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    When aggregate and asphalt binder arecombined to produce a homogenous

    substance, that substance, HMA, takes on

    new physical properties that are related to

    but not identical to the physical properties of its components. Mechanical

    laboratory tests can be used to characterize the basic mixture or predict mixture


    6.1 Mixture Characterization TestsMixture characterization tests are used to describe fundamental mixture parameters

    such as density and asphalt binder content. The three primary mixture

    characterization tests discussed here are:

    Bulk specific gravity

    Theoretical maximum specific gravity

    Asphalt content/gradation

    6.1.1 Bulk Specific Gravity

    Bulk specific gravity is essentially the density of a compacted (laboratory or field)

    HMA specimen. The bulk specific gravity is a critical HMA characteristic because it

    is used to calculate most other HMA parameters including air voids, VMA, and TMD.

    This reliance on bulk specific gravity is because mix design is based on volume,

    which is indirectly determined using mass and specific gravity. Bulk specific gravity

    is calculated as:


    MassGravitySpecific =

    There are several different ways to measure bulk specific gravity, all of which use

    slightly different ways to determine specimen volume:

    1. Water displacement methods. These methods, based on Archimedes

    Principle, calculate specimen volume by weighing the specimen (1) in a

    water bath and (2) out of the water bath. The difference in weights can

    then be used to calculate the weight of water displaced, which can beconverted to a volume using the specific gravity of water.

    Major Topics on this Page

    6.1 Mixture Characterization Tests

    6.2 Performance Tests

    6.3 Summary

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    o Saturated Surface Dry (SSD). The most common method,

    calculates the specimen volume by subtracting the mass of

    the specimen in water from the mass of a saturated surface

    dry (SSD) specimen. SSD is defined as the specimen

    condition when the internal air voids are filled with water and

    the surface (including air voids connected to the surface) isdry. This SSD condition allows for internal air voids to be

    counted as part of the specimen volume and is achieved by

    soaking the specimen in a water bath for 4 minutes then

    removing it and quickly blotting it dry with a damp towel.

    One critical problem with this method is that if a

    specimen's air voids are high, and thus potentially

    interconnected (for dense-graded HMA this occurs at

    about 8 to 10 percent air voids), water quickly drains

    out of them as the specimen is removed from its water

    bath, which results in an erroneously low volume

    measurement and thus an erroneously high bulk

    specific gravity.

    o Paraffin. This method determines volume similarly to the

    water displacement method but uses a melted paraffin wax

    instead of water to fill a specimen's internal air voids (see

    Figure 5.15). Therefore, after the wax sets there is no

    possibility of it draining out and, theoretically, a more

    accurate volume can be calculated. In practice, the paraffin is

    difficult to correctly apply and test results are somewhat


    Figure 5.15: Paraffin Coated Sample

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    o Parafilm. This method wraps the specimen in a thin paraffin

    film (see Figure 5.16) and then weighs the specimen in and

    out of water. Since the specimen is completely wrapped when

    it is submerged, no water can get into it and a more accurate

    volume measurement is theoretically possible. However, in

    practice the paraffin film application is quite difficult and testresults are inconsistent.

    Figure 5.16: Parafilm Application

    o CoreLok. This method calculates specimen volume like the

    parafilm method but uses a vacuum chamber (see Figure5.17) to shrink-wrap the specimen in a high-quality plastic

    bag (see Figure 5.18) rather than cover it in a paraffin film.

    This method has shown some promise in both accuracy and


    Figure 5.17: CoreLok Vacuum

    ChamberFigure 5.18: CoreLok Specimen

    2. Dimensional. This method, the simplest, calculates the volume based on

    height and diameter/width measurements. Although it avoids problems

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    associated with the SSD condition, it is often inaccurate because it

    assumes a perfectly smooth surface thereby ignoring surface

    irregularities (i.e., the

    rough surface texture of

    a typical specimen).

    3. Gamma ray. The

    gamma ray method is

    based on the scattering

    and absorption

    properties of gamma

    rays with matter. When

    a gamma ray source of

    primary energy in the

    Compton range is placed

    near a material, and anenergy selective gamma

    ray detector is used for

    gamma ray counting,

    the scattered and

    unscattered gamma rays with energies in the Compton range can be

    counted exclusively. With proper calibration, the gamma ray count is

    directly converted to the density or bulk specific gravity of the material

    (Troxler, 2001). Figure 5.19 shows the Troxler device.

