Is the soniscope used to evaluate the strength of concrete or is it used to find out whether it has been weakened by cracking?
It is used for both purposes. The strength is evaluated by making pulse velocity measurements; high velocity means high strength. The instrument can also detect the presence of cracks. If a crack is appreciably wide, reasonably long, very roughly perpendicular to the path of the impulse and not filled with water, the signal will be interrupted and not received. If the crack is short, or steeply inclined with respect to the test path, the impulse will detour around it and a signal will be received. However, the velocity will appear to be lower than the velocity in uncracked concrete in the same vicinity because it has taken longer for the signal to travel the longer path.