fairly quickly, before the arrow exceeds 10 millimeters for a range of 1, 5 m between supports. It is therefore interesting to relate this diagram to that of FIG. 11 which indicates, on the ordinate, the cumulative opening of the cracks as a function of the arrow indicated on the abscissa.

Apart from the curve 2 corresponding to the beam 52 with five blockages, it is found that the cumulative opening of the cracks is about the same for the other beams as the arrow remains small.

The curves 4 and 5 corresponding to the beams 54 and 55 show that there is an optimal spacing between blockages making it possible to obtain the greatest number of cracks with a limited cumulative opening, this spacing being a compromise between the resistance of the beam and the number of cracks.

It therefore emerges from this first series of tests that the use of reinforcing bars comprising, according to the invention, a series of sliding zones separated by locking points, makes it possible to distribute the cracking over an area that can reach to 2/3 of the length of the beam and, thus increasing the number of cracks, to limit their openings. It will therefore be possible to comply more easily with the regulations which impose a maximum opening not exceeding 0.2 to 0.3 mm, at most 0.5 millimeters and, consequently, to limit the risk of corrosion over time.

In addition, the alternation of locking points and stall zones makes it possible, at each crack, to distribute the elongation of the tension bars over a fairly long length and, consequently, to avoid the risk of failure by necking. reinforcing bars in the most stressed zones in the event of excessive and / or localized increase in tensile stresses.

However, in this first series of tests, carried out to study the influence of the sliding zones, the blockages, consisting of simple transverse irons were punctual. However, as indicated above, it is preferable, in order to maintain the desired stiffness of the part, that each reinforcing bar remains of the high-adhesion type over the greater part of its length, the sliding zones having a smaller length than the blocking zones between which they are arranged, as shown diagrammatically in FIG.

In addition, the distribution of cracks being random, it is possible that in the most stressed part of the room, blocking zones present a particular point of greater weakness, such as a dusty air bubble or granulate. and that the crack is created there, as diagrammatically shown in FIG.

In this case, as shown in the diagram of FIG. 12a, the tensile stress T1 in the concrete vanishes at the crack 3 and consequently, an increase in the tensile stress T2 which is taken up by the two parts 25b, 25'b of each tension bar 21 extending on either side of the crack 3.

However, as indicated above, a high-adhesion bar is totally blocked in the concrete and withstands a tensile stress that can reach the elastic limit of the steel if it is sealed in the concrete for a minimum length I ₀ which is called sealing length. This sealing length depends on the quality of the concrete and the nature of the rebar. In the case of a round bar, this sealing length can be of the order of 10 to 12 times its diameter for a high-adhesion bar and 20 to 25 times the diameter for a smooth bar.

This can be demonstrated by a peel test carried out, for example, in the manner illustrated in FIG. 13, on a steel bar 6 sealed in a concrete specimen 60.

This bar 6 is extended outside the test piece 60 by a free portion 61 on which is applied a tensile force by clamping jaws 62, by means of cylinders not shown bearing on the front face of the test tube 60.

By varying the forces applied and the length of the sealed bar in the test piece, it is possible to determine the minimum length of seal (I ₀ ) of the bar from which it resists the applied tension, without any disturbance in the concrete, up to the elastic limit of the steel, that is to say until breaking by necking of the bar 6 outside the specimen.

A measuring device 63 such as a load cell, attached to the opposite end 61 'of the bar 6, makes it possible to check whether the length L sealed in the test piece 60 exceeds the minimum sealing length (I ₀ ), the traction applied to the end 61 'of the bar 6 opposite the jaws 62 being, then, zero. Indeed, the tensile stress applied by the jaws 62 on the front end 61 decreases progressively along the sealing length (I ₀ ) and is zero on the remaining part of the bar 6.

