Bill Hime reports that in the early 1950s, at the Research and Development Laboratories of the Portland Cement Association, the analytical chemists were separated from the petrographers, and each worked in near isolation. When Bernie joined PCA, he and I occasionally spoke to each other. One day I received a black powder that was reported to increase the strength of concrete. Using every analytical technique at my command, I failed to identify it. In desperation, I took it to Bernie. A minute later he said, “You stupid chemist (but smart for seeking help from a petrographer), that's fly ash!” At that moment, we decided that a combination of analytical chemistry and petrography was synergistic. Truly two and two made five—and sometimes more—and we have been doing it ever since (but sometimes with different partners).
We once had a request to duplicate a specially formulated lens coating because the powder, purchased by a lens-making outfit, was progressively becoming more expensive to buy. Production costs were skyrocketing! Bill and his chemists sliced and diced it, using an array of up-to-date chemical wonders and procedures that at that time included wet chemistry (the old standby), x-ray diffraction and fluorescence, and infrared spectroscopy. They gave the chemically derived formula to the outfit, whose reply after trying it was, “It doesn't work.”
Recognizing that Bernie's microscopes each used a variety of lenses, the chemists made Bernie their unproclaimed expert on lenses. In the course of his several-minute microscopical observations, he note a few butter-yellow, micron-sized particles—the kind of stuff that starts stampedes, like to the Black Hills in southwestern South Dakota or to Sutter's Creek in California. Gold! Gold! Gold! That was the key to the powder, and the cause of its rising cost; this was at the time when gold was taken off the standard of $35 an ounce and within months rose to $750 an ounce. Perhaps this serves to date the time when that synergism occurred.
But sometimes our hard-earned synergistic efforts were thwarted because of politics. Like the time when a catastrophic concrete failure involved the deaths of several dozen construction workers. We and our staff looked hard to uncover the role of the concrete in the failure. The petrographers found lots of poorly hydrated portland cement particles, peculiar morphologies of calcium hydroxide (from hydration of the cement), and major amounts of fly ash, estimated to constitute 25 to 30 percent of the cementitious materials. The chemists detected components of a water-reducing admixture. And it was winter construction! Our conclusion was that:
- because of low ambient and attendant concrete temperatures, the water-reducing admixture, which contained both a retarder and accelerator, behaved like a retarder
- dilution of the portland cement by the fly ash suppressed the heat normally developed during hydration of the portland cement, which sometimes can overcome low concrete temperatures
- the low temperatures acted as a retarder to cement hydration, so that strength development and strength gain were greatly delayed.
With these contributing factors, the “usual” time of form removal was so premature (because of the low concrete strength) that the anchors embedded in the concrete were not adequately secured. But because of the legal aspects of the case our client never released all of the information we developed.
But we disseminate that and other analytical and interpretative experiences, whenever appropriate. Based on this experience, we urge that when mixture designs are prepared and verified in the laboratory, the concrete be tested at the temperatures anticipated in the field. Any changes in any of the concrete-making components should be incorporated—all in addition to using suitable field-testing techniques for identifying critical concrete strengths.
Each of our disciplines can perform wonders, and often only one or the other is all that is required. But occasionally both are needed. One or the other may be faster and more accurate, but sometimes the answers from one disagree with those of the other. It may take lots of back and forth effort to finalize what has happened. The intellectual challenges provided during failure investigations lead to unsurpassed joy in their solution, but often only anguish until then.
In future columns we will illustrate how chemistry can solve some construction material failure problems, how petrography can resolve others, and how both may be needed.
William Hime is a principal with Wiss, Janney, Elstner Associates and began working as a chemist at PCA 53 years ago.
Bernard Erlin is president of The Erlin Company (TEC), Latrobe, Pa., and has been involved with all aspects of concrete for over 47 years.