Asphalt Concrete Pavement

Asphalt Concrete Pavement INTRODUCTION Asphalt concrete pavements consist of a combination of layers, which include an asphalt concrete surface constructed over a granular or asphalt concrete base and a subbase. Th e 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 ed during pavement construction. Surface treatment acts as a waterproof cover for the existing pavement surface and also provides resistance to abrasion by traffic. Hot mix asphalt is a mixture o f fine and coarse aggregate with asphalt cement binder that is mixed, placed, and compacted in a heated conditi on. 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 ture of emulsified asphalt and aggregate, produced, placed, and compacted at ambient air temperature. The use of cold m ix asphalt is usually limited to relatively low-volume rural roads. For higher traffic applications, cold mix asphalt pavement usually requires an overlay of hot mix as phalt 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 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 n on light- to medium- vo lume roads that may or may not already have an existing bituminous surface.

MATERIALS The components of asphalt concrete inc lude asphalt aggregate and asphalt binder. Mineral filler is sometimes added to hot mix asphalt concrete. Asphalt Aggregate

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

The asphalt binder component of an asphalt pavement typical ly makes up about 5 to 6 perce nt of the total asphalt mixture, and coats an d binds the aggregate particles together. Asphalt cement is used in hot mix asphalt. Liquid asphalt, which is asphalt cement dispersed in water

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: 

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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 pav ing 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: 

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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 polymermodified 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

General Specifications

Test Method

Reference

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 Pavements

 ASTM D693

Gradation

Particle Shape

Particle Texture

Sieve Analysis of Fine and Coarse  Aggregates

 ASTM C136/AASHTO T27

Sizes of Aggregate for Road and Bridge Construction

 ASTM D448/AASHTO M43

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

 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  ASTM D5148 as part of the Hveem mix design procedure)

Particle Strength

Durability

Specific Gravity

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

 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 ASTM C127/AASHTO

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  ASTM D4792 (Developed to measure expansion potential of steel slag aggregates)

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

Property

General Specifications

Rheology

Asphalt binder test procedures

Test Method

Reference

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

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  ASTM D1754 by Thin-Film Oven Test

Incompatibility

SHRP Level 1 Binder Testing

SHRP Mix Design Manual  A-407

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: 

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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

 ASTM D2419

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 elldesigned 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: 

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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

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

Stability and Flow Characteristics (also air voids)

Stripping Resistance

Test Method

Reference

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

Immersion  – Marshall Method

 ASTM D4867

Immersion  – Marshall Method

 AASHTO T283 (Modified Lottman Method)  Asphalt Institute Superpave Series No. 1 (SP-1)

Resilient Modulus

Superpave Mix Design

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

 Asphalt Institute Superpave Series No. 2 (SP-2)

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.

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.