Rock strength is defined as the limit of the ability of a rock to resist stress or deformation without breaking. Testing methods recommended by ISRM (International Society of Rock Mechanics) and ASTM (American Standards Testing Material) include unconfined compressive strength (UCS), point load index (PLI), indirect tensile strength (ITS), Schmidt hammer rebound (SHR), sonic velocity (Vp and Vs), and slake durability index 2nd cycle (Id2).