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SUPERPAVE: A Brand New Promising

Method of Asphalt Concrete Design

1 WHY TO USE SUPERPAVE DESIGN METHOD?

SUPERPAVE stands for Superior Performance Asphalt Pavements. It is a new asphalt mix design method

which was proposed by the Strategic Highway Research Program so as to improve the performance and

durability of United State roads, as well as improving the user’s safety. The use of asphalt materials in

pavement applications has widely increased in the last century and as a consequence, so have the design

methods. Three remarkable design methods have been thoroughly applied through the years:

1.

The Hubbard-Field Method: Developed in the decade of 1920, it was created for asphalt mixtures

with 100 percent passing the Sieve number 4 and then modified to include coarse aggregate. The

strength of the mixture was measured by means of a punching-type shear load test.2.

Hveem Mix Design: method created in de decade of 1930. The method of strength measurement

consist on study the ability of the asphalt concrete to resist lateral movement under vertical load

applications. Created by the California Department of Highways Materials and Design Engineering,

it is still used in some western states.

3.

Marshall mix Design: Developed by the Mississippi State Highway Department for designing of

airfield pavements. The main features that this design provides is the density analysis (voids in

the mixture) and the stability/flow test. The Marshall design method was the most used

methodology in United States. It is widely used in several countries as Colombia.

Not only the AASHTO road tests conducted between 1958 and 1962 set the bases to the AASHTO 1993

design method of pavement structural design, they also provided a general view of the asphalt concrete

performance under several weather and load conditions. However, it did not fit every condition and

results were extrapolated.

The Strategic Highway Research Program (SHRP) goal was to provide a system to relate the characteristic

of hot asphalt mix components to the performance and structural behavior of the pavement. Despite of

the fact that asphalt mixtures and their components have been generally tested with empirical procedures,

field testing was necessary to assure that the laboratory analysis implied satisfactory pavement

performance. The Superpave assessment of pavement structures is based in overcome the three main

distresses of the asphalt mixtures in service: rutting, fatigue cracking, and low temperature cracking.

The Superpave system includes a specification for hot asphalt mixtures based on the asphalt binderphysical properties, aggregate tests and specifications, a HMA design and analysis system and a computer

software to integrate its components. However, field measurement is still necessary so as to ensure a

satisfactory performance.

One of the revolutionary features of the Superpave design method is the capability of testing at

temperatures and aging conditions that more adequately represents the service conditions of the exposed

asphalt concrete layer in a pavement. Another feature of the methodology is the concept of Performance


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Grade (PG) of an asphalt binder, which classifies the binder according not only by its characteristics, but

also by its performance under certain expected climatic conditions, pretending to reduce pavement

distresses.

The Superpave methodology of design can be briefly described by means of four processes:

1.

Material selection,2.

Design of the aggregate structure,

3.

Design of the binder content, and

4.

Moisture sensitivity tests.

The Superpave mix design procedure involves a carefully material selection as well as a cautious

volumetric proportioning in order to give a first approach to produce a mixture that perform successfully.

2 IMPROVEMENT OF HMA PERFORMANCE

Improvement of pavement performance not only requires a dense understanding of the mechanistic

behavior of each component that compounds a hot mix asphalt, but also how they behave as a whole mix.

Both the individual properties and the mix properties do affect the pavement performance during the

mixing, construction and service.

2.1

MECHANICAL ASPHALT BEHAVIOR

Asphalt binder is a viscoelastic material. It possesses properties of both viscous and elastic materials. This

exhibited property highly depends on temperature and loading time, this means that the effects of time

and temperature are related: the behavior at high temperatures with short time periods is equivalent to

the behavior at lower temperatures and longer periods of time.

= + = ∗ + , ,

= = ,


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= + , +

, − = 0

+ − = 0

In hot conditions or under sustained loads, the asphalt mixture tends to behave as a viscous material. It

shows the properties of a liquid and flows. The viscosity, which is used to describe the resistance of liquids

to flow, is used as a description of the asphalt binder. When designing a mixture, viscosity defines the

mixing and compacting temperatures.

Viscous liquids are also understood as plastic materials since when they start flowing, they do not return

to their original position and experience a permanent strain. However, it is more correct to say that and

asphalt mixture behaves like a plastic, more than the asphalt binder. The viscous behavior is partially

responsible of permanent rutting.

On the other hand, in cold climates or under rapid loading, the asphalt mixture shows the behavior of an

elastic solid and any strain is completely recovered. Nevertheless, elastic materials can break under high

load applications. Asphalt mixtures can become brittle when exposed to low temperatures. Not only can

they shrink, generating accumulated internal stresses (low temperature cracking), but also they can

experience fatigue cracking under high repeated loads.

Despite of the fact that the previously exposed behavior occurs on extreme temperature conditions, most

environmental conditions lie between hot and cold situations and the asphalt mixture shows the behavior

of both viscous and elastic materials. Owing to its range of temperature-related behavior, asphalt binder

is a very suitable material to use as an adhesive in paving. However, it may be difficult to explain and

understand. The principle that rules the response of the pavement resembles an automobile shockabsorbing system, which consist on a spring and a liquid filled cylinder. The spring (elastic behavior) tries

to return the car to its original position after a bump whereas the liquid in the cylinder dampens the

reaction to it.

The elastic behavior is due to the aggregate and the asphalt. The viscous behavior (plastic behavior) is due

to the asphalt, particularly in warmer conditions. In spite of the fact that most of the response is elastic


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or viscoelastic and can be recovered with time, some of the response turns out to be plastic and non-

recoverable.

2.2

AGING ASPHALT BEHAVIOR

Since asphalt cements are composed from organic molecules of the petroleum, a reaction with oxygen

from the air is likely to occur. Oxidation may change the structure and composition of asphalt molecules,

causing the asphalt mixture to become brittle. This phenomena is known as oxidative hardening or age

hardening.

In practice, a sizeable amount of oxidation occurs before the construction and asphalt placement. During

the placement procedure, the asphalt covers the surface of the aggregate in thin films, expediting the

oxidation. “Short term aging” is used to describe this reaction.

Aging hardening of the mixture can also occur during service when exposed to air and water. Despite of

the fact that this long term aging occurs at a very low rate, it may give way to cracking distresses, especially

in old asphalt cements.

Several types of hardening may be distinguished as well. Volatilization and physical hardening can alter

the asphalt pavement properties. Volatilization occurs when mixing and placing the asphalt cement, in

which the volatile components of the asphalt evaporate. The physical hardening occurs when asphalt is

exposed to low temperatures for long periods. It is more pronounced when temperatures are below 0°C.

BINDER PROPERTY MEASUREMENTS OF THE SUPERPAVE

One of the goals of the Superpave methodology was to be able to describe the asphalt binder with physical

properties that can be directly related to field performance. Since some of the tests that are currently

used to describe the binder does not have a physical meaning and are basically empirical, a result may not

be suitable for predicting the performance of an asphalt cement and experience is required so as to obtain

meaningful information. The penetration and viscosity asphalt specifications used as a mean to describethe asphalt properties can classify different asphalt with the same grading, even when this asphalts have

different performance and temperature-related peculiarities.

The tests of the Superpave design method are listed below. A brief explanation of their purpose will be

described posteriorly.

Superpave Binder Test Purpose

Dynamic Shear Rheometer (DSR)Measure properties at high and

intermediate temperatures

Rotational Viscometer (RV)Measure properties at high

temperatures

Bending Beam Rheometer (BBR) Measure properties at low

temperaturesDirect Tension Tester (DTT)

Rolling Thin Film Oven (RTFO)Simulate hardening characteristics

Pressure Aging Vessel (PAV)


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2.3

MINERAL AGGREGATE BEHAVIOR

Several type of materials can be used as aggregate to produce hot mix asphalt. Some materials are called

natural, bank-run, or pit-run aggregates. It is due to the fact that they are mined from river or glacial

deposits, and generally the do not experiment any further process to create a mixture. On the other hand,

processed aggregate includes several classes of materials. Since natural aggregate which has been sieved

and separated by size, washed and crushed, to treated aggregate to enhance certain performance levelsof the mixture. Certain aggregates that are not mined nor quarried are called synthetic aggregates and

they mostly represent industrial by-products.

It has been seen lately that the use of recycled and waste materials has increased. Existing HMA can be

recycled to produce new pavements. Reclaimed Asphalt Pavements (RAP) is a growing alternative used to

obtain aggregate. The used of waste materials like scrap tires and glass has been increased as well.

