Asphalt Plant Level II January of Module Aggregate … Plant 2/03 - Module 3 - Agg...Module 3 –...

Asphalt Plant Level II – January 2009 of 41 Module 3 – Aggregate Storage and Feeding Systems

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January 2009 3 - 1

Module 3

Aggregate Storage and Feeding Systems

ConstructionTrainingQualification Program

Asphalt Plant

Level 2

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January 2009 3 - 2

Module 3What you will learn….

• Aggregate Stockpiling Alternatives• Managing Stockpiles for Quality• Managing Feeders for Quality & Accuracy• Blending of Aggregates with Feed Bins• Calibration of Belt Feeders• Unique Aspects of Storing and Feeding RAP• Lime and Mineral Fiber

In this module we will cover aggregate stockpiling alternatives and aggregate feed systems for all asphalt plants. This will include operational concepts for cold feed bins and belt feeders, how to best manage the quality and accuracy, and how they are used on both batch and drum plants to control gradation in the mix. The unique storage and handling problems associated with RAP will also be discussed.

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The information presented in this section is found …

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

– Module 3, pages 3-2 to 3-37– Module 10, pages 10-5 to 10-16

“Hot Mix Paving Handbook 2000”– Section 6, pages 50-61

The information presented in this section can be found in Module 3 (pages 3-2 to 3-37) and Module 10 (pages 10-5 to 10-16) in “Hot Mix Asphalt Production Facilities,” FHWA’s NHI course number 131044; and in Section 6 (pages 50-61) in the “Hot Mix Paving Handbook 2000,” published by the Corps of Engineers.


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Aggregate Quality is Determined at the Quarry

Aggregate quality and gradation is assured at the quarry….cannot be fixed at the asphalt

plant.

It is important to note that regardless of whether the plant is a drum plant or a batch plant, ultimately both the quality and gradation of the aggregate is controlled at the quarry and not the asphalt plant. Plants cannot fix problems associated with aggregate quality (hardness, soundness, deleterious material, etc.). Also, note that while the final composite gradation is controlled by the asphalt plant, gradations of the individual materials cannot be changed at the asphalt plant. The individual stone gradations are controlled at the quarry. The asphalt plant only controls blending of these materials. Even with a batch plant, that has sizing screens in the tower, the screens are only being used to re-separate the materials for re-blending. They cannot re-grade the individual materials.

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Aggregate Quality is Determined at the Quarry

Hot Mix Producer must make sure they have quality aggregates….certified delivery tickets.

(Department requirement)

The producer is responsible for the quality of the aggregates he receives to produce hot mix. He is required to keep certified shipping documents for all aggregate loads.


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Requirement Aggregate Stockpiles

330-5.1

Each aggregate in an individual stockpile

The department requires that each different type aggregate is stored in an individual stockpile (330-5.1).

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330-5.1

Separate aggregates by space or bulkhead and prevent intermingling

Each material must be separated either by sufficient space or a bulkhead to prevent intermingling of the materials (330-5.1). The picture on the left shows material separated adequately by space. The picture on the right shows materials separated adequately by bulkhead.

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January 2009


330-5.1Identify each material including RAP

– sign individual stockpile or map or chart in plant office

3 - 8

Each individual material must be clearly identified. The typical method is to sign the individual stockpile. As an alternative, a map or clearly identified chart can be used in the control house or plant office (330-5.1).


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RequirementAggregate Stockpiles

330-5.2

Form and maintain stockpiles to minimize segregation

Department requirements insist that aggregate stockpiles are formed and maintained to minimize segregation. Management procedures to prevent this from happening are covered later when different stockpiling techniques are discussed (330-5.2).

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Stockpiling Alternatives

• Horizontal Stockpiling– Transport Trucks– Cranes from Barge

• Radial Stacking• Bunker Systems • Bulk Material Bins

Bunker Storage System

Bulk Storage Silos

The producer has several different aggregate stockpiling alternatives at his disposal. These include: •Horizontal stockpiling •Radial stacking •Bunker storage systems – not used in Florida, nor discussed in this course. •Bulk aggregate bins – not used in Florida, nor discussed in this course.

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Horizontal Stockpiling

Aggregates, to be horizontally stockpiled can be delivered by:

• Transport truck and dumped in yard• Barge and unloaded with crane

Horizontal stockpiling is the most popular. Most aggregates are delivered by truck to hot mix facilities and stockpiled horizontally with a loader. Material delivered by barge and crane can be stockpiled horizontally.


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Horizontal Stockpiling

Aggregates delivered by truck to yard is most common stockpiling system and will be

discussed here

Other approaches reviewed at the end of module

Horizontal stockpiling, with material delivered into the plant yard by truck, is by far the most common aggregate stockpiling approach. We will review best management practices of that technique here.

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HORIZONTAL STOCKPILES

This illustration shows material that has been delivered by truck and properly stockpiled at the plant facility. Notice the adequate separation of material by space. Notice how the materials have been deposited by transport truck in individual piles. Notice also how the loader has moved these individual piles to the larger pile by placing the materials in the larger pile in horizontal lifts without driving on the material or segregating the material.

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Best Management PracticesHorizontal Stockpiling withEnd-Dump Trucks/Trailers

• Stockpile materials in horizontal lifts• Do not drive on aggregates• “Lift and place” with front-end loader• Improper or Faulty Handing can cause:

– Segregation– Contamination– Change in Moisture Content

Proper stockpiling management techniques include stockpiling the material in horizontal lifts, by lifting and placing the materials with a front-end loader. This is done to minimize segregation caused by rolling material up the face with the bucket. Aggregates should also not be driven on when stockpiling. This eliminates the possibility of contamination from clay or dirt that may be on the loader’s wheels, and also eliminates any possibility of material breakdown and changing


the size of the materials in the stockpile. Improper or Faulty Handing can cause: Segregation, Contamination, Change in Moisture Content.