In the same way, as shown in FIG. 12a, if, on one side of the crack, the length (h) of the part 25b of the blocking zone 25 is greater than the sealing length (I ₀ ). Δt increase of the traction applied to a bar 21, due to the opening of a crack 3 is maximum to the right of the crack 3 and gradually decreases on either side of it, to become zero at a distance (I ₀ ) of the crack, the traction applied to the bar then taking its average value T2.

On the other hand, if the length (I ₂ ) of the remaining part 25'b of the blocking zone is less than the sealing length (I ₀ ), this high-adhesion part 25'b can absorb only a part of the tensile increase Δt and, at its end 29, there remains an additional stress Δ't which is transmitted to the adjacent sliding zone 26b, the same additional stress Δ't to be absorbed by the concrete d 'coating.

The traction differential 2Δ't between the steel and the concrete is balanced by the tangential stress of adhesion along this smooth part 26b. However, the tests show that, in the case of a smooth bar, the sealing length determining a total blockage of the bar relative to the concrete coating is of the order of 20 to 25 times its diameter. In addition, as indicated above, the length of the smooth zones 26 formed along a tensioned bar 61 must be relatively limited so as not to excessively reduce the stiffness of the part. Therefore, the length d of the sliding zone 26b of the bar is normally less than the sealing length _o of an equivalent smooth bar, and this portion 26b will therefore fall out of the concrete as a result of the traction differential 2 Δ't, if it is greater than the tangential stress of adhesion of this smooth zone 26b. The two curves T1 and T2 then have a bearing along the entire length of the sliding zone 26b, as shown in FIG. 12a.

The excess traction Δ't is therefore applied to the next blocking zone 25 'and absorbed by it, the traction on the bar 61 then returning to its average value T2, just as the traction absorbed on the concrete resumes its value. T1.

However, such a detachment of the sliding zone 26b assumes that at the end of the adjacent blocking zone 25'b, there is still more traction of the steel relative to the concrete, and this is not possible only if the part 25'b of the blocking zone does not exceed the sealing length I ₀ of a high-adhesion bar. In addition, as we have just seen, it is necessary for the length d of the adjacent sliding zone 26b to be such that this zone can be unhooked by this additional traction.

In addition, it appears that, in the case of the formation of a crack at a blocking zone, to prevent total blockage of the bar relative to the concrete resulting in a risk of rupture by necking thereof, the length of this blocking zone must be less than twice the sealing length I ₀ . In this way, indeed, the increase in traction resulting from the formation of a crack at a high adhesion zone will be absorbed only partially by one of the parts of this zone placed on one side of the crack and transmitted to the neighboring slip area that will fall under the effect of the traction differential and thus allow a corresponding elongation of the bar.

Similarly, the length of each smooth zone must not exceed the sealing length of an equivalent smooth bar, so that the traction differential between the steel and the concrete allows its recess at the end of the blocking zone. previous.

Furthermore, as indicated above, it is necessary that the traction differential between the steel and the concrete, at the end of a blocking zone, is sufficient to cause the recess of the adjacent sliding zone. However, this traction differential will be all the more important as the length of the locking zone will be small.

One can thus deduce a correlation between the lengths of the blocking zones and those of the smooth zones which can be even longer than the previous HA zones are short, while remaining lower than the equivalent sealing length.

For the realization of the reinforcing bars according to the invention comprising locking zones and alternating sliding zones, it will therefore be interesting to adapt the number and the relative lengths of these areas, in order to choose an optimal distribution according to the desired result.

For this purpose, a second series of bending tests was carried out under the same conditions, by means of a machine of the type shown in FIG. 4, on beams armed with bars according to the invention, and in which the number, distribution and relative lengths of the blocking zones and the sliding zones have been varied.

FIG. 14 shows, in cross-section on its right-hand side and in longitudinal half-section on its left-hand side, such a test beam 7 in which is embedded a reinforcement cage 2 comprising, as previously, two lower longitudinal bars 71 and an upper longitudinal bar 72 connected at both ends and in the central part of the beam by stirrups 70 of triangular shape.

