“When you hear hoofbeats, think horses, not zebras.”—Theodore Woodward, University of Maryland School of Medicine
Although this advice originated in medicine, it applies equally well to forensic engineering: Look for the most probable cause first. It’s closely related to Ockham’s razor: The simplest explanation is usually the best one.
Part of our work in construction litigation involves reviewing reports by other experts. We’re not necessarily looking for flaws in their methodology or their reasoning. Our role is not to win the case, but to determine the facts. To that end, we examine the other experts’ test programs, results, observations, and conclusions in detail.
Why zebras and not horses?
Occasionally we find expert reports replete with zebras—or even unicorns—in lieu of horses. One reason may be that most experts are specialists, having mastered a limited set of tools. A petrographer, for example, relies primarily on optical microscopy to identify the features of the concrete specimen and the spatial relationships among them. They may also employ electron microscopy and other types of analysis such as chemistry or X-ray diffraction. For practical reasons they’ll examine relatively few specimens, usually relying on others to obtain representative samples.
Sometimes the petrographer lacks the necessary context to understand what to look for and what it means. This is why a team approach and good communication are essential. Studying the context should begin before the site visit. The project specification, history, and weather records all provide clues as to why things went wrong. Other documentation such as concrete batch tickets, requests for information, and meeting minutes may also help. These documents may suggest to the investigator what kinds of samples to take.
Certain sampling techniques or packaging materials may obscure the information you’re looking for. For example, coring usually involves water to lubricate the drill bit. However, if you’re looking for chloride ions, you could wash them away. And don’t use a chloride-containing epoxy to stabilize the concrete. Plastic bags often have a surfactant coating. If you’re looking for surfactant admixtures in a cementitious material, don’t put the samples in plastic bags.
Even if the petrographer can’t visit the site, they should still advise on where and how to take the samples. They also need to be aware of the context of the project as they conduct their examinations. In construction defect cases, it helps if they have some experience of construction sites and field testing.
Looking for horses
Although the observations of scientists are essential, the engineer renders the opinion. Ideally, they’ll base their opinion on all the available information, not just the aspects of the investigation they understand best.
I prefer to look first at the overall picture, then zero in on the details, then verify that the details add up to the complete picture. In litigation, the stakes are usually high enough to justify some redundancy in the investigation. That is, one test acts as at least a partial check on another. The idea is not to repeat the same test several times, but to use different tests that overlap and supplement one another. If microscopy indicates that a harmful substance has infiltrated the concrete from the environment or that something has leached out, chemical analysis of a series of thin layers can verify and quantify the concentration gradient.
Crack patterns, weather records, and project history can point to or eliminate possible causes. Cracks that appeared within the first few weeks could be due to drying or to thermal stresses, but not to long-term effects such as alkali-silica reaction. If a project went well for a while before something went wrong, what changed? Changes between hot and cold weather may necessitate adjustments to the concrete mixture proportions, at least to the admixture dosages. Best practices for concreting and testing also change with weather conditions. Changes connected to time of placement could be due to diurnal temperature changes, or to early stiffening of the concrete.
Construction defects and failures may have more than one contributing cause. That’s why it’s important to verify that you’ve fully explained what went wrong. Whether your client wants to repair the defects or just get compensation from the appropriate party, you need to determine everything that contributed to the problem.