Note: Descriptions are shown in the official language in which they were submitted.
CA 02531705 2005-12-28
Dowel Assembly for Concrete SIabs
held of the invention
The present invention relates to a dowel assembly for concrete slabs on
ground.
s
)gackground of the invention
Newly laid concrete shrinks as it matures. Hardened concrete expands and
contracts with moisture and temperature changes. Theca volume oVangas can
prodLtoe
cracks in a concrete slab unless provxsior~s aze taken to allow for such
volume changes.
to Such provisions include control joints, which are straight grooves formed
in the concrete
slab surface to "control" the cracking. The groove depth can be 1/$'~'to the
full length of
the slab tl~xc~ass, as required. ?he control joints can be formed with a tool
during
concreto hardening or sawcut soon after the slab has hardened.
Control joints are pla,eed at regular intez~~als in th.e slab, typically at 6
to $ meter
~s regular intervals in both- longitudinal and tran~verso directions, thus
forming a grid (see
control joints 40 in slab 30 of Fig. 3). The resulting slab panels (42 in Fig.
3) between
control joints are kept as square as possible. The control joints provide a
zone of
weakness beeween slab panels where the forces which are pulling on the slab
panels will
relieve themselves. This substantially prevents the concrete cracking On itS
owz~, in an
Zo uncontrolled manner. Ideally, the concrete will ztot crack between the
joints, but such
unaesthetic cracks do happen.
Control joints typically include dowels to increase slab strength in the
jøint.
Such dowelled joints include stool dowels, embedded within the cor~ctote at
half the slab
depth, and extending across a cozttrol joint (perpendicular thereto) to
connect slab paa~.els
as on both sides of a control joint. The dowels are spaced at regular
intervals along the
length of the control joint, parallel to each other. The dowels impzove load
transfer across
the conixol joint and maintain the veztic3l alignment of adjaacnt slabs.
The dowels are typically steel round bars with a bond breaker (such as grease,
shrink wrap or plastic sleeve) applied to oz~e and. The dowels arc placed at
half tho
ao required slab height across the expected location of the control joint,
prior to pouring of
the concrete to from the slab. Hurimg hardening of the concrete or shortly
thereafter, the
control jozz~t is formod. ThE bond bzeakers at one end of the dc~vveis
larovide a debonding
effect to the Concrete of the slab panel at that end, whilst the other end of
the dowel is
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bonded to the concrete of the adjacent slab panel. This allows one end of the
dowels to be
movable within the slab panel, allowing expansion of the slab panels relative
to each
other without generating stresses within the slab panels.
Recently, square cross-section dowels, #lat bar dowels azxd plate dowels have
s also been used as they provide a greater load transfer capability than round
bar dowels.
The difficulty rx~ith, dowelled joints is that the dowels must be instahed in
a single
plane, aligned and parallel with each other if they are to work as intended.
Any
misaligned dowel oz a dowel which is eat parallel to the others along a
control joint will
usually result in s 'locked' joint and the slab panel will often crack between
the control
to joints along one end of the dowels. Generally, stresses in the slab paxaels
will be
concentrated between the unparallel dowels which results in undesirable
anaesthetic
cracks {see for example crack SS in Fig. 3b).
Several manufacturers are now providing dowels in pre-fabricated cradles or
cages to overcome the majority of the problems associated with installation.
Such dowel
is cradles/cages include a fabricated wire-frame which rests on the ground and
supports
dowels on one side and bond breakers (sleeves) at ttxe other side. The cradles
position
the dowels and sleeves at the correct half slab height and ensure that the
dowels remain
substantially parallel to each other at the correct dowel intervals. They are
hawever
expensive. rurthcr, the prefabricated dowel cradles do not allo~ov far
installation of the
zo cradles px~.or to the use of a 5omero Laser Screed machine as shown in Fig.
4 (from
So~r~ero Enterprises, www.somero.aozn).
Referring to Fig. 4, the Somero Laser screed machine 50 is a self propelled
four
wheel drive unit vvhicl~ has a telescoping boom 52 with a scxveding/compacting
head 54.
