Ruggedness Testing of the Dynamic Modulus and Flow Number Tests with the Simple Performance Tester (2008) / Chapter Skim
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The experimental design in Table 1 uses seven factors at two levels. This design is considered appropriate for the proposed ruggedness testing for the simple performance tests.
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As shown, the range of dynamic modulus values can be covered using the following temperature and frequency combinations: • High modulus, 4°C at 10 Hz • Intermediate modulus, 20°C at 0.1 Hz • Low modulus, 40°C at 0.01 Hz Project 9-19 has suggested that confined tests may be necessary for gap- and open-graded mixtures. It is likely that the sensitivity of dynamic modulus measurements to confining pressure effects will be different for dense compared to gapand open-graded mixtures.
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Although loading rate has a major effect on the mechanical properties of asphalt concrete, it will not be included in the ruggedness testing because the load standard error computed by SPT software is very sensitive to variations in the frequency of the applied load. Limiting the load standard error to 10 percent or less ensures that the frequency of the applied load will be the same as the specified loading frequency.
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The effects of friction can be minimized by using long specimens and making measurements near the middle. The test specimen size for the simple performance tests was determined in an extensive specimen size and geometry study conducted in Project 9-19 (5)
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To verify that this level of control is acceptable, specimens with sawed ends and milled ends were included in the dynamic modulus ruggedness testing program. 1.3.2.3 Summary Table 3 summarizes the factors and factor levels that were included in the ruggedness testing for the dynamic modulus test.
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Air void content and end parallelism are two specimen properties that must be controlled. With available specimen fabrication techniques, an air void tolerance of ±0.5 percent of the target is obtainable with care8 Mixture Confinement Confining Stress, kPa Deviator Stress, kPa Anticipated Flow Unconfined 0 140 Low Dense-graded Confined 140 965 Moderate SMA Confined 140 965 High Table 6.
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Like the dynamic modulus, this analysis shows that 9 10 100 1000 10000 100000 0 2 4 6 8 10 12 14 Air Void Content, % Fl ow N um be r 3.90 % AC 4.55% AC 5.20 % AC 5.90 %AC 1 10 100 1000 10000 0 2 4 6 8 10 12 Air Void Content Fl ow N um be r 3.90% AC 4.55% AC 5.20% AC 5.90% AC Figure 3. Effect of air voids on unconfined flow number [data from Project 9-19 (10)
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As discussed for the dynamic modulus test, end parallelism was included as a factor in the ruggedness testing for the flow number test. Different conclusions concerning the effects of end parallelism may be drawn from the small strain dynamic modulus test and the large strain flow number test.
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11 Condition Dynamic Modulus Test Flow Number Test Temperature, °C 10 20 35 35 50 50 Confining Stress, kPa 0 0 0 135 0 140 Deviatoric Stress, kPa To obtain 100 µ strain 140 965 Manufacturer Replicates Interlaken 4 4 4 4 4 4 IPC Global 4 4 4 4 4 4 MDTS 4 4 4 4 4 4 Table 8. Equipment effects experiment.
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The responses measured in the dynamic modulus ruggedness experiment are listed in Table 9. These include the measured dynamic modulus and phase angle, and the computed data quality indicators.
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It is likely that the repeatability of the dynamic modulus test will improve in the future as specimen fabrication techniques are improved and operators become more familiar with the equipment. However due to 13 Parameter Type Dynamic Modulus Material Property Phase Angle Material Property Load Standard Error Data Quality Indicator Deformation Standard Error Data Quality Indicator Deformation Uniformity Data Quality Indicator Phase Uniformity Data Quality Indicator Table 9.
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Using these limits, the following observations were made concerning the dynamic modulus test: 1. Temperature control of ±0.5°C is adequate.
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The sections that follow discuss each of the data quality indicators. 15 -14 -12 -10 -8 -6 -4 -2 0 2 4 6 8 10 12 14 Temperature, +0.5 C Time, min -Water / +Air Strain, +25 microstrain -Without / +With Membrane - Milled / +Sawed -Teflon / +Latex Confinement, +2% Change in Dynamic Modulus, % Confined 40 C 40 C 20 C 4 C Figure 6.
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Table 17. Significance of dynamic modulus ruggedness test factors on data quality indicators for the dense mixture tested in FHWA's Laboratory with the IPC SPT.
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The effects of the remaining factors are shown in Figure 9. From Figure 9, it is clear that the deformation standard error for high-temperature tests is higher when water is used as the conditioning fluid and when the unconfined dynamic modulus is measured with the membrane in place.
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2.2.2.4 Phase Uniformity The phase uniformity is a measure of how close the individual phase angle measurements made on a sample agree with one another. During Phase II of this project a maximum phase uniformity of 3 degrees was associated with good quality 18 -10 -8 -6 -4 -2 0 2 4 6 8 10 Temperature, +0.5 C Time, min -Water / +Air Strain, +25 microstrain -Without / +With Membrane - Milled / +Sawed -Teflon / +Latex Confinement, +2% Change in Deformation Standard Error, Percent Confined 40 C 40 C 20 C 4 C Figure 9.
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Phase angles are more variable in unconfined tests when the membrane is used. 2.2.3 Summary Table 20 summarizes the results of the analysis of the ruggedness test data for the dynamic modulus test.
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The current confining pressure control of ±2 percent is acceptable in confined tests. Over this range of control, the dynamic modulus and phase angle are expected to vary by 0.8 percent and 0.2 degrees, respectively.
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50 0 0 50 Axial Stress (135 vs 145 kPa) 0 0 0 0 Contact Stress (5 vs 10 kPa)
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First, the machine control factors of temperature, axial stress, contact stress, dwell time, and confining pressure have little effect on the measured permanent strains over the control range provided by the SPT. Also 22 Permanent Strain at Factors 500 cycles 1000 cycles 2000 cycles 5000*
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23 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 Temperature, +0.5 C Time, min -Water / +Air - Milled / +Sawed -Teflon / +Latex Axial Stress, + 2% Contact Stress, +2% Dwell, +0.01 sec Confining Stress, +2% Change in Permanent Strain, % SMA Confined Dense Confined Dense Unconfined -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 Temperature, +0.5 C Time, min -Water / +Air - Milled / +Sawed -Teflon / +Latex Axial Stress, + 2% Contact Stress, +2% Dwell, +0.01 sec Confining Stress, +2% Change in Permanent Strain, % SMA Confined Dense Confined Dense Unconfined Figure 13. Effect of statistically significant ruggedness factors on the permanent strain after 500 load cycles.
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Apparently, the smooth, milled ends of dense-graded mixture further reduced end friction resulting in an increase in permanent deformation. Because end milling is time consuming sawed ends meeting the specimen end condition requirements in the Equipment Specification for the Simple Performance Test System should be used.
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Of all the factors included in the ruggedness testing, the end friction reducer had the greatest effect on the flow number and the measured permanent deformation. Flow numbers were much lower and permanent deformation much higher when the greased latex friction reducer 25 -30 -25 -20 -15 -10 -5 0 5 10 15 20 25 30 Temperature, +0.5 C Time, min -Water / +Air - Milled / +Sawed -Teflon / +Latex Axial Stress, + 2% Contact Stress, +2% Dwell, +0.01 sec Confining Stress, +2% Change in Permanent Strain, % SMA Confined Dense Confined Dense Unconfined Figure 17.
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The current confining pressure control of ±2 percent is acceptable in confined tests. Over this range of control, the permanent strain is expected to vary by less than 3 percent.
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