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Joints
Users of floors are becoming increasingly aware of the many issues that need addressing at the time of the design and detailing of a new floor slab on ground. Joints are a major consideration and sometimes the need to minimise or eliminate formed joints may be more important than floor flatness or levelness, for example where floors are highly trafficked by heavily loaded pallet trucks with small solid rubber wheels.
To understand the use and positioning of joints it is
important to understand the basic factors that cause
concrete movements. Movements in concrete which can result in cracking if not
controlled can be categorised as follows:
Early movements prior to final set:
Plastic cracking caused through the concrete drying too rapidly.
Plastic cracks are discontinuous and random in nature, but
can lead to continuous shrinkage cracks at a later age. Plastic
cracking is controlled by protection of the slab surface from
rapid drying and/or the use of fibres in the concrete.
Movements commencing 14 hours after casting:
Differential temperature or differential moisture content
through the thickness of a slab can cause the slab to warp
(or curl). Heat of hydration effects, which can be significant
with slabs over say 200mm thick, will result in a positive
temperature gradient from the exposed surface of a slab to
the underside causing the slab to bow upwards at the ends.
A similar effect will occur as the top surface of a slab starts
to dry out after curing has ceased. The heating of pavement
surfaces from the sun will cause the ends to bow downwards.
Figure 1.1 Moisture warping of
slabs
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Warping can cause problems by effecting the flatness of the
floor and slabs can crack across an unsupported edge under
wheel loads. A permeable dry subgrade will reduce moisture
warping by allowing drying to take place from the bottom of
the slab as well as the top. Conversely pouring on a wet subgrade
or on polythene sheet may aggravate warping. Because moisture
warping causes upward curling at a free slab edge, the effect
of warping is apparent at every movement joint. |
Long term drying movements due to concrete shrinkage:
As the excess water which is used in concrete to give the
mix adequate workability evaporates out of the concrete, the
concrete shrinks. To this extent, the greater the amount of
water in the initial mix, the greater will be the drying shrinkage.
The designer can influence this by specifying structural concrete
mixes using appropriate compaction methods for low slump concrete
(e.g. vibrating screed). By comparison, a pumped concrete
mix with a higher slump and sand content could exhibit 50%
higher ultimate shrinkage. Vacuum dewatering reduces long-term
drying shrinkage further by reworking and compacting of the
slab after this excess water has been removed.
Coarse aggregate has a significant role in restraining concrete
shrinkage. Maximising the coarse aggregate size allows a lower
sand content for a given workability which lowers the water
demand. Low shrinkage concrete types are available in most
areas utilising high coarse aggregate volumes and natural
sands. Where joint openings need to be kept to a minimum,
to reduce wear from wheeled traffic for instance, concrete
for such slabs should be specified with a maximum 56-day drying
shrinkage limit of below 750µm (AS 1012.13).
Theoretically, shrinkage would not be a problem if there
was no restraint existing to prevent the concrete shortening.
In a floor slab we have the restraint of the subbase caused
by friction with the underside of the slab. If this friction
was uniform and limitless, shrinkage would not be a problem
either, as the stresses resulting would be transferred straight
into the ground and not taken by the concrete. However, neither
of these extremes is the real world. The concrete will crack
if the stress resulting from the restraint to shrinkage exceeds
the tensile strength of the concrete.
Typically, concrete shrinkage is accommodated by allowing
the slab to move freely at predetermined movement joints,
with allowance between the joints typically using reinforcing
steel to avoid uncontrolled cracking from stress buildup due
to a frictional restraint. Alternatively, jointless solutions
rely on relatively high levels of reinforcement to ensure
that cracks resulting from shrinkage restraint are very narrow
and at close centres (1 to 2m).
A typical concrete shrinkage of say 7mm in 10m length could
be reduced to around 4.5mm due to the restraint of the reinforcing.
For a 100 - 150mm thick slab drying outdoors, 50% of this
could be expected to take place in the first four months drying
and 70 - 90% after twelve months. A wet winter, however, will
slow the rate of drying and hence the initial shrinkage rate
significantly. Floors indoors are likely to shrink at a faster
rate, particularly if the environment is air conditioned.
Curing of concrete will not effect the shrinkage potential
to any extent, however it will optimise concrete strength
gain to resist cracking from shrinkage restraint.
There are a number of design options which cater for concrete
movement. The appropriate option will consider the client's
brief in conjunction with both construction costs and maintenance
costs.
