It highway subject

Asphalt Mixing Plants

Asphalt Mixing Plants

I INTRODUCTION

In this chapter we will explain how an asphalt mixing plant works, how to inspect one

and calibrate some of the plant equipment. Additional information can be found in the “Hot-Mix

Asphalt Paving Handbook 2000" (HMA Paving Handbook).

An asphalt plant technician certification is required for anyone involved in sampling and

testing of asphalt mixtures. The technician may be responsible for obtaining the quality control

or acceptance samples that are tested in the laboratory and they may be responsible for actually

testing the samples.

The contractor is also required to have at least one certified asphalt plant technician who

is in charge of all plant quality control activities such as mix proportioning and adjustment an all

sampling and testing activities. With additional Division approved mix design training the

technician is also allowed to design asphalt mixtures for the plant.

There are two main types of asphalt mixing plants: Batch and Drum. There is a third type, continuous mixing plants; however this type of plant is rarely used and will not be

discussed in this handbook. Batch and Drum plants differ in the equipment that each uses for

mixing and how the materials are proportioned. Each use various combinations of gates, scales,

pumps, weight sensors and computer controls to provide the correct combination of aggregate

sizes and asphalt content for the mixture being produced. Each plant will be describe below, but

for additional information on either refer to Part-II of the HMA Paving Handbook.

Figure 1 - Asphalt Batch Mixing Plant Figure 2 - Asphalt Drum Mixing Plant

II ASPHALT MATERIALS

A) AGGREGATE STOCKPILES

Aggregate should be stored on a clean surface and the various sizes should be separated.

The main things to avoid are contamination, degradation, and segregation. Contamination

occurs when things get mixed in with the stone. If the stockpile is placed on mud or dirt, the dirt

will work its way up into the stockpile. Trucks and loaders working on the stockpile will also

deposit mud and dirt. Dust from the plant can also be a problem. We tend not to worry about

intermingling of the stockpiles, since all of the aggregates will be blended together in the plant.

But this can cause variations in gradation, and is also form of contamination. Placing the

stockpile on a stone base prevents contamination from dirt and helps to drain the stockpile. It is

important to keep stockpiles well drained because drying aggregate is expensive.

Degradation means that the stone becomes smaller through wear or by being broken.

This usually does not change the gradation enough to cause problems, but if it does, the way in

which the stone is handled should be changed. For example, try to avoid using tracked vehicles

on the stockpile.

Segregation means that the aggregate has become separated by size. Segregation must be

kept to a minimum because it can cause large variations in the gradation of the paving mix. A

primary cause of aggregate segregation is cone shaped stockpiles. Any time that aggregate is

dropped, such as off the end of a conveyor belt, it forms a cone shaped pile. The coarse

aggregate rolls down the sides of the pile, while the fine aggregate remains where it was

deposited. To reduce aggregate segregation, use more stockpiles with a smaller range of sizes in

each pile.

Listed below are some methods for reducing segregation.

• Avoid cone shaped stockpiles.

• When trucks are used, deposit each truckload in a single pile, and keep the piles close

together.

• Layer the stockpiles and keep the thickness uniform.

• Avoid any practice that would cause the aggregate to be pushed over the side of the

pile.

• Avoid long drops through the air, such as off the end of a conveyor belt.

• When using a crane to stockpile, dump, don't cast.

• Bulkheads can be placed between stockpiles to prevent intermingling of the

aggregate.

B) ASPHALT

The asphalt supply system consists of storage tanks, a pump, circulating lines to carry the

asphalt between the tank and the plant, a heating system to keep the asphalt hot enough to flow

and a means of metering the asphalt into the mix. The asphalt is usually delivered to the plant in

trucks and stored in storage tanks. The plant may need more than one storage tank to be able to

use more than one grade of asphalt, or to have enough asphalt storage capacity to keep operating

between deliveries.

The usual method of heating an asphalt storage tank is to pump hot oil through tubes in

the storage tank. In some heating systems, hot gases from the burner pass directly through the

tubes. This is called a direct-fired system. When using a direct-fired system, be careful to avoid

localized overheating of the asphalt.

A system of circulating lines carries the asphalt between the storage tank and the mixer.

The lines must be heated to keep the asphalt viscosity low enough that it will flow through the

lines. Low temperature anywhere in the system can clog the lines and stop the asphalt flow.

The combination of heat and air hardens asphalt. The asphalt line should discharge near

the bottom of the tank, in order to minimize contact with the air. The pipe that carries unused

asphalt back from the plant to the tank will normally have a few slots, located above the level of

the asphalt, to break the vacuum when the pump is reversed.

In a batch plant, the asphalt flows continuously from the storage tank, through a pump,

and back to the tank. When a batch of paving mix is being made, the asphalt is diverted to a

weigh bucket, which is filled until it has enough asphalt for one batch of paving mix. In drum

mix plants, asphalt is pumped into the mixer continuously as long as the plant is operating. The

amount is controlled by varying the pump speed, or by using a variable volume pump, or with an

adjustable valve.

C) MINERAL FILLER

Some rock dust is needed in a paving mix. It fills the spaces between the coarse

aggregate particles, reducing the need for asphalt and adding stability. Too much dust causes

problems. The dust coats the coarse aggregate so that the asphalt cannot stick to it, or it may

combine with the asphalt making it brittle. The amount of dust generated depends on the type of

aggregate used and plant's dust collecting system.

Nearly all of the dust can be captured and returned to the mix. If this is more dust than is

needed, part of it is can be removed from the system and disposed of. If there is not enough dust

in the mix, mineral filler must be added. It should be metered into the mix in controlled

quantities. This can be done by packaging the mineral filler in standard sized bags that are added

by hand to the mixer, or the filler can be added in bulk from a conveyor belt. Rock dust, cement

and fly ash are all used as mineral fillers.

