Air brick, concrete floor and ducting

[wadge]

I need to install some ducting to extend the air flow through a new concrete floor in an extension to an air brick in the existing wall. What layer should the ducting go in? I thought about at hardcore stage but wouldn't the ducting get crushed with the weight on top and also I won't be able to get the whacker plate anywhere near it?

[noseall]

Cut it into the insulation layer.