EP1555100B1 - Manufacturing method for prestressed concrete hollow slabs and slabs obtained by that method - Google Patents

Description

The present invention relates to a method of manufacturing a series of hollow core slabs of prestressed concrete by longitudinal reinforcing bars or strands placed between the cells, at the lower part of the slabs, on a prestressing bench of great length and of predetermined width.

The prestressed concrete hollow core slabs that appeared on the market in the 1950s are still highly appreciated by building professionals.

Such slabs have the advantage of being cast not on construction sites but in factories before being transported and delivered as a rule by truck; consequently, their manufacture is not dependent on any bad weather.

These slabs usually have a standard width of about 1.20 m and a variable length adapted to the range, up to 20 meters, and have in their internal part 5 to 11 longitudinal cells. They are manufactured on prestressing benches of great length (of the order of 150 m) and having a width corresponding to that of the slabs by a largely automated process.

During the implementation of this method, a particular casting device called sliding formwork is frequently used which moves continuously along the prestressing bench.

• des guide fils assurant le maintien des torons,
• des organes de coulage de la couche inférieure des dalles, cette couche étant située au-dessous des alvéoles et renfermant les torons d'armature,
• des sabots vibreurs,
• des noyaux profilés déplaçables longitudinalement en va et vient permettant l'obtention des alvéoles longitudinales et coopérant avec des organes de coulage des bandes de béton séparant les alvéoles,
• des patins permettant de vibrer ces bandes en béton,
• des organes de coulage de la couche supérieure des dalles situées au-dessus des alvéoles,
• une table vibrante.

This device essentially comprises, in the direction of movement:

• wire guides ensuring the maintenance of the strands,
• pouring members of the lower layer of the slabs, this layer being located below the cells and enclosing the reinforcing strands,
• vibrating clogs,
• shaped cores displaceable longitudinally back and forth to obtain the longitudinal cells and cooperating with casting members of the concrete strips separating the cells,
• skids for vibrating these concrete strips,
• casting members of the upper layer of slabs located above the cells,
• a vibrating table.

In order to manufacture the prestressed concrete hollow core slabs, the reinforcing strands are placed in parallel with one another, it is tensioned as a general rule with the aid of jacks, and then the concrete is poured on heights which can vary between approximately 12 and 40 cm using the sliding form casting device which moves continuously along the prestressing bench, cut the slabs to the desired length, and after setting the concrete, cut the strands so as to put the latter prestressing.

On site, these slabs are juxtaposed on beams or walls so as to form the floor of the floors of different buildings.

When placed on the beams or walls it is necessary to lift the slabs by means of cranes.

Two main lifting methods are currently used for this purpose.

The first of these methods is to suspend the hook of the crane a large metal beam called rudder equipped at its ends clamps for gripping a slab.

Chains passing below the slab provide additional security.

Such a method does not satisfy all professionals.

The second lifting method is to suspend from the crane hook a four-strand sling whose ends are respectively introduced into four through holes provided for this purpose at the four corners of the slabs.

Two metal bars respectively strung at the ends of two associated strands, below the slabs allow the maintenance thereof.

Such a method has the disadvantage of requiring drilling on the slabs of four through holes which must then be plugged.

FR -A-2,833,285 discloses a method of manufacturing hollow core slabs comprising lifting and handling loops, as well as a prestressed concrete hollow core slab. The object of the invention is to overcome the disadvantages of known lifting methods of hollow core slabs prestressed concrete.

For this purpose, the idea underlying the invention was to incorporate in these slabs lifting and handling devices for gripping by means of a crane, during their manufacturing process by pouring concrete.

It should be noted that it has already been proposed to incorporate lifting hooks to conventional reinforced concrete slabs during their manufacturing process.

Such lifting hooks, however, are not suitable for hollow core slabs prestressed concrete given their specific configuration and the presence of profiled cores to obtain the cells.

Indeed, lifting and handling devices adapted to such slabs must meet particular constraints; in particular, during the manufacture of the slabs using slipform casting devices, these bodies must be able to pass between the vibrating shoes and profiled cores.

It is also essential that these lifting and handling devices are perfectly anchored in the concrete to avoid any risk of breakage that can have dramatic consequences given the weight of the slabs which is of the order of 200 to 600 kg / m ² according to their thickness.

In this context, the invention relates to a method of manufacturing a series of hollow slabs according to claim 1, and a slab according to claim 5.

According to the invention, the fixing of the lifting and handling loops on the associated strands can be carried out manually or be automated.

It should be noted that the specific configuration of the lifting and handling loops, and in particular the planar character of their median bending and the positioning of this bending so that it is located in the same plane as the associated strand correspond to essential features of the invention.

Indeed, given the relatively small thickness of the concrete strips separating the cells, it is imperative that the lifting and handling loops do not occupy a volume too important not to create within these bands of zones. weakness due to an insufficient mass of concrete risking including risks of rupture during handling.

