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Joint free slabs: specification
PREAMBLE
Joint free slabs on grade refers to slabs constructed
without the traditional control and Isolation joints.
The slab is cast over a crack inducer grid with the
intention of inducing fine cracks at close centres in
order to distribute setting and drying shrinkage as
evenly as possible throughout the entire area of the
slab. Nominal reinforcement placed over the grid resists
the opening of the cracks.
The novelty of this method of constructing slabs on
grade is the subject of Australian Patent No New Zealand Patent 523811,
and the intellectual property is held by Joint Free
Slabs Pty Ltd. Joint Free Slabs Pty Ltd is the only
legal supplier of the crack inducer grid, and the cost
of the grid supplied to any project includes a license
fee to use the system on that particular project.
SECTION 1: SCOPE
This specification provides guidance on the preparation
works and the construction of the joint free slab. The
main purpose of this specification is to provide the
necessary guidance on design and construction of joint
free slabs, and to focus attention on issues of good
building practice that can have a critical affect on
the successful performance of the slabs.
All concrete work including but not limited to formwork,
reinforcement supply and fixing, and concrete supply,
placing, sampling, testing, curing and protection, shall
be in accordance with the NZS 3109 Concrete Specification
unless specifically varied by this specification or
the drawings.
SECTION 2: SUB-GRADE PREPARATION
2.1 Bulk Earthworks
The site preparation, bulk excavation, placing and compaction
of fill, and trimming to level shall be in accordance
with the civil earthworks specification where such exists,
and shall comply also with the following clauses. Any
discrepancies shall be referred to the structural consultant
for written instruction. Strip the site of all topsoil,
organic matter and deleterious material.
All earthworks and testing of earthworks shall be completed
in accordance with NZ Standard 4431 Guidelines on Earthworks
for Commercial and Residential Developments.
2.2 Proof Rolling
Proof roll using equipment recommended by the geotechnical
consultant, and remove any soft spots. Make good with
approved low plasticity cohesive fill material compacted
to 98% SRDD.
2.3 Bulk Filling
Raise the building platform to the underside of the
slab if necessary with approved low plasticity cohesive
fill compacted to 98% SRDD, or as otherwise specified
by the geotechnical consultant. The surface shall preferably
be irregular for maximum frictional resistance to shrinkage
of the concrete, but shall not be such that it causes
damage to the membrane.
2.4 Sand Blinding
Do not use a blinding sand layer. Trim the surface such
that the specified slab thickness will be achieved with
a tolerance of +/- 10mm.
2.5 Service Trenches
All service trenches in the sub-floor region shall be
backfilled with fully consolidated granular material
or stabilised material such that there is no appreciable
long term differential settlement between the backfill
and the general sub-grade. The backfill must also comply
with the requirements of the services consultant and
the relevant authority.
2.6 Vapour Barrier
Interior Slabs - Provide a 100 micron (unless noted
otherwise on the drawings) polyethylene moisture barrier
throughout as an underlay to all interior slabs, lapped
200 and taped at all joints and at all penetrations.
The membrane shall be loosely fitted to all ground restraints,
thickenings, setdowns, and other depressions in the
surface of the sub-grade such that the concrete completely
fills such thickenings and depressions and makes continuous
contact with the ground surface.
Exterior Slabs - A moisture barrier underlay is not
required under exterior slabs unless specifically noted
otherwise on the drawings.
2.7 Protection of Natural Surface
Install a fully compacted base course of approved road
base material where the exposed surface of any natural
material exposed in the building platform will be adversely
affected by the elements.
2.8 Wetting Down Prior to Placing
Concrete
Where no moisture barrier underlay is to be installed,
moisten the surface of the sub-grade immediately before
placing concrete sufficiently to prevent absorption
of water from freshly placed concrete but not to the
extent that it makes the surface untrafficable.
2.9 Ground Restraints
Install ground restraints at maximum 15m centres in
two directions approximately at right angles. Ground
restraints shall be a minimum 300mm wide x 200mm deep
from underside of slab, and shall be left rough on excavation.
Throughout, however, the prepared surface of the sub-grade
shall be such that it does not cause damage to the vapour
barrier.
