Q.: My company does concrete sidewalk and driveway work. My crews put in joints that are deep enough and close enough together to control cracking, but occasionally we get a random crack. The owner always wants me to fix the crack. Is there any way to glue a cracked driveway back together so the repair isn't noticeable?
A.: You can blend epoxies to closely match the color of in-place concrete, then inject the material into the crack. But it's usually not worth the effort and expense to do this for a driveway. More often, a standard gray epoxy is used, cleanup isn't too good, and the repair is more obvious than the original crack.
For a slab on grade, a random crack isn't likely to affect slab performance any more than a planned joint would. The crack just looks bad. If you've placed the slab on a well-compacted base and used good quality concrete, it should still perform well. Sealing the crack will keep out water that might soften the subgrade, but the sealing material will also make the crack more noticeable.