asphalt

Question 1

what is asphalt concrete

Answer 1

a complex material consisting of a mixture of asphalt cement and mineral aggregates

Question 2

what is asphalt cement

Answer 2

a viscous cementitious material composed primarily of bitumen

Question 3

what is bitumen

Answer 3

naturally occurring black or brown sticky substance composed principally of high molecular weight hydrocarbons

Question 4

what is viscosity

Answer 4

a fundamental materials property relating the rate of shear strain in a fluid to the applied shear stress (resistance to flow)

Question 5

what is the stiffness of asphalt cement dependant on? what happens at high/low

Answer 5

• dependent on temp and rate or duration of loading or stress application
• low temp, short load rates = elastic behaviour
• high temp, long load rate = viscous/plastic behaviour

Question 6

what is the typical stiffness of asphalt cement in the field

Answer 6

somewhere between the two extrems

Question 7

what is a change in stiffness due to change in temperature associated with

Answer 7

change in viscosity

Question 8

the greater the rate of change of viscosity with temperature … (asphalt cement)

Answer 8

• the more temp susceptible the asphalt cement becomes
• meaning: more likely to fracture/crack at low temp or deform lots at high temp
• more fragile

Question 9

how can you reduce the viscosity of an asphalt cement

Answer 9

by combining it with a liquid solvent of lower viscosity

Question 10

how does the mechanism of turning into liquid asphalt work

Answer 10

• after placement, the solvent evaporates, leaving only asphalt cement
• results in gradual increase in viscosity

Question 11

what are the two types of liquid asphalts

Answer 11

• cutback asphalts
• emulsified Asphalts

Question 12

how are cutback asphalts produced

Answer 12

• by blending an asphalt cement with a hydrocarbon solvent
• lowers viscosity of asphalt cement allowing the asphalt cement to be mixed with aggregates
• solvent evaporates after placement of asphalt concrete

Question 13

cutback asphalts: what are the three types, differing in the rate at which the material cures or hardens

Answer 13

• slow curing: diesel fuel
• medium curing: kerosene
• rapid curing: naptha/gasoline

Question 14

how are emulsified asphalts produced

Answer 14

• breaking asphalt cement into very fine droplets and dispersing them in a mixture of water and a surface-acting emulsifying agent
• can be anionic, cationic or non-ionic, depends on type of ionic charge which is induced on the droplets

Question 15

what does anionic mean

Answer 15

• negatively charged
• more compatible with positive surface charges like limestone

Question 16

what does cationic mean

Answer 16

• positively charged
• more compatible with negative surface charges like siliceous aggregates

Question 17

what are the two types of asphalt concrete

Answer 17

• open graded
• dense - graded

Question 18

describe open graded asphalt concrete

Answer 18

• excess asphalt cement
• asphalt acts as matrix and aggregate as filler
• viscous or viscoelastic in nature

Question 19

describe dense- graded asphalt concrete

Answer 19

• sufficient asphalt cement
• aggregates act as a matrix with asphalt cement as filler
• acts as a solid

Question 20

what is the binder aggregate bonding

Answer 20

• bond formation and strength between asphalt cement and aggregate depends on the pysical and chemical properties of both components

Question 21

what are the asphalt cement properties

Answer 21

• wettability
• viscosity (temp dependant)
• composition
• durability

Question 22

what are the aggregate properties

Answer 22

• surface chemistry
• surface texture
• porosity
• surface condition (cleanliness and dryness)

Question 23

what is the mechanism driving wetting? and what is it controled by?

Answer 23

• adsorption
• controlled by electrostatic interaction of polar asphalt cement molecules

Question 24

how does the mechanism of wetting work?

Answer 24

• water molecules are even more polar and thus have lower surface energy
• thus, water will wet aggregate surface
• but aggregates must be dry!!

Question 25

failure of the asphalt cement - aggregate bond can occur as a result of

Answer 25

1. abnormal loading conditions or stress concentrations 2. displacement or detachment of the binder from the aggregates surface by water result: raveling: progressive disintegration of the asphalt concrete from surface downward as aggregate particules are dislodged

Question 26

what does fatigue failure lead to in asphalt concrete

Answer 26

- leads to longitudinal cracking along wheel paths - later stages cause perpendicular cracking also, result = alligator cracking

Question 27

what is rutting

Answer 27

channelized depressions in the wheel-path resulting from densification or lateral movement of the mixture - affected by asphalt cement content, filler content, shape of aggregate particles

Question 28

what is shoving/corrugation and slippage caused by

Answer 28

caused by shear flow of the mixture or slippage between layers

Question 29

what is the difference between shoving and corrugation

Answer 29

- shoving manifests as an abrupt 'wave' - corrugation takes the form of the ripples or multiple ridges

Question 30

what is stripping of asphalt concrete

Answer 30

breaking of adhesive bond between aggregate and asphalt cement through the action of water (moisture induced damage)

Question 31

what causes contraction cracking

Answer 31

- consequence of the thermal contraction or shrinkage of an asphalt concrete under freezing conditions - occurs when shrinkage is restrained - at low temp, stiffness of asphalt cement is too high

Question 32

how to potholes form

Answer 32

- water penetrates into subgrade below asphalt pavement - erodes/degrades subgrade material, forming water pockets - when temp drops, asphalt pavement becomes brittle - water pockets expand and freeze, cracking asphalt - vehicle loading causes localized failure over voids

Question 33

why does the industry use performance-based specifications for asphalt cement and asphalt concrete rather than prescription-based specifications?

Answer 33

- asphalt is an inherently very variable material bc it is a by-product of another substance - ex: there can be a 400% variation in range of some material components - thus, instead of prescribing a certain ratio of components (hard to regulate) - easier to set a performance criteria (ex: certain strength must be met) - which can be adjusted by controlling the type of binder, aggregate gradation, and production method

Question 34

why should aggregates be dry when being bound to asphalt cement?

Answer 34

- ability of asphalt cement to coat surface of an aggregate is caused by adsoption, an electrostatic interaction between polar asphalt cement molecules (po and neg end) and the surface of the aggregate (pos or neg charged) - water molecules are even more polar than asphalt cement molecules, so they will preferentially wet the aggregates surface and prevent the asphalt from binding to the aggregate - thus aggregates should be dry when bein bound to asphalt cement

Question 35

describe what asphalt concrete is composed of, what its components are composed of, and what the components of those components are composed of

Answer 35

- asphalt concrete is composed of asphalt cement and mineral aggregates - asphalt cement is composed primarly of bitumen - bitumen is composed of high molecular weight hydrocarbons

Question 36

describe how cutback asphalts work

Answer 36

- they are produced by blending an asphalt cement with a hydrocarbon solvent - lowers viscosity of asphalt cement allowing the asphalt cement to be mixed with aggregates - solvent evaporates after placement of asphalt concrete