土木工程材料 課件 Chapter 7 Asphalt and Asphalt Concrete

Materials in Civil

andConstructionEngineering土木工程材料Chapter

7AsphaltandAsphalt Concrete7.1

Introduction7.27.37.47.57.67.7ClassificationAsphalt

CementCutback

AsphaltEmulsified

AsphaltOther

AsphaltAsphalt

ConcreteContent7.1Introduction7.1 IntroductionAsphalt

is

one

of

the

oldest

materials

used

in

construction.The

first

recorded

use

of

asphalt

dates

back

to

3800

BC

in

Mesopotamia

where

the

material

wasused

as

an

adhesive

mortar

for

building

stones

and

paving

blocks.

Asphalt

was

first

used

in

paving

inBabylon

around

625

BC,

in

the

reign

of

King

Naboppolassar.Asphalt,

also

known

as

bitumen

is

a

sticky,

black

and

highly

viscous

liquid

or

semi-solid

form

ofpetroleum.It

may

be

found

in

natural

deposits

or

may

be

a

refined

product.

The

materials

used

by

theseengineers

of

ancient

times

were

naturally

occurring

asphalts.

Such

materials

are

still

used

today,

oftenfor

similar

purposes.

However,

the

vast

bulk

of

today's

bituminous

materials

are

the

products

ofindustrial

refining

processes.7.2Classification7.2 ClassificationBituminousmaterials

areclassified

asasphalts

andtars,as

shownin

Figure7.1.Asphaltisusedmostly

inpavementconstruction,butisalso

used

as

sealing

and

waterproofingagents.Tar

isproduced

by

thedestructive

distillationofbituminouscoalorby

cracking

petroleumvapors.

Taris

used

primarily

for

waterproofingmembranes,

such

as

roofs.

Tarmay

also

beusedforpavementtreatments,

particularly

wherefuelspills

may

dissolveasphaltcement,

such

as

onparkinglotsand

airportaprons.Figure

7.1

Classification

of

bituminous

materials7.3Asphalt

Cement7.3 Asphalt

Cement7.3.1 CompositionAsphalt

cement

is

a

mixture

of

a

wide

variety

of

hydrocarbons

primarily

consisting

of

hydrogen

and

carbonatoms,

with

minor

components

such

as

sulfur,

nitrogen,

oxygen

(heteroatoms),

and

trace

metals.

The

percentagesof

the

chemical

components,

aswell

as

the

molecular

structure

of

asphalt,

vary

depending

on

the

crude

oil

source.Because

asphalt

contains

such

a

large

number

of

molecules

with

differing

chemical

composition

and

structure,complete

chemical

analysis

is

generally

impractical.

In

general,

asphalt

can

be

divided

into

two

broad

chemicalgroups:

asphaltenes

and

maltenes.

The

maltenes

can

be

further

subdivided

into

saturates,

aromatics

and

resins.Chromatographic

techniques

are

the

most

common

methods

to

separate

asphalt

into

four

fractions

on

the

basis

ofthe

size,

chemical

reactivity

and

polarity.

These

four

fractions

are

oftencalled

SARA

fractions

(Saturates,Aromatics.Resins

and

Asphaltenes).

The

characteristics

of

four

fractions

of

asphalt

areshown

in

Table

7.1.Table

4.1

Approximate

oxide

composition

limits

ofComponentColorStateMolecular

weightContributionSaturatesTransparentLiquid--AromaticsPaleyellowLiquid300-500Viscosity

andfluidityResinsYellowSemisolid600-1000AdhesionandductilityAsphaltenesDark

brownSolid-Strength

andstiffness7.3 Asphalt

Cement7.3.2 Colloidal

Structure

of

AsphaltThe

asphaltenes

with

high

molecular

weight

are

wrapped

by

resins,

which

form

micelles

and

are

dispersed

inoilmedia.

This

kind

of

colloidal

system

has

a

uniqueliquidity

and

typical

rheological

properties.Thecolloidal

structurevaries

due

to

different

contentof

each

composition.1.

