Transcript Hot Mix Asphalt Production and Placement

Texas A&M University, CVEN 342
Friday, May 20, 2016
Hot Mix Asphalt
Production and Placement
Gary L. Fitts, P.E.
Sr. Field Engineer
Asphalt Institute
ASPHALT INSTITUTE
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
• 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
ASPHALT INSTITUTE
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•
•
•
•
•
Aggregate and asphalt storage
Aggregate drying
Dust collection, air pollution control
Aggregate and asphalt proportioning
Mixing
Mixture discharge/storage
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Topics
ASPHALT INSTITUTE
• 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
ASPHALT INSTITUTE
• 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
ASPHALT INSTITUTE
Dust
Collector
Cold Feed Bins
Dryer
Cold
Elevator
Batch
Tower
Asphalt
Hot Elevator
Asphalt Trucking, Inc
Storage Silo
(optional)
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Drum Plants
ASPHALT INSTITUTE
• 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
ASPHALT INSTITUTE
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ASPHALT INSTITUTE
Drum Plant-Knippa, Texas
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Drum Plant Configurations
ASPHALT INSTITUTE
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•
•
•
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
ASPHALT INSTITUTE
• 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
ASPHALT INSTITUTE
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
ASPHALT INSTITUTE
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
ASPHALT INSTITUTE
Paving Machine Components
• Tractor unit
• Screed
• Electronic grade controls
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HMA Delivery
ASPHALT INSTITUTE
• 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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Tractor and Screed Units
MS-22, Fig. 5.11 (Courtesy of Blaw-Knox)
Forces Acting on Screed
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Automatic Screed Controls
ASPHALT INSTITUTE
• 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
ASPHALT INSTITUTE
• 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
ASPHALT INSTITUTE
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ASPHALT INSTITUTE
Compaction
The process of compressing a
material into a smaller volume
while maintaining the same mass.
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Compaction
ASPHALT INSTITUTE
• 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
ASPHALT INSTITUTE
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Mixture properties
Base/subgrade support (confinement)
Ambient conditions
Lift thickness
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ASPHALT INSTITUTE
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
Percent Passing
ASPHALT INSTITUTE
100
max density line
restricted
zone
nom max
max size
control point size
0
.075 .3
2.36
12.5
19.0
Sieve Size, mm, raised to 0.45 power
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Design Aggregate Structure
Percent Passing
ASPHALT INSTITUTE
100
Fine graded
nom max
max size
size
Coarse graded
0
.075 .3
2.36
12.5
19.0
Sieve Size, mm, raised to 0.45 power
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Time avail. for Compaction, min
Mix Temp. = 275F
90F 60F 30F
30
30
20
20
~ 6 min
10
10
0
0
1
2
3
Compacted Thickness, in
4
Rolling Phases
ASPHALT INSTITUTE
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•
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Breakdown
Intermediate
Finish
Different equipment and different
techniques for each phase
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Compaction Equipment
ASPHALT INSTITUTE
• 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
ASPHALT INSTITUTE
• 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
ASPHALT INSTITUTE
Time between blows, t
Amplitude
Frequency = 1/t
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Roller Eccentrics
ASPHALT INSTITUTE
Low amplitude
High amplitude
Example from Dynapac CC 501
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Amplitude vs. Frequency
ASPHALT INSTITUTE
• 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
ASPHALT INSTITUTE
• 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
ASPHALT INSTITUTE
Impacts/ft = 1/Impact Spacing
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Impacts per Foot of Travel
Vibratory Rollers
ASPHALT INSTITUTE
VPM
2 mph
2.5 mph
3 mph
3.5 mph
4 mph
2000
11.4
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2500
14.2
11.4
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3000
17.0
13.6
11.4
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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
ASPHALT INSTITUTE
• 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
ASPHALT INSTITUTE
• 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
ASPHALT INSTITUTE
• 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
ASPHALT INSTITUTE
• 2-4 passes, tandem vibratory roller
–
–
–
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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
ASPHALT INSTITUTE
• 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
ASPHALT INSTITUTE
• 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
ASPHALT INSTITUTE
• 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
ASPHALT INSTITUTE
• 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
ASPHALT INSTITUTE
• 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 largescale projects
S-T
Warranty
L-T
Warranty
-
+
Project selection is crucial
--
++
Bonding a concern, full
control must be given to
contractor
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Thanks for your attention!