    The standard bulk specific gravity test is:

    AASHTO T 166: Bulk Specific Gravity of Compacted Bituminous Mixtures

    Using Saturated Surface-Dry Specimens (this is the SSD water

    displacement method discussed previously)

    6.1.2 Theoretical Maximum Specific Gravity

    The theoretical maximum specific gravity (often referred to as theoretical maximum

    density and thus abbreviated TMD) is the HMA density excluding air voids. Thus,theoretically, if all the air voids were eliminated from an HMA sample, the combined

    density of the remaining aggregate and asphalt binder would be the TMD - often

    referred to as Rice density after its inventor. TMD is a critical HMA characteristic

    because it is used to calculate percent air voids in compacted HMA and provide

    target values for HMA compaction.

    TMD is determined by taking a sample of oven-dry HMA in loose condition (versus

    compacted condition), weighing it and then completely submerging it in a 25C

    water bath. A vacuum is then applied for 15 minutes (see Figure 5.20) to remove

    any entrapped air. The sample volume is then calculated by subtracting its mass in

    water from its dry mass. The formula for calculating TMD is:

    Figure 5.19:Gamma Ray Device

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    where: TMD = theoretical maximum density

    A = mass of oven dry sample in air in grams

    C = mass of water displaced by sample at 25C in


    Figure 5.20: Containers Used to Agitate and Draw a Vacuum on Submerged

    TMD Samples

    The standard TMD test is:

    AASHTO T 209 and ASTM D 2041: Theoretical Maximum Specific Gravity

    and Density of Bituminous Paving Mixtures

    6.1.3 Asphalt Binder Content and Gradation

    The asphalt content and gradation test can be used for HMA quality control,

    acceptance or forensic analysis. The three major test methods, solvent extraction,

    nuclear and ignition furnace are discussed here. Each method offers a way to

    determine asphalt content and aggregate gradation from an HMA sample. Solvent Extraction

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    Solvent extraction, the oldest of the three test methods, uses a chemical solvent

    (trichloroethylene, 1,1,1-trichloroethane or methylene chloride) to remove the

    asphalt binder from the aggregate. Typically, a loose HMA sample is weighed and

    then a solvent is added to disintegrate the sample. The asphalt binder/solvent and

    aggregate are then separated using a centrifuge (see Figures 5.21 and 5.22) and

    the aggregate is weighed. The initial and final weights are compared and thedifference is assumed to be the asphalt binder weight. Using this weight and the

    weight of the original sample a percent asphalt binder by weight can be calculated.

    A gradation test can then be run on the aggregate to determine gradation.

    Today, the solvent extraction method is only sparingly used due to the hazardous

    nature of the specified solvents.

    Figure 5.21: Open Centrifuge Used in

    Solvent Extraction

    Figure 5.22: Secondary Centrifuge

    Used in Solvent Extraction

    The standard solvent extraction test is:

    AASHTO T 164 and ASTM D 2172: Quantitative Extraction of Bitumen

    from Bituminous Paving Mixtures Nuclear Asphalt Content Gauge

    A nuclear asphalt content gauge (see Figure 5.23) measures asphalt content by

    estimating the actual number of hydrogen atoms contained within a sample.

    Similar in theory to a nuclear moisture content gauge used in construction, the

    nuclear asphalt content gauge uses a neutron source (such as a 100 Ci specimen

    of Californium-252) to emit high energy, fast neutrons, which then collide with

    various nuclei in the sample. Due to momentum conservation, those neutrons that

    collide with hydrogen nuclei slow down much quicker than those that collide with

    other, larger nuclei. The gauge detector counts only thermal (low energy) or slow

    neutrons thereby making the detector count proportional to the number of

    hydrogen atoms in the sample. Since asphalt is a hydrocarbon, the more hydrogen

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    atoms, the more asphalt. A calibration factor is used to relate thermal neutron

    count to actual asphalt content.

