Final answer:
Roman imperial monuments built using concrete did not have thin walls.
Step-by-step explanation:
The characteristic that is not associated with Roman imperial monuments built using concrete is D) Walls are always thin membranes.
Contrary to the statement, Roman imperial monuments built using concrete did not have thin walls. Instead, they used concrete, brick, and other materials to construct solid and sturdy walls. The concrete allowed for spaces to be carved out, such as alcoves and apses, but the walls were not thin membranes.