March 1, 2008 Engineers developed a spray or paint that coats bridges with carbon nanotubes, allowing inspectors to check for damages without depending on visual indications. The skin conducts electrical current. If the bridge corrodes or cracks, it breaks the current or increases electrical resistance, and the location of that damage can be pinpointed by a computer, alerting inspectors to damage smaller than what human eyes can see.
The sudden collapse of the I-35 Minneapolis bridge that killed 13 people in August, left questions about the safety of our nation's bridges. Now, there is a new device that offers a high-tech approach to inspecting bridges. Each day, we depend on bridges to carry the weight of millions of vehicles.
But all that traffic can weaken a bridge, and many aging bridges are also prone to corrosion. Engineers regularly inspect bridges for damage, but checkups are limited to what the eyes can see. "That type of inspection can only see damage that's occurring on the surface of the bridge," says Jerome Lynch, Ph.D., a civil engineer at the University of Michigan.
Now, civil engineers have a new device -- called a sensing skin -- to help find damage deep inside bridges that may be missed. "So, this skin is applied to the surface of the bridge, and essentially can self-sense whether corrosion is occurring, cracking is occurring on that bridge," says Dr. Lynch. The skin is a thin material, lined with electrical wires. An electric current is sent through the wires. If there is any corrosion or cracking inside the bridge, it will break the electrical current.
A computer then creates a visual map of the change, which alerts inspectors exactly where damage is located. "So, essentially, if the bridge cracks the skin will crack. If the bridge is corroding, the skin will also observe that corrosion," says Dr. Lynch. Researchers expect the skin to also help inspect buildings and planes for hidden damage. Bridges are paving the way first for this high-tech device. "Ultimately, they're going to render bridges safer," says Dr. Lynch … Helping all of us to cross safely to the other side. Researchers plan on field testing the sensing skin on bridges next year.
ABOUT STRUCTURAL DESIGN: All modern high-rise buildings and most bridges feature something called redundant design: when one section fails, the entire structure should still be able to stand. It's a bit like a net: lose one string and the net will be weakened, but as a whole, the net still functions. Most high-rise buildings and bridges also are designed to sway in the breeze, because if the structure is too rigid it can snap under too large an impact. The World Trade Center was designed to withstand strong winds striking it from the side, and was even able to absorb the initial impact of an airplane colliding with it on 9/11, although the Twin Towers ultimately collapsed.
Fiberglass is a composite material. A composite is any material made of more than one component, and therefore has properties of both. Glass fibers are embedded in certain polymers to make them stronger. Fiber-reinforced composites are both strong and light, often stronger than steel. Fiberglass is one such composite. In fiberglass, the glass fibers aren't arranged in any particular direction. By lining them all up in the same direction, scientists can make the composite stronger -- but only in that one direction.
A Bridge with Skin? That's an example of biomimicry, a field in which scientists, engineers, and even architects study models and concepts found in nature, and try to use them to design new technologies.
The American Society of Civil Engineers and the Materials Research Society contributed to the information contained in the video portion of this report.