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7 Table 3. Summary of factors and levels for the dynamic modulus ruggedness test. Unconfined Tests Confined Tests Factor Low High Low High Equilibrium Temperature X 1 C X + 1 C X 1 C X + 1 C Specimen Transfer Time 3 min 5 min 3 min 5 min Specimen Conditioning Fluid Air Water Air Water Strain Level 75 strain 125 strain 75 strain 125 strain Confining Pressure No membrane Membrane 135 kPa 140 kPa Specimen End Parallelism Milled Sawed Milled Sawed Friction Reducer Greased latex TeflonTM Greased latex TeflonTM specimen ends are not perfectly smooth, nor parallel. Since 1.3.3 Ruggedness Testing Plan the friction reducer helps minimize the effects caused by end for the Flow Number Tests roughness, end parallelism is the critical specimen geometry property that must be considered. Based on measurement of This section discusses the ruggedness testing plan that was a number of specimens, a tolerance of 1.0 degree was estab- developed for the flow number test. It discusses the selection lished in Phase I of this project. To meet this tolerance requires of the materials, testing conditions, and factors that were in- careful control of the sawing operation. To verify that this level cluded in the evaluation of control is acceptable, specimens with sawed ends and milled ends were included in the dynamic modulus ruggedness testing Materials and Testing Conditions program. The ruggedness testing for the flow number test included materials and testing conditions that result in a wide range of Summary permanent deformation properties. It also included tests on dense- and gap-graded mixtures because it is likely that the Table 3 summarizes the factors and factor levels that were in- sensitivity of the flow number test to confining pressure cluded in the ruggedness testing for the dynamic modulus test. effects will be different for dense- compared to gap-graded Dynamic modulus tests were performed for each of the combi- mixtures. To evaluate rutting resistance, the flow number test nations of material, confinement, temperature, and loading rate will be performed at a high pavement temperature represen- listed in Table 4. Confined tests were only performed at high tative of the project location and pavement layer depth to temperatures where past research has shown confining effects evaluate the rutting resistance of the mixture. In NCHRP to be significant. Since the dynamic modulus is a non-destruc- Project 9-33, criteria have been developed for the flow number tive test, the testing program required the fabrication of 32 spec- test based on the 50 percent reliability 7-day average maximum imens, 16 for each mixture. Tests on these 32 specimens were high pavement temperatures computed using the LTPPBind performed for the four combinations of temperature and con- software (7). Table 5 summarizes these temperatures for finement listed in Table 4 in the following order, unconfined at selected cities (8). Based on these temperatures, mixtures in- 4C, unconfined at 20C, confined at 40C then unconfined at 40C. For each temperature/confinement combination, the order of the determinations from Table 1 was randomized. The Table 5. LTPPBind design high pavement temperatures for 50 percent reliability. entire ruggedness testing program was performed in two labo- ratories: AAT's laboratory using the ITC SPT and FHWA's Mo- 98 Percent Reliability bile Asphalt Laboratory using the IPC SPT. 50 Percent Reliability High Temperature Design High Pavement Grade, Fast Traffic, 3 to City Temperature, C (8) 10 million ESAL (8) Atlanta, GA 51 64 Table 4. Materials and conditions for Chicago, IL 47 64 the dynamic modulus ruggedness test. Fairbanks, AK 38 52 Fargo, ND 46 64 Temperature, C/Frequency, Houston, TX 52 70 Mixture Confinement Hz Indianapolis, IN 48 64 4/1.0 20/0.1 40/0.01 Miami, FL 51 64 Dense-graded Unconfined X X X Oklahoma City, OK 52 70 Confined X Phoenix, AZ 58 76 SMA Unconfined X X X Reno, NV 51 64 Confined X Washington, DC 49 64

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8 Table 6. Mixture and test conditions equipment specifications currently apply a 2.0 percent tol- for the ruggedness testing for erance on the deviatoric stress. This level of control was taken the flow number tests. from other similar tests for asphalt concrete. Deviatoric stress Confining Deviator Anticipated was included in the ruggedness tests with the factor levels set Mixture Confinement Stress, kPa Stress, kPa Flow at 135 and 145 kPa for unconfined tests, and 945 and 985 kPa Dense-graded Unconfined 0 140 Low for confined tests. Confined 140 965 Moderate SMA Confined 140 965 High Confining pressure. Research has also shown that the flow number test is sensitive to confining pressure (9). Cur- rently the SPT specification requires control of confining corporating PG 64-22 binders should be tested at approxi- pressure to 2.0 percent of the specified value. The maximum mately 50 C. A temperature of 50 C was selected for use in confining pressure available in the SPT is 210 kPa; therefore, the flow number ruggedness testing. the maximum deviation from the target is 4.2 kPa. In the The same two mixtures selected for the dynamic modulus Phase II evaluation, this level of control was easily