Asphalt gradually breaks down; as it is crushed, the proportion of fine particles increases due to the ageing of the bitumen, traffic loads, the characteristics of modern asphalt concrete, and the settings of the crushing plants.
Asphalt, which we are accustomed to seeing as a monolithic road surface, gradually breaks down into small fragments, and this process is accelerating year on year. The reason lies in a combination of external stresses, chemical changes to the binder’s structure and the operational characteristics of the material itself, which leads to an increase in the proportion of fine particles as it crumbles.
The bitumen that binds the asphalt particles together oxidises over time due to exposure to ultraviolet light and temperature fluctuations, becoming more brittle. It loses its elasticity, ceases to absorb loads and crumbles easily even under minor mechanical stress, which immediately increases the yield of fine material during crushing.
The constant movement of vehicles, particularly heavy goods vehicles, creates cyclic compressive and shear stresses within the asphalt layer. These loads gradually break down the bonds between the coarse aggregates, splitting whole blocks into smaller pieces, making it impossible to retain the coarse fraction during reprocessing.
Modern mixtures are often produced with the addition of modifiers and secondary components, which do not always ensure sufficient bond strength during re-crushing. Cutting corners on high-quality aggregate in road construction also results in a looser material structure, which automatically increases the proportion of fine particles during processing.
Most modern crushing plants are designed for high throughput, which involves intense mechanical action on the material. Under such operating conditions, large asphalt particles are inevitably broken down into smaller ones, and adjusting the equipment settings to preserve the coarse fraction is rarely practised due to the high labour costs involved.
