Petrography once regarded as a somewhat esoteric laboratory discipline today increasingly is called upon to contribute to the concrete design and construction team. For example: A propane tanker crashes and burns on an Interstate bridge; concrete piers and deck slab are subjected to the intense heat of the resultant fire. Has the concrete been altered? If so, to what extent and depth? Also, a ready mix company needs to determine the cause of an abrupt decline in concrete strengths. Can the cause be determined through microscopic evaluation of acceptable and low strength concrete samples? Using the science of petrography, augmented at times by other procedures, answers and recommendations have been furnished in cases such as these.
Because petrography frequently offers the quickest, least expensive and, in some cases, only method through which the cause of a failure can be determined, failure analyses occupy much of the petrographer's time. Typical causes of inferior quality of failed concrete which can be readily detected through petrographic techniques and analysis are grouped under the following headings: damaging reactions between cement and aggregate; deterioration of hardened cement pastes due to attack by "aggressive" waters; deterioration caused by exposure to freezing and thawing; presence of unsound aggregates in the mix; use of an improper type or poor quality of cement; deterioration due to abrasion or fire exposure; or inadequate proportioning, mixing, placing, or curing.