Page 158 - AWSAR 2.0

134 || AWSAR Awarded Popular Science Stories - 2019
of useful life, which means that they may not last for 100+ years as expected. Also, if we are not careful and do not monitor correctly, a particular type of bridge (prestressed concrete) can even fall all of a sudden. It has happened in the other parts of the globe. One was in North Carolina Lowe’s Motor Speedway Pedestrian Bridge. One person even died. What has happened can happen again.” Uh oh! Almost all the new bridges in India are made of prestressed concrete, including almost all the metro bridges. What if...? Sigh. What a depressing thought. The critic inside me wins. Always.
Well, you can’t hold on to “ignorance is bliss” when safety is at stake. It is better to face the cold hard truth and Fig. out what can be done to avoid mishaps. Can the sky fall? Yes, I am referring to our enormous prestressed concrete bridges as the sky. And can they fall? on us or with us on top? Yes again. Before you become paranoid and decide
not to step outside your home,
let me tell you something. If we
are able to catch the subtle
signs at the right time, we can
not only prevent this disaster,
but also ensure longevity of our
bridges by performing some
interventions. In short, this
can be made into a ‘happily
ever after’ story if we make a
conscious effort to perform
a good assessment of these
structures, keeping the risks in
mind.
Let me take you through
my little world. I work as a PhD
student under the guidance
of Dr Radhakrishna G. Pillai
in the Building Technology and Construction Management division at IIT Madras. He heads the “Corrosion Group” that focuses on understanding corrosion of embedded
reinforcing steel. My research is on corrosion of prestressing steel, a special steel used in prestressed concrete structures. It has a tensile strength of about 4-5 times that of conventional TMT bars. Structural engineers have worked wonders using this steel and created numerous slender crack resistant structures that make our nation and us proud.
On the down side, as a material, this steel is more prone to corrosion-induced failure. It is made by cold drawing process and is consequently known to have a lot of residual stresses and microcracks. It contains more carbon than conventional TMT bars and is a lot more brittle. Prestressing steel is supplied and used in the form of strands with 6 wires helically wound over a single straight one. This makes strands susceptible to corrosion at the interfaces between the wires (crevice). As a feather on the hat, strands are kept at a stress level of about 0.75 times their ultimate tensile
strength, in the structure. Tensile stress, susceptible material and corrosive environment – perfect concoction for stress corrosion cracking and consequent sudden failure by brittle fracture of steel. Trust me, this is serious.
A unique problem has to be tackled with a unique solution. Even here, problems occur because we try to deal with corrosion in prestressed concrete (PC) the same way we would address corrosion in conventionally reinforced structures (CRC). In CRC, the rust products (bulky iron oxides) formed at the steel surface
exert a pressure on the surrounding concrete and crack it. That is why we see brownish streaks on some of the beams and columns around us. It indicates ongoing corrosion.
If we are able to catch the subtle signs at the right time, we can not only prevent this disaster, but also ensure longevity of our bridges by performing some interventions. In short, this
can be made into a ‘happily ever after’ story if we make a conscious effort to perform a good assessment of these structures, keeping the risks in mind.