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by guest blogger Brianna Ahron

Have you ever seen a crack in a cement bridge and thought, “That can’t be safe?” Well, that crack you are looking at may be cosmetic and nothing to worry about.

Crack in the concrete on the Aurora Bridge

Steel bars in the concrete give it strength

 This is a crack on the Aurora Bridge facing Fremont. The picture was sent to us in early May by a concerned citizen wondering if the cracks concerned us. The answer is no. Our bridge inspectors examine bridges every two years or more depending on structural issues or the age of the bridges. The inspectors noted these cracks in their most recent routine bridge inspection report on Aug. 1, 2010.

Cement bridges have many reinforcing features to help keep them standing, such as steel bars that run through the concrete. These bars help to increase the strength and durability of the bridge. They also have contraction joints, which are grooves in the concrete slabs that help control where cracks occur. The cracks in the Aurora Bridge were considered cosmetic, and were  likely caused by shrinkage in the concrete from moisture in the air and changes in temperature. Cosmetic cracking due to weather is expected in all cement bridges.  This is why cement structures are reinforced with contraction joints, so that the cracks that do occur are in predictable, safe locations.
   
Our engineers are experts on determining whether a crack is structural or cosmetic, but how can you tell? Structural cracks generally occur at locations where the stresses on the bridge cause the reinforcing steel inside the cement to stretch to the point where the concrete cracks. This occurs mostly at high-stress areas in the middle and ends of beams. If a structural crack does occur, it needs to be repaired immediately.

Cosmetic cracks are usually very thin, one-sixteenth of an inch or less, and occur in low- stress areas of bridge support beams. They can become structural over time as water and air reach the reinforcing steel inside the concrete. Once the cracking reaches the steel, it’s considered a structural crack. Structural cracks are typically larger than cosmetic cracks and can cause the steel to rust and corrode. The rust and corrosion takes up more space than the original steel reinforcement. This can put unwanted pressure on the concrete and cause it to crack.

According to bridge engineer Craig Yasuda, cracks occur in every cement bridge. During bridge inspections, engineers look at the size and the width of cracks on the beams of bridges and if the cracks are in areas of high stress. The Aurora Bridge gets inspected at least every two years. Last year it was inspected after one year due to some retrofit work. Some older bridges or bridges with structural issues are inspected even more frequently.

We are a national leader in bridge inspections and maintenance. There are roughly 7,000 bridges in the state that we inspect biennially. The bridge program emphasizes cost-effective preservation programs that increase the life-expectancy of bridges and make them safer. Some of our bridge programs include bridge replacement and rehabilitation work; seismic retrofit work to strength bridges to withstand earthquakes; and bridge foundation scour mitigation to repair areas beneath bridge support pilings that have been washed away by water.

We have 56 trained engineers and technicians who perform bridge inspections, including specialized dive teams and mechanical and electrical inspectors. In 2010, we inspected 1,963 bridges, and 1,859 are scheduled for inspection in 2011.

When asked what makes the our bridge inspection program so great, Yasuda said, “It’s because we inspect our bridges on time.” He also said that we have an excellent bridge maintenance program that helps keep the bridges clean and alerts the bridge team if there is anything wrong with a bridge in between inspections.
So, if you see a crack on a bridge, don’t worry. Our bridge engineers have got a handle on it. For any other information or concerns about bridges or the inspection program, visit www.wsdot.wa.gov/Bridge/Reporting/.