A recent popout problem in a Minnesota parking ramp shows how valuable petrographic analysis can be. The ramp is a multi-level, precast concrete, double-tee bar structure that was constructed during a fall and winter. Shortly after the snow melted, popouts were observed throughout the ramp. Six 4-inch-diameter concrete cores were sent to the laboratory for petrographic examination. To everyone's surprise, three other problems were discovered in addition to the shale in the cores. First, the entrained air content of all six cores was very low, averaging 1.6 percent. Second, the cores showed the classic overfinishing profile- in two cores the aggregate had subsided up to 1 inch from the top surface. Last, two cores showed dark gray bands of cement paste completely lacking sand-sized aggregate.
After the ramp owners received the petrographic analysis results, they requested a more in-depth investigation. Freeze-thaw testing produced failures at a very early age. The ramp's low air content was the most serious concern. It appears to have been caused, at least in part, by the use of too much superplasticizer, overvibration, or both. The low air content would soon cause surface deterioration to begin, causing eventual structural deterioration. This situation makes a case for the importance of petrographic examination as a quality control measure.