Note: Descriptions are shown in the official language in which they were submitted.
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An Articulated Concrete Joint Member
Field of the Invention
The invention relates to the construction of pavements and concrete slabs,
and in particular, the jointing method used in cases of differential movement
between said slabs.
Background of the Invention
Pedestrian thoroughfares whether associated with a road, through a park
or other means, and which fall under the control and maintenance
responsibility
of a Municipal Council will often be formed from concrete. As such pedestrian
thoroughfares carry very light traffic loads, typically, such thoroughfares or
footpaths will have little or no reinforcement within the concrete.
Typically, the footpath will be formed from pavement slabs which are cast
in place in significant lengths so as to economically place the pavement by
limiting the number of concrete pours required when constructing the
thoroughfare between locations.
As the pavement is cast in a unitary mass it is recognized that the unitary
slab will eventually crack as a result of external factors and the lack of
reinforcement. Such external factors can be root intrusion from nearby trees,
soil
heave from saturated expansive clays, soil shrinkage through drying during
summer, or differential settlement of the foundation as a result of a poorly
prepared base course supporting the pavement.
So as to control the number and placement of this cracking, transverse
lines of weakness are placed in the concrete prior to curing. Typically, this
is done
by trowelling a line across the concrete, and thus provide a localised
weakening
of the concrete, as compared to surrounding areas. This has the dual effect of
disguising the crack within this line of weakness, as well as managing the
long
term serviceability of the pavement by ensuring the creation of a plurality of
slab
units from the original unitary slab.
Unfortunately, following the initial crack at the line of weakness the
interfering factor, be it soil or a tree root, will continue to affect the
slab. The slab,
having been broken into discrete and much smaller units, is free to lift.
Further,
as such interfering factors inevitably cause differential movement, in that
adjacent
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slabs will be affected to varying degrees, the movement of one slab compared
to
an adjacent slab will be at a different rate and displacement.
The differential movement of one slab to the next will inevitably cause the
raising of one peripheral edge higher or lower than its neighbour. Thus, the
once
continuous surface will be no longer, with several raised discontinuities
being
formed along the surface.
Thus, depending on the conditions, the once unitary slab having a flat
continuous surface will comprise a plurality of discrete units providing a
disjointed
and discontinuous surface. Such a surface, instead of providing a convenient
path for pedestrian vehicles, such as wheelchairs, prams, etc., will instead
become effectively impassable for such pedestrian vehicles, not to mention
becoming a hazard to foot traffic.
Thus it becomes a serious issue for Municipalities to devote funds from
constrained budgets to expensive maintenance programmes to replace the
pavements that have suffered differential displacements between the pavement
slabs. Further, such Municipalities must maintain a contingent liability
against
litigation brought by pedestrians who may injure themselves by tripping and
falling
as a result of the raised peripheral edges of the slab units.
Statement of the Invention
It is therefore an object of the invention to reduce differential displacement
between concrete slabs, and so diminish the consequential detrimental effects
associated with this displacement.
Hence, in a first aspect of the present invention there is provided an
articulation member arranged to be disposed between two concrete slabs, the
member comprising a resilient core, and connection means in the form of first
and second projecting portions that project from the core, the projecting
portions being angularly spaced apart about an axis of the core and arranged
to
inter-engage with the concrete slabs along an adjacent peripheral edge of each
slab, wherein on application of an out-of-plane displacement to one of the
concrete slabs, the member is arranged to transfer shear between the slabs so
that the displacement is transmitted to the other concrete slab whilst
permitting
pivoting of the slabs about the articulation member.
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In a second aspect of the present invention, there is provided a method
of constructing a pavement having a plurality of slab portions that can
articulate, the method including the steps of: (i) establishing form-work
adapted
to receive wet concrete; (ii) placing a plurality of articulation members
along a
plurality of articulation lines; (iii) pouring wet concrete within the form-
work so
as to engage the articulation members; (iv) curing of the concrete to form a
plurality of adjacent slab portions connected by the articulation members
wherein each articulation member is arranged on application of an out-of-plane
displacement to one of the slab portions to which that articulation member is
engaged to transfer shear to the other slab portions to which that articulated
member is engaged so that the displacement is transmitted to the other slab
portion whilst permitting pivoting of the slab portion about that articulation
member.
