Methods and apparatus for cooling gas turbine rotor blades is provided. The rotor blades include an airfoil having a pressure sidewall and a second suction sidewall connected together at a leading edge and a trailing edge, such that an internal three pass serpentine cooling circuit is formed therebetween. The cooling circuit includes radially extending first, second, and third serpentine cooling cavities partially separated by, in axially aft succession, a first radially extending internal rib and a second internal rib. The second rib includes a radially inner first portion and a radially outer portion wherein the radially outer portion is angled obliquely with respect to the first portion.