Block 19 Compacitons Background 13

Construction Compaction 1

Compaction of HMA Mixes

Background

Senior/GraduateHMA Course

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What you will learn

• Objective of Compaction• Engineering properties related to compaction• Factors affecting compaction

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Safety

• Watch your back!• Keep visual contact with roller operators!• Keep an eye on traffic! • Wear your safety vest!

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Typical Compaction Process

Paving Machine Vibratory Roller Pneumatic Roller

Steel wheeledRoller

TemperatureZones

300° - 240°F 240° - 190°F 170 - 150°F

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Definitions

• Density • Compaction • Pass • Coverage • Time available for compaction (TAC)

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Pass & Coverage DefinedW

idth

of m

at Pass1

Pass 2Make up Pass 5

Pass 3

Pass 4

This is a 5 pass pattern that provides 2 complete coverages.

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Time Available for Compaction

(TAC)

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Importance of Compaction

• Improve Mechanical Stability• Improve Resistance to Permanent

Deformation • Reduce Moisture Penetration• Improve Fatigue Resistance • Reduce Low-Temperature Cracking Potential

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Factors Affecting Compaction

• Properties of the Materials• Environmental Variables • Laydown Site Conditions

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Properties of the Materials

• Aggregate • Asphalt Binder • Mix Properties

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Compact effort increases due to

• Crushed content• Nominal maximum size• Hardness of the aggregate • Angularity of aggregate• Less natural sand• High dust content mixes• Note: Typically, HMA mixes will be harder to

compact.

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Hot Mix Compaction

• Asphalt binder holds particles together– Provides lubrication at high temperatures– Provides cohesion at in-service

temperatures• Limits air and water intrusion into HMA mat

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Environmental Variables

• Layer thickness • Air and base temperature • HMA mix laydown temperature • Wind velocity • Solar flux

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Tim

e f

or

Mat

to

Co

ol t

o 1

75

F,

Min

ute

s

HMA Mat Thickness, inches

0 1 2 3 4 5 6 7 8

50

40

30

20

10

0

Mix

B

as

e °F 3

00,

5

0 300

,

32

300

,

14 250

, 3

22

50,

1

4

250

,

50

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LOWHIGH

LOWHIGH

THINTHICK

Base Temperature

Mix Temperature

Major Factors Affecting Rolling Time

Mat Thickness

allows LESS time

allows MORE time

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Compaction Temperature Range

185 °F(85 °C)

300 °F(150 °C)

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Laydown Site Conditions

• Lift thickness versus aggregate size• Lift thickness uniformity• Base Conditions

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Questions?