As before, the tensioned bars 71 consist, in the tests carried out, of strips of rectangular section, having, for example, a width of 25 mm and a thickness of 3.5 mm. The test beams thus produced were subjected to bending tests on a machine of the type shown in FIG. 4, with a distance of 0.30 m between the points of application of the load 46, 46 'and a bearing. 1.5 m between the fulcrums 44, 44 '.

To easily vary the number, distribution and relative lengths of the sliding zones and the blocking zones, these have been formed by sections of high-adhesion irons (so-called HA) 73 which are welded to the longitudinal bars 71 and separated from each other by free zones 74. The use of flat bars 71 facilitates the welding of the irons 73 on the flat upper face thereof.

In this way, along the length of a longitudinal bar 71, it is possible to vary the number and the relative lengths of the high-adhesion irons 73 which constitute blocking zones and spaces 74 which constitute sliding zones, armature being only constituted, at this level, a smooth band.

It has thus been possible to produce a series of test beams provided with reinforcing bars of different types, which have been subjected to bending tests by applying, on the spaced supports 46, 46 ', a progressively increasing vertical load. . During each test, the applied load and the corresponding deflection taken by the beam in its median plane were measured and the order of appearance and location of the cracks was measured by measuring their thicknesses.

The table in Figure 15 groups together the results of bending tests carried out on three series of five beams all having a length of 1.8 m for a support span of 1.5 m and a distance of 0.30 m between them. load application points 46, 46 '. To allow measurements to be made, the beams were divided into 10 cm wide sections in order to identify the order of appearance of the cracks and to locate them by measuring their distances from the left end of the beam, such as shown in the diagram of Figure 16. Each beam is marked by a number of three digits, the first two digits indicating the length, in centimeters, of the irons HA constituting each blocking zone and the third digit indicating the length, in centimeters, of the smooth zones interposed between two blocking zones. successive. Thus, the P061 beam has 6 cm blocking zones separated by smooth zones of 1 cm.

The five beams of the first series therefore all comprise blocking zones having a length of 6 cm separated by smooth zones whose length varies from 1 cm for the P061 beam to 5 cm for the P065 beam. For each beam, the number of cracks, the thickness of the widest crack and the deflection reached at that moment by the truss were noted, as a function of the load applied and as and when they appeared. in its middle plane. The table in Figure 15 groups these results by columns each corresponding to a maximum width of cracks. For example, the P061 beam with 6 cm blocking areas separated by 1 cm smooth areas shows no cracks under a load of 7.5 kN while the boom is 3 cm in the median plane. On the other hand, under a load of 15 kN, 6 cracks appear whose thickness does not exceed 0.1 mm, the arrow reached under this load being 5 cm. Similarly, under a load of 30 kN, the arrow is 10 cm and we see appear 10 cracks with a maximum thickness of 0.3 mm.

For the beams of the second series, the blocking zones are 10 cm long and are separated by smooth zones whose length varies from 1 cm for the P101 truss to 5 cm for the P105 truss. The beams of the third series are provided with reinforcements having 14 cm locking zones separated by smooth zones whose length ranges from 1 to 5 cm.

As mentioned above, all the test beams are equipped with flat bars having a cross-sectional area of 25 x 3.5 mm which corresponds to that of an equivalent round bar of diameter 10.5 mm for which the sealing length is 10 to 15 cm. Even for the bars of the third series, the locking zones have a length less than double the sealing length and therefore do not risk determining a total blockage in the event of formation of a crack at this level.

The table of FIG. 15 shows that the distribution of the blocking zones and the smooth zones substantially influences the stiffness of the part, that is to say the deflection taken under a certain load, the number of cracks and their thicknesses.

It seems that the best configuration is that of P101 and P102 beams with 10 cm blocking areas as well as P141 and P142 with locking zones of 14 cm. Indeed, for the same maximum thickness of cracks, these beams can withstand a load 25 to 30% higher than the load allowed by the other beams.