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Best Management PracticesHorizontal Stockpiling withEnd-Dump Trucks/Trailers

• Do not push aggregates up/off ramps

• Keep materials properly separated

• Place materials on solid, draining surface

Materials should not be pushed up/off a ramp. Driving on the material creates a condition where materials can be contaminated and the gradation changed. Pushing can cause materials to segregate as they are pushed over the end of the stockpile. Aggregates should be properly separated to avoid contamination of one material to another. If aggregates are placed on a solid, draining surface, they can be protected from sub-grade contamination on removal. The added bonus is that drier materials help maximize plant production rates.

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Stockpiling w/Radial Stackers

• Allows larger stockpiles in smaller footprint• Reduces stockpiling costs• Prone to segregation when dropping from

height

Radial stacking is another technique for stockpiling material. Radial stackers allow a producer to place a large amount of material in a small space, and reduce production costs because the costs associated with stockpiling are eliminated. Because material is being dropped from an elevated conveyor, concerns about the possibility of segregation exist.


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Radial Stacker Stockpiling

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This illustration shows a radial stacking system installed at a quarry, where material is taken directly from aggregate stockpiles and fed into the hot mix plant. Notice the dividing walls between the aggregate stockpiles to adequately separate the different materials.

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Best Management PracticesStockpiling w/Radial Stackers

• Raise stacking conveyors as material is placed (reduces chance of segregation)

• Do not index over while stockpiling (index to new location, build new lift...reduces segregation)

• Re-blend ends with loader on removal

In order to reduce the possibility of segregation, there are recognized best management techniques for this type of system. Building a stockpile by indexing the conveyors to location, then raising the conveyor as the stockpile is built greatly helps reduce segregation. When indexing the conveyor to a new location, the proper technique is to move the conveyor, lower the conveyor, start building the new pile with as low of a drop as possible, then raise the conveyor as the stockpile is built. Removing material from the end of the stockpile, and mixing the ends with the loader as they are removed, helps re-blend the material prior to use.


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Cold Feed Bins

• Most common form of feeding into plants• Typically charged with rubber-tired loader• One bin for each material • Used on both batch and drum plants

Cold feed bins charged with a rubber-tired front-end loader are still the most common feeding system found on asphalt plants in the field. One bin is used for each material. They are found on both drum-mix and batch plants.

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RequirementCold Feed

320-2.4 and 330-5.3 and 330-5.4.1

Provide a separate feed bin/hopper for each material

Specifications require that a separate feed bin/hopper be installed for each material (320-2.5 and 330-5.3 and 330-5.4.1).

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RequirementCold Feed

320-2.4

Must be able to feed the material uniformly to the dryer

By requirement, the equipment must be capable of feeding material uniformly to the dryer (320-2.4).


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RequirementCold Feed Bins

330-5.4.1• No spillage or leakage between bins• Capacity to ensure uniform flow• Bins must have feeder with uniform speed to

ensure proper proportions at all times• If necessary, equip bins with vibrators to

ensure uniform flow

Additional requirements include that bins must be secure enough that no spillage or material can leak between bins, that the capacity of the individual bins are sufficient to provide adequate un-interrupted flow to the plant, and that bins are equipped with some type of feeder with uniform speed (330-5.4.1).

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RequirementAggregate Blending

330-5.3• Blending must occur only from cold feed

hoppers by means of securely positioned calibrated gates or other approved devices

Department specifications (330-5.3) require that materials are blended only from the cold feed hoppers or bins by means of a “securely positioned” calibrated gate or “other approved devices.” Other devices include variable speed motors on belt feeders. The key directive is that the feed opening or flow is adjustable and securable; meaning that once it is calibrated and set it cannot vary or adjust on its own.

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RequirementCold Feed Bin Gates

330-5.4.2• Must be adjustable in vertical

direction• Gates must be able to be

held securely• Measuring device required to

measure the vertical opening

The gates of the individual feed bins must be adjustable in a vertical direction, graduated and marked so that the opening size can be clearly read and recorded for calibration, and securable so that once set it can’t change (330-5.4.2).


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This photograph shows a cold feed system with belt feeders being used on a drum-mix plant.

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Best Management PracticesCold Feed Bins

• Avoid contamination caused by removing material from grade

Best Management Practices for feeding cold feed bins start with operating the loader in such a fashion as to minimize the possibility of contaminating aggregates with material from the grade or stockpile floor.

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Proper removal technique…..remove material from justabove grade (avoids contamination)….....rotate up andthrough material (reduces possibility of segregation)

This illustration shows proper removal techniques. The loader should remove materials slightly above grade, then rotate up and through the aggregate. This technique offers two benefits. It eliminates the possibility of material from the subgrade being picked up, and it reduces the possibility of segregation associated with “pushing” the material up the face to fill the bucket, which can cause larger stones to run outside the bucket and down the face of the stockpile. Material should be removed by inserting the bucket, rotating up to


fill the bucket, and “lifting” aggregates from the stockpile.

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• Advisable to have dividers between cold feed bins (avoids co-mingling materials)

Although not mandatory, it is advisable to have dividers between the cold feed bins. This reduces the possibility of materials flowing from one bin to the other and contaminating the other aggregates.

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This illustration shows a proper set of bin wall dividers. Notice there are no bin extensions on the back sides of the cold feed bins. This reduces the possibility of the loader operator overfilling the bins and materials overflowing from one bin to the other.


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• Advisable to have dividers between cold feed bins (avoids co-mingling materials)

• Do not overfill bins with bin wall dividers (avoids co-mingling materials)

If cold feed bins are equipped with bin wall dividers, they should never be overfilled.

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Material bins should not beheaped abovedivider walls

Heaping bins with material defeats the purpose of the bin wall dividers and creates the very condition the bin wall dividers were designed to eliminate….material flowing from one cold bin to the other.

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Belt Feeders Vary Flow in Two Ways(Belt Speed and Gate Opening)

Variable speed drive is used to increase and decrease flow, and the gate establishes maximum and minimum flow.

