The invention relates to a method for casting prestressed hollow-core concrete products by a continuous extrusion technique, wherein concrete mix is extruded onto a casting bed with the help of auger feeders having a flight diameter smaller than the height of the hollow-core cavities of the product.
The invention also relates to an extruder-type casting apparatus for manufacturing prestressed hollow-core concrete products, the apparatus comprising a slip-form casting machine adapted movable along a casting bed and incorporating a casting mold formed by side troweling beams and a top troweling beam, the feeder augers of the apparatus being connected to drive machinery and each one of them having adapted as an extension thereof a core-forming mandrel that is aligned substantially concentrically with the center axis of the feeder auger and has a height larger than the largest diameter of the feeder auger.
In slip-form casting, the concrete mix is extruded with the help of the feeder augers into a mold or through nozzles, whereby the casting machine is propelled by the reaction forces imparted by the auger feeders. The ready-cast product is left resting on the casting bed. The hollow-core cavities to be made in the concrete product are formed by mandrels of desired external shape over which the concrete is extruded.
When a hollow-core slab with a height of the hollow-core cavity larger than the maximum diameter of the feeder auger flight is being produced, conventional extruder-type casting machines generally have the core-forming mandrel mounted at the trailing end of the feeder auger asymmetrically in regard to the center line of the feeder auger so that the lower edge of the mandrel is substantially at the same level with the lowest point of the feed auger flight, whereby the upper edge of the mandrel is substantially above the highest point of the feeder auger flight. A flaringly slanted surface is adapted to the leading end of the mandrel at the portion of the mandrel that receives the concrete mix flowing onto the mandrel. This kind of a construction is described in patent publication EP 80333.
A problem associated with this type of asymmetrically mounted mandrel is the heavy wear of the upward slanted portion of the mandrel necessitating a replacement of the mandrel even when other surfaces of the mandrel still could be in a working condition. Moreover, also the feeder augers are worn at a higher rate the higher the concrete mix must be extruded.
The present invention is characterized in that the core-shaping mandrel is mounted substantially concentric with the center line of the feeder auger so that the core-shaping mandrel having a diameter larger than that of the feeder auger flight is designed to flare by a substantially equal angle both upward and downward. However, the slanting angles of both the flaring upper surface and lower surface of the mandrel may be different from each other. The width of the mandrel-however remains essentially the same as the maximum diameter of the feeder auger flight.
More specifically, the method according to the invention is characterized by what is stated in the characterizing part of claim 1 and the apparatus according to the invention is characterized by what is stated in the characterizing part of claim 2.
The method and apparatus according to the invention offers the following significant benefits:
Next, the invention will be examined in greater detail by making reference to the attached drawing, wherein
Referring to
During the operation of the casting apparatus, the concrete mix flows from the feeder hopper 1 onto the feeder augers 2 that extrude the concrete mix into a mold formed by a casting bed 6, side troweling beams 4 and a top troweling beam 5. The core-shaping mandrel 3 is mounted at the trailing end of the feeder auger. The mandrel is adapted concentrically with the center axis of the feeder auger.
As shown in the diagram, the concrete mix being cast is flowed along the slanted surfaces of the leading end of the core-forming mandrel 3 to the upper and lower portions of the cross section of the hollow-core slab being cast. This casting technique makes it possible to fabricate hollow-core slabs in which the cavities are substantially higher than the diameter of the feeder auger.
The slanted portions of the core-shaping mandrel according to the invention may be made either asymmetrical relative to the center axis of the feeder auger and the core-shaping mandrel or, alternatively, asymmetrical relative to the center axis, whereby one of the slanted surfaces of the mandrel is longer than the other.
In
The core-forming mandrel embodiment according to the invention also permits casting hollow-core slabs of a height greater than that possible in conventional methods.
Number | Date | Country | Kind |
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20021092 | Jun 2002 | FI | national |
Filing Document | Filing Date | Country | Kind |
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PCT/FI03/00440 | 6/4/2003 | WO |