FR3029444A1 - PRECONTROL BENCH FOR PREFABRICATION OF A CONCRETE STRUCTURAL ELEMENT AND METHOD OF IMPLEMENTING SUCH A BENCH - Google Patents

Abstract

The present invention relates to a prestressing bench (1), for prefabrication of a concrete structural element, the prestressing bench (1) extending between two longitudinal ends (1A) and comprising strands (30), as well as means (40) for tensioning the strands (30) between the longitudinal ends (1A), through a formwork intended to form the structural element. The prestressing bench (1) is characterized in that the tensioning means (40) comprise a lower prestressing system (60) and an upper prestressing system (80), configured to tension and then respectively relax lower strands ( 36) and upper strands (38), so as to preload a lower portion and an upper portion of the structural member. The invention also relates to a method of implementing such a prestressing bench (1).

Description

BACKGROUND OF THE INVENTION 1. Field of the Invention The present invention relates to a prestressing bench, for prefabrication of a concrete structural element. BACKGROUND OF THE INVENTION The invention also relates to a method of implementing such a prestressing bench. The field of the invention is that of the construction of concrete structural elements, such as beams. In practice, a preload bench by adherent wire is a fixed tool of large dimensions. The bench generally comprises two reinforced concrete end massifs connected by a reinforced concrete slab the length of the bench. Metal crutches are anchored vertically in the end massifs. Transverse metal trimmers rest on these crutches. Prestressing strands are anchored behind the trimmers and extend in the longitudinal direction of the bench. Hydraulic cylinders are inserted between the crutches and the headers. These cylinders make it possible to carry out the straining and loosening of the strands once the concrete has acquired sufficient strength. Moreover, it is known prestressing said self-resisting benches, in which the recovery efforts is not provided by large massive ends of reinforced concrete, but by longitudinal stops. The hydraulic cylinders are inserted between the buttresses and the headers. The advantage of this system is to be dismountable, therefore movable, which is particularly advantageous when the manufactured beams mobilize expensive or impractical transport means. In this case, the bench is installed in the immediate vicinity of the place of construction. Longitudinal bumpers are generally containerizable metal sections, less than 12 meters long. The various parts of the system (butons, headers, cylinders) are metallic, which facilitates their disassembly and reassembly, as well as their adaptation in length or width. The object of the present invention is to provide an improved prestressing bench, particularly for the prefabrication of structural elements having significant heights and which are therefore long and heavy elements. For this purpose, the subject of the invention is a pretensioning bench, for the prefabrication of a concrete structural element, the prestressing bench extending between two longitudinal ends and comprising strands, and means for tensioning the strands. between the longitudinal ends, through a formwork provided to form the structural element.

