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6 HMA -TESTING

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

properties.

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

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:

Volume

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.

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

inconsistent.

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

precision.

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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CA

ATMD

=

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

grams

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.

6.1.3.1 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

6.1.3.2 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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6.1.3.3 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

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

Evaluation.

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

parameter.

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.

6.2.1.1 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

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.,

1996).

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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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

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.

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.

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.

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

6.2.1.3 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

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

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

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, =

90.

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

measured

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

evaluated.

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

method.

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)

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

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

and Sample After Test (middle and

right).

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.

6.2.1.4 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.

6.2.1.5 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

environmental conditions of that location are considered.

Wheel tracking devices can reasonably differentiate between binder

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.

6.2.2.1 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

Device

Figure 5.37 (right): Flexural Testing

Device

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

6.2.4.1 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

6.2.4.2 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

Strength

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

described.

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

0.80.

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)