A METHOD OF MANUFACTURING A PRESTRESSED CONCRETE BEAM OR PLATE ELEMENT AND REINFORCING ASSEMBLIES FOR USE IN THE METHOD

Abstract: A prestressed concrete beam (1) is cast with a rectilinear longitudinal profile and a filling piece (13) extending from a surface of the beam (1) to the reinforcing strands (6) is embedded. After hardening of the concrete and removal of the filling piece (13) if necessary, the beam (1) is deflected in a predetermined angle by rotating the beam sections (2, 3) on either side of the recess (4) formed by the filling piece in such a direction that the recess is diminished, following which the beam sections (2, 3) are retained in the predetermined angle. A reinforcement assembly made in two equal halves and connected with a breakable joint may be embedded in the area of the recess to reinforce the beam sections (2, 3).

Description: 1 A method of manufacturing a prestressed concrete beam or plate element and reinforcing assemblies for use in the method. The invention relates to a method of manufac¬ turing a prestressed concrete beam or plate element in which method the reinforcing strands are suspended substantially rectilinearly through the mould and in which the element is cast with a substantially rec¬ tilinear longitudinal profile. Beam or plate elements of the above mentioned type are generally used in building structures, par¬ ticularly for floor-decks and roof constructions, because such elements have a good strength compared to their weight and are cheaper to manufacture than for instance concrete elements with a slack or post- tensioned reinforcement or concrete structures cast on site. However, the prior method does not make it possible to cast the beam or plate elements in roof- shape, viz. designing the beam element as two legs forming an angle with each other, and the plate element, as a roof surface with a ridge with two inclined lateral faces. Such beam elements may be produced in that two rectilinear prestressed concrete elements are joined by casting them together so that they form an angle with each other and by reinforcing the area of the cast joint with cables that are post-tensioned after harden¬ ing of the concrete. This makes, however, the beams more expensive to manufacture and a considerable draw¬ back is further that large V-shaped roof beams may be difficult to convey from the production site to the building site, due to the fact that existing bridges and aerial cables on the conveyance route set a limit to the size of the elements. Moreover, DK patent B 152 999 discloses a method of manufacturing a concrete beam or plate element with a prestressed multiple strand reinfo cement and with a concave underside in which the reinforcement is e - bedded as a tensile reinforcement. According to this method the reinforcing strands extend unbrokenly be¬ tween the ends of the element and the run of strands form one or more bends in which the strands are restrained in a guide intended to take up the trans- verse forces from the reinforcement at the point or points of bending prior to casting of the concrete, said guide extending to the compressive side of the element and is intended to be fixed at said side of the element after hardening of the concrete. According to this prior method the path of the reinforcing strands results in very strong transverse forces on reinforcement guides and moulds when locating and prestressing the strands, and in practice this pro¬ cess only allows for manufacturing beam or plate ele- ments with a modest roof inclination. It is the purpose of the invention to provide a method of manufacturing beam and plate elements of the above mentioned type which do not suffer from the men¬ tioned disadvantages. The invention differs from the above mentioned method in that at least in one point of the element a first filling piece is embedded in the element, said filling piece extending largely from one surface of the element to the reinforcing strands, that the first filling piece, if required, is removed after hardening and prestressing of the concrete, that the element after hardening and prestress- ing of the concrete is deflected to a predetermined angle by a relative rotation of the element sections 3 located on either side of said point so that the recess formed by the first filling piece is diminished, and that the element sections on either side of the first filling piece are fixed in relation to each other in the predetermined angle. The reinforcing strands are usually suspended in a casting bed and in the manufacturing of for example a beam element the reinforcing strands are located in the foot of the beam or at that part of the beam which later on constitutes the tensile side of the beam, and the filling piece that is advantageously wedge-shaped is embedded in the casting mould on the side towards which the element sections are to be rotated during the deflection operation and preferably so that it extends into the area of the reinforcing strands. The concrete is then cast and after hardening of the concrete the reinforcing strands are released, thereby obtaining the desired prestressing in the concrete. The beam element now appears as a rectilinear, prestressed concrete element and may in this form be conveyed to the building site without the risk that bridges, aerial cables and similar installations along the conveyance route hamper the conveyance. If the side of the element accommodating the filling piece in this situation constitutes the compressive side of the ele¬ ment, the filling piece must be structured so that it is capable of taking up the resulting compressive The deflection operation may for instance be carried out by using an erecting crane lifting the element at its ends and comprising a secondary winch to be attached in the area of the deflection point. This area of the