    The nuclear asphalt content gauge offers a relatively quick (4 to 16 minutes

    depending upon desired accuracy) method for measuring asphalt content. Since

    the gauge actually measures hydrogen nuclei and then correlates their number withasphalt content, anything affecting the number of hydrogen nuclei in the sample

    can be a potential source of error. Because water contains a significant amount of

    hydrogen (H2O), anything that adds moisture to the sample (e.g., moisture in the

    aggregate pores) is a potential error source (Black, 1994).

    Figure 5.23: Nuclear Asphalt Content Gauge

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    8/23 Ignition Furnace

    The ignition furnace test, developed by NCAT to replace the solvent extraction

    method, determines asphalt binder content by burning off the asphalt binder of a

    loose HMA sample. Basically, an HMA sample is weighed and then placed in a

    538C (1072F) furnace (see Figure 5.24) and ignited. Once all the asphalt binder

    has burned off (determined by a change in mass of less than 0.01 percent over 3consecutive minutes), the remaining aggregate is weighed. The initial and final

    weights are compared and the difference is assumed to be the asphalt binder

    weight. Using this weight and the weight of the original sample, a percent asphalt

    binder by weight can be calculated. A gradation test can then be run on the

    aggregate to determine gradation.

    A correction factor must be used with the ignition furnace because a certain amount

    of aggregate fines may be burned off during the ignition process. The correction

    factor is determined by placing a sample of known asphalt binder content in the

    furnace and comparing the test result with the known asphalt binder content.

    Based on a limited National Center for Asphalt Technology (NCAT) study (Prowell,

    2002), both traditional and infrared ignition furnaces, if properly calibrated, should

    produce statistically similar asphalt contents and recovered aggregate gradations.

    The standard ignition furnace test is:

    AASHTO T 308: Determining the Asphalt Binder Content of Hot Mix

    Asphalt (HMA) by the Ignition Method

    Figure 5.24: Ignition Furnace

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    6.2 Performance Tests

    Performance tests are used to relate laboratory mix design to actual field

    performance. The Hveem (stabilometer) and Marshall (stability and flow) mix

    design methods use only one or two basic performance tests. Superpave is

    intended to use a better and more fundamental performance test. However,

    performance testing is the one area of Superpave yet to be implemented. The

    performance tests discussed in this section are used by various researchers and

    organizations to supplement existing Hveem and Marshall tests and as a substitute

    for the Superpave performance test until it is finalized. This section focuses on

    laboratory tests; in-place field tests are discussed in Module 9, Pavement


    As with asphalt binder characterization, the challenge in HMA performance testing

    is to develop physical tests that can satisfactorily characterize key HMA

    performance parameters and how these parameters change throughout the life of apavement. These key parameters are:

    Deformation resistance (rutting). A key performance parameter that can

    depend largely on HMA mix design. Therefore, most performance test

    efforts are concentrated on deformation resistance prediction.

    Fatigue life. A key performance parameter that depends more on

    structural design and subgrade support than mix design. Those HMA

    properties that can influence cracking are largely tested for in Superpave

    asphalt binder physical tests. Therefore, there is generally less attention

    paid to developing fatigue life performance tests.

    Tensile strength. Tensile strength can be related to HMA cracking -

    especially at low temperatures. Those HMA properties that can influence

    low temperature cracking are largely tested for in Superpave asphalt

    binder physical tests. Therefore, there is generally less attention paid to

    developing tensile strength performance tests.

    Stiffness. HMA's stress-strain relationship, as characterized by elastic or

    resilient modulus, is an important characteristic. Although the elastic

    modulus of various HMA mix types is rather well-defined, tests can

    determine how elastic and resilient modulus varies with temperature.Also, many deformation resistance tests can also determine elastic or

    resilient modulus.

    Moisture susceptibility. Certain combinations of aggregate and asphalt

    binder can be susceptible to moisture damage. Several deformation

    resistance and tensile strength tests can be used to evaluate the

    moisture susceptibility of a HMA mixture.

    6.2.1 Permanent Deformation (Rutting)

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    Research is ongoing into what type of test can most accurately predict HMA

    pavement deformation (rutting) There methods currently in use can be broadly

    categorized as follows:

    Static creep tests. Apply a static load to a sample and measure how it

    recovers when the load is removed. Although these tests measure aspecimen's permanent deformation, test results generally do not

    correlate will with actual in-service pavement rutting measurements.