The prestressing bench is characterized in that the tensioning means comprise a lower prestressing system and an upper prestressing system, configured to tension and then respectively relax lower strands and upper strands, so as to prestress a part of the prestressing strut. lower and an upper part of the structural element. Thus, the invention makes it possible to dissociate the prestressing actions in the lower part and in the upper part of the concrete structural element, for example a construction beam. The prestressing exerted on the beam during its prefabrication is thus optimized, in particular when the beam has a high height. The prestressing bench according to the invention is versatile, practical and effective. According to other advantageous features of the prestressing bench according to the invention, taken individually or in combination: the lower prestressing system is placed in abutment against longitudinal stops which extend between the longitudinal ends of the prestressing bench. The prestressing bench comprises spacers arranged under the longitudinal buttresses, in particular at regular intervals between the longitudinal ends of the prestressing bench. At each longitudinal end, the prestressing bench comprises an end mass and the upper prestressing system is supported by at least one stand which is vertically implanted in a pit formed in the end mass. - At each longitudinal end, the upper prestressing system is supported by a gusset which is fixed on a longitudinal bumper. - The lower prestressing system and the upper prestressing system 25 each comprise: at each longitudinal end of the prestressing bench, a trestle provided with longitudinal holes for the passage of the strands, and means for fixing the ends of the strands to the trimmer ; and at least one longitudinal end of the prestressing bench, at least one actuator adapted to move the headers to tension and then relax the strands. The lower prestressing system and the upper prestressing system each comprise at least one actuator disposed at each longitudinal end of the prestressing bench. The lower prestressing system and the upper prestressing system comprise headers arranged outside an area delimited by the longitudinal stops in the longitudinal direction of the prestressing bench. 3029444 3 - The lower prestressing system comprises at least one actuator disposed at a first longitudinal end of the prestressing bench, while the upper prestressing system comprises at least one actuator disposed at a second longitudinal end of the prestressing bench. 5 - The lower prestressing system comprises two actuators arranged at one or each longitudinal end of the prestressing bench, on either side of a longitudinal median plane. The upper prestressing system comprises two actuators arranged at one or each longitudinal end of the prestressing bench, on either side of a longitudinal median plane. - The actuators are hydraulic cylinders. The invention also relates to a method of implementing a prestressing bench as mentioned above. The method is characterized in that it comprises at least the following steps: a) a preparatory step, wherein the tensioning means and the strands are installed in the prestressing bench; b) a main tension step of tensioning the lower strands under the action of the lower prestressing system and the upper strands under the action of the upper prestressing system, through the formwork provided to form the structural member; c) a complementary voltage step of stranding the strands individually by action of a monotoron jack; d) a pouring step of pouring the concrete structural element into the formwork; E) a prestressing step of expanding the lower strands and the upper strands so as to preload respectively the lower part and the upper part of the structural element. According to other advantageous features of the method according to the invention, taken separately or in combination, in the preparatory step a): - Longitudinal bumpers are installed between the longitudinal ends of the prestressing bench. Spacers are arranged under the longitudinal buttresses, in particular at regular intervals between the longitudinal ends of the prestressing bench. - The lower stress system is disposed in abutment against the longitudinal stops.

The invention will be better understood on reading the description which follows, given solely by way of nonlimiting example and with reference to the appended drawings in which: FIG. 1 is a perspective view of a bench of prestressing according to the invention; Figure 2 is a partial perspective view on a larger scale of an end of the bench shown in Figure 1, showing a portion of the slab, an end mass, longitudinal buttresses and support struts; Figure 3 is a further enlarged partial perspective view of the end shown in Figure 2 showing strands and tensioning means of these strands; Figure 4 is a top view of the end shown in Figure 3; Figure 5 is a section along line V-V in Figure 4; FIG. 6 is an axial view along arrow VI in FIG. 1, showing a structural element and its formwork in position in the prestressing bench, the tensioning means not being represented; Figure 7 is a top view similar to Figure 4, showing a second embodiment of a prestressing bench according to the invention; and Figure 8 is a side view along arrow VIII in Figure 7.

In FIGS. 1 to 6 is shown a prestressing bench 1 by adherent wire, according to the invention. The bench 1 is fully shown in Figure 1 and partially shown in Figures 2 to 6, for simplification purposes. The bench 1 extends between two longitudinal ends 1A and 1B. The bench 1 comprises a slab 2, two end blocks 3, two longitudinal stops 4 and a set of spacers 5, which are made of reinforced concrete. The elements 2, 3, 4 and 5 are shown as separate pieces but, in practice, they are cast together and are one with the other. The slab 2 extends over the entire length of the bench 1, from the end 1A to the end 1B. The slab 2 is partially shown in Figure 2 and is not shown in Figures 3 to 5, for the sake of simplification. An end wall 3 is disposed at each end 1A and 1B of the bank 1, on the slab 2. Each solid 3 comprises an upper surface 6, a front surface 7 and a rear surface 8. Two pits 9 are formed in each 3, the longitudinal buttresses 4 extend between the ends 1A and 1B of the bank 1, in contact at regular intervals on the spacers 5. At the ends 1A and 1B, a spacer 5 is disposed against the front surface 7 of each frame 3 with which it is a body.