In one preferred embodiment of the invention, the wet concrete may fully
immerse the articulation members, and consequently form lines of weakness at
the articulation lines. In this embodiment, should the assembly be subject to
extraneous loads, such as through soil movement or root intrusion, the
assembly may crack along the lines of weakness and so form the plurality of
adjacent slab units connected by the articulation members.
In a third aspect of the present invention there is provided a concrete
slab, including a plurality of lines of weakness and a plurality of
articulation
members placed along the line of weakness wherein the slab is adapted to
crack along the lines of weakness, resulting in slab portions wherein each
articulation member is arranged on application of an out-of-plane displacement
to one of the slab portions to which that articulation member is engaged to
transfer shear to the other slab portions to which that articulated member is
engaged so that the displacement is transmitted to the other slab portion
whilst
permitting pivoting of the slab portion about that articulation member.
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In the case of a cast in place pavement, the articulation member will be
cast within the unitary pavement. By placing the articulation member along the
transverse line of weakness that is typically trowelled into the pavement when
the
pavement eventually cracks through the interference of soil or tree root, the
unitary pavement will form two discrete slabs adjacent each other, with the
adjacent peripheral edges of each slab essentially co-linear with the
articulation
member.
In laying a unitary pavement so as to connect it to an adjacent unitary
pavement slab, it is common to place expansion and contraction joints between
said slabs. A contraction joint is typically a resilient sheet of material
that on
shrinkage of the concrete during curing, the resilient material will prevent
voids
being created between said slabs. Similarly, and also in application to
expansion
joints it is known for concrete slabs to use steel dowels between said slabs.
Said
dowels are placed within tubes cast within the concrete so as to permit free
uni-
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directional movement between the slabs by sliding along the dowels. The dowels
being made from steel, and thus being relatively stiff, are placed to provide
a
transfer of out-of-plane forces between the slabs. As one slab has a
substantial
force applied to it, said force is transferred to the next slab by the dowels,
and
thus maintain a continuous surface between the slabs. The disadvantage of such
a system is that in transferring this load, localized failure of the concrete
at the
peripheral edges of the two slabs must be prevented. Thus, the concrete
portion
using the dowels must be specifically reinforced and/or have a thickened
portion
designed into the concrete slab. Further, the placement of the dowels adds a
secondary process to the placement of the pavement. Thus while a dowelling
system is a useful tool for transferring displacements and loads between
slabs, it
is also an expensive one and usually inappropriate for general application
footpaths. This can be seen by the preference of Municipalities for using on-
going maintenance programmes to repair pavements rather than the extremely
large capital cost of extensive use of a dowelling system.
The present invention overcomes the disadvantages of the dowelling system by
recognizing, firstly, that whilst a continuous surface for a pavement is
essential,
having that continuous surface flat is not so for a footpath. Thus whilst the
lifting
of a slab unit may not be preventable within reasonable cost constraints,
neither
should it be necessary to prevent, so long as the pavement remains serviceable
and safe for pedestrian traffic.
Thus, it should be appreciated that whilst the articulation member may not
be connected to two slabs in the first instance, but possibly cast within the
unitary
pavement slab, the invention commences functioning immediately following the
controlled cracking of the pavement and thus the creation of the plurality of
slab
units. This should not be construed as rejecting the cast of two adjacent
slabs,
connected by a cast-in-place articulation member.
As discussed, the outer plane displacement that is applied to a concrete
slab may be caused by tree roots, soil expansion, soil drying, or an unstable
base
course laid beneath the pavement.
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As the core is resilient, when displacement occurs the slab will lift the
articulation member which will flex and pivot relative to the two slabs. On
further
displacement the first slab will lift the second slab through pivoting about
said
articulation member.
