FR2698111A1 - Reinforced concrete bridge deck construction method - uses precast units slid onto longitudinal steel beams with starter bars in gap between comprising top flange steel and unit starters and stirrups - Google Patents

Abstract

A method for constructing a reinforced concrete bridge deck supported on longitudinal steel beams (1) comprises placing the beams then sliding precast concrete slab units (4,5) on them and pouring concrete in the gap between. Vertical steel starter bars (13), in rows of three, are fixed to the top flange of the beam before the units are positioned with a holding wedge (16). Reinforcement of the gap between the precast units consists of starter bars (7,8) and longitudinal bars (11,20) with stirrups round them. This cage is lowered after the precast units are positioned. USE/ADVANTAGES - Eliminates localised stress concentration zones, minimal manual operations.

Description

 The present invention relates to a method of constructing a bridge deck of the "mixed" type, comprising a concrete slab supported by longitudinal metal beams.

 The construction of mixed structures poses multiple problems to the designer, as whenever it is necessary to assemble materials having different characteristics, in particular elastic moduli and different coefficients of expansion.

 To this is added, in the case of concrete, the problems resulting from the volumetric evolution of the latter in the time following its pouring (shrinkage).

 The classic technique, which consists, after having set up the longitudinal beams of a bridge, of pouring in place on it the concrete slab using successive recovered formwork, results in significant cracks, in particular on the right supports, resulting from deformations of the beams in bending, and stresses at the location of the attachment zones, located in the vicinity of these supports. Furthermore, it is sought, for cost reasons, to limit the importance of concrete poured in place, in favor of precast concrete elements.

 According to the so-called "pianotage" method, first concrete is poured into place, in the central part of each span which is outside the supports, then concrete is poured in place in the areas around the supports. This technique is complex, and there are added problems resulting from the constraints corresponding to the juxtaposition of concrete of different ages. Cracks due to shrinkage remain.

 One can also constitute the deck by sliding precast concrete elements, contiguous, on the beams, so as to constitute the entire slab of the deck. It is necessary to connect these prefabricated elements with the beams. One solution may consist in providing windows in the prefabricated elements, then, once the elements have been put in place, pouring concrete in these windows, after having put in place, on the part of the upper surface of the beam which is made accessible through the window, attachment pieces intended to ensure a good connection between the concrete thus poured and the beam. These windows are necessarily quite small, so that the installation of the hanging pieces is long and tedious. In addition, there are as many hanging areas as windows, and each of these areas corresponds to a concentration of stresses, which can be the cause of the appearance of cracks. It is also possible to provide that the prefabricated elements include metal inserts at their lower part, these inserts then being welded to the beams. This welding, necessarily manual, is still a long and painful operation.

 The object of the present invention is to provide a method of constructing a "mixed" type deck, which does not have the drawbacks of the methods which have just been mentioned, and which provides a structure devoid of localized areas of concentration of constraint, and in which the operations to be performed manually are as reduced as possible.

To obtain this result, the invention provides a method of constructing a bridge deck comprising a concrete slab supported by longitudinal metal beams, method of the type according to which the longitudinal beams are put in place, then one slides over those - successively precast concrete elements intended to constitute the slab, and then said elements are joined to the beams by pouring concrete into at least one cavity, the bottom of which is constituted by the upper face of a beam, provided with reliefs for the attachment of concrete, and at least one wall of which is formed by an edge of a prefabricated element, the particularity of this process being that
- before sliding the prefabricated elements, said attachment reliefs are fixed on said upper face of a beam,
- Prefabricated elements are used consisting of at least two concrete blocks provided with transverse reinforcement, two adjacent blocks being separated by a longitudinal interval which extends over the entire length of the element and has a transverse dimension equal to that of said cavity, at least a portion of the transverse reinforcements extending through said longitudinal gap to connect the blocks together,
- means are provided to prevent relative movement of the blocks in the longitudinal direction,
- The prefabricated elements are dragged over parts devoid of beam attachment reliefs, the transverse reinforcements which extend across the interval between blocks passing above said attachment reliefs, until the elements have reaches their final position, then the elements and the beam are made integral by pouring concrete into the cavity limited by the upper surface, provided with hooking reliefs, from the beam, and by the longitudinal edges of two adjacent blocks.

 It can be considered that the method of the invention differs from that according to which windows in the blocks are provided by the fact that these windows are replaced by longitudinal openings extending over the entire length of an element. The consequence of this first difference is that the sliding of an element on the beam can respect the hooking reliefs prepared in advance. The installation of these reliefs is therefore greatly facilitated because, instead of making them at bottom of a cramped window, they can be prepared in advance, either on site after placing the beams, or even even before, the beams being put in place when they are already provided with their hooking reliefs .

One could have thought of constituting the prefabricated elements with separate blocks from each other, but this technique would have required precise guiding means, to place with precision, both in the longitudinal direction and in the transverse direction, the different blocks relative to each other. On the contrary, the fact that transverse reinforcement crosses an interval between two adjacent blocks means that these blocks cannot move away from each other, nor to a large extent, move closer to each other. An approximation would however be possible, because the frames constitute with the blocks a series of deformable parallelograms, but relatively light means, provided to prevent relative movement of the blocks in the longitudinal direction, also oppose the approximation of these.