III GENERAL MIXING PLANT COMPONENTS

A) COLD FEED BINS

Aggregate from the stockpiles enters the plant through the cold feed. The cold feed is a set of bins like to that shown in Figure 3. A crane or end loader moves the aggregate from the stockpiles to the cold feed. Each size of aggregate has a separate bin. The purpose of the cold

feed is to provide a uniform supply of aggregate to the plant. The bin capacity must be large

enough that one bin does not run dry while the other bins are being loaded.

The bin outlet gates are adjustable. The amount of each size of aggregate in the mix can

be controlled by changing the gate settings, or adjusting the speed of the conveyor belt that

carries the aggregate from the bin.

In a drum mix plant, cold feed adjustment is the only way that to adjust the gradation of

the mix. In a batch plant additional adjustments can be made at the hot bins.

Figure 3 - Typical Asphalt Plant Cold Feed Bins

To keep the aggregate flowing smoothly and to prevent it from hanging up in the bin, one

of the following mechanical feeders may be used to help pull the aggregate from the bins:

• Continuous belt - a conveyor belt is used to pull aggregate from the bin. This is the

best type of feeder for fine aggregate.

• Vibratory feeder - This shakes the aggregate from the bin.

• Apron flow feeder - A conveyor belt that looks like a tank tread.

Additional information on various feeders types can be found in the HMA Paving

Handbook.

B) THE DRYER

The dryer heats and dries the aggregate. It is an essential part of the plant since asphalt

will not coat wet or cold stone. The dryer is a steel drum, heated by a gas or oil flame. Steel

flanges, called flights are attached to the inside walls of the drum. As the dryer revolves, the

flights raise the aggregate and cascade it through the hot air and flame, heating the aggregate and

removing moisture. The dryer is tilted, with the aggregate entering at the high end and leaving at

the low end. As the stone is raised and dropped by the lifting flights, it moves along to the low

end of the drum, where it leaves through a chute.

There are two types of dryers, Counter Flow and Parallel Flow, the difference is the

direction of which the aggregate flows to or from the burner. In a Counter Flow dryer the

aggregate enters the dryer from the opposite end of the drum from the burner. The parallel flow

drum has the aggregate enter on the same end as the burner. Each of the dryers has a section in

the Hot-mix Asphalt Paving Handbook.

Figure 4 - Typical Asphalt Plant Dryer

Bucket elevators carry the aggregate into and out of the dryer. The elevator that carries

the cold aggregate to the dryer is called the cold elevator and the one that carries the hot

aggregate out of the dryer is called the hot elevator.

A dryer is expensive to operate. Fuel cost makes it the most expensive part of the plant

operation. The amount of aggregate that a dryer can handle depends on its operating efficiency

and the amount of moisture in the aggregate. Often the amount of aggregate that the dryer can

handle limits the plant production rate. It is important to verify moisture contents of the

aggregates being fed into the dryer to monitor the efficiency of the dryer. It is possible that the

dryer must be slowed down in order to thoroughly dry the aggregates. One thing to remember is

that all aggregates don’t have the same absorption and therefore will likely have differing

moisture contents. Fine aggregates typically hold more moisture than coarse.

C) DUST COLLECTORS

One of the by-products of making hot mix asphalt is dust. Dust gets into the plant as a coating on the coarse aggregate and stays there as long as the stone is wet. When the stone dries,

the dust blows off into the air. It can be a nuisance and a major source of air pollution.

Asphalt mixing plants must have a permit from the Air Pollution Control Commission.

Plants must be approved by the air pollution control agency in the state where they are located.

Most dust comes from the dryer. Dust collectors remove the dust from the dryer exhaust

gas, either by mechanical means, or by a water spray, or with a fabric filter. Many plants use a

combination of these methods.

Mechanical collectors remove the larger dust particles. They are located in the pipes that

carry the exhaust gases out of the dryer. They usually consist of baffles in the pipe or an

expansion chamber. An expansion chamber is a wide place in the pipe that slows down the gases

and allows the dust to fall out.

Another type of mechanical collector is a cone shaped device called a centrifugal

collector. As the exhaust gas flows around the cone, the dust hits against the outer wall and falls

to the bottom of the collector.

Wet Scrubbers force the exhaust gas through a spray of water. The water and dust are

collected in a settling pond. This is an efficient way to collect the dust, but the dust must be

discarded in the settling pond and cannot be recovered for further use.

Fabric Filter dust collectors are usually called bag house systems. It works like a vacuum

cleaner. The exhaust gases pass through the fabric, leaving the dust on one side and clean air on

the other side. Fabric filters are widely used and very effective. Overheating of the dryer must

be avoided since the fabric bags will not withstand high temperatures and could melt or catch fire.

D) SURGE AND STORAGE BINS

Surge and storage bins are used for temporary storage of the paving mix. They are

intended to make plant operation more efficient. The bin eliminates the need for the plant to stop

producing while waiting for trucks, or for the trucks have to wait for the plant to produce more

mix. A typical surge or storage bin is a circular silo, which is loaded through the top by a

conveyor. It has a gate at the bottom through which the mix can be dumped into a truck.

A surge bin is used to hold paving mix for a short time, usually until a truck is available

for loading and not for more than two or three hours. Surge bins are usually not insulated.

Storage bins are used for longer storage. They are bigger than surge bins, are always insulated

and usually heated.

