URBANA-CHAMPAIGN, Ill. — It’s not easy making the unsinkable out of the unthinkable.

But at the National Concrete Canoe Competition, civil engineering students use a material that is normally the stuff of dams and parking garages to build a 20-foot-long craft that will float even if completely swamped.

To do so, they replace the gravel and sand of conventional concrete with exotic materials like glass spheres. The result, to judge by the finals of this year’s competition, where 23 teams of 10 or more students gathered at the University of Illinois here, is a concrete that is exceedingly light and, with added fibers, strong as well.

But as the team from the University of Texas at Tyler found out, it is not always strong enough.

On Saturday, after two days of being judged on their engineering know-how and the quality of their final product, the students took to the waters of a nearby lake for races that would count for 25 percent of their overall score. Amid the excitement and noise — the Mississippi State team had brought along cowbells for the occasion — there was also heartbreak, when the Texas-Tyler canoe suffered what engineers soberly call catastrophic failure. That is to say, it cracked in half.