cover the surface uniformly. After the binder cools and cures, it should bind the aggregate tightly to prevent dislodgement  by  traffic. Individual  aggregate  stones should  be  pressed  into  the  binder  but  must  not  be covered by the binder. Approximately one half of the individual aggregate stones should be exposed to traffic. The ROA for the binder material should be between 0.25 and 0.30 gallon of aspalt per square yard. For a single-surface treatment, the bitumen must be heated  and  applied  to  the  surface  while  hot.  The aggregate must be spread and rolled before the bitumen cools. Under no circumstances is traffic permitted to travel  upon  uncovered  fresh  bitumen.  The  distributor should NOT apply bitumen until the aggregate is on hand  and  ready  for  application.  When  the  distributor moves  forward  to  spray  the  asphalt,  the  aggregate spreader should start right behind it. The bitumen should  be  covered  within  1  minute  if  possible; otherwise, the increase in asphalt viscosity may prevent good  binding  of  aggregate. Aggregate Application.—  The size and amount of aggregate, used for surface treatments, are important. You must use a size that matches the bitumen application rate. For a single-surface treatment, one-half inch to sieve number 4 is needed. The amount of aggregate should be 25-30 pounds per square yard. When aggregate is distributed properly, very little hand work is required. At longitudinal joints, the aggregate cover is stopped 8 inches from the edge of the bitumen to ensure ample overlap of the bitumen coat. All bare spots should be covered by hand spreading, and any irregularities of the distribution should be corrected with  hand  brooms.  Excess  aggregate  in  limited  areas should be removed immediately with square-pointed shovels.  When  the  aggregate  spreader  is  properly  set and  operated,  handwork  is  reduced  to  a  minimum. Rolling.—   The  aggregate  is  usually  rolled  by pneumatic-tired   rollers.    Steel-wheeled  rollers  are  not recommended  by  themselves.  If  used,  they  should  make only one pass (one trip in each direction). The rolling operation  should  then  be  completed  with  the pneumatic-tired   rolls.    Steel-wheeled  rollers  produce maximum compaction but must be used with care to prevent  excessive  crushing  of  the  aggregate  particles. Also, these rollers will bridge over smaller size particles and small depressions in the surface and will fail to press the aggregate in these places in the asphalt. Faulty rolling can be eliminated or minimized if you adhere to the following procedures: 1. Rolling should be parallel to the center line of the  roadway  to  reduce  the  number  of  times  the  roller must  change  direction. 2. Succeeding passes should overlap one half of the wheel width of the roller. This action ensures that the aggregate becomes well embedded in the bitumen. 3. Rolling should be completed before the bitumen hardens.  This  will  ensure  that  the  aggregate  becomes well  embedded  in  the  bitumen. 4. Succeeding passes should be made from the low side to the high side of the surface. This operation maintains the surface crown and prevents feathering at the edges. 5. Rolling should be done at a slow speed. 6. Rollers should be only wet enough to prevent bitumen from sticking to the wheels. 7.  The  power  wheel  of  the  roller  should  pass  over the unrolled surface before the steering wheel(s) of the rollers. After rolling and curing, the surface is ready for traffic. When the surface is used as an airfield, excess aggregate must be swept from the surface to avoid damage to aircraft. This practice is also recommended for  roads. MULTIPLE-SURFACE   TREATMENT.—   A multiple-surface treatment is essentially the same as the single-surface   treatment.   However,   the   multiple- surface treatment consists of two or more successive layers  of  binder  and  aggregate. This type of treatment is done in stages. Each stage is accomplished in the same manner as a single-surface treatment.  The  only  difference  is  that  each  additional layer of aggregate should be about one half of the size of the previous layer. This allows the smaller aggregate to interlock with the larger aggregate when rolled. PAVING EQUIPMENT Equipment,  used  in  asphalt  pavement  construction, are  aggregate  spreaders,  asphalt  distributors  and  their associated hand sprayers and spray bars, asphalt kettles, asphalt haul trucks, and asphalt pavers. AGGREGATE  SPREADERS When  a  spreader  is  operated  properly,  it  will conserve  aggregate  and  produce  a  uniform  spread. Spreaders range from a type attached to a truck tailgate to a highly efficient self-propelled machine. 16-25