Care must be taken to avoid mix segregation in the bin. The mix should enter the bin in

such a way that it does not bounce off the side of the bin. The mix must remain hot, but not be

overheated. The mix must be protected from air currents, since oxygen hardens the asphalt. The

bin gate should be sealed. Sometimes the mix is protected from hardening by filling the bin with

an oxygen free atmosphere, such as the exhaust gas from the dryer.

E) TRUCK SCALES

Hot-mix asphalt is usually sold by the ton. The scales are either beam type scales, in

which the mass is determined by sliding weights along a calibrated beam, or dial type scales,

which is read directly from a dial, or an electronic weighing system, which is often part of a

computerized plant operating system.

Computer operated batch plants often use the computer printout of the batch weights,

instead of truck scales, to determine the mass of the mix. However this system will only work if

the mix is loaded directly from the mixer to the truck without passing through a surge or storage

bin.

IV BATCH MIXING PLANT

A) HOT BINS AND SCREENS

A set of vibrating screens, located on the top of the mixing tower, separate the aggregate

by size and deposit it in the hot bins. The aggregate is then recombined to meet the specified

gradation. Screen openings can be square, rectangular or slotted. Square is most common. The

screens can be mounted either on the same level or on top of each other, with the coarsest screen

on top.

As the aggregate falls from the hot elevator it lands on a screen that has openings large

enough to let sand sized aggregate through into the Number One hot bin. Aggregate that is too

large to pass through this screen moves across, or down to the coarser screens, until it either falls

through or falls off the end of the last screen. The aggregate that is too coarse to pass through

any screen is carried out of the plant through an overflow pipe called a scalping chute.

The screens separate the aggregate by size. Each size is stored in its own bin. Usually

two bins are used for a wearing course mix, and up to four bins for a base course. The aggregate

falls into the bins, more or less continuously. When the plant operator is ready to make a batch

of paving mix, he draws aggregate from each bin in the amount needed to make the mix.

Hot bins smooth out small variations in the cold feed gradation. If the gradation is too

variable, or if we try to produce a gradation that cannot be made from the aggregates in the

stockpiles, some of the hot bins will either overfill or run dry.

The bin closest to the hot elevator is called the Number One Bin. Since this bin has the

screen with the smallest openings over it, it will contain the finest aggregate, and the highest

numbered bin will contain the coarsest aggregate.

Figure 5 - Batch Plant Hot Bins and Screens

B) WEIGH HOPPER

The weigh hopper is a large steel box attached to a set of scales. It is used to weigh the

aggregate and hold it until the mixer is ready for a new batch. The plant operator weighs out the

required amount of aggregate from each hot bin, starting with the coarsest aggregate and

finishing with the finest (or from the highest numbered bin to the smallest). This insures that

when the aggregate is dropped into the mixer, there will be some coarse aggregate at the bottom

of the mixer to help blend the other aggregates.

Batch plants have separate scales for the aggregate and asphalt. Older batch plants have

either beam type scales, in which the mass is measured by sliding weights along a calibrated

beam, or dial type scales, which are read directly from a dial. Nearly all of the newer plants use

an electronic weighing system, which is often part of a computerized plant operating system.

The most common scale problems are wearing of the knife-edge balances, caused by dust and

dirt, and aggregate binding in the scale mechanism.

C) ASPHALT DELIVERY

The asphalt needed to make a batch of mix is weighed into the asphalt weigh bucket. The

aggregate is then dropped into the mixer, where after a few seconds of dry mixing, the asphalt is

added. On some mixing plants, a fluidometer takes the place of the asphalt weigh bucket. A

fluidometer is a pump that can be adjusted to deliver a fixed volume of asphalt to each batch of

paving mix.

D) PUG MILL MIXER

A batch plant mixer consists of a mixing chamber, which is lined to reduce wear and two

shafts with paddles. The paddle tips are adjustable and fairly easy to replace. Asphalt is pumped

into the mixer and sprayed over the aggregate through a spray bar. When the mixing is

completed, a gate at the bottom of the mixer is opened and the completed mix drops into a truck.

Figure 6 - Batch Plants Pugmill and Asphalt Delivery System

Dry mixing time is the time from when the aggregate enters the mixer until the asphalt is

first added. Dry mix time should be as short as possible, usually one or two seconds. Wet mix

time begins when the asphalt is added and is normally about 30 seconds.

The hot asphalt spreads in a thin film over the aggregate, which is moving rapidly

through the air. All of this promotes very rapid oxidation. More hardening of asphalt takes

place in the mixer than in any other part of the paving operation, so the mixing time should be

the shortest time that will permit complete coating of the aggregate. When it is necessary to slow

down production, either hold the aggregate in the weigh hopper, or periodically stop the plant,

but do not increase the mixing time for this purpose.

Figure 7 - Live Zone of a Pugmill Figure 8 - Pug Mill Typical Paddle Arrangement

The specifications specify a mixing time of 45 seconds, unless test data shows that a

uniform mix can be obtained in a shorter time.

Any area in the mixer where the material is not thoroughly mixed because the paddles do

not reach it is called a dead zone. Some causes of dead zones are as follows:

• The mixer is so overloaded that the paddle tips are completely covered and the aggregate

above the paddle tips is not mixed.

• Paddle tips are badly worn, or two adjacent paddle tips are broken or twisted.

• The mixer liner is worn, causing too much clearance between paddle tips and liner.

• The fine aggregate or the asphalt is not evenly distributed through the mixer.

• The mixer is underfilled and the materials are just tossed around rather than mixed.

