Note: Descriptions are shown in the official language in which they were submitted.
CA 02243905 1998-07-24
TITLE OF THE INVENTION
OIL CANNING RESISTANT ELEMENT FOR MODULAR
CONCRETE FORMWORK SYSTEMS
FIELD OF THE INVENTION
This invention relates to the structure of an element for a stay-in-place
modular concrete
formwork system.
BACKGROUND OF THE INVENTION
Stay-in-place modular concrete formwork systems are relatively well known.
Such
systems typically include a plurality of vertical modular elements which are
designed to
interconnect to create a wall of formwork into which concrete is poured and is
allowed to
cure and such systems are exemplified by Piccone, PCT/CA95/00281 (WO
96/35845),
Nessa, United States Patent No. 5,216,863 and Rodighiero, Canadian Patent No.
957,816.
It is also well known to fabricate the modular elements out of
polyvinylchloride (PVC).
PVC is well suited to modular formwork systems due to its light weight, which
contributes to reducing transportation costs, its relative ease of manufacture
into the
particular shapes required for a given system, and its resiliency and
flexibility to
accommodate deformation when the concrete is being poured and contraction of
the
concrete as it cures.
However, PVC tends to expand as it warms up. If lengths of PVC material are
fixed at
their ends and sides and are subjected to heat or warming, they will tend to
"oil can", i.e.
buckle or ripple along their length. This problem has been recognized in the
context of
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building siding panels made of PVC in which case the problem has been
addressed by
ensuring relatively loose engagement between interconnected panels as well as
providing
slidable engagement of the siding panels with the underlying surface.
In modular concrete formwork systems, once the concrete is poured, the weight
of the
concrete exerts a very strong expanding force on the joints of the formwork
resulting in
very tight engagement between the various elements. Once the concrete cures,
the joints
become essentially fixed and allow no movement between adjoining elements.
Moreover, the formwork elements are also usually constrained at the top by a
roof or
other such structure and at the bottom by a footing on which the wall and
formwork rests.
As a result each element of the formwork becomes effectively fixed both along
its sides
and at its ends. When such formwork made of relatively thin PVC is erected, a
subsequent rise in temperature can cause the formwork elements to undergo
unsightly oil
canning along their length. This problem tends to occur only when the concrete
is not in
direct contact with the element, such as when a layer of insulation is
interposed between
the element and the concrete in an exterior wall, and the inventors believe
that the reason
the problem does not occur when the concrete is in direct contact with the
element is due
to the concrete presenting a rigid physical barrier against rippling of the
element.
We are not aware of any prior art which offers a solution to the problem of
oil canning in
the context of relatively thin stay-in-place PVC modular formwork systems, and
it is
therefore an object of this invention to provide a modular formwork element
which resists
oil canning even when it is not in direct contact with concrete.
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SUMMARY OF THE INVENTION
The invention comprises an element for use in a stay-in-place modular formwork
system
for pouring concrete which is thin walled, flexible, and made of a material
which tends to
expand when subjected to heat. In one aspect of the invention, the element has
at least
one flat wall segment for use in defining a substantially flat outside wall of
the formwork,
and comprises at least one lengthwise rib extending at an angle, preferably
perpendicularly, from the flat wall segment.
In another aspect of the invention, the element has engagement means at its
longitudinal
edges and a substantially flat wall segment extending between said engagement
means,
comprising at least one lengthwise rib extending at an angle from the flat
wall segment.
In another aspect of the invention, the element comprises a segment defining
the visible
portion of the erected formwork bounded by two longitudinal edges and said
segment
having at least one curved wall portion, and at least one lengthwise rib
extends from the
curved wall portion substantially perpendicularly to the plane defined between
the two
longitudinal edges.
In another aspect of the invention, the element comprises a portion defining a
visible
surface of said formwork. The portion comprises a plurality of longitudinally
continuous
flat sections each of which is angularly disposed in relation to the next
adjacent section.
In another aspect of the invention, the element is made of PVC, is of a
thickness of
between about 1 and about 2 millimeters and the lengthwise ribs or element
edges are
spaced every 80 millimeters or less along the width of the flat wall segment
of the
element. In another aspect of the invention, the element further comprises at
least one
inwardly extending structure for abutting against a layer of insulation, and
preferably said
structure is such a rib.
The invention also comprises a method of use of the elements in an assembly of
formwork in which the concrete is not in direct contact with the wall segment
which
includes the rib, and a method of use in which a layer of insulation is
interposed between
the concrete and the flat wall segment.
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Other aspects of the invention are described in the detailed description of
the best mode
and preferred embodiment below and in the claims.
Brief Description of the Drawings
Other aspects of the invention will be evident from the following disclosure
and
description of the preferred embodiment by reference to the drawings in which:
Figure 1 is a plan view of a prior art assembly of formwork illustrating wall
elements, connecting members and cross connecting elements to produce a wall;
Figure 2 is an end view of an element according to the preferred embodiment of
the invention;
Figure 3 is an end view of an alternative embodiment of an element according
to
the invention;
Figure 4 is an end view of another embodiment of an element according to the
invention;
Figure 5 is an end view of a further embodiment of an element according to the
invention.
