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TRANSCRIPT
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All about gradation
Lecture 2
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Aggregates
"Aggregate" is a collective term for the
mineral materials such as sand, gravel and
crushed stone that are used with an asphalt
binding medium to form compound
materials like hot mix asphalt (HMA).
Aggregate accounts for about 92 to 96
percent by total weight of HMA mixture.
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Aggregates
Aggregate properties of concern in mix design aregenerally physical in nature and can be divided upinto three major categories:
Gradation and sizeParticle size distribution. Physical attributes requirements Properties
associated with physical shape and contaminationmeasurements that can at least partially be
controlled during production. Source properties Properties inherent in the rock
source for the aggregate
http://training.ce.washington.edu/VSL/aggregate_tests/gs/gs_overview.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/aggregate_intro/agg_consensus.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/aggregate_intro/agg_source.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/aggregate_intro/agg_source.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/aggregate_intro/agg_consensus.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/aggregate_intro/agg_consensus.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/gs/gs_overview.htm -
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Physical attributes requirements
Physical attributes requirements requirements are:
Coarse Aggregate Angularity (CAA) Fine Aggregate Angularity (FAA)
Flat & Elongated Particles
Sand Equivalent
http://training.ce.washington.edu/VSL/aggregate_tests/caa/caa_overview.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/faa/faa_overview.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/fe/fe_overview.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/se/se_overview.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/se/se_overview.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/fe/fe_overview.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/faa/faa_overview.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/caa/caa_overview.htm -
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Coarse Aggregate Angularity (CAA)
The coarse aggregate angularity (CAA) test is a
method of determining the angularity ofcoarse
aggregate. Coarse aggregate angularity is important to
ensure better aggregate interlock and prevent
excessive HMA deformation under load (rutting).
Aggregate angularity test conducted by visually
inspecting of a small sample
http://training.ce.washington.edu/VSL/aggregate_tests/caa/caa_overview.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/gs/gs_background.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/gs/gs_background.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/gs/gs_background.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/gs/gs_background.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/caa/caa_overview.htm -
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Aggregate Angularity
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Aggregate Angularity
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Aggregate Angularity
This test is used to help ensure that the
resulting HMA mixture will be resistant to
deformation under repeated loads.
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Fine aggregate angularity (FAA) test
The fine aggregate angularity (FAA) test is
an indirect method of assessing the
angularity offine aggregate. Fineaggregate angularity is important because
an excess of rounded fine aggregate (often
in the form of natural sand) can lead toHMA rutting.
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Fine aggregate angularity (FAA) test
The FAA test estimates fine aggregate
angularity by measuring the loose un-
compacted void content of a fine aggregatesample.
The higher the void content, the higher the
assumed angularity and rougher thesurface.
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FAA test
Calculate the Un-compacted Voids as follows:
U = V(F/G) x 100
V
Where:
U = Un-compacted Voids
V = Volume of Measure
F = Net mass of fine aggregate in the measureG = Bulk Dry Specific Gravity of the Blend of
fine aggregate
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The flat and elongated particle test
The flat and elongated particle test is used to
determine the dimensional ratios for aggregate
particles of specific sieve sizes.
Flat or elongated particles tend to lock up morereadily during compaction making compaction
more difficult. They also have a tendency to
fracture during compaction along their weak,
narrow dimension, which can effectively make
aggregate gradation finer and possibly cause
lower-than-expected HMA.
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The flat and elongated particles
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The flat and elongated particles
This flat and elongated particle test uses a
proportional caliper to help measure
dimensional ratios. The specified ratio isset on the caliper and then about 100
particles are tested for each specified sieve
size.
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Calipers for the flat and elongated particle
test
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Sand equivalent test
The sand equivalent test is a rapid field test toshow the relative proportions of fine dust or clay-like materials in fine aggregate (or granular
soils). The term "sand equivalent" expresses the concept
that most fine aggregates are mixtures ofdesirable coarse particles (e.g., sand) andgenerally undesirable clay or plastic fines anddust. These materials can coat aggregate particlesand prevent proper asphalt binder-aggregate
bonding.
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Sand Equivalent Test Apparatus
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Why Gradation in the Lab???
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Gradationhow much imp???
