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I do not see how the rainwater will get in, the slabs will not let through, and unless you Sika the joints the minimal seep through the joint would not be sufficient to wash anything away. As this is decorative mainly I would bed on the slabs on pit sand and sika the joints, I do not see the need for cement if there is a holding perimeter like house walls and garden walls.

DWD

I see. Most of the people have advised to use 5:1 ratio of sharp sand and cement for the bed.. if using sharp sand alone, do you think the slabs will not move up and down as time progresses having just plain sharp sand under them?

You must do it the way you want. Pit sand or builders sand compacts well and it would work. The Sika stops water seeping in and creating voids under the slabs

DWD