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DETAILED DESCRIPTION OF THE BEST MODE
AND PREFERRED EMBODIMENT
Our commonly owned Canadian Patent Application No. 2,218,600 filed 17 October
1997
entitled "Modular Formwork Elements and Assembly" discloses a modular formwork
system designed to overcome the problem of bulging of wall elements resulting
from the
outward pressure of the concrete. This outward bulging of individual wall
elements,
when compounded across several such elements forming a wall of formwork,
produces an
"accordion" effect tending to shorten the lateral width of the formwork after
it has been
erected and to thereby dislocate it from the surfaces to which the sides of
the formwork
were secured.
The system, illustrated generally in Figure l, relies on the use of connecting
members 10
which serve to simultaneously connect adjacent and facing wall elements 12,
14, 16, 18.
Two T-shaped connectors 20, 22 are provided along the longitudinal (vertical)
extent of
each wall element in spaced relationship to the longitudinal edges of the wall
elements.
Each of the two T-shaped connectors are located about 40 millimeters from each
longitudinal edge of a wall element. A relatively larger central space 24
extended
between the two T-shaped connectors 20, 22. Similar T-shaped connectors 26 are
provided on the connecting member 10 and a cross connecting element 28 extends
diagonally between a portion of the wall element 14 and a connecting member.
The
foregoing structure substantially avoids the problem of bulging and accordion
effect at
wall 11.
Where the elements of Figure 1 are used for an exterior wall, a layer of
insulation 36 is
interposed between the flat wall elements 16, 18 and the concrete. In such
case, no cross
connecting elements 28 are used and the wall elements typically do not include
T-
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connectors. We have observed that in some such cases, notably in warmer
climates, oil
canning occurs across the flat wall elements 16, 18.
We have conducted testing to determine whether oil canning would occur in an
insulated
wall if a wall element having T-connectors (such as elements 12, 14) was used
rather than
a flat wall element such as elements 16, 18. We have observed that oil canning
occurs in
the central space 24 between the two T-shaped connectors. On the other hand,
there is
substantially no oil canning effect in the spaces 30, 32 between each
connector and their
adjacent longitudinal edges.
The inventors have found that the remaining oil canning along the length of
the central
space 24 can also be avoided by including a longitudinal projection or rib 34
along the
center of the central space 24, as illustrated in Figure 2. While no other
element need
necessarily be connected to such projection 34, it has been found that the
presence of the
projection effectively solves the oil canning problem, particularly if the rib
includes a
surface (such as at 35) which is perpendicular to the wall element.
In the embodiment illustrated in Figure 2, the projection 34 is in the same T-
shaped
configuration as the connectors 20, 22. However, while the T-shape is used in
the
preferred embodiment, it is understood that any inward or outward longitudinal
projection
would achieve the same effect of preventing oil canning.
We have found that when wall elements having a thickness of about 1 to 2
millimeters are
used, a sufficient number of projections or ribs should be provided along the
width of the
element such that there is either an edge of the element or a rib spaced at 80
millimeters
or less.
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The longitudinal projection preferably extends perpendicularly to the plane of
the major
surface of the wall element, but the lack of perpendicularity may be
compensated by the
thickness of the extension or by providing a plurality of such non-
perpendicular
extensions, as illustrated in the embodiment of Fig. 3. Fig. 3 illustrates a
wall element 38
for use in conjunction with substantially the same connecting members 10 as in
the
embodiment of Fig. 1 to create a wall of formwork. However, according to this
embodiment, rather than providing a continuous flat portion between the edges
of the
element as in the wall element of Fig. 2, a series of sharp changes in
direction of the wall
surface are provided resulting in diagonal wall sections 40, 42, 44 and 46,
each being
angularly disposed in relation to the next adjacent section.
While none of these wall sections are perpendicular, the cumulative effect of
the diagonal
sections and the sharp corners defining them is to provide significant
resistance to oil
canning.
In a variation on the embodiment of Fig. 3, such variation being illustrated
in Fig. 4,
additional perpendicular projections 48, 50 and 52 are provided centrally of
the flat wall
segments 54, 56 and 58 to thereby provide even further reinforcement. It will
be
appreciated that this arrangement allows the insulation to abut against the
wall element
without substantial interference from the projections.
A further alternative shape of a wall element embodying the invention is
illustrated in
Fig. 5. Wall element 60 comprises two flat portions 62, 64 adjacent the edges
of the
element, and a central curved portion 66. The curved portion provides depth to
the
element so as to accommodate three perpendicular projections 68, 70 and 72
according to
the invention.
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The embodiments of Figs. 3, 4 and 5 also provide useful noise attenuation
properties due
to the angled or curved portions of the walls as well as a pleasing external
appearance to
the completed formwork and the resulting wall.
While the embodiments described herein have all been for separate elements
which
define only the wall portion of the formwork, it will be appreciated that the
invention is
applicable to any wall forming portion of a modular formwork element, even if
the
element also includes non-wall forming components. Such would be the case for
example with an element comprising a combined connecting member and wall
defining
surface, it being understood that the projections or ribs according to the
invention would
be located on the wall defining portion of the element.
Moreover, when the projections or ribs according to the invention abut against
a layer of
insulation, the resistance of the wall element to oil canning is further
enhanced.. In
addition insulation and the central connector appear to cooperate in providing
increased
resistance to bulging.
It will be appreciated by those skilled in the art that other variations of
the preferred
embodiment may also be practised without departing from the scope of the
invention.
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