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202 FINAL > asphalt > Flashcards

asphalt Flashcards

1
Q

what is asphalt concrete

A

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

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2
Q

what is asphalt cement

A

a viscous cementitious material composed primarily of bitumen

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3
Q

what is bitumen

A

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

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4
Q

what is viscosity

A

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

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5
Q

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

A
  • 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
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6
Q

what is the typical stiffness of asphalt cement in the field

A

somewhere between the two extrems

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7
Q

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

A

change in viscosity

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8
Q

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

A
  • 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
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9
Q

how can you reduce the viscosity of an asphalt cement

A

by combining it with a liquid solvent of lower viscosity

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10
Q

how does the mechanism of turning into liquid asphalt work

A
  • after placement, the solvent evaporates, leaving only asphalt cement
  • results in gradual increase in viscosity
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11
Q

what are the two types of liquid asphalts

A
  • cutback asphalts
  • emulsified Asphalts
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12
Q

how are cutback asphalts produced

A
  • 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
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13
Q

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

A
  • slow curing: diesel fuel
  • medium curing: kerosene
  • rapid curing: naptha/gasoline
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14
Q

how are emulsified asphalts produced

A
  • 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
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15
Q

what does anionic mean

A
  • negatively charged
  • more compatible with positive surface charges like limestone
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16
Q

what does cationic mean

A
  • positively charged
  • more compatible with negative surface charges like siliceous aggregates
17
Q

what are the two types of asphalt concrete

A
  • open graded
  • dense - graded
18
Q

describe open graded asphalt concrete

A
  • excess asphalt cement
  • asphalt acts as matrix and aggregate as filler
  • viscous or viscoelastic in nature
19
Q

describe dense- graded asphalt concrete

A
  • sufficient asphalt cement
  • aggregates act as a matrix with asphalt cement as filler
  • acts as a solid
20
Q

what is the binder aggregate bonding

A
  • bond formation and strength between asphalt cement and aggregate depends on the pysical and chemical properties of both components
21
Q

what are the asphalt cement properties

A
  • wettability
  • viscosity (temp dependant)
  • composition
  • durability
22
Q

what are the aggregate properties

A
  • surface chemistry
  • surface texture
  • porosity
  • surface condition (cleanliness and dryness)
23
Q

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

A
  • adsorption
  • controlled by electrostatic interaction of polar asphalt cement molecules
24
Q

how does the mechanism of wetting work?

A
  • water molecules are even more polar and thus have lower surface energy
  • thus, water will wet aggregate surface
  • but aggregates must be dry!!
25
Q

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

A
  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

26
Q

what does fatigue failure lead to in asphalt concrete

A
  • leads to longitudinal cracking along wheel paths
  • later stages cause perpendicular cracking also, result = alligator cracking
27
Q

what is rutting

A

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

28
Q

what is shoving/corrugation and slippage caused by

A

caused by shear flow of the mixture or slippage between layers

29
Q

what is the difference between shoving and corrugation

A
  • shoving manifests as an abrupt ‘wave’
  • corrugation takes the form of the ripples or multiple ridges
30
Q

what is stripping of asphalt concrete

A

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

31
Q

what causes contraction cracking

A
  • 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
32
Q

how to potholes form

A
  • 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
33
Q

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

A
  • 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
34
Q

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

A
  • 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
35
Q

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

A
  • 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
36
Q

describe how cutback asphalts work

A
  • 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
37
Q
A

202 FINAL (7 decks)

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