According to the invention, it is of course necessary that at the end of the process, the upper end of the median bend of the lifting and handling loops is released to allow them to be grasped.

Such clearance can be achieved by digging the concrete around these ends after the pouring step, when the concrete is still fresh.

It is also conceivable to cap the upper end of the median bend of the lifting loops and handling inverted cups whose function is to create reservations after pouring concrete.

As a result, at the end of the process the slabs are equipped at their four corners lifting loops and handling whose median bend is flush with its upper end to the surface of the concrete.

These slabs can thus be grasped by a lifting device comprising a four-strand sling suspended from the hook of a crane.

According to the invention, the lifting and handling loops are generally fixed on the two penultimate strands from the lateral edges of the slabs.

According to the invention, it is also essential that after their clipping on the strands, the lifting and handling loops are perfectly locked and do not risk being dragged into the concrete during casting.

For this purpose, and according to a preferred feature of the invention, during the pouring stage of the concrete, it exerts pressure on the strands associated with lifting and handling loops, upstream and downstream of these loops.

To obtain such a pressure, it is advantageous to modify the conventional slipform concrete pouring device by removing the wire guide at the two strands equipped with the lifting and handling loops and by adding to the right of these strands two pairs of jacks that can exert pressure on them upstream and downstream of the lifting and handling loops to hold them in place as they enter the concrete.

It is of course mandatory that these cylinders retract to the right lifting loops and handling.

The invention also relates to a hollow concrete slab prestressed by longitudinal iron or reinforcing strands placed between the cells at the bottom of the slab.

Such a slab has the features of claim 5.

It should be noted that such a slab is obtained by the implementation of the aforementioned method.

As already indicated, it is essential that the lifting and handling loops do not occupy too much volume not to create within the concrete strips separating the cells weak areas due to insufficient concrete mass.

It is therefore essential that the gripping ring is plane and after fixation the associated strand is essentially placed in this plane.

However, it is also essential that the diameter of the bent rods constituting lifting loops and handling is not too important.

According to a preferred feature of the invention, this diameter is less than 14 mm, and preferably between 8 and 12 mm.

To avoid any risk of breakage during the lifting and handling of the slabs and taking into account the weight thereof, it is necessary to make these rods whose diameter is relatively small in a steel grade having physical and mechanical properties especially high breaking strength.

As a result, and according to another characteristic of the invention, the bent rods constituting lifting and handling loops are made of a steel having a tensile strength of at least 300 MPa, preferably at least 400 MPa, and can advantageously go up to 700 MPa, and parallel ductility sufficient to avoid the risk of rupture.

This constraint requires the use of special steels particularly having undergone hardening treatment.

Another imperative is to use steel grades with sufficiently low tolerances to guarantee the constancy of the geometric characteristics of the loops.

• la figure 1 est une vue en perspective d'une dalle alvéolée en béton précontraint ;
• la figure 2 est une vue en perspective d'une boucle de levage et de manutention mise en place sur un toron.

The characteristics of the method as well as the hollow core of prestressed concrete which are the subject of the invention will be described in more detail with reference to the accompanying non-limiting drawings in which:

• the figure 1 is a perspective view of a hollow core slab of prestressed concrete;
• the figure 2 is a perspective view of a lifting and handling loop set up on a strand.

According to figure 1 , the slab 1 is made of a concrete prestressed by longitudinal reinforcing strands 2 projecting at its ends and is equipped with a series of longitudinal recesses 3.

The number of seven cells and six reinforcing wire selected by way of example is of course in no way limitative of the invention.

The strands 2 are placed between the cells 3 at the bottom of the slab 1. It should be noted that each of the concrete strips 4 separating two cells 3 is not necessarily equipped with a strand 2.

According to figure 1 the slab 1 is equipped on its upper surface 5 and at its four corners with four lifting and handling loops 6 flush with the surface of the concrete and having the function of allowing the slab 1 can be grasped by a device not shown lifting comprising a four-strand sling suspended from the hook of a crane.

According to figure 2 each of the lifting and handling loops 6 is clipped onto an associated strand 2 before pouring the concrete.

The lifting loop 6 is more precisely constituted by a bent metal rod having a diameter of the order of 8 to 12 mm and made of a special steel having a tensile strength of at least 400 MPa and parallel to a sufficient ductility to avoid the risk of breakage.

This metal rod 6 comprises firstly at its central part a median bend 7 and secondly at its respective ends lateral bends 8, 8 'extending substantially symmetrically on either side of the median bend 7.

The median bend 7 defines a planar gripping ring as shown in FIG. figure 1 .

According to figure 2 , the gripping ring and the associated strand 2 are essentially coplanar.

