I’m guessing that you don’t spend all that much time thinking about bridges.
Even if you cross a few on your daily commute,
you probably don’t give any thought to the fact that tons of steel
and concrete are holding you hundreds of feet in the air, and usually over water.
And since there are at least a million bridges around the world,
including overpasses, a lot of us depend on them staying upright.
We really only think about how important they are when something goes wrong,
like that 1-95 overpass that collapsed earlier in 2023.
And it just so happens that one of the biggest dangers to our bridges
is the same thing that’s wrecking so many other things: climate change.
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Let’s be honest: one huge problem with many bridges is that they’re just plain old.
While they’re designed to be sturdy and to last decades,
right now, the age of the average bridge in the US hovers around 45 years old.
And that longevity also means that back when these structures were built,
the bridge construction standards were suited to the condition
that bridge would face… at the time, anyway.
The engineers designing them didn’t include the additional structural components
that would help those bridges withstand the effects of climate change,
because they didn’t necessarily know about those future problems.
But you may be surprised to learn that they still
don’t have to factor in climate change into bridge designs.
See, while our bridge design standards do plan for weather resilience,
that weather is predicted based on climate data from the past 200 years, which
can’t prepare the bridges for the current and future effects of the climate crisis.
So to dig into what can go wrong, and how to fix it,
let’s talk about a few critical design elements that go into making a good bridge.
One thing that has to be considered is the concept of thermal stress.
See, when you expose a material to heat, it expands.
And as a result, part of what you’ve built may not fit together perfectly anymore,
as anyone who has a door in their house
that doesn’t close all the way in summertime will know.
Bridge engineers of the past planned for this thermal stress by including
expansion joints, which are gaps in the bridge structure that give it a little
breathing room so it can safely expand and contract in changing temperatures.
But expansion joints can get clogged up with debris like falling leaves,
and there’s no easy way to keep up with cleaning them out.
And the gunked up expansion joints provide no room for expansion,
so on hotter days, that can now lead to the bridge breaking up and failing.
Since average temperatures are on the rise, this puts
more and more bridges at risk of failure without functional expansion joints.
Depending on how efficient we’re going to be at fighting climate change,
researchers calculate that thermal stress could cause between
60 and 95% of bridges with expansion joints to fail by the year 2100.
Fortunately, there are more modern and leaf-proof bridges
called integral abutment bridges that don’t have these same problems.
These bridges dissipate thermal stress by using
elastic connections between bridge structures instead of just expansion joints.
Unfortunately, integral abutment bridges are more vulnerable to
something called scour, which is the process where water forms eddies
around submerged parts of the bridge, and that eventually
washes away the ground holding down the bridge’s foundations.
Engineers design against that, but here, again, they’re basing their work
on old data about the intensity of rainfall and flooding in a given location.
Climate change has been causing more rain and flooding,
and things are set to get even worse in the future.
This means that scour will eat away at bridges
much faster than our bridge building guidelines currently predict.
The good news is that we already have a lot of solutions
to fight the super-scour of the future.
Some of them involve putting a collar on the pier
to counteract the eddies that dig up the riverbed.
Others use smaller concrete or metal columns called
sacrificial piles to break up the flow of the water before it hits the bridge.
But even if we’re prepared for the scour, there’s also danger in the air.
To make concrete more resistant to any tension,
it’s often bolstered by internal steel reinforcing bars, also known as steel rebar.
But that steel is vulnerable to corrosion.
And this is how the rising levels of CO2 in the air hurt our bridges directly,
even if the heat and flooding can’t get them.
The carbon dioxide in the air can gradually seep
into the cement in concrete, a process called carbonation.
Normally, steel rebar is protected from corrosion by a thin layer of iron oxide,
which forms naturally thanks to the high pH inside the concrete.
Even for the thinnest steel rebar used in concrete,
that protective layer will last around 60,000 years.
But carbonation gradually changes the pH of the concrete
until it’s no longer alkaline enough to make the iron oxide.
And without that protective layer, steel will rust 1,000 times faster,
cutting 60,000 years down to a more human-scale 60.
So more carbon dioxide in the air equals more corroding rebar in concrete bridges.
We have solutions for this, too.
The simplest one is to make the concrete around the steel rebar thicker.
There are also epoxy or acrylic sealers that can
help fill up any tiny pores that might help CO2 get inside the concrete.
So there are solutions if it takes us longer than we hope
to address the climate crisis.
The real problem is that legislation tends to
take decades to include current science.
Climate change itself is a good example.
Even though climate change has been a fact in mainstream science
at least since the 1970s, decades of lobbying sponsored by fossil fuel companies
resulted in our bridges still being built as if the climate crisis has never existed.
Thankfully, legislators in some countries are now updating bridge-construction
standards to include climate resilience.
If others follow, maybe we can all go back to not thinking about bridges again.
And one thing we’d rather be thinking about at SciShow
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Thanks for watching!
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