Thick enough? You mean i should have 2meters of concrete? It doesnt seem like you are in construction mate. Yes, concrete does absorb water. The reason behind this is that concrete is a highly porous material that is made up of gravel, sand, cement and water. The final product appears hard but is full of pores.

Concrete burns even if it starts raining on it. Pouring. You often water a new slab to make it not cure too fast, in that case it will crack.

A packet soil ground is always the first start,under the concrete, and when you can level that you can skip the bottom concrete, (since it has no use at all). That way i save money, work, and height in this low-ceiling bathroom.