E) RECLAIMED ASPHALT PAVEMENT (RAP) IN A BATCH PLANT

Reclaimed asphalt pavement (RAP) is not put in the dryer with the other aggregate

because the heat would damage the asphalt and produce lots of smoke. The usual method of

handling reclaimed pavement is to heat the new aggregate to above the target mix temperature,

add the RAP at some point beyond the dryer (usually the weigh hopper) and let heat transfer

bring the mix to the desired temperature.

In practice the maximum temperature to which the new aggregate can be heated is about

475° F (250°C) since higher temperatures would damage the dryer. The amount of RAP

material in a paving mix can be as high as 50 percent, but often it has to be lower if the moisture

content of the RAP is high. RAP content may also be limited by nonuniform gradation if the

RAP is not further processed into two or more separate stockpiles of different aggregate sizes.

The RAP can be added at the hot elevator, although this is best done at a plant that does

not use screens, since the RAP would tend to clog them. The RAP also can be added at the hot

bins; either with the fine aggregate in the Number One bin, or into a separate bin that is not used

for any other aggregate. The advantage of using the Number One bin is that the hot aggregate

helps to heat the RAP. The disadvantage of using any of the hot bins for RAP is that the asphalt

sticks to the bin walls and can cause major aggregate build-up problems.

The weigh hopper is generally the best place to add RAP. It is fed into the hopper from a

separate conveyor. It can be added in any order, except it should not be the first aggregate

added, since this would cause asphalt build-up on the weigh hopper walls.

V DRUM MIXING PLANT

A drum mix plant is basically a type of continuous mix plant. It is simpler in design or

operation than a batch plant, but it is much more automated and requires computer controls to

operate. The components of a drum mix plant are:

• Cold Feed

• Asphalt supply system

• Drum mixer (which serves as both a dryer and a mixer)

• Surge or storage bin

• Dust control system

A drum mix plant has no screening unit or hot bins. All mix adjustments must be made

at the cold feed. A surge or storage bin is essential, since it produces paving mix continuously,

and cannot easily be stopped to wait for a truck to arrive. A drum mix plant requires more

stockpiles and more narrowly graded stockpiles to control the gradation. Drum mixer plants are

much more portable than batch plants.

Additionally drum mix plant have no screens or hot bins to smooth out variations in the

stockpile aggregates, so care must be taken to see that the stockpiles are properly placed and

handled, and that they are uniform and have the correct gradation. The aggregate should be

stored on a clean surface, and should be split into several stockpiles, each having a small range

of stone sizes.

A) COLD FEED BINS

The cold feed bins have adjustable discharge gates and a conveyor belt to carry the

aggregate to the dryer. The gate settings and often the belt speed are adjustable. The conveyor

belt has a weight sensing device, and a belt speed sensor. The accuracy of the weighing device

shall be within 1.0 % of the actual weight being measured when that weight has been determined

using another measuring device and shall be within 0.5 % when that weight has been determined

using test weights (AASHTO M156). The computer uses this information to calculate how

much aggregate is being fed into the plant, corrects for moisture in the aggregate, calculates how

much asphalt is needed, and adjusts the asphalt pump accordingly.

Some plants have weight sensors on each cold feed belt, but most have only one, located

on the conveyor that carries the aggregate to the dryer. The information supplied by the weight

sensor and the belt speed sensor are used to find the rate at which the cold feed is delivering

aggregate to the dryer.

The conveyor belt should fit tightly around the pulley to which the belt speed sensor is

attached. Otherwise slipping of the belt will cause the belt speed reading to be wrong.

The conveyor belt of a drum mix plant should be equipped with a scraper or brush to

clean wet fine aggregate from the belt. Otherwise this wet material will go around and around

the belt, being weighed each time, and causing the computer to overestimate the amount of

aggregate going into the plant.

The moisture content of the aggregate should be determined, so that the computer can

correct for moisture when setting the aggregate feed rate.

B) DRYER - MIXER

The dryer-mixer looks very much like the dryer on a batch or continuous plant, but it

has a dual purpose. It both dries the aggregate and mixes the asphalt with the aggregate. As the

aggregate first enters the dryer, the lifting flights alternately raise and drop the aggregate through

the heated air from the burner. Then the asphalt is added at a point far enough down the dryer

that the aggregate is no longer in direct contact with the flame, usually around the middle of the

dryer. As the aggregate is lifted and dropped by the lifting flights, it becomes coated with

asphalt. The moisture remaining in the aggregate causes the asphalt to foam. This aids in

coating the aggregate and also traps dust. What comes out of the far end of the dryer is a

completed paving mix, ready to load into trucks and send to the project.

The most frequently used type of drum called a parallel flow mixer. The aggregate enters

the drum at the burner end. The asphalt (and recycle material, if used) is added at about the

middle of the drum. There are other kinds of drums. In some only the aggregate passes through

the dryer, with the asphalt being added in a separate chamber at the end of the drum. There is

also a double drum mixer, in which the aggregate is dried in the inner drum and then passes into

an outer drum, where the asphalt and reclaimed material are added.

C) RECLAIMED ASPHALT PAVEMENT (RAP) IN A DRUM PLANT

Reclaimed material may contain large chunks of pavement, so there is usually a scalping

screen, or sometimes a crusher somewhere in the system. Reclaimed material could be fed into

the plant with the new aggregate, but to avoid problems such as smoke, fire, and asphalt sticking

to the bin walls, it is added elsewhere, usually at either the midpoint or the far end of the dryer.

This allows the reclaimed material to be heated without coming into direct contact with the dryer

flame.

D) ASPHALT DELIVERY SYSTEM

The asphalt metering system controls the rate at which the asphalt is added. The control

system monitors the mass of the aggregate on the belt and the speed of the belt. It uses this

information, and the aggregate moisture, to set and adjust the asphalt feed rate to give the

required asphalt content.