element is then elevated in relation to the ends of the element, e.g. by lowering the ends until the desired deflection angle is attained. Due to its own weight the element bends in the weakest cross- 4 section, i.e. at the recess formed by the filling piece. A fissure is at first formed which from the reinforcing strands extends towards the surface of the element opposite the recess and by continued elevation the fissure opens more and more, the element sections on either side of the deflection point being kept against each other by the pretensioned reinforcing strands. If said filling piece is produced from a deform- able material, e.g. a foam plastic, that is sufficient¬ ly rigid to resist the pressure from the concrete during the casting and vibration thereof in the mould, but which may be deformed without offering any notice¬ able resistance upon deflection of the beam element, the filling piece may remain in its position during the deflection operation. If, on the contrary, the filling piece is made from steel or wood, it has to be removed prior to the deflection step. When the element sections on either side of the deflection point form the desired angle, they are main¬ tained in this position. This may for instance be effected in that brackets previously embedded are connected by means of welding or in any other appropri¬ ate way. Producing the element according to the method described above results in a prestressed concrete member with a deflected or angular longitudinal profile and in which the pretensioned reinforcing strands extend unbrokenly from one end of the element to the ether. The method according to the invention entails the further advantage that the element, as the case may be, is given its final form either on the production site or on the building site. A considerable saving of working hours is obtained when the deflection of the element is effected concurrently with erecting it to its final position. 5 The method according to the invention does not only make provision for producing roof-shaped beam or plate elements but e.g. also for gantry or frame mem¬ bers, such as two-hinge or three-hinge frames with roof and column or facing parts. In a preferred embodiment of the invention the element sections are fastened on either side of the first filling piece in the predetermined angle in rela¬ tion to each other in that the opening between the facing surfaces of fracture belonging to the element sections on either side of said point is filled in with concrete or with a fitting piece secured to the element sections. Filling in the resulting opening with concrete may be advantageous in cases where the element has a simple cross-section and it may be effected quickly and without complicated moulding work, and where the sequence of operations permits the elements to be kept in the desired angle in relation to each other until the filled-in concrete has hardened without incurring substantial costs. In other types of such an element a fitting piece may be inserted in the opening formed between the surfaces of fracture, causing the element when not lifted any longer at the deflection point to maintain its shape and to be immediately able to trans¬ fer compressive forces originating from its own weight and possible external influences between the element sections with the result that temporary supports are saved and that the erecting crane is rapidly liberated to be used for the erection of the next element. An embodiment of the invention is characterized in that a transverse partition extending from the opposed surface of the element towards the reinforcing strands is embedded opposite the first filling piece, and that the transverse partition is removed during the deflection of the element. By using a transverse par- 6 tition as described a well defined weakening of the cross-section of the beam is obtained and the surfaces of the beam sections facing each other on either side of the point of fracture may be given a desired form which for instance facilitates the adaptation of a fitting piece. In this case the transverse partition and the fitting piece may advantageously be cast, e.g. from concrete, by using one and the same set of moulding sides. This ensures that the faces forming the butt seams between the inserted fitting piece and the deflected element fit exactly to each other. In beam and plate elements having larger pre¬ stressing forces it is necessary that a transverse reinforcement is embedded in the element on either side of the point of fracture. Hereby, the transverse forces arising from the change of direction of the reinforcing strands in the point of fracture may be transferred to the compressive side of the beam. As the compressive forces transferred between the two sections of the element are very strong in the area of the prestressed reinforcement, it is preferred that a strengthening member is embedded on either side of the first filling piece and that the faces abutting each other are the faces of the reinforcing members. Said abutment faces of the strengthening members may be constituted by steel plates to which a specific curvature is imparted so that they roil on each other during the deflection of the element, and by varia- tion of the radius of curvature of the surfaces it is possible to obtain that the prestressing force is kept constant or varies in a desired manner during the deflection of the element. Thereby, the changes in the prestressing force occurring in the first period of time after casting the concrete may be compensated for, and other ci cumstances influencing the prestressing force may as well be compensated for. A second embodiment of the invention is char¬ acterized in using strengthening members acting at the same time as reinforcement guides for maintaining the reinforcing strands in a specific position