Asphalt Plant Level II January of Module Asphalt ... Plant 2/02 - Module 2 - HMA...Module 2 –...

Asphalt Plant Level II – January 2009 of 26 Module 2 – Asphalt Production Facilities

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Module 2

Asphalt Production Facilities

ConstructionTrainingQualification Program

Asphalt Plant

Level II

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• The two different types of Asphalt Facilities• How Asphalt is produced in a “Batch Style”

facility• How Asphalt is produced in a “continuous” or

“Drum-Mix” facility• How gradation and asphalt is controlled in

these two different type plants

What we will cover... In this module we will briefly review the two major different types of hot mix facilities, the “batch plant” and the “drum-mixer plant”. We will discuss how the gradation blend and the asphalt content are controlled in these two different style facilities.

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The information presented in this section is found …

“Hot Mix Asphalt Production Facilities”(NHI Course 131044)

– Module 2, pages 2-2 to 2-11

“Hot Mix Paving Handbook 2000”– Section 5, pages 41-47

The information presented in this section can be found in Module 2 (pages 2-2 to 2-11) in “Hot Mix Asphalt Production Facilities,” FHWA’s NHI course number 131044 and in Section 5 (pages 41-47) in the “Hot Mix Paving Handbook 2000,” the development of which was sponsored by AASHTO, FHWA and a number of other agencies. It is available through the National Asphalt Pavement Association and the Federal Highway Administration.


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RequirementAll Plants

320-2.1

Consistently produce a mix within the job mix tolerances and temperatures specified.

In addition to this general requirement (320-2.1), there are some specific requirements for individual plant components. They are primarily found in Sections 320 and 330 of the Department specifications, although 334 and 337 mixes also have some specific equipment requirements. These additional requirements will be discussed as we cover that specific equipment in this manual. In addition to specific equipment requirements, we will also discuss industry recognized Best Management Practices (BMPs) for operation. These BMPs should be taken into consideration when inspecting the facility. This manual and any suggested BMPs, are not the governing document for plant operation in Florida, however. Only the most recent edition of the state specifications is.

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Must also comply with other laws, rules, regulations, statutes…..

Federal GovernmentState Agencies

CountiesCities

…..in which they operate

Plants must also comply with other department and agency regulations. These include federal, state, county, and local city regulations and restrictions. Two important state agencies that affect all Florida producers are the Department of Agriculture and the Department of Environmental Protection.


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Scales for commerce regulated by:

Florida Department of AgricultureBureau of Weights and Measures

Scales used for determining material weights used to pay the contractor must conform to all the requirements of the Bureau of Weights and Measures of the Florida Department of Agriculture and Consumer Services. Batch plant scales and truck scales and weigh hoppers under silos are used for “selling” products, and fall under their jurisdiction. Their specific requirements are in addition to the Department of Transportation requirements.

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Air Quality Permit and Storm Water Permit

(D.E.P.) Department of Environmental Protection

(permit to be posted)

The operation and maintenance of asphalt plants must comply with the Rules of the Department of Environmental Protection (D.E.P.). Water Use Permits fall under the jurisdiction of Water Management Districts. Waste Water Permits are typically issued by the D.E.P. Compliance permits should be posted at the plant site before start of work, although the Department of Transportation does not perform any enforcement duties for the D.E.P.

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Asphalt Facilities

Two Basic Types of Plant Configurations

Batch

Continuous (Drum-Mixer)

Asphalt production facilities have evolved into two basic types of plant configurations: “batch” and “continuous flow” (drum-mix). Both types of facilities, regardless of plant age, can be equipped with all the requirements for modern, up-to-date asphalt production, including computerized control automation, recycling systems, and additional equipment for bulk and liquid material additives. Either type of facility can produce consistent, high quality hot-mix to meet the required job-mix formula.


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Asphalt Batch Plant

Let’s review asphalt batch facilities first.

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Virgin AggregateCold Feed Bins

Aggregate Dryer

Baghouse Dust Collector Dust Silo

Asphalt Tanks

RAP Feed Bin

Control House

Batch Tower

Hot MixStorage

Silos

January 2009

Batch type facilities, as shown here, have been common since the turn of the century and have changed little in overall concept. They produce hot-mix asphalt a “batch” at a time. This means ingredients are weighed up individually, then mixed together and dispensed into a truck or storage equipment on a “batch” basis, one batch at a time. The major equipment components of the plant are identified.

