ᐅ Blinding layer or simply an additional 50mm in the concrete slab

Hello, our site manager plans to lay a 5cm (2 inch) lean concrete layer plus a 0.4 mm polyethylene (PE) sheet under the foundation slab (on top of the gravel layer). Our structural engineer says that since the foundation slab is already 25cm (10 inch) thick, the reinforcement will have enough cover above the gravel (underneath the PE sheet).

How do you handle this?

Regards