Note: Descriptions are shown in the official language in which they were submitted.
WO90/1~909 2~3~ PCT/GB90/00940
Formwork for Formina Structural Beams
The invention relates to a method and apparatus for
forming concrete structural beams used in a building or
building foundation such as, for example, ground beams,
aerial beams or like structures.
In forming the foundations of a building, concrete
ground beams in conjunction with driven concrete piles
are a well known way of providing good foundation
support for later structural work. In preparing the
foundation, concrete piles or pillars formed of concrete
and having embedded within thick metal reinforcement
bars, are pile-driven into the ground as deep as
possible. Where the ground is not particularly firm,
the piles are driven in until the end of the pile
strikes a layer of more solid material. After a pile is
fully driven in, the top end of the pile is capped and
cracked and the ends exposed. Concrete ground beams are
then formed between piles to link up piles. The width
and depth of the beams are determined by what is to be
built on top of the foundations.
Conventionally, a trench is dug, the bottom of -
which is lined with drylene concrete. This is necessary
to provide a good surface to mould the beam and to
protect the beam when formed. Wooden shuttering and
bracing is then erected by a carpenter on the base of
2S the trench between the two piles to provide a mould or
formwork for the ground beam. The mould comprises the -
drylene-coated base of the trench and two walls provided
by the shuttering. The shuttering is then covered with
releasing oil, to enable subsequent removal of the
shuttering, and a reinforcement cage is placed on loose
spacer blocks on the floor of the trench. The
reinforcement cage comprises a metallic meshed cage,
substantially rectangular in cross-section, running the ~-
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WO90/15909 PCT/GB90/00940
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length of the trench inside the two walls of the
shuttering. The cage needs to be raised above the floor
of the trench by the spacer blocks to ensure that cage
will not be exposed to damp when subsequently in place
in the ground beam. The ends of the cage are meshed
with the reinforcement bar ends protruding from the
concrete piles. Then, the concrete is poured into the
mould formed. After the concrete has set, the
shuttering is removed to be used again and the volume of
the trench left by the removal of the shuttering is
;~ backfilled with an inert material.
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; This method of forming ground beams suffers from a
number of disadvantages. Principally, there are many
problems associated with the amount of waste that has to
be excavated. In order to provide sufficient space in
the trench for adequate bracing and support to be
provided to enable the shuttering to hold still while
the concrete is poured, a trench of at least three times
the width of ground beam has to be excavated. This
space is also needed to allow the carpenter to erect the
shuttering and dismantle the shuttering after the beam
is set. Excavation of unnecessary amounts of material
increases both time and labour costs and provides a
problem in stockpiling and disposinq of the unwanted
earth. This problem of 'muck away' is proving
increasingly important due to the difficulty in finding
sites to dump the excess material, the cost of
~ transporting the material etc and is exacerbated by the
- fact that excavated soil expands in volume.
Furthermore, after the beam is set, the remaining space
in the trench must be filled in again with inert
material and the material compacted on each side of
trench.
Also, the conventional method of forming ground
beams by constructing wooden shuttering is extremely
time-consuming. Due to the expense in providing large
amounts of wooden shuttering (including possibly plastic
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WO90/15909 PCT/~B90/0~940
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or metal faced wood for the internal walls of the mould)
it is frequently the case that only part longitudinal
sections of the ground beam can be constructed at a
time. After the concrete has been poured in there then
~ 5 ensues a wait of at least 24 hours while the concrete
-~ sets, after which the shuttering is dismantled and re-
assembled to form the next portion of the groùnd beam.
Not only does this greatly increase the time needed to
construct the foundations, it also results in a ground
beam formed with at least one 'cold joint', rendering
;~ the beam less structurally sound. Furthermore, the job
of shuttering is a skilled job and generally requires
the services of a skilled formwork carpenter to
construct.
These problems of slowness and complexity are also
- associated with conventional methods of forming aerial
beams which are formed by setting concrete around the
metal girders of a building, the concrete being set in a
mould defined by a wooden shuttering similar to that
used in forming ground beams. Furthermore, a relatively
wide platform may have to be provided to support the
shuttering around the beam.
