Sampling may be part of a forensic investigation. As we’ve discussed in previous blogs, it isn’t practical to obtain enough concrete cores for a proper statistical representation. Most owners don’t want holes in their structure. You can patch them, but the patches are conspicuously ugly. Also, many of the analytical techniques we use—petrography, chemistry, chloride gradients, thermal analyses—are expensive to do in quantity.
Another challenge is that the eye naturally gravitates to the anomalous. But if you’re evaluating concrete construction, you need to characterize what’s normal: what’s the overall condition of the concrete? If you sample just what attracts your eye, you’ll mischaracterize what you’re looking at by focusing exclusively on the worst case.
We’ve discussed previously how to conduct a systematic forensic investigation. You look at the big picture first—the project documents and an overall visual survey of the site. Then you zero in on the details, progressing from large scale to small in a series of overlapping steps. At the end, the details should add up to a complete, coherent picture. If they don’t, you investigate further to resolve any gaps or apparent contradictions.
“The good, the bad, and the ugly” is another useful guide to sampling. Some samples should represent the “ugly”—the worst damage or deterioration you observe. But to get a complete picture, you also need the “good” and the “bad”. Differences among them may point to the cause(s) of the problem and/or potential remedial actions. Is the “good” concrete likely to suffer the same problem later? Or did the supplier or contractor do something different that made it perform better there?