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Cive 202 Final > Asphalt > Flashcards

Asphalt Flashcards

1
Q

what is bitumen?

A

Bitumen is a naturally occurring viscous constituent of petroleum. Depending on its exact composition it can be a sticky, black liquid or an apparently solid mass that behaves as a liquid over very large time scales
hydrocarbons!
asphalt = bitumen = binder

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2
Q

describe asphalt cement

A

viscous, cementitious, high molecular weight
very temp sensitive
malleable at high temps
can occur naturally (in venezuela)
used as roofing for flat roofs

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3
Q

describe the viscosity of asphalt cement

A

never becomes truly liquid-y. never flowable enough to cast like portland cement concrete so can’t just put it in to a form
thermoplastic material
even above Tg, high viscosity

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4
Q

how is asphalt different from portland cement in terms of specifications?

A

no specifications. choice of asphalt is based solely on performance. this is because asphalt has super high variability so we rely on performance testing. complex and variable chemical composition

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5
Q

what is age hardening/aging of asphalt cement?

A

asphalt is subjected to heating for variable time periods. this causes substantial changes in the structure and composition of the asphalt molecules.
Oxydation
light hydrocarbon fractions evaporate

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6
Q

how is asphalt’s viscosity affected by time?

A

viscosity of asphalt cement increases with time due to aging or age hardening. this is due to oxidation (exposure to oxygen in environment) and to UV light/degradation

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7
Q

what is consistency and how is it measured?

A

resistance to continuous deformation when subjected to a shearing stress
it is an empirical measure of the resistance offered by a fluid to continuous deformation when under shearing stress. measured using penetration resistance test. depth of a standard needle penetration in a sample under standard conditions. used to measure hardness

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8
Q

what is the flash point test used for?

A

to determine the temp to which an asphalt can be heated before it self ignites. important during manufacturing

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9
Q

what is the ductility test?

A

measures the distance an asphalt cement sample will stretch without breaking.

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10
Q

how is asphalt elastic or plastic under loading circumstances?

A

at low temp or short load durations – elastic behaviour
at high temp or long load durations – behaviour is plastic
High temp - malleable and doesn’t want to go back to original shape, somewhat similar to creep
High variation of asphalt grades
Different roads have different load allowances. roads designed for trucks so car won’t generate damage might just worsen

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11
Q

is asphalt good under tension?

A

no! it’s too deformable to be good under tension. as load duration and temperature increase, strength decreases even more

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12
Q

what are required properties of asphalt?

A

low temp susceptibility
good adhesion and cohesion
good durability
proper viscosity to avoid rutting and shrinkage cracks

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13
Q

why and how do we thin out asphalt?

A

very viscous material so hard to mix
cutback or emulsified

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14
Q

how is cutback asphalt produced?

A

by blending an asphalt cement with a hydrocarbon solvent

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15
Q

what are emulsified aspahlts?

A

Emulsified asphalts are produced by breaking asphalt cement into very fine droplets and dispersing them in a mixture of water and a surface-acting emulsifying agent
can have different ionic charges

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16
Q

what is wettability?

A

how easily a liquid or a fluid covers a surface
Mercury is very low wetting
Something that spreads out over surface is better binding surface - something you want for asphalt cement

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17
Q

what are some properties that affect bonds between asphalt cement and aggregates?

A

Porosity helps - mechanical interlock
Can’t glue anything on dusty surface
Here aggregate has to be DRY unlike portland cement

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18
Q

why do aggregates have to be dry for asphalt?

A

Water is even more polar and creates bigger attraction force and lower surface energy
When aggregate are wet it causes a problem because water getting into asphalt concrete destroys the bond

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19
Q

raveling is caused when asphalt concrete doesn’t bond well with the aggregates

A
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20
Q

what are the layers of asphalt concrete?

A

pavement (asphalt concrete layer) isn’t very thick.
underneath is subbase of crushed rock
don’t want water accumulation
much more aggregates than portland cement concrete.

21
Q

how does asphalt concrete harden?

A

through cooling

22
Q

what are the three ways that asphalt concrete can be classified?
Bagpm

A
  1. type of binder (alone, cutback, or emulsion)
  2. aggregate gradation (dense or open, packed or with void spaces)
  3. production method (hot or cold mixed)
23
Q

difference between dense and open graded?

A

Dense graded is high proportion of aggregate with very little binder between them. larger gradation for better packing
open is gap-graded aggregate and excess asphalt cement – used for pavement
Asphalt cement acts as the matrix and the aggregate as filler
Viscous or viscoelastic in nature
MORE AGGREGATE MORE STIFF

24
Q

How does amount of asphalt cement in concrete affect the mixture?