The bench 1 is intended to prefabricate a structural element 10 made of concrete, in this case a beam 10 in the example of FIG. 6, before it is incorporated into a structure of construction or any other construction. During its manufacture on the bench 1, the beam 10 is cast in a formwork 20, then preloaded by means of strands 30 coupled to tensioning means 40. This prestressing makes it possible to compress the concrete of the beam 10 sufficiently so that in all points the compressions are greater than the tractions which will develop later in use in this beam 10. As shown in Figure 6, the beam 10 comprises a core 14, a lower bead 16 and an upper table 18. The Core 14 is thinner than parts 16 and 18 in a transverse direction. Alternatively, the beam 10 may have any usual section within the scope of the intended applications, comprising a lower portion 16 and an upper portion 18. The formwork 20 has a shape corresponding to that desired for the beam 10, before pouring concrete. The formwork 20 may be arranged on a support 22, 15 resting on the slab 2. Guardrails 24 are preferably arranged on either side of the formwork 20. In the context of the invention, the strands 30 include lower strands 36 and upper strands 38, while the tensioning means 40 include a lower preload system 60 and an upper preload system 80. The strands 30 are longitudinal stress cables, elongated between the ends 1A and 1B of the bench 1, preferably of steel. The strands 36 and 38 have longitudinal ends, respectively 37 and 39. The lower system 60 is configured to tension and then relax the lower strands 36, while the upper system 80 is configured to tension and then relax the upper strands 38 The systems 60 and 80 are independent of one another, thereby independently controlling the tension of the strands 36 and 38. Before casting the concrete into the formwork 20 to form the beam 10, the strands 30 are stretched through the formwork 20 under the action of the tensioning means 40, in the longitudinal direction of the bench 1. More specifically, the strands 36 are stretched through the lower part of the formwork 20 under the action of the system 60, while the upper strands 38 are stretched through the upper part of the formwork 20 under the action of the system 80. During the pouring and setting of the concrete in the formwork 20, the strands 30 are maintained tensioned by the tensioning means 40. After the cast concrete is taken in the formwork 20, the strands 30 are embedded in the concrete parallel to the neutral longitudinal axis 35 of the beam 10. More specifically, the lower strands 36 under tension are embedded in the lower portion 16 of the beam 10, while the upper strands 38 under 3029444 6 voltage are embedded in the upper portion 18 of the beam 10. At this stage, while the concrete of the beam 10 has acquired sufficient strength , the tensioning means 40 are actuated to release the tension exerted on the strands 30, thus making it possible to prestress the concrete of the beam 10, both in the lower part 16 and in the upper part 18. The tensioning means 40 are described hereinafter with reference to FIGS. 2 to 5, showing the end 1A of the bench 1. In practice, the constituent elements of the systems 60 and 80 are distributed at the two ends 1A and 1B. Different variant They are conceivable, as described below.

The lower preload system 60 comprises a tread 61, two hydraulic cylinders 62, two bearing plates 63 and a perforated plate 64. The tread 61 is disposed transversely to the upper surface 6 of the solid 3 or any suitable support. The cylinders 62 and the plates 63 are disposed on either side of a longitudinal median plane, while the tread 61 and the plate 64 pass through this longitudinal median plane. The tread 61 is provided with longitudinal orifices, passing between its front and rear faces, for example in the form of tubes. The plates 63 and 64 are arranged transversely, respectively against the front face and against the rear face of the tread 61. The tread 61 and the perforated plate 64 are traversed by the lower strands 36, so that the longitudinal ends 37 protrude from the side. The ends 37 may be anchored against this plate 64 by means of anchoring means, for example a frustoconical cup and a key, not shown for the sake of simplification. The cylinders 62 are interposed between the buttresses 4 and the strut 61. More specifically, each jack 62 is disposed in abutment against the rear face of a buttrum 4 and against the front face of a support plate 63, which is it itself arranged in abutment against the front face of the headliner 61. The system 60 makes it possible to tension or relax the lower strands 36. When the jacks 62 are actuated in extension, these jacks 62 push back the headdress 61, the plates 63 and 64 and the ends 37 of the strands 36 towards the rear, thereby exerting a longitudinal tension stress on the strands 36. When the cylinders 62 are retracted, the longitudinal tension stress exerted on the strands 36 is gradually released, so that the stringer 61, the plates 63 and 64 and the ends 37 of the strands 36 return to their initial position. The upper prestressing system 80 comprises a trimming 81, two hydraulic cylinders 82, two bearing plates 83 and a perforated plate 84. The cylinders 82 and the plates 83 are arranged on either side of the longitudinal median plane, while that the trimmer 81 and the plate 84 cross this longitudinal median plane. The system 80 is supported by crutches 91 and brackets 92. The crutches 91 are located vertically in the pits 9 formed in the solid 3. The crutch 91 may have a rectangular section or an "H" section. The brackets 92 are fixed on the rear side 5 of the stands 91. The brackets 92 comprise a vertical part attached to the stands 91 and a horizontal part supporting the head 81 and the cylinders 82. In the example of Figures 1 to 5, the parts 91 and 92 are distinct. Alternatively, parts 91 and 92 can be made of material. The tread 81 is provided with longitudinal orifices, passing between its front and rear faces, for example in the form of tubes. The plates 83 and 84 are arranged transversely, respectively against the front face and against the rear face of the tread 81. The tread 81 and the perforated plate 84 are traversed by the upper strands 38, so that the longitudinal ends 39 protrude from the rear side. 81 and the perforated plate 84. Thus, the ends 39 can be anchored against this plate 84 by means of anchoring means, for example a frustoconical cup and a key, not shown for the sake of simplification. The cylinders 82 are interposed between the stands 91 and the tread 81. More specifically, each jack 82 is disposed in abutment against the rear face of a stand 91 and against the front face of a support plate 83, which is even arranged in support against the front face of the trimmer 81.

