Thin-section photomicrograph showing carbonation of concrete along a vertical crack in a concrete slab-on-grade.  The carbonated portion is highlighted by the bright coloration of calcium carbonate in the crossed polarized-light mode in a petrographic microscope.

Lapped section photograph of a concrete core showing deep "V"-shaped carbonation front along a vertical crack.  The lapped section was treated with a phenolphthalein alcoholic solution which turns the non-carbonated concrete into pink and the carbonation portion into light brown.