Nevertheless, some designers consider that the performance might be sacrificed in order to eliminate a

“waste” aggregate and prefer not to use them.

Aggregate is expected to provide to the asphalt concrete a strong stone skeleton to resist repeated load

applications, disregarding its source and mineralogy. Despite of the fact that rounded, smooth-textureand cubical, rough-texture aggregate could have the same inherent resistance, the second ones provide

a better structure since they tend to lock together instead of slide between particles. Interlocked

aggregate particles provide a strong structure, capable of resist shear stress through the asphalt mixture.

Aggregate shear strength is momentous in HAM.

= + tan

While a mass of aggregate has little or no cohesion at all, the asphalt binder contributes to it holding the

particles together. The shear strength is chiefly dependent on the degree of aggregate interlocking.

Moreover, when loaded, the mass of aggregate transmits higher normal stresses which increases the

resistance of the whole solid skeleton, creating a bulk of aggregate almost as strong as an individual piece.

2.3.1

Mineral Aggregate Property Measurements in Superpave Design Method

It was agreed on the SHRP research program results that the properties of the aggregate play a leading

role in outrival the permanent deformation of the asphalt concrete and the whole pavement structure

whereas the low temperature and fatigue cracking are not abruptly dependent on the aggregate. Two

property categories were defined to be used in the pavement system: Consensus properties and Source


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Properties. Specifications for the aggregate gradation were developed so as to ensure a satisfactory design

aggregate structure.

CONSENSUS PROPERTIES

Several pavement experts reached a consensus about certain aggregate characteristics that are

considered critical for a satisfactory asphalt concrete performance. Their use and threshold values weredefined by expertise, which depend on traffic level and position within the pavement. These properties

are listed below.

1.

Coarse aggregate angularity,

2.

Fine Aggregate angularity,

3.

Flat, elongated particles, and

4.

Clay content.

The required values are stricter when the aggregate is closer to the surface and the traffic levels are higher.

This properties are applied to the design aggregate blend rather than every component by their own.

However, some designers are likely to describe every aggregate component of an aggregate blend.

SOURCE PROPERTIES

A set of source properties was recommended since other characteristics of the aggregate particles were

critical for a satisfactory pavement design, being source specific. These properties are listed below.

1.

Toughness,

2.

Soundness, and

3.

Deleterious materials.

2.3.2 Gradation

The Superpave design method uses a modification of an approach used by some companies. It consist on

graphing the gradation on a 0.45 power chart to define a permissible gradation in which some specific

requirements and properties are defined and delimitate the threshold values for a satisfactory aggregate

blend design. One of the features of the 0.45 power chart is the maximum density gradation, which is a

straight line from the origin to the maximum aggregate size.

The Superpave design method uses a set of sieves as described by the ASTM standards. The following

definitions are used to describe the particle size:

1.

Maximum size of the aggregate: one sieve larger than the nominal maximum size, and

2.

Nominal maximum size: one sieve larger than the first sieve to retain more than 10 percent

The maximum density gradation represents a gradation in which the particles fit together in the densest

possible structure. This arrangement of aggregate is not desirable since the space between particles would

be so little that sufficient thick asphalt films will not be developed so as to ensure a durable mixture.

Control points and restricted zones are defined as well. Control points determine master ranges the

gradation curve must pass through. They are defined for the nominal maximum size, an intermediate size

(2.36 mm or N.8) and the dust size (0.075 mm or N.200). The restricted zone is defined along the maximum

density gradation, between an intermediate size (4.75 or 2.36 mm) and a small size (0.3 mm or N.50).

When a gradation pass through the restricted zone, it is known as a bumped gradation and it indicates a


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mixture that possesses too much fine sand as a percentage of total sand. This gradation generates

tenderer mixtures that are not easily compacted in construction and may reduce the resistance of the

mixture to permanent deformation owing to the fact that the skeleton of the mixture tends to be weak

and the sensitivity to asphalt binder content increases dramatically, which easily becomes plastic.

A satisfactory design aggregate structure that meets the requirement of the Superpave design method

lies between the control points and avoid the restricted zone. The Superpave design method recognizesfive aggregate blends according to their size.

Superpave Mixture

Designation

Nominal

Maximum

Size (in)

Maximum

Size (in)

37.5 mm – 1 ½ in 1 ½ 2

25 mm – 1 in 1 1 ½

19 mm – 3/4 ¾ 1

12.5 mm ½ ¾

9.5 mm 3/8 ½

2.4

ASPHALT MIXTURE BEHAVIOR

When the asphalt pavement is subjected to a wheel load, two principal stresses are transmitted within

the mix structure: vertical compressive stress throughout the asphalt concrete, and horizontal tensile

stress at the bottom of the asphalt layer. The asphalt mixture must be resilient and have enough strength

to resist the compressive stress and guard against permanent deformation. In the same way, the asphalt


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concrete must be strong enough to resist tensile stresses and be resilient so as to withstand several load

applications without fatigue cracking. The capability of resist the temperature-related stresses caused by

extreme environmental conditions is also desirable. In order to understand the asphalt mixture behavior

the three main stresses of the asphalt pavement must be comprehend.

2.4.1 Permanent Deformation

The permanent deformation is a distress characterized by a surface cross section in which the position of

the particles is no longer in the design. The permanent deformation is an accumulation of small amounts

of plastic deformation that occurs each time a load is applied. Wheel path rutting is the most common

distress of permanent deformation. In spite of the fact that rutting may have several provenances (e.g.

HMA weakened by high moisture contents, traffic frequency increments and abrasion), it has two main

sources.

On one hand, the rutting is caused by too much repeated stress applied to the layers below the asphalt

concrete layer. This is a structural problem more than a material quality problem, especially related to

insufficient thickness of the layers or increase of moisture. Thus, the rutting occurs throughout the whole

pavement structure rather than the asphalt concrete layer. On the other hand, the rutting may occur due

to the lack of shear strength of the asphalt mixture, which is caused by a low strength of the asphalt binder

and its interaction with the solid aggregate skeleton and a low internal friction. As a consequence, the

rutting appears in the asphalt layer. This kind of rutting occurs especially when the pavement is exposed

to high temperatures. Not only is it desirable for the asphalt binder to provide enough cohesion to the

aggregate particles, but also to behave more like a stiffer elastic solid when exposed to high pavement

temperatures.

Another way to ensure a satisfactory performance under wheel loads, and specifically the shear strength

is to select an aggregate blend composed by cubical, rough-textured particles. When a good interlocking

is achieved, the aggregate skeleton behaves as a single elastic stone. The asphalt binder causes the

concrete to perform as a rubber band under load application and the permanent deformation might behighly decreased.


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2.4.2

Fatigue Cracking.

When the applied loads overstress the asphalt concrete, it is said that the fatigue cracking occurs. It is

recognized because of the several intermittent longitudinal cracks in the wearing course. This kind of

distress is progressive owing to the fact that the initial cracks joint at some point of the service time and

the generated stresses cause more cracks. When the distress is in an advance state of fatigue it is called

Alligator Cracking, characterized by transverse cracking intersecting with the longitudinal cracks. Severedamaged is observed when a pothole is formed due to dislodge of asphalt pieces.

Fatigue cracking may be caused by several factors taking place simultaneously. They include repeated

heavy loads, insufficient layer thicknesses or weak underlying layers. These abnormalities make the

asphalt pavement susceptible to high deflections which increase the tensile stresses within the asphalt

concrete. Poor hydraulic design or the lack of it, and under designed structures can promote fatigue

cracking as well.

In spite of the fact that the fatigue cracking can be caused by an unsatisfactory design, it is often a sign

that a pavement has been exposed to the design number of loads. In this case, the solution results to be

a planned rehabilitation. Nevertheless, this distress should start to be noticeable at the end of the period

design. If it appears much sooner than expected, it might be caused by an underestimated design number

of traffic loads.


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The implications of the causes exposed previously let see several ways to overcome fatigue damage:

1.

Adequate count of the design heavy loads,

2.

Ensure that the design has an efficient drainage in order that the subgrade is dry,

3.

Use ticker pavements,

4.

Use paving materials that in presence of moisture are not severe weakened, and

5.

Use asphalt concrete that when submitted to load can withdraw normal deflections as well as

resist tensile stresses (resilient materials).

So as to ensure that the asphalt layers performance under tensile stress is satisfactory, it is advisable to

set limits for the stiffness of the asphalt cement since this component is responsible of the HMA tensile

behavior. Soft asphalts have better fatigue properties than hard asphalts and they provide a soft elastic

material behavior to the concrete when the pavement is under loading.

