I have a shower Room with a very large shower tray which is accessed with two steps. These are wooden and tiled with marble chip / tiles. The problem is the top step gets very wet. It normally runs off back into the shower tray but every year I have to re-grout them because it cracks. But this year the two layers of 12mm plywood under the tiles has gone soft. So I have taken the tiles off and the plywood and want to replace with concrete.
It's only 790mm x 510mm x 150mm deep. I've got building sand and OPC.
1) But mixture shall I use?
2) I dont want to use gravel, do i need it?
3) and finally should I paint the wood first or something else?
The grout is cracking not because its getting wet but because the substrate is moving.
The ply substrate has got wet because the grout has cracked.
You say two layers of 12 mm plywood. Is that one on top of the other? Not stable enough and by no means does 2 x 12 provide as much support as 1 x 25mm.
What structure is under the plywood?
With a stable base and 25mm WPB ply and a decoupling membrane there is no reason why the tiles / grout should crack.
Anyway to your main question. First I don't think concrete is going to help you at all - try redoing it as above. Second you don't use building sand for concrete you use sharp sand. Third if you don't use gravel in the mix it isn't concrete its mortar - no strength there.
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