Segregation in Asphalt PavingIdentification, causes & prevention

Pennsylvania Asphalt Pavement Association Conference

January 30, 2014

Sam GregoryMunicipal Transportation SpecialistState and Local Road Consultant

What Is Segregation?

Webster defines Segregation as: “to separate from the main mass and collect together in a new body.”

What Is The Effect Of Segregation On Pavement?

• Premature Distress– Raveling– Frost Damage– Potholes

Why Do Premature Failures Occur Due To Segregation?

•Weaker Aggregate Structure and Gradation

•Weaker Mix

•Higher Voids

Sources of Segregation• Stockpiling• Cold Feed Bins• Hot Bins• Truck Loading

Plant Responsibility

• Truck Hauling• Truck Unloading• Paver – Hopper, Slat

Conveyers, Hopper gates, Auger, Screed

• Paving Operation Continuity (Balance)

Field Responsibility

Stockpile Segregation – how does it happen?

• Large Stockpiles

• Single Aggregate Blends

Stockpile Segregation – how is it prevented?

• Well-trained Operators

Stockpile Segregation – how is it prevented?

• Different-sized material

• Separate piles

Stockpile Segregation – how is it prevented?

• Build Horizontal Layers

• Build Sloped Layers

• Bridging

Cold Feed Bin Segregation – how does it happen?

• Reconfigure Opening – uniform feeding

Cold Feed Bin Segregation – how is it prevented?

• Range of Materials

• Size of Bin

• Shape of Bin

• No. 1 Bin

Hot Bin Segregation – how does it happen?

• Install Baffle – dust slides to center

Hot Bin Segregation – how is it prevented?

• Dribbling the material

• Single drop

Truck Loading Segregation – how does it happen?

• No dribbling, no topping off• Multiple Drops

Truck Loading Segregation – how is it prevented?

Truck Hauling Segregation – how does it happen?

• Temperature variations causing:– Crusting – Lumps in mix

due to:– Long hauls– Delays – No tarps– No Insulation

Temperature Segregation in

truck!

Temperature Variation• 1% increase in air voids

over 7% results in a minimum 10% reduction in pavement life

Raveling and Moisture Damage

Fatigue Cracking

Aggregate Segregation

Truck Hauling Segregation – how is it prevented?

• Proper planning

• Proper tarp

• Proper insulation

• Proper Planning /Scheduling– Proper Coordination with Plant– Proper Number of Trucks– Coordination with Paver Speed– Avoid Bunching of Trucks

at Paver or Clean-out Area– Use Material Transfer

Vehicle

Truck Hauling Segregation – how is it prevented?

“Strive for Continuous Paving”

How Does It Happen?In Unloading the Truck•Dribbling the material•Poor Bed Condition•No Truck Release Agent

• Discharge in Mass• Flood Hopper• Truck Release Agent• Modify Dump Enclosure

Truck Unloading Segregation – how is it prevented?

Baffles

Paver Segregation – how does it happen?

• Hopper Wings

• Slat Conveyer

• Hopper Gates

• Auger

• Screed

Proportional Feed Control

Hopper Gates

Slat Conveyors

Hopper Wings

Augers

In the operation of the Auger

Feed Sensor

Gear Box

Material to the midpoint (or just covering) the Shaft

Install Kickback Paddles

Paver Segregation – how is it prevented?

Paver Segregation – how is it prevented?

In the Hopper

Fillet Corners

Caution in Cycling Hopper Wings

Cycle Wings onto full slat conveyors

Paver Segregation – how is it prevented?

In the Slat Conveyer

Never Run the Hopper empty

Preventing Segregation at Slats

• Truck dumper must communicate with paver operator

• MTV operator is responsible to not let hopper insert run “dry”

• Smooth and effective truck exchange when working without an MTV

In setting the Hopper Gates

Gates too HIGH - augers overloaded

Gates too LOW - insufficient material supply

Correct adjustment - uniform material volume/flow

Paver Segregation – how is it prevented?

In the operation of the Screed

Hydraulic Extension

Manual Extension Hydraulic Auger & Tunnel Extension

Auger & Tunnel Extensions missing

Paver Segregation – how is it prevented?

In the Operation of the Paver

• Proper Starting & Stopping

• Continual & Constant Paver Speed

Paver Segregation – how is it prevented?

Goal: Non-Stop Paving

Industry’s Answer

• Material Elevator

• Mobile Conveyors

• Material Transfer Vehicle

Segregation can still occur!

Problem:

Dropping Materials

Solution:

Limit drop

Keep hopper insert full

How to Prevent Segregation

Key Points in Prevention of Segregation

1. Proper Stockpiling2. Proper Plant Operation3. Proper Truck Loading4. Proper Truck Unloading5. Continuity of Paving Operation

Is this Pavement Segregated?

Does Every One Agree?

Segregation can be difficult to identifyAvailable tools to assist in this determination

If Pattern Segregation Is Suspected

• Notify Department and contractor

• Contractor continues work at his own risk

• The Department will initiate an investigation

• Surface texture depth (PTM 751) can be used to determine if segregation is present

PennDOT Specs Section 409.3(h)3.a

P.T.M. 751

P.T.M. 751 PROCEDURES• Perform the test at 3

locations in the suspected segregated area

• Calculate the average radius of these 3 locations

• Obtain the texture depth from Table 1 on page 7 of PTM 751

P.T.M. 751 PROCEDURES• Perform the test at 3

locations in a non- segregated area

• Calculate the average radius of these 3 locations

• Obtain the texture depth from Table 1 on page 7 of PTM 751

P.T.M. 751 PROCEDURES

• If average texture depth between areas exceeds 0.024 inches, suspected area is considered:

“Unacceptable Pattern Segregation”

If Unacceptable Pattern Segregation Is Evident

• [PennDOT Specs Section 409.3(h)3.b]:• Stop Paving• Department will evaluate segregation to

determine corrective work– DO NOT resume paving until Department reviews

corrective actions & authorizes paving to continue

• Test section then placed, <200 tons• Resume normal paving after successful test

section

If Unacceptable Pattern Segregation Is Evident

• Evaluating Segregation [PennDOT Specs Section 409.3(h)3.c]:– Take 6” Cores– Remove & Replace Segregated Areas if

sum of any 2 sieves vary 20% or more from the JMF or core Density is less then 90%

– Remove full lane width of segregated area plus 5 feet minimum beyond each end

SegregationTrouble Shooting

• Begin/end of load segregation

• Material segregated in truck

• Running conveyor deck dry.

• Cycling hopper wings too soon .

SegregationSource & cause

Begin/end of load segregation

• Center line segregation (single)

SegregationTrouble Shooting

• Missing or damaged baffle plates

• Overloaded material feed system

• Worn reversing augers

• Low auger RPM

• Warped or improperly set screed

SegregationSource & cause

Center line segregation (single)

• Stripe in line with outer auger bearing support

SegregationTrouble Shooting

• Excessively slow auger RPM

SegregationSource & cause

Stripe in line with outer auger bearing support

• Stripe in line with conveyors

SegregationTrouble Shooting

• Overloaded feeder system

• Low auger RPM

• Maintain proper head of material

SegregationSource & cause

Stripe in line with conveyors

• Stripe at outboard edge of mat

SegregationTrouble Shooting

• Insufficient material

• Not enough auger extensions

• Cold material dragging at extension

SegregationSource & cause

Stripe at outboard edge of mat

• What is Wrong?

SegregationTrouble Shooting

• Nothing!

• This is one beautiful mat.

SegregationSource & cause

Questions?

Did I go too fast?