2.4.3 Low Temperature Cracking

When adverse environmental conditions are present during the service period of the pavement, in which

the temperature decreases drastically and jeopardize the structure performance, it is said that low

temperature cracking occurs. In spite of the fact that this kind of distress may occur when rapid loadapplications take place over the pavement and its temperature is too low, it is not necessary for the

pavement to be subjected to loading so as to crack.

Low temperature cracks often form when asphalt concrete shrinks due to cold weather, this generates

tensile stresses throughout the asphalt layer eventually, this tensile stresses exceed the tensile strength

and there is failure of the pavement. Depending of the environmental conditions, the cracking can appear

on the pavement from a single cycle of low temperature.

The asphalt concrete capability to resist tensile stresses due to extremely low temperatures is highly

dependent on the asphalt binder properties. Hard binders are more likely to present low temperature

cracking than soft binders. Aged binders, due to the fact that they are bally susceptible to oxidation andtherefore hardening, are prone to present this kind of distress.

Thereby, so as to overcome low temperature cracking, it is recommendable to select a soft binder that is

not overmuch prone to aging. It is also recommended to set strict controls on the air voids of the asphalt

concrete to diminish the effect of oxidation.

3 DESIGN METHOD FEATURES.

Before the Superpave design method, the Marshall Mix design method was by far the most common

procedure used in the design of asphalt concrete mixtures to be implemented in construction. It is still

used in countries as Colombia and is still taught in several superior studies institutions. The procedure of

this method is pointed to develop a satisfactory and suitable asphalt concrete mixture using stability/flow

and density/voids testing, which are two of the features that give strength to the Marshall Design method.

By means of the entailed process, a control of mixture volumetric properties can be accomplished and

therefore, durable HMA concretes can be achieved. Nevertheless, it is considered that the impact

compaction of the mixture, as part of the procedure, does not reflect nor simulate the executed

densification in the construction process. Moreover, the stability/flow test does not reflect the shear


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strength of the asphalt concrete. Thus, the rutting resistant is difficult to ensure by means of the Marshall

procedure.

Notwithstanding that the Marshall Design method is amply used in several countries, the Hveem method

shares some of its features. Firstly, the density/void analyses can be carried out. Moisture sensitiveness

can be determined as well by means of measuring the swelling. The perks of this method locate in that it

is the laboratory kneading method simulates the compaction conditions during construction.

Nevertheless, part of the volumetric properties that are related to the performance and concrete

durability are not determined. Moreover, it is considered that this design method may be overly subjective

and non-durable mixes are often determined as a design mixture.

3.1 SUPERPAVE ASPHALT CONCRETE DESIGN

The wedges of the Superpave design procedure are the laboratory compaction and tests to determine

mechanical properties. The compaction is executed with a Superpave Gyratory Compactor (SGC), which

keeps similarities with the Texas gyratory compactor. The SGC is used to create test samples and with

data capture during compaction, it is possible to perceive the compactibility of a mixture.

Since the asphalt concrete layer performance are highly influenced by the hot mixing and construction,

the procedure of the Superpave design method incorporates an asphalt short-term aging protocol which

requires a treatment of oven aging during 4 hours at 135°C preceding the compaction.

Outputs from the tests could be useful to produce elaborate predictions of the pavement performance.

Not only will it be possible to predict the performance of an asphalt concrete in terms of traffic loads

(ESAL), but also to predict probable levels of distress occurrence. This allows to the designer to develop a

cost-benefit analysis associated to a design mixture and to take decisions.

Two brand new testing devices were developed: the Superpave Shear Tester (SST) and the Indirect Tensile

Tester (IDT). By means of the tests carried out with the Superpave new equipment, it is possible to find

direct indications of mix mechanical performance, estimating the combined effect of binder, aggregate

and mixture proportions; and to generate inputs for performance prediction models, which include

properties like structure, condition and expected traffic loads. The Superpave design method then

becomes a powerful tool for the pavement designer, capable of link material properties with pavement

structural properties and predict future performance.

3.2

SUPERPAVE BINDER TESTS

3.2.1 Aging Methods

One of the features of the Superpave design method is the capability of developing tests that simulate

essential stages during the asphalt’s life: Firstly, during transport, storage and manipulation, secondly,

during mixing and construction, and thirdly, during service period.

Tests performed under unaged asphalt represent the first stage of the asphalt binder. The second stage

is simulated in a Rolling Thin Film Oven (RTFO) and the procedure can be found on the ASSHTO T-240

(ASTM D 2872). The aims of this tests are to expose thin binder films to heat and air conditions that

simulate mixing and placement.


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The third stage of asphalt aging is simulated by means of a Pressure Aging Vessel (PAV), which exposes

the binder to heat and pressure conditions so as to observe the result of years of service in a matter of

hours. The binder samples tested in the PAV are required to have been tested in the RTFO previously. The

PAV binder remnant reflects the characteristics that an asphalt binder is subjected to during production

and in-service.

ROLLING THIN FILM OVEN (RTFO)

The RTFO procedure is detailed in AASHTO T 240. It requires the use of an electrically heated oven which

consist on a vertical circular carriage with 8 holes to accommodate the sample bottles. It is designed to

rotate while working and to blow air into each bottle by means of an air jet.

The preparation of the binder specimens starts with pouring 35g of asphalt into the bottles, heated

enough to flow but never at 150°C or higher. The content of two bottles shall be used to determine mass

loss. The RTFO oven must be preheated at 163°C for at least 16 hours prior to the test. Bottles are then

loaded in the carriage, which rotates at 15 rev/min. The air flow shall be shot at a rate of 4000 ml/min.

the RTFO aging takes 85 minutes to be completed. The residue is then poured, but not scraped, from the

bottles to a single container and stirred to ensure homogeneity. This material can be used to run a DSRtest or transferred to PAV pans for further aging.

PRESSURE AGING VESSEL (PAV)

There are two kinds of pressure aging devices which can be used indiscriminately. Whereas the first type

consist on a stand-alone PAV placed on a temperature chamber, the second type is built as part of it.

Specific equipment and procedure can be found on AASHTO PP1. The PAV is made from stainless steel

and it operates under extreme temperature and pressure conditions (2070 kPa and either 90°, 100° or

110° C) and shall accommodate a minimum of 10 pans in a rack. The Vessel must be tightly closed. The

specimens are made from the RTFO residues, the binder shall be heated to a temperature that allows to

extend, pour and stir the asphalt. Each sample must weight 50g. Approximately two bottles of RTFO

residues are necessary to obtain one 50-g PAV test sample.

The vessel is placed in the temperature chamber, unpressurized. It is necessary that the vessel reaches

the desired test temperature. When the samples are stacked in the sample rack and the test temperature

has been achieved, the rack is placed in the hot vessel. When the temperature is within 2°C of the

temperature test, pressure shall be applied. After applying the total pressure, the test begins and to be

completed, a period of 20 hours must pass. After slowly releasing the pressure, the pans are removed

from the chamber and placed in an oven at 163°C for 30 minutes so as to remove any entrapped air within

the specimens. The places are then stored for further testing.

ROTATIONAL VISCOMETER (RV)

The rotational viscometer allows to evaluate the binder workability when subjected to high temperatures.

The high temperature viscosity is measured to assure that the fluidity when pumping and mixing is enough.

The Superpave guide strongly recommends to use a Brookfield Apparatus (rotational coaxial cylinder

viscometer, which consist on a motor, spindle, and digital readout) rather than a capillarity viscometer.

The procedure of this test is standardized in AASHTO TP48. This test must be run on unaged asphalt.

According to the Superpave binder specifications, the value must not surpass 3 Pa*s at 135° C. The


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viscosity is determined by the rotational torque required to maintain a constant rotational speed at a

constant temperature of the cylinder shaft while submerged in the binder.

Approximately 30 g of binder are required to develop the procedure of a rotational viscosity test and

typically 11 g are used in the test. The sample shall not be heated above 150° C and it must be stirred in

order to eliminate possible entrapped air in the binder. When the asphalt is in the thermo container,

approximately 15 minutes are required to reach a stable temperature throughout the sample and the

container. The spindle generally is set to rotate at 20 rpm. The thermoset consist on a chamber, a thermo

container, and a temperature controller. When the test temperature is achieved, the spindle is lowered

into the binder. The viscosity value should be measured three times with a minute difference between

them, it must be done when the viscosity showed in the equipment screen stabilizes. The test can be

developed at another temperature if desired. The procedure is mainly the same.

