Tuesday, July 6, 2010

Expanding concrete! Really?!?

A portion of Chicago's famous Lake Shore Drive (LSD in local parlance) was closed due to buckled concrete (see the photo from the Chicago Tribune).

Why did this occur? It's simple: the heat.

In grade school science class, kids are taught the concept of thermal expansion: the phenomenon where high temperatures cause materials to expand and cold temperatures cause materials to contract.

Everything undergoes thermal expansion. In fact, some materials noticeably expand or contract with only a small change in temperature. Engineers must factor in thermal expansion of different materials when constructing roads, bridges, and buildings.

Why? Let's take the buckled LSD concrete as an example:

Under normal circumstances, roadways are designed to accommodate thermal expansion and contraction to certain design limits.

The weather over the past few days in Chicago has been anything but normal and likely exceeded the design limits. Unusual warmth both day and night, prevented the concrete sections from cooling and contracting sufficiently. While the concrete was able to withstand several warm days and nights in a row, the intense heat on Monday was the final straw for the section in question. As the concrete expanded beyond engineering limits, several slabs pushed against the other with great force. The only way to relieve strain was to move in the direction with the least resistance (upwards), which is what happened.

Considering the length of Lake Shore Drive, I'm impressed only one section buckled.

Some may ask, why not construct the roadway with greater high temperature tolerances? Since the road way must be used year-round, engineers must also consider the opposite problem of thermal contraction during the winter. If the roadway was only designed with tolerances for the heat of summer, it is likely there would be gaping cracks in the concrete during the cold winter temperatures.

Oh, I forgot to mention that temporary repairs were made overnight to the section of buckled concrete. Thank you City of Chicago Department of Transportation road repair crew!

-Dr. Dave

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