The particle size distribution or gradation of
aggregate is one of the most influential
characteristics in determining how an HMA
mixture will perform as a pavement material.Aggregate gradation influences almost every
important HMA property including stiffness,
stability, durability, permeability, workability,
fatigue resistance, skid resistance and resistance
to moisture damage
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PROPERTIES CONSIDERED IN MIX
DESIGN
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PROPERTIES CONSIDERED IN MIX DESIGN
Good HMA pavements function well because they aredesigned, produced and placed in such a way as to givethem certain desirable properties. There are several
properties that contribute to the quality of HMApavements.They include
stability,
durability,
impermeability,
workability,
flexibility, and
fatigue resistance
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STABILITY
Stability of a HMA pavement is the abilityof the mixture to resist shoving and rutting
under loads (traffic). A stable pavement
maintains the shape and smoothnessrequired under repeated loading; an
unstable pavement develops ruts
(channels), raveling and other signs ofshifting of the HMA.
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Shoving at a busy intersection
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Ravelingdislodgement of aggregate particles
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Rutting
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STABILITY
The stability of a mix depends on internal friction andcohesion. Internal friction among the aggregate particles(inter-particle friction) is related to aggregate
characteristics such as shape and surface texture.Cohesion results from the bonding ability of the binder.
A proper degree of both internal friction and cohesion inHMA prevents the aggregate particles from being moved
past each other by the forces exerted by traffic.
In general, the more angular the shape of the aggregateparticles and the more rough their surface texture, the
higher the stability of the HMA
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DURABILITY
The durability of a HMA pavement is the abilityof the HMA pavement to resist changes in the
binder oxidation and disintegration of the
aggregate. These factors may be the result of
weather, traffic, or a combination of the two.Generally, durability of a HMA may be enhanced
by three methods. They are:
using maximum binder content,
using a sound aggregate, and
designing and compacting the HMA for
maximum impermeability
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Max binder content & durability
Maximum binder content increases durabilitybecause thick binder films do not age and hardenas rapidly as thin films. Consequently, the binder
retains the original characteristics longer. Also, maximum binder content effectively seals
off a greater percentage of interconnected airvoids in the pavement, making the penetration ofwater and air difficult.
A certain percentage of air voids is required to beleft in the pavement to allow for expansion of the
binder in hot weather.
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Dense gradation & durability
A dense gradation of sound, tough aggregate
contributes to pavement durability by
providing closer contact between aggregateparticles that enhances the impermeability
of the HMA, and resists disintegration
under traffic.
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IMPERMEABILITY
Impermeability is the resistance of a HMA pavementto the passage of air and water into or through themixture. This characteristic is related to the voidcontent of the compacted HMA, and much of thediscussion on voids in the mix design relates to theimpermeability. Even though void content is anindication of the potential for passage of air and waterthrough a pavement, the character of these voids ismore important than the number of voids.
The size of the voids, whether or not the voids areinterconnected, and the access of the voids to thesurface of the pavement all determine the degree of
impermeability.
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WORKABILITY
Workability describes the ease with which a paving HMAmay be placed and compacted. Workability may beimproved by changing mix design parameters, aggregatesources, and/or gradation.
Harsh HMA (HMA containing a high percentage of
coarse aggregate) has a tendency to segregate duringhandling and also may be difficult to compact.
Through the use of trial mixes in the laboratory,additional fine aggregate and perhaps binder may beadded to a harsh HMA to make the mixture moreworkable. Care is required to be taken to ensure that thealtered HMA meets all the other design criteria
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WORKABILITY
Excess fines may also affect workability.
Depending on the characteristics of the
fines, the fines may cause the HMA to
become tough or gummy, making the
mixture difficult to compact
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FLEXIBILITY
Flexibility is the ability of a HMA pavement toadjust to gradual settlements and movements inthe sub-grade without cracking. Since virtuallyall sub-grades either settle (under loading) or rise(from soil expansion), flexibility is a desirablecharacteristic for all HMA pavements.
An open graded HMA with high binder content isgenerally more flexible than a dense graded, low
binder content HMA. Sometimes the need forflexibility conflicts with stability requirements,so much care need to be taken.
