Preventing Mix Segregation at the Paver
Transcript of Preventing Mix Segregation at the Paver
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Preventing Mix Segregation at the
Paver
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Outline
• Best practices for prevention of segregation
• Causes/ Remedies of Segregation at the Paver
• Thermal Segregation
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Prevention Begins at the Plant
Segregation can occur any time between production and compaction
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Truck Loading
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Truck Loading
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Truck Loading
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Truck Loading
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Truck Loading
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Trucking Transitions
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Truck-Paver Contact
If trucks hold their brakes, the paver will slow down and the
head of material will go up causing segregation
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Truck Exchanges
When the paver picks up the truck, raise the bed
so the material is in contact with the gate before
discharge
The goal is to move the material as a single mass
RP195
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Material Management
Key Points in Prevention of Segregation:
• Keep Material Contained
• Prevent Excessive Rolling of Materials
• Move Material in a Smooth Uninterrupted Manner
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Forces acting on the screed
Pulling Force
( Force P )
Head Of Material
( Force M )
Shear Force Between
The Material and the
Screed. ( Force F )
Reaction Of Material
Under The Screed
( Force R )
Weight Of The Screed
( Force W )
F R
M
P W
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Managing the Feed System
Components
•Conveyors
•Flow Gates
•Auger Height
•Auger Speed
•Feed Sensor Placement
Paving crews are tasked with syncing the delivery of material
to the screed and the travel speed of the paver
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Balancing Act
Material Level in Hopper
Conveyor Speed
Flow Gates Auger Position
Auger Speed
Feed Sensor Position
Paver Speed Screed Adjustment
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When should I empty the Hopper? When working with segregating materials, the hopper wings should only be
cycled with the hopper relatively full.
Set and keep the conveyor deck covered with a minimum of 6 to 10
inches of material. (152-254mm)
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Flow Gates
Flow gates are designed to Meter the amount of material that is
delivered to the Auger Chamber.
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Head of Material
The head of material is the amount of asphalt carried in front of the screed
Maintaining the correct head of material is CRITICAL for equilibrium/ smooth paving.
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Correct Head of Material
Head of material should consistently
cover ½ the auger height across the
screed box
Augers should be running 100 % of the time.
20-40 RPM
Augers that run at high speeds are constantly
cycyling on/off and creating segregation
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Correct Head of Material
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High Head of Material Large pile of material shown here allows larger aggregates to separate
Large amount of material pushes the screed upward. To compensate, screed depth
has to be lowered and the angle of attack is wrong
The segregated material and unbalanced screed forces will compromise mat quality
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Feed Sensor Placement
Feed sensors control the level of material at
the outboard ends of the augers
A misplaced feed sensor can cause irregular
head of material and ultimately segregation in
the mat.
The feed sensor should be mounted on the
end gate and facing perpendicular to the face
of the material as shown here
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Auger Height Auger height controls the head of material and is determined by mat thickness.
Add 2 inches to the loose thickness you are laying, or more for larger aggregates. (50mm)
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Augers Extension
Auger extensions should always be used when
wider paving is done.
Some material designs may require additional
auger extensions that would not be required
when working with traditional material designs.
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Causes, Effects, and Remedies for
Segregation
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The Usual Suspects
•Longitudinal Segregation Streaks
•End of Load Segregation
•Shadowing
•Erratic surface segregation
•Thermal Segregation
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Streaks Below Chain Case and/or
Auger Bearing Supports
CAUSED BY:
Overloaded or inconsistent head of
material allows larger aggregates to
separate & topple into “Dead Areas”
below chain case and outer auger
bearings
REMEDY
Adjusting flow gates, conveyor speed,
and auger speeds are all ways to
control the head of material.
Auger height and feed sensor position
should also be monitored
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Centerline Streak
CAUSES
-High auger speed is pushing larger
material to the outside, leaving a course
surface under the center of the screed
-As conveyors deliver material to the
augers, the large aggregates are being
thrown forward underneath the paver
REMEDY
Slow the auger speed. Check Auger
height.
Delta Plate is in place to prevent
aggregate from landing beneath the paver
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End of load Segregation
CAUSES
-Hopper wings cycled with low
material level
-Conveyor deck ran dry
-Truck dumped improperly
REMEDY
-If necessary, stop paver to wait on
truck before material level is too low
-Cycle hopper wings with at least
152-254mm material level
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Erratic Flaws and Thermal
Segregation CAUSES
-Sudden change in paver speed
-Thermally Segregated Material
-Low/High Auger height
REMEDY
-Keep Consistent Paving Speed
-Truck Scheduling
- Material Transfer Vehicle
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Issues Created by Thermal
Segregation
•Cold spots
•Increased voids
•Inability to achieve compaction
•Premature pavement failure
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Benefits of MTVs
In summation, a remixing material transfer vehicle can combat
the following enemies of smooth pavement surfaces:
End of Load Segregation
Paving stops due to truck delays (Surge Bin Capacity)
Thermal segregation due to long haul distances
Material segregation during truck delivery
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How mix looks with Shuttle Buggy mixing augers
St. Petersburg, Russia behind SB-2500 of VAD Company only 2 to 3°
Celsius temperature difference across the mat.
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Shuttle Buggy Cures End of Load Segregation
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No matter how simple the truck
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No matter how complex the truck
Insulated Truck owned by a large Dutch Contractor
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• Remove trucks from
paver
• Trucks are not bumping
paver causing it to
change speed and put
marks in mat
Non-Contact Paving
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New NDT Technologies • Few advances in last 10 years in measuring HMA quality
• Localized segregation of new overlays is a major concern
• Need for 100% Coverage devices
Infra-Red Ground Penetrating Radar
• Temperature Uniformity
During Placement
• Location and Duration of
Paver stops
• Directly related to
in-place density
after compaction
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Moba Pave IR Long Life Pavement
Proper Sub-grade
Substantial Structural life
Pavement Smoothness
No Material Segregation
No Temperature Segregation
Uniformity of Density
Speed of Construction
Minimizing Traffic Delays
Low Price
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• Real time infrared logging
• Temperature tied to location using GPS
coordinates
MOBA Pave-IR
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What Happened to the Margin?
Send in the Mill
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If we screw it up bad enough, these guys will
come fix it and we can do it a second time.
Remember it cost a whole lot more to do it
twice than to do it correctly the first time.
Not who you need on your Job
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Thank you