MOLD FOR PREFABRICATED CONCRETE PANELS

Abstract: The concern is the development of reinforced or prestressed concrete precast panels having an open section with inverted channel form, ensuring suitable fire resistances of floor systems built with them. The width of panel ribs in current techniques is insufficient to provide fire resistent floors. This can be achieved employing inverted channel-section panels which form double-wide-ribbed slabs, with the same weight for precast elements. The problem is how to obtain such panels with a strickly rectangular exterior form to ensure that lateral sides are plane and parallel to get the smallest joints between coupled panels in floors so that their floor fire resistance are that of slabs with double-width ribs. The invention solves it by obtaining panels in a fixed continuous formwork bench, without moving any formwork part to take them out. Fabrication of this kind of panels, the procedure and the device to produce inverted channel-section panels are revendicated.

Description: The technique of reinforced or prestressed concrete precast panels with an open section employed as structural floor slab elements in industrialized building systems, corresponds basically to the so-called Π-section precast panels formed by two longitudinal ribs and two lateral and one central flanges. The Π-section precast panels technique was first developed for ribbed floor slabs supported on precast beams. Next it was extended to composite floors comprising the precast ribbed slab units and the reinforced concrete plane beams, in which the overall depth of ribbed floor slabs equals that of the concrete plane beams, cast in situ together with reinforced concrete over the panels. The proposed invention refers to the development of reinforced and prestressed concrete precast panel units with an open section, of the so-called inverted channel section type of panels, formed by two longitudinal ribs, or panel beams, and one central flange, taking into account the adecuate fire resistance requirements for floor systems. The problem when using the technique of reinforced concrete precast panels with Π-section is the insufficient width of usual panel ribs to hold the maximum action-effects in the floor ribbed slabs, during the ocurrence of a standard fire in the area limited by the floors, taking into account both the reduction of steel reinforcement and concrete strenghts for the high temperatures reached in the fire and the temperature distribution on these materials in the floor cross section. Such a problem can be solved without increasing the weight of precast units, from some mimimum width of panel ribs, by employing the new type of panels with an inverted channel section which double the width of ribs of floor slabs that result when they are coupled. This solution keeps the advantages of industrialized prefabrication of panels and efficient transportation, by avoiding an overdimensioned design of the fire resistent structures, making possible the use of usual cranes to handle the precast units in the construction process. In this case, the desideratum from the fire resistance point of view is to make the precast panels with an inverted channnel open section having a strict rectangular section exterior form, ensuring that the lateral sides of panel ribs are plane and parallel. In this way the minimum width for construction joints between the lateral sides of coupled panels can be obtained at the working place; then it is possible to ensure that, regarding the fire resistance in the most unfavourable case of simultaneous action of fire on three slab rib sides, each beam formed by the pair of ribs of coupled panels is equivalent to a double-width beam. The equivalence means that there are the same boundary conditions for maximum temperatures on the surfaces directly attacked by fire and the same temperature distribution in both the resistent concrete core within the cross section and the steel reinforcement, and therefore the resulting floor elements have mechanical strength analogous to that of double-width beam floor. Otherwise, if the exterior form of panels is not rectangular and the rib thickness increases progressively from the bottom to facilitate the extraction of panels from the formwork, the width of resulting construction joints cannot ensure the equivalence of internal and external temperature distributions in the cross sections. Note that this would be the case using the Π-section precast panels when cutting the lateral flanges of the panels. Systems of molds employed so far in fabrication of panels with a strict rectangular section exterior form belong to the class of 'molds in pieces', namely the class of molds in which one or more pieces of the mold must be removed to obtain the cast elements. In particular, the molds for casting rectangular section panels referred to in the known patent documents belong to this class of 'molds in pieces'. Thus, on the one hand, in the case referred to in the document US-3767153A, which is the only one regarding molds for panels having an inverted channel section with a rectangular exterior form, the lower part of the mold is pushed upwards to take out a single panel, by separating the set formed by this lower part and the precast panel lying on it from the mold's lateral pieces and the formwork bench; for this purpose the material should be only partially hardened, such that the separate panel cannot be removed from the movable lower part of the mold because it is not yet self-supporting. In a similar way, in the case referred to in the document FR-53476, related not to precast panels but to small rectangular blocks, the lower part of the mold is moved up and separated from the remainder of the mold to obtain the blocks; for that purpose consistency of block material should be soft as well. On the other hand, in the