Heavy duty cart made out of 3/16" plate and angle iron with four Foot Master heavy duty casters (believe these are the GD Series 2 1/2" leveling caster with a 1210 Lb. load rating, which retail for $42 each at HD) underneath. Looks like it's meant to hold something 3' x 3' inside the angle iron sides. This ought to move a heavy planter around your concrete slab patio.