If the asphalt enters the dryer too close to the burner end of the drum, it will be

overheated. It may be absorbed into the aggregate, and what asphalt is left on the aggregate

surface may become so hard and brittle that it loses its adhesive properties. The plant will give

off blue smoke from the burning of volatile components of the asphalt. The moisture content

may be high because the aggregate was coated with asphalt before the moisture had time to

evaporate.

On the other hand, if the asphalt is introduced too close to the outlet end of the drum, the

asphalt will not do a good job of trapping dust and the aggregate may be too dry to be completely

coated by the asphalt.

VI PLANT INSPECTION

The requirements for asphalt mixing plants can be found in Section 401 of the Standard

Specifications with any revisions available in The Supplemental Specifications.

A) BATCH PLANTS SPECIFIC INSPECTION ITEMS

A weigh hopper is used to proportion the aggregate. It consists of a large bin attached to a

set of scales. Only batch plants have weigh hoppers to weigh the aggregate and weigh buckets to

proportion the asphalt. Some of the things to look for when inspecting a batch plant are as

follows:

• The hot bin gates must close tightly. Otherwise aggregate will leak into the weigh

hopper and cause the gradation of the next batch to be in error.

• The aggregate scales must be accurate to within 0.5% and easily read. A set of ten

50-pound weights and one 5-pound weight (for sensitivity checks) are used to

calibrate the scales.

• A means must be provided for taking hot bin samples. They are usually taken as the

aggregate passes from the hot bins to the weigh hopper. The sampling device must be

long enough to sample the whole stream of aggregate. This is because the finer stone

tends to pass through the screen first, so the gradation is not uniform across the bin.

The finer material is found on the side nearest the hot elevator.

• Hot bin samples are needed to determine the batch weights. If the gradation of the

combined hot bin samples does not meet the plant mix formula, then the batch

weights must be adjusted, or if this does not work, a new plant mix formula must be

established.

• The weigh hopper must not bind against any part of the plant. Any aggregate lodged

between the weigh hopper and its supports must be removed.

• The asphalt weigh bucket must be insulated, and the asphalt scales must return to zero

after the bucket is emptied. The asphalt valve should close tightly and not drip. The

tare weight of the weigh bucket should be checked frequently because asphalt tends

to build up inside the bucket.

• Some batch plants use a fluidometer instead of asphalt scales and a weigh bucket.

This is an adjustable pump that can be set to deliver a fixed amount of asphalt per

batch. To calibrate a fluidometer either use the fluidometer to fill a container of

known volume and compare the volume to the fluidometer reading, or pump the

asphalt into a tared container and weigh it. Some fluidometers are temperature

compensated. If not, a temperature correction factor must be applied.

• The spray bar must be heated and must be long enough (at least 3/4 the length of the

mixer), so that no aggregate is left uncoated.

• All parts of the asphalt system (tanks, circulating lines, valves, weigh bucket, spray

bar) must be heated and insulated.

• It is important to check the condition of the mixer paddles since broken or worn

paddles can result in aggregate segregation and uncoated mix. The Asphalt Institute

recommends that if the broken or worn paddles are widely spaced, they should be

replaced at the end of the working day, but if two adjacent paddles are broken, they

should be replaced immediately.

• The specifications require that hot-mix asphalt produced in a batch plant be mixed for

at least 45 seconds, unless tests show that a shorter mixing time gives a satisfactory

asphalt coating. A time lock is required to insure that the mix is not dumped from the

mixer before the specified mixing time has elapsed.

B) DRUM MIX PLANTS SPECIFIC INSPECTION ITEMS

These plants do not have screens or hot bins and the dryer is also the mixer. The plant

consists of three main units: the cold feed, the dryer/mixer, and a surge or storage bin. The plant

also has an asphalt storage tank and a dust collector.

A drum mix plant often has more cold bins than other types of plants, because the cold

bins are the only means controlling the gradation. Weight sensing devices located on the

conveyor belts measure the amount of aggregate entering the dryer.

A computer controls the asphalt content of the mix by adjusting the output of the asphalt

pump to compensate for changes in the aggregate feed rate. The asphalt is added through a spray

bar at about the midpoint of the dryer. The completed mix then goes from the dryer to a surge or

storage bin. Some of the items to look for when inspecting a drum mix plant are as follows:

• The cold feed must be constructed so that aggregate samples can be obtained from it.

• There should be a weight sensing device and a belt speed sensor on the conveyor belt.

• The dryer/mixer must be able to heat and mix the materials without stripping the

asphalt from the aggregate or causing excessive hardening of the asphalt.

• There must be positive interlock of asphalt and aggregate feed, so that if one changes

there is a proportional change in the other. In practice, this usually means that a

computer controls the asphalt and aggregate feed rates.

VII AVOIDING MATERIAL AND EQUIPMENT PROBLEMS

Asphalt Concrete requires quality materials and a mixing plant in good working order.

The following paragraphs list some of the problems that can occur at the mixing plant.

A) STOCKPILES

The storage yard should be kept neat and orderly. Stockpiles containing different

aggregate sizes should be separated. If there is not enough space, bulkheads may be used to keep

them apart.

Materials such as sand and single sized aggregates can be stockpiled by almost any

method with little segregation. Aggregates with a range of sizes tend to segregate, especially if

the stockpile is formed by dropping the aggregate, such as off the end of a conveyor belt. The

segregation occurs because the coarse aggregate tends to roll down the sides of the pile, while

the fine aggregate stays where it lands, so the coarser aggregate ends up at the outside edge of

the pile. Although cone shaped stockpiles are common, they do not have a uniform gradation

and it is difficult to get a representative sample from them.

