A METHOD OF CASTING CONCRETE OBJECTS AND A FORM HEREFOR

Abstract: When casting in particular elongated, hollow, thin-walled concrete objects, such as concrete posts, concrete is pumped into a form (5) and held under pressure therein by closing a valve means (8) while the concrete hardens in the form. The formation of microcracks in the hardened concrete can be avoided and the hardening or curing process of the concrete can be accelerated by supplying hot air through respective perforated pipes (21, 13) immediately subsequent to filling the form with concrete.

Description: A method of casting concrete objects and a form herefor Technical Field This invention refers to a method of casting elongated, hollow, thin-walled concrete objects, such as concrete posts, in which fluid concrete is pumped into a form or mould. The invention also relates to a form or mould for the manufacture of such cast concrete objects. Background Art In the conventional manufacture of concrete objects, such as concrete posts or pillars for example, wet concrete is normally poured into a form and vibrated or centrifuged. When vibrating the concrete, excesses of air are liable to be drawn into the fluid concrete mass and to form air bubbles therein, which result in irregular surface structures when the concrete hardens. Such air bubbles also amass around the reinforcing elements cast in the concrete object, causing the mechanical strength properties of such elements to be impaired. In conventional concrete casting techniques, attempts are sometimes made to accelerate curing or hardening of newly cast concrete by applying heat thereto. This causes entrapped air bubbles to expand and subsequently the concrete to burst, therewith to form microcracks in the concrete mass. Consequently, it is necessary to wait until the concrete has hardened at least partially, normally after some few hours, before applying heat thereto. This slows down the production rate and lowers the overall efficiency of the expensive form equipment used. There is known to the art - cf. DE-A-a 683 945 (Rocla Concrete Pipes) - a method in the manufacture of concrete pipes which incorporate prestressed reinforcements, in which the inner wall of the concrete pipe is subjected to the effect of a pressurised medium prior to the concrete hardening completely so as to stretch the pipe and tension the reinforcing elements. The method is particularly characterized by using a form constructed from an elastic material, so as to enable the reinforcing elements to be given the tension desired. Since the reinforcing elements can be prestressed in other, simpler ways, however, this particular known method would not seem to be of any great practical significance. SE-B-308 270 (Jasba-Keramikfabriken) describes a method in which hydraulic pressure is employed in the casting of ceramic moulded bodies. The primary object in this case is to force water into the absorbent form more quickly. The method described in this publication, however, cannot be applied when manufacturing, for example, thin- walled concrete posts of great lengths. SE-B-331 646 (Construction Techniques) describes the method of casting concrete objects in a closed form while using a concrete porridge containing an excess of water. In accordance with this known method the form is filled with cement porridge and the cement then subjected to an overpressure applied to the form, so as to drive water out through a filter lining. This method cannot be applied when casting objects of the kind for which the present invention is intended. This also applies to the method described in SE-B-355 745 (Deutsche Bauakademie) in accordance with which concrete objects are produced by compressing fresh concrete in forms with the aid of vacuum techniques. DE-A-2 204 685 (Gismondi) describes a more or less conventional concrete casting process in which concrete is poured into a form and compressed by expanding flexible bodies within the form. DE-C-295 239 (van der Daehle & Co) describes a process in which concrete by gravity is fed from a container into a sloping form. A rotatable means accomplish stirring and an excentric wheel contributes to the feeding and forms a closable valve or damper when the form has been filled. Pressurized air acts on the concrete surface in the container. DE-A1-3 302 469 (VEB Betonleichtbaukombinat) describes an open rather conventional form into which concrete is poured from above. Steam is supplied via a pipe within the form core to accomplish a heat treatment of the concrete. FR-A-619 754 (Froti) describes a process in which concrete in a vertical or sloping container with the aid of pressurized air is supplied into a form which may be conical. FR-A-2 058 742 (Reed) describes a rather complicated movable form-arrangement into which concrete is injected by means of a piston-cylinder unit. DE-B-1 058 908 (Max Giese Stahlbetonbau) describes another known method in which a paper-layer is wound around a metal core in a concrete form. Objects of the Invention One object of the present invention is to provide a method of the aforesaid kind in which the disadvantages of known and conventional methods are avoided, and in which cast concrete