It may go against your initial thinking, but sometimes you actually need to make cracks bigger to properly fill them. Crack chasing blades like the one pictured allow you to widen cracks in concrete and asphalt and taper the edges so you can fill them properly.
MK Diamond designed their V-segment MK-404DV crack chaser for dry cutting. Available in 4″, 4-1/4″, 5″, 7″, and 8″ sizes, these blades can be used in angle grinders with standard threaded or non-threaded arbors depending on the blade model. They run anywhere from $200 to $300 depending on the size.