The present invention relates to a method according to the preamble of claim 1 for producing a precast steel- reinforced hollow-core slab or similar slab-like concrete element.

The invention also concerns an apparatus suitable for implementing said method.

Different types of continuous-casting methods are well known in the production of slab-like precast concrete elements. In these methods, the slab is cast onto a stationary casting bed by means of a movable continuous- casting machine, or alternatively, the casting bed is made movable under a stationary casting machine. Conven¬ tionally, the concrete mix is fed into the form from a hopper placed above the machine and the mix is then com¬ pacted by means of various types of vibrators. Compaction is typically carried out using a trowelling or vibrating plate which shapes the top surface or sides of the ele¬ ment. In the production of hollow-core slabs, the hollow core cavities are formed to the inside of the concrete mix by means of mandrels placed inside the concrete mix during the casting step, and these mandrels are often designed actuated by vibrating or longitudinally reci¬ procating movements that can compact the concrete mix being cast thus supplementing the compacting effect of the externally applied trowelling. Required reinforcing steels are fed into concrete mix from coils during the casting process, or alternatively, are arranged in a pre- stressed state onto the casting bed. Conventionally, very long slabs can be cast in a single run and after curing cut to desired lengths by means of a diamond-blade saw.

In addition to continuous casting, slab structures may also be made by casting into a stationary form with fixed

or adjustable dimensions. Casting into a stationary form is not usually used in the production of hollow-core slabs, because in such a method the core cavities must be formed by inserting core pieces of lightweight filler material into the concrete mix. A disadvantage of these continuous-casting and stationary-form casting methods is the voluminous need of workspace, forms and casting beds, because the cast element must be allowed to cure on the bed a certain time after casting before it can be moved. Further, cutting the slab to length is an expensive and slow operation. On the other hand, the core cavities at the ends of hollow-core slabs, which are made by continuous-casting methods, remain open and their plug¬ ging is costly, too. If the slab end is subjected to a high compressive force, the slab load-bearing capability will be limited by the maximum load permitted by the open ends of the core cavities. Although the casting length of the slabs is designed to be the closest multiple of ready-made slabs, a reject piece from the end of the cast slab will always be left over that cannot be utilized in any practical way. Since a once cured piece of concrete cannot be recycled, such reject stumps must be dumped thus causing entirely unnecessary extra costs and loss of raw materials. Further, sorting of the finished elements into transport bundles is time-consuming and clumsy.

Casting into stationary forms is furthermore handicapped by the work and time lost in stripping/reassembling the forms and the inferior compaction by vibration that cannot penetrate the concrete mix as efficiently as in a continuous-casting method resulting in products of lower density and strength.

It is an object of the present invention to achieve a method and an apparatus capable of overcoming the draw¬ backs of prior-art technology and suitable for made-to-

measure production of composite slab structures compris¬ ing sheet metal and slab of concrete.

The goal of the invention is achieved by casting the slab on a cut-to-measure sheet metal plate forming the bottom surface of the element and bordering the front and rear ends of the element by form walls which during the cast¬ ing process travel through the continuous-casting machine.

According to a preferred embodiment of the invention, the concrete mix is cast in two steps so that first a bottom layer of the mix, intended to form the lower part of the slab remaining below the core cavities, is cast onto the sheet metal bottom plate, and subsequently, the remainder of the concrete mix is cast during a main casting step.

Further according to the invention, the ends of the hollow-core slabs are sealed during the main casting step so that the mandrels are withdrawn from the core cavi¬ ties, whereby the slab remains uncast by about 10 - 30 cm at the end of the core cavity. This space is filled with concrete mix, which is compacted by bottom vibration to achieve a solid end of the slab.

More specifically, the method according to the invention is principally characterized by what is stated in the characterizing part of claim 1.

Furthermore, the apparatus according to the invention is characterized by what is stated in the characterizing part of claim 8.

The invention provides significant benefits.

The method according to the invention makes it possible to produce hollow-core and solid slab elements to length.

The cast slab can be lifted off from the casting bed immediately after the casting steps are completed, and the products can be stacked over one another for storage during the curing step. Hence, the curing step of the slabs can be performed in a relatively small space, whose temperature may advantageously be elevated to speed the curing of the concrete. Since the elements can be removed immediately after casting off from the casting bed, the same bed may without delay be reused for casting a new element. Thus, the footprint need on the casting yard re¬ mains smaller than in conventional methods. As the ends of the hollow-core slabs can be sealed by casting, no expensive separate plugging step is required and the element is finished in a single run. Loss of raw materi- als is minimized, since the elements are cast directly to length, waste pieces are eliminated and expensive sawing to length is unnecessary.

The element lengths can be selected steplessly, and re- quired curvedness of the top and bottom surfaces of the slab, respectively, are accomplished by the automatic control means of the apparatus. Production can be carried out using a semi-dry concrete mix, whereby curing is faster and less water is bled from the mix during vibrating.