Trucks or cranes can be used to make layered stockpiles. These are less likely to

segregate than cone shaped stockpiles, but the trucks may track mud onto the stockpile, and

tracked vehicles may crush the aggregate.

With proper care, segregation can usually be kept within acceptable limits. If not, the

segregated aggregate can sometimes be reblended when transferring it to the cold feed. This is

usually done by alternately scooping from coarse and fine sections of the pile.

Stockpiles of reclaimed material are handled in about the same way as new aggregate. It

may be necessary to limit the height of the stockpile to keep the material from packing together

under its own weight.

Dust, in small quantities acts as an extender and reduces the amount of asphalt needed.

In large quantities, it increases the amount of asphalt needed and makes the mix brittle. When

baghouse dust is added to the paving mix as a mineral filler, it should be added in uniform

quantities. If the dryer has not adequately dried the aggregate, or if the mineral filler has not

been protected from the weather, moisture can cause dust balls to form in the paving mix.

To get a representative stockpile sample, take the sample at several locations and levels.

A metal plate or a piece of plywood can be stuck into the pile above the sample site to keep

aggregate from rolling down the pile into the sampling area. A scoop or shovel with raised sides

should be used to keep the aggregate from spilling off the sides. Larger aggregates require larger

samples. The sample must be big enough so that one large rock does not make a large difference

in test results. Once the sample has been obtained, it can be reduced to test size by quartering or

with a sample splitter.

B) ASPHALT STORAGE

Asphalt must be kept hot both in the storage tank and in the lines between the tank and

the mixer. Asphalt becomes solid when it cools and can completely clog the lines, stopping

production, or partially clog the lines and result in a mix that does not have enough asphalt. It is

essential that both tank and lines be heated and insulated.

A paving mix contains much more aggregate than asphalt, so the temperature of the

aggregate determines the temperature of the mix. If the aggregate is overheated, it also overheats

the asphalt, causing it to oxidize, become brittle and have a dull black appearance.

Asphalt in the storage tank can be heated well above the normal mixing temperature

without damage. But if the asphalt is too hot when it is added to the mix, the asphalt coating on

the stone will be thin, tend to drain from the stone, and the mix will not stick together very well.

The mix will have a brown color, and the stone may show through the asphalt.

A return line from the mixer to the storage tank is needed, so that unused asphalt can be

returned to the storage tank. Otherwise, the asphalt might solidify in the lines. A thermometer is

required in the circulating line and most plants also have one on each storage tank.

C) COLD FEED

The cold feed consists of several bins filled with aggregate. Each size of aggregate

should have its own bin. Anything that causes the output of the cold feed to vary must be

avoided, since variations in the cold feed can cause problems elsewhere in the plant, such as

changes in the gradation or temperature of the mix, or hot bins that overflow or run dry.

The following are examples of problems that may originate in the cold feed:

• Moisture can change the rate at which the aggregate comes out of the cold feed,

causing erratic gradation. It can also act as a lubricant and speed up the aggregate, or

make it stick together and slow it down.

• Variable moisture content of the aggregate causes the mix temperature to be erratic.

• Wet sand tends to arch or otherwise hang up in the bin. This causes the mix to be too

coarse and then when the sand breaks loose, the mix becomes too fine. Using a

vibratory feeder can prevent this problem.

• Large rocks sometimes get into the bin and clog the outlet gate. This can be cured by

placing a steel grid on top of the bin.

• Incorrect bin proportions cause overloaded hot bin screens. This results in carry-over,

hot bins running dry or overflowing, loss of aggregate through the scalping chute, and

variable mix gradation.

While some adjustments in gradation can be made in the plant, in the long run, what goes

in the cold feed comes out the pugmill, so the cold feed determines the gradation of the mix. The

following are some of the things that can be done to reduce cold feed problems and insure that

the gradation meets specifications:

• Use the right size of aggregate

• Try to prevent aggregate segregation

• Avoid intermingling of stockpiles

• Once the gates are calibrated secure them so if they are changed, the plant technician

will know about it.

• Check for obstructions such as tree branches or rocks blocking the gates.

• Keep cold feed bins full.

• Use bulkheads to prevent intermingling from the bins overflowing.

D) THE DRYER

A dryer is designed to provide a certain amount of air and heat. If the aggregate is too

wet, or the feed rate is too high, the dryer can't do its job. Asphalt will not coat aggregate that is

cold or wet. If aggregate is dry on the surface, but wet inside, the moisture may come out of the

mix as steam and strip the aggregate from the stone. This type of problem is best solved by

slowing down the aggregate feed rate into the dryer.

A low mix temperature or high moisture content is an indication of dryer problems.

Other indications that the aggregate is not being dried are: an unusually large amount of steam

coming from the hot bins or from the mix, flattening of the mix in the truck bed, or water

dripping from the truck bed. Too much moisture often causes the mix to look as though it

contains too much asphalt. Increasing the heat, slowing down the dryer, or reducing the

aggregate feed rate can usually correct dryer problems.

The specifications require that the aggregate temperature be measured as it leaves the

dryer. This is usually done with an electric pyrometer. The pyrometer is located in a metal

shield at the discharge end of the dryer. The temperature indicator may either show the

temperature on a dial or record it on a graph. The temperature indicator must be in a location

where the plant operator can see it.

A pyrometer measures small changes in electrical resistance caused by changes in

temperature. Moisture, loose connections and splices in the wire also cause resistance changes,

so the pyrometer may get out of adjustment and have to be recalibrated.