DYNAMIC SHEAR RHEOMETER (DSR)

As exposed previously, the binder shows the behavior of a viscoelastic material. In other words, it

simultaneously presents the behavior of an elastic material (fully recoverable strains) and a viscous

material (non-recoverable strains). The relationship between this apparently two non-related behaviorsis used to study the resistance of the asphalt concrete against several distresses such as fatigue cracking

and rutting. Whereas the resistance against the fatigue cracking in achieved when the binder must be

flexible and elastic, the resistance against rutting requires the asphalt to be stiff and elastic. Therefore,

the balance between stiffness and flexibleness is critical for the good performance of the pavement.

The DSR equipment is used to characterize both behaviors (viscous and elastic) and so as to describe them,

it measures the mechanical response properties of a thin asphalt binder layer, subjected to torsional

torque applied by means of an oscillated and a fixed plate. The procedure that describes the standardized

test is found in AASHTO TP5.

The procedure is quite simple, the asphalt binder specimen is placed between the plates. The oscillating

plate starts from point A and moves to point B. from point B the plate moves back, passing point A on the

way to point C. Afterwards, the oscillation goes back to A, completing one cycle.

As the shear stress is applied to the specimen, the DSR measures the response of the binder in terms shear

strain. Whereas the pure elastic materials have an in-phase response (the strain coincide immediately

with the stress application) and the response time lag is zero, a perfect viscous material presents an out-

of-phase response and there is a time lag between the stress application and the strain occurrence.


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When in service period, the asphalt concrete is exposed to temperature conditions in which its

performance has characteristics of both elastic and viscous materials. The relationship between the

applied shear stress and the mechanical response measured in the DSR test describes the complexbehavior of the asphalt binder. Two fundamental properties of the asphalt binder are measured by means

of this test: the complex shear modulus ∗, and the phase angle ∗ =

The complex shear modulus G* can be understood as the total resistance of a material when subjected to

oscillating shear stress. It consist of an elastic component (horizontal axis) and a viscous component

(vertical axis) as showed below. Meanwhile, the phase angle describes the relative addition ofrecoverably and non-recoverably strain of the asphalt binder. Despite of the fact that G* and are bothfundamental properties of the mechanical response of the binder, they are highly dependent of the

temperature conditions the asphalt is subjected to. Thereupon, knowing the environmental conditions of

the construction site as well as the traffic conditions is critical to the design and specifically, to interpretthe DSR test results.


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In order to calculate the complex shear modulus, the rheometer makes use of the derived equations for

the mechanical response of a pure elastic cylinder subjected to torque along its axis.

= 2

=

ℎ

Therefore:

∗ = 2ℎ Since the properties measured by means of the DSR tests are highly temperature dependent, rheometers

must have a precise temperature controller, accomplished with a circulating air bath or a fluid bath which

normally surrounds the sample with water. In any case, the temperature of the test must not vary more

than 0.1°C. Since the calculations are computed by the rheometer, the operator does not need to worry

about them. Nevertheless, since the radius of the specimen is raised to the fourth power, specimen

trimming must be carefully developed. Specimen height must be carefully controlled as well.

The specimen preparation starts with adjusting the gap with a micrometric wheel between the fixed and

the oscillated plates, before the specimen is mounted on the rheometer at the test temperature. The

thickness of the gap depends on the temperature and the aged condition of the binder. Unaged and RTFO

aged asphalt tested at 46°C or higher require a gap of 1000 µm or 1 mm. PAV aged asphalts, tested in a

range of temperatures from 4° to 40°C need a 2000 µm or 2mm gap. Two plate diameters are used in the

test. Its selection depends on the temperature: higher test temperatures require a 25 mm spindle whereas

intermediate test temperatures use an 8mm spindle. The binder disk can be formed by directly pouring

asphalt onto the plate or by means of a silicone mold.

The Superpave specifications standardize the rotational speed at 10 rad/sec. Shear strain values (or strain

amplitude) vary from one to 12 percent and it mainly depends on the stiffness of the asphalt binderspecimen at the test temperature. Whereas relatively soft materials (e.g., unaged and RTFO aged asphalt)

are tested at strain values of approximately 12 percent, harder materials (e.g., PAV residues tested at

intermediate temperatures) are tested at strain value of one percent.

To begin the test, the rheometer must be calibrated to achieve the specified shear strain and then, to

maintain the stress level precisely along the testing. To ensure that the binder specimen is conditioned to

the torque application, 10 cycles must be applied before data recollection.


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Two properties of the asphalt binder are defined based on the complex shear modulus and the phase

angle: The High Temperature Stiffness (HTS), factor that aims rutting; and the Intermediate Temperature

Stiffness (ITS), factor that addresses fatigue cracking.

= ∗

sin

= ∗ sin BENDING BEAM RHEOMETER (BBR)

The bending beam rheometer test is used to measure the stiffness of the asphalt binder at low

temperatures. The test uses the engineering elastic beam theory to determine the rigidity of a specimen

under application of a creep load which simulates the stresses that build up in the pavement when the

temperature decreases. Two parameter are used to describe the binder beam behavior when subjected

to creep loading: Creep Stiffness and m-Value. Whereas the creep stiffness describe how the asphalt resist

constant loading, the m-value measures how the asphalt stiffness changes when a load is applied. The

procedure of this test is specified in AASHTO TP1.

The BBR equipment mainly consist on a loading frame, a controlled-temperature bath of ethylene glycol,

methanol, and water, and a control and data acquisition software. The load application is developed with

a blunt-nosed shaft, which applies a constant load at the midpoint of the binder beam supported on its

ends. The loading shaft is enclosed by an air bearing so as to eliminate any frictional resistance. A

deflection transducer is installed so as to measure any change in the beam geometry due to loading.

The beam is created by pouring liquid asphalt onto an aluminum mold, which dimensions must be

6.35x12.7x125 mm. After a cooling period between 45 and 60 minutes, the beams can be unmolded, this

procedure must be made only when the testing procedure is going to be developed, but no more than

three hours must pass between the unmold and the beam testing. So as to unmold the specimens, they

must be cooled in a freezer for 5 to 10 minutes. An ice bath for 30 to 45 seconds can be used as well.When the beams are ready, the must be temperature conditioned, which is accomplished by placing the

beams into the test bath for an hour. Then the load application can begin.

The operator must apply a pre-testing load of 3 grams to ensure that the beam is correctly placed on the

supports. A 100 grams seating load is immediately applied to the beam for one second. After this step,

the load must be reduced to the pre-testing load for a 20-seconds recovery period. Then a 100-grams load

is applied on the beam for 240 seconds. The deflection of the beam is recorded during this load application.

The deflection measured during this load application is plotted against time to determine the creep

stiffness and the m-value.

So as to obtain the creep stiffness, the software makes use of the following equation:

=

4ℎ∆ Where is the creep stiffness, is the applied load, is the distance between supports, is the beamwidth, ℎ is the beam thickness, and ∆ is the deflection measured at 60 seconds. This equation istaken from the theoretical deflection of a simply-supported beam when subjected to loading in its


17/44

midpoint. The creep stiffness is desired to be found after two hours of loading. Nevertheless, it has been

found that by raising the temperature 10°C, an equivalent stiffness is found at 60 seconds of loading.

The m-value is calculated automatically by the control software. Nonetheless, it can be measured byplotting the log creep stiffness against log time. The slope of the generated curve is the m-value at any

time.

DIRECT TENSION TESTER (DTT)

The DTT measures the ultimate tensile strain of the asphalt binder under low temperature conditions. The

test can be developed at relatively low temperatures, from -36° to 0°C, in which the asphalt exhibits a

brittle behavior. The tests must be developed with specimens made from RTFO and PAV aged binder so

as to measure the performance of in-service asphalt concrete when subjected to extreme environmental

conditions.

The test is performed on a boned-shaped specimen subjected to tension loading at a constant rate of 1

mm/min. The measured strain at failure is the change in length divided by the original effective length.

The failure of the material is defined as the moment in which the tensile stress reaches its maximum value,

and is equal to the applied load divided by the original cross section of the specimen. The procedure of

the test is standardize in AASHTO TP3.


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Since the stress-strain behavior of the pavement is highly dependent on the temperature conditions, three

tests must be run so as to describe several mechanical responses of the binder: Brittle, Brittle-Ductile, and

Ductile.

DTT specimens are created on a silicone mold, which allows the fabrication of four bone-shaped binder

specimens to produce one test result. The specimens weight approximately 2 grams and are 100 mm long.

The effective length of the specimen is 27mm. the cross section is 6 mm by 6 mm.

