Q.: We are having trouble with slabs cracking before we saw the control joints. Our procedure is to place the concrete slab, then come back the next morning to saw the joints. Before we start sawing, there are already some cracks in the slab. However, it's too early to start sawing the same day the concrete is placed, and we don't want to pay workers overtime to saw the joints in the middle of the night. The specifications say to saw the joints as soon as possible. Since the specifications don't tell us the exact time to saw the joints, we believe we've met the specifications. What do you think?

A.: We don't believe you have met the specifications, but there is a way to solve your problem. Remember, it's impossible for a specification to indicate an exact time to saw joints. Because of variations in weather and mix designs, the time to saw joints after concrete has been finished varies from around four hours in hot weather to as long as three days in cold weather. Engineers don't specify a time to saw joints because they don't know what time of year the concrete will be placed or even what time of day the contractor will decide to place the concrete.

Unfortunately, the time to saw joints is based on experience and trial and error. Experience with local weather conditions and mix designs provides a time window for sawcutting, but the exact time is still determined by trial and error. At the beginning of the time window for sawing, make a sawcut in the slab. If the edges of the sawcut ravel, dislodging aggregate, then it's too early to saw. Continue to saw at regular time intervals until the cut edges show only minimal raveling. When this happens, the slab is ready for sawcutting.

Sometimes concrete cracks before the saw can cut the joint without causing raveling. In these cases, consider using a lightweight saw that allows you to cut within a few hours after final concrete finishing. Using this equipment, you can place concrete in the morning, then cut the slab during regular working hours the same day. This eliminates costly overtime and minimizes the waiting time associated with the trial-and-error procedure used to determine sawcut timing.

Gasoline-powered saws with 8- or 10-inch-diameter blades are used for runways, pavements, and industrial floors. They feature automatic depth control and sawing speeds from 7 to 30 feet per minute. Sawcut depth is up to 13/8 inch and sawcut widths range from a 1/8-inch straight groove to a 1/2-inch T-cut. Electric-powered lightweight models use a smaller-diameter saw blade and cut notches 3/4- to 1-inch deep at sawing speeds of 4 to 11 feet per minute. For more information on these saws, see "Saw Cuts Concrete Immediately After Finishing," Concrete Construction, March 1988, pages 336-338.