When forming a slab floor, concrete is dischargEd in strips fo match the width
of the
zs screeding/eompaciing head 54, up to a height approximately 1" higher than
the hr~al slab
thickness required. The Somero machine 50 moves info position and the
telescopic booze
52 is extended over the discharged concrete. The screeding/cornpacting head 54
is then
lowered to the slab thickness required as detennined by a laser-level
transmitter.
Retraction ofthe boom 52 causes the screed head 54 to be drawn across the
fresh concrete
~o which is leveled and compacted in a single pass. Once a pass is com~plated,
the machine
SU is moved to a position for forming the adjacent slab panel rwith some
overlap on the
previously screeded concrete.
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The dowel cradles mentioned have to be placed in a grid azzarlge,rnent
corresponding to the control joints to be formed. As these cradles include a
wire frame
for locating the dowels and sleeves at the correct height, a Sorr~sro machine
cannot be
driven oven these czadles without damaging them. It is thus necessary to
remove the
s cradles to allow the Somero machine to move onto the adjacent slab panel to
be fon~xed,
or to place the dowel cxadles only iznu~ediately pziar to forming of the slab
panel. This
signifleantly slows forming of the slab floor.
Further (referring to Fig_ 3b) such cradles are only available in 3m Iangths,
which requires the use of two dowel cradles 44 fez each coz~tral joint of a
6rra x 6zx~ slab
to panel 42. This allows the possibility that the dowels of cradles along the
same control
joint 40 will be misaligned and increases the chance of undesired cracks
forming in the
slab panr3l.
It is the abject of the present invention to substantially overcome or at
least
ameliorate one or more of the prior art disadvantages or at least provide a
useful
i s alternative.
Summary of the Invention
The present invention provides an assembly to be ac least partially embedded
within a concrete: slab, the assembly including:
io a fizst bar to be embedded within the concrete slab and located on a first
side of
a joint;
a second bar to be embedded within the concrete slab, the second bar extending
substantially parallel to and transversely spaced trom the first bar, the
second bar to be
located on said first side of said joint; and
zs a plurality of dowels extending substatitiahy parallel to and transversely
spaced
from each other, the dowels being aitaahed ~to the first and second bars so as
to be
substantially perpendicular thereto, the dowels being provided to extend
across said joint.
hi the preferred e~~nbodiment, the first and second bars are sub$tantially co~
extensive with each other. The dowels are ;also pzeferably substantially co-
extensive with
3o each other.
The dowels are preferably spaced at regular intervals along the first and
second
bars. Preferably, the dowels include a sleeve covering the dowel portions to
extend
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across said joint. The dowels are preferably attached to the first and seoond
longitudinal
bars by welding.
Each dowel preferably includes a first end and a second end and the first bar
is
attached adjacent the dov~rel first ends. Preferably, the distance between the
first and
s second bars is less than half the length of each dowel. The first and second
bars and the
dowels are preferably steel bars, of a round, square or rectangular cross-
section.
1n another aspect, the present invention provides a concrete slab having the
assembly t~f the above at least partza~lly embedded therein. The slab is
preferably a slab on
ground. 'fhe bars ~d dowels are preferably substantially parallel to a top
surface of said
)o concrete slab. The assembly is preferably embedded within the concrete at a
depth lower
than 40 mm from the top surface of the concrete slab, and bigher than 40 mm
from the
lower surface of the concrete slab. The assembly is preferably embedded at
half the
concrete slab thiclrness.