    Repeated load tests. Apply a repeated load at a constant frequency to a

    test specimen for many repetitions (often in excess of 1,000) and

    measure the specimen's recoverable strain and permanent deformation.

    Test results correlate with in-service pavement rutting measurements

    better than static creep test results.

    Dynamic modulus tests. Apply a repeated load at varying frequencies to

    a test specimen over a relatively short period of time and measure thespecimen's recoverable strain and permanent deformation. Some

    dynamic modulus tests are also able to measure the lag between the

    peak applied stress and the peak resultant strain, which provides insight

    into a material's viscous properties. Test results correlate reasonably

    well with in-service pavement rutting measurements but the test is

    somewhat involved and difficult to run.

    Empirical tests. Traditional Hveem and Marshall mix design tests. Test

    results can correlate well with in-service pavement rutting

    measurements but these tests do not measure any fundamental material


    Simulative tests. Laboratory wheel-tracking devices. Test results can

    correlate well with in-service pavement rutting measurements but these

    tests do not measure any fundamental material parameter.

    Each test has been used to successfully predict HMA permanent deformation

    characteristics however each test has limitations related to equipment complexity,

    expense, time, variability and relation to fundamental material parameters. Static Creep Tests

    A static creep test (see Figure 5.25) is conducted by applying a static load to an

    HMA specimen and then measuring the specimen's permanent deformation after

    unloading (see Figure 5.26). This observed permanent deformation is then

    correlated with rutting potential. A large amount of permanent deformation would

    correlate to higher rutting potential.

    Creep tests have been widely used in the past because of their relative simplicity

    and availability of equipment. However, static creep test results do not correlatewell with actual in-service pavement rutting (Brown et al., 2001).

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    Figure 5.25: Unconfined Static

    Creep Test

    Figure 5.26: Static Creep Test Plot

    Unconfined Static Creep Test

    The most popular static creep test, the unconfined static creep test (also known as

    the simple creep test or uniaxial creep test), is inexpensive and relatively easy.

    The test consists of a static axial stress of 100 kPa (14.5 psi) being applied to a

    specimen for a period of 1 hour at a temperature of 40C (104F). The applied

    pressure is usually cannot exceed 206.9 kPa (30 psi) and the test temperature

    usually cannot exceed 40C (104F) or the sample may fail prematurely (Brown etal., 2001). Actual pavements are typically exposed to tire pressures of up to 828

    kPa (120 psi) and temperatures in excess of 60C (140F). Thus, the unconfined

    test does not closely simulate field conditions (Brown et al., 2001).

    Confined Static Creep Test

    The confined static creep test (also known as the triaxial creep test) is similar to

    the unconfined static creep test in procedure but uses a confining pressure of about

    138 kPa (20 psi), which allows test conditions to more closely match field

    conditions. Research suggests that the static confined creep test does a better job

    of predicting field performance than the static unconfined creep test (Roberts et al.,


    Diametral Static Creep Test

    A diametral static creep test uses a typical HMA test specimen but turning it on its

    side so that it is loaded in its diametral plane.

    Some standard static creep tests are:

    AASHTO TP 9: Determining the Creep Compliance and Strength of Hot

    Mix Asphalt (HMA) Using the Indirect Tensile Test Device

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    12/23 Repeated Load Tests

    A repeated load test applies a repeated load of fixed magnitude and cycle duration

    to a cylindrical test specimen (see Figure 5.27). The specimen's resilient modulus

    can be calculated using the its horizontal deformation and an assumed Poisson's

    ratio. Cumulative permanent deformation as a function of the number of loadcycles is recorded and can be correlated to rutting potential. Tests can be run at

    different temperatures and varying loads. The load varies is applied in a short

    pulse followed by a rest period. Repeated load tests are similar in concept to the

    triaxial resilient modulus test for unconfined soils and aggregates.

    Repeated load tests correlate better with actual in-service pavement rutting than

    static creep tests (Brown et al., 2001).

