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. [♪ INTRO] 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 is our patrons over on Patreon.

Our community of supporters helps us  make videos like this one possible. And our patrons aren’t just doing it  out of the goodness of their hearts. Patrons get a lot of awesome perks, like  access to our private Discord server, behind the scenes material  like a monthly blooper reel, and access to our patrons-only podcast.

If you’d like to learn about becoming a patron, head over to patreon.com/scishow. That’s patreon.com/scishow Thanks for watching! [♪ OUTRO]