Fires in long traffic tunnels are rare. But when they occur, they tend to burn long and hot, inflicting damage on the concrete walls that requires months of closure and costs millions of dollars in economic loss.
At least 40 people died in the March 24 fire in the Mont Blanc tunnel connecting France with Italy, which burned for two days and reached a temperature of 1,832 degrees.
When fire burned for 10 hours in the 31-mile tunnel connecting France and England in 1996, the concrete walls peeled away layer by layer like an onion, destroying entire portions of the 20-inch-thick concrete ring and leaving the chalky soil exposed in places. Repairs took six months. Tunnel closure caused an estimated loss of $1.5 million per day.
A researcher at MIT now believes he understands exactly what happens to concrete in extreme heat, a first step to being able to design a more fire-resistant concrete for use in tunnel walls.
In two papers published in the Journal of Engineering Mechanics last month and prompted by the 1996 fire in the tunnel under the English Channel (the Chunnel), Franz-Josef Ulm and two colleagues analyzed the failure of tunnel walls. They examined the mechanical properties of concrete at the molecular level and created a computer model to mimic the material's reaction to intense heat.
"We used an interdisciplinary approach to this problem. Using chemistry and mechanics, we considered the mechanisms at the scale of a few water molecules to explain what happens to a 45cm tunnel ring during fire," said Professor Ulm of the Department of Civil and Environmental Engineering, lead author of both papers.
"Whatever money you put into research in this area will pay off," he said. "There is no comparison between the enormous economic loss and the cost of research."
He and two coauthors -- Professor Olivier Coussy of the Laboratoire Central des Ponts et Chaussees in France and Professor Zdenek Bazant at Northwestern University -- discovered that when mature dried concrete is exposed to extreme heat for long periods of time, the chemical bonds between the water molecules in the concrete break, destroying molecular bridges that bind together the various materials that make up concrete.
As the water molecules are pulled out of the skeleton through dehydration, the concrete loses its cohesion and weakens, pushing pieces of the concrete off the tunnel walls in very thin layers resembling onion peel. This phenomenon, called spalling, can eventually work its way through the entire concrete ring lining a tunnel, layer by layer.
"After the [Chunnel] fire, there were pieces of the spalled concrete on the tunnel floor. You could actually see the aggregates in the material in these thin slices," said Professor Ulm.
If the soil between the tunnel wall and the English Channel had not been stable at the points where this happened to the four-year-old Chunnel, water would have rushed in, he said. "There's no recovering a tunnel. Once the water comes in, you can't pump it out again."
One possible solution to the spalling problem is to include tiny plastic fibers in the concrete mixture. When heated, the plastic would melt and reduce the risk of spalling. Another answer could be to use certain paints or coatings on the concrete.
Concrete buildings receive ratings indicating how long the structure can withstand fire. For instance, a class 60 building can withstand 60 minutes of fire; a class 90 building, 90 minutes. Tunnel fires are an animal of a different color, with very hot temperatures lasting much, much longer. The tunnel itself sometimes works like a convection oven, drawing air in to fuel the fire. The air temperature during the Chunnel fire reached more than 1,000 degrees, heating the concrete to nearly 1,300 degrees.
Professor Ulm pointed out that the United States hasn't seen a big tunnel fire like those two in Europe, but it's not impossible. Subway tunnels generally include sprinkler systems, giving them a tremendous advantage over long underground or underwater traffic and rail tunnels. Those longer tunnels are usually equipped with modern information technology equipment that monitors traffic, provides safeguards against fire and swift communication when it does occur. But those can fail, as they did in the Mont Blanc and English Channel tunnels.
According to Professor Ulm, who began this research while at the Labor-atoire Central des Ponts et Chaussees, people in Europe are more concerned in general about the degradation of concrete in infrastructure.
"Research into concrete degradation was triggered by the problem of nuclear waste disposal, where you need a clear prediction over a long period of time," he said. "In Europe, people are also aware of the costs which are coming for infrastructure renewal and the research and development necessary to monitor it. This will be one of the great challenges to the United States in the new millennium."
A version of this article appeared in the April 14, 1999 issue of MIT Tech Talk (Volume 43, Number 26).