The system 80 makes it possible to tension or relax the upper strands 38. When the cylinders 82 are actuated in extension, these cylinders 82 push back the tread 81, the plates 83 and 84 and the ends 39 of the strands 38 towards the rear, thus exerting a longitudinal tension stress on the strands 38. When the cylinders 82 are retracted, the longitudinal tension stress exerted on the strands 38 is gradually released, so that the tread 81, the plates 83 and 84 and the ends 39 of the strands 38 return to their original position. In the context of the invention, it is noted that the headers 61 and 81 are located on the other side of the buttresses 4 relative to a vertical plane P4 positioned at the end of the buttresses 4, as shown in Figures 4 and 5. In other words, the trimmers 61 and 81 are disposed outside an area delimited by the buttresses 4 in the longitudinal direction of the bench 1, between two planes P4. In the prior art, it is known inverted trimmers, which are disposed within the space delimited by the end of the buttresses. This method makes it possible to reduce the length of the strands between the end of the beam and the end of the butons, but represents a certain complication, since it doubles the number of headers and imposes the use of two distinct types of steel ( strands and threaded bars) on the same tension axis. Moreover, this method does not simplify the calculation of prestressing losses. According to a first variant embodiment, the systems 60 and 80 comprise elements movable at each of the ends 1A and 1B. In other words, the systems 60 5 and 80 comprise at least one jack 62 and 82 disposed at each of the ends 1A and 1B, so as to move the trimmers 61 and 81 and the ends 37 and 39 of the strands 30 at each end 1A and 1B of the bench 1. According to a second variant embodiment, the system 60 comprises moving elements at the end 1A and fixed elements at the end 1B, whereas the system 80 comprises fixed elements at the end 1A and movable elements at the end 1 B. In other words, the system 60 comprises a jack 62 and a headmember 61 movable at the end 1A, and a fixed head 61 but no jack at the end 1B. For its part, the system 80 comprises a fixed head 61 and no cylinder at the end 1A, as well as a jack 82 and a tread 81 movable at the end 1 B. The system 60 tends the 15 strands 36 in the direction of the end 1A, while the system 80 tends the strands 38 towards the end 1B. According to a third embodiment, the systems 60 and 80 comprise moving elements at the end 1A and fixed elements at the end 1B. The systems 60 and 80 tend the strands 36 and 38 towards the end 1A.

As shown in FIGS. 3 to 5, the tensioning means 40 also comprise a monotoron jack 42, the operation of which is known. In practice, the cylinders 62 and 82 exert a collective tension on the strands 36 and 38, which do not all have the same original soft. The monotoron jack 42 makes it possible to check and adjust, if necessary, the optimum tension of each strand 30. The monotoron jack 42 is supported by a bracket, not shown for purposes of simplification. The invention also relates to a method of implementing the bank 1, comprising at least the successive steps a), b), c), d) and e). In a preparatory step a), the tensioning means 40 and the strands 30 are installed in the bench 1. In a main tension step b), the strands 30 are stretched through the formwork 20 under the action of the systems 60. and 80. In a complementary tension step c), the strands 30 are tensioned individually under the action of the monotoron jack 42. In a casting step d), the concrete beam 10 is poured into the formwork 20. In a step preloading e), the strands 30 are expanded so as to preload respectively the parts 16 and 18 of the beam 10.