case referred to in the document DE-4333080A1, which is solely related to stiffly hardened panels with rectangular section, the mold lateral pieces are horizontally removed toward the sides by opening the mold to take out the panel while the lower part of the mold remains fixed on the formwork bench. Document US-3767.153A or ES-2 076 856 A2 is considered constitute the closest prior art, and corresponds therefore to the preamble of claim 1. It shows however a mold for the production of concrete panels, where the mold belongs to the type of "mold in pieces", which means that one or more parts of the mold must be removed to take the casted panels out of the mold. However, because of fast production, eficiency and economy, interest in this invention is focussed in fabrication of open section panels in a fixed mold in one piece, longer than usual molds in pieces for a single panel, on which the spacers and headers of panels are placed, given the advantages for industrial fabrication of continuous placing and compacting of the concrete and posterior straight extraction of many precast panels from the formwork. The problem is, therefore, to obtain reinforced or prestressed concrete precast panels with an inverted channel section having a strict rectangular section exterior form, ensuring also that lateral panel sides are plane, in a mold of the class of 'molds in one piece', namely the class of molds in which panels can be taken out without moving any part of the mold and the formwork bench, therefore in a radically different way than in the class of 'molds in pieces'. The procedure that solves this problem consists in placing along the formwork two stiff linear elements of triangular shaped section with a right angle, being the opposed side to this angle plane or near-plane as a cylindric concave form. These elements are placed inside the formwork in a way such that the right angle edge of each element lies exactly on the edge of corner formed by planes molding the lateral and lower sides of panel ribs. Once the suitable angles and dimensions of the triangular elements as well as the convenient stiffness of formwork elements are fixed, this arrangement determines that precast panels can be taken out by straight and vertical pulling from hooks previously anchored in them, avoiding the failure of panels due of friction forces against the formwork. Indeed, when taking the panel out from the formwork, the latter becomes elastically deformed laterally making the compression normal forces and the friction vertical forces against the panel to be located near the lateral corners inside the formwork. Moreover, stresses due to friction along the panels are concentrated just in those zones near the fasteners stiffening the different elements of the formwork. Both effects, developed when taking out the panels, would cause the shear fracture of the concrete close to these zones, approximately according to an inclined failure plane, parallel to the edge of each lateral inferior corner of the panels. Figure 1. The position of the above mentioned stiff linear pieces in the formwork corners raises the possible fracture plane, with the same angle for this plane than in the fracture case; it increases considerably the area of potential fracture surface making that normal and shear stresses on this surface, which are related to the same normal and friction forces, become smaller than the failure stresses defined by the intrinsic resistance curve of concrete at the age of extraction of panels. Figure 1 represents how these linear elements are disposed in the rectangular section exterior form which molds the panels in the formwork. In this figure the symbol e indicates the direction for extraction of paneles. The symbol 1 represents the linear pieces with a triangular section, and symbols 2 and 3 designate respectively the initial inclined fracture plane and the more elevated potential parallel fracture plane, in which the failure stresses will not be reached. When the width of construction longitudinal joints between two ribs of coupled panels is minimum and the reinforcement covers and spacement are adequate, then the change in the initial rectangular section resulting when molding panels with the linear triangular pieces, does not alter significatively the fire resistance of the floor, maintaining the internal temperature distribution on the steel reinforcement and the resistent concrete core in the cross section. Figure 2 represents the arrangement of two precast panel with the inverted channel section which are jointly placed in the floor which form a double-ribbed slab beam with the alteration made by the linear pieces 1. In this figure the symbol f represents the three directions in which the standardised fire attacks the double-ribbed slab beam. Symbols 4 and 5 design respectively the construction joint between panels and the reinforced concrete over the panel in the floor. Symbol 6 corresponds to the cross holes, which are disposed where appropiate to open ways through the slab ribs to avoid accumulation of gases under the floor. The function assigned in this invention to the linear rigid elements 1 in the case of a 'mold in one piece' is only to make possible the extraction of the precast panels having an inverted channel section with a strict rectangular exterior form, in a way that these elements become essential in the procedure. It differs from the functions of placing the linear elements similarly along the formwork in the cases of 'molds in pieces', given in the patent documents US-3767153A, DE-4333080A1 and FR-53476. Along these documents it is only referred to that the lower corners are bevelled to avoid the sharp edges of the precast elements having a tendency to failure when managing them as well as to avoid unpleasant aesthetic results in construction joints of precast panels, but not to take