across the cross section of the drum. The aggregates must  not  only  rotate  with  the  revolving  motion  of  the drum but must also spread out sufficiently to make heating and drying of all particles quick and efficient. The  spiral  flights  are  located  at  the  charging (burner)   end   of   the   drum-mixer   and   direct   wet aggregates into the drum in such a manner as to attain uniform drum loading. Tapered lifting flights pick up the aggregates and drop them in an even veil through the burner flame. Burner  Operation The burner inside the drum-mixer provides the heat necessary to heat and dry the aggregates used in the final mixture. The burner provides this heat by burning fuel oil, gas, or both. When oil is burned, a low-pressure air draft is used to atomize the fuel oil for burning. Depending on the type of fuel used for the burners, the fuel feed and air blower must be balanced to ensure that the proper proportions of fuel and air are being introduced into the burner to ensure efficient combustion. Lack of balance can lead to incomplete burning of the fuel. Especially, when fuel oil or diesel fuel is used, this can leave an oily coating  on  the  aggregate  particles.  An  imbalance between the fuel feed and air flow can be adjusted by either decreasing the fuel rate or increasing the blower or draft air. Surge Silo The drum-mix plant produces a continuous flow of fresh asphalt hot mix and has a surge silo for temporary storage of the hot-mix material. The surge silo is also used for controlled loading of trucks. A weight system is  normally  connected  to  the  holding  bin  of  the  silo  to monitor the amount of hot mix loaded into each truck. Weight measurements are normally recorded by the weight  system  control  panel. BITUMINOUS SURFACING MATERIALS Bituminous materials are tremendously important in  the  construction  of  roads  and  airfields  for  both military and civilian use. A basic knowledge of these bituminous materials, their origin, composition, types, and grades are essential for an understanding of their use in  construction. Bituminous surfaces are composed of compacted aggregate   and   bitumen   (binder).   The   aggregate transmits the load from the surface to the subgrade, takes the abrasive wear of the traffic, and provides a nonskid surface. The binder binds the aggregate together, thus preventing the displacement and loss of the aggregate. The binder also provides a waterproof cover for the base that  keeps  surface  water  from  seeping  into  and weakening the material. Bituminous surfaces are particularly adaptable to stage construction. Additional courses can be added to existing pavements to provide further reinforcement when  loads  or  traffic  density  increases.  The  flexibility of bituminous surfaces permits slight adjustment caused by settlement of the subgrade without detrimental effect. Properly  designed  bituminous  wearing  surfaces,  when compared  with  concrete,  are  less  affected  by temperature   strains.   The   surfaces   resist   wear, weathering, and deterioration from aging with only minimal  maintenance. Bituminous materials are highly versatile and serve admirably in temporary, expedient, and light traffic situations where concrete is not justifiable. It is equally true that thicker bituminous pavement designed for heavy  and  continuing  duty  is  fully  comparable  to concrete designed for heavy volumes of traffic or heavy wheel loads; however, bituminous wearing surfaces lack appreciable bearing action to carry wheel loads over weak spots in the subbase. For this reason, the subgrade must have an adequate, uniform bearing strength  and  the  base  course  must  have  adequate thickness,  bearing  capacity,  and  cohesion. TYPES AND GRADES OF ASPHALT Asphalt is a natural or man-made by-product of petroleum  distillation.  Natural  asphalt  is  found  in  nature as either lake (or pit) asphalt or rock asphalt. The common bituminous surfacing materials are asphalt cements, asphalt cutbacks, asphalt emulsions, road tars, and road tar cutbacks. For identification purposes, these materials  are  divided  into  three  classes: asphalt bitumens, emulsions, and tars. The classification of these materials is based on the extent to which they dissolve in a distillate of petroleum or coal. Asphalt cements  and  asphalt  cutbacks  are  asphalt  bitumens  (or asphalts). Road tars and road tar cutbacks are tars. Asphalt  Cements Asphalt cements are solid products of petroleum refining (fig. 8-19). An asphalt cement is designated by the letter symbol AC, followed by the penetration grade that  represents  its  relative  hardness.  The  numbers  range 8-18