Happens in Batman (1940) #689.
This Clayface already has a few awesome feats of overpowering heroes much stronger than he is (well, mainly Wonder Woman but that's probably because of his powers) as well as lifting and throwing trucks with apparent ease.
This is just as good to confirm his strength on that front.
Using the average window length of 1.5 meters and the octahedron volume calculator...
As well as the density of clay (1,746 kilogram per cubic meter) and concrete (2,400 kilograms per cubic metre), since it's not clear if these rocks he's throwing are pieces of himself or chunks he's ripped off the building he's in.
Barely over Class 10, but easily Class 25 given how consistently he's chucking these projectiles.
Class 25, in case there was any doubt. Funny that both Post-Crisis Croc after his Hush virus and possibly Bane with his Venom are in this Class as well, goes to show you who Batman's heavy hitters usually are.