Sink or Swim: A Concrete Example

By: Christine Mapili

Cross Section

The job of an engineer revolves around solving problems, from calculating the maximum load on a structure to planning out the timeline for a major project. Sometimes the key issues are obvious from the beginning and the solutions need to be planned out well in advance, while in other instances, obstacles pop up later down the road and must be surmounted on the fly.

Both of these types of problem solving are called for in conquering one Virginia Tech design team’s objective: to build and race a canoe constructed entirely out of concrete. The average initial reaction might be one of abject disbelief, due to the apparent difficulties involved. “Wouldn’t it just sink to the bottom?” many might ask. Concrete may not be the most practical material for a watercraft compared to other options, but for the students involved, the project is more about the journey than the finish line. The experience allows aspiring engineers to explore a fundamental skill common to the discipline: creating a final product within a set of unique constraints with the best possible performance.

Nadeem

Though the American Society of Civil Engineers adopted it as an annual competition in 1988, the concrete canoe challenge has existed in some form since the 1960s. Virginia Tech’s team, run by the Department of Civil Engineering, builds its entry from scratch each year and races it in the contest. This current project is headed by captains Brian Hutter, Heather Hicks and Nadeem Khan.

The Virginia Tech squad tested three possible concrete mixtures this year, aiming to make the canoe lighter and faster compared to last year’s model. On a basic level, the craft needs to be less dense than water in order to float, so the ideal material would maximize volume while minimizing mass. This is accomplished by including glass spheres, fibers, and other components in the mix, along with a shape designed for high buoyancy. After testing key parameters, the team decided on a composition that was lighter than last year’s mixture, yet still strong enough to use in construction.

After the best material was selected, the students made several batches and ran compression tests to measure the strength of the concrete throughout the curing phase. The first batch was tested after seven days, as the concrete reached about 60 to 70 percent of its maximum strength. The second batch was examined after two weeks, and the third after a month, as the concrete approached its final durability.

Foam is cut to the shape of the canoe design as a mold for the concrete, and reused for transportation to protect it. To keep the craft structurally sound while in the water, it is composed of three concrete layers with reinforcing material between each. Reinforcing agents are also used in the concrete mixture itself, preventing the canoe from cracking. This is particularly important since it must resist two forces: the weight of people in it from the top, putting the structure there under compression, and the opposing normal force of the water on the bottom, resulting in tension on the concrete. Because concrete is stronger under compression than tension, the reinforcement gives the bonds of the concrete something to latch on to, allowing it to better resist the latter force.

After the reinforcement process, the canoe then must undergo a “wet cure” for four weeks to hydrate the cement, during which the boat is sprayed with water every day, and burlap is layered on top to retain moisture. Although concrete can dry in a short amount of time after being poured, keeping it moist helps the mixture develop stronger bonds to further decrease the likelihood of cracking. Once this is complete, the concrete is removed from the mold, which can be a tricky process since the material shrinks during curing. While this effect can be reduced using special chemicals in the mixture, it is impossible to eliminate completely. Once successfully extracted, the craft is sanded down and decorated, making it ready for competition.

This year’s concrete canoe contest takes place the first weekend in April at the Virginia Military Institute. Every year, the rules dictating the shape, size, composition and other elements change, forcing students to create fresh designs each time. In the first three of the competition’s four phases, entrants submit a design report prior to the contest, team captains present to the judges and the organizers evaluate the aesthetics of each boat. For the main event, each group competes in a series of races as the students responsible for each canoe try to be the first to paddle across the finish. The greater momentum of concrete compared to more conventional materials makes this task difficult, as steering becomes much more challenging. Teams cannot usually practice before the competition due to the risk of cracking if the concrete is exposed to water prematurely, so the crew can’t get a sense of the canoe’s handling until they jump in the craft for the first race. However, finding out how their vessel behaves in the water has to happen quickly for the Virginia Tech crew to have a chance, a process which team captain Hutter calls “problem solving on the spot.”

This year’s squad hopes to qualify for the national competition, and to do so will require every bit of their skill and talent, organized into task forces to solve specific problems. One such sub-group works on creating a stand to support past canoes, such as the “pencil boat,” the most recent design to reach nationals and showcased at Patton Hall, as well as last year’s entry currently on display at the Ware Lab. Another committee conducts structural analysis to ensure the craft withstands the stresses and loads it will face during throughout the competition.

Any willing and able student can contribute to the project, and participants are free to select the jobs they do best rather than being assigned an arbitrary task. “One of our biggest goals this year was just getting the entire team involved, and I feel like we made that goal at this point…making different little groups to tackle different problems,” says team captain Khan. With such diversity and dedication, the concrete canoe squad is sure to be successful.