MIT researchers have developed a bioinspired framework for use by engineers to design more durable concrete.

Today’s concrete is a random assemblage of crushed rocks and stones bound together by a cement paste. “If we can replace cement, partially or totally, with some other materials that may be readily and amply available in nature, we can meet our objectives for sustainability,” says Oral Buyukozturk, professor in MIT’s Department of Civil and Environmental Engineering.

Concrete’s strength and durability depends partly on its internal structure and configuration of pores. The more porous the material, the more vulnerable it is to cracking. However, there are no techniques available to precisely control concrete’s internal structure and overall properties.

The researchers hope the framework will help engineers identify ingredients that can improve concrete’s performance and longevity. Image credit: Pixabay. “It’s mostly guesswork,” Buyukozturk says. “We want to change the culture and start controlling the material at the mesoscale.”

For Buyukozturk, the “mesoscale” represents the connection between microscale structures and macroscale properties. For instance, how does cement’s microscopic arrangement affect the overall strength and durability of a tall building or a long bridge? Understanding this relationship would help engineers identify features at various length scales that would improve concrete’s overall performance.

To start to understand this connection, he and his colleagues looked to biological materials such as bone, deep sea sponges and nacre (an inner shell layer of mollusks), which have all been studied extensively for their mechanical and microscopic properties. They reviewed the scientific literature for information on each biomaterial and compared their structures and behavior at the nano-, micro- and macroscales with that of cement paste.

They searched for connections between a material’s structure and its mechanical properties. For example, the researchers found that a deep sea sponge’s onion-like structure of silica layers provides a mechanism for preventing cracks. Nacre has a “brick-and-mortar” arrangement of minerals that generates a strong bond between the mineral layers, making the material extremely tough.

Applying the information they learned from investigating biological materials, as well as knowledge they gathered on existing cement paste design tools, the team developed a general methodology with which engineers can design cement “from the bottom up.”

The framework is a set of guidelines that engineers can follow to determine how certain additives or ingredients of interest will impact cement’s overall strength and durability. To see whether volcanic ash would improve cement paste’s properties, for instance, engineers following the group’s framework would first use existing experimental techniques, such as nuclear magnetic resonance, scanning electron microscopy and X-ray diffraction, to characterize volcanic ash’s solid and pore configurations over time.

These measurements could then be plugged into models that simulate concrete’s long-term evolution to identify mesoscale relationships between, say, the properties of volcanic ash and the material’s contribution to the strength and durability of an ash-containing concrete bridge. These simulations could then be validated with conventional compression and nanoindentation experiments to test actual samples of volcanic ash-based concrete.

Ultimately, the researchers hope the framework will help engineers identify ingredients that are structured and evolve in a way, similar to biomaterials, that may improve concrete’s performance and longevity.

“Hopefully, this will lead us to some sort of recipe for more sustainable concrete,” Buyukozturk says. “Typically, buildings and bridges are given a certain design life. Can we extend that design life maybe twice or three times? That’s what we aim for. Our framework puts it all on paper, in a very 'concrete' way, for engineers to use.”