A

Too high: stability of mixture drops and reduction of aggregate contact
too low: aggregates move under load

25
Q

what is marshall stability test?

A

empirical. measures the max load defined as stable.

26
Q

what is flexibility and why is it important in ashaplt?

A

Ability to deform and to return to shape
Rocks underneath asphalt pavement need to allow for deformation
Subgrade might not want to come back after it deforms
Need to reach compromise between flexibility and stability

27
Q

what kind of asphalt gives greater fatigue resistance?

A

higher asphalt cement concrete and well graded aggregates

28
Q

what is alligator cracking?

A

longitudinal cracking that is so extensive that the pavement starts cracking in other directions as well

29
Q

when is tensile fracture important? 3

A
  1. heavy loads at low temperatures
  2. Weak subgrade beneath pavement layer
  3. tensile stress from temp changes and volume changes
30
Q

what are the four ways of asphalt concrete permanent deformation?

A

crss
corrugation
rutting
shoving
slippage

31
Q

what is rutting?

A

most common form of permanent deformation
Plastic deformation cau`sed by wheels
A wheel load is not a point load, it’s a moving point load - line load
Deformation in material is cased by material trying to relieve the stress caused by wheel
Plastic deformation is caused away by that line - sideways

32
Q

why is Florida the worst place for rutting?

A

high temp makes asphalt more deformable and lots of rain too and when rain accumulates in the ruts so they fix the problem right away
Car could hydroplane if is driving in water
note: a car will not damage a highway because highways are designed for trucks

33
Q

what are shoving and slippage?

A

caused by shear flow of the mixture or slippage between layers
caused by friction due to vehicles stopping.
slippage occurs due to poor bonding between the hot-mix asphalt surface and the underlying layer
usually seen in high speed intersections

34
Q

when do corrugation cracks occur? and what do they look like?

A

ripples/parallel lines
cracks occurs when asphalt is not viscous enough to not crack

35
Q

what is joint reflection cracking?

A

Portland cement concrete and expands and contracts pretty easily so you need to put joints in order to allow room for that to prevent damage from high stresses due to temp
when putting a layer of asphalt over the layer of concrete with the joints, this causes joint reflection cracking for the asphalt

36
Q

what is the solution to joint reflection cracking?

A

more joints. Break everything up into smaller pieces so you don’t have as much strain in just one point

37
Q

does hardened asphalt have low or high permeability?

A

low! also low transport properties. path of least resistance is for water to just run off, especially encouraged by the crown on roads. once cracking occurs, water can get into subbase

38
Q

what is moisture induced damage?

A

Water will destroy bond between asphalt cement and aggregate surface
Aggregates start coming loose and cars pull them further away – stripping

39
Q

how can moisture induced damage be reduced?

A

reducing void content of the concrete
washing aggregates to remove coating
high temp in mixing to drive off water and reduce asphalt cement viscosity

40
Q

wide ranges of temp could cause problems
Difference of coefficients causes stresses at areas of bonding

A

different coefficients of thermal expansion for asphalt cement vs aggregates

41
Q

what is contraction cracking?

A

when asphalt concrete wants to contract due to low temp but its ends are restrained by continuous asphalt layer. restrained shrinkage! stiffness is too high when it’s cold.

42
Q

describe asphalt concrete under chemical attacks

A

highly resistant to acids, alkalis, and salts. does dissolve in petroleum products (think of vehicles leaking gas - bad and concentrated!)

43
Q

when do potholes occur? how?

A

in spring, not winter!
Potholes happen when water gets into subgrade. Pocket of water underneath the asphalt. If it freezes creates a pocket of a frozen solid. In spring it thaws out and that leaves a void which collapses when driven over. once it deforms downwards, the edges degrade easily and potholes worsen

44
Q

when are anti-stripping agents added? why?

A

they repel water so improve bond stability between aggregates and asphalt cement. better to add during actual mixing process than on existing pavements.

45
Q

why are antioxidants added to asphalt?

A

delay aging process and retard the oxidative aging

46
Q

why are rubbers added to asphalt?

A

good way to recycle old tyres!
will increase the viscosity of an asphalt cement, improve its adhesion to the aggregate particles, retard its rate of oxidation, and reduce its temperature susceptibility

47
Q

why are fillers used? limestone 🤪

A

to reduce the amount of asphalt needed
Addition of filler results in a “hardening” of the asphalt cement (increase in viscosity and tensile strength; decrease in ductility)

48
Q

is asphalt recyclable?

A

yes! highly!
Take off old pavement, condition it, and use it again
condition it by milling/grinding it

49
Q

What are the problems with high vs low ductility in asphalt (think graph)

A

low ductility has poor adhesive property and doesn’t bond well. high ductility has high temp susceptibility

Cive 202 Final (5 decks)

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