For example, for a maximum thickness of cracks of 0.2 mm, the beams P101 and P102 support a load exceeding 30 KN while, for the other beams, such a load causes the opening of cracks having a thickness of 0.3 or even 0.5 mm.

On the other hand, it seems preferable to limit the length of the sliding zones to 30 mm, preferably 10 or 20 mm, the load supported, for the same maximum thickness of cracks, being lower for slip zones of 40 and 50 mm. In practice, the length of the sliding zones should be of the order of 5 to 30 mm. However, the table in FIG. 15 shows that an interesting result can also be obtained with beams P062 and P063 which combine pairs of low HA lengths and larger smooth zones.

It therefore appears that, in certain cases, reduced lengths of blocking zones may be of interest if they are associated with relatively large smooth zones allowing greater energy dissipation during their stall.

Such an association would be particularly advantageous for structures built in seismic risk areas or for applications presenting a risk of explosion or violent impact.

By way of example, the table in FIG. 16 indicates the evolution of the cracking for the P102 truss, which seems to give the best results since it can withstand a load of up to 39 kN, with a deflection of 12 cm. for a maximum thickness of cracks of 0.3 mm.

The beam is shown schematically above this table, to indicate the order of appearance and location of cracks. In the table, the first two columns indicate the applied load and the arrow measured in the middle of the beam, respectively, under this load.

The other columns indicate, for each of the cracks and in their order of appearance, the thickness of this crack as a function of the load applied.

It would have been expected that the crack f ^ which appears first in the central part of the beam always has a greater thickness than the others.

In fact, it appears that, if the cumulative opening of the cracks, indicated in the last column of the table, increases according to the applied load, four cracks f ₂ , f ₃ , f ₄ , f ₅ open very quickly to from a load of 15 kN, which then allows to limit the opening of the first crack fi which has the same thickness as the cracks f ₃ , f ₄ , f ₅ to a relatively large load, 39 kN , for which this thickness of 0.3 mm is still acceptable, the second crack f ₂ , farther from fi, having a slightly smaller thickness. Moreover, it is from this maximum load of 39 kN that the compressed part C of the beam begins to burst, the load can then increase only slightly until the ruin of the beam, without breaking the steels.

On the other hand, the tests show that the cracking extends over a length of about 2/3 of the span of the beam between the supports 44, 44 'and that, from the beginning of the crack, the zone where the first cracks appear is not limited to the central part of the beam, between the points of application of the load 46, 46 '. For example, in the case of the P102 beam, the cracks f ₂ and f ₃ form outside this central portion 46, 46 '.

As indicated above, all the beams were armed, in the stretched portion T, of flat bars 71 having a cross section of 25 × 3.5 mm, equivalent to a round bar having a diameter of about 10 mm. For the beam P102, the locking zones therefore have a length of the same order as the sealing length I ₀ .

These two series of tests thus confirm that the use of reinforcing bars comprising, according to the invention, an alternating series of locking zones and sliding zones makes it possible to distribute the cracking over a great length of the part, which can to go up to 2/3 of the span between supports and, thus, by multiplying the number of cracks to reduce their thicknesses and to increase substantially the load supported for a maximum thickness of cracks respecting the regulations, in order to avoid, in particular, the risk of corrosion of reinforcement. In addition, by providing, along the frame, smooth areas that can unhook the concrete and, thus, to lie freely in the right or near a crack, it avoids the risk of rupture of the frame by necking. This advantage is particularly important in areas with seismic risks, or in case of explosion or violent shock. Indeed, a part of a structure such as a beam or slab, for example, may eventually undergo a relatively large deformation without breaking frames and, therefore, without risk of sudden collapse of the structure, because the distribution of cracking over practically the whole range of the room and the dissipation of energy by detachment of certain smooth zones. Similarly, a bridge span accidentally subjected to too high a load, for example, to the passage of an exceptional convoy, may be deformed with, possibly, opening of many cracks that can be repaired thereafter, but without major risk for the holding of the work.

