Hot Mix Asphalt Production and Placement

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Hot Mix AsphaltProduction and Placement
Texas AM University, CVEN 342 Friday, August 21,
2020

• Gary L. Fitts, P.E.
• Sr. Field Engineer
• Asphalt Institute

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ASPHALT INSTITUTE

• International association of petroleum asphalt
  producers, manufacturers, and affiliated
  businesses, established in 1919
• Promotes the use, benefits and quality
  performance of petroleum asphalt through
  engineering, research and educational activities.
• HQ office-Lexington, KY

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References

• Asphalt Institute
• www.asphaltinstitute.org
• 859-288-4960
• Texas Asphalt Pavement Association
• www.txhotmix.org
• 512-312-2099
• National Asphalt Pavement Association
• www.hotmix.org
• 888-468-6499
• Asphalt Pavement Alliance
• www.AsphaltAlliance.com

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HMA Plant Functions

• Aggregate and asphalt storage
• Aggregate drying
• Dust collection, air pollution control
• Aggregate and asphalt proportioning
• Mixing
• Mixture discharge/storage

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Topics

• Basic information on
• HMA Production
• HMA Placement
• HMA Compaction
• Considerations for developing HMA specifications

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Batch Plant
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Batch Plants-Features

• Aggregates dried, separated by size
• Aggregates recombined by weight in weigh hopper
• Aggregates introduced into pugmill, briefly mixed
• Asphalt introduced by weight, mixed with
  aggregates
• Completed HMA discharged or stored

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Batch Plant Layout
Dust Collector
Cold Feed Bins
Batch Tower
Dryer
Asphalt
Cold Elevator
Hot Elevator
Storage Silo (optional)
Asphalt Trucking, Inc
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Drum Plants

• Aggregates are dried, mixed with asphalt in a
  continuous operation
• Quality control entirely dependent on
• stockpile management
• plant calibration
• Mixture must be stored in surge bin or silo

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Drum Plant Layout
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Multipav, S.A., El Salvador
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Drum Plant-Knippa, Texas
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Drum Plant Configurations

• Parallel flow
• Counter flow
• Coater (continuous dryer/pugmill)
• Drum in a drum
• double
• triple

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Weigh Bridge
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HMA Production

• Objective is to produce a mixture meeting the
  specified design requirements
• Volumetric
• Mechanical
• Samples are normally obtained from trucks at the
  plant, taken to the laboratory, and tested to
  confirm these qualities

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Surface Preparation

• Prime Coats
• Applied to unbound surface
• Moderate, uniform application (0.15-0.35 gal/sy)
  of low viscosity liquid asphalt
• Example materials
• MC-30
• MC-70
• AEP

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Surface Preparation

• Tack Coats
• Applied to bound surface
• Light, uniform application of liquid asphalt
  (0.03-0.05 gal/sy residual)
• Example materials
• Asphalt emulsions
• SS-1h, CSS-1h, MS-2
• Special Tack Emulsions
• Paving grade asphalt
• PG 64-22

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Tack Coat Application
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Paving Equipment

• Paving Machine Components
• Tractor unit
• Screed
• Electronic grade controls

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HMA Delivery

• Paver pulls up to meet the truck
• DONT BUMP THE PAVER!
• Break the load before opening tailgate
• Charge the hopper before its empty

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Tractor and Screed Units
MS-22, Fig. 5.11 (Courtesy of Blaw-Knox)
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Forces Acting on Screed
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Automatic Screed Controls

• Electronic adjustment to screed height using
  sensing and reference system
• Sensor detects elevation changes, adjusts height
  of tow point
• Slope (transverse) controls

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Paving Operations

• Maintain uniform resistance to face of screed!
• Keep uniform head of material at the face of the
  screed
• Operate paver within a narrow range of forward
  speed
• Coordinate mixture delivery, paver speed and
  compaction operations

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Uniform Head of Material
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Compaction

• The process of compressing a material into a
  smaller volume while maintaining the same mass.

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Compaction

• Essential to good performance!
• Need to compact to desirable air voids level
• Fine graded mixtures 4-8
• Coarse or gap-graded mixtures 3-6
• Compaction can only achieved if
• Mixture is confined
• Mixture is hot (workable)

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Factors Affecting Compaction

• Mixture properties
• Base/subgrade support (confinement)
• Ambient conditions
• Lift thickness

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Asphalt Binder Properties

• Binder grade
• increase high temperature grade ? stiffer binder
• neat or modified?
• PG grades with 92C or more temp. difference are
  usually polymer modified
• Temperature
• must complete compaction while mix temp exceeds
• 85C (185F) for neat binders
• 115C (240F) for modified binders

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Compaction-Lift/Layer Thickness

• Coarse-graded mixtures, mixtures using modified
  asphalts
• Minimum 4X nominal maximum size
• ½ in NMS minimum 2 lift thickness
• Fine-graded mixtures
• Minimum 3X nominal maximum size
• Thicker lifts also conserve heat, providing more
  time to complete compaction

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Aggregate Gradation
100
max density line
restricted zone
max size
nom max size
Percent Passing
control point
0
.075 .3 2.36
12.5 19.0
Sieve Size, mm, raised to 0.45 power
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Design Aggregate Structure
100
max size
nom max size
Percent Passing
0
.075 .3 2.36
12.5 19.0
Sieve Size, mm, raised to 0.45 power
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Mix Temp. 275F
90F
30F
60F
Time avail. for Compaction, min
6 min
1
2
3
4
Compacted Thickness, in
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Rolling Phases