Provide a ground restraint to the full perimeter of
a joint free slab, and provide a ground restraint at
all construction joints. A perimeter ground restraint
may be omitted wherever the slab edge is continuously
tied into tilt-up panels, footings, retaining walls
or the like in such a manner that there is full restraint
to in line and transverse shrinkage movement of the
slab.
Remove all loose material from the excavated ground
restraints.
SECTION 3: CRACK INDUCER GRID
3.1 Supply
All components of the crack inducer grid shall be obtained
from Joint Free Slabs Pty Ltd. A written order is required
to initiate any supply, and a minimum 14 days shall
be allowed for delivery to site after a written order
is received by Joint Free Slabs Pty Ltd.
3.2 Size of Crack Inducer Tube
The overall dimension of the crack inducer tube shall
be varied to suit the design thickness of the slab.
For slabs reinforced with steel wire mesh, the overall
height of the crack inducer tube shall be approximately
25% of the slab thickness.
3.3 Assembly
All on site labour and equipment required to take delivery
of, assemble and install the crack inducer grid shall
be provided by the builder.
The crack inducer tubes are supplied to site cut to
length suitable for assembly in a 1m x 1m square grid.
The grid is to be assembled dry (without adhesive) by
pressing the tubes firmly into or onto the connectors
as appropriate. When assembled the connectors are to
be at 1m centres in both directions with a tolerance
of +/- 5mm. The centre-to-centre dimensions are to be
regularly checked when assembling and installing the
grid to ensure there is no creep. If necessary adjustments
are to be made in the way the grid is assembled to ensure
the 1m grid dimension is maintained.
3.4 Installation
The assembled crack inducer grid is to be laid directly
on the moisture barrier or the sub-grade, with the base
of each connector and the underside of each tube in
contact with the membrane or the sub-grade as appropriate.
Connectors shall not generally be located to coincide
with ground restraints and the job shall be set out
accordingly. Adhere to any setout dimensions provided
on the drawings. All connectors shall be installed with
the blank base facing downwards.
All crack inducer grids shall be installed with the
tubes aligned in straight lines in both directions so
that each connector can be used as a bar chair to support
the steel mesh reinforcement.
Crack inducer tubes around the outermost perimeter of
a joint free slab shall be cut on site as and if necessary
so that they terminate no less than 100mm and no more
than 500mm clear of any visible vertical face.
Reasonable care shall be taken during the construction
to ensure the crack inducer grid is not damaged, dislodged
or displaced. Any damaged components shall be replaced,
and any displaced components shall be re-positioned.
SECTION 4: REINFORCEMENT
4.1 Reinforcement types
Joint free slabs shall be constructed with reinforcement
throughout. General reinforcement shall be in the form
of hard drawn wire mesh. All trimmer bars shall be Grade
500Y deformed bars or as otherwise specified on the
drawings. All reinforcement bars shall comply with NZS
3402 1989, and all reinforcement mesh shall comply with
NZS 3402.
4.2 Cover
The minimum top and edge cover to reinforcement shall
be 30mm in applications where the slabs are not exposed
to the elements, and 40mm in applications where slabs
are permanently exposed to the elements.
4.3 Support
All mesh reinforcement shall be supported on the crack
inducer grid junctions at 1m spacing in both directions,
except around the perimeter where standard plastic bar
chairs shall be used. Special attention shall be paid
to the support of mesh and trimmer bars at re-entrant
corners and penetrations. Install conventional bar chairs
as necessary. Do not allow the reinforcement to lay
on the ground.
4.4 Laps
All reinforcement mesh shall be placed with rows of
sheets lapped end to end. Successive rows of sheets
shall be staggered by one half of a sheet length so
that not more than three sheets are lapped at any one
location. All side laps shall be nested to minimise
the bulking up of the laps, whereas end laps shall not
be nested. Nominal 200mm side laps and end laps shall
be used so that a 200mm square grid of bars is maintained
throughout. This is required so that the mesh can be
supported on each and every grid connector. Bar reinforcement
shall be lapped a full tensile development length where
required.
4.5 Trimmer Bars
Trimmer bars shall be installed around the perimeter
of all joint free slabs and along the edge of all pour
breaks as detailed. Diagonal trimmer bars shall be installed
at the corners of all penetrations and at all re-entrant
corners. A minimum HD12 bars shall be installed around
the perimeter.