If a drum mix plant is operating efficiently, the temperature of the exhaust gas should be

no more than about 10° C higher than the mix temperature, except that higher temperature

differences may be unavoidable if a high percent of reclaimed material is used. High exhaust

temperature indicates that the veil of aggregate in the dryer is not being properly maintained, or

there may be an air leak in the dust control system.

Fuels used the dryer include gases (natural gas or liquefied petroleum gas), liquids (fuel

oil), or solids (pulverized coal). With any of these fuels, the burner adjustment can cause

problems. If the balance of the air and fuel is not right, the dryer will produce lots of smoke and

not much heat. Dryer problems are commonly caused by soot on the burner, leaks in the air

system, or the blower not working properly.

Incomplete combustion occurs when the fuel to air ratio is too high. This wastes fuel and

often results in underheated aggregate. The unburned fuel can coat the aggregate with an oily

film or dilute the asphalt. Signs of incomplete combustion are black smoke coming from the

plant exhaust; dark sooty stains on the aggregate leaving the dryer, or a sputtering sound from

the burner. Other signs of incomplete combustion are an oily film on the surface of the settling

pond, or dark stains on the filter bags in the bag house, or an increase in the pressure across the

bag house.

The material coming from the dryer should have a uniform color and the fine aggregate

should be evenly distributed through the mix. A dark stain on the coarser stone indicates

incomplete combustion of the fuel.

If a plant uses a highly absorptive or very wet aggregate, modifications to the dryer may

be needed. For example, reducing the slope of the dryer, or rearranging the lifting flights to slow

down the aggregate, or a second dryer may be needed.

E) SCREENS

Screens can develop rips or tears, or become so worn that they allow oversized aggregate

to pass through. The screens must be able to handle the aggregate that is fed to them and have

some excess capacity since some of the openings will eventually become blocked by aggregate

sticking in them. Even screens that are in good condition can become clogged, or be fed

aggregate faster than they can handle it. When this happens, the aggregate that should fall

through a screen, passes over it and is deposited in the bin that should contain the next larger

size. This is called carry-over. There is always some carry-over and there is no way to eliminate

it completely. Up to 10 percent carry-over is considered acceptable. It becomes a problem when

it becomes so large or variable that the gradation cannot be controlled.

Screen problems are often discovered by testing samples from the hot bins. Carry-over

should be suspected when undersized aggregate is found in a bin. The opposite problem of

oversized aggregate in a hot bin, is usually caused by a hole in a screen.

F) HOT BINS

Hot bins provide temporary storage for the aggregate as it comes from the screening unit

and remix it to meet the required gradation. Drum mixer plants do not have hot bins. The

aggregates are mixed entirely at the cold feed.

Hot bins often overflow or run dry. An overflowing hot bin wastes aggregate. A hot bin

that runs dry slows production because the plant operator must wait for more material to fall into

the bin before he can continue weighing out the batch. The operator may also be tempted to pull

heavily from the bin that has plenty of aggregate, causing the gradation to be considerably off.

Although the problem shows up at the hot bins, it usually originates somewhere else. The cold

feed may have been set wrong, or the stockpile gradation may have changed, so that the hot bins

are getting too much or too little of one of the aggregate sizes. Hot bin problems can usually be

corrected by adjusting the cold feed.

Clogged screens, or holes in the bin walls, also cause problems. These should be checked

periodically.

Moisture in the aggregate may condense on the bin walls and cause fine aggregate to

build up in the corners of the hot bins. This tends to break loose all at once, causing a surge of

fines in the mix. It can be corrected by installing fillet plates in the corners of the number one

bin, or by installing a steel plate at the top of the bin to deflect the fines toward the center of the

bin.

If the overflow pipes are too small or they are stopped up, oversized aggregate may

overfill the whole hot bin system. As the bins fill up, aggregate overrides the bin partitions and

ends up in the wrong bins. The screens may be damaged by riding on top of aggregate, and the

gradation will not be anything close to what was expected.

Loss of aggregate through the overflow pipe should be rare, since it means that the

money that was paid to buy and heat the aggregate is being wasted.

Aggregate is sometimes deposited in the hot bins in alternating coarse and fine layers.

This is called stratification, and it is usually caused by variations in the stockpiles or erratic

operation of the cold feed. Stratification makes it impossible to obtain a representative hot bin

gradation, and may show up in the road as variations in the appearance of the mix.

G) SURGE AND STORAGE BINS

Surge bins are used to hold paving mix for a short time. Their purpose is to permit the

plant to keep running when there are no trucks available for loading. Storage bins are used for

longer-term storage.

Dropping the mix through the bin can cause aggregate segregation, especially when it is

dropped a long distance. For this reason, operating at a low bin level is not good practice, and

dropping the mix straight through the bin into a truck is prohibited by the specifications.

Aggregate segregation across the bin can happen when the mix is dumped into the bin

from a conveyor belt, since the larger pieces of aggregate tend to fall to the far side of the bin.

This sometimes shows up as a visible change in gradation across the road. This side-to-side

segregation can be corrected by using baffles or a rotating chute to spread the mix around the

bin.

When the mix drops to the bottom of the bin, it forms a cone shaped pile. This may

cause aggregate segregation, especially if the mix is coarse or gap graded. A small surge hopper

at the top of the bin can be used to drop the mix in large enough batches that it flattens out when

it hits the bottom of the bin, instead of forming a cone.

Paving mix that is stored in a heated, sealed bin can usually be stored for several hours,

or under ideal conditions, for days. Densely graded mixes can be stored longer than coarse or

open graded mixes, since air currents do not pass through them as readily. When bins are used

for long-term storage, they may have an inert gas system to prevent oxidation of the asphalt. The

inert gas is often the exhaust gas from the dryer. The exhaust gas has had most of the oxygen

burned out of it, and so it doesn't react with the asphalt as fresh air would.