    Figure 5.27: Repeated Load Test Schematic

    Note: this example is simplified and shows only 6 load repetitions, normally there are

    conditioning repetitions followed by a series of load repetitions during the test at a

    determined load level and possibly at different temperatures.

    Most often, results from repeated load tests are reported using a cumulative axial

    strain curve like the one shown in Figure 5.28. The flow number (FN) is the load

    cycles number at which tertiary flow begins. Tertiary flow can be differentiated

    from secondary flow by a marked departure from the linear relationship between

    cumulative strain and number of cycles in the secondary zone. It is assumed that

    in tertiary flow, the specimen's volume remains constant. The flow number (FN)

    can be correlated with rutting potential.

    Figure 5.28: Repeated Load Test Results Plot

    Unconfined Repeated Load Test

    The unconfined repeated load test is comparatively more simple to run than the

    unconfined test because it does not involve any confining pressure or associated

    equipment. However, like the unconfined creep test, the allowable test loads are

    significantly less that those experience by in-place pavement.

    Confined Repeated Load Test

    The confined repeated load test is more complex than the unconfined test due to

    the required confining pressure but, like the confined creep test, the confiningpressure allows test loads to be applied that more accurately reflect loads

    experienced by in-place pavements.

    Diametral Repeated Load Test

    A diametral repeated load test uses a typical HMA test specimen but turning it on

    its side so that it is loaded in its diametral plane. Diametral testing has two critical

    shortcomings that hinder its ability to determine permanent deformation

    characteristics (Brown et al., 2001):

    1. The state of stress is non-uniform and strongly dependent on the shapeof the specimen. At high temperature or load, permanent deformation

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    produces changes in the specimen shape that significantly affect both the

    state of stress and the test measurements.

    2. During the test, the only relatively uniform state of stress is tension

    along the vertical diameter of the specimen. All other states of stress are

    distinctly nonuniform.

    Shear Repeated Load Test

    The Superpave shear tester (SST), developed for Superpave, can perform a

    repeated load test in shear. This test, known as the repeated shear at constant

    height (RSCH) test, applies a repeated haversine (inverted cosine offset by half its

    amplitude - a continuous haversine wave would look like a sine wave whose

    negative peak is at zero) shear stress to an axially loaded specimen and records

    axial and shear deformation as well as axial and shear load. RSCH data have been

    shown to have high variability (Brown et al., 2001).

    Some standard repeated load tests are:

    AASHTO TP 7: Determining the Permanent Deformation and Fatigue

    Cracking Characteristics of Hot Mix Asphalt (HMA) Using the Superpave

    Shear Tester (SST) - Procedure F

    AASHTO TP 31: Determining the Resilient Modulus of Bituminous

    Mixtures by Indirect Tension

    ASTM D 4123: Indirect Tension Test for Resilient Modulus of BituminousMixtures Dynamic Modulus Tests

    Dynamic modulus tests apply a repeated axial cyclic load of fixed magnitude and

    cycle duration to a test specimen (see Figure 5.25). Test specimens can be tested

    at different temperatures and three different loading frequencies (commonly 1, 4

    and 16 Hz). The applied load varies and is usually applied in a haversine wave

    (inverted cosine offset by half its amplitude - a continuous haversine wave would

    look like a sine wave whose negative peak is at zero). Figure 5.29 is a schematic of

    a typical dynamic modulus test.

    Figure 5.29: Dynamic Modulus Test Schematic

    Dynamic modulus tests differ from the repeated load tests in their loading cycles

    and frequencies. While repeated load tests apply the same load several thousand

    times at the same frequency, dynamic modulus tests apply a load over a range offrequencies (usually 1, 4 and 16 Hz) for 30 to 45 seconds (Brown et al., 2001).

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    The dynamic modulus test is more difficult to perform than the repeated load test

    since a much more accurate deformation measuring system is necessary.

