What material properties should be taken into account when selecting ballast?

When selecting ballast, key material properties to consider include gradation, hardness, toughness, durability, and resistance to degradation. Gradation refers to the particle size distribution of the ballast material. A well-graded ballast will have a mix of particle sizes that provides good interlocking and stability. Hardness refers to the resistance of the ballast particles to abrasion and wear. Harder ballast materials will last longer and generate less dust. Toughness refers to the ability of the ballast particles to resist impact and crushing. Tougher ballast materials will be less likely to break down under heavy loads. Durability refers to the resistance of the ballast particles to weathering and chemical attack. Durable ballast materials will be less susceptible to degradation from freeze-thaw cycles, exposure to sunlight, and chemical reactions. Resistance to degradation refers to the ballast's ability to maintain its properties over time, avoiding breakdown and the creation of fines (small particles). Excessive fines can impede drainage. Ballast should be made from a material such as granite, traprock, or quartzite, chosen for their superior hardness and durability. For example, limestone, while sometimes locally available, is generally less suitable because it is softer and more prone to degradation.