Slipform casting is a commonly used concrete element casting method. Slipform casting can aiso be carried out by means of core pipes. In a first stage, before the core pipes are mounted, fresh concrete that will form the lower part of the slab to be cast is fed onto the bed. After mounting the core pipes, fresh concrete that will form the upper part of the slab is fed on top of and between the pipes. Usually, slipform casting is used for producing a long, continuous cast that, once hardened, is sawn into sections of the desired length. From a handling point of view, however, it is more preferable in some cases to produce castings having their final length.

A solution is known from patent publication FI 102 253 (WO 97/07949) in which a wall delimiting a casting space is placed at the front and back edges of a metal piate forming part of a composite construction. No form is used in the casting.

After the casting, the front wall is lifted away. The back wall moves back and forth, thus pressing fresh concrete under a vibrating plate as casting proceeds. This known solution has the problem of not being useful for casting pre-stressed elements, since it is not possible to anchor wires to the movable end walls. In the cited solution, reinforcing rods running in the longitudinal direction along the top of the metallic base plate are used instead of pre-stressing wires. A steel base plate with reinforcement rods, on the other hand, increases the price of the cast product.

The characteristics of the method according to the present invention are set forth in Claim 1. The characteristics of the device according to the invention are set forth in Claim 2.

The method according to the invention makes it possible to provide the correct final shape both at the front and the back end of the cast. Thus, it is possible to cast elements or beams of the desired length that do not have to be sawn into sections of the right length after the casting. Consequently, no waste pieces are produced. In the production of short slabs, the portion of slab end waste pieces would be proportionally greater than in the production of long, continuous slabs that are afterwards sawn into sections of the desired length. It is therefore particularly important to avoid producing waste pieces in the case of short slabs.

In the device according to the invention, the ends of the concrete pre-stressing wires can be anchored to a headplate while the other ends thereof can be anchored to an endptate. Furthermore, because short elements do not always have to be pre-stressed, the invention can be implemented using non-tensioned wires or reinforcement bars.

In a preferable embodiment of the invention, a movable anchor plate is employed that forms either the headplate or the endplate. It can be mounted in a desired position on the casting bed according to the length of the element to be cast.

In the following, the invention and details thereof will be described in more detail with reference to the accompanying drawings in which Figure 1 is a schematic side view of the apparatus according to the invention at the beginning of casting, Figure 1 a is an axonometric view of a detail of Figure 1, Figures 2 to 4 show the apparatus of Figure 1 at different stages of the casting, and Figure 5 shows an alternative way of fastening an end wall of the form (the headplate or the endplate) to the sides of the form, and Figure 6 shows a way of fixing the movable end wall of Figure 5.

In the device according to the invention (Figure 1), the frame 1 is movable in relation to the bed 2, in the direction of arrow A, either the frame moving relative to the stationary bed, or the bed moving relative to the stationary frame. The power drive of the device is not shown in the drawings. Two feed tanks 3 and 4 are supported on the frame, and below these belt conveyors 5 and 6 that feed fresh concrete into feeding hoppers 7 and 8.

In order to form hollow cores, the apparatus comprises parallel tubular vibrating shoes 9, having a bottom sloping to position its front end higher than its back end, and parallel core pipes 10 behind them. The vibrating shoes and core pipes are supported on the frame at their upper parts by means of flat irons 11. The vibrating shoes are vibrated by a vibrator 12. At the rear end of the apparatus, a top plate 14 vibrated by a vibrator 15 is provided. Moreover, a motor 13 sets the core pipes 10 into a continuous longitudinal back and forth motion, each pipe moving in the opposite direction in relation to the adjacent pipe.

At the front end of the casting bed, a headplate 16 is provided, forming a vertical end watt at the front end. The headplate comprises parallel openings 17 to receive the moving core pipes (Figure 1 a). The upper edge of the headplate comprises cuts 18 to receive the core pipe supports. Further, the upper and the lower edge of the plate comprise slots 19 for pre-stressing wires. At the back end of the bed, an endplate 20 of the same shape is provided (Figure 3).

The invention is applicable not only to the casting of hollow core slabs but also to the casting of other shapes of concrete elements, such as T beams. In this case, forming means according to the profile of the beams are used instead of vibrating shoes 9 and core pipes 10, and in contrast to the casting of hollow core slabs, the upper surface of the cast is not covered.

When casting hollow core slabs, the device according to the invention operates as follows: At the first stage (Figure 1), the vibrating shoes and core pipes are situated behind the headplate 16. In order to form the layer of stab below the cores pipes, fresh concrete, which will form the bottom part of the slab, is fed onto the bed from a first feed hopper 7, initially located in front of headplate 16.

The casting device moves in the casting direction in relation to the bed and the vibrating shoes 9 and the heads of the core pipes 10, as well the supports 11 thereof, are received in the headplate 16. The vibrating shoes vibrate the concrete beneath them, thus making it compact. After passing over the headplate 16, to the front of the headplate, the rear feed hopper begins to feed fresh concrete between and onto the core pipes (Figure 2).

The casting continues and the device moves towards the endplate 20 at the end of the bed while the top plate 14 compacts the upper surface of the slab by means of the vibrator 15. The core pipes moving back and forth in the longitudinal direction simultaneously carry out shear compaction, and the concrete forming the intermediate structure 23 between cores becomes more compact. Once the front feed hopper 7 has reached the endptate, the feeding of concrete therefrom is stopped (Figure 3).

The apparatus passes the endplate 20, and the vibrating shoes 9 and the core pipes 10, as well as their supports 11, pass through it (Figure 4). Once the rear feeding hopper 8 has reached the endplate, the feeding of concrete therefrom is stopped. Thus, the hollow core slab cast is completed.

The headplate 16 or the endplate 20 can be removably mounted at the desired position on the bed, so that the position of the plate can be adjusted according to the length of the siab to be cast. In Figure 5, a solution is shown in which the stop is fastened to the sides of the bed. The fixing is performed by means of a plate slide 28 having end openings 21. In the sides of the bed, a number of holes 22 are provided, and the slide is locked into the desired position by pushing a pin through the aligning holes 21 and 22. Pre-stressing wires 23 are tensioned by means of the slide. The holes 21 and 22 do not have to be made in horizontal surfaces but they can also be provided in the vertical surfaces of the slide and the sides of the bed.

Figures 6a and b show a manner of fixing the movable plate of Figure 5, in cross- section and from above, respectively. The hole 21 in the stop is oval and its larger diameter is transverse to the casting direction. A circular plate 24 is arrange in the hole 21, having a diameter corresponding to the smaller diameter of hole 21. A pin 25 fitting into holes 22 in the sides of the bed is eccentrically fastened to the plate 24. The top plate 26 is provided with an operating and locking lever 27. Thus, the stop can be moved by turning the plate 24 in the oval hole 21 by means of lever 27. When the holes 22 have the correct spacing with respect to the size of holes 21 and to the eccentricity of the pin 25, the position of the stop can be changed stepiessly.

There are further suitable ways of providing adjustable fixing of the plates, and the locking can be done not only to the sides, but also to the end wall of the bed.

If the wires are fixed by means of an adjustable head-or endplate, less steel is consume. If the wires are anchored to fixed bed ends, part of their length is wasted. Further, because it is possible to cut the wires only after the stretching stage, use can be made of the wire elongation.