Belt feeders vary flow in two ways. A strike-off gate that is adjustable vertically can cause more or less material to flow from the bin as the gate setting changes. Variable speed belts drive motors can also be fitted to the belts. Most drum plants have both adjusting mechanisms; that is they have adjustable gates and variable speed drive motors. Batch plants are typically equipped with fixed speed motors and adjustable gates only. The flow from the cold bin on a batch plant is not as critical as the flow of the cold bin on a drum plant. With a drum


plant, gradation is being controlled at the feeder. With a batch plant, varying the cold feed flow is useful only in balancing cold feed to hot bin pulls. Gradation is controlled with the hot bin pulls, not at the cold feed. Because variable speed drive motors are becoming so common, however, many batch plant operators have fitted their cold bins with these devices. It makes adjusting cold flow much easier for the operator.

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Using Belt Feeders onBatch Plants

• Traditionally varied gate openings only to match hot bin requirements

• Chart variable flow from each bin to match hot bin requirements

• Calibration procedures and examples for setting cold feed bins by varying gate openings in Batch Plant Appendix

• Most batch plants now varying belt speed like drum-mixers (simpler, easier)

The output from each feeder on a batch plant can be controlled by simply changing the gate opening on the bin. This is the way belt feeders were used on batch plants. Gate openings were set to match the hot bin requirements, based on the mix formula. Calibration procedures and example problems for this approach are included in the Batch Plant Appendix, for those interested. Most batch plants now vary the belt speeds like a drum-mixer plants. Variable speed drives have gotten very inexpensive over the last few years. This makes it more convenient to adjust flow during production, which is often a requirement when making different types of mixes during a day. Batch plants that have removed their screens, and are running “screenless”, use the variable speed drives to adjust the feeder blend just like a drum plant.


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Using Belt Feeders onDrum Plants

• Varying belt speed in addition to gate opening is typical of drum-mixer plant

• Gates only used to establish maximum and minimum flow

• Calibration procedures and examples for setting drum-mixer cold feed belt speeds will be completed later in this module

With a drum-mixer, the cold feed bin flow is typically adjusted by varying the belt speed. The gate setting is used to ensure the feeder motor is not operating too slow or too fast, and is operating in a range where it is designed to function best.

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“Bridging” = Major Concernwith Cold Feed Bins

• Concern is partial flow in feeders…..”bridging” is typical culprit

• Equipment designs and features reduce chance of bridging

One of the biggest concerns at the cold feed is partial flow from the feeder. A phenomenon called “bridging” is the typical culprit. Equipment designs have evolved to help reduce the possibility of this condition from developing in the feeder.

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“Bridging” is caused by the cohesive nature of certain aggregates. They can adhere to one another in the bin. When bridging develops, material directly above the feeder is typically removed but the material further up in the bin is sticking together and material will not continue to feed out from the bin.


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Vibrators and Air Cannons Help “Break the Bridge”

Vibrators or air cannons can be used to dislodge the material in the bin and “break the bridge.” Fine, wet aggregates, such as sand, are more prone to bridging than coarse aggregates. RAP is also very prone to bridging. Operators frequently keep vibrators constantly operating on hard to feed materials, and use air cannons to dislodge a particularly difficult bridge.

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TrapezoidalOpening Shape

Steep Side Walls

Closely Spaced Idlers

Long Feeder Opening

Other feeder design elements help reduce bridging. These include longer feeder openings, closely spaced idlers, trapezoidally shaped openings, and steep side walls, in addition to the vibrators and air cannons. While these are not mandatory, these good design elements help promote the even flow of material from the cold feed bin. Newer feeders typically have all these design features built in to them. Notice the steep side walls, the long feeder opening onto the belt, the trapezoidal shape, the closely spaced idlers, and again, the vibrator and air cannon.


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January 2009 3 - 39Feeding Effect of Trapezoidal Opening

This illustration demonstrates the advantage of a trapezoidal opening on a cold feed bin. As material moves from the bin the wider opening helps reduce resistance, which promotes a more consistent material flow from the bin. Most new bins have this design feature.

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“No flow paddles” detect lack of flow and warn operator Paddles are frequently installed in the openings of cold feed bins to warn the operator of developing bridges. These paddles swing down when material stops flowing from the bin and are usually wired to an alarm or light in the control house.

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Cold Feed Proportioning“No-flow” warning

“No-flow” paddles warn of lack of feed.

Positioned in opening of feeder.

Lights or interlocks to plant shutdown in control room for operator (not a department spec).

One of the chief concerns in cold feed flow in a drum-mix plant is “no-flow” or “loss-of-flow” at the individual feeders. Even if flow is interrupted for a short period of time, gradation of the mix is affected. Most plants are equipped with no-flow alarms for the operator. They operate from a paddle or limit switch positioned in the opening of the feeder. If the material from the cold feed bin stops, the paddle drops or limit switch is “made” and an alarm light warns the operator of the interrupted flow. While department specifications


do not require “no-flow” devices on drum plants, most plants are equipped with these devices to provide the operator with the information he needs to make quality mix. The department position on no-flow indicators is that mix gradation is being controlled by end-result based on the testing outlined in Module 10. The decision to use no-flow indicators for gradation accuracy is left to the discretion of the contractor, but it is highly recommended that this feature is not disabled.

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No-Flow Paddle

This picture shows a no-flow paddle positioned in the opening of the feeder. The gate is shown in the background.

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No-Flow Alarm

This photograph shows the no-flow alarm light on a drum-mix cold feed control panel. The plant can be programmed to either alarm the operator or shut down the operation if the problem persists. Again, this is not a department requirement, but a best management practice. Material gradation is the responsibility of the contractor, and tested in the laboratory to verify proper gradation blend. This equipment just protects the contractor from gradation errors caused by lack of flow.


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Cold Feed Calibration & Blending

If there is a mix gradation problem, consider checking the belt feeders

and feed bin operation first …

The following slides detail standard procedures for calibrating cold feed bins and performing cold feed blends. Generally, if there is a mix gradation problem (other than dust) check the cold feed bins and belt feeders first to make sure they are calibrated and are operating correctly.

