Asphalt Delivery Placement

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Screed UnitScreed Unit

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Operational Principles of the Operational Principles of the ScreedScreed

• Self-leveling Concepts

• Screed Response versus Distance

• Forces Acting on a Screed

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Self Leveling - Rubber Tired Paver

Rear Drive Tires Front Bogie Wheels

Line of Pull

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Self Leveling – Track Paver

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1 inch movement at Tow Point

1/8 inch movement at Leading Edge of Screed

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Detail of Screed PathDetail of Screed Path

Tow Point PathTow Point PathStep Amplitude = 100%Step Amplitude = 100%

L = Leveling Arm LengthL = Leveling Arm Length

Screed PathScreed PathDirection of PavingDirection of Paving

63%63% 87%87%

6L6L5L5L4L4L3L3L2L2L1L1L00

96%96% 98%98% 99%99%

L

Screed Responses Verses Distance

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Main Forces Acting on Screed Main Forces Acting on Screed

1. Towing Force

2. Force of Material in Front of Screed

3. Frictional Forces

4. Weight of Screed

5. Additional compactive force applied by the screed

The self-leveling action of the screed means it will respond to the various forces acting on it to find its

balance or equilibrium.

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Free Body Diagram of Forces on ScreedFree Body Diagram of Forces on Screed

Lowering the pivot point increases the angle of attack

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Tow Point

Screed DepthCrank

Pivot Point

F1

F2

F3

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Constant Head of Material Volume

Head of Material Volume Too High

Head of Material Volume Too Low

Correct Depth ofMat Maintained

Screed Rises Due toExcess Material ForcedUnder Nose of Screed

Screed Settles Due toInadequate Supporting

Material

How does the head of material affect HMA thickness?

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Correct Correct ElevationElevation

Auger Auger OverloadedOverloaded Auger Auger

UnderloadedUnderloaded

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Travel

Angle of Attack

Increase the angle of attack, increase the thickness.

Decrease the angle of attack, decrease the thickness.

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HOT STOP ConceptHOT STOP Concept

• If constant material flow cannot be maintained, before HMA mix drops in hopper below gates...

• “Hot Stop” the paver as quickly as possible• Wait for next truck• Pick up truck on run and resume speed as

quickly as possible.

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RidabilityRidability

• Noticeable pavement defects that destroy smoothness are caused by– Stop and Go Operation– Transverse Joint Construction

• To avoid these irregularities– Keep the paver moving– Pick up trucks on the run– Adjust paver speed to match the mix arrival rate– Keep a uniform head of material– Take time to make good transverse joints

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Screed Control SystemsScreed Control Systems

• Who’s Responsible for Changes • Sticking the Mat • Manual Controls • Automatic Controls

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Pavement ProfilePavement Profile

Direction of PavingDirection of Paving

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Detail of Screed PathDetail of Screed Path

Tow Point PathTow Point PathStep Amplitude = 100%Step Amplitude = 100%

L = Leveling Arm LengthL = Leveling Arm Length

Screed PathScreed PathDirection of PavingDirection of Paving

63%63% 87%87%

6L6L5L5L4L4L3L3L2L2L1L1L00

96%96% 98%98% 99%99%

L

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Screed Compaction Screed Compaction

• Screed With Tamping bar– Moved up and down 4mm at a rate of 1500

tamps per minute

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• Screed With Tamping bar– This system has large number of moving

parts, required regular maintenance and very heavy

– Slight Indentations in the HMA surface were produced by each stroke of the bar

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Screed CompactionScreed Compaction

• Vibrating Screed– Consists of a curved, trough-shaped plate

that has an electric vibrator (3600 vpm with a few thousands of an inch amplitude)

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Screed CompactionScreed Compaction

• Vibratory Screed with Strike-Off Plate

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Screed HeatersScreed Heaters

• Heating the screed plate to 150ºC• Ensures that the first material placed does

not stick to the screed plate and tear, giving mat a rough texture

Crown ControlCrown Control

• Assist in draining rainfall off the road surface

• adjust a turnbuckle device to flex the bottom of the screed

•

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Screed ExtensionsScreed Extensions

• Basic width of the paver : 2.4 m or 3.0m• Paving width can be extended

– Rigid Screed Extensions– Hydraulically-operated Screed Extensions

• Hydraulically extendible screed are more economical

• Surface texture is different and density is lower under the screed extension than the basic screed unit

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Types of Grade Reference Types of Grade Reference

Three Basic Types of Grade References• Erected Stringline • Mobile reference• Joint matching shoe

• Sonic sensor• Laser Technology

Establish a single grade or dual grade for both

Grade and slope control

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Grade sensorsGrade sensors

• Longitudinal Grade Sensor – Transverse cross slope Device

• Dual Grade Sensors

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Stringlines

Supports at every 25ft on tangents and 10ft on

horizontal curves

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Stringline in a Runaway Overlay Stringline in a Runaway Overlay

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Stringline Grade Reference Stringline Grade Reference

• Disadvantages– Difficult to maintain correct elevation– Need high tensile strength line– Time consuming (need survey crew)– Truck drivers, equipments and all personal must

stay away• Advantages

– Controls the actual grades– Essential for Airfield paving– Prevents “bird baths”

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Balancing ProductionBalancing Production

TruckingTrucking CompactionCompaction

PavingPavingHMA HMA FacilityFacility