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Materials Asphalt and Aggregate

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The 3 A’s of Hot Mix Asphalt

Asphalt,

Aggregates, and Air

HMA = Asphalt + Aggregates + Air

Asphalt

also known as Asphalt Cement or

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What is Asphalt?       

Black, glue-like, waterproof material A product of the petroleum refining operation Used extensively in the paving, roofing industries Is a thermoplastic material Must be in liquid form for roadway applications (heat, dilution in solvent, emulsification) Can be used in pavements effectively in the full range of climate and loading conditions Can be polymer modified to improve properties for better performance when necessary

Background 

Asphalt Soluble in petroleum products  Generally a byproduct of petroleum distillation process  Can be naturally occurring 



Tar Resistant to petroleum products  Generally byproduct of coke (from coal) production 

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Sources of Asphalt  Natural

asphalt deposits  Petroleum asphalts

Natural Asphalt Deposits 

Each lake asphalt source very consistent 

Used solubility test to determine source 



Insolubles differed substantially between sources

Demand for paved roads exceeded the supply of lake asphalts in late 1800’s 

Led to use of petroleum asphalts

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Asphalt Types  Petroleum

asphalt

 Produced

through the process of distillation of crude petroleum  Accomplished

by raising the temperature of the crude in stages  Different fractions separate at various temperatures  Lighter fractions - Simple distillation  Heavy distillates - Vacuum distillation, solvent extraction

Petroleum Asphalt

Asphalt is one of the products of the petroleum industry

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Refinery Operation LIGHT DISTILLATE

FIELD STORAGE

PUMPING STATION

MEDIUM DISTILLATE HEAVY DISTILLATE

TOWER DISTILLATION REFINERY

RESIDUUM

STORAGE GAS

TUBE HEATER

OR

CONDENSERS AND COOLERS

ASPHALT CEMENTS AIR BLOWN ASPHALT

PETROLEUM SAND AND WATER

PROCESS UNIT

AIR

FOR PROCESSING INTO EMULSIFIED AND CUTBACK ASPHALTS

STILL

Asphalt Classification 3

Ways to “Liquefy” Asphalt  Heat  Thin

w/ solvents (cutbacks)  Make a waterbased emulsion

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Asphalt Classifications Asphalt material must be liquid to be used in highway applications. 





Asphalt cements  Generally refinery produced material  Heated to liquify for spraying, mixing with aggr. Cutback Asphalts  Asphalt cements “cut” with petroleum solvents Emulsified Asphalts  Mixture of asphalt cement, water, and emulsifying agent

Cutback Asphalt Paving asphalt liquefied by blending with petroleum solvents  Resulting material can be sprayed/mixed at lower temperatures  Primary uses: 

penetrating prime coat  binders for storable cold mix asphalt  binders in some cold mix recycling processes 

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Cutback Asphalt Grades 

Rapid cure (RC) (Naphtha or Gasoline) High volatility of solvent  Tack coats, surface treatments 



Medium cure (MC) (Kerosene) Moderate volatility  Stockpile patching mix 



Slow cure (SC) (Low viscosity oil) Low volatility  Tack coats, prime coat, dust control 

Asphalt Emulsion Microscopic asphalt droplets suspended in water.  Mostly 1-5 m diameter  Emulsifiers or surfactants hold these droplets in suspension. 

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How is Emulsion Made? Introducing molten asphalt and treated water under pressure into a colloid mill produces an emulsion.  The colloid mill is a high-shear mixing device specially designed for this purpose. 

Emulsified Asphalts “Break” 

Broken = complete separation of phases Base Asphalt  Water 

Color change from brown to black  For tack – allow emulsion to break prior to placing HMA 

Emulsion Oil in Water Emulsion

Broken

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Classes Of Emulsified Asphalt: 1.

Anionic:

2.

Cationic:

3.

Nonionic:

Electro-negatively charged asphalt particles Electro-positively charged asphalt particles Neutral charge on asphalt particles

Types of Emulsified Asphalt 1. 2. 3. 4.

Rapid setting – RS or CRS Medium setting – MS or CMS Slow setting – SS or CSS High Float – HFMS

(Specifications for quick setting emulsions are under development)

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Tack Coats Promote Bonding between Layers

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Prime Coats Prevent Slippage  Prevent Base Shifting  Protect Against Weather 

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Asphalt Binders

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Old Methods of Measurement of Asphalt Properties vacuum 0 sec

5 sec penetration

100 g

100 g

Penetration Viscosity

Asphalt Grading Systems  Hot

Mix Asphalt Binders

 Performance

grades (PG)  Viscosity grades  original

(unaged)-AC  recovered (aged)-AR  Penetration

grades

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Superpave Asphalt Binder Specification 

