You may have heard that concrete shoes were used by mobsters to resolve certain “disagreements”. As it turns out, the composition of the concrete the mobsters used would determine the shoes’ buoyancy.
Concrete is a composite mixture of cement, sand, aggregate, and water. The ratio of water in the mix, or “dilution ratio”, plays a significant role in determining the strength properties of the mixture.
By definition, cement has a true density of 190 lb/ft^3 while the density of water is 62.4 lb/ft^3. As the water/cement ratio increases, the compressive strength decreases, but its workability (concrete's ability to take shape of its form) increases.
As Archimede’s Principle states, in order for anything to float in water the substance placed in it must have a lower density (ie, displace more volume per unit of weight). Even if we dilute cement as much as possible with water, it will never be less dense than water. So how do we get something with a greater density to float?
One way is to mix the cement with less dense materials to lower the density of the composite. In the concrete canoe competition, some teams use a latex-modified base mixture to lower their overall density to between 45-50lb/ft^3. They can then increase the tensile strength with fiber-reinforced-plastic (FRP) or wire mesh. There are other methods that use foam board as part of the canoe structure, but the end result is the same.
Concrete boat construction has some obvious pitfalls. Forms must be built to house the concrete while it cures, which can in itself take up to 28 days. And, concrete is severely disadvantaged in its workability when compared to wood or steel.
Concrete canoes have proven to be a good premise for a college student engineering competition. Just don't expect to see the next cruise ship constructed out of it.
For more information on engineering basics, visit Joel Erway’s website: www.30minuteeit.com