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Batch Plants(Traditionally vary gate opening to

match production requirement)

(See Batch Plant Appendixfor an example)

In a batch plant, the traditional approach to varying the cold feed flow was to vary the gate opening to match production. This traditional approach will not be covered here because most batch plants now use variable speed belts to control output like a drum-mixer plant. Examples of this traditional approach are covered in the Batch Plant Appendix, for those interested.

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Drum-Mix Plants(Vary each feeder belt speed to match production requirement)

(Information repeated in Module 7, Drum Plants)

The following slides outline the procedures for calibrating cold feed bins and performing cold feed blends in a drum-mix plant, based on varying the speed of the belts. This is the same approach typically used in modern batch plants.


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Cold Feed Proportioning

Composite gradation is controlled at cold feed by proportioning material from individual bins, varying the speed of the individual belts to control the blend AND the production rate.

Gradation and quality of the individual materials is controlled at the quarry, not at the cold feed bins. The cold feed controls the blend.

With a drum plant, the composite gradation of the mix is controlled at the cold feed by proportioning the material from the individual cold feed bins. It is important to note that the gradation and quality of the individual materials is controlled at the quarry, not at the hot-mix plant. This is even more true with a drum plant than a batch plant. With a drum plant there is no opportunity for re-sizing or re-screening the cold feed materials what-so-ever. The only thing a drum plant can do is blend the individual materials together. The gradation of the individual aggregates must be correct coming from the aggregate source.

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Cold Feed Proportioning

Proportion material flow from each bin:• Material output is charted against belt

speed in calibration charts• Adjustable manual gates help control

minimum and maximum flow• Different charts are created for different

gate settings (min and max flow)• Different materials will produce different

charts

Proportioning flow from each feeder on a drum plant is controlled two ways, by using a variable speed belt on the belt feeder, and adjusting the manual gate of the feeder opening to affect the minimum and maximum flow possible from the feeder.


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Belt Feeders Vary Flow in Two Ways(Belt Speed and Gate Opening)

Variable speed drive is used to increase and decrease flow, and gate establishes maximum and minimum flow.

This illustration shows the two elements used in varying individual feed bin output on a drum plant clearly; the variable speed drive motor and the manual strike-off gate.

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Variable speed motor, manual gate, and no-flow paddle This bin opening photograph clearly shows the variable speed drive motor, the adjustable manual gate, and the no-flow paddle at the feeder opening.

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“Safe” “Linear” Zone

Feed bin = 2Material = 3/4” Agg. Code #

Notice how on this calibration chart the feeder speed on the x axis is plotted against the feeder output in tons per hour (TPH) on the y axis. Lines 2, 3, and 4 are for gate position 2, 3, and 4 on the feeder. These could be 2”, 3”, or 4”. Florida requirements insist that gates are clearly graduated, and it is better to mark the gates in inches to be consistent with Florida guidelines. It is not advisable to run a feeder below 10% or above 90% of speed. Notice how the output of the feeder is non-linear below 10% and above


90% speed. Due to the nature of electro-mechanical devices, it is hard for any motor-controller package to be stable or perform consistently in these ranges. This is why manual strike-off gates are useful. In the example shown in this illustration, if one desired 30 tph from this feeder, then it would be advisable to change the manual gate setting to “2”, and run the feeder at approximately 20% speed.

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Best Management PracticesCold Feed Proportioning

It is best to run each feeder above 10% and below 90% speed to ensure that the feeder is operating in a stable range, and output is more predictable.

It is best to run each feeder above 10% and below 90% speed to ensure that the feeder is operating in a stable range, and output is more predictable. This point cannot be overemphasized.

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BMP - Class DiscussionCold Feed Calibration

Feeder calibration charts should be created with dry material weight figures not wet. WHY?

(Because the same amount of dry material flows from a cold feed bin whether it has 5% moisture or 7%! Therefore, charts should always be recorded using dry weights.)

Feeders should always be charted or calibrated showing dry output on the graphs. Likewise, computerized plants should be calibrated using dry weights. Most computerized plants do this automatically, by simply asking you what the moisture of the material is when you calibrate. This is because the same amount of dry material will flow from a bin that has slightly different amounts of moisture. For example, the same amount of dry material will flow from a 1/2” stone bin whether there is 5% surface moisture on this stone or 7%.


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This slide shows where moisture is indicated during the calibration process. The computer will use this value while reading the belt scale to determine the dry weight during the calibration process. Different brands of computer automation do this differently, but the theory is the same. If you are making manual calibration charts or graphs, you need to make sure that you make this adjustment manually. Charts should be constructed in dry weights.

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Drum-Mix Setup Cold Feeds for

ProductionStep 1 - Calibrate Belt ScalesStep 2 - Calibrate FeedersStep 3 - Establish the Belt Speeds

When setting up the cold feeds on a drum-mixer for production of mix, first the belt scale has to be calibrated, then the belt scale is used to calibrate the feeders. Using the belt scale greatly simplifies this process. One only needs to set the feeder speed, then read the tph on the belt scale. Once the feeders are calibrated, the proper belt speeds are established for the mix formula and production rate. This either has to be done manually, or computer controls on the plant will do it for you. Now you are ready to produce mix.

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Step 1 - Calibrating Belt Scales

(Covered in Module 7)

A detailed explanation of how belt scale calibration is accomplished is covered in Module 7. At this point in the program and text, we will assume this has already been accomplished.


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Step 2 - Belt Feeder Calibration

• After belt scale has been calibrated ...• Develop a chart of aggregate rate (tons/hr)

vs. belt speed for each cold feed bin

(Most plant computers do this for you)(Manual blending process shown below)

Once the belt scale is calibrated, calibration charts for the different feeders at different speeds and gate openings have to be completed. Most plant computers do this for you through their cold feed calibration screens. In fact, often with these type of plants, the calibration charts are then printed out for you for backup manual operation. With a drum plant with manual cold feed control, this must be done manually. A manual process is shown below.

