Timing is critical for saw-cut joints; cutting too soon spoils the concrete while cutting too late fails to fulfill the purpose. Typically concrete joints should be cut in less than a day, but the specific timing depends on numerous factors, including local conditions, weather and concrete mix.
Concrete Control Joints
Concrete can crack from thermal expansion and contraction caused by soil movement. Extensive cracking can lead to the deterioration of concrete, but cracking is unavoidable. Control joints in concrete slabs control the location and extent of cracking. They're placed at regular intervals in pavements. Some control joints are dummy joints, placed to balance the specified joints aesthetically. Control joints are tooled or sawn to a depth of about one-third the depth of the concrete.
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Timing for Cutting Concrete
Cutting the concrete has to be timed so that the concrete is set enough that sawing won't damage it, but not so long that the concrete cracks either before it's sawn or just ahead of the saw. The timing varies according to local conditions, but typically saw cuts can be done 4 to 12 hours after the concrete has been finished. In hot weather early-entry dry-cut joints are cut 1 to 4 hours after finishing; they're not cut as deeply as conventional cuts.
Early Concrete Cutting
If the saw cuts are made too soon, the joint edges may ravel. A test area can be set aside or a scratch test conducted first. If the scratch affects the surface texture, it's too early to cut. A green concrete saw blade may avoid tearing the concrete and permit cutting earlier than the usual diamond blade; it is lightweight with a special skid plate, although the green blade wears out three times as fast as the diamond type.
Concrete Cutting Tips
Cutting too early can mark the pavement surface as well as cause joint raveling. Late sawing can result in random concrete cracks. Shallow cracks aren't sufficient to prevent uncontrolled cracking while deep cuts are excessively labor intensive and undermine the aggregate interlock in the concrete. Once cutting starts, it has to continue until the joints are finished and before the concrete surface temperature begins to fall. Stop the saw cut 1/2 inch before the edge of the pavement.