Counterweights are typically made by metals. The cast iron counterweights and concrete counterweights are the most often used.
Cast iron features the best mechanical properties for the most reliable performance. It is denser, stronger, harder and requires only paint or a coating, so the manufacturer prefer using cast iron to produce counterweights Although concrete is cheaper than iron, it's wiser to choose iron, since concrete applications are prone to many problems, that cast iron counterweights can overcome.
Advantages of cast iron counterweights compare to the concrete ones:
Cast iron is denser than most materials, thus cast metals for counterweights have in average three times more density than pure concrete. This enables iron to achieve the target weight in less space and volume compared to concrete which eliminates the cost for additional material.
Concrete's density can be improved by adding dense aggregates such as limonite, hematite, or magnetite, or metal bits and scraps into the concrete mix. This increases the concrete's cost substantially, but its density is still less than those of the iron.
Cast iron is indeed stronger and harder than concrete which automatically makes counterweights more resistant to impact and strains and guarantees longer fatigue life against cyclic stress. Usually counterweights need screws for fastening or mounting additional equipment. The concrete's surface cracks easily and hardly supports any additional constructions.
In usages where counterweights need to fit into specific shapes, for example tractor suitcase counterweights are differently shaped according to space on rear axle of tractors; concrete turns to be more expensive to carve out the desired shape into it. Cracking is less likely to occur, when creating a finer shape on cast iron.
Cast iron is a product of recycling and doesn't require any extraction of new raw materials, which is not the case with concrete.
To sum up, in terms of quality, performance and overall reliability, cast iron suitcase weights are much better than concrete weights.