SolStructureThe

quantities

of

resin/aromatic

fraction

are

adequate

to

make

the

asphaltenes

peptized,

and

the

attractionamong

the

micelles

is

low.

This

kind

of

asphalt

is

characterized

by

the

great

fluidity

but

inferior

viscosity.

Thestraight-run

asphalt

isof

the

sol

structure.7.3 Asphalt

CementGel

StructureIf

the

quantity

of

the

aromatic/resin

fraction

is

insufficient

to

peptize

the

asphaltenes,

the

asphaltenes

canassociate

to

form

large

micelles

or

even

a

continuous

network

throughout

the

asphalt.

This

kind

of

asphalt

ischaracterized

by

the

inferior

fluidity,

plasticity

and

temperature

sensitivity,

but

of

great

viscosity.

The

oxidizedasphalt

has

thegelstructure.Sol-gel

StructureThe

asphalt

contains

appropriate

ground

asphaltenes

and

micelles,

and

the

distances

among

micelles

arerelatively

small,

and

there

is

certain

attraction

among

them,

and

the

structure

between

sol

structure

and

gelstructure

isformed,called

sol-gelstructure.

In

practice,

most

asphalt

isof

intermediate

characteristics.The

structure

of

petroleum

asphalt

depends

not

only

on

the

content

of

the

composition

but

also

on

thetemperature.(a)

sol

structure(b)

gel

structure(c)

sol-gel

structureFigure

7.2

Colloidal

structure

sketch

map7.3 Asphalt

Cement7.3.3 Physical

PropertiesDensityThe

density

of

asphalt

is

the

mass

per

unit

volume

at

the

given

temperature.

Generally,

the

value

is

between0.97-1.01g/cm3.

Thedensity

is

related

to

the

chemical

composition

of

asphalt.StiffnessAt

low

temperatures,

and

in

particular

for

short

duration

of

loading,

asphalt

binder

shows

elastic

properties,deforming

under

load

and

recovering

on

the

removal

of

load.

If

loads

are

applied

for

an

appreciable

time,

however,viscous

flow

develops.Ithas

been

demonstrated

for

asphaltbinderthat

therateof

viscous

flow

decreaseswith

time.This

would

not

be

the

case

for

true

liquids,

and

it

therefore

follows

that

this

apparently

viscous

flow

is

made

up

oftwo

components,

a

true

viscous

flowas

in

liquids,together

with

a

delayed

elastic

deformation.7.3 Asphalt

CementViscosityViscosity

is

the

fundamental

material

property

relating

the

rate

of

shear

stain

in

a

fluid

to

the

applied

shearstress;

in

practicalterms,

itis

theresistance

to

flow.

The

rheological

behaviorof

asphalt

cements

is

verytemperature

dependent,

and

therefore

the

temperature

of

a

material

must

be

stated

when

reporting

its

rheologicalproperties.ThixotropyThe

property

of

asphalt

binder

wherebyit

"sets"

when

not

stirred.

Thixotropy

is

thought

to

result

fromhydrophilic

suspended

particles

that

form

a

lattice

structure

throughout

the

asphalt

binder.

This

causes

the

asphaltto

increase

in

viscosity

and

hardening.

Thixotropic

effects

can

be

somewhat

reversed

by

heat

and

agitation.

Theproperty

ismost

marked

in

blown

or

oxidized

asphalts,

although

it

isexhibited

to

some

extent

by

many

binders7.3 Asphalt

CementSoftening

PointSoftening

point

of

asphalt

is

taken

as

the

temperature

at

which

the

sample

soft

enough

to

allow

the

ball,enveloped

in

the

sample

material,

to

fall

a

distance

of

25

mm.

Asphalt

binder

does

not

change

from

solid

to

liquidat

any

definite

temperature.

It

gradually

become

softer

and

less

viscous

asthetemperature

rises.

Softening

pointisuseful

in

the

classification

of

certain

asphalt

binder.