When the bench 1 is in use, the buttresses 4 of reinforced concrete are subjected to high compressive stresses, for which the concrete naturally has a satisfactory behavior. In particular, the low prestressing exerted on the concrete in the lower part 16 of the beam 10, by the strands 36 and the system 60, is taken up by the buttresses 4. Preferably, the buttresses 4 are made of reinforced concrete while the struts 61 and 81 are metal. Alternatively, the butons 4 may be metal. According to another alternative, the trimmers 61 and 81 may be made of concrete. In practice, the length of the butons 4 is limited by the buckling phenomenon, affecting any slender structure subjected to a compressive force. Thanks to the transverse struts 5 arranged under the buttresses 4, this problem is solved simply and economically. The spacers 5 are positioned at regular intervals 10 between the ends 1A and 1B, for example every 9 to 12 meters. The spacers 5 reinforced concrete are simple and inexpensive to produce, directly on the site. The spacers 5 take up the horizontal deformation forces of the buttresses 4. Also, the spacers 5 take up the deformation forces in the vertical plane of the buttresses 4, ensuring a ballast function.

Advantageously, the buttresses 4 in reinforced concrete may be poured directly on the site, near the bench 1, which reduces the cost of transport in comparison with metal bumpers. Nevertheless, this advantage decreases when the center of gravity of the prestressing increases and the height of the buttresses 4 increases. Another disadvantageous case occurs when the beam 10 has a large inertia, and therefore a great height. In this case, to be as effective as possible, the prestressing is located as low as possible in the lower part 16 of the beam 10. However, the prestressing concentration in the lower part 16 of the beam 10 will cause partial traction forces upper 18. To optimize the prestressing of the beam 10, it is appropriate to resume these efforts by exerting a prestressing in the upper part 18.

The two objectives of prestressing in the lower part 16 and in the upper part 18 of the beam 10 are contradictory. If the same prestressing system were used to tension the lower strands 36 and the upper strands 38, the buttresses 4 would have a very great height, which would be unfavorable economically. It is therefore particularly advantageous to dissociate the high and low prestressing actions, as is the case thanks to the invention. In Figures 7 and 8 is shown a prestressing bench 1 according to a second embodiment of the invention. Some elements are similar to the constituent elements of the first embodiment, described above, and bear the same numerical references. Other elements differ from the first embodiment and carry numerical references increased by 100.

The main differences with the first embodiment relate to the weights 103 and the tensioning means 140. The weights 103 have smaller or similar dimensions to the masses 3 of the first mode. At the ends 1A and 1B of the bench 1, the longitudinal bumpers 4 rest on the weights 103 with which they form a body. Between the ends 1A and 1B, the buttresses 4 rest on the spacers 5 with which they form a body. The tensioning means 140 comprise a lower preloading system 160 and an upper preloading system 180. The lower preloading system 160 comprises a truss 61, two hydraulic cylinders 62, two bearing plates 63 and a perforated plate 164. The system 160 10 is comparable to the system 60 of the first mode. In practice, the dimensions of the plates 64 and 164 depend on the number and distribution of the strands 36. The system 160 may be based on a dedicated support, not shown for the sake of simplification. The upper prestressing system 180 comprises a header 181, two hydraulic cylinders 82, two bearing plates 83 and a perforated plate 184. The system 180 is comparable to the system 80 of the first embodiment, except for the dimensions of the hook 181 and the plate 184, more transversely than the tread 81 and the plate 84. In practice, the dimensions of the tread 181 and the plate 184 depend on the spacing of the buttresses 4. The system 180 is supported by gussets 191 and brackets 192 preferably metal. The gussets 191 have a triangular shape, with a rear side supporting the brackets 192 and a lower side fixed to the upper surface of the buttrum 4, by any suitable means. The brackets 192 support the tread 181 and the cylinders 82. The jacks 82 are interposed between the gussets 191 and the tread 181. More specifically, each jack 82 is disposed in abutment against the rear face of a gusset 191 and against the front face. a support plate 83, which is itself disposed in abutment against the front face of the trestle 181. In a simple and economical way, the dimensions of the weights 103 may be increased to resume the lifting efforts of the buttresses 4 generated by the gussets 191. Furthermore, the prestressing bench 1 may be shaped differently from Figures 1 to 8 without departing from the scope of the invention.

