Q.drainage from garden soil planted on top of concrete?
Weight is not the problem, but how and where does the water drain when soil is directly on top of concrete? Also, how deep should I plant my garden surrounded by 2 x 6 boards in a 4′ x 8′ square?
With concrete underlaying soil, the water will naturally flow toward the lowest area of the concrete and collect there, or flow away, depending on what surrounds the concrete.
Raised beds should have a depth of soil from 6 - 12". This article will help you. https://www.gardeningknowhow.com/edible/vegetables/vgen/raised-vegetable-gardens.htm