Fine polygonal cracks were visible on the surfaces of concrete piers. When cores were recovered, a pattern of closely spaced, axial cracks were revealed. At the microscopic level, all cracking was traced to alkali-silica reaction or ASR. The coarse aggregate shown here is a variety of geologically deformed quartzite. Though hard with a high elastic modulus, the aggregate is highly reactive in alkaline cement paste. The reaction has produced a gel that is seen to saturate cracks that have propagated into the surrounding cement paste causing expansive failure of the concrete.

A newly placed and unfinished concrete slab on a high-rise tower was exhibiting flaking and extensive surface distress within days of placement. It was suspected that the ready-mix supplier had provided a poor concrete product. Through petrography, we determined that inadequate protection had been provided for the cold weather placement. The arrows in the micrograph illustrate microscopic linear openings that disrupt and compromise the hydrated cement paste. These are the impressions of ice crystals that formed in the outer half inch of the slab prior to the concrete hardening.

Investigation of Concrete Cracking
Cracking terminology in the concrete industry is nearly endless. Delamination, scaling, pop-outs, crazing, map cracking, pattern cracking, D-cracking are all familiar terms and the list goes on. Regardless of the name, cracking is always a cause for concern and when occurring in recently placed concrete often a source of heated speculation. 
Some common causes of material failures in concrete are:
These causes alone or in combination are readily investigated through petrography obviating the need for speculation. As a direct observational tool, petrography can be used to pinpoint the reaction products, deficient cement paste structures or finishes, and trace unstable components responsible for the visually apparent cracking. In cases where a determination of root cause is inconclusive, many potential cracking mechanisms can be disregarded through careful examination.