Asphalt Concrete Pavement

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Asphalt Concrete Pavement

INTRODUCTION

Asphalt concrete pavements consist of a combination of layers, which include an asphalt concrete

surface constructed over a granular or

surface constructed over a granular or asphalt concrete base and a subbase. Thasphalt concrete base and a subbase. The entiree entire

pavement structure, which is constructed over the subgrade, is designed to support the traffic

load and distribute the load over the roadbed. Pavements can be constructed using hot mix or

cold mix asphalt. Surface treatments are sometimes us

cold mix asphalt. Surface treatments are sometimes used during pavement construction. Surfaceed during pavement construction. Surface

treatment acts as a waterproof cover for the existing pavement surface

treatment acts as a waterproof cover for the existing pavement surface and also providesand also provides

resistance to abrasion by traffic.

Hot mix asphalt is a mixture o

Hot mix asphalt is a mixture of fine and coarse aggregate with asphalt cement binder that isf fine and coarse aggregate with asphalt cement binder that is

mixed, placed, and compacted in a heated conditi

mixed, placed, and compacted in a heated condition. The components are heated and mixed at aon. The components are heated and mixed at a

central plant and placed on the road using an asphalt spreader.

Cold mix asphalt is a mix

Cold mix asphalt is a mixture of emulsified asphalt and aggregate, produced, placed, andture of emulsified asphalt and aggregate, produced, placed, and

compacted at ambient air temperature. The use of cold m

compacted at ambient air temperature. The use of cold mix asphalt is usually ix asphalt is usually limited to relativelylimited to relatively

low-volume rural roads. For higher traffic applications, cold mix asphalt pavement usually

requires an overlay of hot mix as

requires an overlay of hot mix asphalt or surface treatment to resist traffic action. Thephalt or surface treatment to resist traffic action. The

components of cold mix asphalt can be mixed at a central plant or in-situ with a traveling mixer.

Surface treatments consist of an application (or sometimes multiple applications) of emulsified or

liquid asphalt and select aggregate, placed over a prepared granular base or existing s

liquid asphalt and select aggregate, placed over a prepared granular base or existing surface.urface.

Following placement of the aggregate, the mixture is rolled and compacted to provide a drivable,

dust-free surface. This type of pavement is commo

dust-free surface. This type of pavement is common on light- to medium- von on light- to medium- volume roads that maylume roads that may

or may not already have an existing bituminous surface.

MATERIALS

The components of asphalt concrete inc

The components of asphalt concrete include asphalt aggregate and asphalt binder. Mineral fillerlude asphalt aggregate and asphalt binder. Mineral filler

is sometimes added to hot mix asphalt concrete.

Asphalt

Asphalt AggregateAggregate

Aggregates used in asphalt mixtures (hot mix asphalt, cold mix asphalt, surface treatments)

comprise approximately 95 percent of the mix by mass. Proper aggregate grading, strength,

toughness, and shape are needed for mixture stability.

Asphalt Binder Asphalt Binder

The asphalt binder component of an asphalt pavement typical

The asphalt binder component of an asphalt pavement typically makes up about 5 to 6 percely makes up about 5 to 6 percentnt

of the total asphalt mixture, and coats an

of the total asphalt mixture, and coats and binds the aggregate particles together. Asphaltd binds the aggregate particles together. Asphalt

cement is used in hot mix

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with the aid of an emulsifying agent or s olvent, is used as the binder in surface treatments and cold mix asphalt pavements. The properties of bin ders are often improved or enhanced by using additives or modifiers to improve adhesion (stripping resistance), flow, oxidation characteristics, and elasticity. Modifiers include oil, filler, powders, fibres, wax, solvents, emulsifiers, wetting agents, as well as other proprietary additives.

Mineral Filler

Mineral filler consists of very fine, inert mineral matter that is added to the hot mix asphalt to improve the density and strength of the mixture. Mineral fillers make up less than 6 percent of the hot mix asphalt concrete by mass, and generally less than about 3 percent. A typical mineral filler completely passes a 0.060 mm (No. 30) sieve, with at least 65 percent of the particles passing the 0.075 mm (No. 200) sieve.