After pouring, trimming, and demolding the specimens, they must be tested within 60 minutes. Specimens

are tested individually. A normal test requires less than a minute from load application to failure. A test is

considered to be successful when fracture, when the behavior is brittle, occurs in the center position of

the specimen. The Superpave specification requires a minimum value of ultimate tensile strain of one

percent.

4 S

UPERPAVE

A

SPHALT

B

INDER

S

PECIFICATION

As an intend to improve the performance of asphalt pavements, the Superpave characterizes the asphalt

binder with a complete specification, based on the concept of limiting the potential of the asphalt to


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contribute to rutting, low temperature cracking, and fatigue cracking. The specification provides a full

characterization of the physical properties, measured with the Superpave equipment and tests set. The

binder classification is not only used to relate the physical properties to pavement performance, but to

select an appropriate asphalt binder for specific project conditions as well.

Despite of the fact that the physical properties remain constant for all of the performance grades (PG),

the temperatures at which these properties must be achieved highly vary with the environmental

conditions expected for the project in which the asphalt binder will serve.

The selected notation for the PG classification is exposed below:

− Where XX is the temperature grade that suitably supports the conditions of an environment in which the

average of the seven maximum pavement temperatures are ° C and the minimum environmentalcondition is– ° . As an example, PG 58-22 is highly used in the state of Wisconsin since it covers therequired temperature-related service conditions. A general review chart of the characterization and

physical property requirements is exposed below.


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Several investigation has been furthered in the characterization of binder materials for construction and

available software can be used to provide a guide for the design.

4.1

A

DDRESSING

S

UITABILITY

A

GAINST

A

SPHALT

C

ONCRETE

D

ISTRESSES

.

4.1.1 Permanent Deformation/Rutting.

As exposed before, the asphalt binder shows a mechanical load-related response of both elastic

(recoverable strains) and viscous (non-recoverable strains) materials. It has also been exposed that the

permanent deformation, or rutting, occurs in the pavement when the non-recoverable response of the

asphalt concrete under action of repeated load and high temperatures accumulates over time. Since this

distress often occurs early in the in-service pavement, the tests run on unaged and RTFO aged materials

are suitable for characterize the performance of the asphalt binder against rutting.

Several requirements are addressed by the Superpave guide on a rutting factor, = ∗/sin, whichrepresents the high temperature viscous component of the overall stiffness. The

must be at least

1.00 kPa for an unaged binder and 2.20 kPa or higher for RTFO aged binders. Binders with lower values

may be too soft to have a satisfactory performance against rutting.

Since high stiffness and elastic properties are desirable for an asphalt binder to resist permanent

deformation, high ∗ values and low values are strongly desiderated.


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4.1.2

Fatigue Cracking

∗ and are used in the Superpave design guide to describe and characterize the fatigue failure in asphaltpavements. Fatigue cracking generally occurs under low to moderate temperature conditions and very

often after a service period. The Superpave characterization for fatigue cracking requires tests developed

in RTFO and PAV aged binders. The = ∗ sin is a factor related to the performance of the binderagainst fatigue. The specification places a maximum of 5000 kPa.

Since the ability to easily recover from several load applications and behave as a soft elastic material is

strongly desirable, binders with low values perform better since they are more elastic and improvefatigue properties. Nevertheless, it is possible that the ITS value would be so large that both viscous and

elastic parts would become too high and the asphalt binder would no longer be suitable to effectively

resist fatigue cracking.

4.1.3

Low Temperature Cracking.

When the pavement is subjected to low temperatures it tends to shrink. While at the bottom of the layer

the friction with lower layers prevent the particle movement, tensile stresses build in the upper part of

the asphalt concrete layer. When this stresses overcome the tensile strength of the asphalt mixture, low

temperature cracks appear, which is a very difficult distress to alleviate. The BBR test is used to describe

the performance of the asphalt binder when subjected to creep loading. If creep stiffness is high, the

behavior of the asphalt tends to be brittle and cracking is more likely to occur. So as to prevent and control

it, the Superpave specification stablish a maximum of 300 MPa. A high m-value is desirable since as the

temperature changes and thermal stresses accumulate, the capability of rapidly change the stiffness

means that the built stresses will tend to shed instead of accumulate and cause cracking. The minimum

specified value for the m-value is 0.300.

On the other hand, owing to the fact that shrinkage causes stresses to build in the pavement, and if the

binder is ductile enough, it is possible that the asphalt concrete may perform satisfactory under low

temperature conditions. Studies have shown that when the binder can show tensile strains of more than

one percent under extreme temperature conditions, low temperature cracking is less likely to occur.

Despite of the fact that the DTT is used to describe the response of the asphalt binder under severe

environmental conditions, its analysis is only required when the creep stiffness is between 300 and 600

MPa. Values of creep stiffness below this range do not require further analysis with DTT.

4.2

MISCELLANEOUS SUPERPAVE SPECIFICATION CRITERIA.

Other specifications related with safety control and handling are exposed below. The flash point test,

standardized by the AASHTO T 48, is used to address safety issues. Its minion value is 230° C, this test must

be developed in unaged binders.

To address easy pumping and handling of a hot mix, the Superpave specification requires a maximumviscosity of 3 Pa*s for all grades.

The mass loss calculated by means of the RTFO, under any circumstance, cannot be more than one percent

so as to prevent excessively aging due to volatilization.


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5 ASPHALT BINDER SELECTION.

Asphalts binders, according to the Superpave Specification of “Performance Grade”, must be selected

according to the environmental condition in which they are expected to render service. As exposed below,

the distinction among grades is done by means of the maximum and minimum pavement temperature

binder performance required for the project.

Whereas the general specification shows several classifications, the PG grades are not limited to the given

characterization and extending unlimited in both temperature limits. The PG high and low temperatures

extend as far as required in standardized six-degree increments. The Superpave software, found as Long

Term Pavement Performance (LTPP), assists the designer in selecting binder grades. The LTPP software

offers three main methods in which an asphalt binder can be selected:

1.

By Geographic Area: the LTTP Bind provides an interactive map in which a database from 7920

North American weather stations (US and Canada) can be found. A performance grade map can

be created based on weather and/or policy decisions,

2.

By Pavement Temperature: The LTPP Bind Database allows to find an estimated pavementtemperature by means of a correlated equation which is a function of the air temperature and

the geographic location of the project, and

3.

By Air Temperature: By determining the environmental conditions, it is possible to find the

expected pavement temperature range.

Since weather stations that are less than 20 years are not used, the LTTP Bind software results to be a

reliable method to give a first approach to the asphalt binder selection. In the selection, the software

defines the high pavement design temperature at 20 mm below the pavement surface whereas the low

pavement design temperature is defined at the pavement surface. The software allows to choose any

other depth, which causes the PG to change. Nevertheless, variations are generally unnoticeable.

The Superpave system allows the designer to select a level of reliability as well. The capability of selecting

a reliability level enables a degree of design risk for high and low temperature grades. The reliability is

defined as the probability in a single year that the actual temperature will not surpass the design

temperatures (PG specification of a binder). The binder selection is flexible since the Superpave system

allows to select different reliabilities for high and low design temperatures.

So as to explain the concept, we will consider a design in West Allis Co, WI, which has a mean high air

temperature of 32.3°C and a standard deviation of 1.3°C. In an average year, there is a probability of 50%

that the seven-day maximum air temperature will exceed 32.3°C. However, there is only a 2% probability

that the temperature will exceed 35°C. A design for 35°C would provide a 98% reliability design.


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Starting from the air temperature, observe that the average seven-day maximum air temperature is

32.3°C with a standard deviation of 1.3°C, whereas the average low air temperature is -25.7°C. For a really

cold condition the temperature is -32°C with a standard deviation of 3.6°C.

When converting to pavement temperature, it is important to remember that it is only necessary to do

for high design temperatures. For a wearing course at the top of a pavement section, the pavement

temperatures are -25.7° and 51.4° C for a 50% reliability. For a 98% reliability the design temperatures are

-32° and 54°C.

For a reliability of 50%, the asphalt binder selection must be PG 52-28. For a 98% reliability, the asphalt

binder selection will be PG 58-34. Please note that the selection of an asphalt binder immediately results

in a higher level of reliability due to the rounding up to the next standard grade. Furthermore, it is possible

to select different reliability levels for high and low design temperature.

For high temperature design situations, in which the binder properties related to permanent deformationare critical for a satisfactory performance, the traffic speed has an additional effect on the mechanical

response of the pavement. For slow moving traffic loads, the binder should be selected one grade above

the grade specified by means of temperature analysis so as to offset the effect of slower traffic speed. In

the example, the selected binder when subjected to slow moving loads, should be PG 58-28 or 64-34,

according to the desired reliability. For standing traffic loads, the asphalt binder should be selected two

grades above the original selected binder.


