Asphalt Plant Road Construction

Drum Asphalt Mixing Plants

The drum asphalt mixing plants  process uses the dryer drum to simultaneously heat, dry and mix the aggregate with bitumen. A cold feed unit proportions the aggregate from stockpiles and feeds it into the rotary drum where the bitumen is added. Hot screens , hot bins and pugmills are dispensed with. Output from these asphalt mixing plants varies from 150-800 tonnes per hour.

You can see the components of a typical drum asphalt mixing plant and following pictures illustrates the flow of material through such a asphalt plant.

Material Flow Diagram of drum asphalt mixing plants

Drum Asphalt Mixing Plant Details

Cold Feed Unit

The cold feed unit is normally a self contained unit with three to five bins and mechanical feeder for each unit. The aggregate gradation of drum asphalt mixing plants is controlled by a combination of variable speed belts and variable gate opening on each bin. An alternative system is individual weigh belts for each bin.

Aggregate Conveyor

The combined aggregate then moves to the drum of asphalt plant on a conveyor belt which is fitted with a load  cell type of weighing  system. The  belt  scales are  interlocked with the  bitumen pump which adjusts the bitumen content to match the amount of combined aggregate passing over the  belt  scales.  Moisture  is  added  to  the  aggregate  to  produce  steam  when  the  aggregate enters the dryer thereby providing an inert atmosphere in which asphalt mixing takes place.

Binder Feeding System of asphalt mixing plants

The  bitumen  binder can  be  introduced  in two ways. It  may  be sprayed onto the stream of cold wet aggregate in the heat chute or the bitumen spray line may extend some distance into the drum and the bitumen introduced at this point.


The predetermined bitumen-aggregate material drops off the conveyor and enters a hopper at the upper end of the drum in drum asphalt mixing plants. The lifting flights are designed such that the bitumen aggregate blend is directed away from the hottest part of the flame.

The drum is fired by a conventional type burner using either diesel or propane-type fuel. A combination of heating, drying and mixing produces a uniform asphalt paving material in about 3-5 minutes in the drum of  asphalt plants .

The low-oxygen atmosphere resulting from the steam given off by the wet aggregate reduces hardening  of  the  bitumen.  The  discharge  temperature  of  the  asphalt mix  is  usually  maintained between 850 – 1050 C depending upon conditions. This is lower than that for the other two types of  asphalt plant such as batch and continuous asphalt plants and the mixture may contain between 1% and 2% of water, which improves the handling and compact ability at lower temperatures.

Dust Extraction

One  of  the  primary  objectives  in  the  development  of  the  drum-asphalt mixing plant  process  was  the elimination  of  dust  particles  in  the  exhaust  gases.  This  objective  has  not  been  completely realized and in built up areas a scrubbing system is required.

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Asphalt Batch Plants Controls

Asphalt batch plants controls fall into three categories, depending upon the degree of automation used in the operation of the plant:-

• Manual
• Semi-automatic
• Automatic

It is normal in all these systems for the power control of the weighing and mixing process to be automated. Even in the so called manual plants, air or hydraulic cylinders activated by electric switches have replaced hand lever systems. They operate supply bin gates, feeders bitumen valve and the weigh batch and pugmill discharge gates.

In semi-automatic plants, all operations from the weigh batch to pugmill discharge are under automatic cycle control. The entire measuring and mixing phase of the plant is handled automatically.

In a fully automatic system an operator would:-

(a) Select the total weight required to be carried by the truck.

(b) Select the mix required either by pressing a button for say one of the six most common mixes or by inserting the appropriate punch card into the control panel.

(c) Wait for results.

The automatic mechanisms do the rest including monitoring the mix temperature, hot bin levels, weighing materials, controlling mixing times and mixing the correct number of batches and part batches to fill the truck.

(b) Continuous Plant Controls

The inherently automatic operation of the continuous mix plant can be extended by the addition of several additional automatic controls, viz:-
(a) Automatic burner controls
(b) Automatic mix discharge
(c) Automatic mixer and gradation cut-offs in case of hot bin shortage or improper feed
(d) Electric interlocks which shut down the plant in case of failure anywhere in the

This Asphalt Batch plants Controls can be change with technology.

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Hot Mix Asphalt – Temperature Control

Temperature  control of  hot mix asphalt concrete  is  required  in  all  plants  except those  producing  cold  mixes  using bitumen  emulsion.  Thermometers  (mercury  or  electric)  has to be  located  in  the  flow  of aggregate  from  the  dryer,  in  each  hot  bin,  and  in  the  binder  supply  line.  The  number  of thermometers used and their locations have to be such that the readings are valid indications of the  material  temperatures.  The  thermometers  should  have accurate  to  within  I  percent  and  their registrations clearly and conveniently visible to the operator.

