Under the floor

Just a day after the concrete base was poured, the joists for my extension have been put in - on the right. My neighbour's new floor will be concrete so their base, on the left here, is beam and block. (Yes, their extension is much bigger than mine.) That's what I was planning to have until the builders discovered that the existing joists in my house were so flimsy they all needed replacing and it made sense to make my floors the same throughout. 

Also, although I love the look of polished concrete floors in the catalogues and magazines, when I went looking at real ones in use I couldn't find any without cracks and stains. I know floorboards get damaged and need maintaining but it looks like concrete gets damaged and can't be maintained.