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FATIGUE RESISTANCE
Fatigue resistance is the pavement's resistance to repeatedbending under wheel loads (traffic). Air voids (related tobinder content) and binder viscosity have a significant
effect on fatigue resistance. As the percentage of air voids in the pavement increases,
either by design or lack of compaction, pavement fatiguelife (the length of time during which an in-service
pavement is sufficiently fatigue-resistant) is drastically
shortened. Likewise, a pavement containing binder that has aged and
hardened significantly has reduced resistance to fatigue.
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Thickness & Strength of pavement
The thickness and strength characteristics of the
pavement and the supporting strength of the sub-
grade also have an effect on the pavement lifeand prevention of load associated cracking.
Thick, well supported pavements do not bend as
much under loading as thin or poorly supported
pavements. Therefore, thick well supportedpavements have longer fatigue lives.
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Gradationhow much imp???
Theoretically, there exists a particular
gradation that, for a given maximum
aggregate size, will produce the maximumdensity. This gradation would involve a
particle arrangement where successively
smaller particles are packed within thevoids between larger particles
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Smaller particles are packed within the voids
between larger particles
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Three different aspects
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what is the best gradation?
Gradation has a profound effect on material
performance. But what is the best gradation? This
is a complicated question, the answer to which
will vary depending upon the material (HMA)
its desired characteristics,
loading, environmental
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what is the best gradation?
It might be reasonable to believe that the best
gradation is one that produces the maximum
density. This would involve a particle
arrangement where smaller particles are packedbetween the larger particles, which reduces the
void space between particles. This creates more
particle-to-particle contact, which in HMA would
increase stability and reduce water infiltration
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But we require space/voids
However, some minimum amount of void
space is necessary to:
Provide enough volume for the binder(asphalt binder) to occupy.
Promote rapid drainage and resistance to
frost action for base and sub-base courses
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Fuller and Thompson Eq
A widely used equation to describe a maximum
density gradation was developed by Fuller and
Thompson in 1907. Their basic equation is:
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Maximum density curves for 0.45 Power gradation graph
each curve is for a different maximum aggregate size)
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Gradation types
Dense or Well-Graded
Refers to a gradation that is near the FHWAs
0.45 power curve for maximum density. Themost common HMA mix designs tend to use
dense graded aggregate. Typical gradations are
near the 0.45 power curve but not right on it.
Its primary use is as a surface course for locationswith high traffic levels or when the potential for
rutting within the HMA layer exists.
http://training.ce.washington.edu/wsdot/modules/02_pavement_types/02-2_body.htmhttp://training.ce.washington.edu/wsdot/modules/09_pavement_evaluation/09-7_body.htmhttp://training.ce.washington.edu/wsdot/modules/09_pavement_evaluation/09-7_body.htmhttp://training.ce.washington.edu/wsdot/modules/02_pavement_types/02-2_body.htm -
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Dense-graded mix
A dense-graded mix is a well-graded HMA
mixture intended for general use. When
properly designed and constructed, a dense-
graded mix is relatively impermeable.
They can further be classified as eitherfine-
graded or coarse-graded. Fine-graded mixes
have more fine and sand sized particles thancoarse-graded mixes
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Gap gradedMIX
Gap graded. Refers to a gradation that contains
only a small percentage of aggregate particles in
the mid-size range. The curve is flat in the mid-
size range. Some PCC mix designs use gap
graded aggregate to provide a more economical
mix since less sand can be used for a given
workability. HMA gap graded mixes can beprone to segregation during placement.
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A gap-graded HMA
SMA is a gap-graded HMA that is designed tomaximize deformation (rutting) resistance anddurability by using a structural basis of stone-on-
stone contact. Because the aggregates are all in contact, rut resistance
relies on aggregate properties rather than asphalt binderproperties. Since aggregates do not deform as much asasphalt binder under load, this stone-on-stone contact
greatly reduces rutting. SMA is generally moreexpensive than a typical dense-graded HMA because itrequires more durable aggregates, higher asphalt contentand, typically, a modified asphalt binder.
http://training.ce.washington.edu/VSL/aggregate_tests/gs/gs_background.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/gs/gs_background.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/gs/gs_background.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/gs/gs_background.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/gs/gs_background.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/gs/gs_background.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/gs/gs_background.htmhttp://training.ce.washington.edu/VSL/aggregate_tests/gs/gs_background.htm -
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Use
Improved rut resistance and durability.