The lateral bends 8, 8 'define at the ends of the loop 6 positioning tabs 9, 9' which bear on the not shown prestressing bench on which the slabs are cast.

The positioning tabs 9, 9 'thus enable the precise positioning of the lifting and handling loop 6.

The positioning tabs 9, 9 'are directly contiguous to essentially U-shaped fixing brackets 10, 10' which are situated in a plane substantially perpendicular to the plane of the gripping ring.

The fixing brackets 10, 10 'allow the torsion clipping of the lifting and handling loop 6 along the associated reinforcing bar 2, on either side of this strand, in the position shown in FIG. figure 2 .

Claims (8)

1. Method for producing a series of hollow slabs of concrete prestressed by longitudinal reinforcement bars or strands which are positioned between the cells, at the lower portion of these slabs, on a prestressing bed, having a length in the order of 150 m and a width corresponding to that of the slabs, via which the reinforcement strands are positioned parallel with each other, they are placed individually under tension, in particular using actuators, then the concrete is poured using a specific pouring device which is known per se and which is referred to as a sliding formwork, which is moved continuously along the prestressing bed, the slabs are cut to the desired length and, after the concrete has set, the strands are cut in order to place the concrete in a prestressed state,
   comprising the following steps:

   - there are formed lifting and handling loops (6) which are each constituted by a bent metal rod which comprises, on the one hand, at the central portion thereof a central bent portion (7) which defines a planar gripping ring and, on the other hand, at the respective ends thereof, lateral bent portions (8) which extend substantially symmetrically at one side and the other of the central bent portion (7),
   the lateral bent portions defining at the ends of the loop positioning lugs (9, 9') which are directly contiguous with substantially U-shaped fixing clips (10, 10') which are located in a plane which is substantially perpendicular to the plane of the gripping ring, and

   - before the step of pouring the concrete, upstream of the pouring device, in the movement direction of this device, a series of lifting and handling loops (6) are clip-fitted by means of torsion along two lateral strands (2) which are preferably located substantially symmetrically relative to the longitudinal centre axis of the prestressing bed, at one side and the other of these strands, at locations corresponding to the corners of the slabs (1), so that the gripping ring of each of these loops (6) and the associated strand are substantially coplanar,
   and in that the positioning lugs (9, 9') thereof are supported on the prestressing bed, and

   - the concrete is poured so that the central bent portion of the lifting and handling loops is flush, via the upper end thereof, with the surface thereof.
2. Method according to claim 1,
   characterised in that,
   after the step of pouring the concrete, the upper end of the central bent portion (7) of the lifting and handling loops (6) is released by hollowing out the concrete around it.
3. Method according to either claim 1 or claim 2,
   characterised in that
   the lifting and handling loops (6) are clip-fitted to the two penultimate strands (2) from the lateral edges of the prestressing bed.
4. Method according to any one of claims 1 to 3,
   characterised in that,
   during the step for pouring the concrete, a pressure is applied to the strands (2) associated with the lifting and handling loops (6), upstream and downstream of these loops, in order to fix them in place during their introduction into the concrete.
5. Hollow slab of concrete prestressed by longitudinal reinforcement bars or strands placed between the cells, at the lower portion of the slab, which is obtained by carrying out the method according to any one of claims 1 to 4,
   and which is provided in the region of the four corners thereof with lifting and handling loops (6) which are flush with the surface of the concrete, the loops being clip-fitted by means of torsion along an associated strand (2), at one side and the other of this strand, respectively, and which are each constituted by a bent metal rod which comprises, on the one hand, at the central portion thereof a central bent portion (7) which defines a planar gripping ring, this gripping ring and the associated strand (2) being substantially coplanar, and, on the other hand, at the respective ends thereof, lateral bent portions (8, 8') which extend substantially symmetrically at one side and the other of the central bent portion (7) and which define at the ends of the loop (6) positioning lugs (9, 9') which enable precise positioning of the lifting and handling loop (6) on the prestressing bed on which the concrete is poured, the positioning lugs being directly contiguous with substantially U-shaped fixing clips (10, 10') which are located in a plane which is substantially perpendicular to the plane of the gripping ring.
6. Hollow slab of prestressed concrete according to claim 5, characterised in that
   the diameter of the bent rods which constitute the lifting and handling loops (6) is less than 14 mm, preferably between 8 and 12 mm.
7. Hollow slab of prestressed concrete according to claim 6, characterised in that the bent rods which constitute the lifting and handling loops (6) are produced from a steel which has a minimum tensile strength in the order of 300 MPa and preferably at least 400 MPa and in parallel sufficient ductility to avoid risks of fracture.
8. Hollow slab of prestressed concrete according to claim 7,
   characterised in that
   the steel from which the bent rods which constitute the lifting and handling loops (6) are produced is a special steel which has been subjected to a strain hardening processing operation.

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