But the invention is obviously not limited to the details of the embodiments and examples that have just been described.

In particular, as indicated above, it has been used, for the production of test beams, flat reinforcing bars consisting of metal strips, the locking zones can thus be constituted simply by HA bars welded to the faces. planes of said bands. Indeed, such an arrangement allowed, for testing, to easily vary the length of the blocking areas and their spacing. However, the use of flat strips, such as reinforcing bars, which was the subject of patent application EP 1 191 163 by the same applicant, has many other advantages. In particular, as indicated above, the adhesion of the steel to the concrete being proportional to the contact surface and, therefore, to the perimeter of the steel, a flat bar which has a perimeter approximately 1, 6 times larger than that of an equivalent round bar having the same cross section, has a better adhesion. On the other hand, the strength of the tensile steels is a function of their section and their lever arm, that is to say the distance separating the center of gravity of the steel from that of the compressed part of the concrete. However, geometrically, this lever arm is substantially increased by the use of flat strips instead of round bars of the same section because, as indicated in the document EP 1 191 163 cited above, the connecting stirrups between the two layers bars can be welded or glued on the inner faces thereof, which allows to place the longitudinal bars closer to the corresponding facing faces of the room, while respecting the minimum coating distance. The result is, in addition, that one can, thus, achieve thinner pieces and, consequently, lighter for the same resistance.

Furthermore, in the tests carried out, the locking zones consisted of simple HA bars welded to the inner faces of the flat strips. In reality, these blocking zones could be realized differently. For example, the flat strips used as reinforcements could be made of a slit sheet after rolling. It would then be possible, during rolling, to make embossed or recessed impressions on both sides of the sheet forming, after slitting, the broad faces of the strip.

However, if the use of flat bars as reinforcements has many advantages, the invention can also be applied to all bar profiles, in particular round bars with round section. In this case, as indicated schematically in FIGS. 6 and 12, the bars according to the invention would differ from conventional high-adhesion bars in that, during rolling, the notches or blocking ribs are not produced in a continuous manner. , over the entire length of the bar, but only on remote blocking areas, alternating with smooth sliding areas.

Moreover, the invention has been described in the case of a beam or a slab but can be applied to all kinds of structures and to all forms of concrete parts such as beams, floors, slabs, sails, etc.

However, the use of reinforcing bars according to the invention has other advantages.

Indeed, the dissipation of energy required for the stepping of the steels relative to the concrete absorbs part of the energy causing the cracking such as a seismic shock, a movement of ground or an accidental shock and thus allows a better overall resistance of the work. In this respect, it will be possible to modulate the resistance to the offset of the reinforcements in order to adapt it to specific constraints and, in particular, to determine the distribution and the relative lengths of the locking zones and sliding zones according to the purpose. research. For example, the tests have shown that by producing locking zones having a length of the order of the length of seal, associated with rather short stall zones, not exceeding 20 mm, it was possible to increase the maximum permissible load without exceeding a maximum thickness of cracks of 0.3 mm, corresponding to the regulations. It would be possible, however, to increase the length of the sliding zones so that their recess dissipates a maximum of energy in the event of an accidental shock or seismic shock while admitting, then, a greater deformation under the effect of the applied loads. or by reinforcing the reinforcement.

But it is also possible to modulate the adhesion which is proportional to the contact surface of the concrete on the steel, acting on the profile, in cross section, of the reinforcing bars. In particular, as indicated above, the use of reinforcing bars consisting of flat strips with an oval or rectangular section makes it possible, for the same cross-sectional area, to enlarge the perimeter and therefore the contact surface and the surface area. energy required for the recess.

To further increase this contact surface without creating asperities in the direction of the future recess, it would also be possible to provide on the surface of the steel continuous impressions parallel to the longitudinal axis of the bar.

FIG. 17, for example, shows a round bar and a flat bar with a rectangular cross-section, both of which, in cross-section, have a corrugated profile with longitudinal, recessed and protruding portions 24, which are extend parallel to the longitudinal axis of the bar over the entire length of each sliding zone.

