Contraction Joints
The installation of Connolly key joint prior to concrete placement
is one of the best methods of providing a contraction joint.
Unlike tooled joints or saw cutting, which can be ineffective
if not done correctly or in the narrow timeframe required,
Connolly key joint makes a definite break in the slab and
virtually eliminates many of the problems associated with
concrete shrinkage.
Tooled joints, which are tooled in during the
finishing operation, are time consuming and interrupt the
finishing operations. In many cases they are not installed
to a sufficient depth (recommended as ¼ of slab thickness)
and as a result do not control all random cracking. If joints
are not of sufficient depth they cannot be guaranteed to induce
the fracture through the remaining thickness of the slab.
Also, if the joint is not of sufficient depth the induced
fracture may "wander" away from the vertical plane
and increase the likelihood of stepping at the joint when
it is subjected to vertical loads.
Saw cuts will control shrinkage
cracking if they are installed early enough in the curing
stage and they are to the minimum depth as mentioned above.
To be effective, saw cutting must be carried out before the
concrete starts to cool. If using a conventional wet saw,
this usually means that cutting must be done between 4 and
12 hours after the surface finishing operations have ceased.
The waiting period for early entry dry-cut saws is usually
between 1 and 4 hours.
If the saw cutting is not carried
out in this narrow window of opportunity, then it is likely
that it will not be effective in controlling cracking. In
many situations it is not practical to saw cut during this
timeframe due to saw availability, darkness, noise considerations
or other factors. If a slab is poured in the early morning,
it is generally too late to saw cut effectively the following
day.
