Asphalt Mixture Performance Tester (AMPT)


Background


Present HMA volumetric mix design method used by the majority of state highway agencies was developed in the asphalt component of the strategic highway research program between 1987 and 1993. Though the utility and soundness of the HMA mix design method are evident by its present day use, mix designers from the beginning have asked for complementary simple performance tests to quickly and easily proof-test candidate mix designs. In 1996, work sponsored by the Federal Highway Administration, started to identify and validate simple performance tests for permanent deformation and fatigue cracking.

Created as part of the National Cooperative Highway Research Program (NCHRP) Project 9-19, "Superpave Support and Performance MOdels Management". The NCHRP Project 9-19 recommended three test and parameter combinations as simple performance tests for permanent deformation: (1) the dynamic modulus, E*, determined the triaxial dynamic modulus; (2) the flow time, FT, determined with the triaxial static creep test; and (3) the flow number, FN, determined with the triaxial repeated load test. The dynamic modulus, E*, also was chosen as the simple performance test for fatigue cracking. Under the NCHRP Project 9-29 "Simple Performance Tester (SPT) for Superpave Mix Design," Advanced Asphalt Technologies, LLC was assigned the task of designing, procuring, and evaluating an SPT (Figure 1) for (1) proof-testing for permanent deformation and fatigue cracking in HMA mix design and (2) materials characterization for pavement structural design with the Mechanistic Empirical Pavement Design Guide (MEPDG).

Currently, only the dynamic modulus, E*, test is the only widely performed test that the SPT is capable of running. After years of research and development the SPT was renamed the Asphalt Mixture Performance Tester (AMPT), due to the non-simplistic nature of the tests. The rest of this wiki will focus on the dynamic modulus, E*, test. The two main reasons for the development of the AMPT would be (1) asphalt mixture characterization and (2) inputs into the MEPDG.

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Figure 1 - Asphalt Mixture Performance Tester



Test Description




To obtain the dynamic modulus, a haversine axial compressive stress is applied to a cylindrical asphalt concrete specimen at a specified temperature and different loading frequencies. The applied stress and the resulting axial strain of the specimen are measured and used to calculate the dynamic modulus and phase angle. The dynamic modulus is defined as the peak stress divided by the peak strain at a specific frequency and temperature combination. This is the overall stiffness of the asphalt concrete mixture at the given conditions. The phase angle is defined as the angle, in degrees or radians between a haversine applied peak stress, and the resulting peak strain in a controlled stress test. This is shown schematically in Figure 2. Once the dynamic modulus values are measured over a range of temperatures and loading frequencies, they can be combined or shifted into a single curve as shown in Figure 3. This curve is known as the dynamic modulus master curve. The master curve is created using an equation developed as part of the NCHRP Project 9-29, this is Equation 1 shown below.



Equation_1.JPG





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Figure 2 - Schematic of Dynamic Modulus Test



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Figure 3 - Dynamic Modulus Master Curve With Shifted Values



The master curve along with the shift factors, provides information about the mechanical response of the specific asphalt mixture at any given load frequency and temperature. The values obtained from this master curve can be used for performance prediction and analysis. The values of dynamic modulus and phase angle can also be used as performance criteria for control/quality assurance during Hot Mix Asphalt (HMA) design and construction.




Specimen Preparation


Preparation of the asphalt concrete specimens used for this test consists of making a Superpave Gyratory Compactor (SGC) sample that meets the requirements of AASHTO T 312. The SGC sample is then cored to a nominal 102 mm diameter using a water-cooled diamond bit core drill. The specimen is then cut at both ends of the core to obtain a nominal 150 mm tall test specimen. This process is shown in Figure 4. Actions are taken to secure and support the specimen during coring and cutting to make sure it meets the requirements presented in Table 1.


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Figure 4 - Standard Gyratory Compactor


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Figure 5 - Specimen Coring (left) and Speciment Cutting (right)


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Table 1 - Test Specimen Dimensional Tolerances

Once the specimens have been cored and cut and meet the specifications presented in Table 1, they are ready to start the testing procedure for the AMPT. As shown in Figure 5, six gauge points are attached to the specimen using a standard epoxy. These six gauge points will house the three Linear Variable Differential Transformers (LVDT’s) that measure the axial deformations during testing. The specimens are then placed in an environmental chamber to condition them for the determined testing temperature. A dummy specimen with a mounted thermocouple at the center is also placed in the environmental chamber for temperature verification. The chamber of the AMPT is allowed to equilibrate at the testing temperature for at least one hour before testing begins.