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Furthermore, a shift is included in the Superpave system to select the asphalt binder when subjected to

towering numbers of heavy traffic loads, defined as any design lane traffic that exceeds 10.000.000 ESALS.

When the expected number of ESALs is between 10.000.000 and 30.000.000, it is recommended to

consider the selection of a grade above the original selection based on climate. When the traffic is

expected to be greater than 30.000.000 ESALs, the designer should select one grade higher than the

temperature grade.

6 SUPERPAVE SPECIFICATIONS FOR AGGREGATES

6.1 C

ONSENSUS PROPERTIES

As exposed previously, it was accorded by the SHRP researchers that certain characteristics are critical for

a satisfactory performance of an asphalt concrete when in service. These characteristics, denominated as

consensus properties, were proposed after an agreement of their importance and specified values. The

properties are briefly exposed below.

6.1.1

Coarse Aggregate Angularity:

This property is strongly related with the degree of aggregate internal friction and resistance against

permanent deformation of the pavement. It is defined as the percentage by weight of the coarse

aggregate larger than 4.75 mm that have one or more fractured surfaces. This procedure is standardize in

ASTM D 5821. It mainly consist in manually counting particles to determine fractured surfaces, defined as

any surface that occupies 25% or more of the total surface. The required minimum values for coarse

aggregate angularity are a function of the traffic level and its position within the pavement. The

requirements must be applied to the design aggregate blend.

Superpave Coarse Aggregate Angularity Requirements

Traffic Level

(x106 ESALs)

Minimum percentage

Depth 100 mm

< 0.3 55/- -/-

0.3-1.0 65/- -/-

1.0-3.0 75/- 50/-

3.0-10.0 85/80 60/-

10.0-30.0 95/90 80/75

30.0-100.0 100/100 95/90

>100 100/100 100/100

6.1.2 Fine Aggregate Angularity:

This property is related to the degree of fine aggregate internal friction and resistance against rutting. It

is defined as the percentage of air voids present in loosely compacted aggregates smaller than 2.36 mm.

Higher void contents are related with more fractured faces in the fine aggregate. The test procedure is

standardized in AASTHO T 304. The procedure is developed by pouring a sample of fine aggregate into a

calibrated cylinder ok known volume, flowing through a standard tunnel. The weight of the fine aggregate

in the filled cylinder is measured and void content can be calculated by means of the following equation:


25/44

= 1 −

Where V is the calibrated volume, W is the weight of the uncompacted fine aggregate into the cylinder,

and is the bulk specific gravity. The required values for fine aggregate angularity are a function of thetraffic level and the position of the material within the pavement. The requirements should be applied to

the final blend. Nevertheless, estimates can be made on individual aggregate piles.

Superpave Fine Aggregate Angularity Requirements

Traffic Level

(x106 ESALs)

Minimum percentage

Depth 100 mm

< 0.3 - -

0.3-1.0 40 -

1.0-3.0 40 40

3.0-10.0 45 40

10.0-30.0 45 40

30.0-100.0 45 45

>100 45 45

6.1.3 Flat Elongated Particles

This property concept is the percentage by weight of the coarse aggregates that have a maximum to

minimum dimension ratio equal or greater than five. As exposed before, elongated particles are

undesirable since they are easily breakable when subjected to bending load and tend to fail during

construction and in service. The standardized procedure is found in ASTM D 4791. This test is valid for

coarse aggregate greater than 4.75 mm. The procedure equipment consist on a caliper device to measure

the dimensional ratio of an aggregate sample. Every particle is first placed on its largest dimension

between the swinging arm and a fixed post in position A. Then the swinging arm remains stationary while

the smaller dimension is measured between the arm and a fixed post in position B. If the particle passes

through the gap in position B, it is said to be elongated and it is counted.

The required maximum values of flat, elongated particles are a function of traffic level. The Superpave

specifications are applied to the design aggregate blend. However, this test may be developed for every

single aggregate stockpile.

Superpave Flat, Elongated

Particle Requirements

Traffic level

(x106 ESALs)

Maximum

percentage

1.0 10

6.1.4

Clay content and Sand Equivalent:

Clay content is understood as the percentage of clay material present in the aggregate fraction smaller

than 4.75 mm. the standardized procedure is found in AASHTO T 176. The test briefly consist in placing

fine aggregate in a graduated cylinder with an agitated flocculating solution so as to loose clayey fines.

The flocculating solution causes the clay and silt to be suspended while the bigger particles (sand)


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sediment. The height of suspended clay and sand is measured and then, the sand equivalent is computed

by means of the following equation:

= ℎ ℎ The minimum values for the Sand Equivalent of Superpave specifications for fine aggregate are a functionof traffic level.

Superpave Clay Content Requirements

Traffic level

(x106 ESALs)

Minimum Sand

Equivalent


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6.2.3

Deleterious Materials:

The deleterious materials are contaminants such as shale, wood, mica, coal and any organic material

present in the aggregate blend. It is defined by a weight percentage. The specified procedure is

standardize in AASHTO T 112. It can be developed for fine and coarse aggregate. The tests briefly consist

on wet sieving materials and measuring the material lost as a percent of clay lumps and friable materials.

Values range from 0.2 to 10% according to the composition of the contaminant.

6.3

GRADATION

The Superpave system uses the 0.45 power chart to define satisfactory gradations. Some of the features

associated with the characterization are the maximum density gradation, control points and restricted

area within the chart. The maximum density gradation, as exposed previously, is defined by a straight line

between the origin and the maximum aggregate size. The densest arrangement is undesirable owing to

the fact that very little voids may be developed and the asphalt binder may not behave suitably.

The restricted zone resides along the maximum density gradation. It forms a band in which the gradation

should not pass owing to the fact that it would produce a “humped gradation”, characterized by having

too much fine sand in relation with the total amount of sand. This gradations generally produce tender

mixtures which are difficulty compacted and reduces the resistance to rutting because of a weak

aggregate skeleton that highly depends on the asphalt binder to achieve strength.

An aggregate blend that passes through the control points and avoid the restricted zone meets the

requirements of the Superpave design method.

The Superpave system recommends the gradation to pass bellow the restricted zone, although it is not

necessary. When the project is expected to perform under heavy traffic levels, it is recommended the

gradation to be coarse control points (lower points).

Gradation requirements are exposed below for every Superpave Mixture Designation:


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37.5 mm Nominal Size

Sieve Control PointsRestricted Zone

Boundaries

Standard (mm) Mesh Min. Max. Min. Max.

50 2 100

37.5 1 1/2 90 10025 1 90

19 3/4

12.5 1/2

9.5 3/8

4.75 N. 4 34.7 34.7

2.36 N. 8 15 41 23.3 27.3

1.18 N. 16 15.5 21.5

0.600 N. 30 11.7 15.7

0.300 N. 50 10 10

0.150 N.100

0.075 N. 200 0 6


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25 mm Nominal Size

SieveControl

Points

Restricted Zone

Boundaries


37.5 1 1/2 100

25 1 90 10019 3/4 90

12.5 1/2

9.5 3/8

4.75 N. 4 39.5 39.5

2.36 N. 8 19 45 26.8 30.8

1.18 N. 16 18.1 24.1

0.600 N. 30 13.6 17.6

0.300 N. 50 11.4 11.4

0.150 N.100

0.075 N. 200 1 7


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19 mm Nominal Size

SieveControl

Points

Restricted Zone

Boundaries


25 1 100

19 3/4 90 10012.5 1/2 90

9.5 3/8

4.75 N. 4

2.36 N. 8 23 49 34.6 34.6

1.18 N. 16 22.3 28.3

0.600 N. 30 16.7 20.7

0.300 N. 50 13.7 13.7

0.150 N.100

0.075 N. 200 2 8


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12.5 mm Nominal Size

SieveControl

Points

Restricted Zone

Boundaries


19 3/4 100

12.5 1/2 90 1009.5 3/8 90

4.75 N. 4

2.36 N. 8 28 58 39.1 39.1

1.18 N. 16 25.6 31.6

0.600 N. 30 19.1 23.1

0.300 N. 50 15.5 15.5

0.150 N.100

0.075 N. 200 2 10


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9.5 mm Nominal Size

SieveControl

Points

Restricted Zone

Boundaries


12.5 1/2 100

9.5 3/8 90 1004.75 N. 4 90

2.36 N. 8 32 67 47.2 47.2

1.18 N. 16 31.6 37.6

0.600 N. 30 23.5 27.5

0.300 N. 50 18.7 18.7

0.150 N.100

0.075 N. 200 2 10


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7 SUPERPAVE MIXTURE DESIGN

7.1 ASPHALT MIXTURE TESTS:

7.1.1

Superpave Gyratory Compaction.