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The Asphalt BatchPlant Facility

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This series of illustrations shows how material actually moves through and is processed in a batch type facility.


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Materials from thedryer are transportedto the top of the “tower”using a “bucket elevator” also known as a “hot stone elevator.”

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Once crushed aggregate is dried it is conveyed to the top of the screening section with a device known as a bucket elevator.

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Materials are then sizedwith the batch tower’s“Screen Deck” andstored in the “Hot Bins”in different sizes.

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At the screening unit, the aggregate flow passes over different screens that separate the aggregate into different sizes. Sized aggregates are then stored in the “hot bins”, so called because they contain the hot, dried aggregate which is waiting to be dispensed into the aggregate weigh hopper.

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The final mix gradationblend is determined by“drawing” different amounts of materialfrom the hot bins usingthe aggregate “weighhopper.”

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Below the “hot bins” are “supply gates” or “hot bin gates.” The plant operator manually or automatically “draws” different amounts of material from each hot bin to match the final job-mix formula weighing each material with the aggregate “weigh hopper,” which is positioned directly below the hot-bin gates. This is where the final mix gradation blend is determined, or is the last place where gradation is affected. We refer to this as the “final” place, because one can easily see that the gradation in the hot bins is affected by the flow of different materials from the cold


feed bins. This is a key element in managing mix consistency in a batch type plant, and is covered later in this section and also in the separate Appendix addressing batch plant production.

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The asphalt content isdetermined by weighingup the proper quantityof asphalt using theasphalt “weigh bucket.”

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The asphalt content is determined by pumping asphalt into a device known as the asphalt “weigh bucket,” where it is weighed to the required amount. In modern, automated plants, the aggregate weighing and asphalt weighing is done simultaneously to shorten the batch cycle.

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-The aggregate andasphalt materials areblended together withthe “pugmill / mixer” during a “dry mix cycle”and a “wet mix cycle.”

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To produce a homogenous mix, aggregate is discharged from the aggregate weigh hopper into the “pugmill”, where it is mixed for a few seconds (normally 2-5 seconds) without asphalt cement in a “dry-mix cycle.” After the dry-mix cycle, the asphalt cement is discharged from the asphalt weigh bucket into the pugmill, where it is mixed with the blended aggregate in a “wet-mix cycle,” normally about 30-35 seconds.


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-Mix is dispensed intoa truck or a transferdevice to a storage silo

- HMA is produced a“batch” at a time(not continuously)

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The hot-mix is then dispensed into a waiting vehicle or into transfer equipment for storage in a silo. Note that with a batch type plant the aggregate gradation in the final mix is determined at the aggregate “weigh hopper” by “drawing” different quantities of each hot bin. Asphalt content is determined in the asphalt “weigh bucket” by weighing up the desired quantity of binder. Mix is produced a batch at a time.

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The Asphalt Batch Plant Facility This photograph shows an asphalt batch plant facility. The size of the “batch” will depend on the size of the facility and the discretion of the plant operator. Batch plants are sized for the tonnage in the batch. While facilities have been produced over the years with capacities from 3/4 ton to 20 ton, typical sizes found in the field range from 2 ton to 6 ton.

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Screen Deck

BIN 1 2 3 4

One of the most important components in a batch style plant is the screening unit or “gradation unit” (FDOT term). Here material passes over several different sized screens, referred to as “decks.” As material successfully passes down through the screen “cloth” on each “deck,” it is directed into one of the hot bins in the batch tower. Material that passes through all the decks is deposited into the hot bin closest to the hot elevator. This is commonly referred to as the “fines bin” or No. 1 bin. In Florida, this bin typically has either minus 8


or minus 10 material in it, and is the fine aggregate bin. Material retained on the deck over the fines bin, but passing through all other screen decks ends up in the No. 2 bin. Material that is retained on the screen sizing for the No. 2 bin, but passing all other screens is directed to the No. 3 bin. This type of sizing process continues to fill all the hot bins on the plant. Material that is retained on all screens is rejected from the tower through the overflow chute. This performs the “scalping” function for oversized material. Bins are numbered from the hot elevator. Over 80% of the batch plants in the field have four hot bins, but it is not unusual to find a plant with five. Some plants were also manufactured with three hot bins, and some with six, but these are rare.

