Behavior of Asphalt Binder and Asphalt Concrete

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)

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

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

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

Factors Influencing the Behavior Behavior depends on: Temperature Time of loading (Traffic Speed) Aging (properties change with time) This specification uses tests which evaluate the fundamental material properties (stress, strain, and strain rate). Changes in asphalt properties due to temperature, rate of loading and the effect of aging are considered. Asphalt is a viscoelastic material. That is, it can both exhibit elastic and viscous properties at the same time. To demonstrate, take a penetration tin of asphalt cement at room temperature. Press your thumb into the asphalt; this will leave a large depression in the surface. Then use a hammer to hit the a remaining flat section of the asphalt; this will not leave much of an impression. Under light but long term loads, the asphalt has a noticeable viscous behavior (it flows out from under the load). Higher, but much shorter duration loads result in a primarily elastic response.

Permanent Deformation Ruts can be very visible in extreme cases such as the one shown in this photo. Other places where rutting can be observed are at stop lights. In many cases, the crosswalk lines can highlight this type of distress. Courtesy of FHWA Function of warm weather and traffic

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

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

Stability

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

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

Tensile (Fracture) Strength resistance to thermal cracking important @ 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

Low Temperature Behavior Cold Climates Winter Rapid Loads Fast moving trucks At cold temperatures, or under very quick loads, the binder response is predominately elastic.

Thermal Cracking Courtesy of FHWA Thermal cracks are transverse cracks, usually at relatively evenly spaced intervals. The spacing gets closer together with increasing binder stiffness the colder the temperatures. Courtesy of FHWA

Aging Asphalt reacts with oxygen Short term Long term “oxidative” or “age hardening” Short term Volatilization of specific components During construction process Long term Over life of pavement (in-service) Aging also needs to be considered in the specification as oxidation and heat hardening during tank storage, mixing and placement (short term aging) of the asphalt concrete change the properties of the original binder. Long term aging refers to the changes in binder property after 7 to 10 years of exposure to environmental factors.

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

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

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

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

Mix Design

Mix Design selection of aggregate blend selection of binder content 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

Thermal Cracking Courtesy of FHWA Thermal cracks are transverse cracks, usually at relatively evenly spaced intervals. The spacing gets closer together with increasing binder stiffness the colder the temperatures. Courtesy of FHWA

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

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

Binder-Aggregate Bonding

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

Composite Material

Permanent Deformation Ruts can be very visible in extreme cases such as the one shown in this photo. Other places where rutting can be observed are at stop lights. In many cases, the crosswalk lines can highlight this type of distress. Courtesy of FHWA Function of warm weather and traffic

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

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.

Parallel Model Vp = volume of particulate Vm = volume of matrix The particulate and matrix carry the same strain. Used to describe soft particles in a hard matrix

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

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

Viscoelastic Behavior of Asphalt Concrete Viscoelastic response = Immediate elastic + Time dependent viscous Strain Elastic Stress to time tr Strain to time tr Viscous to tr time

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.

Basic responses Strain Elastic to time tr Stress Strain Viscous to to

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

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

Burger Model Constant Stress (Creep) Strain time

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

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

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

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.

Permanent Deformation Ruts can be very visible in extreme cases such as the one shown in this photo. Other places where rutting can be observed are at stop lights. In many cases, the crosswalk lines can highlight this type of distress. Courtesy of FHWA Function of warm weather and traffic