Since previous design methodologies, such as Marshall procedure, have some issues with the relation

between the laboratory and construction compaction methods, the SHRP research was aim to produce a

device in which a realistically compaction of trial specimens were developed under climate and traffic

conditions of a real project. The developed equipment is capable of accommodating large aggregates in a

similarly to an in-construction compaction procedure, and measure the compactibility of the mixture so

as to easily determine potentially tender behavior of an asphalt concrete design. Due to the fact that no

compactor was suited to perform under the required features. The Superpave Gyratory Compactor (SGC)

was developed.

The SGC operation is based on two previously developed machines: the Texas Gyratory Compactor and

the French Gyratory Compactor. The first one was taken into account since it is capable of accomplishrealistic specimen densification. SHRP researchers modified the Texas compactor by lowering the angular

offset and adding real time specimen height measurement. The SGC mainly consist on a reaction frame,

a rotating base and a motor, a loading system, a height measurement and recordation system, and a mold

and base plate. The base of the SGC rotates at 30 revolutions per minute as the specimen is compacted

and supports the mold while it occurs. The SCG mold nominal dimensions are 150 mm diameter and a

height of 250 mm minimum. The mold is positioned in an angle of 1.25 degrees. The loading system can

be hydraulic or mechanical. It applies a constant 600 kPa compaction pressure.

The measurement of the height is an extremely important feature in the SGC compaction. Using the mass

of the specimen and the diameter of the mold, an estimate of the density can be developed in any instant

during the compaction process.

So as to normalize the effect of the binder, the specimens are required to be mixed and compacted under

equiviscous temperature conditions. This are standardize as 0.170±0.20 Pa*s for mixing and 0.280±0.30

Pa*s for compaction as determined for the temperature-viscosity behavior determined by means of a

Rotational Viscometer (RV) test. Mixing temperatures of 170° C and higher may indicate that the mixture

is created with a modified asphalt binder. Nevertheless, temperatures above 177°C may lend to

degradation and shall not be used.

The mixing procedure must be prosecuted by means of a mechanical mixer. After mixing, the loose

mixture specimens shall be subjected to four hours of short term aging in a RTFO oven. During this

procedure, the specimens are mean to be spread resulting in a thickness of 21 to 22 kg per square meter

and stirred every hour to ensure homogeneous aging. The compaction molds must be subjected to

temperature treatment at 135° C for at least 45 minutes before compaction.

The Superpave specifications use three different specimen sizes to determine several properties:

1.

For volumetric properties determination, the compacted specimens must be 115±5 mm height. It

requires approximately 4500 grams of asphalt concrete loose mixture. Specimen trimming is not

necessary,


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2.

For performance testing, specimens shall be approximately 135 mm height. This is accomplished

by using around 5500 grams of mixture. The specimens must be trimmed to 50 mm before testing

in the SST or IDT. At least one loose sample is required to determine maximum theoretical specific

gravity, as specified in AASHTO T 209, and

3.

For moisture sensitivity tests, the specimens are fabricated with a height of 95mm, requiring

about 3500 grams of loose asphalt concrete.

7.1.2 Procedure Overview

After short term aging of the loose specimens the compaction can be started. The vertical pressure must

be set to 600±18 kPa and the gyration counter must be zeroed and set to stop when the desired number

of gyrations is achieved. Three gyration numbers are of interest:

1.

Design number of gyrations (),2.

Initial number of gyrations (), and3.

Maximum number of gyrations ().The specimens are compacted until

gyrations, the relationship between the relevant number of

gyrations are exposed below:

log = 1.10log log = 0.45log

The is a function of the climate conditions and the traffic level of the project. A range of values for, , and are listed below:Design ESALs

(x10^6)

Average Design High Air Temperature

< 39° C 39° - 40° C 41° - 42° C 43° - 44° C

N Nde NMx N Nde NMx N Nde NMx N Nde NMx < 0.3 7 68 104 7 74 114 7 78 121 7 82 127

0.3-1.0 7 76 117 7 83 129 7 88 138 8 93 146

1.0-3.0 7 96 134 8 95 150 8 100 158 8 105 167

3.0-10.0 8 96 152 8 106 169 8 113 181 9 119 192

10.0-30.0 8 109 174 9 121 195 9 128 208 9 135 220

30.0-100.0 9 126 204 9 139 228 9 146 240 10 153 253

>100.0 9 142 233 10 158 262 10 165 275 10 172 288

When has been reached, the compactor automatically stops. After releasing the angle and thepressure and after a cooling period, the specimen is extruded from the mold. Bulk Specific gravity is

required to be measured by the Superpave design method. It can be measured according to the standard

procedure in AASHTO T 166.

Recorded data from the SGC procedure is analyzed by computing the maximum theoretical specific gravity

for each desired gyration. The following example illustrates the analysis. After compaction, must bemeasured after the compacted specimen is cooled down. The ratio between and is the % at . So as to calculate the % at any number of gyrations ( ), the % at must bemultiplied by the ratio of heights at and .


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= = 2.489 ℎℎ = = 2.563

Specimen 1

Total Mass 4869 g

N Height (mm) % 8 127 86.5%50 118 93.1%

100 115.2 95.3%

109 114.9 95.6%150 113.6 96.7%

174 113.1 97.1%

This example is for a mix design in which the average percent values of two specimens has been usedfor further analysis. The Superpave specifications require at least the compaction of two samples for

number of gyrations versus percent of

anaylisis.

7.2 A

SPHALT

M

IXTURE

R

EQUIREMENTS

The Superpave design method requires the compacted asphalt mixture to meet certain specified

parameters of the following properties:

1.

Mixture volumetric requirements,

2.

Dust proportion, and


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3.

Moisture susceptibility.

7.2.1 Mixture Volumetric Requirements:

Some of the Superpave specifications for volumetric properties share some features of design methods

used in the past. Properties such as content of air voids, voids in the mineral aggregate, voids filled with

asphalt, and density at

and

must be quantified and compared with assessment values because

they are used as a criteria for binder content selection. In Superpave, the design air void content is four

percent (4%)

= 4.0% VOIDS IN THE MINERAL AGGREGATE

The voids in the mineral aggregate () are defined as the sum of the volume of air voids and the effective(i.e., unabsorbed) binder in an asphalt concrete. It represents the space between aggregate particles. The

aims of an asphalt concrete design are to ensure enough space for the asphalt binder so as to provide

adequate adherence between particles, avoiding bleeding when exposed to high temperatures. Specified

minimum values of the Superpave design method are a function of nominal maximum aggregate size:

Superpave requirementsNominal Maximum

Aggregate size (mm)

Minimum (%)

9.5 15

12.5 14

19 13

25 12

37.5 11

VOIDS FILLED WITH ASPHALT

The voids filled with asphalt ( are defined as the percentage of the volume of voids in the mixture ()that contains asphalt binder (). In other words, it is defined as the volume of effective binder expressedas a percentage of the total voids.

= = − =

− 4 The main effect of S in the asphalt concrete is to limit maximum values of , and as a consequience,limiting the asphalt content levels. The acceptance criteria for S are a function of the traffic level.

Superpave S requirementsTraffic level

(x106 ESALs)Design (%)

< 0.3 70 – 80

0.3-3.0 65 - 78

>3.0 65 – 75

DENSITY REQUIREMENTS FOR COMPACTED SPECIMENS


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The Superpave design system stablish maximum density criteria values of 89 percent for and 98percent for .The compaction characteristic curve for an asphalt mixture allows to perceive the strength of the

aggregate structure as well as the effect of the asphalt binder in the concrete density. At the same asphalt

content, a weaker aggregate skeletons tend to be denser than stronger blends. Nonetheless, their

compaction characteristic curve presents a flatter slope which indicates that their densification occurs

slower. On the other hand, for a same aggregate structure, denser mixtures are achieved as the binder

content increases.

Consequently, the maximum density limit for is aimed to avoid tender asphalt mixtures composed byweak aggregates and high asphalt binders (lower internal friction between particles). Maximum density

limits stablished for are aimed to prevent the design of a mixture that will compact excessively whensubjected to the traffic design, becoming plastic, and leading to permanent deformation. The represents an equivalent denser traffic than expected. By limiting the density at , excessivelycompaction under traffic is less likely to occur.

