2Mixture Classification type of binderasphalt cementliquid asphaltaggregate gradationdense-graded (well-graded)open-gradedproduction methodhot-mix (hot-laid)**cold-mix (cold-laid)
3AC Mix Design WEIGH components in production Asphalt Concrete = binder + aggregateselect & proportion components that provide adequate performance over design reasonable costVOLUMETRIC processVair > 3% to preclude bleeding, instabilityVair < 8% for durabilityVasp to coat, bind, & satisfy (absorption) aggWEIGH components in production
4AC Mix Design fracture (tensile) strength thermal characteristics adequate performance assessed based on MIXTURE PROPERTIESstiffnessstabilitydurabilityflexibilityfatigue resistancefracture (tensile) strengththermal characteristicsskid resistancepermeabilityworkability
6Factors Influencing the Behavior Behavior depends on:TemperatureTime 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.
7Permanent 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 FHWAFunction of warm weather and traffic
8Stability resistance to permanent deformation under repetitive loading rutting, shovingMarshall Stability
9Stabilitymechanical / frictional interlock between aggregate particlessame factors that influence creeprough, angular, dense-graded aggregate binder (w/ voids filled) Sac degree of compaction (> 3% air)Stability
15Low Temperature Behavior Cold ClimatesWinterRapid LoadsFast moving trucksAt cold temperatures, or under very quick loads, the binder response is predominately elastic.
16Thermal 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
17Aging Asphalt reacts with oxygen Short term Long term “oxidative” or “age hardening”Short termVolatilization of specific componentsDuring construction processLong termOver 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.
18Permeability ease w/ which air & water can pass through or into AC moisture damage, accelerated aginginversely proportional to durabilitydense graded aggregate degree of compaction binder Permeability
19Durability Durability resistance to weathering & abrasive action of trafficexposure to air (aging), water, & trafficmoisture damage (stripping, loss of stiffness),accelerated aging Sac binderstrong, hard, clean, dry aggregate resistant to polishing, crushing, freeze-thaw effects; not water sensitivedense graded aggregate degree of compactionDurability
20Mix Designselect & proportion component materials to obtain desired reasonable costproperties of component materialsproperties of composite materialeconomic factors & availability of materialsconstruction methods
23Mix Design selection of aggregate blend selection of binder content aggregate properties (primarily gradation)compactibilityselection of binder contentsurface area of aggregatesvolumetrics of mixture (air voids, voids between aggregates)mechanical properties of mixture from laboratory testing
24Thermal 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
28Composite Material 2 components physically combined w/ some AIR VOIDS 1 continuous phasebinder - viscous, viscoelasticaggregate** - soliddense aggregate skeleton w/ sufficient binder to bind and provide durability> 90% by weight aggregate
30Permanent 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 FHWAFunction of warm weather and traffic
31Description of Asphalt Concrete Particulate composite material that consists of:Aggregates.Asphalt.Air voids.
32Review 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.
33Parallel Model Vp = volume of particulate Vm = volume of matrix The particulate and matrixcarry the same strain.Used to describe soft particles in a hard matrix
34Series Model The particulate and matrix carry the same stress. Used to describe hard particles in a soft matrix
35Hirsch’s ModelggX: represents the degree of bonding
37Viscoelastic ModelsViscoelastic 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.
38Basic responses Strain Elastic to time tr Stress Strain Viscous to to
39Maxwell Model Constant Stress (Creep) Constant Strain (Relaxation) timetime
40Kelvin Model Constant Stress (Creep) Constant Strain (Relaxation) timetime
42Asphalt Binder Behavior Temperature scaleElastic partis negligibleViscousbehaviorTemperatureValue dependson asphalt typeViscoelasticbehaviorfluidSemi solid or solid
43Viscous Behavior of Fluids ShearStressShearStressSlope = (Viscosity)yieldYieldstressShearRateShearRateNon NewtonianBingham behaviorNewtonian
44Viscous Behavior of Fluids ShearStressShearStressShearRateShearRateNon NewtonianShear ThinningNon NewtonianShear ThickeningIncrease in viscosity withincrease in strain rateDecrease in viscosity withincrease in strain rate
45Why 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 conditionsUse the model to predict performance under different loads and applications.
46Permanent 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 FHWAFunction of warm weather and traffic