It

is

indicative

of

the

tendency

of

the

material

to

flow

atelevated

temperaturesencountered

in

service.PenetrationAsphalt

binder

is

classified

bythe

depthto

whicha

standardneedle

will

penetrate

under

specified

testconditions.

This

"pen"

test

classification

is

used

to

indicate

the

hardness

of

asphalt,

lower

penetration

indicating

aharder

asphalt.

Specifications

for

penetration

graded

asphalts

normally

state

the

penetration

range

for

a

grade,

e.g.50/70.7.3 Asphalt

Cement7.

Temperature

SusceptibilityThe

consistency

of

asphalt

is

greatlyaffected

bytemperature.

Asphalt

gets

hard

and

brittle

at

low

temperatures

andsoft

at

high

temperatures.

The

viscosity

of

the

asphalt

decreases

when

the

temperature

increases.

Asphalt'stemperature

susceptibility

can

be

represented

by

the

slope

of

the

line

shown

in

Figure

7.3;

the

steeper

the

slope,

thehigher

the

temperature

susceptibilityof

the

asphalt.

However,

additives

can

be

used

to

reduce

this

susceptibility.Duetotemperature

susceptibility,

thegradeof

theasphaltcementshould

be

selected

according

tothe

climate

ofthearea.

Theviscosity

oftheasphalt

shouldbemostly

within

theoptimum

rangefor

thearea'sannual

temperature

range;

soft-gradeasphalts

areused

for

cold

climatesandhard-gradeasphalts

forhot

climates.Figure

7.3

Typical

relations

between

asphaltviscosity

and

temperature7.3 Asphalt

Cement8.Penetration

IndexThe

penetration

index

represents

a

quantitative

measure

of

the

response

of

bitumen

to

variation

in

temperature.Knowing

the

penetration

index

of

particular

bitumen,

it

is

possible

to

predict

its

behavior

in

an

application.Therefore,

asphalt

binders

with

high

penetration

numbers

(called

"soft")

are

used

for

cold

climates

while

asphaltbinders

with

low

penetration

numbers

(called

"hard")

are

used

for

warm

climates.

All

bitumen

displaysthermoplastic

properties

i.e.

they

become

softer

when

heated

and

harden

when

cooled.

Several

equations

exist

thatdefine

the

way

that

the

viscosity

(or

consistency)

changes

with

temperature.

One

of

the

best

known

is

thatdeveloped

by

Pfeiffer

and

Van

Doormaal

which

states

that:

if

the

logarithm

of

penetration,

P,

is

plotted

againsttemperature,

T,

a

straight

line

is

obtained

suchthat:lgP

=

AT

+

KIn

this

formula:

P

isthe

penetration

at

temperature

T;A

istemperature

susceptibility

(or

temperature

sensitivity);K

is

theconstant.7.3 Asphalt

Cementpenetration

index

(PI)

has

been

defined

for

which

the

temperature

susceptibility

would

assume

a

value

of

zerofor

road

asphalt,

as

given

by:PI

=20(1－25A)/(1+50A)The

value

of

A

(and

PI)

can

be

derived

from

penetration

measurements

at

two

temperatures,

T1and

T2,

usingthe

equation:A=[lgPT1,m,t－lgPT2,m,t]/(T1－T2)Inthisformula:mis

themass

ofthetest

sample;t

isthe

set

timeof

thetest.7.3 Asphalt

Cement7.3.4 Asphalt

Grading

Methods1.Penetration

GradingPenetration

grading's

basic

assumption

is

that

the

less

viscous

the

asphalt,

the

deeper

the

needle

will

penetrate.

Thispenetration

depth

is

empirically

(albeit

only

roughly)

correlated

with

asphalt

binder

performance.

Therefore,

asphaltbinders

with

high

penetration

numbers

(called

"soft")

are

used

for

cold

climates

while

asphalt

binders

with

lowpenetration

numbers

(called

"hard")

are

used

for

warm

climates.

