Asphalt Flashcards
What is Asphalt Cement?
A viscous cementitious material composed primarily of high molecular weight hydrocarbons.
What is Asphalt Concrete?
A complex material consisting of asphalt cement, mineral aggragates and possibly some admixtures.
Bitumen occurs?
- Naturally: as Rock asphalt (sandstone and limestone) and as Lake asphalt (40% bitumen mixed with silt and clay)
- Industrially: The Leftover obtained when lighter constituents of crude oil are distilled away during refining.
Why is Asphalt Cement ideal for the construction of pavements?
- Waterproof
- Ability to adhere to solid particles such as aggregates
Asphalt Cement can be made temporarily fluid for construction operations by?
- Melting: heating above 140C
- Dissolving in selected petroleum solvents
- Emulsifying or dispersing the asphalt in water (12-15% water)
Types and Applications of Cutback Asphalts?
- Slow Curing: Low viscosity/high penetration used for cold-laid pavement bases (solvent: Diesel)
- Medium Curing: Medium viscosity/medium penetration used for cold-pavement bases (solvent: Kerosene)
- Rapid Curing: High viscosity/low penetration used for mixed in place pavements (solvent: Naphtha)
What emulsifying agents are used to make water and asphalt mixable?
- Soap
- fine clay
What is the chemical composition of Asphalt Cement?
CnH2n+bXd
What is the typical composition of Asphalt Cements?
- Carbon (80-87%)
- Hydrogen (9-11%)
- Oxygen (2-8%)
- Nitrogen (0-1%)
- Sulphur (0.5-7%)
- Trace Metals (0-0.5%)
What are the three phases of asphalt cement?
- Dispersion (Oil) which contributes to its viscosity and fluidity
- Dispersed (Asphaltene) which contributes to its strength and stiffness
- Interfacial (Resin) which contributes to its adhesion and ductility
What is meant by chemically stable structure?
When the three phases exist as continuum with no distinct boundaries between them: Oil act as a solvent for the resins and the resin acts as a solvent for the asphaltenes
What happens when the temperature goes up?
Asphaltenes become more soluble in the resins, which in turn become more soluble in the oils. The material becomes less viscous
What happens when the temperature goes down?
Asphaltenes become less soluble, for micelles, and the material becomes more vicious
What is the glass transition temperature?
The temperature at which the asphalt cement goes from visco-elastic to liquid (Newtonian fluids). Tg = 140C
What happens if the temperature decreases below 140C?
The structure is effectively frozen and the material becomes rigid and brittle.
What kind of relation between temperature and viscosity?
Indirect non-linear relation
What are the methods to evaluate the changes due to aging in asphalt cement?
- Rolling Thin Film Oven procedure
- Provinsional standard PP1
What is asphalt cement graded on?
- Penetration
- Viscosity
- Performance Grading
Which grading method is the most commonly used?
Viscosity
What are the two categories in which viscosity is graded?
- Viscosity on Asphalt Cement (AC)
- Viscosity on Aged Residue (AR)
Describe AC grades
Based on measurements of absolute viscosity of asphalt tested at 60C
Describe AR grades
Based on measurements of absolute viscosity of asphalt tested at 60C after being aged in rolling thin-film oven
What are the various techniques that measure the viscosity of asphalt cements?
- Capillary flow under gravity
- Capillary flow under vacuum
- Rotational coaxial-cylinder viscometers
- Dynamic shear rheometers
Explain Performance grading
Based on the idea that any asphalt cement properties should be related to the conditions under which it is used.
Explain PG XX YY
XX: 7 day maximum pavement temperature, 52 to 70C
YY: minimum expected pavement temperature, -16 to -40C
What are the perfomance grading tests?
- Rolling Thin-Film Oven Test
- Rotational Viscometer
- Pressure Aging Vessel
- Bending Beam Rheometer
The deformational behaviour of asphalt cemets depends substantially on?
- Temperature
- Rate of loading
- Duration of loading
What conditions need to be met to have a Elastic Behaviour?
Low temperature and/or short duration
What conditions need to be met to have an Viscous Behaviour?
High temperature and/or long duration
What is Asphalt Concrete?
A complex material consisting of asphalt cement, mineral aggregates, and possibly some admixtures
What are the types of binder used for asphalt concrete?
Asphalt cement are primarily used, but liquid asphalt (cutbacks and emulsions) are commonly used for certain applications
What are the two types of aggregate gradation?
- Dense graded (or well compacted)
- Open graded
What are the two productions?
- Hot-mixed hot-laid mixtures: mixed, placed and compacted at elevated temperatures.
- Cold-mixed cold-laid mixtures: mixed, placed and compacted at or slighty above ambient temperature using liquid asphalt as the binder.
What are the responses to applied loads?
- Stiffness: Depends on the stiffness of asphalt cement and the volume fraction of coarse aggregates and their geometric properties.
- Stability: Indicates the resistance of asphalt concrete to permanent deformation under traffic loading
- Flexibility: Allows asphalt concrete to adjust without fracturing, to minor long-term settlements of the base or sub-grade layers beneath the pavement structure.
- Tensile Strength: depends on the tensile strength of the binder and the aggregate, the amount of binder in the mixture and voids content of the mixture
- Permanent Deformation: occurs under high shear or normal loads.
What are the different forms of permanent deformation?
- Rutting
- Shoving (or pushing)
- Slippage
- Corrugation
Which case of deformation is the most common?
Rutting
Describe rutting
It consists of channelized depressions in the wheel-path of heavy trucks resulting from densification and lateral movement
Why does rutting occur?
- High asphalt cement content
- High filler content
- Rounded aggregates
- High temperature
- High Loads
What are the effects of moisture in the permeability?
High asphalt cement content, dense aggregate gradations and good compaction result in low permeability and thus good quality
Explain stripping?
Refers to the breaking of the adhesive bond between the aggregate particles and the asphalt cement due to the action of water
How can you decrease stripping?
- Reducing the void content of asphalt concrete
- Avoiding the use of hydrophilic aggregates
- Washing aggregates to remove coatings that hinder the bond
- Pre-coating aggregates with bitumen prior to mixing
- Using higher temperatures in the mixing phase
Response to temperature
- Thermal contraction and expansion of asphalt concrete is largely affected by the asphalt cement
- Under thermal contraction, especially when it is resisted by friction with an underlying layer, tensile stresses can exceed the tensile strength of the mixture causing thermal cracking. Mixtures that have high stiffness at low temperature are most prone to this effect
Response to chemicals
Asphalt cement are highly resistant to most acids, alkalis and salts. However, asphalt cements readily dissolve in petroleum solvent, which could be a problem for pavements in parking areas (spills of fuels and lubricants from vehicles can result in disintegration and failure of asphalt concrete)