objects of higher quality can be produced more quickly with more efficient use of the form equipment. A further object is to provide a method for the manufacture of elongated, thin-walled hollow concrete constructions, for example posts and pillars, which exhibit advantageous mechanical strength properties and superior surface structures. Disclosure of the Invention In its widest aspect the method according to the present invention is mainly characterized by utilizing the generated pump pressure to maintain the concrete under pressure while the concrete hardens in the form. By holding the concrete under pressure during the hardening or curing process, the formation of cracks, which always occur when applying conventional concrete casting techniques, is practically completely avoided. The concrete mass is suitably held under a pressure 2 of 1-2 Mpascal, i.e. from about 10-20 kp/cm , or at higher pressures in certain cases. Pronounced positive results, however, can be obtained by pumping at far lower overpressures, for example, pressures in the region 2 of 1-2 kp/cm or even less. A slight overpressure as small as a few hundred pascal may in some cases be sufficient to obtain the desired result, i.e. no cracks in the surface of the cast product. A concrete pump of conventional design used when applying the method according to the invention generates 2 in the concrete mass a pressure of from 10-20 kp/cm . Consequently, when applying the method according to the invention the pump can be kept running subsequent to having filled the form with concrete, during the time taken for the concrete to harden. In practice, however, subsequent to filling a form with concrete, the concrete is held under pressure by holding a valve means located in the vicinity of the form-inlet opening closed while the pump is still operating. The valve means is preferably of simple construction, and may advantageously comprise a flat plate or disc arranged to close and open the form inlet. Heat is applied to the concrete mass pumped into the form, in order to accelerate the curing process. Distinct from prior art techniques in this respect, the present invention constitutes an essential step forward in the art, since in accordance with the invention heat can be applied in direct, or substantially direct connection with pumping the concrete into the form. When the concrete is held under pressure in the form, there is no tendency towards the formation of microcracks in the concrete. Because heat can be applied at a much earlier stage in the curing or hardening process, the concrete grows in mechanical strength much more quick¬ ly than would otherwise be the case. This means in practice that in the event of a core being used in the casting process, the concrete has become sufficiently strong, or rigid, for the core to be removed within the space of only from 1 to 2 hours. " After a further 1-2 hours have lapsed, i.e. from 2-4 hours of commencing the casting operation, the outer form can be opened and the hardened concrete object removed from the form, therewith leaving the form free for a further casting operation. Heat is applied through the agency of a suitable fluid, such as air, and/or steam, ■ which is distributed through a first perforated tube, extending centrally through the form core, and through a second perforated tube, located beneath the form. The fluid is preferably heated to a temperature of at least 40°C, and suitably to a temperature of from 60-90°C. When concrete is pumped into the form and held under pressure therein, air enclosed in the form will be effectively expelled therefrom. This, in turn, affords the following advantages: the air content of the concrete will be at a minimum; resultant concrete surfaces exposed by the air will be devoid of air pores; and the concrete will seat firmly around the reinforcing elements, located in the cast concrete objects, i.e. no air cushions will form in the immediate close vicinity of the reinforcing elements. Another important advantage is that when applying the method according to the invention it is not necessary to vibrate the form or the concrete mass therein. The invention relates to an advantageous method of manufacturing elongated, thin-walled, concrete objects, such as concrete posts, for example. In accordance with the invention, the concrete is preferably pumped into the form through one end thereof in an axial direction, and air present in the form is permitted to leave in the vicinity of the other end of the form. When manufacturing posts, or similar concrete objects, of conical tapering cross-section, the concrete is preferably pumped into the narrow end of the form. When manufacturing - conical objects, where the form has arranged therein a core which tapers conically towards one end thereof, the narrow end of the core is preferably given a greater conicity than the remainder of the core. The core therewith obtains a pointed tip which facilitates the flow and distribution of the pumped concrete in the form. In addition hereto, there is formed in the cured concrete object a thickened and stiffening end part which improves the mechanical strength of the object. When manufacturing a hollow