Wednesday, June 17, 2009

Concrete Creep Slowed

This item is something that likely does not attract much note, but concrete is and has been the foundation of our material civilization for so long that we take no notice of it. We forget that a concrete structure can stand out in the weather for centuries and never lose its strength. The Roman Coliseum is one of our oldest examples and it is two thousand years old.

Science has been finding clever ways to improve its performance and other characteristics and here we discover a nifty fix that allows a concrete structure to last a quoted 16000 years.

This means using concrete for long term sequestration just became a lot more practical. Why not now simply cast radiated scrap in cement blocks and store down in a mine shaft somewhere. It will be there whenever we go back.

In the event, precision building with concrete has become a lot more secure.

They also talk about silica vapor. I wonder were they might get a worthwhile supply. In fact that is likely a major business opportunity right now. A lot of materials need to be produced in large quantities at nano particle sizes.

http://4.bp.blogspot.com/_VyTCyizqrHs/SjbOltuZgnI/AAAAAAAAD_c/uvEfMXnBRw4/s400/creep-1-enlarged.jpg

The image shows the imprint left by a nanoindenter in a particle of cement paste. The round blob at the top center is actually an extremely fine piece of dust on the surface. Photo / Chris Bobko

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.

Franz-Josef Ulm stressed that UHD concrete could alter structural designs, as well as have enormous environmental implications, because concrete is the most widely produced man-made material on earth: 20 billion tons per year worldwide with a 5 percent increase annually. More durable concrete means that less building material and less frequent renovations will be required.

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.

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