But we can also act on the surface of the steel by creating, on the surface of the bar, asperities consist of particles releasably attached to the outer surface of the bar and protruding into the concrete in order to increase the adhesion bond and the limit value of the adhesion stress from which an increase in the tensile stresses causes the stall of the bar. Advantageously, these protruding particles can be detached gradually one after the other remaining in the concrete, as the tensile stresses increase, so as to maintain the adhesion stress to a limit value on a range of increase of said constraints. These particles could be fixed by gluing on the outer surface of the bar, for example by sprinkling on it of the large sand applied under pressure on the bar, at high temperature, at the output of the rolling mill. We could also use metal particles such as steel chips, balls or filings fixed on the outer surface of the bar by heat sealing.

Such methods would make it possible to modulate the shear strength of the projections thus produced. For bonding, it would be possible to use more or less resistant adhesives and vary the size of the projections and, therefore, their bonded surface brought into contact with the steel.

It would also be possible to vary the size of the grains sprinkled on the bar and the pressure subsequently applied or the amperage of the weld when it is made electrically. Such methods would make it possible to maintain a plane contact surface between the steel and the concrete after delamination, since the breaking of uncoupling would not occur inside the concrete, as in the case of HA steels, but at the interface steel / protrusion, by breaking the bonding or welding, each protruding particle remaining included without disorder inside the concrete, after detachment.

It therefore appears that the use, according to the invention, of reinforcing bars having an alternation of locking zones and sliding zones has multiple advantages. Firstly, the distribution of cracking over a long length of the part makes it possible, by increasing the number of cracks, to reduce their thickness, and consequently, the risk of corrosion of the reinforcements over time. It will also be possible, because of the small opening of the cracks, to protect the reinforcements from the risk of corrosion by means of a layer of paint or a suitable coating product. On the other hand, in case of excessive opening of a crack, it avoids the risk of rupture of the reinforcement by necking allowing it to unhook concrete on a length that can, so lie down.

But this recess also causes energy dissipation and it will be possible to determine the distribution and relative lengths of the locking zones and smooth areas so as to modulate the ability of the part to withstand abnormal stresses without risk of collapse of the structure as a result of an accidental breaking of the reinforcements. In practice, for each structure, the distribution of the blocking zones and the sliding zones can be determined according to the normal operating load and the accidental loads against which it is necessary to protect oneself, so that, in normal service, the tensile reinforcement behaves in the usual way with a blocking of the bar with respect to the encasing concrete over its whole length and that, in case of accidental overload, the recess of certain sliding zones, due to the stress differential Steel / Concrete allows, on the one hand an elongation of reinforcement avoiding the risk of rupture and, on the other hand, causes a dissipation of energy able to avoid a brutal collapse of the structure. The invention thus provides the possibility of solving a whole set of problems without calling into question the general design and the calculation method of the reinforcement cages, using only rebar of a new type but which can be realized industrially in a simple and inexpensive way.

Claims

1. A method for producing a reinforced concrete part (1) comprising, on either side of a neutral line (10), a compressed part (C) and a tensioned part (T) subjected to stress traction and having a tendency to elongate under the effect of the load supported by the workpiece, and in which is embedded in a reinforcement cage (2) comprising, in the tensioned portion, at least one longitudinal tensioned bar (21) secured with the concrete by a bond of determining adhesion, along said bar (21), a tangential stress of adhesion varying according to the tensile stresses applied, respectively, on the bar (21) and on the concrete of coating (16), an increase of the tensile stress in the concrete above a limit value resulting in the opening of at least one crack (3) with a transfer of the tensile stress on the bar (21) and a corresponding elongation thereof, in which process, at least in part la more stressed of the workpiece, said tensioned bar (21) is provided with a plurality of spaced anchor means (23) forming abutments bearing on the encasement concrete (16), characterized in that the means of anchoring (23) of the bar (21) are distributed in a discontinuous series of spaced locking zones (25) each comprising a plurality of anchoring means (23) and separated from each other by sliding zones (26). ) without anchoring means, in each of which a local increase in the traction differential between the bar (21) and the concrete above a limit value, causes the bar (21) to disengage with respect to the concrete ( 16) which encases, on at least a portion (27) the length of said sliding zone (26) between two blocking zones (25a, 25'a), said hooked portion (27) being able to lengthen without disorder in the embedding concrete (16) under the effect of the tensile stresses applied to the extended bar (21).

