SCDOT defines a mass concrete pour as "any pour in which the concrete being cast has dimensions 5 feet or greater in three different directions. For pours with a circular x-section, a mass concrete placement shall be defined as any pour that has a diameter of 6 feet or greater and a length of 5 feet or greater."
For SC bridges, we don't have to specify because the definition is in the SCDOT Standard Specs. However, if your project is subject to codes that do not address a mass concrete pour, and if internal cracks could be detrimental to the concrete member, I think you would be responsible for dictating how that pour is made (concrete specification, and limits of temp. differentials - and how these are to be tested or controlled).
Many mass concrete pours will have rebar (at least nominal) and for members such as drilled shafts, there may be significant rebar. Although the rebar is near the exterior, there is still a concern about internal cracks/flaws in the interior of the section.
good luck.