The end result of a successful mix design is a recommended mixture of aggregate and asphalt binder. This recommended mixture, which also
includes aggregate gradation and asphalt binder type is often referred to as the job mix formula (JMF) or recipe.
Summary
HMA mix design is a laboratory process used to determine the appropriate aggregate, asphalt binder and their proportions for use in HMA. Mix design is a process to manipulate three variables:
(1) aggregate, (2) asphaltbinder content and (3) the ratio of aggregate to asphalt binder with the objective of obtaining an HMA that is deformation resistant, fatigue resistant, low temperature crack
resistant, durable, moisture damage resistant, skid resistant and workable. Although mix design has many limitations it has proven to be a cost-effective method to provide crucial information that can be used to formulate a high-performance HMA.
Static Creep Tests
A static creep test (see Figure 1) is conducted by applying a static load to an HMA specimen and then measuring the specimen’s permanent deformation after unloading (see Figure 2). 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 correlate well with actual in-service pavement rutting (Brown et al., 2001[1]).
Figure 1. Unconfined Static Creep Test
Figure 2. 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 40°C (104°F). 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 et al., 2001[1]). 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[1]).
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[2]).
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
Repeated Load Tests
A repeated load test applies a repeated load of fixed magnitude and cycle duration to a cylindrical test specimen (see Figure 3). 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 load cycles 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[1]).
Figure 3. 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 4. 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 4. 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 confining pressure 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]):
1. The state of stress is non-uniform and strongly dependent on the shape of the specimen. At high temperature or load, permanent deformation 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[1]).
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 Bituminous Mixtures Dynamic Modulus Tests
Dynamic modulus tests apply a repeated axial cyclic load of fixed magnitude and cycle duration to a test specimen (see Figure 1). 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 is a schematic of a typical dynamic modulus test.
Figure 5. Dynamic Modulus Test Schematic
Dynamic modulus tests differ from the repeated load testsin 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 of frequencies (usually 1, 4 and 16 Hz) for 30 to 45 seconds (Brown et al., 2001[1]). 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 a complex number called the “complex modulus”
(E*) (Witczak et al., 2002[3]) as seen in the equation below:
where E = complex modulus
: *
= dynamic modulus
φ =
phase angle – the angle by which εo lags behind σo.
For a pure elastic material, φ = 0, and the complex modulus (E*) is equal to the absolute value, or dynamic modulus. For pure viscous materials, φ = 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[3]):
where
: = dynamic modulus
o = peak stress amplitude(applied load / sample cross sectional area)
eo =
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 (φ), which is the lag between peak stress and peak recoverable strain. The complex modulus, E*, is actually the summation of 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 haversine load 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[1]). Unconfined dynamic modulus tests do not permit the determination of phase angle (φ).
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 (φ). 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[1]). Figures 6 and 7 show a prototype Superpave Simple Performance Test (SPT). The SPT will provide a performance test for the Superpave mix design method.
Figure 6. A Prototype Superpave SPT Figure 7. 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 replacedby
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 8). To conduct the test the HMA sample is essentially glued to two plates (see Figures 9 through 11) 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 et al., 2002[3]). 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 8. Superpave Shear Tester (SST) Figure 9. Loading Chamber
Figure 10. Prepared Sample Figure 11. Prepared Sample (left) and Sample After Test.
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 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 Chausées (LCPC) wheel tracker – also known as the French Rutting Tester (FRT). Cooley et al. (2000[4]) reviewed U.S. loaded wheel testers and found:
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 site’s 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 1: Asphalt Pavement Analyzer - A Wheel Tracking Device