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Screen Deck

BIN 1 2 3 4

This illustration is another good depiction of the internal workings of the batch plant screen deck or “gradation unit” (FDOT term). It is important to understand that batch type plants were designed in an era when it was not really possible to obtain sized aggregate from a quarry facility. The screening unit or “gradation unit” was used to process a “crusher run” aggregate product and natural sands and size them into fractions for producing different hot mix formulas. In today’s world, where availability of sized aggregates is the norm, all this seems somewhat unnecessary and redundant. With batch plants today, producers take


sized aggregates from the material suppliers. And establish the blend percentages required to achieve the job mix formula from the cold feed bins like a drum-mixer plant. They then examine the material in the hot bins by taking individual gradations of each hot bin. At that point, new blend percentages have to be established based on the gradations of the hot bins not the cold feed bins. Most newcomers to the business find this somewhat bewildering, but looking at the history of the batch plant, and realizing that drum-mixers are a relatively new phenomenon, having only been introduced in the last 40 years, helps makes this a little more understandable.

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Aggregate Sampling at Hot Bin Gate

Final Gradation Blend Control at Hot Bin

This photo shows how samples of the hot bin gradations are actually taken. You can see a sampling pan positioned on rails directly below the middle hot bin gate. A sample is taken from each bin. From the gradations of each hot bin, percentages are established to achieve the target Job Mix Formula. The process used to do this is exactly the same as that used to determine blend percentages from the cold feed bins for drum-mix plants. So you can see that in today’s world, with a batch plant, and with the availability of sized aggregates from the material producers, this blending process is done at least two times. An example will be done in Section 3, so you can see how the process is executed.


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Running “Screenless”in a Batch Plant

In fact, many of today’s batch plants either are fitted with a “screen bypass” chute, or have removed the sizing screens altogether, leaving only one screen to act as a “scalping screen” for oversized material. This permits all the materials to fall into the “No. 1” bin already blended, but requires that the composite gradation is established at the cold feed bins like is done with a drum-mix plant. When the producer “batches,” he weighs up just one aggregate and then he drops this into the pugmill for processing. Florida DOT specifications do not prohibit you from producing in this fashion, and this is the most popular approach being used with batch plants producing mixes for FDOT. (Note that FDOT specifications require a screen on both drum-mix plants and batch plants to prohibit oversize aggregate particles in the mix. With a drum-mix plant, the “scalping screen,” which is described later, provides this function. When the producer leaves one deck in the batch tower screen assembly he is complying with this requirement.)


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More on Batch Plants in the Appendices There are many specifications and operational issues that impact mix quality with a batch plant. Because over 95% of the mixes produced for FDOT are produced in drum-mixer plants, we are not going to take precious class time reviewing them. You will find an excellent treatise on these points in Appendix 3.

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Drum-Mix (Continuous)Plant Facility

Now let’s look at making mix continuously in a drum-mix facility.

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Drum Dryer/Mixer(Drum-Mixer)

Asphalt Tanks

Baghouse Dust Collector

Virgin AggregateCold Feed Bins

RAP Feed BinControl House

Dust Silo

Hot MixStorage

Silos

Although the concept of continuous flow plant facilities is not new, and has come and gone since the early part of the 20th century, the "drum-mix" facilities we are familiar with mostly include parallel flow drum-mixers popular from the early 1970's to the late 1980's, the counter-flow drum-mixers popular since the late 1980's, and the modern dryer/mixer facilities such as the “Double Barrel” plant. It was the availability of consistently sized aggregate from the quarry, together with the development of storage silos for batch plants in the 1960’s that


helped lead to the development of the drum-mixer plant.

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BLEND

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The primary difference between "drum-mix" facilities and a "batch" style facility is that aggregate is blended, dried, and mixed with liquid asphalt in a one-step continuous basis, rather than the batch-at-a-time basis common with a batching style facility. Unlike a batching style plant that has sizing screens at the top of the batching tower to size the aggregate into fractions, and holding bins and a weigh scale for proportioning the aggregate blend, drum-mixers control gradation directly at the aggregate feeders. Both the availability of sized aggregates from the quarry, and the availability of inexpensive variable speed motors made this possible.

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BLEND

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Variable speed belt feeders, fitted below the cold feed bins, allow the proportioning of the aggregates based on calibrating the output of each feeder at different speeds. Varying the speeds of each individual cold feed bin belt controls the final mix gradation. The plant automation does this with computerized precision, based on the desired production rate and the mix recipe entered into the computer (percentages of each cold feed bin desired).