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Figure 6 - Specimen with LVDT's Attached and Placed in the Environmental Chamber



When the dummy specimen and the testing chamber reach the target temperature, the chamber is opened and the specimen is set in place. Friction reducers are placed on the top and the bottom of the specimen to mitigate the friction effects of the loading platen on the specimen. The LVDTs are then installed between the gauge points. Two compensating springs are attached on the LVDTs to counteract the force generated by the LVDT. The LVDT’s are then tared and checked to confirm that they are within their calibrated range. After everything has been installed, the chamber is closed and allowed to equalize to the testing temperature.

While the chamber is returning to the testing temperature, the required identification and control information is entered into the Dynamic Modulus software. The frequencies and temperatures chosen to evaluate the specimens come from suggested values in a pending standard from the American Association of State Highway and Transportation Officials (AASHTO). These temperatures and corresponding frequencies can be seen in Table 2.

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Table 2 - Recommended Testing Temperatures and Loading Frequencies for Different Asphalt Binders


After running the test at the specified temperatures and frequencies, the software included in the machine calculates needed information such as the dynamic modulus, phase angle, and data quality statistics. This data is presented by the program and can be exported into a comma separated values (.csv) file for further analysis.


testing Procedures

Data Obtained


external image clip_image002.png (1)
Where:

external image clip_image004.png =Minimum Modulus used as a fitting parameter, kPa (psi);
external image clip_image006.png = Maximum Modulus obtained from volumetrics, kPa (psi);
external image clip_image008.png = Fitting Parameter;
external image clip_image010.png = Fitting Parameter;
external image clip_image012.png = Activation energy, fitting parameter;
external image clip_image014.png = Testing frequency, Hz;
external image clip_image016.png = Testing temperature, °C;
external image clip_image018.png = Reference temperature, °C ;

Once the data is obtained it is initially evaluated for quality based on the requirements presented in Table 3.

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Table 3 - Data Quality Requirements


The dynamic modulus software uses a standard procedure for calculating needed information. The initial analysis of the data, as well as the calculations for determining the dynamic modulus, phase angle, and data quality statistics are some of the calculated information. After the initial data quality has been met the calculated data is then exported to a .csv file. This is currently the only method for exporting and analyzing the data that the AMPT software collects.


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Figure 7 - Data Exported as .csv file from the Dynamic Modulus Software


When all 4 replicate specimens have completed the required testing at the recommended temperatures and frequencies, the data is then compiled in a spreadsheet (Figure 8) and prepared for the development of a dynamic modulus master curve. A dynamic modulus master curve is a composite curve constructed at a reference temperature by shifting dynamic modulus data from various temperatures along the log frequency axis. Currently we use Mastersolver Version 2.3, a spreadsheet developed by Dr. Ramon Bonaquist of Advanced Asphalt Technologies, for the development of the dynamic modulus master curve. Other programs are currently being evaluated.

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Figure 8 - Data from four samples entered into the MasterSolver Version 2.3



Using Equation 1 that has been shown above, the data is then fitted to a single dynamic modulus master curve (Figure 9) at a selected reference temperature. This is done by varying the fitting parameters in Equation 1 to minimize the errors.

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Figure 9 - Solving of the Fitting Parameters


In the Mastersolver version 2.3 there is a report tab that presents recommended information (Figure 10 - Example of Report Information). The report includes: mixture identification, measured dynamic modulus and phase angle data for each specimen at each temperature and frequency combination, average measured dynamic modulus and phase angle at each temperature and frequency combination, Coefficient of variation of the measure dynamic modulus data at each temperature and frequency combination, standard deviation of the measured phase angle data at each temperature and frequency combination, VMA and VFA of each specimen tested, average VMA and VFA for the specimens tested, reference temperature, parameters of the fitted master curve, goodness of fit statistics for the fitted master curve, plot of the fitted dynamic modulus master curve as a function of reduced frequency showing average measure dynamic modulus data, plot of shift factors as a function of temperature, plot of the average phase angle as a function of reduced frequency, and tabulated temperature, frequency, and dynamic modulus for input into MEPDG.

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Figure 10 - Example of Report Information


Once the data has been analyzed, the fitting parameters can be recorded and stored in a database for comparison to other mixtures or similar mixtures for mixture verification. An analysis has been done on the variability of the AMPT and found that the CV value between the 4 specimens was typically below 10%. From that evaluation it has been determined that the AMPT is repeatable and has the ability to generate precise data.