Today's BFI blog shares an excerpt from a petrography report prepared for a project on a distressed concrete warehouse slab. BFI performed investigations and gave concrete expert witness opinion and testimony.

Enjoy the excerpt from this petrographic report.

"The cracking was due to drying shrinkage, rather than expansion that can result from the infiltration of external environmental agents. The study found that the concrete represented by the cores possessed a higher potential for shrinkage due to inadequate and inconsistent aggregate gradation as well as inconsistent water content. In addition, there was evidence of inconsistent finishing practices that related to the timing and amount of finishing.

Evidence for drying shrinkage was presented by cracks that spanned a wide range of scales. Fine crazing cracks that ranged up to 25 μm in width were observed on nearly every sample. Such cracks can result from inadequate curing. Shrinkage microfractures that measured up to 150 μm in width were observed on most of the samples. This scale of fracturing is common when drying shrinkage occurs, although the abundance of such cracks was excessive in the concrete represented by the cores. Several samples presented macroscopic microfractures that ranged up to 2.5 mm (0.10 in.) across and cut through the full thickness of the concrete slab. This scale of fracturing was unusual in that it was larger than typical for drying shrinkage cracking. Cracking of this magnitude resulted from improper construction practices, such as inadequate saw cutting depth.

There was evidence that the proportioning of the concrete increased the potential for the material to undergo drying shrinkage. The grading of the coarse aggregate was poor for limiting drying shrinkage, as it appeared to be gap graded (note that all reported percentages in this summary and the following report are based on visual estimations). The nominal maximum aggregate diameter size ranged from 19-37.5 mm (¾ to 1 ½ in.); in most cases more than 65% of the coarse appeared to consist of particle sizes that measured less than 12.5 mm (½ in.). In several cases it appeared that the coarse aggregate fraction of the concrete constituted less than 40% of the volume of the concrete. The fine aggregate consisted of well-graded quartz rich sand--there was not an excessive amount of fines or of clay or micaceous particles. No evidence of alkali silica reaction was observed; particles of potentially alkali-silica reactive rocks (see ASTM C 294)

2 were identified as components in both the coarse and fine aggregate."