Asphalt Plant Road Construction

Proportioning Materials of Asphalt Batch and Continuous Plants

proportioning materials of asphalt batch and continuous plants, there  is  little  difference  in  principle between batch and continuous plants.

However beyond this point the two types of plant differ considerably and therefore each will be discussed separately.

(a) Batch Plants – Weigh Batching

The  fine  and  coarse  aggregates  are  dropped  from  the  hot  storage  bins  through  gates, which are either manually or mechanically operated , onto the weigh batch hopper. The gate  of  each  aggregate  bin  is  closed  when  the  correct  weight  of  material  has  been discharged into the weigh batch hopper.

The  filler  should  be  weighed  separately  because  of  the  relatively  small  amounts involved.  Weighing  may  be  done  manually,  but  modern  plants  provide  for  automatic

The weigh batch hopper empties directly into the mixing chamber or pugmill.

Bitumen may be weighed in a special bucket or it may be measured by a meter for each batch.  With  either  system  the  bitumen  should  be  distributed  uniformly  over  the  full
length of the pugmill.

(b) Continuous Plants – Proportioning Devices

Accurate proportioning materials of the mix components is achieved volumetrically by means of calibrated gates on the bins for aggregate and filler and a metering pump for the binder. The binder is sprayed onto the stream of aggregate as it enters the pugmill.

A  positive  interlock  between  the  controls  of  the  binder,  aggregate  and  filler  feeders  is essential  for  a  uniform  mix.  The  positive  interlock  ensures  that  aggregate,  filler  and binder are supplied in constant proportions even if the total output varies.

In both the batch and the continuous plant the mixing unit consists of a pugmill (paddle )The  function  of  the  pugmill  is  to  mix  the  separate  constituents,  (coarse  and  fine
aggregate,  filler  and  binder)  to  achieve  complete  coating  of  all  particles  and  homogeneity throughout the mix. This should be achieved as quickly as possible, and hence the mixing unit should be of adequate capacity and mixing efficiency.

The main parts of the pugmill are paddles, paddle tips, liner, shafts, discharge gate and heated jacket.

(a)     Pugmills for Batching Plants

Pugmills  may  be  of  the  single  or double  shaft  type  but   double  shaft  pugmills,  in which the blades or paddles rotate in opposite     directions,     are     more efficient  and  will  be  the  only  ones followed.

The  capacity  of  the  pugmill  should  be  such  that  the  maximum  batch  does  not  fill  the chamber  to  a  greater  depth  than  the  maximum  height  of  the  blades.  The  zone  between  the maximum height of the blades and the bottom of the pugmill is termed the live zone.

In order to mix efficiently, a pugmill should be neither over-filled nor under-filled.

Overfilled Pugmill

The  mix  is  discharged  from  the pugmill  by  opening  a  rapid  operating, sliding  gate  in  the  bottom  of  the  pugmill, This  allows  the  material  to  fall into a truck (or alternatively  into a skip  for delivery  to  a  surge  bin  or  hot  storage  silo). The  discharge  gate  must  have  a  positive action,  opening  quickly  to  allow  the  mixed batch to drop into the truck as a mass. If the batch is discharged as a flow, there is a risk of segregation.

(b) Pugmills for Continuous Plants

The pugmill in a continuous flow plant is similar to that in a batch plant except that it is usually longer and narrower. In order to move the mix forward, most of the paddles are
turned in one direction, except the last few which are usually reversed to prevent the mix from being discharged too rapidly.

The mix is discharged over a level-control gate at one end of the pugmill. Mixing time is controlled by this gate which, when raised or lowered, increases or decreases the amount
of material in the pugmill and consequently the time taken for the mix to pass through. You can see more details of  proportioning of materials in batch and continuous plants .

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