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By JAE O. HAROLDSEN
How can that be? Concrete weighs roughly 150 pounds per cubic foot (2,400 kilograms per cubic meter). Compare this with water's density at 62 pounds per cubic foot (995 kilograms per cubic meter). How can concrete float? In a solid chunk, concrete sinks like a rock. But what if your teacher gave extra credit to students who participated in designing, building, and racing a concrete canoe? That's what happened to Myia Redic, a mechanical engineering student at the University of Alabama at Huntsville (UAH). When Myia first heard of the annual concrete canoe competition sponsored by the American Society of Civil Engineers (ASCE) and Master Builders, Inc., she thought, "Concrete? Do bathtubs masquerade as canoes?" We accept steel ships. But, wait! Steel weighs three times more than concrete.
The same properties that enable steel ships to float apply to concrete canoes.
Thinning concrete, spreading its weight over a 6-meter length, and creating an inside cavity with a depth of one foot (30
centimeters), causes a concrete canoe to weigh less than the water that it displaces.
So, it floats! UAH conquered the 2001 national competition with Survivor, a 76-pound (34-kilogram), 22.3- foot-long (6.8-meter) concrete canoe. Survivor was not only light, but it was also flexible. Regular concrete is rigid and withstands compression or pushing forces. However, tension or pulling forces break it. Reinforcement - usually steel - placed inside concrete withstands tension forces. UAH used graphite (soft carbon) mesh for reinforcement. The mesh had a checkerboard-like pattern and spread like fabric. With this elastic reinforcement and the addition of latex (a rubbery material) to the concrete mix, Survivor gently flexed without breaking. The canoe flexed during paddle strokes and released stored energy to surge forward between strokes. Instead of using heavy aggregate (sand and stone) in their concrete, UAH used microscopic glass beads, which are so fine that they sift like dust through your fingers. To glue glass beads together, the heavy-duty boat builders used Portland cement and water, which normally binds concrete; latex, which gives rubberlike flexing capacity; and acrylic fortifier, a glassy, heated plastic.
And to form this innovative concrete into a racing canoe, UAH used computer graphics to create a physical design. From their computer drawings, they built a mold of the inside of the canoe. Usually, concrete is poured into a mold, but UAH applied concrete to the outside of theirs. A flat wooden frame, measuring the inside width and length of the canoe, laid the mold's foundation. Using a jigsaw, they cut plywood sheets to the canoe's inside width and depth and fixed them vertically to the frame every 18 inches (46 centimeters). They hot-glued together 2-inch-thick (5-centimeter), trapezoidal-shaped foam strips across the plywood sheets, running the canoe's length. This generated a complete mold of the canoe's cavity. Rough sanding and patches of putty refined the mold's shape, while fiberglass hardened the surface.
UAH laid a plastic sheet over the mold, so that Survivor could eventually be removed with ease. On top of the plastic, they applied the first layer of graphite mesh. To enable them to place a thin, even layer of concrete over the mold, UAH ran a wire at the same height every 3 inches (7.6 centimeters) along the canoe's length. Small batches of concrete, which smelled and felt like putty, were mixed and placed by hand. Students leveled the wet concrete to the top of the marking wires, and then left it to dry for 12 hours. Once dry, the marking wires were pulled out, a second middle layer of graphite mesh was laid down, and the process of stringing marking wires and placing concrete was repeated. Twelve hours later, UAH repeated the process for the third and last time. After three additional days of drying, UAH perfected Survivor's ability to cut through water. Students used soft lighting to create shadows that enabled them to identify high and low spots. They then smoothed the rough surface by hand, sanding or applying additional concrete where needed. And then the moment of truth arrived. Myia recalls, "I held my breath the entire time we were taking the canoe off the mold." Muscles strained and jaws clenched while students gently pulled. Would the canoe break? No! A champion emerged!
It was time to add the finishing touches. UAH covered the graphite mesh on Survivor's interior with concrete, formed gunwales, and painted Survivor in UAH's colors, blue and white. She looked good, but was she seaworthy? Suruivor's maiden launch would tell. Per contest rules, each canoe must float with the exterior end points breaking (above) the water surface when the canoe's hull is flooded. Survivor passed this test easily. All concrete canoes entered were judged on the team's written, oral, and final product presentations, as well as on race results. Sprint races involved paddling 330 feet (100 meters), making a 180- degree turn, and sprinting back. Survivor's unique flexing made practice in handling the canoe mandatory. Myia describes paddling Survivor as "challenging." In fact, "I almost fell out of the canoe during the women's sprint final at the regional level," she says. "If I had, our team's combined score wouldn't have sent us to the national competition." Could Myia help paddle the team to victory at the national competition in San Diego? Clemson University from Clemson, SC, with its 180-pound (82-kilogram) canoe, Good
Fortune, provided fierce competition. Both Survivor and Good Fortune had strengths and weaknesses.
Survivors lightness produced fast starts, but made maintaining a constant cruising speed difficult.
Good Fortune's starts required heavy muscle power, but once the craft was brought up to speed, that speed was easily maintained.
However, Survivor's finesse in making a 180-degree turn, midway through the race, helped Myia place second in the coed race and fourth in the women's sprint.
UAH's high racing scores helped the team edge out Clemson by 6.5 points in the overall competition, and made UAH's Survivor the 2001 National Concrete Canoe Champion. It's lightweight, it flexes, and it's inert (Not readily reactive with other elements; forming few or no new compounds.), making it less vulnerable to radiation and corrosive conditions encountered in space. Imagine: From a concrete canoe to a concrete lunar colony in space!
Jae O. Haroldson is a civil engineer who as a student participated in the ASCE's concrete canoe competition. |
Huh?
Creating
a Survivor
