This invention relates to a method of producing a prestressed continuous concrete product, according to which method

- mould units are connected one after another to form a sequence of moulds,

- said sequence is provided with longitudinal prestressing wires,

- concrete is supplied in said mould units and shaped into a concrete blank,

- the hardened concrete blank is continuously displaced away from the concrete supply to a hardening area simultaneously as said prestressing wires are retarded, and

- said mould units are discharged from the sequence of moulds after the concrete blank has hardened and are added to said sequence before the concrete supply.

It is known per se from mould casting to use so called mould circulation lines, which comprise several separate moulds connected one after another into a sequence which moves below a stationary concrete supply. This kind of casting method is disclosed in U.S. Patent Specifications 2,607,099 and 2,745,164. The separate moulds are thereby connected in a pulling connection with each other either by means of special coupling means provided between the moulds or the prestressing wires or a conveyor supporting said moulds.

In the methods according to USSR Inventor's Certificate 837,871 and U.S. Patent Specification 2,607,099, the prestressing steels are prestressed by passing the hardened concrete blanks through or over a pulling device simultaneously as said prestressing

steels are retarded at the opposite end. A disadvantage of this kind of method, wherein a sequence of moulds is pulled forwards either from the finished product or from the mould by means of a stationary pulling device positioned at the discharge end, is that the fixed distance between the concrete supply point and the pulling point prevents altering of the length of the production line, when the product to be manufactured is changed. The use of a stationary pulling device at the discharge end of the mould sequence also prevents any increase in the capacity of the production line, because it is not possible to position more than a predetermined number of mould units between the concrete supply point and the pulling point. In the method according to U.S. Patent Specifi¬ cation 2,745,164, the sequence of moulds is pushed forward by means of a pushing force exerted separately oneach mould unit by means of alower displacing belt. Numerous gripping means provided on said displacing belt grip each mould unit and push it independently towards a discharge point. Because each mould unit must be guided by said belt up to the discharge point, and a stationary conveyor, accordingly, must be provided between the concrete supply point and the discharge point, this method, either, does not allow choosing of the length of the production line and increasing of the capacity. The mould in the mould sequence are separate from each other and do not form any continuous casting surface. U.S. Patent Specifications 4,240,776 and

4,186,169 disclose a method of mould casting, wherein prestressing of the wires is effected by pushing the mould sequence forward step by step by using locking means for the wires and by receiving the tension of the wires by means of a cross-head during the pushing

movement. The locking points of the wires must be changed for each mould unit in the sequence of moulds in order to couple a new mould unit to the sequence. Such continuous alternately occuring fastening and opening of the locking points is, however, very laborious and time consuming. Further, it is difficult to ensure an even prestressing of the wires, because the locking points thereof are constantly altered during the pushing movement. This kind of casting method is not suitable for producing a continuous concrete product, either, because said pushing method requires separate moulds, which are limited with respect to the casting surface, and because the moulds allowing cast¬ ing of such concrete products only which have a length not more than one mould unit length.

Said publications do disclose moulds which are positioned end to end in a sequence, but there are transversal bars positioned between the carriages under and above the prestressing wires. These bars prevent movement of the concrete mass and the moulds with respect to the prestressing wires in spite of the prestressing of the wires. Such transversal bars do not allow any continuous casting surface to be formedin the sequence of moulds. EP Patent Application 0078,767 discloses a casting apparatus for the production of single concrete products by means of a kind of assembly line principle. A mould sequence formed by adjacent mould units is positioned on supporting rails, said sequence being pushed step by step towards a discharge end. New mould units are added to the supply end of the sequence when mould units are released from the discharge end. The mould units are formed by longitudinal boards, trans¬ versal boards and bottom boards. In order to provide the moulds with sufficient firmness and dimensional

stability, said transversal and longitudinal boards are provided with matching coupling means. The longitudinal boards of the moulds are thus indispensable with respect to the firmness of the mould sequence. Therefore the - _ apparatus is not suitable for the production of an endless concrete product, because the casting surface is not uninterruptedly open in the direction of movement of the mould sequence.

The apparatus according to said EP publication is not designed for the use of prestressing wires, but the required reinforcement is provided separately for each mould. Consequently the mould sequence is not exposed to any kind of pulling force caused by the prestressing wires. The mould units must be fastened to.each other only to such an extent that the moulds maintain their shape and are prevented from sliding off their place in the mould sequence, when the sequence moves forward during the mounting of the reinforcement, during the supplying of concrete, etc. According to the EP publication, the mould sequence is pushed step by step forward, because the pushing device must be drawn aside, whenever a new mould is positioned between the mould sequence and the pushing device. Slide casting of an endless concrete product, however, requires that the mould sequence is continuous¬ ly moved forward below the concrete supply.