 The blocks can slide on parts of the upper face of the beams which are devoid of attachment reliefs, and / or on other horizontal surfaces of the beams.

 The concrete poured on site, which fills a kind of narrow longitudinal trench, ensures a continuous connection between the slab and each underlying beam, without significant concentration of stresses.

 Those skilled in the art will however observe that the transverse reinforcements are found, according to the process just indicated, necessarily above the attachment reliefs when we know that it is preferable that there are reinforcements located below the level of the top of the hanging reliefs.

 Among the attachment reliefs, parts are known in L-section, with a vertical wing, arranged longitudinally or transversely, this wing being pierced with passages which are filled with concrete during casting. It is often preferred to use reliefs constituted by vertical rods, the lower part of which is welded or screwed onto the beam, while the upper part forms an enlarged cap.

According to the preferred method of the invention
- provision is made for the attachment reliefs to be discontinuous in the longitudinal and transverse directions,
a mobile structure is provided comprising at least one transverse frame linked to longitudinal bars, and
- After the elements have reached their final position, but before the concrete is poured in the interval between blocks, the said mobile structure is placed in a final position within the interval, the lower part of the transverse frame and at minus certain longitudinal bars extending between the attachment reliefs, below the apex.

 As we can see, the quality that we have just described is not limited to attachment reliefs formed of a rod with an enlarged cap, but extends to any type of attachment relief, provided that it is discontinuous in the longitudinal and transverse directions.

 The longitudinal bars of the mobile structure may be discontinuous and short enough to pass between the transverse reinforcements which connect one block to the other across the interval, but it is clear that a simplification of the work on site and greater resistance will be obtained if these longitudinal bars are continuous, for example over the entire length of an element. In this case, it is no longer possible to prepare the mobile structure separately, and to bring it to the deck under construction only when all the prefabricated elements have been put in place. Indeed, the transverse reinforcements oppose this implementation.

According to a preferred method
the mobile structure comprises longitudinal bars, the length of which is greater than the interval between the reinforcements crossing the interval between blocks,
- the mobile structure is placed in the interval between adjacent blocks before the prefabricated elements are slid in place, so that it is mobile between a displacement position, where its lower part is at a height sufficient for it passes over the hanging reliefs, and a final position, where its lower part is lower than the top of the hanging reliefs,
- during the sliding of the elements on the beams, the mobile structure is kept in its displacement position, and when the elements have reached their final position, the said structure is lowered by passing the frames and the bars between the hooking reliefs, then pour the concrete into the cavity.

 It is understood that, in its displacement position, the mobile structure can be held simply by a few provisional links, which it will suffice to dismantle so that it descends, by its own weight and with a minimum of effort, towards its final position .

Advantageously, for optimal mechanical strength, provision is made on the facing surfaces of two blocks, waiting frames,
and, after having put the mobile structure in its final position, the waiting reinforcements are joined or brought together with said structure in order to obtain a transverse continuity or quasi-continuity of the deck reinforcements below the top level of the hanging reliefs.

 To prevent relative displacement in the longitudinal direction while it slides on the beam, one solution could be to provide that the adjacent blocks are connected across the interval, in addition to the transverse reinforcement, by some oblique reinforcement in the longitudinal directions and transverse. This simple solution has the disadvantage of complicating the reinforcement program. We simply prefer, in general, to provide a temporary bracing, which is removed when the elements have reached their final position.

The invention will now be described in more detail with the aid of a practical example illustrated with the aid of the drawings, among which
FIG. 1 is a schematic perspective view of a step in the process, where the mobile structure is not shown,
Figure 2 is a cross section showing in solid line the movable structure in its displacement position, and, in phantom, the movable structure in its final position.

 Figure 1 shows a longitudinal beam 1, on which slides, according to arrow 2, a prefabricated element, designated as a whole by 3, and which is composed of two concrete blocks 4, 5, poured together, but separated by a gap 6, crossed by transverse reinforcements 7, 8, which are the extension of internal reinforcements 9, 10 of each block. The longitudinal reinforcements 11 connected to the transverse reinforcements 7 are also provided in the interval 6.

 The upper face 12 of the beam 1 carries, in its central part, a series of attachment reliefs each consisting of a vertical rod 13, welded at its lower part to the surface 12, and the upper part of which is widened to form a hat 14.

 The edges of the surface 12 constitute kinds of tracks 15, devoid of attachment reliefs, and over which the edges of the blocks 4 and 5 slide. A bracing has been represented in FIG. 1, quite symbolically. temporary 16, placed on the edges of two adjacent blocks, and which serves to prevent their coming together, and consequently, a relative longitudinal displacement of one block with respect to the other, by a deformable parallelogram effect constituted by the blocks themselves and the transverse reinforcements 7, 8.

