We placed a 46x90-foot concrete floor slab that's 5 inches thick. The plans called for sawed control joints at 15- to 18-foot spacings. The slab was placed during cold weather so we used heated mixing water and 2% calcium chloride in the concrete. Concrete temperature at delivery was 60 F and the air temperature was 35 F. We finished the pour yesterday at 5 p.m. and covered the slab with plastic sheeting and 2-inch-thick insulated pads. The temperature stayed at or slightly below freezing last night and today it's still in that range with sleet and snow forecast. That means we won't be able to saw joints until tomorrow or even later and I'm concerned about random cracking. What do you suggest doing?
Under the conditions you had to work with, random cracking is likely to occur despite your best efforts. The cold weather is working against you in two ways. Concrete gains strength more slowly in cold weather. Thus you have to wait longer before cutting so that joint edges don't ravel. And as the slab cools it wants to contract. This restrained movement causes tensile stresses in the concrete. Because of these stresses, the concrete may crack before you can saw it. On the other hand, the cold weather may work for you. Because concrete gains strength slowly in cold weather, it's not as stiff and may creep more under the tensile stress. We suggest getting on the slab as soon as possible to see if you can saw without excessive raveling. If it's possible, saw the joint that's nearest the middle of the 90-foot length. Sawing contractor Rick Younger advises that you use a diamond blade and cut the joint at least 2 inches deep. This will divide the slab into two nearly square sections and may also release some of the stress that could cause random cracking. Cut the remaining joints as soon as weather permits.