The problems associated with formation of ground
~- beams are to some degree addressed in EP 0141465 which
discloses a method for forming a foundation in which a
-~ shuttering unit is used comprising a polystyrene box -
- formed from four walls and a bottom and having a
reinforcement cage within, the box remaining in place
around the ground beam formed after the concrete has
set.
By using a disposable formwork that remains in
place after the concrete has set, the trench can then be
backfilled immediately prior to pouring in the concrete,
; the backfill then providing the main support for the
walls of the formwork against outward displace~ent as
the concrete is poured in. Using the backfill to
provide the support also means that the trench can be
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~; wo9~159092~`?~ ' PCT/GB90/00940
~- excavated to a width not much greater than that of the
ground beam, reducing the amount of material to be
excavated, and resulting in a saving of time and money
in excavation, storage and muck away. The amount of
S material to be backfilled is also less than in the usual
method which results in further cost saving if special
inert material is used.
Backfilling at the earlier stage also enables easy
access to fill the formwork with concrete and removes
the need for providing any further working platforms or
the like.
The shuttering unit of EP 0141465 suffers from a
i~ number of disadvantages, particularly with regard to
; stability and convenience of use. In order to enable
the unit of EP 0141465 to be sufficiently supported to
both stand upright and to hold its shape during backfill
the reinforcement cage is provided on spacers inside the
box and the walls and floor of the box bound against the
cage by bracing wire. Even using this bracing it seems
likely that the unit is not particularly stable
particularly during backfill and binding the unit may
prove an awkward and time consuming process.
Furthermore, using a box of set dimensions clearly
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reduces the adaptability of the structure in forming
ground beams of different lengths and widths. To
produce a ground beam of double the length of the box,
two boxes must be placed adjacent to each other,
: resulting in the formation ground beam of broken length.
According to one aspect of the invention there is
provided a method of forming a structural beam for a
building or building foundation comprising providing a
formwork having two side walls arranged to provide a
mould for the beam, characterized in that the formwork
comprises at least two pre-fabricated self-standing
opposing parts, each part comprising a side wall formed
integral with a supporting foot extending outwardly from
the side wall, stabilising the supporting feet to assist
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WO90/1~909 PCT/GB90/00940
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in maintaining the position of said parts, filling the
mould with poured concrete and allowing the concrete to
set.
Where used in the formation of a ground beam, the
method comprises excavating a trench, positioning the
formwork parts in the trench so that the supporting foot
of each extends outwardly from the respective side wall
to the adjacent wall of the trench, backfilling with
material onto the supporting feet at least part of the
volume between the side walls of the formwork and the
adjacent walls of the trench and, after backfilling,
filling the mould with concrete and allowing the
concrete to be set, the formwork permanently remaining
in place around the set concrete.
The use of a formwork having two pre-fabricated
self-standing parts shaped to define the mould provides
a number of significant practical advantages over prior
art methods regarding installation time and cost,
manufacturing cost, and adaptability. Location of the
mould parts is simple to achieve, and stability may be
provided e.g. by depositing material onto the feet to
weigh each part down, fixing the feet by using a road
p~n or the like extending through the feet or by merely
stopping movement of the parts by using a block at the
end of the feet. In each case the use of complex
bracing is avoided.
~ sing integral feet to provide self-standing parts
means that each part can ~e quickly and easily
positioned and, by eliminating the need for further
bracing, can provide a mould free of obstruction such
that a person can work inside the mould, if desired.
- As compared with the prior art box structure
disclosed above, the formation of a mould by separate
opposed self-standing parts also leads to improvements
concerning ease of manufacture, transportation and
storage, and the lack of end walls means that a
reinforcement cage may conveniently extend continuously
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PCI /GB90/00940
from the central region between the parts to around a
pile for intermeshing with the reinforcement wires
extending from the pile.
When used in the formation of a ground beam, the
feet will be weighted down and stabilised by the initial
backfill of material onto the feet and will provide good
support or anchorage to the side walls during any
further backfill and during the subsequent pouring in of
concrete. After this first backfill minor adjustments
of position of the formwork may then be conveniently
made prior to further backfill and concrete pouring.
Whilst it will be usual that the remainder of the volume
between the side walls and the trench walls is
backfilled prior to the pouring of the concrete the
stability of a preferred formwork is such that this may
not be necessary, particularly if additional supports
such as road stakes (see below) are provided.