that is dif- ferent from the position they occupy at the ends of the element. In the proximity of the point of deflection it is desirable that the reinforcing strands are disposed in one layer extending perpendicularly to the direction of deflection while it is most frequently desirable to locate the reinforcing strands at the ends of the ele¬ ment at a reasonably large mutual distance and uni¬ formly distributed throughout the cross-section in oder to obtain a good anchoring of the strands and a suit¬ able distribution of the prestressing force throughout the concrete. It is desirable that the reinforcing strands after the deflection operation extend at an appropri¬ ately large radius of curvature through the fracture zone. According to the invention this may be achieved in that a second filling piece is embedded between the reinforcing strands and the first filling piece. The second filling piece may have a plane or grooved sur¬ face facing the reinforcing strands and be thickest opposite the point of fracture from which it tapers towards the ends. The second filling piece may be made from a deformable material, e.g. an appropriately rigid foam plastic. During the deflection operation the rein¬ forcing strands will exert a pressure on the second filling piece and press it together until the rein¬ forcing strands at last rest against the concrete on the opposite side of the second filling piece. A suitable design of the second filling piece ensures that the reinforcing strands are given exactly the desired path during the deflection operation. The second filling piece may be accommodated in the reinforcement guide which may further be built together with the transverse reinforcement. 8 Such an assembly according to the invention is advantageously made in two identical halves to be em¬ bedded on either side of the point of deflection. The two halves are assembled prior to embedment with a con- nection securing an exact, mutual position during em¬ bedment and vibration, but which is no stronger than it will break upon starting the deflection operation. Such joining of the two halves of the assembly permits the above mentioned adjustment of the prestressing force. According to a second embodiment of the inven¬ tion the reinforcement assembly is made in the form of a single, integral unit having a box-like or U-shaped cross-section, the ends of which over a suitable length are fixedly embedded in the concrete on the respective sides of the point of deflection. During the deflection operation the prestress force is transferred directly through the continuous piece serving as reinforcement guide which is bent in a uniform curve during the deflection operation while being supported by the embedded transverse reinforcement. The invention will now be described in detail using examples of embodiments and with reference to the drawings, in which Fig. 1 is a side view of part of the beam or plate element according to the invention, Fig. 2 is a side view of the element in Fig. 1 after deflection, Fig. 3 is a side view of a beam section cons¬ tituting a gantry or frame corner, Fig. 4 is the same beam section after deflec¬ tion. Fig. 5 is a side view of part of a beam element in a second embodiment, Fig. 6 is a side view of a section of the beam element in Fig. 5, on a larger scale, Fig. 7 is a section along the line V I-VTI in j_ __y . O , Fig. 8 is a view corresponding to Fig. 6 after deflection of the element, Fig. 9 is a side view of a reinforcement assembly according to the invention, Fig. 10 is an end view of the reinforcement assembly in Fig. 9, Fig. 11 is a side view of part of a beam element with a reinforcement assembly in a second embodiment, Fig. 12 is a sectional view along the line XIΪ-XII in Fig. 11, and Fig. 13 is a side view corresponding to Fig. 11 after the deflection operation. Fig. 1 illustrates part of a beam or plate ele¬ ment of prestressed concrete produced by suspending reinforcing strands 6 rectilinearly throughout a casting mould on the bottom of which a wedge-shaped filling piece 13 is disposed prior to casting the concrete and further including two reinforcement stirrups 7. Fig. 1 shows a section of the beam element in the stage of the manufacturing process in which the concrete has been cast and hardened and the reinforcing strands 6 have been released. The filling piece in the element forms a wedge-shaped recess extending to the reinforcing strands 6 and having sides 5. By lifting the element in the proximity of the recess in such a direction that the recess has a ten¬ dency to close, the beam cross-section at the recess will be subjected to a bending movement of such a magnitude that the beam deflects sharply, thereby giving rise to a fissure or an opening confined by the surfaces of fracture 8. By further lifting the portion of the beam containing the fracture in relation to the beam ends, the two beam sections 2, 3 are on either side of the point of fracture pivoted a certain angle relative to each other. The beam is then retained and as indicated in Fig. 2 concrete 9 is cast into the opening between the surfaces of fracture 8. Fig. 3 illustrates a part of a frame member manufactured substantially as a rectilinear prestressed element, in principle like the beam element illustrated in Fig. 1, but with the prestressed reinforcement 6 located at the upper side of the element. On either side of the filling piece 13 anchored steel brackets 20 are embedded in the concrete. Filling piece 13 remains in its position until the deflection is to be effected and it forms until then part of the compress- ive side of the element during storage and conveyance. During this step brackets 20 serve to transfer forces between filling piece 13 and the concrete. Fig. 4 illustrates the element in Fig. 3 after the deflection operation has been accomplished. This