Concrete isn't thought of as a plastic, but plasticity at small scales boosts concrete's utility as the world's most-used material by letting it constantly adjust to stress, decades and sometimes even centuries after hardening. Rice University researchers are a step closer to understanding why.
The Rice lab of materials scientist Rouzbeh Shahsavari performed an atom-level computer analysis of tobermorite, a naturally occurring crystalline analog to the calcium-silicate-hydrate (C-S-H) that makes up cement, which in turn holds concrete together. By understanding the internal structure of tobermorite, they hope to make concrete stronger, tougher and better able to deform without cracking under stress.
Their results appear this week in the American Chemical Society journal ACS Applied Materials and Interfaces .
Tobermorite, a key element in the superior concrete Romans used in ancient times, forms in layers, like paper stacks that solidify into particles. These particles often have screw dislocations, shear defects that help relieve stress by allowing the layers to slide past each other. Alternately, they can allow the layers to slip only a little before the jagged defects lock them into place.
The researchers built the first computer models of tobermorite "super cells" with dislocations either perpendicular to or in parallel with layers in the material, and then applied shear force. They found that defect-free tobermorite deformed easily as water molecules caught between layers helped them glide past each other.
But in particles with screw defects, the layers only glided so far before being locked into place by the tooth-like core dislocations. That effectively passed the buck to the next layer, which glided until caught, and so on, relieving the stress without cracking.
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(Score: 2) by fishybell on Thursday January 05 2017, @03:12AM
A major source of concrete problems today are also a result of the iron reinforcement. The rebar expands as it rusts/corrodes, breaking the concrete from the inside.
(Score: 2) by Runaway1956 on Thursday January 05 2017, @03:23AM
Interesting. Do you have citations for that? In my experience, it seems that rebar doesn't rust or corrode until/unless the concrete is first damaged, allowing moisture and oxygen to get to the rebar. I don't recall ever removing undamaged concrete, to find rusted rebar inside. Then again, maybe I wasn't really watching for rusted rebar. In cases where the concrete is badly cracked, flaked, and chipped, I've often seen rusted rebar extending down into the undamaged concrete. Are there studies on this, or are you, like me, basing your conclusion on personal experience?
I will note, in support of your claim, that the state of Texas often specifies vinyl clad reinforcement bar in their structures. The first time I used vinyl clad was in the New Boston bridge over the Red River. There is no naked iron in that structure, everything is heavily coated, most of it in vinyl.
(Score: 4, Interesting) by JoeMerchant on Thursday January 05 2017, @04:05AM
https://en.wikipedia.org/wiki/Spall [wikipedia.org]
http://www.hdb.gov.sg/cs/infoweb/residential/living-in-an-hdb-flat/home-maintenance/spalling-concrete [hdb.gov.sg]
https://failures.wikispaces.com/Concrete+Bridge+Failures+-+Deterioration+and+Spalling [wikispaces.com]
https://www.concretenetwork.com/fix-spalled-concrete/ [concretenetwork.com]
http://www.truesdellcorp.com/index.php/services/articles/41-general/429-corrosion-protection-and-repair-of-reinforced-concrete [truesdellcorp.com]
it's a big enough problem to have extensive study, publication, and service providers who specialize in maintenance to prevent it, and repair work after it starts.
🌻🌻 [google.com]
(Score: 2) by Runaway1956 on Thursday January 05 2017, @10:40PM
Thank you, Joe, great job!
(Score: 0) by Anonymous Coward on Thursday January 05 2017, @04:49AM
Two words: Road salt