- By using a pre-fabricated formwork that remains in
place, a complete section of the ground beam can more
readily be formed at once, without the need to dismantle
and reassemble a support assembly. Thus, this method is
particularly applicable to form a single piece ground
beam linking two or more piles, avoiding problems such
as cold joints and reducing the time needed to form the
ground beam. Furthermore, by virtue of the simple
construction of the invention, two lines of opposing
parts can be arranged along the length of the trench to
provide a mould of desired and unbroken length.
Advantageously, each part may ~e formed with a tongue
and groove arrangement in each end such that a line of
parts can be matingly engaged.
~ he method is also applicable to the forming of
ground beams or like foundation structures above ground
and outside of a trench where full backfilling is not
- 35 desired but rather only a small amount of material
placed onto the feet may be used to assist in
stabilising the formwork.
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WO90/15909 PCT/GB90/00940
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The method may also be used in the formation of
aerial beams around girders used in the structure of a
building above ground. In such an application the
formwork may be positioned around the girder on a
platform some distance above ground. Stability may be
best provided by use of road stakes or by use of a block
or rail fixed to the platform at the end of the feet to
stop lateral movement of the parts.
In these above ground situations the formwork may
be left in place or removed, as desired.
In a particularly preferred embodiment, each
formwork part has a foot also extending inwardly of its
respective side wall such that in use the formwork as a
whole defined by the two parts has an internal U-shaped
cross section and has two side walls and an impermeable
base. Provision of a base as part of the formwork means
-~ that the surface of floor of the trench does not have to
be covered with drylene concrete. Instead, the method
can include lining the floor of the trench with a layer
of sand reducing the cost considerably. Furthermore,
the substantial inverted T-shape cross-section of this
embodiment is especially stable when put in position by
the very nature of its shape. Advantageously, each or
one of the parts can be designed to be of an asymmetric
T-shape, such that different internal formwork widths
can be formed by re-arrangement of the two parts.
Alternatively or in addition to having a foot
extending inwardly to form a base, a separate base may
be placed between opposing parts to vary the width of
the formwork.
~ he method may further include the step of placing
one or more restraint straps or top braces between the
top of the side walls to provide extra initial support
; to the walls, in particular where backfill material is
used to stabilise the parts. These can be removed after
the concrete has set and need not form part of the beam.
Stakes may also be driven through the foot of each
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WO90/15909 PCTt~B90/00940
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part to provide additional initial support to the side
walls.
Viewed from a further aspect the invention provides
a formwork having two side walls which can be arranged
in use to provide a mould for forming a structural beam
for a building or building foundation characterized in
that the formwork comprises at least two pre-fabricated
separate self-standing opposing parts, each part
comprising a side wall formed integral with a supporting
foot extending outwardly from the respective side wall.
For the reasons explained earlier, the formwork of
the invention when used to form a ground beam or the
like is particularly stable in use during backfill, as
the backfill weighs down and anchors each part in
position. Advantageously therefore, an outwardly
extending part of each foot may be provided with a
recessed portion or portions to catch the backfill.
Preferably, each foot also extends inwardly of its
respective side wall, such that each part has a
substantially T-shape cross-section. In use, as
explained before, opposing parts may then be arranged to
form a formwork having an impermeable base.
Alternatively, each part may have a simple L-shaped
cross-section.
Advantageously, each or one of the parts can be
designed to be of an asymmetric T-shape cross-section.
Preferably, each part includes supporting webbing
- integrally formed with and between the side wall and
foot. This webbing lends extra support and rigidity to
the side wall.
Preferably, an inwardly extending part of each foot
is pre-fabricated with integral spacers upon which a
reinforcement cage may be placed. This removes the need
- for having loose spacers and the problems associated
with keeping them secure during the formation of a
ground beam. These integral spacers may be formed as
protruding ridges or as blocked areas separated by
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WO90/15909 PCT/GB90/00940
9 ~ 3 ~ f-
grooves.
The formwork parts in either aspect ~f the
invention may be formed from concrete as this proves an
inexpensive, easily manufactured and reliable
construction. Alternatively, other materials could be
used, such as metal or plastic. The material chosen
should be resistant to the effects of moisture to which
the formwork may be exposed after many years in situ in
a building or in a buildings foundations.
