|
Joint free slabs: guideline
This guideline has been prepared to provide users of
the Joint Free Slab (JFS)
system a basic understanding of the system, and to provide
guidance for the preparation and installation of the
system. This Guideline and the accompanying Construction
Checklist should be made available to all site personnel
that are involved in the construction of a joint free
slab.
PREAMBLE
A joint free slab on grade is very similar in effect
to a conventional slab on grade except that the shrinkage
is dissipated as evenly as possible throughout the length
and breadth of the slab instead of being concentrated
at formed and sawn control joints. A crack inducer grid
is installed in the slab to promote a 1m grid of fine
cracks in the long term, and thereby avoid the large
shrinkage movements that are designed to take place
at traditional formed and sawn control joints.
As with a conventional slab on grade, the quality of
the site preparation, and the proper placing, compaction
and curing of the concrete is fundamental to a satisfactory
end result. There should be no compromise on any of
these aspects of the construction.
Site Preparation
The building platform should be of uniform compaction
with a firm surface. All topsoil must be removed and
any soft zones should also be removed upon proof rolling,
and made good with fully compacted select fill.
The JFS design incorporates a perimeter and an internal
grid of ground restraints (thickenings). The primary
function of these is to provide resistance in the form
of continuous restraint to shrinkage movement. Ground
restraints are intended to provide resistance in the
longitudinal as well as the transverse direction. Irregularities
in the excavation of the ground restraints are an advantage
in this regard.
It is essential that the concrete is able to completely
fill all surface deformations and all ground restraints,
and hence it is essential that the membrane is loosely
fitted to all ground restraints, and it is most desirable
that any membrane is of light duty.
All loose material on the building platform and particularly
in the excavated ground restraints should be removed.
Sand blinding is not required and is not considered
desirable. If the surface of the building platform is
to be raised slightly, compacted crusher dust or the
like is recommended.
The height of the crack inducer tubes is ideally 25-30%
of the slab thickness, hence thicker slabs require larger
crack inducers. It is very important therefore to construct
slabs without significant variations in the thickness.
A tolerance of +/-10mm on the specified slab thickness
is recommended.
Compressible separation between the slab edges and walls
and columns is not generally required.
Assembly and Installation of the
Crack Inducer Grid
The JFS crack inducer grid is a one metre grid of
extruded plastic tubes connected using four way injection
moulded plastic junctions. The tubes are pre-cut to
a length that provides a one metre grid when assembled.
The junctions have a dual role – they connect
the tubes and hold them in position, and they provide
support for the reinforcement. The junctions generally
replace all conventional bar chairs except those around
the perimeter, and around penetrations.
Note that the flat base of the junctions is to sit on
the ground. Do not install the junctions upside down.
The crack inducer tubes could be thought of as saw-cuts
installed in the underside of the slab prior to pouring
the concrete. They work in the same manner as saw cuts.
The JFS crack inducer grid is assembled dry (i.e. no
adhesives or the like are required) and the tubes should
fit snugly onto the junctions. The tubes must be pushed
home onto the junctions to ensure that the one metre
grid dimension does not grow.
Set out dimensions may be provided on the plan for the
set-out of the grid in both directions. The dimensions
will be referenced to some readily identifiable feature
such as a column centre line, and when given these are
simply to provide guidance on the positioning of the
grid such that junctions are generally located clear
of all ground restraints, and such that the need to
cut tubes to length around the perimeter of the slab
can be generally avoided.
When set-out dimensions have not been provided, install
the grid such that these objectives are achieved.
It is recommended that the grid be assembled with the
help of a string line in both directions to maintain
true and straight lines at right angles. This is necessary
if the junctions are to provide the intended support
for the reinforcement mesh.
The ends of the crack inducer tubes should not be visible
around the formed edge of the slab. They are generally
terminated 100 to 400 mm short of the edge form. The
crack inducer tubes can be cut to length if necessary
but this should not generally be necessary if the job
is properly set out.
The installation of the JFS grid is very simple and
very fast. An assembly team of two labourers should
be able to install 1,500 to 2,000 square metres of grid
per day.
Reinforcement
The reinforcement mesh in joint
free slabs is typically lighter than in conventional
slabs on grade, and it is supported on the JFS junctions.
Specific mesh lap details are provided on the drawings,
and it should be noted that the side lap of the mesh
sheets is varied to maintain the 200 spacing of the
bars throughout. Trimmer details are also provided on
the drawing and these must be strictly followed. All
slab perimeters in each pour and all penetrations and
re-entrant corners are to be trimmed using HD12 bars
as detailed.
It is generally possible to install most sheets of mesh
without the need to cut them, hence steel fixing time
should be reduced. The placing and tying of the reinforcement
is a simpler and quicker task than in conventional slabs
on grade and can readily be undertaken using unskilled
labour.
Sheets of mesh are not always welded with the bars square
to each other. Each sheet of mesh should be supported
on the JFS grid junctions and any discrepancies taken
up in the side and end laps. This will minimise the
need for conventional bar chairs. Bar chairs will normally
be required however around the outer perimeter of the
slab and around all penetrations where the mesh has
been cut.
Concrete
The concrete grade should suit the purpose of the
slab. The concrete in Joint Free Slabs should also have
a target 56 day drying shrinkage of 600 microstrain
maximum.
In most slabs that have no applied coverings such as
vinyl or ceramic tiles, a shrinkage-reducing admixture
may also be specified so that the best long-term aesthetic
results are achieved.
In order to properly control the quality of concrete
installed in any project, the addition of water after
a truck leaves the batching plant should be prohibited.
Added water will have an adverse affect on the shrinkage,
the strength and the durability of the concrete surface.
All concrete should be compacted using mechanical vibrators,
and all concrete should be cured for seven days after
placing using a method or a curing compound that complies
with the Australian Standard. In adverse weather conditions
(hot and windy days) the use of aliphatic alcohol is
recommended to prevent premature drying of the surface
prior to the application of a curing compound.
Refer to the Joint Free Slabs
specification
for more comprehensive information on the construction
of joint free slabs and the application of surface finishes.
|
