hot mix asphalt production and placement
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Hot Mix AsphaltProduction and Placement
Hot Mix AsphaltProduction and Placement
Gary L. Fitts, P.E.Sr. Field EngineerAsphalt Institute
Texas A&M University, CVEN 342Wednesday, April 19, 2023
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 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
DryerDryer BatchBatchTowerTower
DustDustCollectorCollector
AsphaltAsphalt
Cold Feed BinsCold Feed Bins
Asphalt Trucking, Inc
ColdColdElevatorElevator Hot ElevatorHot Elevator
Storage SiloStorage Silo(optional)(optional)
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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 Configurations
• Parallel flow
• Counter flow
• “Coater” (continuous dryer/pugmill)
• Drum in a drum– double– triple
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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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Paving Equipment
Paving Machine Components
• Tractor unit
• Screed
• Electronic grade controls
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HMA Delivery
• Paver pulls up to meet the truck– DON’T BUMP THE PAVER!
• Break the load before opening tailgate
• Charge the hopper before it’s empty
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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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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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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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100
0
Sieve Size, mm, raised to 0.45 power
.075 .3 2.36 12.5 19.0
Pe
rce
nt
Pa
ss
ing
control pointcontrol point
restrictedrestricted zonezone
max density linemax density line
maxmaxsizesize
nomnommaxmaxsizesize
Aggregate Gradation
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100
0
Sieve Size, mm, raised to 0.45 power
.075 .3 2.36 12.5 19.0
Pe
rce
nt
Pa
ss
ing
maxmaxsizesize
nomnommaxmaxsizesize
Design Aggregate Structure
Fine graded
Coarse graded
30
20
10
0
1 2 3 4
Tim
e av
ail.
for
Com
pact
ion,
min
Compacted Thickness, in
30F60F90F
30
20
10
0
~ 6 min
Mix Temp. = 275F
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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
Amplitude
Time between blows, t
Frequency = 1/t
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Roller Eccentrics
Low amplitude
High amplitude
Example from Dynapac CC 501Example 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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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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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 owner’s (or
contractor’s) quality monitoring tests– Delivery, placement, and compaction accepted based
on inspection by owner/owner’s agent
• Examples of method spec:– TxDOT, Item 340 (all Standards Spec’s 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
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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