hello..im a civil engineer though a fresh one..i think he's right..usually we go for r6 which 6mm in diameters for a minor construction..r8 are rarely used because we usually just go for r10 and they are usually used in taller buildings..r6 is also widely available although the contractor might want to save money on his side..anyway u should try to ask the engineer if he can consider the usage of r6 in his design..hope this help..