concrete step treads or 600 x 600 mm strong pavers - Spence

I am installing a large (about 2 m x 0.6 m) raised planter box that includes a water tank underneath to capture the used water - enough said, it will be heavy when full and will need a strong level foundation on the ground beneath.

So I am after some strong concrete step treads (any one doing renovations?) OR some 600 x 600 mm *strong* concrete pavers.

Thanks

Jim
Spence