The sheet metal bottom plate offers a plurality of bene¬ fits, of which particularly should be noted the bottom surface quality, which is neater than that of a pre- stressed slab, and the well-controlled precast curved¬ ness, which is attained by the longitudinal anchorage of the sheet metal bottom plate acting as tensional-stress- carrying reinforcement thus replacing the pretensioning cables used in conventional prestressed precast elements. The latter type of prestressed slab is often hampered by a "propeller" type of dimensional distortion, which com¬ plicates the installation. The composite construction

according to the invention is entirely free from this drawback.

The invention is next examined with the help of the annexed drawings, in which

Figure 1 is a side view of an apparatus according to the invention;

Figure 2 is a sectional view along the plane A - A of the apparatus shown in Fig. 1;

Figure 3 is an enlarged cross-sectional view of the apparatus shown in Fig. 1;

Figure 4 is a cross-sectional view of a hollow-core composite structure fabricated by means of the method according to the invention; and

Figure 5 is a partially sectional perspective view of the construction shown in Fig. 4.

In the apparatus according to the invention, casting takes place on a casting bed 1 formed by an endless belt adapted to run over two guide rolls. Over the casting bed 1, in the casting process direction, are first located the casting means for the bottom layer concrete mix, the means comprising a feed belt 2 followed by a feed hopper 4 for laying the concrete mix of the bottom layer and sliding mandrels 15 for shaping the bottom layer. Next to the casting means of the bottom layer concrete mix is a main casting machine in which actual casting of the element takes place. The casting space is enclosed by the casting bed 1 with side walls 13 placed to its both sides, while the top side of the casting space is formed by a belt 10 and a trowelling/vibrating plate 8. The belt 10 runs endlessly in a triangular path over three guide

rolls 9, and over the length of the top of the casting space, the belt 10 is spanned between two guide rolls 9 so as to run essentially parallel with the top surface of the casting space. Between these two rolls is adapted the trowelling/vibrating plate 8 so as to press the belt 10 from above. The belt 10 and the trowelling/vibrating plate 8 are inclined slightly downward in the direction of the casting process thus making the casting space somewhat tapering in this direction, whereby the compact- ing effect is augmented.

One leg of the triangular path of the belt 10 forms one side of the feed hopper 5. The feed hopper 5 discharges behind the trowelling/vibrating plate 8 and the concrete mix is loaded into the hopper by means of a conveyor 3. Under the casting bed 1 are mounted vibrators 7 so that the bottom plate can slide over them during the casting process. The first one of the vibrators 7 is located approximately underneath the discharge opening of the feed hopper 5, while the second one is placed immediately next thereto in the casting process direction. These vibrators form a vibrating region on the casting path over which the concrete mix is compacted. To border the front end of the element being cast, subsequent to the main casting machine is placed a form wall 11 of the element front end, said wall being movable with the help of actuator means 22. The other end of the element is bordered by means of a movable rear wall 12 through which core-forming mandrels 14 are adapted to pass.

The casting process is started by transferring the bottom plate 6 of the element onto the casting bed 1 and aligned correctly with respect to the feed hopper 4 of the bottom layer concrete mix. The bottom plate 6 is fabricated from a sheet of steel or stainless steel into a plate having ridges 15 folded thereto that act as anchorage to ensure the adherence of the plate to the cast concrete. After

the bottom plate 6 is properly located under the feed hopper 4, the casting of the bottom layer concrete mix 21 is commenced. The concrete mix 21 for the bottom layer is metered by means of a feeder 2 into the hopper 4, where- from it flows into the space between the sliding mandrels 15 and further to the space below them. The flow of the concrete mix is augmented by a vibrating table adapted close to the sliding mandrels. The concrete mix flow rate is adjusted to match the casting speed, and after the first casting step, the bottom plate 6 which is now covered by the bottom layer concrete mix 21 is trans¬ ferred on the casting bed 1 to the main casting machine.

In the main casting machine, the end of the bottom plate 6 is stopped underneath the end of the concrete feeder 3 and the discharge opening of the hopper 5. The mold wall 11 of the form front end is moved at the end of the bottom plate 6, whereby the metering and casting of the top layer concrete mix 18 can be started. The mix 18 is metered by a feeder 3 which may be a belt conveyor or any other suitable feeder means. The mix is advantageously cast in semi-dry consistency, whereby its handling is more readily accomplished by means of a belt conveyor than with, e.g., auger conveyors or similar means. During the progress of the main casting step, the front wall 11 is moved backward away from the point of mix feed and the mix is rammed by means of a reciprocatingly moving rear wall 12 under the belt 10 of the trowelling/vibrating plate 8. In the case that sealing by casting of the core cavities of the element is desirable, the front wall 11 and the bottom plate 6 are moved so that a void space remains between the ends of the mandrels 14 and the front wall 11. This void space is first rammed full of concrete mix, and the actual main casting step and the forward movement of the bottom plate 6 are started only subsequently.

