How to Do Concrete – Guide to Controlling Cracks – Slab on Grade Concrete – Do it Yourself Guide to Controlling Cracks (DIY)
Control Joints (Contraction Joints)
Control Joints (Contraction Joints) are formed, sawed, or tooled grooves which weaken the concrete so it cracks at the joint. These joints accommodate the movement caused by temperature changes and drying shrinkage.
Joint Spacing is Critical – As shown in the drawing above an un-jointed slab will usually develop a similar pattern of uncontrolled cracking cause by shrinkage. The patterns will change depending on the dimensions of the slab, (including depth), the psi of the concrete, curing, and the temperature extremes the slab is exposed to especially during the first few hours and days after the pour. If joints are spaced to far apart uncontrolled cracks can occur between the Control Joints.
Two rules that will help when pre-planning control joints.
- Rule #1 – Keep joints as square as possible.
- Rule #2 – In order to prevent intermediate cracking, space joints (in feet) no more than 2 – 3 times the slab thickness (in inches). Refer to the table below for the recommended joint spacing. The thicker the slab the farther apart these joints can be place without violating Rule #1.
Joint Depth is Critical – It is extremely important for the joints to be cut at least ¼ of the slab depth. This will weaken the slab along this joint line and let the slab crack at this controlled point. If the depth is not deep enough the crack may not follow the desired path along the joint. Refer to the table below for the recommended joint depth.