In a particularly advantageous embodiment, however,
- the formwork parts are formed from a lightweight foamed
plastic material, such as polystyrene, by, for example,
a conventional moulding process. In addition to ease of
manufacture and cost benefits, such a formwork is easily
transportable and positionable, leading to a reduced
time in setting up the formworK. Also such formworks
may be c~t and shaped on site and in use to provide for
trenches of different widths, to arrange two adjacent
formworks in a mitred joint to vary the width of a
particular trench etc. Furthermore, formworks formed
from foamed plastics such as polystyre~e have the
additional advantage of being flexible and less rigid
than those made from concrete so that, after the
formwork is in place in the foundation, ground movements
can be accommodated by the formwork, which will bend or
flex. This reduces the chances of cracks appearing in
the formwork and results in better resistance to damp
etc. and better weathering. When left in place
surrounding a ground beam, a foamed plastic formwork
also provides an advantageous insulation effect.
Viewed from a further aspect the invention
comprises a formworK part moulded from a foamed plastic,
such as polystyrene, for use in formation of a
structural beam for a building or building foundation,
characterized by a side wall formed integral with a
supporting foot extending at least outwardly in one
- direction perpendicular to the side wall, said
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WO90/15909 PCT/CB90/00940
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~p`~ rting foot having a recessed portion or portions
adapted to receive material so as to anchor the formwork
part in use. Such part may also include a foot portion
extending in the opposite direction for defining a base
part for a mould in use.
Embodiments of the invention will now be described,
by way of example only, and with reference to the
accompanying drawings in which:
Figs. 1 to 3 illustrate the conventional method of
forming a ground beam, showing a cross-sectional views
of the trench, shuttering and ground beam,
Figs. 4 to 7 illustrate a method and apparatus
according to an embodiment of the invention for forming
a ground beam, showing cross-sectional views of the
trench, formwork and ground beam so formed,
Fig. 8 is a side or cross-sectional view of the
formwork of Figs. 4 to 7,
Fig. 9 is a perspective view of the formwork of
Figs. 4 to 7 and
Figs. lO and 11 are a side view and a perspective
view respectively of a further formwork part according
to the invention.
Conventionally, and with reference to Figs. l to 3,
first a trench l is dug, of approximately the same
height as the ground beam to be formed, but of
approximately three times the width of the ground beam.
The floor of the trench is then covered with drylene
concrete mix and compacted. Typically, for a ground
beam of cross-sectional dimension 6 m by 6 m, a trench
of approximately 6.5 m by 20 m would be dug, and the
bottom of the trench lined with 0.05 m of drylene.
Then, plywood shuttering and timber bracing is erected
by a carpenter on the floor of the trench 1, as shown in
- Fig. 2. The inside of the shuttering 3, forming the
mould 4, is then coated with releasing agent in
preparation for the concrete. Spacer bars 5 are placed
on the floor of the trench 1, and the reinforcement cage
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WO90/1~909 PCT/GB90/00940
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6 is placed loosely on top of the spacers. Then, the
concrete 7 is poured into the mould 4 and allowed to set
accordingly. After a wait of 24 hours, the shuttering 3
is dismantled and moved to the next part of the ground
beam 8 to be formed. Once a complete length of ground
beam between two piles has been formed, the space
between the walls of the trench and the ground beam 8 is
backfilled with a large quantity of expensive inert
material.
As mentioned, the conventional method suffers from
a number of disadvantages, particularly relating to the
size of the trench to be dug, the amount of backfill
required, and the time required to construct the ground
` beam.
lS Figs. 4 to 9 show a method and apparatus for
-~ preparing a ground beam according to embodiments of the
; invention. Firstly, a trench 10 of approximately the
same cross-sectional dimensions of the ground beam to be
- formed is dug. Typically, to form a ground beam of
cross-sectional dimension 6 m by 6 m, a trench of height
,- 6.5 m and width 9 m is dug. The bottom of the trench is
then lined with approximately 0.05 m of sand 11. A pre- -
fabricated formwork 12 is then placed in the trench.