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Variable Speed Motors for Feeder Belts(controls flow from each cold feed bin)

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This is a photograph of a typical cold feed bin system, and the variable speed motor can be clearly seen. One bin is used for each aggregate. The calibration process is the process of establishing the output from the bin at different speeds. The computer will later select the proper speed for that material, based on the Job Mix Formula and the target tons per hour (tph). A sample cold feed calibration problem will be done in Section 3, so you can see exactly how the computer arrives at the target feeder speed. It is important to note that with a drum-mixer plant, the gradation is established from this calibration process. Materials are not weighed from the individual bins. The belt scale in the system is used to measure only the combined flow off all the cold feed materials, and is used to proportion the liquid asphalt cement and other ingredients to this combined aggregate flow. Several things can go wrong with the cold feed flow and the calibration process, affecting the quality of the mix. Those points are covered in Section 3.


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BLEND

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A “scalping” screen is used to keep oversized materials from the mix. This is an FDOT requirement. The “scalping” screen is not a sizing screen. It is sized only to keep oversize materials from entering the drum, and therefore the mix.

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Scalping Screen

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This photograph shows a typical “scalping screen” on a drum-mix plant. It is typically positioned in the transition between the cold feed bins and the conveyor belts.

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BLEND

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With a drum-mixer plant, asphalt is proportioned and blended on a continuous basis. This is accomplished first by weighing the combined aggregate flow from the cold feed as it travels to the dryer with a belt scale. The asphalt cement is then proportioned on a continuous basis with an asphalt pumping and metering system to match the weight of this aggregate stream. The responsibility for the proper proportioning and the proper timing of the asphalt cement flow falls on the electronic, automated controls.


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Belt Scale

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This is a picture showing the location of the belt scale on that same drum-mixer plant. The “scale conveyor” is typically positioned between the scalping screen and the drying drum.

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This illustration shows how the belt scale actually works. One “idler” or “roller” on the conveyor is not attached to the conveyor frame, but is connected to a “load cell” or “cells.” This provides a continuous real time static (steady state) weight reading over a span of belt. A speed sensor on the conveyor simultaneously reads the conveyor speed, and the two values together are used to establish “ton per hour.” The electronic device that processes these signals is either a separate electronic device or is integrated directly into the plant automation. The calibration process is the electronic process required to make this scale read correctly. We will do a sample calibration problem in Section 7.


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BLEND

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Proportioning the asphalt flow to the belt scale signal is then accomplished with an assembly that both varies the flow of the asphalt and measures the flow of the asphalt with a meter.

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Variable Speed Motor(for flow control)

Feed Line from Tank

PumpAC Meter

(measures flow)

Feed Line To Drum

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This photograph shows a typical asphalt pumping and metering system and identifies the key components for flow control and metering. Using a variable speed asphalt pump is currently the most popular approach for asphalt flow control.

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This illustration shows the dynamic relationship between the aggregate belt scale, the controls, and the asphalt pumping and metering system. The automation reads the scale signal, adjusts it for the moisture entered by the operator (later to be driven off by the dryer), calculates the flow required for the asphalt based on the target asphalt content (typically coming from a recalled stored mix recipe), then adjusts the asphalt flow control device until the signal coming back from the meter is what the control system desires. Mix is made continuously in this


fashion, with the asphalt tracking, following, or proportioned to the belt scale signal.

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Types of Drum-Mixers …

New types of drums have been introduced since the 1980’s.

Environmental excellence is the goal.

All are called “drum-mixers”.

In the late 1980's plant manufacturers began experimenting with more environmentally sensitive designs for recycling, drying, and mixing. Each manufacturer's goal was to create a plant design with the cleanest possible gaseous emissions and the highest possible percentage of reclaimed pavement that could be used in the new hot-mix. Several new styles of hot-mix plants evolved from this design period. Today there are counter-flow drum-mixer facilities, unitized dryer/mixer facilities, and separate dryer-mixer facilities. To minimize any confusion, and to standardize specifications by regulatory agencies, all of these facilities are typically referred to under the umbrella of "drum-mix facilities" or "drum-mix plants”, even though the term was originally coined for the parallel-flow drum-mixers that started appearing in the field in the 1970’s.