[s Brief flesc~p~tian of the Drawings
Preferred forms of the present invention will now be described by way of
examples only, with reference to the accompanying drawings, wlxexeiz~:
Fig. I is a perspective view of an assembly according to a preferred
embodiment
of the present invention;
xo Fig. 2 is a cross-section of a floor slab with the assembly eel' Fig. 1
embedded
therein, the cross-section being along line 2-2 of Fig. 3;
Fig. 3a is a schematic top view of pan of a slab panel having control joints
and
with assemblies according to Fig. 1 embedded therein;
Fig. 3b is a schematic top view of part of a slab panel having control joints
and
zs with prior art cradle assemblies embedded therein; ~d
Fig. 4 is a photograph of a commercially available Sor~aexo Laser Screed
machine.
betailed rleaerlption of tire 1'relk'erred Embodiments
3U fig. 1 shows an assembly 10 according to a prefer~d embodiment of the
present
invention. The assembly 10 includes a first bar 12, a second bar 14 and a
plurality of
dowels 16. The first and second bars 12 and 14 extend substantially parallel
to and
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CA 02531705 2005-12-28
transversely spaced from each other. The first and second bars 12 and 14 are
also
substantially co-extensive with each other-
The dowels 16 extend substantially parallel to and transversely spaced from
each
other. The dowels 16 are also substantially co-extensive with each other and
each dowel
s 1G includes a fZZSt ex~d 1.9 and a second ez~d 20- The dowels 1G are
attached to the i;lrst
and second bars 12 and 14 at regular distance intervals 21 by welding as
indicated at 17,
and are substantially perpendicular to the first and second bars 12 and 14, as
indicated at
18. The first bar 12 is attached adjacent the dowel first ends 19, such that
each dowel 16
extends ;from the fizst bar 12 and past the second bar 14. 'T'he distance 22
between the
~o first and second bars 12 and 14 is less than half the length 24 of each
dowel 1E.
The first and second bars 12 and 14 and dowels 16 are preferably steel bars,
of a
round, srluare or rectangular cross..section_
The use of the assembly 10 will now be described with reference to Figs. 2 and
3a. The assemblies 10 are placed an the ground prioz to °pouring of the
concrete at
is locations specified by engineers. The assemblies 10 are located such that
the first and
second bars 12 and 14 will be located on one side of a (to be formed) control
joint 40 and
substantially parallel thereto. The dowels 16 are located to extend across the
(to be
formed) control joint 40 and substantially perpendicular thereto. The dowel
second ends
30 are inserted into individual plastic sleeves 25 which will act as coxacrete
bond breakers.
zo E~.einforcement mesh 32 is then placed aver the assemblies 10 and the
assemblies
axe tied by wire to the reinforcement mesh 32. Workers and the Somero Laser
Screed
machine can then walk or dive veer the mesh 32 and assembly 10 without
substanti al
risl~ of damaging or moving the bars 12 and 14 and dowels 16 out co
aligzunent. Prior to
pouring a:f coztcxete for forming a slab section, the mesh and assembly 10 are
'chaired up'
zs (raised via spacers to (preferably) about half the slab depth) as one
Lulit. The actual height
the unit is chaired up is determined by engineering requzrexxaents, but is
generally at a
depth lower than 40 mm from the top surface of the concrete slab, and higher
than AO n~~m
from the lower surface of the concrete slab. Concrete is then poured and the
Somero
machine is used to screed the laid concrete to compact and level same as
described above.
3a In fiber reinforced concrete which does not include the reinforcement mesh
32, the
assemblies 10 are chaired by themselves prior to pouring of the concrete.
During hardening of the concrete, the bars I.2 and 14 and dowel first ends 19
are
bonded to the concrete of slab panel 42a whilst the sleeves 25 are bonded to
the slab panel
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42b. 'The slight shrinking of the concrete during hardeung z~~oves the sleeves
25 slightly
away from the bars 12 and 14. This creates a small space between the second
end 20 of
the dowels 16 and the closed end of the sleeves 25, which allows the dowels 16
to move
longitudinally inside sleeve 25. This allows expansion of the slab panels 42a
and 42b
s relative to each other without generating stresses within the slab panels
42a and 42b. The
sleeve 25 can be made of metal, plastics material or shxinlt wxap. The
assembly 10 will be
typically supplied with the sleeves 25 installed over the dowel second ends
20. During or
after hardening of the concrete, the control joint 40 xs ~Farcned.
