The best way to prevent costly structural problems down the line is to crack down on fissures in concrete driveways, walkways, stairs, or the foundation when they're still small. Use a putty knife to fill hairline cracks with concrete patching compound or masonry crack filler. To rebuild cracked concrete on stair edges or corners, use wood planks to build a form for the desired shape of the concrete, then trowel premixed vinyl compound into the form. While severe foundation cracks call for professional help, you can fill foundation cracks less than one-quarter of an inch in depth on your own. Be sure to cut away the crumbling concrete with a chisel or sledgehammer, then moisten the crack with water, and fill it with a mixture of dry vinyl concrete patch powder and latex.