I haven't seen a recommendation, but you had better cover the hatch area with a sheet of plywood or an equivalent or you will destroy the flimsy rear deck.
You can safely carry weight up to the GVWR of the car, but dense weight like concrete bags concentrated only at the rear will make your steering wheels dangerously light, and overload the rear wheels, even if the total weight is under the vehicle's GVWR.
It would be a better idea to spread the load out onto a sheet over the entire rear of the car.
Probably an even better idea is to rent or borrow a vehicle designed for the task - a pickup truck - rather than messing up a new car interior...
You can safely carry weight up to the GVWR of the car, but dense weight like concrete bags concentrated only at the rear will make your steering wheels dangerously light, and overload the rear wheels, even if the total weight is under the vehicle's GVWR.
It would be a better idea to spread the load out onto a sheet over the entire rear of the car.
Probably an even better idea is to rent or borrow a vehicle designed for the task - a pickup truck - rather than messing up a new car interior...