The cross-section shape and dimensions of the bars 12 and 14 and dowels 16,
]o length 24 of dowels 16, distance 22 between the bans 12 and 14 and distance
21 between
the dowels 16 are determined by an engineer depending on the environment and
expected
leads of the floor slab to be Laid. The distances 21 and 22 can be set at any
from 10 mm
to five rrae~ers, and the dowel length 24 can be set at any from 10 mm to one
meter, as
specified by an engineer. The present invention allows flexibility to the
engineer's
l s requirements for any such distances 21 and 22 and dowel length 24.
The assembly 10 provides accurate dowel spacing and alignment and maintains
this accuracy even after being wallcEd on or driven over by a Somero machine.
The
dowels can be placed in their intended position in conventional or fiber
reinforced slabs,
prior to the use of the Somero machine. The Somero machizie can be driven over
the
zo assemblies 10 without causing damage andlor misalignment of the dovxrels
16. This,
decreases the ~~slc of cracking in the slab pa~.~:~ls due to dowel
~ixisalignxnent. The present
invention can be used with a Somcro machine or with conventional methods of
concrete
pouring aJ.ld f1I11Shlrig.
The assemblies 10 are typically stacked inside a rectangular steel frame,
which
2s allows for a greater mobility around the building site. The assemblies 10
are thus easier
to haa-~dl~ than the dowel cradles presently available.
The assembly 1 p k~as been primazily described above with reference to control
joints. The assembly 10 can however also be used for othex types ref concrete
joints, such
as stop work joints, expansion joints and edge joints-
3o Slop warlf joints are formed at an edge of a laid concrete slab. Edge
boards are
erected to support the laid concrete on such edges, and the assembly 1 U is
located such
that the bars 12 and 14 will be embedded in the laid concrete, with the
sleeved dowels 16
extending thxough l~Ql~s formed in the edge board. Once the concrete slab
hardens and
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the cdge board is removed, an adjacent slab embedding the sleeved dowels 16
can be laid.
An e~cpansion joint is similar to a stop work joint, with the addition of 2t
corxapressible
board material between the slabs. Moles are provided in the material for the
dowels 16 to
extend through.
s An edge joint is formed at an edge of a laid concrete slab adjacexrt a
structure,
such as a wall. Zn this case, hales are formed in the str<tet<tre for the
dowels 16 to be
inserted into prior to pouring concrete. The bars 12 and 14 are then embedded
in the laid
concrete.
The assemblies 10 can be fabricated in standard six (6) meter lengths but can
be
to varied to suit any required control joint and or saw cut specifications,
providing flexibility
to tailoring the length of the assembly 10 to each individual requirement. The
standard
six (6) zr~eter lengths also provides higher accuracy of dowels remaining
parallel over
longer distances, resulting in less craclting, where compared to the 3 meter
dowel cradles
described.
[ s The assembly I O is fabricated off site by welding the doweis 16 to the
bars 12
and 14 by a qualified welder to the required engineering dimensions. They are
then
transported to site for installation only, ~rrithout reqttiz~ng ;further
assembly, Welding of
the dowels to the bars 1~ and 14 substantially guarantees that the dowels 15
will remain
substantially perpendicular to the bars 12 and 1~4, and that the dowels 15
will remain
zo substantially parallel throughout ilea placement process and the pouring of
concrete. This
also lessens the chance of having unparallel dowel by human installation
error. Further,
efficiency on site is improved as no further assembly is required, thus
requiring less labor.
Accurate placement and installation of the dowels becomes easier.
as The preferred ernbodimex~t of tl~e present invention thus provides the
following
advantages.
~ Tinno saving on dowel installation and placement;
~ No far assembly required;
Significant cost savings in comparison to other prefabricated dowel
so cradle systems;
~ Somero machine can be driven over assezzxblies 10, which etumot be
done to other prefabricated dowel cradle systsn~s;
~ The assemblies 10 can be stacked;
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~ The assemblies 10 remain stable under concrete placement.
~ Easier handling of dowels an site leading try fewer injuries as less work
is required.
No need for wire cage as opposed to pae~abricated dowel cradle systems
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