Cracks in sidewalks or driveways that are larger than hairline cracks must be enlarged before they can be satisfactorily repaired. Enlarge the crack along its entire length with a cold chisel and hammer (Fig. 1). Make the crack wider at the bottom than at the top (Fig. 2). This is known as undercutting. It helps to bond the new concrete with the older concrete. Undercut the crack to a minimum depth of 1″. The depth of the undercutting depends on the size and depth of the crack to be repaired. After the crack has been thoroughly undercut, remove all loose material and brush the area with a wire brush before filling it with a new concrete patch.