SECTION 5: CONCRETE SUPPLY AND
PLACING
The objective with joint free slabs is to place quality
concrete with low shrinkage characteristics, and with
a hardness and surface finish that suits the proposed
in-service conditions. An increase in concrete compressive
strength usually results in increased shrinkage, hence
concrete compressive strength should not be over specified.
Steps must also be taken to minimise water loss after
placing concrete.
5.1 Compressive Strength
The concrete is to be of a compressive strength that
suits the purpose and that satisfies the durability
requirements of the client andthe NZ Building Code.
5.2 Slump
The design slump shall be 80mm.
5.3 Drying Shrinkage
The design 56 day drying shrinkage shall be 600 microstrain
for slabs which are to be covered with applied finishes.
This shall be reduced to 450 microstrain for slabs which
have no applied finishes and for slabs where the applied
finish is an epoxy paint.
5.4 Mix Design
The concrete mix design is to be carried out by the
concrete supply company using readily available local
aggregates. Details of the mix design are to be provided
to the design engineer for review, and a copy of the
final mix design is to be provided to Joint Free Slabs
Pty Ltd for their records.
5.5 Admixtures
Water reducing admixtures may be used to improve workability
provided they have negligible effect on the shrinkage
characteristics of the concrete, and on the durability
of the surface.
5.6 Addition of Water
No water shall be added to the concrete after the truck
has left the batching plant. Any concrete that has had
water added shall be rejected and the truck ordered
to leave the site.
5.7 Placing and Compaction
All concrete shall be placed in such a manner as to
avoid segregation or loss of materials, and all concrete
shall be thoroughly compacted by means of mechanical
vibration. See also NZS 3109.
5.8 Hot Weather Placing
In hot and/or windy conditions, evaporation of water
from the surface of a freshly placed slab shall be minimised
by the use of aliphatic alcohol sprayed onto the surface.
5.9 Pour Size
The size of a concrete pour shall be limited only by
the availability of supply, and the capacity of the
contractor to place and finish the concrete. Daily pour
sizes of 1,500 to 2,000 square metres are readily achievable.
Any partial pour of a floor slab or pavement shall be
restrained around its full perimeter with ground restraints
or physical attachment to other building elements. All
construction joints shall be formed as detailed on the
drawings.
5.10 Preparation and Detail at Construction
Breaks
Before concrete is placed against hardened concrete,
surface shall be cleaned of all laitance, dust and loose
material, and shall be wetted down immediately prior
to placing concrete. A smooth vertical surface at construction
joints is to be avoided, and the mesh reinforcement
is to continue through the joint and be fully lapped
with the mesh in the subsequent pour.
5.11 Compressible Fillers
Compressible jointing is not generally required at internal
columns and walls, as these elements help restrain the
shrinkage movement of the slabs. Refer to the details
on the drawings for special cases.
SECTION 6: CONCRETE CURING
The exposed surfaces of all joint free slabs on grade
shall be cured for seven days after the placing of concrete.
Except in extreme adverse weather conditions, curing
components may be applied the morning after the pour.
SECTION 7: APPLIED FINISHES
Joint free slabs are suitable for applied finishes
such as vinyl tiles, sheet vinyl, carpet, ceramic tiles,
terrazzo, epoxy concrete topping and the like provided
correct procedures are adopted in the preparation of
the surface and the fixing or application of the materials.
7.1 Vinyl Tiles
Prior to the fixing of vinyl tiles, the concrete surface
shall be thoroughly cleaned of all dust and foreign
material, and any irregularities in the surface ground
flush and smooth. Pressure sensitive adhesive shall
be used for fixing the tiles and this shall be spread
using a notched spreader of a type and size recommended
by the adhesive manufacturer. The tiles may be placed
continuously over all induced cracks in the joint free
slab.
7.2 Sheet Vinyl
Prior to fixing sheet vinyl, the concrete surface shall
be thoroughly cleaned of all dust and foreign material,
and any irregularities in the surface ground flush and
smooth. Acrylic adhesive shall be used for fixing the
vinyl and this shall be spread using a notched spreader
of a type and size recommended by the adhesive manufacturer.
The sheet vinyl shall be placed continuously over all
induced cracks in the joint free slab.