5 The connection means must be capable of transferring the force
associated with the change of displacement. These forces will include the mass
of the slabs, friction of the second slab as it is lifted from the base
course, and
any cohesive force due to surrounding soil. Thus, the connection means must be
capable of resisting, first tensile loads within the plane of the slab, then
shear
forces as the first slab is lifted out of the original plane and begins to
displace the
second slab.
Preferably the material of the resilient core may include rubber. The
pivoting action of the articulation member is central to the core idea of the
invention. Thus in order to achieve the pivoting action a resilient material
such as
rubber may provide an advantageous effect.
More preferably, the resilient core material may further include recycled
rubber crumb. The present invention may not require a high degree of
dimensional tolerance in order to function satisfactorily. Thus it may be that
the
formation of the articulation member is a suitable application of recycled
material
and thus provide an environmental benefit.
Even more preferably, the connecting means may be made from rubber
also. Thus if the connecting means is also made from rubber this may provide
an
opportunity to form the articulation member from a single unitary extrusion of
rubber, and reduce the manufacturing costs of the articulation member.
Alternatively, the connection means may be made from a substantially stiffer
material than rubber. As the function of the connecting means is somewhat
different from the resilient core, in that it must connect to the concrete and
transfer loads between the concrete slabs via the resilient core, it may be
that an
advantage can be gained from making the connection means from a different
material that is economically and functionally more suited to this
application.
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More preferably, the connection means may be made from steel. As
discussed the connection means may have an economic advantage in being
made from a stiff material and may be further advantageously made from steel.
Preferably, the connecting means may be projections emanating from the
resilient core. Whether the material from which the connecting means is made
is
rubber, steel or any other material, having the connecting means being
projections may be well suited to the articulation member being placed at the
time
of pouring the cast in place pavement.
Alternatively, the connecting means may be separable projections which
may connect with the slab and the resilient core. In one embodiment of the
connecting means being separable projections, the connecting means may be
steel spikes that pass through the core and have a "cog-type" end profile for
casting within the concrete.
Alternatively, the connecting means may be an adhesive material. For
instances where the pavement is pre-cast, and so use slabs which are
subsequently laid, the articulation member in order to function, may be
adhered to
the peripheral edges of adjacent slabs during the pavement laying.
Preferably the concrete slabs may be pavement slabs for foot traffic.
Alternatively the concrete slabs may be decking for a bridge or may be slabs
for a
cosmetic finish to said decking.
Preferably the articulation members may further include crack propagation
means. Said propagation means are intended to assist in the controlled
cracking
of the slab by providing a line of weakness in the cast in place pavement.
Thus,
as an alternative to a line of weakness being trowelled into the surface of
the
concrete, the articulation member having a crack propagation means may provide
the same or similar function and advantageously avoid the secondary process of
placing these lines of weakness. Alternatively, said crack propagation means
may
further assist in defining the line of weakness when used with a trowelled
surface.
More preferably, the crack propagation means may be projections directed
away from the resilient core towards the upper and/or lower surface of the
concrete, but not penetrating. Thus the thickness of concrete between the
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surfaces and the articulation member will be considerably less than that of
the
surrounding concrete, and thus on application of an interfering factor, the
concrete will crack precisely at the required line of weakness and ensure the
efficient functioning of the present invention. Preferably the articulation
means
may further include separation means. The separation means may be projections
from the resilient core to the upper and/or lower surface of the concrete and
actually penetrating said surface. As an alternative to the crack propagation
means, the separation means may be projections that provide a dividing barrier
between adjacent portions of the pavement, and thus may separate the pavement
into discrete slabs at the time of pouring rather than as a result of
cracking. In
essence, the separation means may be considered transverse formwork which is
conveniently placed at the same time as the articulation member. In addition
to
the advantages of controlling the cracking of the pavement and clearly
defining
adjacent slabs, the separation means may further act as a contraction joint
between the slabs. Thus the articulation member may provide the multiple
functions of articulating the slabs, providing contraction joints to limit
gaps in the
pavement caused as a result of concrete shrinkage and act as expansion joints
to
accommodate thermal expansion.
