The term "spacing factor" refers to the distance between air bubbles in hardened concrete. All concrete has some air bubbles, usually in the range of 1 or 2%, referred to as "entrapped air". These bubbles provide no freeze/thaw protection. Where freeze/thaw protection is desired, air bubbles are intentionally introduced, or entrained, into the plastic concrete mixture. These microscopic bubbles protect the mortar portion of the concrete by providing space for water in the concrete to expand during the freezing process. If these bubbles were not available for this purpose, the expansion of the water would damage the mortar. An important characteristic of a good air-void system is the spacing factor. Bubbles need to be in close proximity so the water migrating through the concrete does not have to travel far to find a bubble in which the water can expand. Ideally the spacing factor will be less than 0.008 in. This analysis is performed on hardened concrete by a trained petrographer using test method ASTM C 457. There usually is some slight variance between petrographers evaluating the same concrete sample.