object, for example, a hollow concrete post, where the form accommodates a core supported by core support means, the core supports can be withdrawn to a location outside the form, subsequent to filling the form with concrete and leaving the pump operating, so that the core is supported in the form substantially solely by the concrete under pressure in its form. This results in a fully homogeneous concrete object exhibiting totally smooth and uniform surfaces. The resul¬ tant concrete object compares very closely to a cast steel object. The concrete used in accordance with the invention is suitably a high-quality class concrete, K 80 MPa-K 120 MPa. The concrete shall have a pumpable consistency and a setting or slump value of little more than 25 cm. The reinforcement may comprise pretensioned rod, a box reinforcement or shear reinforcement being optionally arranged in the close proximity of the ends of the object. The invention also relates to a form for use when applying the method according to the invention, the main characterizing features of the form being set forth in the following claims. An embodiment of the invention will now be described in more detail with reference to the accompanying schematic drawings. Brief description of the drawings Fig. 1 is a side view of a two-part form for manu¬ facturing hollow, conical concrete posts having lengths, for example, of 30 meters, and an associated concrete pump. Fig. 2 is a sectional view, in larger scale, of the forward and rearward end of the form illustrated in Figure 1 , in a state ready to receive concrete from the pump. Fig. 3 is a corresponding sectional view of the form illustrated in Figure 1 , subsequent to concrete having been pumped into the form and the concrete has begun to harden and removal of the core from the form has commenced. The concrete pump illustrated in Figure 1 may have a working pressure of 1-3 MPa. The pump co-acts with a concrete-mass collecting cone 2 and is connected, via a distribution line 3, and a connecting flange 4, with a form 5, described in more detail hereinafter. When seen horizontally the form 5 narrows conically towards the connecting flange 4, and may have a length, for example of 30 meters. The form is divided into two parts longitudinally and is constructed to enable it to absorb the axial forces to which the reinforcements are subjected when tensioning the same. The form is also able to absorb radially acting forces generated through the concrete pump 1 and as a result of expansion of the concrete as it hardens. The narrow end of the form 5 merges with a conduit 6, having at one end thereof a connecting flange 7 which lies against the flange 4 of the distribution line 3. The conduit 6 is also arranged to co-act with a closable and openable valve plate 8, and is provided with a holding-on device 9 for co-operation with the prestressing steel. The reinforcing lines 10 are tensioned by means of tensioning devices 11, having an active dolly-plate 12, at the other end thereof. Located beneath the form is a longitudinally extending perforated tube or pipe 13, through which steam is dispensed for heating the concrete. The form accommodates a core 20, the major part of which has a conicity corresponding to that of the form. The conicity of the narrower end 20a of the core is greater than that of the major part of the core, therewith to form a pointed tip 20b. In this way, the core will facilitate distribution of the pumped concrete in the form. Located centrally along the core is a perforated pipe or tube 21 , through which hot air is distributed to the surrounding concrete, through the outer surface of the core. The core is reinforced and held together by lines 22, extending therewithin, these lines being anchored in the vicinity where the core exhibits its greatest conicity, and at the thick end 23 of the core. The lines are tensioned to the extent desired by means of a tensioning device 24, located at the thick end of the core. As concrete is pumped into the form 5, the core 20 is held in position in the outer form 5, by means of radially movable core supports 25, which support the core . radially, and by means of bolts 26, having nuts 27, which secure the position of the core in the axial direction. When a post is to be cast in the form 5, both the outer form 5 and the core 20 are first treated with a suitable form oil. Reinforcements in the form of lines 10 are then positioned in the form 5 together with the core 20, which is held in position by core supports 25 and nuts 30, respectively. The outer form is then closed and the reinforcement elements tensioned with the aid of the with concrete. As will be understood the valve-plate 8 may be replaced with other means for ensuring that the concrete is held under pressure in the form 5. Alternatively, the valve means may be dispensed with altogether and the pump allowed to run until the concrete has hardened sufficiently. Another alternative is one of providing suitable means for this purpose. When the pump is shut off and the valve or other means brought into place the overpressure in the concrete mass is normally somewhat reduced for instance from 1.5 Mpascal to about 0.8 Mpascal or the like.