In the production of Hot mixes the aggregate must be heated only sufficiently to obtain a mix at the required temperature. Overheating may result in damage by oxidation of the binder while it is in the form of thin films during the mixing process.

If the mix is too hot, the binder will drain from the particles of aggregate with the result that  some  portions  of  the  mix  may  be  dry  and  others  fatty.  This  applies  particularly  to  open graded mixes. See related posts of hot mix asphalt concrete for further knowledge.

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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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Filler Feeding in asphalt Plant

 Filler feeding  is separate storage for filler and the means for its controlled introduction into the pugmill should be provided. Measurement may be by weight or volume.

Filler is not passed through the dryer, as the flow of air to the dust extractor would carry most of the fine particles to the dust collector, and into the wet scrubber (if installed) or into the atmosphere.

Feeding of the filler is usually by mechanical means, which should be checked regularly to ensure that no obstructions have occurred to affect uniform feeding.

In  batch  plants  the  filler  is  proportioned  by  weight.  In  continuous  mix  plants  the proportioning  is  achieved  by  varying  the  filler  gate  opening  and  interlocking  the  filler  feed system  with  the  aggregate  feed  system.  The  average  proportions  of  filler  in  the  mix  can  be checked by comparing the total quantity of filler used and the quantity of mix produced. Do you need to more details about filler feeding and see batch and continuous plants.

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Dust Extraction in Asphalt Plant

 Dust extraction  of aggregate is the process to remove dust after drying cold aggregate. A certain amount of dust is carried away in the hot gases from the dryer and it is usual for this to be collected in a dust collector.

The type of extractor normally used consists of a set of high energy cyclones and a low energy water scrubbing system. In isolated areas the scrubber may be eliminated.

The fan in the dust extractor creates suction from the dryer, hot bins etc. and delivers the dust to the cyclone type collector, where it is precipitated by centrifugal force. If the dust is found to be suitably graded, it can be returned to the plant for inclusion in the mix

Aggregate  from  the  dryer  is  fed  to  screens  mounted  over  the  hot  bins.  The screens separate the aggregate into selected sizes for proportioning, and reject oversize particles. Vibrating screens are preferred. This screening and recombining of aggregate  is carried out to correct segregation resulting from passing the combined aggregate through the dryer.

It will also help to correct variations in relative proportions of sand and coarse aggregate. However, as the smallest hot screen size is approximately 4 mm, hot screening will not correct any grading variations in material below this size.

For efficient screening the screens should be:-

• Of Adequate effective area

• Of the correct size opening

• Clean and in good condition

• Property arranged and at the correct slope

• Vibrating in the correct manner.

Hot storage Bins and Screens

For a general purpose plant there should be at least four bins to hold the hot aggregates, but  the  number  in  use  will  depend  on  the  type  and  size  of  mix  being  produced.  Bin  capacity should be adequate for continuous operation at maximum output. Each bin should be fitted with a  contents  indicator  in  full  view  of  the  operator,  and  with  an  overflow  outlet  to  ensure  that should  a  bin  become  overfull,  the  excess  will  not  spill  into  an  adjoining  bin.  A  thermometer should  be  installed  in  each  hot  bin.  The  hot  bins  function  as  small  reservoirs  to  dampen variations in supply rate and to improve batching.

Dust extraction is one step of the asphalt batch and continuous  plants. It is recommended to see that post.

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Aggregate Heating and Drying

 Aggregate Heating and Drying , Dryers are inclined rotating steel drums ranging in size from 1 m to 3 m in diameter and from 5 m to 16 m in length depending on the capacity of the plant and the moisture content of the materials being handled.

A  dryer  may  not  be  required  in  a  plant  producing  cold  mix  but  it  is  an  essential component of a hot mix plant.

In  a  hot  mix  plant,  the  dryer  performs  two  functions:  it  removes  moisture  from  the aggregate and heats it to the required mix temperature.

Cold aggregate is fed in one end of the dryer and is continually lifted by lifting flights which line the cylinder and dropped in veils through the burning flame and hot gases. The slope of the cylinder, its speed of rotation, diameter, length and arrangement of flights all control the time taken by the aggregate to pass through the dryer.

The  amount  of  aggregate  entering  the  dryer  should  be  such  that  it  can  be  dried  and heated  to  a  temperature  slightly  higher  than  mixing  temperature,  before  passing  out  at  the opposite end of the drum. The hot aggregate is carried to the screen deck by means of a bucket elevator.