may be carried out as described with reference to the beam element illustrated in Fig. 2. After the deflec¬ tion operation element sections 2 and 3 are fixedly secured in the new position by welding brackets 20 together or joining them by bolts. The external recess formed by the deflection may, if desired, be filled in later on with concrete 9 or other material. The frame element in Fig. 4 may be a partial member of a 3-hinge frame, a so-called semi-frame, with the roof portion 2 and the column portion 3. The element may as well be a 2-hinge frame. Fig. 4 illus- tating then one of two symmetrically positioned points of deflection. The element may for instance have a third point of deflection in the centre of element sec¬ tion 2 analogously with Fig. 2. Fig. 5 illustrates a beam element manufactured as that illustrated in Fig. 1 but with two embedded reinforcing plates 11 adjacent each other and which in the area of reinforcing strands 6 are simply curved, thereby rolling on each other during the deflection of the beam. The figure further shows a first filling piece 13 inserted between reinforcing 11 plates 11 and a transverse partition 14 of concrete. The transverse partition is removed prior to the bending of the beam, thereby providing well defined "surfaces of fraction" 8 in the beam concurrently with obtaining a well defined positioning of the point of fracture. As illustrated in more detail in Figs 6 and 7 a second filling piece 15, for instance of rigid foam plastic and shaped as a double-wedge, is embedded below the reinforcing strands 6, the underside of said second filling piece being formed as two inclined sur¬ faces 12 respectively curved as a part of a cylinder surface. Fig. 8 illustrates how a fitting piece 10 of concrete after the deflection operation is inserted in the beam in Figs 5 to 7 in the opening between sur¬ faces 8 in beam sections 2 and 3 on either side of the point of fracture. The surfaces of the transverse partition 14 and the fitting piece 10 corresponding to surfaces 8 are cast by use of the same mould sides so that the sides of fitting piece 10 may form tight joints with surfaces 8. To secure fitting piece 10 notches may be provided in the above mentioned mould sides, thereby locking surfaces 8 and the correspond¬ ing surfaces of the fitting piece in relation to each other, or grooves may be provided in the joint surfaces in which grooves concrete is later on filled in. It also appears from Fig. 8 that reinforcing strands 6 during the deflection operation have com¬ pletely compressed second filling piece 15 and then exert a pressure against concrete surfaces 12 which now form a continuous uniform surface, thereby impart¬ ing to the reinforcing strands a uniform circular path throughout the fracture zone. It further appears that the compression of second filling piece 15 creates a cavity 16 above the reinforcing strands. This cavity 12 may later on be filled in with a thin casting material, e.g. cement mortar through a casting channel 17 in fitting piece 10. The reinforcement assembly is made from steel as two interconnected, halves assembled by a weld 18 that is no stronger than it still breaks during the deflection operation. The reinforcement assembly is made as a single assembly containing strengthening plates 11, abutment faces 12 for the reinforcing strands after deflection and transverse reinforcements 7 and embedded filling pieces 13 and 15. The assembly is located in the casting mould prior to laying and tensioning the reinforcing strands. Transverse reinforcements 7 are shaped as open stirrups with reinforcing plates 19 at the top in order to facilitate the positioning of the reinforcing strands. The anchoring plates 19 also function as guides for the transverse partition 14. Figs 11 and 12 illustrate a section of a prestressed concrete beam 2, 3 with an embedded reinforcement assembly of an alternative type. The reinforcement section or the reinforcement guide 11 is in this case a continuous steel profile of U-shaped cross-section whose ends are solidly embedded in the respective element sections 2 and 3. The profile is sized to support the full prestressing force of the reinforcing strands and is provided with ribs 11a to contribute to transferring the prestress load between the reinforcement guide and the concrete, and the transverse rein orcements 7 are welded to the guide. Dependent on the size of the desired deflection angle, one or more filling pieces 13 with associated trans¬ verse partitions 14 are inserted. Fig. 13 illustrates the same element after accompl shment of the deflection operation which, more¬ over, is likely to be effected by the same means as 13 previously described. Filling pieces 13 are in this case made from a durable and resilient material, e.g. rubber, which is left in the element and forms a uni¬ formly curved transition between element sections 2 and 3 and a piece 22 disposed between the trans¬ verse partitions. During the deflection operation the reinforcement guide is cold-deformed in the fracture zones. Fitting pieces 10 are finally inserted between embedded brackets 23 and the reinforcement plates 19 and may for instance be locked in position by welding. Such a locking may also be obtained by a suitable design of the corresponding surfaces of the butt seams between the element sections and the fitting pieces. The reinforcing strands abut on the bottom of the guide and the cross-section of the guide is designed so that its centre of gravity is located exactly in relation to the reinforcing strands so as to effect upon deflection a desired adjustment of the prestressing in the frac¬ ture zones. The invention is not limited to the above described examples of embodiments illustrated in the drawings and other embodiments are possible without thereby deviating from the idea of the invention.

