Concrete isn't flammable. Yet the fire that broke out after a crash that killed four people on Interstate 84 so impaired the concrete overpass above that the state has decided to spend millions of dollars to replace it.
The crash occurred under the Cloverdale Road overpass about 11:30 p.m. Saturday, June 16, as eastbound traffic was being herded from four lanes to one for overnight highway maintenance. Three Mountain Home Air Force Base airmen died. So did a commercial truck driver, identified Friday as Illya D. Tsar, 42, of Rochester, New York.
The Boise Fire Department was alerted to the fire at 11:32 p.m. and put it out by 11:50 p.m., spokeswoman Char Jackson said.
How could a fire that burned for 20 minutes do so much damage?
The answer lies in the composition of one of mankind’s most prolific materials: concrete.
Concrete is the single most widely used material in world, second only to water. The reason is its versatility as a building material.
It is essentially a stone version of fruit jello. The role of the fruit is played by chunks of sand and stone, known as aggregate, and the jello is the cement that holds everything together. That cement is a complex mix derived from limestone, clay and gypsum. When water, which is between 14 and 21 percent of concrete, is added, the cement hardens and holds everything together.
It turns out that all that water is the root cause of damage when concrete is heated.
With heat, that water turns to steam, which damages the concrete as it is expelled, causing the material to crack and flake apart in a process known as explosive spalling.
When spalling occurs on the exterior of the concrete, material can break off and fall onto the road below. Jake Melder, a spokesman for the Idaho Transportation Department, said this did not occur and ITD deemed it safe to reopen I-84 underneath the bridge.
The damage was internal. For added strength, the concrete girders are filled with steel cables and rebar. The cables create a tension in the beams toward their centers, holding everything together and increasing the weight load they can handle. Under extreme heat, the concrete separates from the steel inside, and the metal weakens.
ITD engineers determined that this separation had occurred.
First, they figured out the temperature the concrete was exposed to by the color it had turned after the fact. Most of the bridge structure was blackened by soot but not directly damaged, Melder said. But some sections had turned pink and white, indicating temperatures upward of 1600 degrees that changed the concrete's chemistry.
Second, the engineers conducted acoustical tests on the girders, hitting them with hammers and observing the sound. The girders gave off a hollow sound, indicating that the concrete had indeed pulled away from the steel inside, creating open space.
If the bridge was subjected to a lot of weight, such as large trucks, the concrete girders beneath would likely fracture, dropping pieces to the freeway.
There are two options for repair.
One is to remove the road above and replace the girders, which would only serve as another patch on a bridge built in 1966 and repaired in 2002 and 2007. ITD estimated that this would cost between $1 million and $1.5 million.
"That investment on an aged bridge that is inadequate for today's traffic and pedestrian needs is not a good use of taxpayer funds," said Julie DeLorenzo, a member of the Idaho Transportation Board, in a news release.
The second option is to rebuild the bridge at a cost of $6 million to $8 million. That's what the Transportation Board chose Friday.
The bridge will expand to four travel lanes and a pedestrian walkway, in accordance with the Ada County Highway District's master plan for Cloverdale, which is a county road. The new bridge will have a higher clearance over I-84 and will be long enough to accommodate future expansion of the freeway to five lanes each direction.
The replacement is expected to take a full year to complete. The state plans to seek the highway district's approval next week.