MATERIAL PROPERTIES AND TESTING METHODS

Asphalt Aggregate

Since aggregates used in bituminous mixtures (hot mix asphalt, cold mix asphalt, surface treatments) comprise approximately 95 percent of the mixture by mass and roughly 80 percent by volume, the aggregate material(s) used in asphalt concrete have a profound influence on the properties and performance of the mixture. The following is a listing and brief comment on some of the more important properties for aggregates that are used in asphalt paving mixes:

 Gradation - the size distribution of the aggregate particles should be a combination of

sizes that results in the optimum balance of v oids (density) and pavement strength.

 Particle Shape - aggregate particles should be angular and nearly equidimension al or

cubical in shape to minimize surface area. F lat or elongated particles should be avoided.

 Particle Texture - particles should have a ro ugh, rather than smooth, texture to

minimize the stripping of asphalt cement.

 Particle Strength - particles should be of sufficient strength to resist degradation or

breakdown under compaction or traffic.

 Durability - particles must be durable enough to remain intact under variable climatic

conditions and/or chemical exposure.

 Specific Gravity - the specific gravity of an aggregate is needed in order to properly

design and proportion an asphalt mix.

 Absorption - the absorption of an aggregate refers to the amount of vo id spaces within

a particle that may be filled with asphalt binder (or air or w ater), and is a measure of the tendency of an aggregate to absorb asphalt. The higher the absorption, the more asphalt cement will be needed.

 Unit Weight - the unit weight of an aggregate is an indicator of the compacted density

of an asphalt paving mix contain ing this aggregate and the pavement yield (the volume of pavement that will be required for a given pavement mass).

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 Volume Stability - certain aggregates may undergo volumetric expansion following

prolonged exposure to moisture, deicing salts, etc., which may contribute to popouts, ravelling, and random cracking in asphalt pavements.

 Deleterious Components - some aggregates may contain harmful amounts of

potentially reactive components (shale, chert, sulfates, alkalis, expansive silicates, etc.), which may contribute to popouts, ravelling, and cracking in pavements.

Asphalt Binder

Although the asphalt binder component typically comprises approximately 5 to 6 percent by mass of an asphalt paving mixture, the selection of the proper grade of asphalt (asphalt cement or emulsion) for the traffic and climatic co nditions to which the paving mixture is to be exposed is essential to the performance of the mix. Some of the more important properties of asphalt cement that are used to distinguish between different cements and to evaluate their quality include:

 Penetration - a measure of the relative softness or hardness of an asphalt cement (or

emulsion) at a given temperature.

 Viscosity - a measure of the resistance of an asphalt cement to flow at a given

temperature.

 Ductility - a measure of the ability of an asphalt cement to undergo elongation under

tensile stress at a given temperature.

 Incompatibility - a measure of phase separation of the components of

polymer-modified asphalt binders during storage and use. Such a s eparation is undesirable since it results in significant variation in the properties o f the binder and the asphalt in which it is used.

Table 1 provides a list of standard test methods th at are used to assess the suitability of conventional mineral aggregates for use in asphalt paving applications.

Table 1. Asphalt paving aggregate test procedures.

Property

Test Method

Reference

General

Specifications

Coarse Aggregate for Bituminous Paving

Mixtures

ASTM D692

Fine Aggregates for Bituminous Paving

Mixtures

ASTM

D1073/AASHTO M 29

Steel Slag Aggregates for Bituminous

Paving Mixtures

ASTM D5106

Aggregate for Single or Multiple Surface

Treatments

ASTM D1139

Crushed Aggregate For Macadam

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Gradation

Sieve Analysis of Fine and Coarse

Aggregates

ASTM C136/AASHTO

T27

Sizes of Aggregate for Road and Bridge

Construction

ASTM D448/AASHTO

M43

Particle Shape

Index of Aggregate Particle Shape and

Texture

ASTM D3398

Flat and Elongated Particles in Coarse

Aggregate

ASTM D4791

Uncompacted Void Content of Fine

Aggregate (As Influenced by Particle

Shape, Surface Texture, and Grading)

(Test is part of SHRP Superpave Level 1

design procedure for hot mix asphalt)

ASTM

C1252/AASHTO TP33

Particle Texture

Accelerated Polishing of Aggregates Using

the British Wheel(Not widely recognized in

North America)

ASTM D3319/T279

Insoluble Residue in Carbonate

AggregatesIndirect measure of resistance

of aggregate to wear, by determining

amount of carbonate rock present)

ASTM D3042

Centrifuge Kerosine Equivalent(Only used

as part of the Hveem mix design procedure)

