Modern life is taking its toll on
an ancient bulwark of strength: concrete.
The Egyptian pyramids and the Roman coliseum are still standing,
their concrete holding fast after thousands of years of
weathering. But modern concrete structures can fail after only
20 years.
The difference? Modern structures are forced to withstand a lot
more use, bear a lot more weight, and must cope with more
intense, strength-withering pollutants.
Vanderbilt’s Florence Sanchez is looking to the future for
answers. The assistant professor of civil engineering is
exploring the tantalizing world of nanoscience for ways to
strengthen concrete by adding randomly oriented fibers ranging from nanometers to micrometers
in diameter and made of carbon, steel or polymers. (A nanometer is
roughly the size of three to ten atoms.)
“Concrete is an ancient material that has been used for centuries,
but its chemistry is still not well understood,” Sanchez says. “We
mix cement with aggregate to create concrete, which we often
reinforce with steel rebar that corrodes over time, leading to
significant problems in our transportation and building
infrastructure.”
Sanchez is working on making modern concrete structures stronger and
more resilient well into the future, using novel microfibers and
nanofibers that may help concrete stay strong over time, in a
variety of conditions due to weathering. She has won a National
Science Foundation CAREER award for her research into the effects of
using these fibers to reinforce concrete’s strength and to give it
such exotic capabilities as electrical heating and self-assessment.
Since nano/microfibers made of carbon can conduct electricity, a
nano/microfiber-reinforced concrete bridge could be heated during
winter or could monitor itself for cracks. Or concrete mixed with
polymers would be pliable and flexible, and can be used to make
buildings earthquake resistant.
“All of these ideas to reinforce concrete with fibers look good in
theory and in the short term, but we need to make sure that the new
concrete structures will still be holding firm 50-100 years from
now,” Sanchez says.
All in the mix
Concrete may look crude, but its chemistry is quite sophisticated.
There are many different types of concrete in use today, each type
with its own particular combination of properties and
characteristics. Builders use the type of concrete that will best
carry out the job the material needs to perform in the structure.
Concrete is an odd material, really. It gets hard by adding water,
for one thing. For another, although concrete achieves most of its
strength after 28 days, it continues to cure for years after it is
first set. And it’s tricky to make: Clinker, the base material for
cement, is made in a tubular kiln that is up to 750 feet long and
heats up to 2700 degrees Fahrenheit and is mixed with small amount
of gypsum to produce cement.
Other ingredients such as aggregates and water are then added.
However, too much water and concrete gets too weak; too little water
and it stops solidifying before it’s hard enough.
Like bones, concrete gets its strength from calcium. And it’s the
leaching of calcium out of concrete by acids in the environment that
is the biggest threat to the strength and integrity of concrete
structures.Microfibers and nanofibers are expected
to strengthen concrete and give it other desirable properties, but
no one knows how these new materials will affect the durability of
concrete structures over the long haul.
“We know that nano/microfibers improve the strength of concrete at
28 days, but we don’t know how well it resists
weathering over the long term,” Sanchez
says, “We do not fully understand the chemistry, the mechanisms of
interaction, or how molecular level chemical phenomena at the
fiber-cement interface influence the material performance during
environmental weathering.”
Accelerating aging
To determine the long-term performance of concrete reinforced with
fibers, Sanchez is accelerating the aging process of concrete so she
can determine how concrete mixed with new materials will withstand
weathering over time.
Sanchez is using a solution of ammonium nitrate to increase the
solubility of calcium, which allows it to leach out of the concrete.
The accelerated aging process is based on the same chemical process
as occurs in the natural environment, but at a much faster rate.
“First we leach out calcium hydroxide, which makes up 20-30 percent
of the cement mixture,” Sanchez says. “Then we leach out calcium
silicate hydrate, which is the gel that holds cement together.”
She and her associates then subject the decalcified cement to a host
of tests, including thermal analyses, infrared spectroscopy,
electron microscopy, and other analyses that help elucidate the
chemical reactions and to infer the chemistry of the interfaces
between the fibers, the crystalline calcium hydroxide and the
gel-like calcium silicate hydrate.
Her team will also subject the material to a variety of typical
weathering forces, such as varying temperatures, water, acids,
sulfates, and chlorides,. They will then analyze the material to see
how these forces change the chemical processes within the concrete
Because microfibers and nanofibers have a great deal of surface area
for their mass, understanding how these fibers will interface with
other substances within the cement is crucial to using the material
effectively. Cement’s adhering capability is at the heart of its
usefulness in holding concrete together, so researchers need to know
whether the increased surface areas of fibers will ultimately
strengthen or weaken the bonds within concrete.
“Our research will help provide a roadmap for civil engineers and
builders to harness the unique properties of microfibers and
nanofibers,” Sanchez says. “We hope to also add to the world’s
understanding of how this unique material works.”