    The dynamic modulus test measures a specimen's stress-strain relationship under a

    continuous sinusoidal loading. For linear (stress-strain ratio is independent of the

    loading stress applied) viscoelastic materials this relationship is defined by acomplex number called the complex modulus (E*) (Witczak et al., 2002) as seen

    in the equation below:

    where: E* = complex modulus

    |E*| = dynamic modulus

    = phase angle - the angle by which o lags behind o.For a pure elastic material, = 0, and the complex

    modulus (E*) is equal to the absolute value, or

    dynamic modulus. For pure viscous materials, =


    i = imaginary number

    The absolute value of the complex modulus, |E*|, is defined as the dynamic

    modulus and is calculated as follows (Witczak et al., 2002):

    where: |E*| = dynamic modulus

    o = peak stress amplitude

    (applied load / sample cross sectional area)

    o = peak amplitude of recoverable axial strain = L/L.

    Either measured directly with strain gauges or

    calculated from displacements measured with linear

    variable displacement transducers (LVDTs).

    L = gauge length over which the sample deformation is


    L = the recoverable portion of the change in sample

    length due to the applied load

    The dynamic modulus test can be advantageous because it can measure also

    measure a specimen's phase angle (), which is the lag between peak stress andpeak recoverable strain. The complex modulus, E*, is actually the summation of

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    two components: (1) the storage or elastic modulus component and (2) the loss or

    viscous modulus. It is an indicator of the viscous properties of the material being


    Unconfined Dynamic Modulus Test

    The unconfined dynamic modulus test is performed by applying an axial haversineload to a cylindrical test specimen. Although the recommend specimen size for the

    test is 100 mm (4 inch) in diameter by 200 mm (8 inches) high, it may be possible

    to use smaller specimen heights with success (Brown et al., 2001). Unconfined

    dynamic modulus tests do not permit the determination of phase angle ().

    Confined Dynamic Modulus Test

    The confined dynamic modulus test is basically the unconfined test with an applied

    lateral confining pressure. Confined dynamic modulus tests allow for the

    determination of phase angle (). Although the recommend specimen size for the

    dynamic modulus test is 100 mm (4 inch) in diameter by 200 mm (8 inches) high,

    it may be possible to use smaller specimen heights with success (Brown et al.,

    2001). Figures 5.30 and 5.31 show a prototype Superpave Simple Performance

    Test (SPT). The SPT will provide a performance test for the Superpave mix design


    Figure 5.30: A Prototype

    Superpave Simple Performance

    Test (SPT)

    Figure 5.31: The SPT is a Confined

    Dynamic Modulus Test

    Shear Dynamic Modulus Test

    The shear dynamic modulus test is known as the frequency sweep at constant

    height (FSCH) test. Shear dynamic modulus equations are the same as those

    discussed above although traditionally the term E* is replace by G* to denote shear

    dynamic modulus and o and o are replaced by 0 and 0 to denote shear stress

    and axial strain respectively. The shear dynamic modulus can be accomplished by

    two different testing apparatuses:

    1. Superpave shear tester (SST). The SST FSCH test is a is a constant

    strain test (as opposed to a constant stress test). Test specimens are

    150 mm (6 inches) in diameter and 50 mm (2 inches) tall (see Figure

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    5.32). To conduct the test the HMA sample is essentially glued to two

    plates (see Figures 5.33 through 5.35) and then inserted into the SST.

    Horizontal strain is applied at a range of frequencies (from 10 to 0.1 Hz)

    using a haversine loading pattern, while the specimen height is

    maintained constant by compressing or pulling it vertically as required.

    The SST produces a constant strain of about 100 microstrain (Witczak etal., 2002). The SST is quite expensive and requires a highly trained

    operator to run thus making it impractical for field use and necessitating

    further development.

    2. Field shear tester (FST). The FST FSCH test is a is a constant stress test

    (as opposed to a constant strain test). The FST is a derivation of the

    SST and is meant to be less expensive and easier to use. For instance,

    rather than compressing or pulling the sample to maintain a constant

    height like the SST, the FST maintains constant specimen height using

    rigid spacers attached to the specimen ends. Further, the FST shears the

    specimen in the diametral plane.

    Figure 5.32: Superpave Shear Tester

    (SST)Figure 5.33: Loading Chamber

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    Figure 5.34: Prepared Sample Figure 5.35: Prepared Sample (left)

    and Sample After Test (middle and


    Standard complex modulus tests are:

    Unconfined dynamic modulus. ASTM D 3497: Dynamic Modulus of

    Asphalt Mixtures

    Shear dynamic modulus. AASHTO TP 7: Determining the Permanent

    Deformation and Fatigue Cracking Characteristics of Hot Mix Asphalt

    (HMA) Using the Simple Shear Test (SST) Device, Procedure E -

    Frequency Sweep Test at Constant Height. Empirical Tests

    The Hveem stabilometer and cohesiometer and Marshall stability and flow tests are

    empirical tests used to quantify an HMA's potential for permanent deformation.

