Yes, concrete can float.

Considering the weight and mass of concrete, it would seem that concrete that is placed in water is more than likely to sink straight to the bottom. However, concrete can float on water, according to the American Society of Civil Engineers, because it is lighter than water. Composite concrete mix has a weight of forty-five point six pounds per cubic foot, while water weighs sixty two point four pounds per cubic foot, making concrete lighter than water, thus giving it the ability to float.

Buoyancy and Archimedes' Principle

Buoyancy is defined as the tendency of a body to float or rise when it is submerged in a fluid. According to the famous Greek mathematician, Archimedes, who discovered what is called as "Archimedes' Principle", objects have the ability to float when immersed in water if the buoyancy of the object is greater than the weight of the object. The reverse is also true, thus if the object's buoyancy is less than the water, the object will sink.

What is Concrete?

Concrete is substance that is made out of paste and aggregates. The paste is composition of gravel, sand or cement combined with water. The cement and aggregate mixture turns into concrete through the process of hydration.

Concrete Ships

The discovery of the idea that concrete can float is not a new one. In fact, during the time of World War I, concrete instead of steel hulls were used in building ships, because there was a shortage of steel supply during that period. The use of concrete in building ships was an innovation attributed to an inventor from Norway around the year of 1912. It is not only ships that were being built out of concrete during the early twentieth century. Floating barges, tugboats, and fishing boats were also built out of concrete.

Concrete was and is an acceptable alternative to steel as a material for building ships only if the density of the ship is less than the density of the water it displaces. In other words, the ship must be buoyant. Density is the mass of an object divided by its volume.  If the ship has a greater density than the water below and surrounding it, the ship will sink.