Table

7.2

shows

the

penetration

grading

system

ofasphalt

cement.Table

7.2

Penetration

grading

system

of

asphalt

cementGradePenetration

minPenetration

maxComments40-504050Hardgrade60-706070Typical

gradesusedonpavement85-10085100120-150120150Soft

grade.Used

forcoldclimates200-3002003007.3 Asphalt

Cement2.

Viscosity

GradingViscosity

grading

can

be

done

on

original

(as-supplied)

asphalt

binder

samples

(called

AC

grading)

or

agedresidue

samples

(called

AR

grading).

The

AR

viscosity

test

is

based

on

the

viscosity

of

aged

residue

from

therolling

thin

film

oven

test.

With

AC

grading,

the

asphalt

binder

is

characterized

by

the

properties

it

possessesbefore

it

undergoes

the

HMAmanufacturing

process.

The

AR

grading

system

is

an

attempt

to

simulate

asphaltbinder

properties

after

it

undergoes

a

typical

HMA

manufacturing

process

and

thus,

it

should

be

morerepresentative

of

how

asphalt

binder

behaves

in

HMA

pavements.

Table

7.3

and

Table

7.4

show

standardviscositygrades

for

the

AC

and

AR

gradingsystems(ASTM

D3381).Table

7.2

Penetration

grading

system

of

asphalt

cementTable

7.4

Standard

viscosity

grades

based

on

aged-residue

asphalt(AR)GradingAC-2.5AC-5AC-10AC-20AC-30AC-40AbsoluteViscosity(Poises)250±50500±1001000±2002000±4003000±6004000±800GradingAR-1000AR-2000AR-4000AR-8000AR-16000AbsoluteViscosity(Poises)1000±2502000±5004000±1

0008000±200016000±40007.3 Asphalt

Cement3.SuperpavePerformance

GradingPenetration

grading

and

viscosity

grading

are

somewhat

limited

in

their

ability

to

fully

characterize

asphaltbinder

for

use

in

HMA

pavement.

Therefore,

as

part

of

the

Superpave

research,

new

binder

tests

andspecifications

were

developed

to

more

accurately

and

fully

characterize

asphalt

binders

for

use

in

HMApavements.These

tests

and

specifications

are

specifically

designed

to

address

HMA

pavement

performance

parameterssuch

asrutting,

fatigue

cracking

and

thermal

cracking.Superpave

performance

grading

(PG)

is

based

on

the

idea

that

an

HMA

asphalt

binder's

properties

should

berelated

to

the

conditions

under

which

it

is

used.

For

asphalt

binders,

this

involves

expected

climatic

conditions

aswell

as

aging

considerations.

Therefore,

the

PG

system

uses

a

common

battery

of

tests

(as

the

older

penetrationand

viscosity

grading

systems

do)

but

specifies

that

a

particular

asphalt

binder

must

pass

these

tests

at

specifictemperatures

that

aredependent

upon

the

specificclimatic

conditions

in

thearea

of

use.7.3 Asphalt

CementTable

7.5

shows

how

the

Superpave

PG

system

addresses

specific

penetration,

AC

and

AR

grading

systemgeneral

limitations.Table

7.5

Prior

limitations

vs

Superpave

testing

and

specification

featurescontinueLimitations

ofPenetration,

AC

and

AR

Grading

SystemsSuperpave

Binder

Testing

and

Specification

Features

that

AddressPrior

LimitationsSuperpave

performance

grading

(PG)is

basedon

the

ideathat

an

HMA

asphalt

binder's

properties

should

be

related

tothe

conditions

under

which

itis

used.

For

asphalt

binders,

thisinvolves

expected

climatic

conditions

as

well

as

agingconsiderations.