ASTM D5148

Particle Strength

Resistance to Degradation of Large-Size

Coarse Aggregate by Abrasion and Impact

in the Los Angeles Machine

ASTM C535

Resistance to Degradation of Small-Size

Coarse Aggregate by Abrasion and Impact

in the Los Angeles Machine

ASTM C131/AASHTO

T96

Degradation of Fine Aggregate Due to

Attrition

ASTM C1137

Durability

Aggregate Durability Index

ASTM

D3744/AASHTO T210

Soundness of Aggregates by Use of

Sodium Sulfate or Magnesium Sulfate

ASTM C88/AASHTO

T104

Soundness of Aggregates by Freezing and

Thawing

AASHTO T103

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and Absorption

Specific Gravity and Absorption of Coarse

Aggregate

T85

Specific Gravity and Absorption of Fine

Aggregate

ASTM C128/AASHTO

T84

Unit Weight

Unit Weight and Voids in Aggregate

ASTM

C29/C29M/AASHTO

T19

Volume Stability

Potential Expansion of Aggregates from

Hydration Reactions

(Developed to measure expansion potential

of steel slag aggregates)

ASTM D4792

Deleterious

Components

Sand Equivalent Value of Soils and Fine

Aggregate(Indirect measure of clay content

of aggregate mixes)

ASTM D2419

Clay Lumps and Friable Particles in

Aggregates

ASTM C142

Table 2 provides a list of standard test methods us ed to characterize asphalt binder properties. Table 2 Asphalt binder test procedures

Property

Test Method

Reference

General

Specifications

Recovery of Asphalt from Solution by

the Abson Method

ASTM D1856

Graded Asphalt Cement for Use in

Pavement Construction

ASTM D946

Graded Asphalt Cement for Use in

Pavement Construction

ASTM D3381

Emulsified Asphalt

ASTM D977

Rheology

Penetration of Bituminous Materials

ASTM D5

Preparation of Viscosity Blends for

Recycled Bituminous Materials

ASTM D4887

Kinematic Viscosity of Asphalts

ASTM D2170

Ductility of Bituminous Materials

ASTM D113

Effect of Heat/Air on Asphaltic Materials

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SHRP Level 1 Binder Testing

SHRP Mix Design Manual

A-407

Incompatibility

Storage Stability Test

Shell Bitumen Industrial

Handbook, 1995

Mineral Filler

Mineral fillers consist of finely divided mineral matter such as rock dust, slag dust, hydrated lime, hydraulic cement, fly ash, loess, or other suitable mineral matter.

Mineral fillers serve a dual purpose when added to asphalt mixes. The portion of the mineral filler that is finer than the thickness of the as phalt film and the asphalt cement binder form a mortar or mastic that contributes to improved stiffening of the mix. The particles larger than the

thickness of the asphalt film behave as mineral aggregate and hence contribute to the contact points between individual aggregate particles. The gradation, shape, and texture of the mineral filler significantly influence the performance of hot mix asphalt.

Some of the more important properties of mineral filler used in asphalt concrete applications are as follows:

 Gradation - mineral fillers should have 100 percent of the particles passing 0.60 mm

(No. 30 sieve), 95 to 100 percent passing 0.30 mm (No. 40 sieve), and 70 percent passing 0.075 mm (No. 200 sieve).

 Plasticity - mineral fillers should be nonplastic so the particles do not bind together.  Deleterious Materials - the percentage of deleterious materials such as clay and shale

in the mineral filler must be minimized to prevent particle breakdown.

Table 3 provides a listing of applicable test methods con taining criteria that are used to

characterize the suitability of conventional filler materials for use in asphalt paving applications. Table 3. Mineral filler test procedures.

Property

Test Method

Reference

General

Specifications

Mineral Filler for Bituminous Paving

Mixtures

ASTM D242/AASHTO

M 17

Gradation

Sieve Analysis of Mineral Filler for Road

and Paving Materials

ASTM D546

Plasticity

Liquid Limit, Plastic Limit, and Plasticity

Index of Soils

ASTM D4315

Deleterious

Materials

Sand Equivalent Value of Soils and Fine

Aggregate

(Indirect measure of clay content of

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aggregate mixes)

ASPHALT CONCRETE MATERIAL

The mix proportions for a properly comp acted asphalt concrete paving mixture are determined in the laboratory during mix design testing. The ability of a properly proportioned asphalt paving mix to resist the potentially damaging effects of the asphalt binder stripping from the aggregate particles is also routinely evaluated in the laboratory. To perform properly in the field, a w ell-designed asphalt paving mixture must be placed within the proper temperature range and mu st be adequately compacted. Asphalt concrete paving mixtures should be evaluated for the

following properties:

 Stability - the load that a well-compacted pavin g mixture can accept before failure.