 As best shown in Figure 2, the transverse reinforcements 17 located at the bottom of blocks 4 and 5 do not cross the gap 6, because, during the progression of the elements, they would meet the rods 13 of the hooking reliefs . They are folded upwards, forming waiting frames 18.

 Figure 2 shows the sectional structure of the movable structure. This figure shows a frame 19, with a closed contour, located in a transverse plane. Longitudinal frames 20 are connected to the lower part of the frames 19, and interconnect several of these frames, only one of which is shown. In the displacement position, shown in solid lines, the mobile structure is in the highest position that it can occupy, the reinforcements 20 being retained against the transverse reinforcements 8. In the final position shown in phantom, the structure is , on the contrary, in the lowest position it can occupy, because the frame 19 rests on the upper frames 11. It can be seen that, in the displacement position, the frame 19 is located above the top 14 of the reliefs On the contrary, in the final position shown in phantom, the lower part of the frames 19 as well as the longitudinal frames 20, are located below the top 14 of the hooking reliefs. It will be observed that the position of the longitudinal reinforcements 20 has been calculated so that it passes between the heads 14. Likewise, although this is not shown, it is obvious that the longitudinal position of the frames 19 of the structure is such that these frames pass in the longitudinal interval which exists between two heads 14.

 It will also be observed that, in the position shown in solid lines in FIG. 2, the longitudinal reinforcements 21 of the mobile structure come to rest, or are in the vicinity, of the hooks 18. As a result, a continuity, or quasi-continuity, between the lower transverse reinforcements 17 of each of the blocks 4 and 5, is established by means of the lower part of the frame 19, and, possibly, the reinforcements 21.

 It will be observed that, in the position of displacement, the movable frame is projecting above the upper surface of the blocks 4 and 5. This does not present any difficulty, and it is easy to predict that there is no of interference with the temporary bracing structure 13. In the final position, the frame 19 no longer projects above the upper surface of the blocks 4 and 5, which makes it possible to establish a continuity of the upper face of the slab after pouring concrete.

Claims (5)

 1. Method for constructing a bridge deck comprising a concrete slab supported by longitudinal metal beams (1), method according to which the longitudinal beams are put in place, then concrete elements are slid over them successively prefabricated (3) intended to constitute the slab, and then said elements are joined to the beams by pouring concrete into at least one cavity, the bottom of which is constituted by the upper face of a beam, provided with reliefs (13, 14) for the attachment of concrete, and at least one wall of which is formed by an edge of a prefabricated element,  characterized in that  - before sliding the prefabricated elements, said attachment reliefs (13, 14) are fixed on said upper face of a beam,  - Prefabricated elements are used consisting of at least two concrete blocks (4, 5) provided with transverse reinforcement, two adjacent blocks being separated by a longitudinal interval (6), which extends over the entire length of the element and has a transverse dimension equal to that of said cavity, at least a part (7, 8) of the transverse reinforcements extending through said longitudinal gap to connect the blocks together,  - means (16) are provided for preventing relative movement of the blocks in the longitudinal direction,  - The prefabricated elements are slid onto parts (15) devoid of beam attachment reliefs, the transverse reinforcements (7, 8) which extend through the interval between blocks passing above said attachment reliefs, up to '' that the elements have reached their final position, then the elements and the beam are secured by pouring concrete into the cavity limited by the upper surface, provided with attachment reliefs (13, 14), of the beam (1 ), and by the longitudinal edges of two adjacent blocks (4, 5).  2. Method according to claim 1, characterized in that  - provision is made for the attachment reliefs (13, 14) to be discontinuous in the longitudinal and transverse directions,  a mobile structure is provided comprising at least one transverse frame (19) linked to longitudinal bars (20), and  - After the elements have reached their final position, but before the concrete is poured in the interval between blocks, the said mobile structure is placed in a final position within the interval, the lower part of the transverse frame and at minus certain longitudinal bars extending between the attachment reliefs, below the apex.  3. Method according to claim 2, characterized in that  the mobile structure comprises longitudinal bars (20) whose length is greater than the interval between the reinforcements crossing the interval between blocks,  - the mobile structure is placed in the interval between adjacent blocks before the prefabricated elements are slid in place, so that it is mobile between a displacement position, where its lower part is at a height sufficient for it passes over the hanging reliefs, and a final position, where its lower part is lower than the top of the hanging reliefs,  - During the sliding of the elements on the beams, the mobile structure is kept in its displacement position, and when the elements have reached their final position, the said structure is lowered by passing the frames (19) and the bars (20) between the attachment reliefs (13, 14), then the concrete is poured into the cavity (6).  4. Method according to claim 2 or 3, characterized in that waiting frames (18) are provided on the facing surfaces of two blocks,  and in that, after having put the mobile structure in its final position, the waiting armatures (18) are joined or brought together with said structure (19, 20) in order to obtain a transverse continuity or quasi-continuity of the reinforcement of the deck below the top level of the hanging reliefs.  5. Method according to one of claims 1 to 4, characterized in that the means for preventing a relative displacement of the blocks in the longitudinal direction are constituted by a temporary bracing (16), which is removed when the elements have reached their position final.