A  temperature  control  in  the  exhaust  line  is  generally  used  to  adjust  the  burner  (or burners) to maintain a reasonably uniform temperature in the dried aggregate.

A  check  should  be kept on the temperature of the discharged aggregate at all times to ensure that it is within the specified range  because overheating of the aggregate may result  in damage to the binder by oxidation. This description about aggregate heating and drying will be part of the process of asphalt batch and continuous plants.

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Batch and Continuous Plants


The flow of material through both batch and continuous plants is similar up to the point of measuring and mixing. The difference in  the  types  is  that  in  the  batch  plant,  aggregate  proportioning  is  by  weight  whilst  in  the continuous plant, aggregate proportioning is by calibrated gate openings which allow a constant flow of hot aggregate and filler into the pugmill mixer.

The essential features of these plants are:-

• Cold aggregate bins fitted with feeders to meter the cold aggregate onto a belt conveyor.

• A  dryer which  is  a  rotating  steel  drum  where  flames  and  hot  air  are  forced through the cold aggregate.

• Screens to separate the hot aggregate which is then remixed in the correct proportions.

• Bins to hold hot aggregate and filler.

• Binder storage tanks

• Weight batching or continuous feeding mechanism.

• A pugmill mixer.

• Dust collectors.

• A manual, semi-automatic or automatic control system.

Surge bins  may  be provided to overcome the disruption due to dump truck delays whilst hot storage silos for the storage of hot mixed material are often provided at batch and continuous plants.


Aggregate is fed into the plant from one of the following sources:-

(a) Small open-topped bins fed by a front end loader.

(b) Large bins fed by trucks either directly or via a conveyor transfer system.

(c) Large open stockpiles (separated by bulk heads) and situated over a conveyor tunnel. Materials are stockpiled over the tunnel by belt conveyor, truck etc.

The number of bins necessary will depends on the type of mix being produced, and may vary from one, (for a patching premix plant), to four or more (for a plant producing dense graded mixes).

When charging the cold bins, care is required to minimize aggregate segregation and degradation. Enough materials must be maintained in all bins to provide a positive and uniform flow.

When a front-end loader is used to feed the cold bins, the operator should not pick up material from the storage stockpile at ground level. The bucket should be held high enough above the ground to prevent contamination when filling.

If trucks are used to charge the bin, they should deposit their loads directly above the feeder.


Each size of aggregate must be metered out by the cold feeders in correct proportions and at the correct rate otherwise variation in the final mix, or overflow of the hot bins, may occur.

The feeder units are located beneath the storage bins or stockpiles, and have controls that can be set to produce a uniform flow of aggregate to the cold elevator.

The four types of feeders in common use are:-

• Tilting Chute. These are definitely not suitable for sand and are not particularly good for aggregates as they do not provide a uniform flow. The rate of flow is very much a matter of trail and error as they cannot be calibrated accurately.

• Continuous Belt.  These  are  the  most  popular  for  handling  sands.  They  generally  have  an infinitely variable speed control which is easily adjustable and calibrated. Continuous Belt Feeder

• Reciprocating  Plate.  These  Deliver  a  uniform  flow  and  are  capable  of  handling  most materials. However, flow rates cannot be varied as readily as with the continuous belt and conveyor types.

Reciprocating Plate Feeder

• Electromagnetic  Vibrating  Chutes.  They  are  generally  used  for  feeding  coarse  aggregates.

Electromagnetic Vibratory Feeder

Close control over the feeding of cold aggregates into the dryer is necessary in order to avoid  variations  in grading; temperature and  moisture content of dried aggregate; to carryover on screens.

To ensure that the correct quantity of each aggregate is  fed to the dryer, the cold  bins should be supplied with materials if uniform grading and moisture content, and the levels of the materials in the bins should be kept constant. The feeder gates should be accurately calibrated, securely held in the set position, and kept free from obstructions.

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Types of Asphalt Mixing Plants

Asphalt Mixing Plants generally fall into three categories:-

Batch mixing
Continuous mixing
Drum-Mixing plants

Within each of those categories there occur various types of mixing plants ranging from small rotary paddle  mixers used  for the preparation of small quantities of patching premix, to large plants for producing tonnes of asphalt in a single batch.

The four main functions of any mixing plant are:-

• Heating and drying the aggregates.

• Proportioning  the  various  sizes  of  aggregate,  filler  and  binder  to  give  a  correctly proportioned batch, or continuous flow of material.

• Measuring  the  required  quantities  of  aggregate,  filler  and  binder  to  give  a  correctly proportioned batch, or continuous flow of material.

• Mixing the ingredients to produce a homogeneous material

See the following details of each asphalt plants functionality & usage