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Feed bin = 2Material = 3/4” Agg. Code #

This chart shows feeder output for one feeder, with a given material, at different gate openings. To make manual calibration charts, this process must be repeated for each bin and each material for that bin, at every contemplated gate opening. The process takes a considerable amount of time, and most plants now have computerized cold feed calibration, and the operator simply follows the prompts. If one charts the feeders based on actual belt speeds measured with a tail pulley tachometer, then a linear chart is typically achieved in the 10-90% range, as shown here and covered previously in the module. Remember to produce these charts in dry weights.


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This photograph shows a computerized feeder calibration screen. You can see how much easier it would be over manually calculating each bin. Some brands of automation will print out manual charts for you after completing the calibration process, so you can see the speed to output relationships the computer is using. Remember to enter the aggregate moisture as is seen in the upper left hand corner so the calibration of the feeders is accomplished in dry weights.

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0

20

40

60

80

100

120

10% 30% 50% 70% 90%

Aver

age

Del

iver

y -T

ons

per H

our

Percent of Belt Speed

CALIBRATION GRAPH FOR COLD FEED FOR DRUM PLANT

BIN # 1 BIN # 2 BIN # 3 BIN # 4

Let’s go back to our manual process. This graph shows four different cold feed bins, each at a given gate opening, all posted to one chart. This has been done here for simplicity in working an example for this course. Obviously, if we decided we needed to change any of the gate settings for any of the bins, we would have to re-post different lines for the bins changed. Notice the chart is graphed between 10% and 90% speed. If we use a tachometer to actually measure the belt speed, as most control systems now do, the relationship between the feeder belt speed and the output should be linear between these ranges. We will use this chart to determine the feeder belt speed settings we should use for each bin to meet our job mix target, but first let’s look at how we determined the individual cold feed bin percentages.


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Step 3 -Establishing Feeder Belt Speeds

(Establishing feeder speeds first starts with target material percentages from the Job Mix

Formula - established during mix design)

In order to establish the feeder speed settings, we first need to know the target aggregate percentages for each material. The job mix target blend (target aggregate percentages for each material) first had to be determined in the mix design process. Initially, the percentages of component materials from the mix design are used to set the target cold feed blend percentages. Using an example, we will show how to determine a composite blend of materials. When we get done with this, and the target bin percentages are set, we’ll continue looking at how we set the actual belt speeds for each feeder.

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Blending of Aggregates

• Combined gradation of component materials– Mix Design– Cold Feed Bins– Hot Bin Blends

• Batch Plant Only

Note that this process is often done as many as three times in the production process: •Once during the initial design phase, as shown here. •A second time if the initial mix production doesn’t hit the job mix formula target blend, and small changes need to be made to the cold feed bin percentages. •A third time if a batch plant is used, to determine the percentages required from the hot bins. (Remember, in a batch plant the gradations of the hot bins will be different than the gradations of the cold feed bins feeding the dryer. A new set of percentages to “pull” from the hot bins, then, have to be calculated. To see an example of how this is done, see the Appendix on “Calibration of Hot Bins” in Module 9.)


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Blending of Aggregates(Check of mix design blend)

Sieve Aggregate 1 Aggregate 2 Aggregate 3 Aggregate 4 Blend JMF

3/4 in 100 100 100 100 100

1/2 in 65 100 100 100 95

3/8 in 30 95 100 100 88

No. 4 5 33 100 100 66

No. 8 4 7 91 100 54

No. 16 4 4 67 93 41

No. 30 3 4 50 85 33

No. 50 3 4 34 57 23

No. 100 3 3 13 22 9

No. 200 2.0 2.0 6.0 1.2 4.0

15 30 41 14

Gradations were determined from stockpile samples for each component material. The proportions shown for each component are from the mix design.

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Blending of Aggregates(weighted average)

· P = Aa + Bb + Cc + ….– Where:

• P = % of material passing a given sieve for the blended aggregates A, B, C, …

• A, B, C, … = % material passing a given sieve for each aggregate A, B, C, …..

• a, b, c, …. = Proportions (decimal fractions) of aggregates A, B, C, … to be used in blend

This is the formula used for the procedure. Determining the blend is basically a “weighted average” process.

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Blending of AggregatesP = Aa + Bb + Cc + Dd

Sieve Blend JMF

a b c d

3/4 in

1/2 in

3/8 in

No. 4 A Aa B Bb C Cc D Dd P

No. 8

No. 16

No. 30

No. 50

No. 100

No. 200

Aggregate 1 Aggregate 2 Aggregate 3 Aggregate 4

·P = Aa + Bb + Cc + Dd ….

This slide shows the calculation graphically. The blend is the sum of the percentages from each sieve.


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Blending of AggregatesAggregate 1


15 30 41 14

3/4 in 100 15.0 100 100 100 100

1/2 in 65 9.8 100 100 100 95

3/8 in 30 4.5 95 100 100 88

No. 4 5 0.8 33 100 100 66

No. 8 4 0.6 7 91 100 54

No. 16 4 0.6 4 67 93 41

No. 30 3 0.5 4 50 85 33

No. 50 3 0.5 4 34 57 23

No. 100 3 0.5 3 13 22 9

No. 200 2.0 0.3 2.0 6.0 1.2 4.0

P = Bb + Cc + DdAa +

The calculations shown here are for proportioning aggregate 1.

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Blending of Aggregates2, 3, 4


15 30 41 14

3/4 in 100 15.0 100 30.0 100 41.0 100 14.0 100

1/2 in 65 9.8 100 30.0 100 41.0 100 14.0 95

3/8 in 30 4.5 95 28.5 100 41.0 100 14.0 88

No. 4 5 0.8 33 9.9 100 41.0 100 14.0 66

No. 8 4 0.6 7 2.1 91 37.3 100 14.0 54

No. 16 4 0.6 4 1.2 67 27.4 93 13.0 41

No. 30 3 0.5 4 1.2 50 20.5 85 11.9 33

No. 50 3 0.5 4 1.2 34 13.9 57 8.0 23

No. 100 3 0.5 3 0.9 13 5.3 22 3.1 9

No. 200 2.0 0.3 2.0 0.6 6.0 2.5 1.2 0.2 4.0

Bb + Cc + DdP = Aa +

The calculations are shown here for proportioning aggregate 2, 3, & 4.