In addition, the technical features of the various embodiments and variants mentioned above may be, in whole or in part, combined with one another. Thus, the prestressing bench 1 can be adapted in terms of cost, ergonomics, functionality and performance.

Claims (10)

REVENDICATIONS1. Prestressing bench (1), for prefabrication of a concrete structural element (10), the prestressing bench (1) extending between two longitudinal ends (1A; 1B) and comprising strands (30), as well as strand tension means (40; 140) between the longitudinal ends (1A; 1B) through a formwork (20) provided to form the structural member (10); characterized in that the tension means (40; 140) comprises a lower biasing system (60; 160) and an upper biasing system (80; 160) configured to tension and then relax lower strands (36) respectively. and upper strands (38) for prestressing a lower portion (16) and an upper portion (18) of the structural member (10). 2. Prestressing bench (1) according to claim 1, characterized in that the lower prestressing system (60; 160) is disposed in abutment against longitudinal stops (4) which extend between the longitudinal ends (1A; B) of the prestressing bench (1). 3. preload bench (1) according to claim 2, characterized in that it comprises spacers (5) disposed under the longitudinal buttresses (4), in particular at regular intervals between the longitudinal ends (1A; 1 B) of the bench prestressing (1). 4. prestressing bench (1) according to one of claims 1 to 3, characterized in that at each longitudinal end (1A; 1B), the prestressing bench (1) comprises an end mass (3) and the upper prestressing system (80) is supported by at least one stand (91) which is vertically implanted in a pit (9) in the end mass (3). 5. prestressing bench (1) according to one of claims 1 to 3, characterized in that at each longitudinal end (1A; 1B), the upper prestressing system (180) is supported by a gusset (191) which is fixed on a longitudinal buton (4). 3029444 12 6. Prestressing bench (1) according to one of claims 2 to 5, characterized in that the lower prestressing system (60; 160) and the upper prestressing system (80; 180) comprise trimmers (61; 81 181) disposed outside an area delimited by the longitudinal stops (4) in the longitudinal direction of the prestressing bench (1). 7. Prestressing bench (1) according to one of the preceding claims, characterized in that the lower prestressing system (60; 160) and the upper prestressing system (80; 180) each comprise: 10 at each longitudinal end ( 1A; 1B) of the prestressing bench (1), a treadle (61; 81; 181) provided with longitudinal holes for the passage of the strands (30), and means (84) for fixing the ends (37; 39) strands (30) to the header (61; 81; 181); and at least one longitudinal end (1A; 1B) of the preload bench (1), at least one actuator (62; 82) adapted to move the trimmers (61; 81; 181) in order to tension and then relax the strands (30). 8. Prestressing bench (1) according to claim 7, characterized in that the lower pretensioning system (60; 160) and the upper prestressing system (80; 180) each comprise at least one actuator (62; 82). disposed at each longitudinal end (1A; 1B) of the prestressing bench (1). 9. Prestressing bench (1) according to claim 7, characterized in that the lower prestressing system (60; 160) comprises at least one actuator (62) arranged at a first longitudinal end (1A; prestressing (1), while the upper prestressing system (80; 180) comprises at least one actuator (82) arranged at a second longitudinal end (1A; 1B) of the prestressing bench (1). 30 10. A method of implementing a prestressing bench (1) according to one of claims 1 to 9, characterized in that it comprises at least the following steps: a) a preparatory step, wherein the tensioning means (40) and the strands (30) are installed in the prestressing bench (1); B) a main tension step of tensioning the lower strands (36) under the action of the lower preload system (60) and the upper strands (38) under the action of the upper prestressing system (80); ), through the formwork (20) provided to form the structural member (10); c) a complementary voltage step of stranding the strands individually by action of a monotoron jack; D) a casting step of pouring the concrete structural member (10) into the formwork (20); e) a preloading step of expanding the lower strands (36) and the upper strands (38) so as to preload respectively the lower part (16) and the upper part (18) of the structural element (10) .