Sufficient mix stability is required to satisfy the d emands of traffic without distortion or displacement.

 Flow - the maximum diametric com pressive strain measured at the instance of failure.

The ratio of Marshall stability to flow approximates the mix’s load -deformation characteristics and therefore indicates the material’s resistance to permanent deformation in service.

 Air Voids - the percentage of void spaces within the aggregate-binder matrix that are

not filled with binder. Sufficient voids should be provided to allow for a slight amount of additional compaction under traffic and a slight amou nt of asphalt expansion due to temperature increases, without flushing, bleeding, or loss of stability.

 Stripping Resistance - the ability of a paving mixture to resist the loss of tensile

strength due to stripping of the asphalt cement from the aggregate. Low resistance to stripping could result in mix disintegration.

 Resilient Modulus - a measure of the stiffness of a well-compacted paving mixture

under prescribed conditions of load application. A mix having a low resilient modulus would be susceptible to deformation, whereas a high resilient modulus indicates a brittle mixture.

 Compacted Density - the maximum unit weight or density of a properly designed

paving mixture compacted under prescribed laboratory compaction procedures.

 Unit Weight - a measure of the density of a pav ing mixture compacted in the field in

accordance with project specifications.

Table 4 provides a list of standard laboratory tests that are presently used to evaluate the mix design or expected performance of paving mixes.

Recent developments in asphalt pavement design research which was conducted under the Strategic Highway Research Program (SHRP), has resulted in the development of a n ew asphalt mix design procedure, referred to as Superpave (Superior Performing Asphalt Pavement Design Procedure). Where the traditional mix design approach (us ing Marshall mix or Hveem design methods) was based on empirical laboratory design procedures, the Superpave mix design

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approach represents an improved system for s pecifying asphalt binder and mineral aggregates, developing an asphalt mixture design, and analyzing and es tablishing pavement performance prediction. The system includes an asphalt binder specification (performance graded binders), a hot mix asphalt design and analysis system, and computer software that integrates the system components. The unique feature of the Superpave system is that it is a performance-based specification approach, with the tests and analyses having direct relationship to field

performance.

Table 4. Asphalt paving material test procedures.

Property

Test Method

Reference

Stability and Flow

Characteristics

(also air voids)

Marshall Method

AASHTO T245

Hveem Method

AASHTO T246, T247

Asphalt Institute Recommended Cold

Mix Method

Asphalt Institute Cold

Mix Manual

Resistance to Plastic Flow of

Bituminous Mixtures Using Marshall

Apparatus

ASTM D1559

Stripping Resistance

Immersion

_–

Marshall Method

ASTM D4867

Immersion

_–

Marshall Method

AASHTO T283

(Modified Lottman

Method)

Resilient Modulus

Superpave Mix Design

Asphalt Institute

Superpave Series No. 1

(SP-1)

Asphalt Institute

Superpave Series No. 2

(SP-2)

Unit Weight

Theoretical Maximum Specific Gravity

and Density of Bituminous Paving

Mixtures

ASTM D2041

Compacted Density

In-Place Density of Compacted

Bituminous Paving Mixtures

ASTM D2950

Superpave mix design and analysis is performed at one of three increasingly rigorous levels o f performance. Superpave Level 1 is an improved materials selec tion and volumetric mix design procedure; Level 2 uses the same vo lumetric mix design procedure as Level 1 as a starting point, in conjunction with a battery of tests to predict the mix performance; and Level 3 involves a more comprehensive array of tests to ac hieve a more reliable level of performance prediction.

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At present, only the performance-graded asphalt binder specification and Superpave Level 1 approach has been finalized, with the performance prediction models us ed in the Level 2 and Level 3 procedures still being validated.

Users are referred to the Asphalt Institute Superpave Series No. 1 and No. 2 publications listed in the reference section for detailed information on the Superpave mix design equipment and test methods and on the performance-graded asphalt binder requirements.