How does the gradation of aggregates affect the long-term performance of asphalt pavement surface treatments?

Aggregate gradation refers to the distribution of different particle sizes within an aggregate mixture used in asphalt pavement surface treatments like chip seals, slurry seals, and microsurfacing. The gradation significantly impacts the long-term performance of these treatments. A well-graded aggregate, meaning it contains a balanced distribution of coarse, intermediate, and fine particles, provides a dense and stable structure. This interlocking of particles creates a strong resistance to deformation and wear under traffic loads. A dense gradation minimizes voids within the mixture, reducing the potential for water infiltration, which can weaken the bond between the aggregate and the asphalt binder, leading to raveling (loss of aggregate). If the gradation is too coarse, with a lack of smaller particles to fill the voids, the surface treatment will be more susceptible to raveling and loss of aggregate. This also increases the risk of bleeding, where excess asphalt binder rises to the surface. If the gradation is too fine, the surface treatment may become smooth and slippery, reducing skid resistance, which is critical for safety. A fine gradation can also lead to premature rutting or deformation under heavy traffic loads. The appropriate gradation depends on the type of surface treatment and the traffic conditions. For example, chip seals typically use coarser aggregates to provide a durable, skid-resistant surface, while slurry seals and microsurfacing use finer aggregates to fill minor surface cracks and provide a smooth, waterproof layer. Therefore, selecting the correct aggregate gradation is essential for achieving the desired performance characteristics and ensuring the long-term durability of asphalt pavement surface treatments.