Fire, Metal and Concrete: Why the Atlanta Bridge Collapsed

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The Atlanta bridge on I-85 that collapsed this week was not on the list of America’s most structurally deficient bridges. Georgia actually ranks pretty well against the likes of New York and Pennsylvania in terms of percentage of deficient bridges. Georgia is 45th for percentage of structurally deficient bridges. Only 5 states are better (Nevada, Texas, Florida, Arizona and Utah).

So why did it fall if it was not rated as dangerous?

Americans love to solve mysteries. We are a curious lot. We love Mythbusters and How Things Work, or at least I do. So it did not surprise me that when an Atlanta bridge fell down because of a fire under the bridge, not on or in the bridge, that I would get a lot of questions.

When I was Inspector General of the U.S. Department of Transportation, aviation and rail were always front and center in the press. But the biggest federal expenditures and the most urgent of needs were — and remain — our inventory of deficient bridges and our roads.

For years just outside the door of our law firm’s Charleston office, we said our prayers, crossed our fingers and drove over two of the most dangerous and decrepit bridges in America. Thank you, United States citizens, for buying us a beautiful new bridge that we all now adore. The Ravenel Bridge is the biggest tourist attraction in Charleston — and not far behind is the USS Yorktown Aircraft Carrier, conveniently parked in concrete right next to our gorgeous bridge.

Which brings me to the important variable in all this–concrete.

People think concrete is fireproof, and the real answer is it – and it isn’t. Concrete does not burn but it is compromised by exposure to high temperatures. How compromised it becomes in a fire is a matter of several variables, so to this question I get to give the lawyer’s favorite answer: it depends.

Doctoral dissertations have been written, replete with hundreds of formulas including lots of sigmas, about the formulation of concrete and the rate of degredation in a fire. Concrete is a science and a darned important and complicated one in fact –one to which we trust our lives without much thought.

While concrete does not burn, high temperatures cause concrete to suffer from thermal strains. High heat effects the strength and adhesion of the concrete, called compressive strength, and can cause something called explosive spalling.

When subjected to high heat, the water that is chemically bound with the cement is removed and can become vapor within the concrete. At 500 degrees Celsius, Portland cement can decompose.

Next consider the stones or aggregate within the mix. Different stones have different thermal strains.  When fire temperatures reach 1000 degrees Celsius, some of the aggregate may melt. This releases gases and causes expansion, further weakening the concrete. Different rock textures have different porosity and that too can affect what happens to the concrete in a fire.

Spalling is similar to crumbling. Exposed to high heat, the fascia comes off first but it can also result in crumbing that looks like a jackhammer has been taken to the cement. This type of damage was exhibited in the Channel Tunnel fire in 1996, the Mont Blanc Tunnel fire in 1999 and the Gotthard Road Tunnel fire in 2001.

Now let’s add rebar to the redi-mix. When temperatures hit 500 to 600 degrees Celcius, the strength of the metal is decreased by one-third or more. The failure of the World Trade Center towers on September 11, 2001, reminded us all of the possibility of failure of reinforced concrete in fire. Common rebar will fracture under elevated temperatures. Of course it also matters if there corrosion inhibitors applied to the rebar used in the concrete.

And there are expansion joints or plates. These are typically metal and they, too, are weakened by high heat and environmental conditions.

The temperature of the fire is important, but so is the rate and duration of heating. A really odd thing is that concrete can recover, but it takes time—it has to make it through the fire, and then be given time to cool, relaxing the stresses from the heat event.

So while the investigators are carefully examining what caused a bridge not on the decrepit bridge list to collapse, let’s not forget the human element. Unused construction materials were stored under this bridge for well over 5 years, and it appears those materials had not been used or touched during that time. It’s a colossal waste of taxpayer money, both in the now destroyed bridge and the years gone unused construction materials which were clearly also a hazard to our precious infrastructure. It may be an important warning that hoarding construction combustibles under a bridge is a dangerous thing to do and we should be thankful we learned the lesson without loss of human life.

Why is this so important right now? Because we are about to have an infrastructure building boom. President Trump has encouraged Congress to fund a nationwide infrastructure repair effort. This is very much needed and an important national goal, but as projects begin we must remember that our building standards matter. What’s in our concrete and steel matter, and we must remember to require our contracts meet our safety standards — because our lives depend on it.