Evaluating crack width measurements can help contractors address owners' concerns about the appearance, long-term durability, and structural integrity of reinforced concrete. The methods described here are for evaluating cracks that are roughly perpendicular to the concrete surface. Larger cracks can be measured with limited accuracy by using a calibrated scale, perhaps with the aid of a magnifying glass. Optical comparators make it easy to measure crack widths to an accuracy of 0.002 inch or 0.001 inch. They come with interchangeable reticles that hold the measuring scale against the work. To use the optical comparator, first focus the instrument's scale to your eye while looking at a light source. Then position the reticle tightly against the crack to be measured and orient the scale to the proper measuring position across the crack.
If the crack size is stable and there isn't excessive deflection or other indication of reduced load-carrying capacity, the only other threat could be rebar corrosion. Whether or not cracking significantly increases the chance of corrosion depends on the crack width and the exposure and configuration of the structure. This article provides charts that can be used as an aid to determine tolerable crack widths for reinforced concrete.