Mixes for quality concrete canoes must be in the range of 50 to 80 pounds per cubic foot with strengths of 500 to 1500 pounds per square foot.
With regard to concrete, lightweight means less strength, so a medium must be found.
Lightweight concrete is achieved through lightweight aggregates.
Cracking is a major problem and canoe concrete must be somewhat flexible.
The concrete mix revolves around the aggregate. We, like many other schools, use perlite as our aggregate. Perlite is very fine and has an extremely low specific gravity. Another product that is used by many schools are glass bubbles, which are similar to perlite but can produce higher ultimate strengths in lightweight concrete. We use a combination of both in our mixes.
Fiber Reinforcement
Fiber reinforcement is another common element in canoe concrete. We use a brand called "Fibermesh". These fibers are thin, short filaments mixed in with the concrete to provide tensile resistance in the concrete matrix.
Latex
Latex is a material added to the mix water that provides some flexibility to the concrete. That is, it allows the concrete to bend somewhat without cracking or breaking. This is vital to allowing the canoe to have thin walls (approximately 0.25 inches) and still remain water tight.
Superplasticizer
This concrete admixture increases workability in the concrete mix. Concrete strength goes down as the water content goes up. With concrete canoes, the quantity of lightweight aggregate required to get the low unit weights means that strength needs to be maintained with any means possible. In addition, since the concrete must be placed on forms and stand vertically in places, it must be stiff, but also workable enough to work its way through any reinforcing mesh that is used to reinforce the concrete. By using a very low water content and a high dose of superplacticizer, workability and strength can be adjusted to fit specific needs.
This sums up our mix design. I did not go into specific quantities of materials because those really need to be determined on a trial and error basis considering the type of reinforcing used, the type of form, and the goals regarding strength and unit weight.