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1 Behavior of Asphalt Binder and Asphalt Concrete

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2 Mixture Classification type of binder asphalt cement liquid asphalt aggregate gradation dense-graded (well-graded) open-graded production method hot-mix (hot-laid)** cold-mix (cold-laid)

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3 AC Mix Design Asphalt Concrete = binder + aggregate select & proportion components that provide adequate performance over design reasonable cost VOLUMETRIC process V air > 3% to preclude bleeding, instability V air < 8% for durability V asp to coat, bind, & satisfy (absorption) agg WEIGH components in production

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4 AC Mix Design adequate performance assessed based on MIXTURE PROPERTIES stiffness stability durability flexibility fatigue resistance fracture (tensile) strength thermal characteristics skid resistance permeability workability

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5 CONCRETE ASPHALT CONCRETE MIXTURES Asphalt Concrete = binder + aggregate 3 stages of Life mixing (fluid asphalt cement) curing (viscoelastic solid) aging (environmental effects & loading)

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6 Behavior depends on: Temperature Time of loading (Traffic Speed) Aging (properties change with time) Factors Influencing the Behavior

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7 Permanent Deformation Function of warm weather and traffic Courtesy of FHWA

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8 Stability resistance to permanent deformation under repetitive loading rutting, shoving Marshall Stability

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9 Stability mechanical / frictional interlock between aggregate particles same factors that influence creep rough, angular, dense- graded aggregate binder (w/ voids filled) S ac degree of compaction (> 3% air) Stability

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10 Stability

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11 Flexibility ability to conform to long-term variations in underlying layer elevations settlement (clay), heave (frost, moisture) open-graded aggregate binder Flexibility

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12 Fatigue Resistance resistance to fracture caused by repetitive loading (bending) fatigue (alligator) cracking dense-graded aggregate binder degree of compaction Fatigue Resistance

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14 Tensile (Fracture) Strength resistance to thermal cracking low temps large induced stresses (restrained contraction) weak subgrade transverse cracking primarily controlled by binder limiting tensile strength (4-10 MPa) ~ limiting stiffness dense graded aggregate degree of compaction binder Tensile Strength

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15 Low Temperature Behavior Low Temperature Cold Climates Winter Rapid Loads Fast moving trucks

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16 Thermal Cracking Courtesy of FHWA

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17 Aging Asphalt reacts with oxygen “oxidative” or “age hardening” Short term Volatilization of specific components During construction process Long term Over life of pavement (in-service)

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18 Permeability ease w/ which air & water can pass through or into AC moisture damage, accelerated aging inversely proportional to durability dense graded aggregate degree of compaction binder Permeability

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19 Durability resistance to weathering & abrasive action of traffic exposure to air (aging), water, & traffic moisture damage (stripping, loss of stiffness), accelerated aging S ac binder strong, hard, clean, dry aggregate resistant to polishing, crushing, freeze- thaw effects; not water sensitive dense graded aggregate degree of compaction Durability

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20 Mix Design select & proportion component materials to obtain desired reasonable cost properties of component materials properties of composite material economic factors & availability of materials construction methods

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21 Mix Design select aggregate blend determine optimum binder content balance desired properties

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22 Mix Design

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23 Mix Design selection of aggregate blend aggregate properties (primarily gradation) compactibility selection of binder content surface area of aggregates volumetrics of mixture (air voids, voids between aggregates) mechanical properties of mixture from laboratory testing

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24 Thermal Cracking Courtesy of FHWA

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25 Binder-Aggregate Bonding wettability viscosity (temp) composition (oxygen) durability surface chemistry (mineral composition) surface texture porosity surface condition (cleanliness, moisture) BinderAggregate

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26 Binder-Aggregate Bonding ac wetting the aggregate surface low surface energy need dry aggregates polar nature of ac / electrostatic interaction mechanical bonding failure interface stripping

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27 Binder-Aggregate Bonding

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28 Composite Material 2 components physically combined w/ some AIR VOIDS 1 continuous phase binder - viscous, viscoelastic aggregate** - solid dense aggregate skeleton w/ sufficient binder to bind and provide durability > 90% by weight aggregate

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29 Composite Material

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30 Permanent Deformation Function of warm weather and traffic Courtesy of FHWA

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31 Description of Asphalt Concrete Particulate composite material that consists of: Aggregates. Asphalt. Air voids.

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32 Review of the Properties of Particulate Composites The properties of the composite can be calculated from the properties of the constituents. For simplicity, assume asphalt concrete to be represented by particulate (aggregates), and matrix (asphalt and air). Also, assume elastic behavior.

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33 Parallel Model The particulate and matrix carry the same strain. V p = volume of particulate V m = volume of matrix Used to describe soft particles in a hard matrix

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34 Series Model The particulate and matrix carry the same stress. Used to describe hard particles in a soft matrix

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35 Hirsch’s Model X: represents the degree of bonding

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36 toto trtr Stress toto trtr time Strain toto trtr time Strain Elastic Viscous Viscoelastic Behavior of Asphalt Concrete time Viscoelastic response = Immediate elastic + Time dependent viscous

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37 Viscoelastic Models Viscoelastic Model: Mathematical expression for the relationship between stress, strain, and strain rate. Combinations of basic rheological models. The combinations mean that there are different mechanisms due to different chemical and physical interactions that govern the response.

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38 Basic responses Viscous toto trtr Stress toto trtr time Strain toto trtr time Elastic time Viscous toto trtr time Strain

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39 Maxwell Model Constant Stress (Creep) Constant Strain (Relaxation) time Strain time Stress

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40 Kelvin Model Constant Stress (Creep) Constant Strain (Relaxation) time Strain time Stress

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41 Burger Model Constant Stress (Creep) time Strain

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42 Asphalt Binder Behavior Viscoelastic behavior Temperature Value depends on asphalt type Elastic part is negligible Viscous behavior Temperature scale Semi solid or solid fluid

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43 Viscous Behavior of Fluids Shear Stress Shear Rate Slope = (Viscosity) Shear Stress Shear Rate yield Yield stress Newtonian Non Newtonian Bingham behavior

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44 Shear Stress Shear Rate Shear Stress Shear Rate Viscous Behavior of Fluids Non Newtonian Shear Thinning Non Newtonian Shear Thickening Increase in viscosity with increase in strain rate Decrease in viscosity with increase in strain rate

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45 Why do we need to model the response? Conduct a creep or a relaxation test. Fit a model to the data. Determine the material parameters. Describe the material parameters based on design conditions Use the model to predict performance under different loads and applications.

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46 Permanent Deformation Function of warm weather and traffic Courtesy of FHWA

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