Final answer:
The characteristic that is not found in Roman imperial monuments built using concrete is A) Walls are always thin membranes.
Step-by-step explanation:
The characteristic that is not found in Roman imperial monuments built using concrete is A) Walls are always thin membranes. Roman buildings were built with brick-faced concrete, which allowed for spaces to be carved out of the wall's thickness. The walls were not always thin membranes. Concrete was a cheaper and lighter material than most other stones used for construction, enabling the Romans to build taller and more complicated structures.