    They are discussed in their mix design sections. Simulative Tests - Laboratory Wheel-Tracking Devices

    Laboratory wheel-tracking devices (see Video 5.1) measure rutting by rolling a

    small loaded wheel device repeatedly across a prepared HMA specimen. Rutting in

    the test specimen is then correlated to actual in-service pavement rutting.

    Laboratory wheel-tracking devices can also be used to make moisture susceptibility

    and stripping predictions by comparing dry and wet test results Some of these

    devices are relatively new and some have been used for upwards of 15 years like

    the Laboratoire Central des Ponts et Chauses (LCPC) wheel tracker - also known

    as the French Rutting Tester (FRT). Cooley et al. (2000) reviewed U.S. loaded

    wheel testers and found:

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    Results obtained from the wheel tracking devices correlate reasonably

    well to actual field performance when the in-service loading and

    environmental conditions of that location are considered.

    Wheel tracking devices can reasonably differentiate between binder

    performance grades.

    Wheel tracking devices, when properly correlated to a specific sites

    traffic and environmental conditions, have the potential to allow the user

    agency the option of a pass/fail or go/no go criteria. The ability of the

    wheel tracking devices to adequately predict the magnitude of the

    rutting for a particular pavement has not been determined at this time.

    A device with the capability of conducting wheel-tracking tests in both air

    and in a submerged state, will offer the user agency the most options of

    evaluating their materials.

    In other words, wheel tracking devices have potential for rut and other

    measurements but the individual user must be careful to establish laboratory

    conditions (e.g., load, number of wheel passes, temperature) that produce

    consistent and accurate correlations with field performance.

    Video 5.1: Asphalt Pavement Analyzer - A Wheel Tracking Device

    6.2.2 Fatigue Life

    HMA fatigue properties are important because one of the principal modes of HMA

    pavement failure is fatigue-related cracking, called fatigue cracking. Therefore, an

    accurate prediction of HMA fatigue properties would be useful in predicting overall

    pavement life. Flexural Test

    One of the typical ways of estimating in-place HMA fatigue properties is the flexural

    test (see Figures 5.36 and 5.37). The flexural test determines the fatigue life of a

    small HMA beam specimen (380 mm long x 50 mm thick x 63 mm wide) by

    subjecting it to repeated flexural bending until failure (see Figure 5.38). The beam

    specimen is sawed from either laboratory or field compacted HMA. Results are

    usually plotted to show cycles to failure vs. applied stress or strain.

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    Figure 5.36 (left): Flexural Testing


    Figure 5.37 (right): Flexural Testing


    Figure 5.38: Flexural Test Schematic (click picture to animate)

    The standard fatigue test is:

    AASHTO TP 8: Determining the Fatigue Life of Compacted Hot-Mix

    Asphalt (HMA) Subjected to Repeated Flexural Bending

    6.2.4 Tensile Strength

    HMA tensile strength is important because it is a good indicator of cracking

    potential. A high tensile strain at failure indicates that a particular HMA can

    tolerate higher strains before failing, which means it is more likely to resist cracking

    than an HMA with a low tensile strain at failure. Additionally, measuring tensile

    strength before and after water conditioning can give some indication of moisturesusceptibility. If the water-conditioned tensile strength is relatively high compared

    to the dry tensile strength then the HMA can be assumed reasonably moisture

    resistant. There are two tests typically used to measure HMA tensile strength:

    Indirect tension test

    Thermal cracking test Indirect Tension Test

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    The indirect tensile test uses the same testing device as the diametral repeated

    load test and applies a constant rate of vertical deformation until failure. It is quite

    similar to the splitting tension test used for PCC.

