Aggregate, the granular filler in portland cement concrete, occupies 60% to 75% of most concrete volume. It influences the way both fresh and hardened concrete perform. Aggregate can be naturally occurring sand and gravel, crushed stone, or manufactured sand. It can also be man-made, such as blast-furnace slag or recycled concrete. Generally, desirable aggregate used in concrete is square or triangular shaped, hard, dense, well-graded, clean, inert, sound and durable. When considering using a new or existing aggregate, adhere to accepted industry guidelines and specifications. Perhaps the most important document is ASTM C 33, "Standard Specification for Concrete Aggregates." Although ASTM C 33 is widely referenced as a blanket specification for normal-weight aggregate, it is intended to be used as a discretionary and project-specific document. Both the specifier or purchaser can significantly limit misunderstandings and field problems by becoming familiar with ASTM C 33. ASTM C 33 requires that aggregate pass several laboratory tests. For the most part, these tests ensure satisfactory aggregate quality. Other tests, such as alkali-reactivity tests and long-term expansion tests, may yield a clearer picture of an aggregate's potential durability. The most reliable way to assess an aggregate's performance is to examine its service history and ask how it has performed in similar concrete in similar service environments.