Grading System Based on Climate

PG 58 - 22

Performance Graded Binder Specifications Superior to viscosity and penetration grading systems  Have resulted in higher quality binders  Allow appropriate grade of binder for specific location based upon: 

air and pavement temperature  loading conditions  traffic speed 

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Superpave Asphalt Binder Specification 

Grading System Based on Climate

PG 64 - 22 Performance Average 7-day Min pavement Grade max pavement design temp design temp

Performance Graded Binder Specification 

Old Specifications – Viscosity Graded 

(AC-30)

Superpave System included PG Binder Grading System  SC DOT adopted PG grading system in late 1990s  Two grades are used (Section 401.2.1.1) 

PG 64-22  PG 76-22 

most work high volume, heavy loading

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LTTP 98% Pavt High Temperatu Green – 58°C Blue – 64°C

Superpave Asphalt Binder Specification 

Grading System Based on Climate

PG 76 - 22 High temperature performance is improved. This grade is used for heavily loaded roadways and where trucks are stopped or slowed

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Which PG asphalt should perform best in resisting thermal cracking? PG 64-22  PG 76-22  PG 64-28  PG 58-34 

Which PG asphalt should perform best in resisting thermal cracking?  PG

64-22  PG 76-22  PG 64-28  PG 58-34

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Which PG asphalt should perform best in resisting rutting? PG 82-22  PG 76-28  PG 70-28  PG 76-22 

Which PG asphalt should perform best in resisting rutting? PG 82-22  PG 76-28  PG 70-28  PG 76-22 

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Aggregate For Hot Mix Asphalt (HMA)

The 3 A’s of Hot Mix Asphalt

Asphalt

Aggregates and Air

HMA = Asphalt + Aggregates + Air Bob Horan, P.E. Mechanicsville, VA

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Aggregate Materials Summary    

Aggregate = “Rocks” (stone, gravel and sand) used in pavements and pavement treatments Aggregate is the primary ingredient in pavements (95% by weight?) Aggregate can be produced from a variety of sources (quarries, gravel sources) Measured and specified aggregate properties include gradation, shape, angularity, toughness, soundness, cleanliness

Aggregate Types Natural  Processed  Synthetic 

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Aggregates in Asphalt Construction 

Mineral aggregate has a great influence on the performance of an asphalt pavement as aggregates comprise 90 to 96 percent of the mixture weight and/or 82 to 88 percent of the aggregate volume.

Aggregate Processing  Excavation  Crushing  Sizing  Stockpiling

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Excavation

Excavation

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Crushing

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Crushing River Gravel

Partially Crushed River Gravel

Sizing

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Stockpiling

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Aggregate Properties Particle Size  Gradation  Cleanliness  Toughness 

Particle Size

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4.75 mm

19.0 mm

9.5 mm

25.0 mm

12.5 mm

37.5 mm

Aggregate Gradation

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Standard Aggregate Sieves 2 in

#8

1.5 in

#16

1 in

#30

3/4 in

#50

1/2 in

#100

3/8 in

#200

#4

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Aggregate Gradation Sieve Size

Percent Passing

Gradation Tolerance

3/4 in. 1/2 in. #4 #8 #16 #40 #200

100 92 70 45 37 18 5.8

100 75-95 45-75 30-55 20-45 10-30 3.0-7.0

Toughness * Los Angeles Abrasion (AASHTO T96, ASTM C131): Resistance of coarse agg to abrasion and mechanical degradation during handling, construction and use * Aggregate at standard gradation subjected to damage by rolling with prescribed number of steel balls in large drum for a given number of rotations * Result expressed as % changes in original weight

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LA Abrasion Test

- Approx. 10% loss for extremely hard igneous rocks - Approx. 60% loss for soft limestones and sandstones

Stripping The loss of adhesion between the asphalt cement and the aggregate surface primarily due to the action of water Other contributing factors are:  Aggregate surface coatings  Smooth aggregate surface texture  Antistrip additives help solve stripping

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Aggregate Specific Gravity Both Coarse and Fine Aggregates

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Recycled Asphalt Pavement (RAP) 

SC-M-407 replaces Section 401.2.2.6 Stockpile Approval / Records  Fractionated vs. Non-Fractionated  Maximum % RAP 

 Varies

by RAP fractionation and mix type

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Maximum % RAP SC-M-407 (06/11) – Section 7

SCDOT Aggregate References Standard Specifications Section 401.2.2  Supplemental Spec. SC-M-402 Hot-Mix Asphalt Material Properties  Supplemental Spec. SC-M-407 Recycled Asphalt Pavement (RAP) and Asphalt Shingles  SCDOT Qualified Product List 1 – Fine  SCDOT Qualified Product List 2 - Coarse 

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Aggregates for HMA

Materials Asphalt and Aggregate Questions?

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