Concrete that can bend
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.
N.N. Sachitanand
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