Note: Descriptions are shown in the official language in which they were submitted.
CA 02230794 1998-02-27
PCTAFI96/00456
WO 97/07949
Method and apparatus ~or producing concrete elements
The present invention relates to a method according to
the preamble of claim l 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 ismade 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.
. 35
In addition to continuous casting, slab structures may
also be made by casting into a stationary form with fixed
,
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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.
0 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
2~ 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
3~ method and an apparatus capable of overcoming the draw-
backs of prior-art technology and suitable for made-to-
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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.
~o
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 lO - 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 l.
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.
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WO 97/~7949 PCTrFl~G~'~0~'~
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
0 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- c
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
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WO 97/07949 PCT~F196/001~6
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 l is a side view of an apparatus according to the
invention;
0 Figure 2 is a sectional view along the plane A - A of the
apparatus shown in Fig. l;
Figure 3 is an enlarged cross-sectional view of the
apparatus shown in Fig. l;
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 l formed by an endless belt
adapted to run over two guide rolls. Over the casting bed
l, 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 l with side walls l3 placed to its both
sides, while the top side of the casting space is formed
by a belt l0 and a trowelling/vibrating plate 8. The belt
l0 runs endlessly in a triangular path over three guide
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rolls 9, and over the length of the top of the casting
space, the belt l0 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 l0 forms oneside 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 l 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 ll 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 l 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
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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 mandrels15 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
0 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.
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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 11 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
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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
0 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
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WO 97/07949 PCTAF196/00456
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,
0 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
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11
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.
