The basic types of joints are: construction joints (occur between concrete lifts of slabs in which one part is allowed to harden before the next is placed); contraction joints (used to permit shrinkage without random cracks being formed); connection detail joints (between structural members); and isolation joints (to prevent random cracking of slabs of different thicknesses or shapes). Use an isolation joint between concrete slabs of different shapes or thicknesses and around all columns, footings and peripheries of buildings. The use of skewed joints to reduce impact at the joint should be considered.
To place the joints, mark the exact location of all joints on plans and indicate the means of forming them. Place joints at the inside point of V-shaped rustication strips and at the top edge of the inner face of rectangular rustication strips. Joints should be exactly horizontal or vertical in walls. Waterstops should be used for joints subject to hydrostatic pressure.
For fillers, clean all joints before inserting joint filler materials. Don't allow workers to handle hot lead for joints without first instruction them in its proper use and handling. Fill sawed joints immediately to prevent debris from becoming lodged in them.
The time to saw a joint is crucial. If sawed too early, the concrete will ravel badly at the joint; if sawed to late, blade wear will be excessive and/ or random cracks might form. The time to saw joints will vary from 4 to 12 hours after casting. Concrete cast in the morning should be sawed the same afternoon, but evening concrete may be sawed the next morning. If sawing is going to be postponed, use relief joints ever 60 to 100 feet.
Good construction practices include: using a mix with the amount of coarse aggregate reduced by one half for the first 5 inches of a lift at construction joints; positioning formed joint material firmly in place; and before placing the top layer of the concrete at a construction joint, always clean the top surface and provide a cushion of mortar.