A common misconception on the grade is that a walk-behind milling machine reacts the same way to all hard surfaces. The reality is that milling concrete and milling asphalt are two entirely different thermodynamic processes. I have trained operators who were experts at concrete surface prep, only to watch them completely ruin an asphalt parking lot in ten minutes.
Concrete is a rigid, crystalline structure. When the scarifier flails hit it, the concrete fractures and chips away in dry dust and shards. It is a strictly mechanical failure. Asphalt, however, is a viscoelastic material. It is essentially aggregate suspended in a highly sticky, petroleum-based binder (bitumen). When you drop a scarifier drum into asphalt, the friction of the cutters generates immense heat. Within minutes, the asphalt begins to melt.
If you run a scarifier too slowly over asphalt, the melted bitumen will completely coat the tungsten carbide cutters. Once the cutters are "gummed up," they lose all their cutting edges and simply act as hammers, beating the asphalt and causing the machine to violently bounce. To combat this, I change my operational rhythm. On asphalt, I run the machine at a faster walking pace, taking shallower passes—usually no more than 3 mm [approx. 1/8 inch] at a time. I also frequently use the water suppression system not just for dust, but to act as a coolant. The water drastically drops the temperature of the drum, preventing the bitumen from melting and keeping the flails spinning freely.