    Standard indirect tension test is a part of the following test:

    AASHTO TP 9: Determining the Creep Compliance and Strength of Hot

    Mix Asphalt (HMA) Using the Indirect Tensile Test Device Thermal Cracking Test

    The thermal cracking test determines the tensile strength and temperature at

    fracture of an HMA sample by measuring the tensile load in a specimen which is

    cooled at a constant rate while being restrained from contraction. The test is

    terminated when the sample fails by cracking.

    The standard thermal cracking test is:

    AASHTO TP 10: Method for Thermal Stress Restrained Specimen Tensile


    6.2.5 Stiffness Tests

    Stiffness tests are used to determine a HMA's elastic or resilient modulus. Although

    these values are fairly well-defined for many different mix types, these tests are

    still used to verify values, determine values in forensic testing or determine values

    for new mixtures or at different temperatures. Many repeated load tests can be

    used to determine resilient modulus as well.

    Of particular note, temperature has a profound effect on HMA stiffness. Table 5.13

    shows some typical HMA resilient modulus values at various temperatures. Figure

    5.39 shows that HMA resilient modulus changes by a factor of about 100 for a 56

    C (100 F) temperature change for "typical" dense-graded HMA mixtures. This

    can affect HMA performance parameters such as rutting and shoving. This is one

    reason why the Superpave PG binder grading system accounts for expected service

    temperatures when specifying an asphalt binder.

    Table 5.13: Typical Resilient Modulus Values for HMA Pavement Materials

    Resilient Modulus (MR)Material

    MPa psi

    HMA at 32F (0 C) 14,000 2,000,000

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    HMA at 70F (21 C) 3,500 500,000

    HMA at 120F (49

    C)150 20,000

    Compare to other materials

    Figure 5.39: General Stiffness-Temperature Relationship for Dense-Graded

    Asphalt Concrete

    6.2.6 Moisture Susceptibility

    Numerous tests have been used to evaluate moisture susceptibility of HMA;

    however, no test to date has attained any wide acceptance (Roberts et al., 1996).

    In fact, just about any performance test that can be conducted on a wet or

    submerged sample can be used to evaluate the effect of moisture on HMA by

    comparing wet and dry sample test results. Superpave recommends the modified

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    Lottman Test as the current most appropriate test and therefore this test will be


    The modified Lottman test basically compares the indirect tensile strength test

    results of a dry sample and a sample exposed to water/freezing/thawing. The

    water sample is subjected to vacuum saturation, an optional freeze cycle, followedby a freeze and a warm-water cycle before being tested for indirect tensile strength

    (AASHTO, 2000a). Test results are reported as a tensile strength ratio:

    where: TSR = tensile strength ratio

    S1 = average dry sample tensile strength

    S2 = average conditioned sample tensile strength

    Generally a minimum TSR of 0.70 is recommended for this method, which should

    be applied to field-produced rather than laboratory-produced samples (Roberts et

    al., 1996). For laboratory samples produced in accordance with AASHTO TP 4

    (Method for Preparing and Determining the Density of Hot-Mix Asphalt (HMA)

    Specimens by Means of the Superpave Gyratory Compactor), AASHTO MP 2

    (Specification for Superpave Volumetric Mix Design) specifies a minimum TSR of


    In addition to the modified Lottman test, some state agencies use the Hamburg

    Wheel Tracking Device (HWTD) to test for moisture susceptibility since the test can

    be carried out in a warm water bath.

    The standard modified Lottman test is:

    AASHTO T 283: Resistance of Compacted Bituminous Mixture to

    Moisture-Induced Damage

    6.3 Summary

    All pavements can be described by their fundamental characteristics and

    performance. Thus, HMA tests are an integral part of mix design because they

    provide (1) basic HMA characteristics and (2) the means to relate mix design to

    intended performance. Without performance tests, mix design has no proven

    relationship with performance (Roberts et al., 1996). The Hveem and Marshall mix

    design methods use two basic performance tests (Hveem stabilometer and the

    Marshall stability and flow), but these tests are empirical and limited in their

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    predictive ability. New and better performance tests are still being developed and

    evaluated. In fact, Superpave has yet to implement performance testing because

    of this. The performance tests presented in this section are those that are most

    commonly used in the industry today, although it is quite likely that these tests will

    change in the future as better methods and equipment are developed.

    (all photos from Cooley et al., 2000)