How Many Elephants Does It Take To Crush Concrete?

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And Other Questions Answered During The Summer Camp Engineering Field Trip

It was a bright and blazingly hot day in Austin, but it was cool and dark down in the basement of the Ernest Cockrell Jr. Engineering Building. Here on the University of Texas campus, twenty-six first and second grade girls craned their necks to get a look at a chunk of concrete floating in a bucket. “Concrete can float,” the presenter told the girls, “but not all concrete does float.” The presenter, an engineer herself, specialized in the field of concrete. She told the group about a competition that UT enters in every year, where engineering students must design and build a canoe out of concrete and race it against other schools. The Longhorns had been known to win, although it depended on the year. The girls were enthralled. Who knew that concrete could be so interesting? I sure didn’t.

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Our “Engineer It” summer camp group had spent the first part of the week learning the basics of engineering. They made boats, balloon cars, paper airplanes, and kites. They paid a visit to Huston-Tillotson University and attended the “Chemistry Circus” at UT. They got their hands messy with oobleck and glitter slime. The girls were passionate about engineering at this point.

The field trip to the engineering department kicked off with a presentation about the history and need for concrete. Did you know that concrete and cement are not the same thing? Cement is made of limestone and other minerals and creates a chemical response when combined with water. Concrete is made up of cement and water and rocks. The cement binds the mixture together and creates a base for many of our buildings and other civil projects. The chemical reaction created by the mixing of cement and water produces quite a bit of heat, so one of the engineering assistants made up a batch and cracked a fresh egg over the top. We watched the egg cook right in front of our eyes. It turns out that you can cook an egg on the sidewalk, as long as the cement has just been mixed! The presenter had a volunteer from the group help her make a batch of “test concrete”—a.k.a. brownies. Eggs and water stood in for the binding agents and M&Ms and white chocolate chips denoted the rocks. For the record, concrete brownies are much more delicious than the real thing.

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The girls split into two groups for their afternoon activities. Group 1 stayed in the classroom and practiced mixing cement. They stirred cement and water in a cup and when the mixture was finished, they added beads. The finished product was a bejeweled medallion that they would get to take home to show their families. Group 2 walked over to a testing area. One of the engineers gave us a demonstration of how much weight it would take to crush a relatively small cylinder of concrete. UT’s pressure testing machines track the weight placed upon an object. Using the approximate metric of elephants (assuming that one elephant weighs around 10,000 pounds), we stood back and watched the machine do its thing.  As the number was climbing higher, one of the girls exclaimed, “I can’t imagine it getting up to eight elephants!” You can imagine our surprise as the counter ticked past 150,000 pounds. We determined that it would take at least SIXTEEN elephants as the concrete finally cracked under the weight.

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The girls swapped activities and eventually they took the bus back to The Thinkery. We all left with a greater sense of gratitude for the building blocks that hold our city together…and for the civil engineers who keep us safe. My bet is that more than one of the girls on that field trip will end up in the engineering field someday.

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