Bridge deck cracking is an ongoing challenge for transportation agencies. Even worse, when those cracks form soon after the bridge is poured—a process known as “early-age” cracking—it can lead to costly maintenance and significantly reduce the lifespan of a bridge deck.
To combat this problem, researchers at the University of Minnesota Duluth set out to determine why this cracking happens and what can be done to prevent it. The research was led by Swenson College of Science and Engineering Associate Professor and CTS scholar Brock Hedegaard and sponsored by the Minnesota Department of Transportation (MnDOT).
There are two main causes of early-age bridge deck cracking. The first is the stress of thermal demand: as the temperature of the concrete deck changes, either by cooling after being poured or in response to weather changes, it tries to expand or contract. The second problem is shrinkage, which happens naturally as concrete hardens—much like a sponge shrinking as it dries. In both cases, the bridge’s steel structure restrains the concrete’s movement, which causes the stress that can lead to cracking.
According to prior research, two of the top strategies for preventing early-age cracking are adjusting the concrete mixture and reducing the spacing between reinforcement bars, Hedegaard says. “MnDOT needed us to evaluate these strategies to identify the most cost-effective solutions that engineers could incorporate into new bridge design and construction practices.”
In their search for solutions, researchers conducted a two-part investigation. First, in the lab they created and tested 12 different concrete mixtures. They focused on testing particular additives, known as shrinkage-reducing admixtures (SRAs), with the goal of finding a mix that was strong but less prone to cracking. Next, they built a virtual bridge using advanced computer modeling to simulate real-world conditions that included temperature swings and shrinkage. This allowed them to see exactly where and why stresses build up.
The computer modeling revealed a crucial insight: shrinkage is the primary culprit in early-age bridge cracking. Lab tests showed the best way to combat shrinkage was by adding SRAs to the concrete mix; these additives reduced shrinkage by about 25 percent and the associated stresses by about 50 percent. Surprisingly, researchers also found that steel reinforcement did not prevent cracks from forming, though it did help prevent cracks from widening after they appeared.
“The results of using SRAs in the concrete mix are encouraging,” says Paul Gronvall, MnDOT Bridge Office principal engineer, adding that the findings will lead to pilot projects to further evaluate the mix’s performance in preventing early-age bridge cracking.
The study’s results provide a clear path forward for MnDOT, Hedegaard says. The researchers recommend incorporating SRAs into high-performance concrete mixes, controlling temperature differences between the newly poured deck and the bridge girders, and adjusting steel reinforcement to limit cracking. With these changes, they believe the durability of new bridges can be improved, saving money and serving drivers for decades to come.
—Megan Tsai, contributing writer