The formwork 12, which is seen in better detail in
Figures 8 and 9, comprises two parts 14,15 of an
~; inverted asymmetric T-shape arranged in use opposing
each other. The formwork so formed defines an internal
U-shape cross-section and two outwardly protruding feet
or ledges 16,17. During backfill, material will fall on
- 30 these feet and anchor the assembly, giving good support
to the integral side walls 18,1g. Inwardly protruding
feet portions 16',17' are provided to define a base for
the mould provided by the parts 14,15. By reversing one
or both of the parts 14,15 so that either one or both
feet 16,17 face inwardly and the corresponding portions
16',17' face outwardly, a formwork of reduced internal
width can be formed. This is advantageous in that a
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WO90/15909 ~c?~ 12 PCT/~B90/00940
number of different ground beam widt~s can be formed
from the same shape components. Any number of parts may
be used to cover the whole length of the trench between
the piles, and may be conveniently assembled in situ in
the trench. In order to provide part of the support for
the next stages, a number of stakes or road pins (not
shown) may be driven through the base into the ground,
extending above the ground to support and engage with
the side panels 18 and lg. Also, a restraint bar 21 may
be placed over the top of the formwork, holding the tops
of the side walls 18 and 19 at a constant spacing.
Where a number of side panels are used to cover the
length of a trench, the restraint bar may additionally
be arranged to bridge adjacent side panels along the
length to provide additional support. However, these
additional supports are not essential.
Next, a reinforcement cage 22 is placed between the
parts 14,15, optionally on integrally formed spacer
blocks (not shown), and the space between the walls of
the trench 10 and the side walls 18 and 19 is backfilled
: with material 24. The support provided by the anchoring
of the feet 16,17 at the first load backfilled together
with the support provided by the restraint bar 21 and
stakes (if used) ensures that the formwork retains its
shape during backfill.
After backfill, concrete 23 is poured into the
mould formed by the formwork 12 and allowed to set.
During this time, the backfill material provides support
for the side walls and the additional support provided
by the restraint bar 21 (if used) is no longer required
and this may be removed, to be used again. The presence
of the backfill 24 also enables easy access to the
formwork 12 during this stage. The formwork 12 is
allowed to remain in place, and the finished ground beam
and formwork arrangement is shown in Fig. 7.
Although the formwork of Fig. 8 could be made of
- any material, such as concrete, solid plastic or metal,
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WO90/15909 PCT/GB90/00940
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advantageously it is formed from a foamed plastic such
as polystyrene.
In forming a structural beam in an above ground
situation the formwork parts are arranged to define the
mould for the beam, the formwork stabilised by use of
material placed on the feet or by any other means, after
which concrete is poured into the mould and left to set.
After the beam has been formed the formwork may be left
in place or removed as desired.
Figs. lO and ll show side and perspective views of
an asymmetric T-shape formwork part according to a
preferred, especially stable, embodiment of the present
invention, usable in the same manner as the embodiment
described above. Part 25 comprises side wall 26
integrally formed with a foot 27 having outwardly and
inwardly extending portions 28,29 respectively. Part 25
also includes recessed portions 42,43 adapted to receive
and contain backfill in use to enhance stability.
Further hollowed portions 30-33 may also be formed to
receive backfill and to provide a weakened portion
through which a road stake may be driven. Webbing
supports 34-36 may be formed integral with the part 25
to provide additional support to the side wall 26 and
spacers 37-39 separated by grooves may also be formed
integral with the part 25 to support the reinforcement
cage of the ground beam. Due to the inherent rigidity
and stability of this design restraint bars are not
needed to provide additional support.
In order to facilitate interlocking of adjacent
parts in a line so as to strengthen adjacent parts, each
part may formed with a respective tongue 40 and groove
- 41.
Whilst the part 25 may be formed from any suitable
material, advantageously it is formed by moulding a
suitable foamed plastic such as polystyrene. In order
to speed up the moulding process, the part may include
scalloping 42 on the side not used in forming the ground
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WO90/15909 ~ PCT/GB90/009qO
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beam. Similarly, the central webbing support 35 is
split into two parallel supports 45,46 to reduce the
-;: moulding time and a further recessed portion 47
introduced between them. A further hollowed portion
(not shown) may be formed at the base between the two
supports 45, 46 to receive backfill and to provide a
weakened portion through which a road stake may be
driven.
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