Q: How do I cut contraction joints across a wide slab of fresh concrete without using a bridge?

We do outdoor flatwork and sometimes in hot weather the concrete cracks within a matter of hours after finishing. To avoid this trouble we want to form control joints immediately after screeding and bull floating. Walking groovers with long handles don't seem to be the answer because they dig in and are hard to manage at a distance.

A.: One contracting company, Northern Illinois Concrete and Paving Inc., of Sandwich, Illinois, devised its own tool, shown in the accompanying picture. It's a modified bull float that has a blade just off center on the bottom surface. This particular blade can't be set at the center because its simple clamp won't accommodate the handle, but the company says it works fine in an off-center position.The company bull floats the slab, then uses this tool to cut the fresh concrete, then bull floats again. They establish the joint line by snapping a string. They feel that the accuracy of locating the string line is more critical for driveways than for parking lots.

When cutting across any joint previously cut at right angles, the operator has to be careful not to leave a ball of concrete in the older joint. The whole operation is said to require a good eye and a good feel for the use of the modified bull float.The modern recommendation for depth of cut is 1/4 the thickness of the slab. This tool, however, has a bit that is only about 7/8 to 1 inch deep. It has nevertheless been successful in controlling cracking in slabs both 4 inches and 5 inches thick, perhaps in part because the cut is made so early. It is thought that with a deeper bit it would be hard to move coarse aggregate particles out of the joint when the bit is in a location some distance from the operator. The company says that for slabs 6 inches thick or more they rely chiefly on sawing.

At least one tool supplier (Kraft Tool Company, 1901 Wyandotte, Kansas City, Missouri 94108) now offers a groover attachment for a Fresno trowel that looks much like the attachment in the picture.

Northern Illinois Concrete Paving used the tool shown here to joint a 4-inch-thick concrete overlay on an old 3-acre blacktop lot. Joints were located at 10-foot intervals in both directions. After one year there is reported to be only one random crack in the three acres.When planning joint locations to control cracking, with this tool or any other, it's wise to divide the slab into areas that are as nearly square as possible.