Claims: / V Claims 1. A method of casting elongated, hollow, thin-walled concrete objects such as concrete posts, in a form or mould intended herefor, comprising pumping the concrete into the form, characterized by utilizing the generated pump pressure to maintain the concrete under pressure while the concrete hardens in the form. 2. A method according to Claim 1, characterized in maintaining the concrete under pressure in the form by holding a valve means located in the vicinity of the form inlet opening closed subsequent to filling the form with concrete and with the pump still working. 3. A method according to Claim 1 or Claim 2, in which heat is applied to the concrete to accelerate the hardening or curing process thereof, characterized by supplying heat in direct or substantially direct connection with pumping the concrete into the form. 4. A method according to Claim 3, characterized in that the heat is supplied through the agency of a hot fluid distributed through a perforated pipe arranged centrally in the mould core, and through a perforated pipe located beneath the form. 5. A method according to any one of Claims 1-4, for manufacturing elongated, for example post-like objects, characterized by pumping the concrete axially into one end of the form; and by permitting air entrapped in the form to depart therefrom through holes located in the vicinity of the other end of the form. 6. A method according to Claim 5 for manufacturing concrete objects of conically tapering cross-section, characterized by pumping the concrete into the form at its narrow end. 7. A method according to Claim 5 or Claim 6, in which the form accommodates a core, which tapers conically towards one end thereof, characterized by giving the narrow end of the core a conicity greater than the remainder of said core, therewith to form a pointed tip, effective to facilitate the flow and distribution of the concrete pumped into the form, so that there is formed a relatively thick and rigidifying end part in the hardened concrete object. 8. A form for casting elongated hollow, thin-walled, concrete objects, such as concrete posts ' , said form having an inlet opening (6a) for connection to the outlet or supply line (3) of a concrete pump, characterized in that the inlet opening (6a) is provided with or adapted to co-act with means, for example valve means (8) , effective to hold the concrete pumped into the form under pressure during the hardening process of the concrete. 9. A form according to Claim 8, having arranged therein a core (20) supported by core-supports (25) for the manufacture of hollow objects, characterized in that the core supports (25) can be withdrawn to a position externally of the form (5) , wherewith subsequent to filling the form with concrete and with the pump still working, the core supports (25) are arranged to be with¬ drawn so that the core is supported substantially by the concrete under pressure in the form. 10. A form according to Claim 8 or 9, for manufacturing elongated tapering, concrete objects, such as posts, characterized in that the inlet (6a) is located at the narrow end of the form. 11. A form according to any one of Claims 8-10, and accommodating a conically tapering core (20) characterized in that the narrow end (20a) of the core has a conicity that is greater than the remainder of the core, to form a pointed tip, facilitating the flow and distribution of the concrete pumped into the form and contributing to provide a thickened and rigidifying end part of the cast concrete object.

Inventors: SCHOEN INGEMAR, SVENSSON SVEN OLOF, SCHOEN, INGEMAR, SVENSSON, SVEN, OLOF
Applicants: A BETONG AB, A-BETONG AB
Application year: 1985
Main classification: B28B21
IPC codes: B28B21