A concrete floor slab has to be subdivided into small areas
for two reasons:
- To control tensile stresses due primarily to moisture
change and thermal contraction of the slab, and thus to
limit random cracking.
- For convenience during construction. The size of the area
may be governed by practical considerations arising from
the method of construction and resources available.
Joint Types
There are two primary types of joint used: free movement joints
and tied joints
1. Free movement joints
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Isolation Joints: These joints permit horizontal and vertical movement between
abutting elements, allowing the elements to behave independently
of each other. They should be provided between a pavement
panel and fixed parts of the building (such as columns, walls,
machinery bases, pits, etc).
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Figure
1.2 Free Isolation
Joint
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Isolation joints should also be provided at the junction
when an extension is being made to an existing pavement, and
at junctions between internal and external pavements, to prevent
the development of stresses that may result from differential
movements.
However, provision for load transfer may well be
required where such extensions occur and the designer needs
to address the detail necessary to achieve this. Isolation joints are generally formed by casting against
a compressible, preformed filler material (eg self-expanding
cork) over the full depth of the joint to provide a complete
separation.
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Expansion Joints: Expansion joints are used in pavements to provide for thermal
and moisture-induced movement of the slab. However, these
joints may also be required in areas or rooms subject to large
temperature fluctuations. Designers should satisfy themselves
that there is a definite need for expansion joints, thereby
minimising their unnecessary installation and the relatively
wide gap required between panels.
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Figure
1.3 Expansion Joint
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In many instances expansion
joints will not be required because the drying shrinkage is
the dominant linear movement. These joints within pavements
require the provision of load transfer, usually by the provision
of dowels fitted with a cap to accommodate the moving dowel
as the joint opens and closes.
Contraction Joints: Contraction
joints control the random drying shrinkage cracking
of concrete by inducing the slab to crack at the contraction
joints. They allow horizontal movement of the slab at
right angles to the joint and act to relieve stresses
which might otherwise cause random cracking. In order
to ensure that shrinkage cracking occurs at a contraction
joint, a plane of weakness must be created by forming
(using crack-inducing tapes or formers) or cutting a
groove to a depth of at least one-quarter of the slab
thickness. However, if the cut can be formed early enough,
by a suitable grooving tool or early-age saw cutting,
some reduction in the groove depth may be warranted.
The spacing of contraction joints in jointed unreinforced
pavements should be selected to suit the geometry of
the pavement being constructed, but should be such that
the joint movement does not mean that load transfer
by aggregate interlock is lost. If it is, load transfer
has to be maintained by dowels or other devices such
as sleeper beams. Otherwise the slab thickness should
be designed as a free edge. |
Figure
1.4 Free Contraction Joints (Induced)
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Contraction joints are usually constructed either by forming
a groove in the top of the freshly-placed concrete (Formed
Joint) or by sawing one in the panel after the concrete has
hardened but before uncontrolled cracking occurs (Sawn Joint).
Formed Joints are constructed after the concrete has hardened sufficiently
that it will not be damaged by the sawing, but before shrinkage
cracking can occur. The appropriate time for sawing varies
with the many conditions, eg concrete strength and ambient
temperature, that influence the hardening of concrete. The
initial saw cut should be 3 to 5mm in width. If required,
for the installation of a joint sealer, the joint can be widened
later.
Sawn Joints
are constructed after the concrete has hardened sufficiently
that it will not be damaged by the sawing, but before shrinkage
cracking can occur. The appropriate time for sawing varies
with the many conditions, eg concrete strength and ambient
temperature, that influence the hardening of concrete. The
initial saw cut should be 3 to 5mm in width. If required,
for the installation of a joint sealer, the joint can be widened
later.
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Tied Joints
Tied joints are used in two ways:
- to restrict the movements at the
joint in unreinforced pavements; and
- to provide relief for warping stresses
in reinforced pavements.
Keyed longitudinal joints should
be held together with deformed tie-bars. However, such
tie-bars should not be used in panels with a total width
of more than 10m unless dowelled longitudinal contraction
joints are also provided at a spacing not exceeding 10m.
The tie bar spacing relates to the overall design of the
slab between free joints. Typical bars and centres used
are D12 at 300mm for slabs up to 150mm thick, or D16 at
350mm for slabs over 150mm. |
Figure
1.5 Tied Construction Joints
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