the precast panels out of the mold. The arrangement of these stiff linear elements 1 in the case of a 'mold in one piece' is quite different because such elements, placed on the corners inside the formwork, are fixed both on the lower side part as well as on the lateral side part of the mold, whereas in the cases of 'molds in pieces' they are only fixed on one of the mold side pieces, either the lateral or the bottom, in order to be possible to remove the other side piece of the mold for taking the precast panels out of the mold. Moreover the simple arrangement of linear elements in each interior corner of a 'mold in one piece' is not sufficient to allow the extraction of precast panels with an strict rectangular inverted channel section without failures in panels. Both the appropriate transversal and longitudinal stiffness of the mold and a suitable shape and dimensions of the linear elements 1 are required to pull out the panels; such dimensions have to be also stated to ensure the due fire resistance of panels. The system of normal and friction forces and the stress field originated in the case of the 'mold in one piece' for panels with an inverted channel rectangular section are also different from the system of forces and the stress field in the case of 'molds in pieces' referred to in the above patent documents. Thus in the cases described in US-3767153A and FR-53476 in which the material is partially hardened or has soft consistency, the applied forces should mobilize only the adherence to fixed pieces of the mold, since the said material may be yet deformed. In the case referred in DE-4333080A1, in which panels are stiffly hardened, lateral forces should mobilize only adherence stress since there is not friction. The system of forces and the stress field are also quite different from those in the usual Π-section precast panels technique, with trapezoidal section ribs, in which the applied forces should only mobilize the adherence to the mold. Hence, the panels with an inverted channel rectangular section obtained by this procedure are distinguible from other possible panels made in a mold in which only the adherence force requires to be mobilized, by action of friction on the surfaces of lateral sides of panels, which however are undamaged. Figure 3 represents one realization of a formwork bench for fabrication of precast panels with inverted channel section. The parallel lateral sides of the panels are molded by the formwork elements formed by U-section steel profiles with parallel flanges and the smooth and corrosion protected steel plates covering the inside web faces in the formwork, respectively denoted by symbols 7 and 8. The mold is completed with the steel plates of the same kind, remarked as the symbol 9, which are folded according to the required form for the ribs and flange of panels, and fixed on the stiffness cross plates of the formwork denoted by symbol 10. The symbol 11 designs the longitudinal main beams of the formwork bench frame which rest anchored on metallic supports through elastomeric blocks, represented respectively by symbols 12 and 13. In the same figure it is represented how the triangular linear elements 1 are placed on the formwork and the cylindric elements non-adherent to concrete are cross placed inside the formwork and adjusted to the folded steel plate sides, to mold the panel holes 6, opening cross ways through the slab ribs in the floors. On this logitudinal formwork having an inverted channel open section, the reinforced an prestressed concrete precast panels are fabricated by continuous placing, compacting and vibration and curing of the concrete, and posterior extraction by means of a traveller crane which vertically pulls the panels out from the formwork. These are then stored until the age they reach the adequate strength to be handled and arranged on the building floors.

Claims: A mold for the production of reinforced or prestressed concrete panels having an inverted channel concrete section with a strictly rectangular external form, having plane parallel lateral sides in order to form longitudinal ribs with plane parallel lateral sides, that part of the mold defining the inside panel section being formed such that the longitudinal ribs have a variable width, characterized in that the mold is of the type "mold in one piece", and that two linear stiff linear elements (1) of triangular shaped section with a right angle are provided inside the mold in a way such that the right angle edge of each element lies on the edge of the corner between the plane lateral side and the underside of the rib forming part of the mold. The mold according to claim 1 characterized in that the plane parallel sides of the mold in order to form the longitudinal ribs with plane parallel lateral sides consist of U-section steel profiles with parallel flanges with a smooth and corrosion protected steel plate covering their webs, and in that the steel plate of the same kind is folded to shape the inside section of the mould with variable rib width. The mold according to claim 1 or claim 2 characterized in that cylindric elements non-adherent to concrete are provided inside the mold in order to create holes (6) communicating the spaces between slab ribs in the floors. A method of producing reinforced or prestressed concrete panels having an inverted channel concrete section with a strictly rectangular external form, using the mold as defined in one of the claims 1 to 3, the method comprising the steps of: filling the mold with concrete, taking the panel out of the mold once the panel is stiffly hardened, by straight and vertical pulling, thus leaving the mold a "mold in one piece".

Inventors: SOLANA GOMEZ VICENTE, SOLANA GOMEZ, VICENTE
Applicants: SOLANA GOMEZ VICENTE, SOLANA GOMEZ, VICENTE
Application year: 1996
Main classification: B28B7
IPC codes: B28B7, E04B5