Description of Preferred Embodiment
It will be convenient to further describe the articulation member with
respect to the accompanying drawings, which illustrate possible arrangements
of
the invention. Other arrangements of the articulation member are possible and
consequently the particularity of the accompanying drawings is not to be
understood as superceding the generality of the preceding description of the
invention.
Figure 1 is an elevation sectional view of the articulation member,
according to the present invention.
Figure 2 is an elevation sectional view of the articulation member,
according to another embodiment of the present invention.
Figure 3 is a further elevation sectional view of the articulation member,
according to the present invention.
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Figure 4 is a further elevation sectional view of the articulation member,
according to another embodiment of the present invention.
Figure 5A is an elevation view of the support means, according to one
embodiment of the present invention.
Figure 5B is a plan view of the support means according to Figure 5A.
Figure 6 is an elevation sectional view of the articulation member
according to a further embodiment of the present invention.
Figure 1 shows the articulation member (1) cast within a pavement. The
articulation member (1) includes a core (2) about which the articulation
member
(1) can pivot. Further included are the connection means (3), in this case
sideways projecting portions of sufficient size to engage the concrete (5, 6),
and
transfer loads from one side of the articulation member (1) to the other.
In this embodiment of the invention the articulation member (1) is made
from a single elastomeric extrusion, such as rubber, which may also include a
proportion of, or possibly entirely from, recycled rubber crumb. The
environmental benefits of being able to use recycled rubber crumb from
granulated car tyres will be clear.
It will be clear to the person skilled in the art that the core (2) can
incorporate a portion of the articulation member (1), that is of a thicker
section
than the connection means (3), or be of the same size. In defining the portion
of
the core (2) rather than purely geometric description, it is important to
recognize
that the core (2) is that portion of the articulation member (1) that is
subjected to
the greatest flexural stress as load is transferred between adjacent slabs
(5 and 6). In placing a stress based functionality to the articulation member
(1)
the connection means provide, in terms of stress, an engagement with the
concrete which may be achieved through pure friction, or through a mechanical
jointing with the concrete (not shown), and thus at least for a frictional
engagement the primary consideration is one of maximizing surface area without
unduly reducing the thickness of the concrete pavement above and below the
connection means (3). The core (2) however, is required to transfer load from
one pavement to the next through the transfer of flexural stress and thus, in
terms
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of geometry need only be of a size to handle the expected flexural load, the
most
significant portion being the tensile load in the upper portion of the core
(2).
Also included are the crack propagation means (4), in this case upwardly
and downwardly directed projections, which create a concrete section of
reduced
thickness, and thus promote cracking of the concrete at that point.
On cracking, the pavement is effectively divided into two slab units 5 and 6,
which without the articulation member (1) would effectively act independently
of
each other. Having the articulation member (1) in place, however, provides for
the transfer of displacement and load from one slab unit (5) to the next (6).
Finally, as is typically done for such pavements, a troweiled notch (7) is
placed transversely across the pavement, so as to further reduce the section
thickness of the concrete, and so promote the formation of a crack. Whilst
this
may be standard practice without the installation of the articulation member
(1), it
can become notionally superfluous, from a functional standard point to include
such a notch (7) when using an articulation member (1) having the crack
propagation means (4). Nevertheless, in certain circumstances, the addition of
such a feature, can provide an aesthetic benefit by hiding the crack from the
pavement users.
Figure 2 shows another embodiment of the articulation member (1), having
separation means (8). As an alternative to using crack propagation means (4),
the articulation member (1) may incorporate projections upwardly, and possible
downwardly from the core (2), such that a divide is placed within the pavement
(5, 6). Thus, the slab units (5 and 6) are defined prior to pouring, without
having
to rely on a crack forming first. Among the benefits provided by the inclusion
of
the separation means (8) is the saving in expansion joints. Thus, whilst
expansion joints may be provided for pavements, so as to accommodate concrete
shrinkage during curing, the separation means (8) provides a full thickness
buffer
between the slab units (5 and 6), which may limit the formation of gaps in the
pavement due to shrinkage.