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Blending of AggregatesThe Answer


15 30 41 14

3/4 in 100 15.0 100 30.0 100 41.0 100 14.0 100.0 100

1/2 in 65 9.8 100 30.0 100 41.0 100 14.0 94.8 95

3/8 in 30 4.5 95 28.0 100 41.0 100 14.0 88.0 88

No. 4 5 0.8 33 9.9 100 41.0 100 14.0 66.0 66

No. 8 4 0.6 7 2.1 91 37.3 100 14.0 54.0 54

No. 16 4 0.6 4 1.2 67 27.4 93 13.0 42.3 41

No. 30 3 0.5 4 1.2 50 20.5 85 11.9 34.1 33

No. 50 3 0.5 4 1.2 34 13.9 57 8.0 23.6 23

No. 100 3 0.5 3 0.9 13 5.3 22 3.1 9.8 9

No. 200 2.0 0.3 2.0 0.6 6.0 2.5 1.2 0.2 3.5 4.0

Aa + Bb + Cc + DdP =

The calculations are shown here for combining the proportions for aggregates 1, 2, 3, & 4 into a combined blend. You will note that the minus 200 material is low (3.5 % versus 4.0 %). Therefore, an adjustment in proportioning of the aggregates should be considered to increase the minus 200 material. Note: some aggregates tend to break down more in the drum and this should be considered in the mix design stage.


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Blending of AggregatesAdjusted Blend Proportions (a, b, c, d)

• Experience– Old guys

• Empirical Method– Shown in batch plants Appendix 3b

• Computer program– Used by most everybody

You can determine adjusted blend proportions of each component material (or bin) by: 1. Experience 2. Empirical procedures 3. Computer programs Today, most people use a computer program.

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Classroom Problem

So we will try an adjusted blend of the aggregate components. This time we will do it as a classroom problem.

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Blending of AggregatesClassroom Problem


3/4 in 100 100 100 100 100

1/2 in 65 100 100 100 95

3/8 in 30 95 100 100 88

No. 4 5 33 100 100 66

No. 8 4 7 91 100 54

No. 16 4 4 67 93 41

No. 30 3 4 50 85 33

No. 50 3 4 34 57 23

No. 100 3 3 13 22 9

No. 200 2.0 2.0 6.0 1.2 4.0

15 30 45 10

We will use “experience” to adjust the blend for aggregates 3 and 4 to 45 and 10 percent respectively to increase the amount passing the No. 200. Note: Even though the adjustment of blends gets closer to the JMF gradation, volumetric properties could be affected by a change in the blends. A full size version of this chart can be found at the end of this chapter.


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Blending of Aggregates- Classroom Problem –

The AnswerSieve Aggregate 1 Aggregate 2 Aggregate 3 Aggregate 4 Blend JMF

15 30 45 10

3/4 in 100 15 100 30 100 45 100 10 100 100

1/2 in 65 10 100 30 100 45 100 10 95 95

3/8 in 30 4.5 95 28.5 100 45.0 100 10 88.0 88

No. 4 5 0.8 33 9.9 100 45.0 100 10 65.7 66

No. 8 4 0.6 7 2.1 91 40.9 100 10 53.7 54

No. 16 4 0.6 4 1.2 67 30.1 93 9.3 41.2 41

No. 30 3 0.5 4 1.2 50 22.5 85 8.5 32.7 33

No. 50 3 0.5 4 1.2 34 15.3 57 5.7 22.7 23

No. 100 3 0.5 3 0.9 13 5.8 22 2.2 9.4 9

No. 200 2.0 0.30 2.0 0.60 6.0 2.70 1.2 0.12 3.7 4.0

You can see this blend brings us closer to our target JMF.

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• Desired plant production - 200 tph• Aggregate proportions

– Bin 1 15%– Bin 2 30%– Bin 3 45%– Bin 4 10%

• Asphalt content - 6 %

Step 3 -Establishing Feeder Belt Speeds

Once the bin percentages are established with the Job Mix Formula, together with accurate calibration charts and the known target production rate, one can determine the proper speed settings for each feed bin.

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Step 3 a = Correct aggregate percentages for total weight

• Bin # 1 - 15% x .94 = 14.1 % mix

• Bin # 2 - 30% x .94 = 28.2 % mix

• Bin # 3 - 45% x .94 = 42.3 % mix

• Bin # 4 - 10% x .94 = 9.4 % mix

Convert the aggregate bin blend percentages (they total 100%) to the percentages these bins represent of the total mix by taking out the asphalt content of the mix. A simple way to do this is to multiply by a factor of 1 minus the asphalt content expressed as a decimal. In this example this is 1-0.06 or 0.94.


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Step 3 b - Compute demand for each aggregate in tons/hr

• Bin # 1 - 14.1 % x 200 TPH = 28.2 TPH

• Bin # 2 - 28.2 % x 200 TPH = 56.4 TPH

• Bin # 3 - 42.3 % x 200 TPH = 84.6 TPH

• Bin # 4 - 9.4 % x 200 TPH = 18.8 TPH

Multiply these numbers by the target production rate to determine the dry material required per bin in tph. Remember, the feeders are graphed dry. Daily variations in surface moisture do not affect dry output from a feeder.

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0

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120

10% 30% 50% 70% 90%


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BIN # 1 BIN # 2 BIN # 3 BIN # 4

This graph shows our four different cold feed bins, each at a given gate opening, all posted to one chart, as previously explained. As a reminder, and to avoid confusion, remember this was done for simplicity for this course. In reality you will have four different charts, one for each bin. We will now use this chart to determine the feeder belt speed settings we should use for each bin to meet our job mix target.

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10% 30% 50% 70% 90%


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BIN # 1 BIN # 2 BIN # 3 BIN # 4

28 TPH

17 %

The 28.2 tph requirement from Bin 1 can be achieved at 17% speed.