Therefore,the

PG

system

uses

a

limitations

ofpenetration,

AC

and

AR

grading

systemsSuperpave

bindertesting

andSpecification

features

that

addressprior

limitationsPenetration

and

ductility

tests

are

empirical

and

notdirectlyrelated

toHMA

pavementperformanceThe

physical

properties

measured

are

directly

related

tofieldperformance

byengineering

principlesTests

are

conducted

at

one

standard

temperature

withoutregard

to

the

climate

in

which

theasphalt

binder

will

be

usedTest

criteria

remain

constant,however,

the

temperature

at

which

thecriteria

must

be

met

changes

in

consideration

of

thebinder

grade

selectedfortheprevalent

climatic

conditionsThe

range

of

pavement

temperatures

atanyone

site

is

notadequately

covered.For

example,

there

is

notest

method

forasphaltbinder

stiffness

atlow

temperatures

to

controlthermal

crackingThe

entire

range

of

pavement

temperatures

experienced

at

aparticular

site

is

covered7.3 Asphalt

CementSuperpaveperformancegrading

uses

thefollowing

asphaltbinder

tests:

rollingthinfilm

oven(RTFO),pressure

aging

vessel

(PAV),rotational

viscometer(RV),dynamic

shearrheometer(DSR),

bending

beamrheometer(BBR),direct

tension

tester

(DTT).Limitations

of

penetration,

AC

and

AR

gradingsystemsSuperpave

binder

testing

and

Specification

features

thataddressprior

limitationsTestmethods

only

consider

short-term

asphalt

binderaging

(thin

film

oven

test)

although

long-term

aging

is

asignificant

factor

in

fatiguecracking

and

lowtemperaturecrackingThree

critical

binderages

aresimulated

and

tested:Originalasphaltbinder

priortomixingwith

aggregate.Aged

asphalt

binderafterHMA

production

andconstruction.Long-term

aged

binderAsphalt

binders

canhave

significantly

differentcharacteristics

within

thesame

grading

categoryGrading

ismorepreciseandthereis

less

overlap

betweengradesModified

asphaltbindersarenot

suitedfor

thesegrading

systemsTests

andspecifications

areintended

for

asphalt

"binders"

toinclude

bothmodified

andunmodifiedasphalt

cements7.3 Asphalt

CementRolling

Thin

Film

Oven

(RTFO)The

rolling

thin-film

oven

(RTFO)

procedure

is

used

to

simulate

the

short-term

aging

that

occurs

in

the

asphaltduring

production

of

asphalt

concrete.

In

the

RTFO

method

(ASTM

D2872),

the

asphalt

binder

is

poured

intospecial

bottles.

The

bottles

are

placed

in

a

rack

in

a

forced-draft

oven,

at

a

temperature

of

163°C

(325°F)

for

75min.

The

rack

rotates

vertically,

continuously

exposing

fresh

asphalt.

The

binder

in

the

rotating

bottles

is

alsosubjected

to

an

air

jet

to

speed

up

the

aging

process.

The

performance

grade

specifications

limit

the

amount

ofmass

loss

during

RTFO

conditioning.

Rolling

thin-film

oven

conditioning

is

used

to

prepare

samples

forevaluation

for

rutting

potential

with

the

dynamic

shear

rheometer

and

prior

to

conditioning

with

the

pressureaging

vessel.

Under

the

penetration

and

viscosity

grading

methods,

the

aged

binder

is

usually

tested

forpenetration

or

viscosity

and

theresults

arecompared

with

those

of

new

asphalt.Pressure

Aging

Vessel(PAV)The

pressure

aging

vessel

(PAV)

provides

simulated

long

term

aged

asphalt

binder

for

physical

property

testing.Asphalt

binder

isexposed

to

heat

and

pressureto

simulatein-serviceaging

over

a

7-10

years'

period.

According

toASTM

D6521,

the

basic

PAV

procedure

takes

RTFO

aged

asphalt

binder

samples,

places

them

in

stainless

steelpans

and

then

ages

them

for

20

hours

in

a

heated

vessel

pressurized

to

305

psi

(2.10

MPa

or

20.7

atmospheres).Samplesarethen

stored

for

use

in

physical

property

tests.7.3 Asphalt

CementRotational

Viscometer

(RV)The

rotational

(Brookfield)

viscometer

test

(ASTM

D4402)

consists

of

a

rotational

coaxial

cylinder

viscometerand

a

temperature

control

unit.

The

test

is

used

to

determine

the

viscosity

of

unaged

asphalt

binde