Figure 3 shows the articulation member (1) of Figure 1 following the
application of a severe displacement to slab unit (6), caused by a tree root
(9). As
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pavements for pedestrian traffic are commonly placed in proximity to trees, it
is
common for a root (9) to extend underneath the pavement (6). As the tree
grows,
so does the root (9), the consequence being uplift of the pavement (6).
Without
the installation of the articulation member (1), slab unit (6) would be
displaced
5 upwards, independent of slab unit (5). Thus, a discontinuity in the pavement
would be created as a result of the step the effected of slab unit (6). Not
only
does this cause a problem for vehicles permitted access to pedestrian
pavements, but also becomes a hazard for foot traffic, where a user may trip
and
fall.
10 By including the articulation member (1), slab unit (5) is also displaced
upwards to maintain the relatively continuous surface of the pavement. Whilst
a
crack (10) may form, detritus along the pavement, or even a maintenance
programme of filling such a crack, is all that is required to eliminate any
serviceability or aesthetic problems which may be caused. If such remedial
action is required, this is minor in comparison to the maintenance cost to
replace
such pavements.
In consideration of the connection means (3), there may be a number of
useful profiles of the concrete engaging end of the connection means (3) can
adopt. It should be noted that this discussion is based entirely on the cast-
in-
place situation, where the articulation member (1) is placed prior to the
pouring of
the concrete, and thus the articulation member (1) becomes integral with the
concrete. An alternative to this is the use of pre-cast slabs, where the
connection
means may include an adhesive, or other engaging means, to connect with the
pre-cast slabs.
Figures 4, 5A and 5B show a support means (12) which is used to assist in
the placement of the articulation member (1). In some circumstances it may be
advantageous to hold the articulation member (1) in place during the pouring
of
the concrete in order to form the pavement. In certain circumstances, because
the specific gravity of rubber is such that the articulation member (1) may
float in
the denser concrete (typical specific gravity of 2.2 to 2.4) it may be
advantageous
to have a bracket which when connected to the form-work (16) can resist the
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floatation of the articulation member (1) as the concrete approaches and
amerces
the connection means (3). Therefore, in one embodiment of the invention, there
is provided a support means (12) comprising an assembly of angle members
(14A, 14B and 17). Member (17) is fixed to the form-work (16) through nails
(13).
Further elements (14A and 14B) are fixed to the angle (17) and are oriented so
as
to run parallel to the direction of the articulation member (1). Members (14A
and
14B) are oriented so as to provide a close fitting gap into which the upper
separation means (4) can slide with the fit such that the members (14A and
14B)
once engaged with the articulation member (1) will enclose the upper portion
of
the separation means (4) and bear down upon the connection means (3). Thus,
is the fully installed condition, the support means will comfortably engage
the
articulation member (1) ready for the pour of concrete. The degree to which
the
support means (12) is required will depend upon a number of factors, and so
the
level of concrete at which the support means (12) is no longer required may be
at
a point (15A), and thus just up to the connection means (3), or to a point
(15B)
where the concrete has amerced the connection means (3). In either case and
as determined by those installing the articulation member (1), once the
articulation member (1) is securely engaged with the concrete, the support
means
(12) may be removed and the concrete pour continued.
Figure 6 shows an alternative arrangement of the support means (19),
which is used as a means to finish a portion of a pavement (5), but where it
is
expected that further work is required, and so the articulation member (1) is
used
as a terminating barrier, ready for further additions to the pavement to be
added.
The support means (19) includes parallel members oriented so as to enclose the
connection means (3) on the side of the articulation member (1), to which the
future pavement will be added. A lower member (18B) is placed abutting the
lower portion (4B) of the separation means and the connection means (3), and a
further member (18A) placed abutting the upper portion (4A) of the separation
means (4), and the opposing side of connection means (3). These members are
fixed to the form-work (16) through nailing a connecting bracket (20) which is
fixed to the supporting elements (18A and 18B).
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Thus, on completion of a portion of the pavement (5) there will be
projecting from that portion an articulation member (1) having a protruding
connection means (3) which is supported and confined by the support means
(19), which is subsequently fixed to the form-work (16).
