James' World 2

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Every bridge has parts that drivers never see: steel buried in concrete, welds tucked under girders, and soil packed around foundations below the waterline. A bridge can look fine from the road while rust spreads around steel hidden inside concrete. A small fatigue crack can lengthen. A flood can wash soil away from a pier. By the time cracks, loose concrete or lane closures appear, the cheapest repair window may already have closed.

When it comes to these damaged bridges, this problem is national. The United States has more than 624,000 highway bridges. About 220,000 need major repair or replacement, and 41,677 are rated poor, also called structurally deficient. While “poor” does not mean unsafe, it does mean at least one key bridge element received a poor rating, indicating deterioration or cracking that will require significant repair.

As a researcher who studies photonics and quantum sensing, I work on devices that measure faint or hidden signals. My lab applies physics to develop devices, including quantum sensors. Advanced sensors of this type might one day be able to help engineers pinpoint where to look to determine whether hidden damage in infrastructure is worsening. However, they cannot replace human inspectors.

Inspections keep bridges safe, but are snapshots

Federal bridge inspections—rooted in National Bridge Inspection Standards mandated by Congress in 1968—exist because past failures showed that small defects can threaten large structures.

Under current federal rules, many bridges must be inspected in, at most, 24-month intervals. Higher-risk bridges, such as those carrying heavy interstate traffic, those with aging structures or known defects, or those built over saltwater, may require shorter intervals. Lower-risk bridges with lighter traffic and sound materials may qualify for longer intervals.

Those inspections remain essential, but they are snapshots. A bridge may change during the months between visits. Corrosion can spread below a deck that looks sound. A small crack can sit inside a weld. A river can displace soil from a foundation while the roadway above looks unchanged. Sensors extend inspections by tracking these change that form between scheduled checks.

Hidden damage can grow quietly

The three common hidden threats to bridges are corrosion, fatigue, and scour. Corrosion begins when water, oxygen and salts reach steel. A concrete layer usually protects steel, but cracks, salt spray, and chloride ions from seawater or deicing salts can break that protection. The rust then expands, much like ice widening a crack in a sidewalk. It pushes the concrete outward and can cause the material to come loose or the layers to separate.

Fatigue damage is the bridge version of bending a paper clip back and forth. Just as a paper clip eventually snaps after repeated bending, a bridge’s steel components weaken and break down under continuous cycles of stress. Thousands of heavy vehicles can make tiny cracks grow near welds, bolted connections, or older steel details.

Scour damage is different: Moving water removes soil around the bridge’s foundations. The bridge above can look stable, while the support below loses the ground it needs.

Waiting costs more

The earlier engineers can identify damage to aging bridges, the more time and options they have to fix them. The average U.S. bridge is about 47 years old. Many bridges are near or past the 50-year life they were designed for, and about 45 percent have exceeded their planned design lives.

Typically, it’s less costly to preserve bridges in fair condition than those already in poor condition. Making all the identified necessary U.S. bridge repairs would cost about $467 billion.

Past failures show why small details matter. As one example, the 2007 I-35W bridge collapse in Minneapolis was partially due to undersized gusset plates—steel plates that connect the intersecting beams in a bridge’s structural framework – along with added weight and construction loads. The collapse killed 13 people and injured 145.

Sensors alone are not a cure for such failures, but better measurements can help engineers notice when important details are changing.

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