Claims: 14 P A T E N T C L A I M S 1. A method of manufacturing a prestressed concrete beam or plate element in which method the reinforcing strands are suspended substantially recti- linearly through the mould and in which the element is cast with a substantially rectilinear longitudinal pro¬ file, characterized in that at least in one point of the element (1) a first filling piece (13) is embedded in the element, said filling piece extending largely from one surface of the element (1) to the reinforcing strands (6), that the first filling piuece (13), if required, is removed after hardening and prestressing of the concrete, that the element (1) after hardening and pre- stressing of the concrete is deflected while maintain¬ ing the prestressing throughout the entire length of the element (1), to a predetermined angle by a relative rotation of the element sections (2, 3) located on either side of said point so that the recess (4) formed by the first filling piece (13) is diminished, and that the element sections (2, 3) on either side of the first filling piece (13) are fixed in relation to each other in the predetermined angle. 2. A method as claimed in claim 1, characterized in that the opening between the facing surfaces of fracture (8) belonging to the element sections (2, 3) on either side of said point are filled in with concrete or with a fitting piece (10) secured to the element sections {2, 3) . 3. A method as claimed in any of the preceding claims, characterized in that a transverse partition (14) extending from the opposed surface of the element (1) towards the reinforcing strands (6) is embedded opposite the first filling piece (13), and that the transverse partition (34} is removed during the deflec- - • ? the elemen . 4. A method as claimed in claim 3, characterized in that the transverse partition (14) and the fitting piece (10) are cast, e.g. from concrete, by using one and the same set of moulding sides. 5. A method as claimed in any of the preceding claims, characterized in that a transverse reinfor¬ cement (7) is embedded in the element (1) on either side of the point of deflection. 6. A method as claimed in any of the preceding claims, characterized in that a reinforcing member (11) with surfaces abutting on each other during the deflec¬ tion operation is embedded on either side of the first filling piece (13). 7. A method as claimed in claim 6, characterized in using strengthening members (11) acting at the same time as reinforcement guides (12) for maintaining the reinforcing strands (6) in a specific position that is different from the position they occupy at the ends of the element. 8. A method as claimed in any of the preceding claims, characterized in that a second filling piece (15), preferably consisting of a deformable material and advantageously together with a transverse rein¬ forcement, is embedded between the reinforcing strands (6) and the first filling piece (12). 9. A reinforcement assembly for use in the method according to any of the preceding claims, char¬ acterized in that it is made as two similar halves to be embedded on either side of the point of fracture, said two halves being assembled prior to casting with a connection (18) securing an exact, mutual position during casting and vibration, but which is no stronger than it will break upon starting the deflection opera¬ tion. 10. A reinforcement assembly for use in the method according to any of claims 1 to 8, characterized ιe i n that it is made in the form of a single, integral un i t ( 11) having a box-like or U-shaped cross-section the ends of which over a suitable length are fixedly embedded on either side of the point of fracture.

Inventors: LIND ERIK, LIND, ERIK
Applicants: DANSK SPAEND AS, A/S DANSK SPAENDBETON
Application year: 1989
Main classification: E04C2
IPC codes: E04C3