engineers discover what causes concrete creep

[amor de cosmos]

MIT engineers find way to slow concrete creep to a crawl
Denise Brehm, Civil and Environmental Engineering
June 15, 2009

MIT civil engineers have for the first time identified what causes the most frequently used building material on earth -- concrete -- to gradually deform, decreasing its durability and shortening the lifespan of infrastructures such as bridges and nuclear waste containment vessels.

In a paper published in the Proceedings of the National Academy of Sciences (PNAS) online Early Edition the week of June 15, researchers say that concrete creep (the technical term for the time-dependent deformation that occurs in concrete when it is subjected to load) is caused by the rearrangement of particles at the nano-scale.

"Finally, we can explain how creep occurs," said Professor Franz-Josef Ulm, co-author of the PNAS paper. "We can't prevent creep from happening, but if we slow the rate at which it occurs, this will increase concrete's durability and prolong the life of the structures. Our research lays the foundation for rethinking concrete engineering from a nanoscopic perspective."



In their PNAS paper, the researchers show experimentally that the rate of creep is logarithmic, which means slowing creep increases durability exponentially. They demonstrate mathematically that creep can be slowed by a rate of 2.6. That would have a truly remarkable effect on durability: a containment vessel for nuclear waste built to last 100 years with today's concrete could last up to 16,000 years if made with an ultra-high-density (UHD) concrete.

http://web.mit.edu/newsoffice/2009/creep-0615.html

[photoLith]

They hadnt figured out what caused cement creep? That seems pretty obvious to me that a ton of weight on something over a long period of time will squish and put load onto the atomic structure of the material causing it to deform.

[scalziand]

Originally Posted by photoLith

They hadnt figured out what caused cement creep? That seems pretty obvious to me that a ton of weight on something over a long period of time will squish and put load onto the atomic structure of the material causing it to deform.

That concrete under load for a prolonged period will creep has been known for a long time, but the exact way the CSH particles move under creep wasn't known before. Now that the actual mechanism of creep is known, it led to an insight of how the nanostructure of concrete can be changed to reduce the creep.

Ulm, who has spent nearly two decades studying the mechanical behavior of concrete and its primary component, cement paste, has in the past several years focused on its nano-structure. This led to his publication of a paper in 2007 that said the basic building block of cement paste at the nano-scale -- calcium-silicate-hydrates, or C-S-H -- is granular in nature. The paper explained that C-S-H naturally self-assembles at two structurally distinct but chemically similar phases when mixed with water, each with a fixed packing density close to one of the two maximum densities allowed by nature for spherical objects (64 percent for the lower density and 74 percent for high).

In the new research revealed in the PNAS paper, Ulm and co-author Matthieu Vandamme explain that concrete creep comes about when these nano-meter-sized C-S-H particles rearrange into altered densities: some looser and others more tightly packed.

They also explain that a third, more dense phase of C-S-H can be induced by carefully manipulating the cement mix with other minerals such as silica fumes, a waste material of the aluminum industry. These reacting fumes form additional smaller particles that fit into the spaces between the nano-granules of C-S-H, spaces that were formerly filled with water. This has the effect of increasing the density of C-S-H to up to 87 percent, which in turn greatly hinders the movement of the C-S-H granules over time.