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Parallel-Flow Drum-Mixer

Parallel-flow drum-mixers, where the burners are on the “feed end” of the drum, were the first drum-mixer designs and are still popular today.

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Parallel-Flow Drum-Mixer

January 2009

The term “parallel-flow” refers to the direction of the gas relative to the aggregate flow. In a “parallel-flow” drum-mixer, the aggregate and process gases are flowing in the same direction, hence the term parallel-flow. This difference between counter-flow and parallel-flow drying will be discussed more in Module 5. Parallel-flow drying was very effective for both the heating of the virgin aggregate and of the RAP (Reclaimed Asphalt Pavement). The gaseous emissions of these facilities, however, have proven to be less than desirable over time. The new liquid asphalt cement and the RAP is introduced in the hot process gas stream, and they can fume and cause visible hydrocarbon vapor emissions in the exhaust stack under the right field conditions.


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Counter-Flow Drum-Mixer

This is a picture of a “counter-flow drum-mixer”.

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Counter-flow drum-mixers have the burner up inside the dryer, and the material dries and heats as it passes through the dryer toward the flame combustion area. The new liquid asphalt cement is introduced behind the flame and the outside process gas stream. This keeps any hydrocarbon fume or aerosol from the process gas stream and the stack emissions are much more environmentally friendly under all drying conditions. The cold feed blending and asphalt proportioning equipment remains unchanged. The only difference with this type of drum-mixer is the drying technology. In fact, many parallel-flow drum plants have changed their dryers to this technology without changing the controls, aggregate feeding, or asphalt proportioning equipment.


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This cutaway view of a counter-flow drum-mixer clearly shows the combustion area and the mixing area. The drying and heating area is to the left of the flame. The aggregate enters on the left, is dried and heated, then passes below the flame to the mixing area. It is discharged just above the first tire you see in the portable drum’s flame.

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Unitized dryer/mixer plants (also referred to by their trade name of “Double Barrel” plants) are also very popular. These plants are a marriage of a continuous pugmill and an aggregate dryer.

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In this style of counter-flow drum-mixer the aggregate is heated and dried in the drying portion, then falls out into a mixing area. There, new liquid asphalt cement, RAP, and other mix ingredients are introduced outside of the hot gas stream. The paddles and tips welded to the outside of the drum mix the materials together and move them up the mixing area to the discharge in the center of the unit.


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Unitized Dryer- Mixer“Double Barrel” Plant

This photograph shows the inner workings of this style plant. The openings where the aggregate “falls out” into the mixing area are clearly seen. The sweep paddles move the aggregate into the area with the mixer tips. Here RAP and baghouse fines and other additives are introduced. Further “uphill” or toward the right in this photograph the liquid asphalt is injected, with the rest of the mixing tips blending the liquid asphalt and aggregate materials together for a homogenous mix. The right angle paddle flights at the right discharge the finished mix from the mixing chamber into the slat conveyor.

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Unitized Dryer-Mixer“Double Barrel” Plant

This close-up photograph shows the working portion of this style of plant. You can see the insulated covering over the mixing area. This part does not turn. The drum turns inside this cover, and the tips and paddles attached to the outer surface of the drum act as a pugmill for mixing. Notice how the mix exits to the middle of the mixer. You can see the slat conveyor for the storage silo positioned at the end of the insulated mixing area to the left.


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Other Types of Drum-Mixers

Counter-Flow Dryers with Separate Mixers

New or converted plants.

These are referred to as “drum-mixers” too.

Mix is made continuously, with gradation atcold feed and asphalt pumped and meteredcontinuously.

Other manufacturers offer “drum-mixer” or continuous flow plants with separate dryers and mixers. These are also popular as retrofit conversions of batch style facilities. All are referred to in the industry as “drum-mixer” plants. Any time mix is made continuously, with gradation controlled at the cold feed bins, and asphalt pumped and metered continuously to the aggregate flow, the term “drum-mixer” is used to characterize this type of facility.

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Separate Dryer / Mixer Plant

This photograph shows a new counter-flow dryer with a separate mixing drum. Aggregate passes from the drying drum to the mixing drum where asphalt cement and other mix ingredients (such as lime and fibers) are added outside the gas stream. RAP on this plant is being mixed with the virgin aggregate as it enters the mixing drum. A fan (upper left) pulls both the steam from the RAP and fumes from the mixing drum and introduces them back into the aggregate dryer for mixing with the combustion gases and evacuation to the baghouse. This helps regulate visible emissions from the plant. Proponents of this equipment approach favor the ease of adding extra ingredients in the mixing drum, the advantage of independently controlling the mixing and coating in the mixing drum separate from drying of the aggregate, and the advantage of being able to make “cold mix” more safely since there is no direct flame in the mixing drum.


