Concrete that can bend
By N. N. Sachitanand
The Hindu
Thursday, June 30, 2005
For most of us, concrete represents a hard, stiff, strong
and brittle material, prone to crack failure. For a long
time now, engineers have been able to impart a very
slight ductility in concrete by reinforcing it with
fibres. Now, a team of researchers at the University of
Michigan, Ann Arbor, under engineering professor Victor
Li, have come up with a concrete that is actually
flexible.
Gives under great strain
Belonging to the class of materials called Engineered
Cement Composites (ECC), the new material looks like
regular concrete, but is 500 times more resistant to
cracking and 40 percent lighter in weight. ECC is a
strain-hardening material while the normal fibre
reinforced concrete (FRC) exhibits tension -- softening
behaviour. Under excessive strain, the ECC gives, because
the specially coated network of fibres veining the cement
is allowed to slide within the cement, thus avoiding the
inflexibility that causes brittleness and breakage.
In traditional reinforced concrete structural design, the
most common and most important material parameter of
concrete is its compressive strength. For this reason,
structural strength (and more generally, structural
performance), is often perceived to be governed by
material strength. This means that higher material
(compressive) strength is expected to lead to higher
structural strength.
This concept is correct, points out Professor Li, only if
the material strength property truly governs the failure
mode. However, if fracture failure occurs, a high
strength material does not necessarily mean higher
structural strength. Rather, a high toughness material,
and in the extreme, a ductile material like an ECC, can
lead to a higher structural strength.
The key to the behaviour of ECC is that it is engineered,
which means that in addition to reinforcing the concrete
with microscale fibres that act as ligaments to bond the
concrete more tightly, scientists design the ingredients
in the concrete itself to make it more flexible.
"The broad field of micromechanics has tried to
understand how composite materials behave," says Li. "We
went one step further and used the understanding as a
material design approach in the development of our
material."
Bendable concrete is made mainly of the same ingredients
in regular concrete minus the coarse aggregate. Tiny
Polyvinyl Alcohol (PVA) fibres comprise about 2 per cent
of the material's volume.
When you put steel-reinforced concrete under load, it
cracks and the steel fibres tend to slip out. PVA fibres,
on the other hand, form a molecular bond with the
concrete during hydration, so you cannot easily pull them
out. Micromechanics provides guidelines to tailoring the
fibre and the interface to minimize the critical fibre
volume fraction.
Higher surface coating content (between 0.8 and 1.2 per
cent by weight of fibres) tends to lower the interface
chemical and frictional bond properties to a level that
causes the critical fibre volume fraction to drop to a
minimum of about 2 per cent.
Lasts twice as long
A University of Michigan release says that ECC is
expected to address traditional concrete's many problems:
lack of durability and sustainability; failure under
severe loading.
Comparison studies have been done by the university's
School of Natural Resources and Environment's Center for
Sustainable Systems.
They show that over 60 years of service on a bridge deck,
the ECC is 37 per cent less expensive, consumes 40 per
cent less energy, and produces 39 per cent less carbon
dioxide than regular concrete.
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