For the same reason you won't be starting your bonfire with the big logs. It has a huge thermal mass and it takes a lot of external energy to make the fire self

For the same reason you won't be starting your bonfire with the big logs. It has a huge thermal mass and it takes a lot of external energy to make the fire self-sufficient. It's also easier to repair, as you can just replace the damaged wood. With concrete that's not so easy, and in some cases it can be hard to even gauge how damaged concrete is from fire damage.