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Batch tower replaced with mixing drum to create RAP capable “drum-mixer” style plant

This producer removed the screens, hot bins, weigh hopper and pugmill from his batch tower and installed a mixing drum on the tower structure, changing his batch plant into a “drum-mixer.” RAP is heated in the combustion area of the dryer for high RAP percentage capability. Belt feeders on the cold feed bins control the gradation. A drum-mix style asphalt pumping and metering unit was installed. This conversion allowed the producer the benefits of a drum-mixer, including higher RAP percentages, with less expense than purchasing an entirely new drum-mix facility. As previously mentioned, batch plant producers can also remove the screen decks in the tower and turn the batch tower into one or two large hot storage bins, controlling the gradation at the cold feed drum-mix style, and weighing up the pre-blended aggregate in one “hot bin pull.” Do not be surprised if you encounter these types of plant conversions in the field.

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Batch or Drum-Mixer?

Which is Best?

It is often debated in the field whether a batch plant or a drum-mix plant is the best choice of equipment for getting the job done. As is expected, it is not unusual to find opinions held with conviction on both sides of the argument.


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Choice based on business factors.Both make equal quality mix.

Batch Drum

Ultimately the choice between a batch plant and a drum-mixer plant depends on business factors such as purchasing costs, operating costs, and the flexibility required for local market conditions. Both type plants can produce equal quality mixes.

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What ARE the Business Factors?:– Drums are less expensive to buy– Drums are less expensive to maintain– Drums run high percentages of RAP

(Reclaimed Asphalt Pavement)– Drums are more cost effective

Drum-mixer plants are typically less expensive to buy and maintain, and offer higher RAP percentage capabilities. This leads a new drum-mixer plant to be more cost-effective when evaluating the total costs of the two different type plant facilities. This is why most new plants you see are drum-mix plants.

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What ARE the Business Factors?:– Batch provides more flexibility– Fitting drums with silos helps flexibility– Small runs with drums, however, are

difficult– This helped create market for “combo

plants”

Batch type plants offer more flexibility, however. It is very difficult to produce small quantities or a short “run” of mix with a drum-mix plant. Also, changing back and forth between different mixes in “short runs” is very difficult. When making periodic changes between long production runs this is not an issue - making several different mixes and storing them in different silos is very practical with a drum-mixer. If the drum-mixer does not have multiple storage silos, however, and the market requires frequent mix changes, batch plants have the edge.


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In Summary...A “Batch” Plant Produces Mix a “Batch at a Time”...

– The final gradation blend is established from the hot bins using the aggregate “weigh hopper” to weigh up each quantity of the individual hot bin materials

– The asphalt content is established in the asphalt “weigh bucket” by “weighing up” the right quantity for each batch

– Mix is made on a batch type basis

In summary, a batch plant produces mix a “batch at a time.” The final gradation blend is established from the hot bins using the aggregate “weigh hopper” to weigh up each quantity of the individual hot bin materials. The asphalt content is established using the asphalt “weigh bucket” to weigh the right quantity of asphalt for each batch. Mix is made on a batch type basis, a batch at a time.

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In Summary...A “Drum-Mix” Plant Produces Mix “Continuously”...

– The final gradation blend is established at the cold feed bins by regulating the flow from the individual cold feed bins - each containing a specific material from the quarry

– The asphalt content is established with an asphalt pumping and metering unit that changes the flow of the asphalt to match the aggregate flow

– Mix is made continuously in a non-stop fashion

In summary, a drum-mix plant produces mix continuously using a dryer and mixer to dry and heat the aggregates and mix them with the proper AC content in non-stop fashion. The final gradation blend is established at the cold feed bins by regulating the flow from each of the individual bins to meet the composite aggregate gradation of the Job Mix Formula (JMF). The asphalt content is established with a continuous pumping and metering unit that changes the flow of the asphalt to match the aggregate flow. Mix is made continuously in a non-stop fashion.


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QUESTIONS ?

Any questions?

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