Yes. Concrete can float. Like, it can be submerged in a bucket of water and actually resurface. As a long-time participant of the ASCE National Concrete Canoe Competition, I think this is the most common question I get from outsiders. This competition requires engineering students across the world to design and construct canoes from concrete and battle head to head in paddling, aesthetics, a technical design paper, and an oral presentation.
It’s funny that so many people are in awe about the idea of a concrete vessel. I realize conventional concrete is a dense material, but have you ever thought about a steel ship? Most large boats are made from steel (think: cruise ships, aircraft carriers, and cargo ships). But steel is three times as dense as concrete! Archimedes’ principle is the basic reason why these ships don’t sink like bricks. Basically, if the weight of fluid displaced by the object exceeds the weight of the object itself, then it floats!
Surprisingly enough, concrete war ships were constructed during World War I and World War II because of the wartime steel shortage. According to Wikipedia, they were also commonly used as barges in Europe in the early 1900s. However, ships made from other materials were easier to construct and cheaper to operate, so you don’t see many concrete ships around today. Some of the old wartime ships can still be seen today, and Amusing Planet has a great article about these concrete ships.
So now the question remains: why do civil engineering students make canoes from concrete? Well, the only reasonable answer is that ASCE made us do it. They have a national competition to determine who can build the most efficient canoe from concrete. However, unlike these old warships, our concrete is actually designed to be less dense than water so they float when completely submerged.
To explain how we achieve this, we must start by talking about the basics of concrete. Concrete is made from three main ingredients: cement, water, and aggregates (gravel, sand, etc). Note that cement is an ingredient to concrete and it chemically changes from a powdered substance to a hardened construction material when mixed with water and cured over time. It is analogous to using flour as an ingredient for cake. You add flour to a mixture of ingredients, bake it (i.e. chemically alter it), and then get a delicious cake in the end. I wouldn’t pick up a bag of flour and refer to it as cake, so please don’t point at the sidewalk and refer to it as cement.
Note that all of the ingredients to concrete are pretty heavy. Cement is about three times as dense as water and aggregates are generally about twice as dense as water. Intuitively, when you combine these ingredients, you get some final product that is much heavier than water. What we do is remove the aggregates and replace them with very small recycled glass beads (some are about 10% the density of water) to decrease the overall weight of the concrete. The end result is a concrete that is light-weight and relatively durable.
The concrete we make for these concrete canoes is quite different than the normal concrete you would see. Our concrete is generally not very fluid and has a consistency more like cookie dough. Rather than pouring it into a form, it is hand-packed onto a mold. The challenge is to come up with the optimum ingredients to make your team’s canoe lightweight, durable, and aesthetically pleasing.