DUST PROPORTION

The dust proportion is computed as the ratio of percentage by weight of aggregate finer than 0.075 mm

(


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practices, environmental and traffic conditions, and poor drainage design can contribute to stripping as

well.

The procedure is standardized in AASHTO T 283. Despite often fact that this test is not related to pavement

performance, it is used as a criteria to identify moisture susceptible mixtures and to measure the

effectiveness of anti-stripping additives.

The procedure requires the creation of two subsets of specimens with a height of 95 mm and an air void

percentage of 7±1 %. One subset is moistures by vacuum saturation to a constant saturation degree

between 55 and 88 %, followed by an optional freeze cycle. The last step is a hot water soak. After the

treatment, both the conditioned and unconditioned subset of specimens are analyzed by means of an

Indirect Tensile Test. The moisture susceptibility is computed as the strength ratio between the two

conditions, called Tensile Strength Ratio (TSR). The Superpave specifications require a minimum TSR of

80%. Lower values may indicate that the asphalt pavement has a tendency to show stripping.

8 ASPHALT MIXTURE VOLUMETRIC PROPERTIES

Asphalt mixture behavior is extremely influenced by the volumetric proportions of asphalt binder and

aggregate components, called Asphalt Mixture Volumetric and Gravimetric Properties. Several volumetric

properties are strongly related to the mixture’s probable pavement service performance. The analysis of

the volumetric properties of the asphalt plays an important role in several design methodologies, included

Superpave.

8.1 COMPONENT DIAGRAM: VOLUMETRIC AND GRAVIMETRIC PROPERTIES.

When analyzing the properties of HMA, it is useful to rethink the extremely complex interaction between

asphalt concrete components by means of a component diagram so as to individually describe every

component in terms of mass and volume relationships.


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The following values can be found in the component diagram and they all are related directly or indirectly

to each other, all of the gravimetric and volumetric properties can be expressed as a percentage of the

total mass and total volume.

8.1.1 Volumetric Properties

: Total bulk volume of the compacted mix.

: Volume of voids between the mineral aggregate. : Volume of mineral aggregate : Volume of absorbed asphalt. = + . : Volume of air. : Volume of asphalt between aggregate (effective asphalt binder). = + .

8.1.2

Gravimetric Properties:

: Wight of air=0

: Weight of asphalt between aggregate particles (effective asphalt binder) : Total Weight of asphalt. : Weight of aggregate. : Total Weight of mixture. : Weight of absorbed asphalt binder.

As exposed previously, several relations between volumetric and gravimetric properties are used to

describe the mixture composition such as air voids (), voids in the mineral aggregate (), voids filledwith asphalt (), asphalt content (), effective asphalt content (), and absorbed asphalt content(). Furthermore, this relations between mass and volume can be found by means of the differentspecific gravities of the asphalt mixture.

8.2

SPECIFIC GRAVITY

The specific is defined as the ratio of the mass of a unit volume of a material to the mass of the same

volume of water at any determined temperature. It is better understood as the ratio of densities between

a material and water. Since at 25°C the density of the water is 1.000 g/cm3, the specific gravity can be

computed by dividing its mass by its volume since density and specific gravity would be numerically

identical. This property of the materials is fundamentally important for the Superpave system owing to

the fact that by knowing the mass of a material, its volume can be known as well, and vice versa.

8.2.1 Aggregate Specific Gravities:

Aggregate particle structure, and moreover, its interaction with asphalt binder is extremely complex.Mineral aggregate is porous and can absorb water and asphalt. Moreover, the ratio of water to asphalt

binder absorption varies with the aggregate source. The Superpave system uses three main specific

gravities so as to take this variations in consideration and describe the asphalt concrete volumetric and

gravimetric properties. These methods are bulk, apparent, and effective specific gravities.

BULK SPECIFIC GRAVITY


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The bulk specific gravity (), is understood as the ratio of the aggregate mass to the volume of theaggregate, including both permeable and impermeable voids in the aggregate.

= 1 + + = 1 ( )

APPARENT SPECIFIC GRAVITY

The apparent specific gravity (), is understood as the ratio of the aggregate mass to the volume of theaggregate, without including the volume of surface pores (water permeable voids) in the aggregate.

= 1 + = 1 EFFECTIVE SPECIFIC GRAVITY

The effective specific gravity (), is understood as the ratio of the aggregate mass to the volume of theaggregate, including the volume of water permeable voids that cannot be reached by the asphalt binder.


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= 1

+ + −

= 1

8.2.2

Mixture Specific Gravities:

Two measurements of the specific gravity of the HMA are fundamentally important in the determination

of volumetric properties of the asphalt concrete: the maximum theoretical specific gravity (), and thebulk specific gravity ().The maximum specific gravity, , is the ratio of the mass in air to a unit volume of the asphalt andaggregate in the mixture. In other words, it can be computed as the mass of the asphalt and aggregate

components divided by their volumes, not including the air voids.

= + + = −

The bulk specific gravity, , is the ratio of the mass in air to a unit volume of the compacted mixture.In other words, it can be computed as the mass of the asphalt and aggregate components divided by the

volume, including the air voids.

= + + + =

Because of the fact that volume quantities are not easily determined. The mixture quantities must be

firstly determined by weight.

8.3

ANALYZING A COMPACTED PAVING MIXTURE

The Superpave analysis system makes use of two methods to evaluate the volumetric properties of a hot

asphalt mixture. Whereas the first one is based on the analysis of the component diagram, computing by

means of aggregate and mixture specific gravities, the second one uses the same specific gravity

measurement and a set of mathematical relations to directly determine the mixture properties.


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The author recommends to use the diagram component so as to have a clear idea of what is the

interaction between asphalt concrete components in terms of quantities. Nevertheless, the use of

mathematical equations not only ease the laboratory mix and design, but also allows the designer to

observe the variations in certain properties of the mixture.

8.3.1 Mathematical Equations Method.

The following measurements are required by the Superpave system so as to run an air void analysis:

1.

Bulk specific gravity of the coarse aggregate ( ) and the fine aggregate (), (AASHTOT 85 and AASHTO T 84),

2.

Specific gravity of the asphalt cement () and mineral filler (), (AASHTO T 228 and AASHTOT100),

3.

Bulk specific gravity of the aggregate blend (),4.

Maximum specific gravity of the loose asphalt mixture (), (AASHTO D 2041), and5.

Bulk specific gravity of the compacted asphalt mixture (), (AASHTO T 166).The following calculations are required as well:

1.

Effective specific gravity of the aggregate (),2.

Maximum specific gravity for several asphalt contents,

3.

Asphalt absorption of the aggregate (),4.

Effective asphalt content in the mixture,

5.

Voids in the mineral aggregate in the compacted mixture,

6.

Air voids I the compacted mixture, and

7.

Voids filled with asphalt in the compacted mixture.

BULK SPECIFIC GRAVITY:

= + + ⋯ + + + ⋯ +

Where is the bulk specific gravity of the aggregate blend, is the percentage by mass of the differentaggregate components of the final aggregate blend, and is the bulk specific gravity of every aggregatethat compose the final aggregate blend.

EFFECTIVE SPECIFIC GRAVITY:

= −

−

= 1 − 1

−

Where is the effective specific gravity of the aggregate blend, is the maximum specific gravity ofthe loose mixture, is the percentage by mass of total loose mixture (100%), is the percentage bymass of the binder in the mixture, and is the asphalt binder specific gravity.MAXIMUM SPECIFIC GRAVITY OF MIXTURES WITH DIFFERENT ASPHALT CONTENTS


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= +

= 1 +

Where is the maximum specific gravity of the mixture as a function of the asphalt binder content,

is the percentage by mass of total loose mixture (100%),

is the percentage of aggregate content

in the mixture, is the percentage of asphalt binder in the mixture, is the effective specific gravityof the aggregate blend, and is the asphalt binder specific gravity. Notice that + = 1.ASPHALT ABSORPTION

= − Where is the absorbed asphalt, is the asphalt binder specific gravity, is the effective specificgravity of the aggregate blend, and is the bulk specific gravity of the aggregate blend.EFFECTIVE ASPHALT CONTENT OF AN ASPHALT MIXTURE

= − Where is the effective asphalt binder content, is the content of asphalt binder in the mixture, and is the absorbed asphalt.VOIDS IN THE MINERAL AGGREGATE OF A COMPACTED MIXTURE.

= 1 − Where is the void content in the compacted mixture, is the

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