Therefore, SMA is almost exclusively used
for surface courses on high volume
interstates ROADS
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SMA Gap graded
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Open-graded HMA
An open-graded HMA mixture uses open-
graded aggregate and is designed to be
water permeable (dense-graded and SMAmixes usually are not permeable)
Typically result in smoother surfaces than
dense-graded HMA. Their high air voidstrap road noise and significantly reduce
tire-road noise.
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Dense vs Open graded mix
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(NMAS)
Nominal maximum aggregate size
(NMAS). The largest sieve that retains
some of the aggregate particles butgenerally not more than 10 percent by
weight. We can define nominal maximum
aggregate size as "one sieve size largerthan the first sieve to retain more than 10
percent of the material"
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Gradation Specification
Gradation and size are specified by a series of
gradation control points
Control points give the allowable percent passing
(or retained) range for given sieve sizes. For instance, the gradation control points for a
0.5 inch (12.5 mm) mix specify a maximum of
58% passing and a minimum of 28% passing on
the No. 8 (2.36 mm) sieve.
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HMAVariables
HMA is a rather complex material.
It must resist deformation and cracking, be
durable over time, resist water damage,provide a good tractive surface, and yet be
inexpensive, readily made and easily
placed
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A HMA roadis it real??? Maui
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Qualities of a well-made HMA mix Deformation resistance (stability). HMA
should not distort (rut) or deform (shove) undertraffic loading. HMA deformation is related toone or more of the following:
Aggregate surface and abrasion characteristics.Rounded particles tend to slip by one anothercausing HMA distortion under load while angular
particles interlock with one another providing a
good deformation resistant structure. Brittleparticles cause mix distortion because they tendto break apart under agitation or load.
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Aggregate gradation
Gradations with excessive fines (either
naturally occurring or caused by excessive
abrasion) cause distortion because the largeamount of fine particles tend to push the
larger particles apart and act as lubricating
ball-bearings between these larger particles
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Asphalt binder content
Excess asphalt binder content tends
to lubricate and push aggregate
particles apart making theirrearrangement under load easier.
The optimum asphalt binder content
as determined by mix design shouldprevent this.
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Asphalt binder viscosity
In the hot summer months, asphalt binder
viscosity is at its lowest and the pavement willdeform more easily under load. Specifying an
asphalt binder with a minimum high
temperature viscosity ensures enough high
temperature viscosity.
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Fatigue resistance
HMA should not crack when subjected to repeated loadsover time. HMA fatigue cracking is related to asphalt
binder content and stiffness.
Higher asphalt binder contents will result in a mix that
has a greater tendency to deform elastically (or at leastdeform) rather than fracture under repeated load.
The use of an asphalt binder with a lower stiffness willincrease a mixture's fatigue life by providing greaterflexibility. However, the potential for rutting must also be
considered in the selection of an asphalt binder. Note thatfatigue resistance is also highly dependent upon therelationship between structural layer thickness andloading
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Durability
HMA durability is related to one or more of thefollowing
oAsphalt binder film thickness around each
aggregate particle. If the film thicknesssurrounding the aggregate particles isinsufficient, it is possible that the aggregatemay become accessible to water through holesin the film. Water will displace the asphaltfilm and asphalt-aggregate cohesion will belost. This process is typically referred to asstripping.
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Air voids
Excessive air voids (on the order of 8
percent or more in a dense-graded HMA)increase HMA permeability.
oHMA mix design seeks to adjust items such as
asphalt content and aggregate gradation toproduce design air voids of about 4 percent.
http://training.ce.washington.edu/VSL/mix_design/fundamentals/fundamentals_wv.htmhttp://training.ce.washington.edu/VSL/introduction/intro/pvmt_mixtures.htmhttp://training.ce.washington.edu/VSL/introduction/intro/pvmt_mixtures.htmhttp://training.ce.washington.edu/VSL/introduction/intro/pvmt_mixtures.htmhttp://training.ce.washington.edu/VSL/introduction/intro/pvmt_mixtures.htmhttp://training.ce.washington.edu/VSL/mix_design/fundamentals/fundamentals_wv.htm -
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