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BIN # 1 BIN # 2 BIN # 3 BIN # 4

56 TPH

45 %


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10% 30% 50% 70% 90%


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BIN # 1 BIN # 2 BIN # 3 BIN # 4

85 TPH

74 %


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BIN # 1 BIN # 2 BIN # 3 BIN # 4

18 TPH

10 %

And, the 18.8 tph requirement from Bin 4 can be achieved at 10% speed. This is close to the belt speed where it might be advisable to close the gate down and recalibrate the feeder in order to run at a higher speed.


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Step 3 c - Pick Belt Speeds

• Bin # 1 - 28.2 TPH = 17 % speed

• Bin # 2 - 56.4 TPH = 45 % speed

• Bin # 3 - 84.6 TPH = 74 % speed

• Bin # 4 - 18.8 TPH = 10 % speed

Based on the calibration chart, then, the following belt speeds were selected. Produce mix and check the extracted gradations.

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Step 3 c - Pick Belt Speeds

Most control systems determine the feeder belt speeds for you!

They calculate the required speeds for the desired production rate and drive the feeder belt motors to these speeds.

We went through this process manually to show you what the control system does.

The example was a manual process, so you could see how the process works. Most of today’s plants are computerized, and the control system does these calculations for you. This is much easier.

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Practice Problem

To reinforce how the process of establishing belt speeds from

calibration charts works, there is a practice problem in Module 7a.

To reinforce how this process of establishing belt speeds from calibration charts works, there is a practice problem in Module 7a. You are encouraged to do this problem so that you know how this process works, both manually and “behind the scenes” with the automation.


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Cold Feed Control Panels

Many styles of feeder controls found in field.

All can perform satisfactory proportioning.

When it comes to actual feeder controls for drum-mixer plants many different styles are found in the field. They all can perform satisfactory blending of different cold feed materials.

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This panel shows a simple increase/decrease button for each feeder. A digital readout off the tachometer on the tail shaft shows actual feeder speed. The digital meters are calibrated to read 0-100%. They could just as easily be calibrated to read in linear speed of the belt (fpm), or revolutions per minute of the pulleys on the feeders (rpm).

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This panel has digital readouts in addition to manual speed controls. The three position switch on the bottom of the unit allows the operator to select the feeder in manual control, automatic control, or “off”. On this system, the “run” setting is automatic and the computer controls the feeders. If the operator wants to run the feeder in manual mode, he selects “start” and controls belt speed by punching in the set point on the digital switch (00.0 to 99.9 available). The feeder speed or rate is shown in RPM of the feeder for this brand of plant. Manual charts can be constructed, if


desired, in either 0-100% speed settings or 0-Maximum RPM settings. In this brand of plant the operator typically makes a note of his RPM settings while a mix is being run at a target production rate. A quick glance to the panel, then, reassures him that the gradation blend is accurate.

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This drum-mix plant control screen shows the stored mix formula entered by the operator. Each individual feeder ingredient is listed in percentage in the lower left portion of the screen. Notice that the mix formula automatically calculates the desired tph of each material. Notice also that the feeder on and off times are part of the mix formula. This minimizes waste for the contractor on start-up and shut-down.

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RAP Cold Feed Bins

Now lets take quick look at RAP cold feed bins. There are some design and operation differences due to the nature of the material.


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RAP Cold Feed Bins

Special Requirements and Designs:

• More horsepower (larger output required)• Special designs to promote flow of material• RAP more prone to bridge • “Lump breaker” or “scalping screen” often found

at discharge to remove oversize particles

RAP bins have special requirements over conventional cold feed bins. Because the percentage of RAP is sometimes high on a mix design, RAP bins often have to be larger and be equipped with more horsepower than conventional bins. RAP is more prone to bridging due to the large amount of fine particle sizes in the RAP and high moisture percentages often found in RAP.

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RAP Handling – Best Practice

• Don’t dump RAP from high in the air – can compact it.

• Don’t leave RAP in bin for extended periods –can stick.

• Check Moisture – RAP does not drain like sand.

Don’t dump RAP from high in the air – can compact it. Don’t leave RAP in bin for extended periods – can stick. Check Moisture – RAP does not drain like sand.

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320 HOT BITUMINOUS MIXTURES-COLD FEED.(REV 6-30-08) (FA 8-21-08) (1-09)

• 320-2.4 Cold Feed: Provide a separate cold bin for each component of the fine and coarse aggregates required by the design mix. Equip the cold bins with accurate mechanical means for feeding the aggregates uniformly into the dryer in the proportions required for the finished mix to maintain uniform production and temperature. When using RAP as a component material, use a grizzly or grid over the RAP cold bin, in-line roller crusher, screen, or other suitable means to prevent oversized RAP material from showing up in the completed recycled mixture. If oversized RAP material appears in the completed recycled mix, take the appropriate corrective action immediately. If the appropriate corrective actions are not immediately taken, stop plant operations.

The current specification addresses both virgin aggregates and RAP.


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Requirement RAP Cold Feed

320-2.4 Prevent Oversize particles by

• Grizzly/grid over bin• In-line roller crusher• Screen• Other suitable means

Taking this last RAP tendency into consideration, specification 320-2.4 is written to require that plants are fitted with a grizzly/grid, or in-line crusher, or scalping screen to ensure that no oversize RAP makes its way into the plant process. Discretion is left to the Engineer in case other suitable means to ensure a consistent RAP size are met.

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Requirement RAP Cold Feed

320-2.4

Immediate corrective actions required if oversize RAP particles appear in the final

mix. If appropriate corrective actions are not immediately taken, stop plant operations.

The specification requires immediate corrective action if oversize particles make it to the final product. This obviously implies the Engineer should inspect the plant mix and final product for oversize particles (320-2.4).

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Typical RAP Feed System on Drum-Mixer

This photo shows an example of a RAP bin in use today. Designs have evolved in order to produce a consistent even flow.


