Soil is typ about 1T/cu yd
Sand is typ about 1.5T/cu yd
Concrete, about 2T/cu yd

Wheelbarrow capacity is typ 1/6 cu yd

So, sizing with concrete's density in mind
suggests it would never see more than ~700lb load.

Anything that you'd EXPECT to encounter with a
wheelbarrow that would EXCEED that? (i.e., you
wouldn't be lugging gold bullion around in one)