You would need slightly less than 1 cubic yard of concrete (0.90 to be exact)
However, 15mm thickness of concrete is not realistic. It would have to be a special skim coat as the aggregate size in normal concrete is larger than 15mm (5/8 inch dia)
Perhaps you meant 15cm which is about 6 inches thick. Then you would need 9 cubic yards of concrete.
Dodgeman
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