The object of the present invention is to provide a casting method, which avoids the above disadvantages. This object is achieved by means of a method according to the invention, which is characterized in that

- said mould units are connected into a rigid, buckling resistant sequence of moulds withstanding the traction of the prestressing wires and forming a casting bed, which is continuously open in the longi- tudinal direction of the sequence of moulds up to the

hardening area of the concrete blank, and

- that said rigid sequence of moulds is pushed continuously from that end where the mould units are added to said sequence. The invention is based on the idea that a pushing force caused by a displacing device is exerted only on the mould unit positioned at the supply end of the mould sequence in such a manner that said displacing device is thereby not at all .engaged with the prestressing wires. Consequently, no locking means are required between the prestressing wires and the mould units over a distance between a retarded reel of the prestressing wires and the hardened end of the concrete blank. Therefore, the discharge end does not have any fixed discharge point in the production line, but the length of the production line can be varied according to the hardening time of the concrete and the required produc¬ tion capacity simply by coupling more mould units to the supply end of the mould sequence. The casting bed can be made continuous over the whole length of the mould sequence and the casting bed is thus suitable for continuously casting endless concrete products. Because the prestressing wires are not exposed to any other forces except the continuous uniform pushing force caused by the pushing device, it is possible to maintain the stress of the prestressing wires accurately constant. Besides, it is easy to interconnect and accurately synchronize the pushing movement of the displacing device and the retarding movement of the wire reel, bacause the displacing device and the wire reel are positioned very close to each other.

The inventon is also concerned with an apparatus developed for carrying out said method, which apparatus is characterized by the features stated in claim 4. By means of such an apparatus the above-mentioned

advantages of the method are achieved with a simple construction.

The invention will be described more closely in the following with reference to the attached drawings, wherein

Figures 1 and 2 are schematical side and a schematical top view respectively of an apparatus for carrying out the method according to the invention,

Figure 3 is a perspective view of a casting bed provided by the apparatus, and

Figure 4 is an enlarged side view of a locking means between two mould carriages.

Figures 1 and 2 of the drawings illustrate a horizontal casting bed 1 , which is formed by mould carriages 2 connected one after another into a sequence, whereby said carriages are displaced on wheels along rails 3. A stationary supplying and compacting device 4 for concrete is mounted above said casting bed for supplying of concrete on the casting bed moving there- under so as to form a concrete blank 5 which continuous¬ ly fills the mould carriages. The supply device is followed by a hardening zone 6 for the concrete blank and a discharge point 7 for disconnecting and removing of the mould carriages from the hardened concrete blank. A cutting device 8 is positioned on the path of movement of the concrete blank for cutting off pieces of a desired length from the concrete blank.

Longitudinal prestressing wires 10 extend from retarded wire reels 11 to the mould bed so that they become surrounded by concrete at the concrete supply.

A mould carriage 2' disconnected and removed from the sequence of moulds is passed along a returning rails 12 back to the supply end of the sequence so as to be connected to the end of the casting bed before the supply device to form a fixed extension of

said casting bed.

In order to advance the casting bed formed by the mould carriages, a pushing device 13 is mounted under the casting bed before the supply device. The pushing device comprises an endless chain 15 which extends around traction wheels 14 and is provided with uniformly spaced gripping means 16. The bottom of each mould carriage is provided with corresponding gripping means 17, which are engaged by the gripping means 16 when the chain passes around the traction wheels. A pushing force caused by the pushing device thus acts on the above mould carriage, whereby said carriage pushes all the mould carriages ahead of it towards the discharge point 7. Because the prestressing wires are immovably engaged with the hardened concrete blank, the wires are tightened to a desired prestress by means of the pushing device, when the mould carriages push the concrete blank forward.

As appears from Figures 3 and 4, the casting bed 1 is formed by several mould carriages 2, each of which is provided with a planar casting surface 18, said surfaces forming a continuous, gaples casting plane, when the moulds are interconnected. The ends of the mould carriages are provided with locking means 19, by means of which the carriages can be connected with each other into a rigid, buckling resistant sequence. Said locking means comprise a locking arm 20 fastened on one carriage and a counter plate 21 provided on another carriage, whereby said arm extends through a hole 22 provided in the plate, when the carriages are interconnected. The protruding end of the locking arm is provided with a hole 23, in which a tightening wedge 24 can be inserted.

A retaining device 25 is fastened under the mould carriage, which retaining device comprises a roll 26

which is fixed on a support and engaged under the rail of the mould carriages, thus preventing the carriage from raising off the rails and the sequence of moulds from buckling upwards on account of the pushing action. In general, the concrete blank and the mould carriages as such are sufficiently heavy to prevent such raising up.

It is noted that because the displacing of the sequence of moulds in a direction away from the supply device for concrete is effected by means of a pushing action before the concrete supply, the discharge of the mould carriages is not confined to any stationary point, but the length of the production line can be easily altered. In addition, the casting bed is, if required, wholly continuous from one end to another so that the apparatus can be used for continuously casting concrete blanks. The sequence of moulds can be pushed forward continuously, which ensures an even stress in the prestressing wires. The drawings and the description related thereto are only intended to illustrate the idea of the invention. In their details, the method according to the invention and the apparatus developed for carrying out said method can vary within the scope of the claims, The method is also suitable to be used in mould casting, whereby the casting surface of the mould carriages is shaped in a manner required in each individual case and is divided into separate casting spaces when necessary. The mould carriages can also be e.g. through-shaped so that they form side edges for a concrete blank or for a single piece. Also other kinds of power transmission means can be used as a pushing device, e.g. a cogwheel-cograil transmission. The pushing device can also act on the sides of the mould carriage.