If you have a paving business and purchase concrete for your business, is the purchase of the concrete an asset or an expense, and when the need arises to refill the truck with more concrete, is this another purchase of an asset or an expense?

Concrete would represent inventory, an asset. When your business is performing a job (paving) the concrete would move from inventory to cost of goods sold based on some type of cost structure.

Purchase of concrete
DR Inventory XXX
CR A/P XXX

Use of concrete in paving job
DR Cost of goods sold XXX
CR Inventory XXX