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Atomic-level Forces at Play in Concrete

Wednesday, January 14, 2015

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By examining concrete at the atomic level, researchers at Rice University have suggested new ways to improve the most widespread building material's crack resistance.

Researchers Rouzbeh Shahsavari and Saroosh Jalilvand have published a study focused on what happens at the nanoscale when “structurally complex” materials like concrete—a random jumble of elements rather than an ordered crystal—rub against each other.  

Rice University
Shahsavari Group / Rice University

The Rice University researchers studied how atomic-level forces in particulate systems, like concrete, interact when friction is applied.

The scratches left behind can say a lot about the material’s characteristics, according to a Jan. 8 research announcement.

The Study

The team is the first to run sophisticated calculations that show how atomic-level forces affect the mechanical properties of a complex particle-based material, according to Rice University.

Their techniques suggest new ways to fine-tune the chemistry of such materials to make them less prone to cracking and more suitable for specific applications.

The study used calcium-silicate-hydrate (C-S-H)—the glue that binds the small rocks, gravel and sand in concrete.

Friction at Work

Though it appears to be a paste before hardening, C-S-H actually consists of discrete nanoscale particles, the team says.

The van der Waals and Coulombic forces that influence the interactions between the C-S-H and the larger particles are the key to the material’s overall strength and fracture properties, according to Shahsavari.

Shahsavari
Jeff Fitlow

Rouzbeh Shahsavari is an assistant professor of civil and environmental engineering and of materials science and nanoengineering.

“Classical studies of friction on materials have been around for centuries,” he said.

“It is known that if you make a surface rough, friction is going to increase. That’s a common technique in industry to prevent sliding: Rough surfaces block each other.

“What we discovered is that, besides those common mechanical roughening techniques, modulation of surface chemistry, which is less intuitive, can significantly affect the friction and thus the mechanical properties of the particulate system.”

The team tested their theories using computer models.

Controlling Element

Shahsavari said it’s a misconception that the bulk amount of a single element—for example, calcium in C-S-H—directly controls the mechanical properties of a particulate system.

Rice University
Shahsavari Group / Rice University

A top-down view at the tip of a virtual cement probe shows the position of atoms, dominated by calcium and silicate. The team used simulations to show the chemical makeup of particulate systems affects their material strength by simulating friction with various surfaces.

“We found that what controls properties inside particles could be completely different from what controls their surface interactions,” he said.

While more calcium content at the surface would improve friction and thus the strength of the assembly, lower calcium content would benefit the strength of individual particles, the team reported.

“This may seem contradictory, but it suggests that to achieve optimum mechanical properties for a particle system, new synthetic and processing conditions must be devised to place the elements in the right places,” he said.

In this video simulation, the team shows how atoms in a smooth substrate are displaced by the force of a calcium-silicate-hydrate tip. The researchers’ calculations help predict the fracture toughness of materials and show how they might be improved by fine-tuning chemical bonding.

Shahsavari said atomic-level analysis could help improve a broad range of non-crystalline materials, including ceramics, sands, powders, grains and colloids.

About the Researchers

Shahsavari is an assistant professor of civil and environmental engineering and of materials science and nanoengineering and a member of the Richard E. Smalley Institute of Nanoscale Science and Technology at Rice.

Jalilvand is a former graduate student in Shahsavari’s group at Rice and is now a Ph.D. student at University College Dublin.

They recently published their research in the American Chemical Society journal Applied Materials and Interfaces.

   

Tagged categories: Concrete; Concrete defects; Concrete repair; Cracking; Research

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