2. The method of claim 1 wherein, the piece (1) having, in the concrete (15), zones of weakness inherent to the quality of the concrete and randomly distributed, at which an increase in the tensile stresses applied. above the elastic limit of the concrete causes, in the most stressed part of the part, the appearance of at least one crack (3) localized at least in line with one of the said zones of weakness, the opening said crack (3) determining, at this level, the cancellation of the tensile stress in the concrete and a correlative local increase in the tensile force applied to the reinforcing bar (21), with a corresponding increase in the tendency to elongate it under the effect of the applied stresses, characterized in that the local increase of the tensile force on the bar (21) at a crack (3) determines a stall of the bar (21) by to the embedding concrete (16), at least in the sliding zone (26a) closest to said crack (3) and over a length (cT) such that the stall force of the bar (21) by to the concrete (16) at least partially offsets the traction differential between the two materials when this differential causes the adhesion stress to exceed the length considered.

3. Method according to claim 2, characterized in that, a part of the traction differential at a crack (3) being compensated by the stall of the concrete (16) in a first sliding zone (26), the additional traction remaining applied on the bar (21) is absorbed, at least in part, by the adjacent blocking zone (25'a) extending beyond the first sliding zone (26a), on the opposite side to the crack (3).

4. Method according to claim 3, characterized in that from the appearance of a first crack (3) in a first zone of weakness, the reinforcing bar (21) is detached from the concrete coating in at least one first sliding zone (26a), the closest to said crack (3) and that an increase in applied tensile stresses successively determines the opening of at least one secondary crack (31) in another zone weakness of the concrete part (1) and the stall of the bar (21) in at least one other sliding zone (26b), the closest to said secondary crack (31), and so on as and when measurement of the increase in tensile stresses, the sum of the thicknesses of the first crack (3) and secondary cracks (31, 32 ...) open at a given instant, being a function of the increase in the elongation of the bar resulting from the increase in the constraints applied at this moment and this increase in elongation is distributed over all of the slip zones (26a, 26b ...), as and when the occurrence of secondary cracks (31, 32 ...).

5. Method according to one of the preceding claims, characterized in that, in the case where a first crack (3) is formed at a first sliding zone (26a), the local increase in the stress of traction applied to the tensioned bar (21) resulting from the opening of the crack (3) causes the rod (21) to be unhooked on either side of said crack (3) over a total length (d ¹ ) for which the stall force of the bar (21) relative to the concrete compensates for at least a portion of the traction differential between the two materials.

6. Method according to one of claims 1 to 4, characterized in that, in the case where a first crack (3) is formed at a first blocking zone (25a), causing a local increase of the tension applied on the tensioned bar (21), at least a first part of this tensile increase is absorbed by the two parts of the first blocking zone (25a) extending on either side of the crack (3). ) and the remaining portion of the traction increase on the bar (21) is compensated by the stall force of the tension bar (21) relative to the concrete at least over part of the nearest sliding area.

7. Method according to one of the preceding claims, characterized in that the number, length and distribution of the blocking zones (25) and the corresponding lengths of the sliding zones (26) are determined according to the distribution and predictable values of the tensile stresses along each tension bar (21), taking into account the applied loads, so that the thickness of each of the cracks (3,31,32 ...) does not exceed a given limit.

8. Method according to one of the preceding claims, characterized in that the relative lengths of the locking zones (25) and sliding zones (26) distributed along each tension bar (21), are determined taking into account their position, so as to give the piece (1) the stiffness necessary to remain within a range of values allowed for the arrow of the room under a given load.