maintained ruggedness were used in the ruggedness testing for the flow by the two devices. The ruggedness testing included confined tests. Table 6 summarizes the testing conditions for the flow tests with confining pressures of 135 and 140 kPa to verify that tests. Tests were performed on the dense-graded mixture with the current level of confining pressure control is adequate. and without confinement, but only confined tests were per- formed on the SMA mixture. All tests were performed at 50 oC. Contact stress. The contact stress used in the flow num- ber test applies a small creep load to the specimen during the test. The effect of this loading has not been evaluated in past Factors and Levels research. A contact stress of 5 percent of the deviatoric stress Many of the same factors discussed for the dynamic modu- was recommended in the Project 9-19 test procedures (2). lus ruggedness test were included in the ruggedness testing for In the ruggedness testing, contact stresses of 3.7 and 7.5 per- the flow number test. The sections below discuss each of these cent were evaluated. factors. End friction reducer. A major assumption in the flow Temperature. The same temperature factors: equilibrium number test is that the stresses are distributed uniformly over temperature tolerance, transfer time, and conditioning fluid the specimen. Friction between the loading platen and the were included in the ruggedness testing for the flow number specimen produces shear stresses which result in a deviation test. The factor levels were 1.0 degree for equilibrium tem- from this assumption. The effects of friction can be minimized perature, 3 min and 5 min for specimen transfer time, and air by using long specimens. The test specimen size for the simple and water as conditioning fluids. performance tests was determined in an extensive specimen Loading rate. The duration of the load pulse and dwell size and geometry study conducted in Project 9-19 (5). The time between load pulses are important factors affecting the specimen diameter of 100 mm was selected to provide flow accumulation of permanent deformation in the flow number data that are independent of specimen size. The height to test. The duration of the load pulse was not included in the diameter ratio of 1.5 was selected to provide dynamic modulus ruggedness testing because the load standard error computed and flow data that are independent of specimen height. In the by the SPT software is very sensitive to variations in the Project 9-19 specimen size and geometry study, an end fric- duration of the load pulse. Limiting the load standard error tion reducing element consisting of two latex sheets separated to 10 percent or less ensures that the load pulse will be by silicon grease was used. The reduction of end friction in sinusoidal with a duration of 0.1 sec. The equipment specifi- these tests was probably a significant factor in the conclusions cations currently do not include a tolerance on the dwell time concerning specimen size. The greased latex sheets are not between load pulses. It is specified as 0.9 sec, and current conducive to production testing; therefore, in Project 9-29 computer control systems are very accurate allowing it to be TeflonTM sheets were used in the evaluation testing. The type controlled within a millisecond or less. A tolerance should be of end friction reducer, greased latex versus TeflonTM was included in the specification; therefore, the dwell time was in- included in the ruggedness evaluation to verify that either cluded in the ruggedness testing. The levels for this factor approach is acceptable. were set at 0.85 and 0.95 sec. Only the IPC equipment had the capability to adjust the dwell time in the flow number test. Specimen properties. Air void content and end paral- lelism are two specimen properties that must be controlled. Deviatoric stress. Research has shown that the flow num- With available specimen fabrication techniques, an air void ber test is sensitive to the applied deviatoric stress (9). The tolerance of 0.5 percent of the target is obtainable with care-

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9 100000 10000 Flow Number 3.90 % AC 4.55% AC 1000 5.20 % AC 5.90 %AC 100 10 0 2 4 6 8 10 12 14 Air Void Content, % Figure 3. Effect of air voids on unconfined flow number [data from Project 9-19 (10)]. ful control. It is desirable to increase the air void tolerance data, the air void content has a large effect over the 4 to 7 per- to minimize the number of specimens rejected. Project 9-19 cent air void range likely to be used in laboratory testing. included a subset of flow number tests on mixtures with Using the trend lines shown, a 1 percent change in air voids varying asphalt and air void contents (10). Flow number data produces a 56 percent change in the flow number for uncon- from this study are plotted in Figure 3 and Figure 4 to show fined tests and a 20 percent change in flow number for con- the effects of air voids. Although there is large scatter in the fined tests. Like the dynamic modulus, this analysis shows that 10000 1000 Flow Number 3.90% AC 4.55% AC 100 5.20% AC 5.90% AC 10 1 0 2 4 6 8 10 12 Air Void Content Figure 4. Effect of air voids on confined flow number [data from Project 9-19 (10)].