Rotor blade assembly and methods for forming rotor blade assemblies are provided. A rotor blade assembly includes a rotor blade including a shell and defining a pressure side, a suction side, a leading edge and a trailing edge each extending between a tip and a root. The rotor blade further defines a span and a chord. The shell includes an inner skin, an outer skin, and a core disposed between the inner skin and the outer skin. The rotor blade assembly further includes a reinforcement assembly bonded to the shell, the reinforcement assembly comprising a reinforcement core.