Reinforcing fibers, such as aromatic polyamide (“aramid”) fibers, are treated by coating with or embedding in a binder such as a Fischer-Tropsch wax. The treated fibers are divided into suitably-sized units, which are added to bitumen, aggregate and other ingredients in an asphalt cement concrete mix. The units shed fibers into the mix during the manufacturing process so that the fibers are distributed fairly evenly throughout the asphalt mixture. The binder may also impart beneficial qualities to the finished asphalt concrete.