• Breakdown
• Intermediate
• Finish
• Different equipment and different techniques for
  each phase

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Compaction Equipment

• Screed unit
• weight of screed
• external force applied to screed
• vibratory unit
• 35 Hz (2100 VPM)
• tamper bar
• Rollers
• vibratory steel-wheeled
• pneumatic
• static steel-wheeled
• combination

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Vibratory Rollers

• Commonly used for initial (breakdown) rolling
• 8-18.5 tons, 57-84 in wide (heavy rollers)
• 50-200 lbs/linear inch (PLI)
• Frequency 2700-4200 impacts/min.
• Amplitude 0.016-0.032 in.
• For thin overlays ( 2 in.) use low amplitude or
  static mode
• Operate to attain at least 10 impacts/ft
• 2-4 mph

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Amplitude Frequency
Time between blows, t
Frequency 1/t
Amplitude
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Roller Eccentrics
Low amplitude
High amplitude
Example from Dynapac CC 501
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Amplitude vs. Frequency

• High amplitude generates most force
• At same frequency, high amplitude does more work
• Is it logical to use high amplitude with high
  frequency?

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Vibratory Frequency

• Frequency is drum impacts per minute
• Working speed must match frequency
• Best results when impact spacing is 10-14 per
  foot

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Frequency
Impacts/ft 1/Impact Spacing
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Impacts per Foot of TravelVibratory Rollers
VPM 2 mph 2.5 mph 3 mph 3.5 mph 4 mph
2000 11.4 --- --- --- ---
2500 14.2 11.4 --- --- ---
3000 17.0 13.6 11.4 --- ---
3500 19.9 15.9 13.3 11.4 10.0
4000 22.7 18.2 15.2 13.0 11.4
Reed Tachometer-used to check frequency of
vibratory rollers
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Static Steel-Wheeled Rollers

• 10-14 ton rollers normally used for HMA
  compaction
• Commonly use vibratory rollers operated in static
  mode
• Lighter rollers used for finish rolling
• Drums must be smooth and clean
• For initial compaction, drive wheel must face
  paver

AI MS-22, Figure 6.05
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Static Steel-Wheeled Rollers
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Pneumatic Tire Manipulation

• Overlap manipulates mat under and between tire
• Tight finish resists moisture penetration
• Manipulation increased by lowering tire pressure
• Static force increased by high tire pressure

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Rolling Pattern

• Speed lap pattern for each roller
• No. of passes for each roller
• Min. temperature by which each roller must
  complete pattern
• IMPORTANT
• Paver speed must not exceed that of the
  compaction operation!!!

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Typical Pattern, 2 inch Overlay

• 2-4 passes, tandem vibratory roller
• Mix temperature above 250F
• Lowest amplitude setting, highest frequency
• Vibration on one or both drums
• Travel speed to obtain 10 impacts/ft (3 mph)
• 2-4 passes, pneumatic roller
• Mix temperature 180-230F
• Roller speed 3 mph
• 2-4 passes, static rolling
• Mix temperature 140-180F
• As necessary to remove blemishes

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HMA Specifications

• General types of project specifications
• Method
• Statistically-based acceptance
• QC/QA
• End result
• Warranty
• Short-term (workmanship related)
• Long-term (design-build)
• No one type is appropriate for all conditions

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Method Specifications

• Contractor is required to follow specific
  instructions during all phases of construction
• Owner provides mixture design
• Mixture is accepted on the basis of owners (or
  contractors) quality monitoring tests
• Delivery, placement, and compaction accepted
  based on inspection by owner/owners agent
• Examples of method spec
• TxDOT, Item 340 (all Standards Specs through
  1982)
• 2004 Standard Specifications to include
  method-type Item 340

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Statistically-Based QC/QA

• Requires a minimum established level of
  contractor quality monitoring tests
• Contractor usually furnishes the mixture design
• Defines lots and sublots for production and
  placement, requires stratified random sampling of
  materials
• Acceptance/payment performed on a lot-to-lot
  basis
• Usually includes penalty/bonus provisions on key
  materials qualities
• Many agencies applying percent-within-limits
  (PWL) criteria, whereby upper and/or lower limits
  of key criterion are defined
• Most commonly applied specification type for
  highways and airports
• 1993 TxDOT Standard Specifications, all 2004 HMA
  items (except 340)
• Most common PWL specification FAA P-401

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Short-Term Warranty

• Acceptance based on observed performance under
  limited term
• Performance defined through measurement of
• Roughness/ride quality
• Cracking (non-wheelpath)
• Rutting
• Surface friction
• Contractor controls materials selection, mixture
  design, and all construction requirements
• Term may vary from 1 to 7 years, becoming more
  popular for use on highway projects
• Indiana, Wisconsin, Michigan, other states

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Long-Term Warranties

• Contractor is required to deliver a pavement with
  performance characteristics at defined levels at
  the end of an extended period (15-30 years)
• Contractor responsible for pavement structural
  design and construction
• Most often used outside USA (design/build/operate/
  concessions projects in Central and South
  America, Europe)
• Best US example, NM 44 project

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Comparison of Different Types of Specifications
Relative Risk Relative Risk
Type Owner Cntrctr Comment
Method - Requires diligent inspection
QC/QA Best applicable to large-scale projects
S-T Warranty - Project selection is crucial
L-T Warranty -- Bonding a concern, full control must be given to contractor
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Thanks for your attention!