The pressure applied to the concrete mix being cast can be controlled by adjusting the ramming force imparted by the rear wall 12, and the vibrating energy imposed on the cast mix can be varied by adjusting the input power to the vibrators 7 and 8. Additionally, the core-forming mandrels 14 can be provided with vibrators or arranged reciprocatingly movable. The upper surface of the cast concrete mix is shaped by means of a trowelling/vibrating plate 8 adapted to press a moving belt 10 against the surface of the cast mix. The belt 10 is arranged to move at the same speed with the moving cast element thus rendering a level and smooth upper surface thereto. Also the wear of the trowelling/vibrating plate 8 is thus reduced, because the plate is isolated from direct contact with the concrete mix which is a most abrasive medium. Additionally, the belt 10 augments the feed of the concrete mix through the hopper 5 and so prevents bridging in the hopper and reduces the dynamic flow friction of the concrete mix in the casting space.

During casting, the form wall ll of the front end follows the progress of casting through the main casting machine after which the wall can be lifted up. The core-forming mandrels 14 follow the cast element until they are coincident with the concrete mix feeder means. The mandrels 14 must extend so far that the vibrating energy applied by the vibrators 7, 8 is not transmitted in excessive amounts to an area where the core cavities formed are not anymore supported by said mandrels.

Slabs intended for use in floor and ceiling structures or similar applications are provided with flexural precom- pensation which means that the slab is cast slightly upward concave, whereby it can assume a maximally level state after installation on-site. Such a precompensation can be accomplished by moving the feed hopper 5 and the vibrator means 8, 9, 10 in a controlled manner vertically

during the casting step. Then, the discharge opening of the hopper 5 initially shapes the top surface of the element and the vibrator 8 smooths the surface accurately to design height. Also the movement of the core-forming mandrels 14 is here adapted to follow the precompensated shape of the element top surface.

As shown in Fig. 3, the core-forming mandrels 14 are comprised of two vertically invertedly superimposed troughs, which are adapted to slide vertically tele- scopingly into one another. This arrangement permits easy adjustment of the height (and thus, the cross section) of the core cavities 19 to optimally suit each application.

During the casting of the rear end of the element, the core-forming mandrels 14 are withdrawn through the rear wall of the form off from the core cavities, whereby openings are formed in the cast concrete mix at the with¬ drawn mandrels. Now, the end of the element is stopped underneath the discharge opening of the main casting machine feed hopper 5 and the core cavity openings are sealed with concrete mix. Additionally, the mix can be further compacted by ramming the mix into the ends of the core cavities by the ends of the mandrels 14. After the element end is filled with concrete mix 20, the end is pushed through the casting machine, whereby the trowelling/vibrating plate 8 and the vibrating table 7 perform the compaction of the concrete mix. The core- forming mandrels 14 and the rear wall 12 follow the cast element through the main casting machine. After passing through the casting machine, the element is ready for transfer to the curing step, and the casting of a new element can be started immediately.

Referring to Figs. 4 and 5, the finished composite struc¬ ture shown therein comprises a sheet metal bottom plate 6 having longitudinal anchor ridges 15 made thereto for

anchorage to the overlying bottom layer concrete mix 21. Over the bottom layer concrete mix 21 is laid the remain¬ ing part of the concrete layer 18 in which are located the longitudinal reinforcing steels 16, transverse reinforcing steels 17 and the core cavities 19. In Fig. 5 is shown the sealed end part 20 of the core cavities 19.

In addition to those described above, the invention can be implemented in alternative embodiments. For instance, the trowelling belt can be replaced by a trowelling beam working the top surface of the concrete layer directly. Besides the bottom plate, the element may incorporate additional reinforcements such as steel fabrics or rebars placed into the body of the element. In addition to load- bearing hollow-core elements, the method may be applied to the fabrication of other kinds of products, e.g., load-bearing beams and solid-core slabs. The members of the apparatus may be varied by using alternative constructions, e.g., different types of concrete mix feeders. While in the above-described embodiment the casting machines are made stationary, an alternative arrangement can be contemplated having a stationary casting bed on which the casting machines are adapted to move. However, such an embodiment requires a larger footprint and makes the constructions more complicated.

Owing to its low cost, the bottom plate is conventionally made from sheet steel having suitable ridges made thereto that perform as anchorage for the cast concrete. Obvious¬ ly, the needs of special applications can be covered by elements having their bottom plate made from any other material of sufficient stiffness and strength under tensile stress.

In stead of vibrators, the concrete can be compacted by other suitable means like pressing rams and movements of parts of the machine. The ends of the product can be made

angular by adjusting the front and/or end wall on an angle in relation to the longitudinal axis of the product. The adjustment can be made easily for example by hydraulic cylinders arranged to move the plates durins casting. The lower and upper part of the concrete can be provided with a reinforcing grid. The grid can be arranged on a suitable height automatically by gripping and positioning means included in the machine.

It must be particularly noted that the method and apparatus according to the invention are also applicable to the manufacture of hollow-core and solid beams, pillars and wall elements.