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Asphalts
Francisco AguíñigaAssistant Professor
Civil Engineering ProgramTexas A&M University – Kingsville
CEEN 3144 Construction Materials
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One of the oldest materialsUsed in 3000 B.C.Originally used from natural pools
La Brea Pitch Lake, Trinidad and Tobago
History of asphalts
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Classification of bituminous materials
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Distillation of crude petroleum
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Asphalt cementCannot be used without heating (highly viscous at normal temperatures) – high molecular weightMost common binder in pavements –better quality than liquid asphalts
Solid road asphalts
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Asphalt cutbackMade by dissolving asphalt cement in a light molecular weight hydrocarbon solventWhen used to make roads the solvent evaporates, leaving the cutback as the binderNot used because has three disadvantages
Solvents are too expensiveHazardous because of the volatile solventsApplication releases environmentally unacceptable hydrocarbons into the atmosphere
Liquid road asphalts
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Asphalt emulsionMicron size globules60 to 70% asphalt residue, 30 to 40% water, fraction of a percent of soapAlmost same specific gravity as water so asphalt does not float nor sinkWhen emulsion is mixed with aggregates and the water evaporates the asphalt acts as a binderEvaporation of water is called breaking or settingAggregates that bear positive surface charges (limestone) tend to be compatible with anionic emulsionsAggregates that bear negative surface charges (siliceous aggregate) tend to be compatible with cationic emulsionsAdvantage
Do not require hazardous and costly solvents
Liquid road asphalts
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Main uses – pavement construction and maintenanceAlso used in sealing and waterproofingAsphalt cements
Hot-mix asphalt concrete – asphalt pavements
Liquid asphaltsPavement maintenance: fogs, seals, chip seals, slurry seals, and microsurfacingSealing cracksMixed with aggregates to make cold asphalt
Uses of asphalt
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Asphalt applications
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Asphalt applications
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Paving applications of asphalt
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Low temperatures – brittleHigh temperatures – softViscosity decreases with temperature
Temperature susceptibility of asphalt
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Adjust asphalt grade to climateSoft grade – cold climateHard grade – hot climate
Temperature susceptibility of asphalt
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Asphalt contains mainlyHydrocarbons (hydrogen and carbon atoms) with a small amounts of sulfur, nitrogen, and oxygen
Asphalt consists of asphaltenes and maltenes
AsphaltenesDark brown friable solids responsible for viscosity and adhesive properties of asphalt
MaltenesConsist of resins and oilsResins are dark with a viscosity dependent on temp.Resins act as agents to disperse aspahltenes in the oilsOils are clear or white liquids
Chemical composition
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Strategic Highway Research Program -1987Superior Performing Asphalt Pavements
Mix design methodPerformance grading methods for asphalt binders
Performance based specifications for asphalt binders and mixtures to control
RuttingFatigue crackingThermal cracking
Superpave
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Mainly characterize asphalt cementConduct test at a given temperaturePerformance grade characterization
Test properties at lower, middle, and upper operating temperaturesPrevious tests – perform test at fixed temperature, then vary requirements for different grades of asphaltNew tests – perform test at critical temperature and fix criteria for all grades of asphalt
Characterization of asphalt
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Perform tests at three temperaturesMaximum temperature – Highest successive 7-day average maximum pavement temperatureMinimum temperature – Minimum expected temperature for the pavement over its service lifeIntermediate temperature – Average of maximum and minimum plus 4 °C
Temperatures used to evaluateRutting potential - maximumFatigue potential - intermediateThermal-cracking – minimum plus 10 °C
New tests of asphalts
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Rolling thin-film ovenPressure-aging vesselFlash pointRotational viscometer testDynamic shear rheometer testBending beam rheometer testDirect tension test
Performance grade binder characterization
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Used to simulate short-term aging that occurs in asphalt during the production of asphalt concrete
Asphalt binder poured into special bottles, the placed in a rack, the rack rotates vertically in an oven at 163 °C for 75 min.Aged binder is used in dynamic shear rheometer and pressure aging vessel
Rolling thin-film oven
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Designed to simulate oxyidative aging that occurs during pavement service
Asphalt is aged under specified temperature for 20 h at 305 psiResidue from the test can be used to estimate physical or chemical properties at 5 or 10 years of service
Pressure-aging vessel
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Safety test – measures temperature at which asphalt flashes
Fire temperature – cause vapors to igniteFlash temperature – sustain burning > 5 sec
Flash point – Cleveland open cup
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Used to measure the viscosity of asphalt
Test performed on unaged bindersPlace sample in chamber at 135 °C, then placed in a thermocell, then spindle is placed in asphalt and rotatedViscosity determined by amount of torque required to rotate spindle
Rotational viscometer test
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Used to evaluate rutting and fatigue potential
Consists of two parallel metal plates, an environmental chamber, a loading device and a data acquisition systemRutting potential evaluated at upper temperature (about 60 °C)
Sample 25 mm diameter and 1 mm thickSample preconditioned in rolling thin-film oven
Fatigue potential evaluated at intermediate temperature (about 25 °C)
Sample 8 mm diameter and 2 mm thickSample preconditioned in rolling thin-film oven followed by pressure-aging vessel
Dynamic shear rheometer test
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Used to evaluate thermal crackingMeasures midpoint deflection of an asphalt binder beam under constant load
Loading frame, controlled bath, and data acquisitionSpecimen tested at 10 °C above the lower temperatureSample preconditioned in rolling thin-film oven and pressure-aging vesselDeflection measured at 8, 15, 30, 60, 120, and 240 sec
Bending beam rheometer test
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Displacement controlled tensile loading machine in a temperature controlled chamber
Specimen preconditioned in rolling thin-film and pressure-aging vesselPulled at 1mm/minTest temperature is low temperature plus 10 °CReport failure load, stress, and strainFailure strain is a measure of elongation without cracking
Direct tension test
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PenetrationMeasures asphalt cement consistencyAsphalt prepared at 25 °CPlace 100 g needle on asphalt surfaceMeasure penetration in 5 sec
Absolute and kinematic viscosity testsMeasures asphalt consistencyHeat asphalt, put it in a water or oil bath at 60 °CViscosity measured as flow of asphalt in a U-tube
Traditional asphalt characterization tests
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Distillation of cutback and emulsionCutback
Measures amount of volatile constituentsDetermine percentage by volume constituents at specified temperature
EmulsionDetermines percent of residue and oil distillates by weight
Saybolt viscosity of emulsionWhen applied in a spray must be thin enough to be uniformly applied through the spray bar of the distribution truckEmulsion tested at 25 or 50 °C and allowed to flow through the orificeTest result = time in seconds required to fill a flask
Characterization of emulsion and Cutback
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Asphalt binders are graded by 4 methodsPerformancePenetrationViscosityViscosity of aged residue
Classification of asphalt
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Performance grading of asphalts
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ASTM D946
Penetration grading
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ASTM D3381
Viscosity grading