Joints
in House Slabs & AS2870
Many builders have been using Connolly Key Joint for a number
of years with great success in driveways and paths. Some have
recently expressed an interest in having key joints included
in their house slab designs to prevent cracking of floor tiles.
Thickening beams in slabs can restrain the slab
from shrinking and result in cracks occurring parallel and
adjacent to the beams. Some builders have decided to use Connolly
key joints in their slabs to avoid these problems. With careful
design, key joints can be placed strategically in slabs, usually
adjacent to walls, where they will not interfere with brittle
floor coverings and many of the problems associated with concrete
shrinkage are avoided.
Although AS2870 does not include any joints
in the standard designs that it contains, Clause 3.1.1 (c)
states that the standard designs "shall not apply to
slabs containing permanent joints, e.g. contraction or control
joints". So in fact AS2870 does not preclude the use
of joints in slabs it simply does not apply to slabs that
contain these joints.
Furthermore, a slab designed with joints to
control shrinkage cracking is considered to be good engineering
practice in other slab applications so why should it not be
considered good practice for house slabs. Clause 1.1 of AS2870
states that the "Standard shall not be used to prevent
the use of locally proven designs, or alternative designs
in accordance with engineering principles". This clause
recognises the fact that other designs, including those with
contraction joints, are acceptable if designed correctly.
While some engineers are reluctant to
incorporate joints into slabs for fear of penetration by termites,
past research shows that only 4.7% of termite entry points
are through joints as opposed to 20.2% through cracks in the
slab. With the majority of houses now using the slab as a
physical termite barrier, all proprietary barrier systems
have installation methods to protect key joints. Surely it
is far better to have a protected joint than an unprotected
crack in a slab.
Disclaimer
The information contained in this manual has been provided
by Connolly Key Joint Pty Ltd as a part of its commitment
to help designers and contractors produce high quality trouble
free slabs and pavements. As the information is largely of
a general nature and there are many variables associated with
individual site requirements it is not intended to replace
professional engineering advice. Please consult a structural
engineer or other professional consultant for design details
and advice specific to your project.