9. Method according to one of the preceding claims, characterized in that each locking zone extends over a length at least equal to a so-called sealing length (I ₀ ) of the bar (21) determining a stress of adhesion at least equal to the maximum allowable tensile stress of the bar (21).

10. Method according to one of the preceding claims, characterized in that each locking zone extends over a length less than twice a sealing length (I ₀ ) of the reinforcing bar for which said bar ( 21) supports, without displacement relative to the concrete coating, a tensile force up to the elastic limit of the bar.

11. Method according to one of claims 9 and 10, characterized in that each sliding zone extends over a length less than the sealing length ( _1'o ) of a smooth bar with equivalent round section.

12. The method of claim 11, characterized in that each sliding zone (26) extends over a length of the order of 5 to 30 mm.

13. A method of reinforced concrete for carrying out the method according to one of the preceding claims, characterized in that each sliding zone (26) of a longitudinal stretched bar (21) has a smooth outer surface in the direction longitudinal.

14. The method of claim 13, characterized in that, each longitudinal stretched bar (21) having, in cross section, the area required for the desired tensile strength, the profile of said bar (21), in each sliding zone (26), is adapted to give it the perimeter necessary for the contact surface between the bar and the concrete to provide a connection by gluing and friction making it possible to reach the desired limit value of the tangential stress of adherence in said sliding zone (26).

15. The method of claim 14, characterized in that each longitudinal stretched bar (21) has, in cross section, a flattened profile with a width greater than the thickness, so as to increase its perimeter relative to that of an equivalent circular bar having the same transverse area.

16. The method of claim 15, characterized in that each longitudinal stretched bar (21) has, in cross section, a corrugated profile with parts longitudinal, recessed and protruding, extending parallel to the axis of the bar, along the entire length of each sliding zone (26).

17. Method according to one of claims 14, 15, 16, characterized in that, in each sliding zone (26), the outer face of the bar comprises a layer of particles detachably attached to the outer surface of the rod and protruding into the embedding concrete so as to increase the adhesion bond with the concrete and the limit value of the adhesion stress from which an increase in tensile stresses causes the stall to the bar, said particles gradually becoming detached one after the other from the bar, remaining included in the concrete, as and when the tensile stresses increase, so as to maintain the adhesion stress to its value limit over a range of increase of said tensile stresses.

18. The method of claim 17, characterized in that the particles are fixed by gluing on the outer surface of the bar.

19. The method of claim 17, characterized in that the particles are sprinkled and applied under pressure on the outer surface of the high temperature bar at the output of the rolling mill.

20. The method of claim 17, characterized in that the particles consist of chips, metal balls or filings and fixed on the outer surface of the bar by electrofusion contact.

21. Method according to one of claims 17 to 20, characterized in that the particles fixed on the outer surface of each sliding zone of the bar have various dimensions so as to detach gradually, depending on the size of the part fixed, as the tensile stresses applied increase.

22. Reinforced concrete part produced by the method according to one of the preceding claims and comprising a reinforcing cage (2) embedded in cast concrete (15) and comprising at least one longitudinal bar tension (21) of the high type. adhesion, along which are formed a plurality of spaced apart anchoring means (23) forming abutments bearing on the encasement concrete (16), characterized in that the anchoring means of the bar (21) are divided into a discontinuous series of spaced locking zones (25) each comprising a plurality of anchoring means (23) and separated from each other by sliding zones (26) devoid of anchoring means, in each of which a local increase in the traction differential between the bar (21) and the concrete (16) above a limit value, causes the bar (21) to disengage with respect to the concrete (16) which coats it, on at least a part (27) of the length of said zoo no slip (26) between two blocking zones (25a, 25'a), said unhooked portion (27) being able to elongate without disorder in the embedding concrete (16) under the effect of the tensile stresses applied on the tension bar (21).