In Serial Concrete Mixing System, each aggregate grade is stored in a separate bin, and the individual bins form a row. The material is discharged onto a conveyor system. Again, feed hoppers or inclined conveyors are used to transport the material to the mixer. The bins may be made of steel or, in stationary plants, also of concrete. In the latter case, they include an underground conveyor system. Serial Concrete Mixing System are frequently used in concrete plants, and the number of possible aggregate grades is larger.