The concrete test most drivers will never hear about

by Ines Aviles-Spadoni, M.S., M.A., Research/Communications Coordinator, UFTI

There is something we often don’t think about when we drive over a Florida bridge: How can it hold up the weight of thousands of vehicles every day?

Especially when the bridge is over a vast body of water such as the Banana River in Florida. The view is spectacular, but it’s also very easy to forget that bridges such as the Peebles Bridge, which safely take you over to Cocoa Beach, are a result of years of meticulous engineering, design and rigorous testing to make sure the materials used to build it will hold up for years to come.

One of those materials is concrete. Kyle Riding, Ph.D., professor and head of the UF Department of Civil and Coastal Engineering, was the principal investigator of a Florida Department of Transportation (FDOT) grant-funded study that investigated quality control tests engineers use to evaluate concrete when it becomes part of a road or bridge. Christopher Ferraro, Ph.D., associate director and associate professor in the same UF department, served as the study’s co-principal investigator.

“Concrete bridges can be very durable options for departments of transportation,” Riding said. “Concrete producers in Florida have become very good at delivering high-quality concrete for long-lasting bridges. Test methods to verify concrete durability, especially when exposed to harsh seawater environments, can help transportation agencies ensure they are getting the concrete that they pay for.”

The study primarily focused on surface resistivity. This test gives engineers information about how resistant concrete is to moisture and chloride salts. Over time, water and salts can work their way through concrete and affect the steel rebar inside the concrete, eventually leading to corrosion, and we don’t want that.

But Riding and Ferraro found that samples made at construction sites did not produce the same results as those made back in their laboratory at UF. So, before they could recommend using this test out in the field as a quality control measure, the UF researchers wanted to find out why there were inconsistencies.

They studied fifteen concrete mixtures and tested them. They wanted to identify which characteristics were influencing the differences between those prepared in the lab and those made at construction sites. While no single issue stood out to the researchers, they found that moisture conditions during testing, how the samples were cured, how long they remained in their molds, and the type of testing equipment used could all be causing differences in the test results. Based on these results, they also developed recommendations for FDOT as guidance for having more consistent field measurements.

“Concrete test results depend on more than the concrete mixture itself,” Ferraro said. “How a specimen is prepared, cured, stored, and tested can influence the measured results. By making those procedures more consistent, FDOT can better distinguish actual differences in concrete performance from differences introduced during the testing process.”

A good way to think about all this is when you make cookies. You have all the ingredients and the recipe, but if you bake one batch at a different temperature or if the butter is too cold instead of at room temperature, then the cookies will not be the same. The same thing can happen when you mix and test concrete. So, in this case, Riding and Ferraro wanted to know whether the differences in the measurements were caused by the concrete or by the way the samples were prepared, cured and tested.

While most of us will never think about the term surface resistivity, what matters is that every one of these bridges, overpasses, and highways depend on these tests before they even open up to be used by the public. The research funded by FDOT and conducted by Riding and Ferraro will eventually provide state engineers with more information as they test the concrete used to build the infrastructure we use every day.