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Plants & Production
Asphalt Materials, Mix Designs, and Plants
Danny Gierhart, P.E.
Asphalt Institute Regional Engineer
Tuttle, Oklahoma
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Basic Functions of Plants
• Store component materials
• Combine aggregates in proper proportions
• Heat and dry aggregates
• Add and mix liquid binder in proper proportions
• Coat aggregates uniformly
• Store mixture and load trucks without segregation
Reproduce the mix design !
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Types of Facilities
• Batch
• Continuous (drum mix)
• Combination
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Drum Mix Plant - Typical Layout
Storage silos
RAP bin
Cold feed bins
Asphalt storage tanks Control room
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Stockpiling Aggregates
• Good stockpiling
procedures are crucial to
HMA production.
• If the material in the
stockpile is segregated or
contaminated, the HMA
will be also
• Coarse aggregate piles
segregate easier than fine
aggregate piles
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Stockpiling Aggregates
Good spacing between stockpiles
Use of bulkheads to maintain separation
Horizontal stockpiles with separate aggregate piles
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Stockpiling Aggregates
• Angularity of the aggregate is an essential component of good HMA.
• Each time the aggregate is moved and stockpiled, the aggregate degrades and the edges become more rounded
• Handling should be minimized as much as possible
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Stockpile Problems
Segregated stockpile
Contaminated stockpile
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Paved & Sloped Area
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Covered Stockpiles
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Horizontal Asphalt Tanks
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Vertical Asphalt Tanks
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Cold Feed Bins
Vertical Dividers
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Cold Feed Bins
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Bin Feed is determined by:
Gate Opening
Belt Speed
Cold Feed Bin Belt
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Collector Belt
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Feeder
Collector Belt
Belt Scale
• Collects material and transfers to dryer
Collector Belts/Conveyors
Cold Feed Bin
Plant
Computer
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Single Deck Scalping Screen
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Heat And Dry Aggregates
Aggregate Dryers
• Gas or liquid fueled burner
• Removes moisture from aggregate
• Surface and internal
• Heat aggregate to production temperature
• “Super-heat” aggregate for heat transfer to cooler RAP being blended
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Two Types of Dryers
Aggregate & Air
• Parallel-flow
– Same direction
• Counter-flow
– Opposite direction
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Parallel-Flow Drum-mixer Zones
Combustion Heating Drying
Air
Liquid Feed
Hot Mix Asphalt
Mixing
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Counter-Flow Drum-mixer Zones
Air
AC
Hot Mix Asphalt
Aggregate Combustion Heating Drying Mixing
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Effective Aggregate Veiling
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Emission Control Equipment
Clean air
Dust collected and returned to
plant process
Exhaust Dryer or Drum
Secondary Primary
Larger Fines
collected
Air
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Cutaway of Baghouse
Captures Dust
Returns to mix
Accurate metering
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Baghouse Filter Bags
Clean air Impulse
reverse air
Dirty air
Filtering mode
Dust falls from bag
Dust clings to bag
Bag cleaning mode
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Insufficient Drying
• Trapped moisture
• Poor asphalt - aggregate bond
• Tender mixes
• Rapid heat loss after mixing
• Asphalt content errors
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Insufficient Heating
• Poor mixing
– Rich and lean spot
– Uncoated Aggregate
– Poor compaction
• Insufficient heat transfer to RAP
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Stationary Silo Installations
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1 2 3
Proper loading of trucks helps reduce segregation
Truck Loading
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Plants and Production
• Summary
– Batch and drum plants (drum most common)
– Aggregate must be properly stockpiled to prevent segregation, contamination, high moisture content
– Aggregate and binder must be carefully proportioned before drying and mixing
– Sufficient mixing to heat, dry aggregate and obtain uniform coating of binder
– Final step in production is to temporarily store and then transport HMA to project site without segregation
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Questions?