In 1987 the event went national after it picked up sponsorship from Master Builders Inc. At the 1997 National Concrete Canoe Competition, held in Cleveland, Ohio, last June, 25 regional winners from the U.S. and Canada were represented. Their sleek, hydrodynamically sound racing crafts showcased cutting-edge concrete technology, and many of the 16- to 20-foot boats had cut their weights to less than 100 pounds. Some entries weighed less than 70 pounds (a typical 17-foot Grumman or Old Town weighs about 85 pounds), and all were made with the ASCE regulation minimum of 75 percent Portland (water-based) cement.
It hardly seems possible and, until very recently, it wasn’t. People have been trying to get concrete to float for a long time–with mixed results. In 1848, the French nobleman Joseph Louis Lambot built concrete boats for use on his estate in Miraval, France, but for some reason the idea never caught on. Following this, the infant technology languished until severe steel shortages during World War I forced U.S. naval engineers to take another look at it. One notable attempt, the Atlantus, can still be seen 150 yards off the beach at Cape May, New Jersey, where for the past 70 years or so it has served as an apartment building for fish.
So why can’t these folks just face facts and let it go?
Because, in the sapient words of former ASCE president, Edward O. Groff: “The value of the competition should not be minimized because it is focused on a seemingly absurd concept. Many of the wonders of civil engineering were projects originally dismissed as ludicrous or impossible, such as the Brooklyn Bridge.
RECIPE FOR FLOTATION
The secret to making concrete float is not in the cement, but in the stuff it’s mixed with; that is, the aggregate (concrete = cement + water + an aggregate). The aggregate in traditional concrete is sand and gravel; it’s heavy and things made with it tend to sink like stones. The aggregate in a concrete canoe, on the other hand, might be composed of tiny, hollow ceramic spheres; foam beads from beanbag chairs; or Perlyte, which is made from volcanic dust that pops like popcorn when it reaches a certain temperature and, just like a kernel of corn, its volume increases enormously while its weight stays the same. When you mix any of these substances with water and cement and then let it cure for a few weeks, the result is a concrete that actually floats, even in slab form.
But a canoe cannot win by flotation alone.
“We were at the Southeastern Regionals,” explains Tomas Montemayor, captain of the Florida Institute of Technology‘s ’97 concrete canoe team. “Vanderbilt was there, and as soon as they put their canoe in the water it crumbled into little pieces, just like a cracker. I guess they hadn’t taken their loads and stresses into consideration.”
Apparently not, and this is why you don’t see more concrete boats around. Because, while concrete is great under compression, it doesn’t flex worth a damn, and next to flotation, flexion is what naval architecture is all about. This is where steel mesh and geotech plastic weave (otherwise known as gutter guard) come in to take the place of steel reinforcement rod, which supports concrete when it’s under stress. To further increase flexibility, polymers like latex are added to the concrete mixture.
Part 3: http://www.energiezukunft-fuer-deutschland.info/2015/06/racing-concrete-canoes-takes-both-physics-and-physiques-part-3/