Prolonged storage may cause the asphalt to strip from the aggregate. The stripping is

usually caused by moisture from incompletely dried aggregate. A silicone additive can prevent

this.

H) TRUCKS

The paving mix must arrive at the project while it is still hot enough to be compacted.

Long haul distances and unexpected delays can cause major problems. The following are some

of the items to check when inspecting trucks:

• The truck bed must be insulated.

• A canvas cover is required. This protects the mix from rain and from air currents.

Oxygen in the air reacts with asphalt, making it brittle and difficult to compact, so it

is important that the canvas cover be used even on hot days and that it extends over

the sides of the truck.

• The truck bed must be lubricated to keep paving mix from sticking to it. This is to

prevent the formation of lumps of cold paving mix that stick to the truck bed and later

work loose and end up in the pavement.

• Soapy or oily liquids can be used to lubricate the truck bed. Several silicone-based

materials are also made for this purpose. Petroleum products, such as diesel fuel,

should not be used because they dilute the asphalt.

• Check for holes in truck bed, deep indentations, or anything harmful to the mix.

I) OBSERVATION OF THE PAVING MIX

Here are some of the things to look for when checking the paving mix.

• Temperature - a leading cause of trouble. Check frequently.

• Blue Smoke - indicates overheating

• Peaked pile - indicates under heating

• Mixer gate opens slowly or incompletely - causes segregation

Some easy to spot indications of problems with mix:

• Appearance not uniform

• Lean or dry looking mix, usually brown in color

• Fat or over-asphalted mix

• Brown stripped asphalt on top of pile caused by escaping moisture.

• Sluggish appearance as mix settles in the truck (underheating)

• Mix slumps in the truck (moisture or too much asphalt)

Cold Feed Controls gradation and

production rate.

Adjust gates & belt speed

to control gradation &

production rate.

Adjust gates & belt speed

to control gradation and

production rate.

Dryer Heats and dries the

aggregate.

Adjust burner & feed rate

to control mix

temperature & moisture

content.

Adjust burner & feed rate

to control mix

temperature & moisture

content. Asphalt added

here.

Screening Unit Smooth out variations in

gradation. Separates

aggregate by size.

Change mix type by

changing screen sizes.

Replace worn out screens

when needed.

Doesn’t have a screening

unit. Gradation is

controlled at cold feed.

Hot Bins Aggregate proportioning

and temporary storage.

Adjust bin proportions to

control gradation Check

scales for accuracy.

No hot bins. Aggregates

proportioned at cold feed

and mixed in dryer.

Mixer Mixes the aggregates &

asphalt.

Check for worn paddles

or liner. Time lock

regulates mixing time.

No mixer. Mixing is

done in dryer. Computer

controls mixing time.

Surge & Storage

Bins

For temporary mix

storage.

Optional equipment.

Check periodically. Mix

may cool, harden, or strip

during storage.

Needed to store mix

between trucks. Mix

may cool, harden, or strip

during prolonged storage.

VIII CALIBRATIONS

A) TEMPERATURE CALIBRATIONS

Temperature is very important to the quality of the pavement and should be checked both

at the plant and at the project. Mix temperature may be checked with a dial type thermometer,

placed in a hole in the side of the truck. An infrared thermometer may also be used, and is

generally preferred because it’s greater speed and ease of use.

B) SCALE CALIBRATION

The procedure for scale calibration and accuracy checks is given in Section 708 of the

Construction Manual. MP 700.00.30 provides information pertaining to the calibration of the

50-pound weights. The scales must be approved by the Division of Weights and Measures. A

summary of this procedure is as follows.

• A zero balance check is required twice a day. Zero balance means that the scale

reading must return to zero when the weigh hopper is emptied. If it does not, the

cause should be found and corrected immediately.

• A sensitivity check is required twice a day. When a 50-pound weight is placed on the

loaded weigh hopper, the scale reading must increase by at least 30 pounds for

aggregate scales and 49 pounds for asphalt scales. If this requirement is not met, the

scales must be inspected as soon as possible.

• A simplified calibration of the aggregate scales is conducted weekly and a complete

calibration approximately twice a year. The scales must meet the requirements of the

Division of Weights and Measures.

• An asphalt scale calibration consists of alternately placing weights and aggregates on

the scales at 500 pound intervals, until the total mass is slightly above the expected

batch weight. This is usually done twice a year.

C) AGGREGATE SCALE CALIBRATION

On the following page is an example of Form T603. This is the form used for calibrating

aggregate scales. The procedure for checking the accuracy of aggregate scales is as follows:

• Place the weights on the weigh hopper. The scale dial should now read 500 pounds.

The difference between 500 pounds and what the dial does read is the error.

• Remove the weights.

• Add aggregate to the weigh hopper until the dial reads the same as it did in Step 1.

• Put the weights back on the hopper. The theoretical mass is now 1000 pounds (500

pounds of aggregate and 500 pounds of weights). The difference between 1000 and

what the dial reads is the error.

• Repeat, adding 500 pounds more of aggregate each time, until the mass is equal to or

greater than the largest weight that you intend to weigh when batching the mix.

• The allowable error is 0.5 percent (1/2 pound in 100 pounds). If any of the observed

errors exceed this, the scales must be repaired.

Table 1 - Calibration of Scales Form

IX SUMMARY

We have covered how a hot-mix asphalt plant works, plant inspection, calibration and accuracy checks of scales and thermometers, and checking the accuracy of batch

weights.

More detailed information on how a hot-mix asphalt plant works can be found in the

HMA Paving Handbook.