7.3 Carpet
Prior to the fixing of carpet, the concrete surface
shall be thoroughly cleaned of all dust and foreign
material, and any irregularities in the surface ground
flush and smooth. The carpet shall be placed continuously
over all induced cracks in the joint free slab and fixed
in the normal manner for the particular carpet.
7.4 Ceramic Tiles
Prior to fixing ceramic tiles, the concrete surface
shall be thoroughly cleaned of all dust and foreign
material, and any irregularities in the surface ground
flush and smooth. All relevant requirements of AS3958.1-1991
Ceramic tiles - Guide to the installation of ceramic
tiles shall be complied with.
Ceramic tiles shall not be laid on freshly placed concrete.
A quality flexible adhesive shall be used for fixing
all ceramic tiles. The adhesive shall be spread using
a 10mm x 10mm or 12mm x 12mm square notched trowel,
and the tiles fully bedded such that there are no voids.
The finished thickness of the adhesive shall be no less
than 3mm, and tiles shall be removed from time to time
so that the thickness of the adhesive can be check measured.
The ceramic tiles shall be laid continuously over all
induced cracks in the joint free slab, however expansion
joints shall be installed at centres recommended by
the tile manufacturer for the particular tile being
laid.
7.5 Terrazzo
Insitu terrazzo shall be placed on a minimum 50mm thick
bed of sand and cement reinforced with galvanised chicken
wire. The sand and cement bed shall be separated from
the surface of the joint free slab by means of a 200
micron virgin polyethylene membrane or a suitable bituminous
membrane so that there is no bond.
Prior to installing the bond breaker, all irregularities
in the slab surface shall be ground or treated so that
the surface to receive the bond breaker is flat and
smooth.
Concrete control joints shall be installed in the terrazzo as
required to control shrinkage of the terrazzo itself
and the bedding material, and to achieve the desired
surface pattern.
7.6 Epoxy Concrete Topping
Prior to placing epoxy concrete topping, all irregularities
in the slab surface shall be ground or treated to produce
a flat and smooth slab surface.
Install a 200 micron virgin polyethylene membrane or
a suitable bituminous membrane so that there is no bond
between the epoxy concrete topping and the slab surface.
Install the epoxy concrete topping continuously over
the induced cracks in the joint free slab, however install
control joints in the epoxy concrete topping as and
if recommended by the product manufacturer to control
shrinkage or thermal movement of the material.
SECTION 8: STRUCTURAL DESIGN &
CERTIFICATION
8.1 Consulting Engineer
All joint free slabs on grade shall be designed by an
experienced and competent practising structural engineer
in accordance with all of the requirements of this specification.
All designs and construction details shall be reviewed
by Joint Free Slabs’ consulting engineer prior
to commencement of construction.
8.2 Design loads
All slabs shall be designed for all applied dead loads
and the minimum design live load specified in AS1170.1
unless a heavier design live load is specified in the
client design brief, in which case the heavier design
live load shall be adopted.
The builder shall take every necessary care to ensure
the slabs are not overloaded or damaged in any way during
construction. Any damaged sections of slab shall be
reinstated to the detail provided by the consulting
engineer.
8.3 Construction Phase Inspections
The design engineer or his representative shall attend
to periodic site inspections of the joint free slab
on grade on at least the following occasions:
o upon preparation of the sub-grade, including the excavation
of ground restraints, prior to placing the plastic vapour
barrier for at least the first slab pour.
o upon installation of the crack inducer grid and all
reinforcement, but prior to the placing of concrete
for every slab pour
o during the placing and finishing of the concrete in
at least the first slab pour.
Any representative of the design engineer who attends
to periodic site inspections on his or her behalf, shall
also be a suitably qualified and experienced practising
structural engineer.
Prior to departing the site after each inspection the
inspecting engineer shall provide to the builder a hand
written inspection report noting the scope of the inspection,
the extent and detail of any works outstanding at the
time, and approval or otherwise to place concrete. Concrete
shall not be placed unless such inspection report is
received.
8.4 Structural Certification
Upon completion of the slab, the inspecting engineer
shall prepare a certificate for submission to council
stating that the slabs have been inspected during construction,
and that they have been constructed generally in accordance
with the intent of the structural design and documentation.
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