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In-line Roll/Roller Crusher or “Lump Breaker”

This illustration shows a typical in-line roller crusher or “lump breaker” fitted between the RAP bin and the conveyor taking the RAP to the plant.

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RAP Bin with Scalping Screen

This photo shows a scalping screen installed between the RAP bin and the plant.

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Hydrated Lime

Required for 337 Friction (FC-5) Course mixes using granite aggregates.

Introduced two different ways:• Wet on aggregate at cold feed• Dry in mixing process (batch or drum)

(Wet method discussed in this Module)

Plants need to be equipped to treat aggregate with hydrated lime for 337 Friction Course (FC-5) mixes using granite aggregates. Several techniques are allowed, and specifications divide these methods into “wet” methods and “dry” methods. The “wet” or “slurry” method is discussed in this module. The “dry” methods are discussed later in Modules 7 and 8.


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Hydrated Lime RequirementFor 337 (FC-5) mixes with Granite

337-9.2 All Plants• Separate feed system• Accurately proportioned• Coat the aggregate before the AC is injected• Must not be entrained in the air stream• Interlock the proportioning device• Control within ± 10 % accuracy• No-flow = plant shutdown

Specification 337-9.2 requires that plants are fitted with a Hydrated Lime Supply System when producing FC-5 containing granite. •Separate feed system •Accurately proportioned •Coat the aggregate before the AC is injected •Must not be entrained in the air stream •Interlock the proportioning device •Control within ± 10 % accuracy •No-flow = plant shutdown

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RequirementHydrated lime for 337 (FC-5) mixes

337-9.2.2 Method B - Wet

• Add based on the dry weight of the lime• Lime/water concentrations directed by the

engineer• Blend and maintain lime in suspension• Mix with the aggregate uniformly and

proportionately

The “slurry” or “wet” method, outlined in 337-9.2.2, involves treating wet aggregate with lime prior to the drying process. This can be accomplished in several ways. One method mixes hydrated lime with water, making sure the lime is kept in suspension, then meters this mixture on the aggregate prior to the drying operation. Lime must be controlled by dry weight to the aggregate feed. The lime must be adequately mixed with the aggregate. This method is not commonly used because it adds additional moisture to the aggregate cutting production rates substantially. Another method is designed to take advantage of the moisture in the aggregate and simply mix the hydrated lime with the aggregate prior to the drying process.


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RequirementMineral Filler

330-5.5

Feed mineral filler separate from the other aggregates

(mineral filler typically introduced dry,similar to returning baghouse finesand will be covered in Module 6)

Florida specs (330-5.5) also require that mineral filler, if called for in the mix formula, is introduced as a separate material. Most plants are equipped to meter this material in as a dry ingredient, and it is not introduced at the cold feed. Plant systems that are used to introduce mineral filler are virtually identical to systems used to re-introduce baghouse fines, and this equipment will be covered in Modules 6, 7, and 9.

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InspectionAggregate Storage - General

• Each material separated? (330-5.1)

• Each material identified? (330-5.1)

• Stockpile management to ensure minimal segregation? (330-5.2)

Personnel inspecting a plant facility and operation will want to include the following areas in their routine: Check to make sure each material is identified, stockpiled separately and properly separated by space or bulkhead or dividers (330-5.1). Verify stockpiling techniques are being employed that will minimize segregation and contamination (330-5.1 and 5.2). Refer to this course text for guidelines and best management practices.

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InspectionCold Feed Bins

• Bin for each individual material? (320-2.4 and 330-5.3 and 330-5.4.1)

• Feeders adequately control flow of material to dryer? (320-2.4)

• Secure, adjustable, marked gates? (330-5.3 and 330-5.4.2)

If cold feed bins are being used, verify there is an individual bin for each aggregate material in the job mix formula (320-2.4 and 330-5.3 and 330-5.4.1). Feeders should be adequately designed to provide consistent and reliable flow to the dryer (320-2.4). This can typically be checked by simply watching the feeder flow. Feeder gates must be secure, adjustable, and graduated with markings (330-5.3 and 330-5.4.2).


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InspectionRAP Feed Bins

• RAP systems with scalpers, breakers, or screens to regulate oversize material from mix? (320-2.4)

If RAP is being used in the mix formula, make sure the RAP system has a sufficient provision to ensure that oversize RAP particles are either broken up or rejected from the mix (331-2.5.1).

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InspectionHydrated Lime

• Lime addition system (337-9.2)

• Separate feed system• Accurately proportioned• Coat the aggregate before the AC is

injected• Must not be entrained in the air stream• Interlock the proportioning device• ± 10 % accuracy• No-flow = plant shutdown

If hydrated lime is being added on the wet side or aggregate side of the plant, verify all the provisions required for introducing lime are adequately covered. The provisions for all plants are listed below, and enumerated in 337-9.2. •Separate feed system •Accurately proportioned •Coat the aggregate before the AC is injected •Must not be entrained in the air stream •Interlock the proportioning device •± 10 % accuracy •No-flow = plant shutdown

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InspectionHydrated Lime

• Lime slurry or wet lime system being used ? (337-9.2.2)

• Adding based on the dry weight of the lime?• Lime/moisture concentrations adequate?• Mixing with the aggregate uniformly and

proportionately?

Specific additional requirements for wet lime systems include the following (337-9.2.2): •Adding based on the dry weight of the lime? •Lime/moisture concentrations adequate? •Mixing with the aggregate uniformly and proportionately?


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InspectionMineral Filler

• Mineral filler (if required) being introduced as separate material? (330-5.5)

And finally, if mineral filler is called for in the mix formula, you need to verify that the filler is being introduced as a separate material in the plant process somewhere (330-5.5). This is typically introduced dry in the mixing cycle, rather than on the cold feed end of the plant.

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Module 3What we covered….

• Aggregate Stockpiling Alternatives• Managing Stockpiles for Quality• Managing Feeders for Quality & Accuracy• Blending of Aggregates with Feed Bins• Calibration of Belt Feeders• Unique Aspects of Storing and Feeding RAP• Lime and Mineral Fiber

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QUESTIONS ?

Any questions?

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