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7/31/2019 apostila mistura asfltica
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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
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:
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.
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
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
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
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
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.,
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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6.2.1.2 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
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
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, =
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)
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
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
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.
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)