Quantcast Pugmill Mixing

Asphalt Introduction From  the  weigh-hopper,  the  aggregates  are deposited into the plant pugmill (mixing chamber) and are blended with the proper proportion of asphalt. In a typical  plant  system,  asphalt  is  weighed  separately  in  a weigh bucket before being introduced into the pugmill. When the asphalt reaches a predetermined level in the weigh  bucket,  a  valve  in  the  delivery  line  closes  to prevent  excess  asphalt  from  being  discharged  into  the bucket.  The  asphalt  is  then  pumped  through  spray  bars into  the  pugmill.  Asphalt  buckets  should  be  checked  for accuracy in the mornings. New asphalt loosens some of the old asphalt that accumulated the previous day on the sides and bottom of the bucket. Loss of this accumulated asphalt changes the tare weight of the bucket. Pugmill Mixing Asphalt  and  aggregates  are  blended  in  a  chamber called  the  pugmill.  The  pugmill  consists  of  a  lined mixing chamber with two horizontal shafts on which several  paddle  shanks,  each  with  two  paddle  tips,  are mounted. The paddle tips are adjustable and fairly easily replaced. The paddle areas are adjusted to ensure there are no “dead areas” in the pugmill. A “dead area” is a location where aggregates can accumulate out of reach of the paddles  and  not  be  thoroughly  mixed.  Dead  areas  can be  avoided  by  making  sure  the  clearance  between  the paddle tips and the liner is less than one half of the maximum  aggregate  size. Nonuniform  mixing  can  occur  if  the  pugmill  is overfilled (fig. 8-15). When the plant is operating at full production,  the  paddle  tips  should  be  barely  visible  at the surface of the material during mixing. If the material is  too  high,  the  surface  aggregates  will  tend  to  “float” above   the   paddles   and   will   not   thoroughly   mix. Conversely, in a pugmill containing too little aggregate (fig. 8-15), the tips of the paddles rake through the material  without  mixing  it.  These  problems  can  be avoided by following the manufacturer’s pugmill batch rating recommendation. Normally, the rating is based on a percentage of the capacity of the pugmills “live zone.” This live zone (fig. 8-15) is the net volume in cubic feet below a line extending across the top are of the inside body shell radius with shafts, liners, paddles, and tips deducted. Figure 8-14 presents the mixing cycle during which the aggregates, mineral filler, and asphalt are blended in asphalt hot mix in the pugmill. The length of time Figure 8-15.-Typical pugmill. between  the  opening  of  the  weigh  box  gate  (Step  6  in the figure) and the opening of the pugmill discharge gate (Step 9) is referred to as the batch mixing time. The  batch  mixing  time  must  be  long  enough  to produce an homogeneous mixture of evenly distributed and  uniformly  coated  aggregate  particles.  If  the mixing time is too long, the lengthy exposure of the thin asphalt film to the high-aggregate temperature in the presence of air can affect the asphalt and reduce the durability of the mix. The speed of the mixer shafts and  the  arrangement  and  pitch  of  the  paddles  are factors governing the efficiency of the mixing. Most job specifications require the use of a timing device to  monitor  batch  mixing  time. 8-14


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