|FHWA > Engineering > Pavements > An Investigation of the Endurance Limit of Hot-Mix Asphalt Concrete > Part 1|
An Investigation of the Endurance Limit of Hot-Mix Asphalt Concrete Using a New Uniaxial Fatigue Test Protocol
|Range in Cycles||Number of Cycles between Measurements|
|First 100 cycles of Stage||1|
|Cycles 101 to 1,000 of Stage||20|
|Cycles 1,001 to 1,000 of Stage||100|
|Cycles 1,001 to 100 cycles prior to end of Stage||1,000|
|Last 100 Cycles of Stage||1|
Eighteen thermocouples (TC) were used for the measurement of temperature at various points. The specimen surface temperature was measured using nine thermocouples (Figure 2). One TC was placed in the middle of each pair of transducer contact points (thermocouples 2, 5, and 8). Above and below each transducer, three thermocouples were placed at the very top of the specimen (thermocouples 1, 4, and 7), and three thermocouples were placed at the very bottom of the specimen (thermocouples 3, 6, and 9). TC 10 was placed in the water bath and TC 11 was placed next to the RTD that controls the test chamber temperature. Thermocouples 12 and 13 were placed on the upper and lower aluminum heads and TC 14 was placed on the load cell. The air temperature of the walk-in chamber was monitored with the built-in thermocouple of the multiplexer. TC 15 was placed at the mid center of a dummy specimen and TC 16 at its center. The laboratory ambient air temperature was monitored with the built-in thermocouple of the Micrologger.
An AM25T multiplexer and a Campbell Scientific CR23x Micrologger were used to collect and store temperature data. Temperature measurements were obtained once per minute during the fatigue testing and synchronized in the time domain with the dynamic measurements obtained with the FastTrack™ software.
The complexity of the data acquisition system requires caution with respect to the synchronization of the data. If the computer in the Fast Track 8800 cannot acquire and process data fast enough to prevent buffer overruns, data from the various channels will not be properly synchronized in time. The results of this problem can be seen as excessive variability in the plots of modulus versus loading cycles as shown in Figure 3.
Figure 2. Location of thermocouples on the test specimen.
As discussed above, the data are collected from two separate GPIB boards (number 3 and 4) within the Fast Track 8800. Except for transducer 3 all the channels are located on GPIB 3. The Fast Track 8800 system is in charge of keeping GPIB 3 and 4 synchronized so all the collected data are obtained at the same time. Data synchronization is well maintained by Fast Track 8800 as long as the buffer is not full. If for any reason the computer does not read the data fast enough from the buffer, the buffer becomes full and Fast Track 8800 loses its ability to keep the two GPIB boards synchronized. If such an event occurs, data from GPIB 4 (where transducer 3 is connected) are no longer synchronized with GPIB 3 (where the rest of channels are connected). Unfortunately from the moment Fast Track 8800 loses its ability to maintain synchronization, all the collected data are desynchronized, even after the buffer is fully read and empty.
In normal situations the speed of reading the data by the Fast Track 8800 computer is sufficient to prevent buffer overruns. This keeps the buffer empty almost all the time, ensuring that no data are lost by overwriting. In the event that the computer processor is busy with some other tasks, then there is a risk that the buffer will overflow. To avoid this situation all the possible actions were taken to be certain that the computer was fully and exclusively devoted to the task of data acquisition. All possible services were turned off; the computer was disconnected from the network, the antivirus was disabled, all the services from the operating system were deactivated, and temperature data acquisition was loaded on a separate computer. Although these actions reduced the number of times that this problem occurred, only at the end of this project was it realized that some unknown application(s) was (were) occasionally taking processor time and causing the data not to be synchronized.
Figure 3. Example of the effect of data synchronization on variability in the measured modulus.
At the beginning of the project, when the data synchronization problem was first realized, the only solution immediately available was to stop and restart the loading when the buffer overflow occurred. This could add some other risks to the testing in addition to the fact that any discontinuity in the loading applied to the specimen affected its modulus. As a consequence a new module was programmed and added to the control and data acquisition software. This module allowed the operator to reset the buffer and to synchronize the data without any interruption in the loading. The only downside of this procedure was that it was not automatic and required that the operator be continuously present during the test and intervene manually. This meant that from the moment the problem occurred until the moment the operator noticed the problem and reset the buffer, the collected data were not synchronized.
During the testing in this project the difference in time between the first appearance of unsynchronized data collected from transducer 3 and the two others was limited to a maximum of 2 seconds (or 20 cycles). During those time periods in the testing where the variation of modulus was very small, the effect of unsynchronized data on the analysis was negligible. This is especially true for the calculation of the slope of modulus at the end of the stages where there were a large number of data points.
For future testing either a system upgrade will be needed to solve this problem or a module must be added to the control and data acquisition system to periodically check whether the data are synchronized or not. The added module will automatically reset the buffer in the event that the data are not synchronized.
A dual chamber scheme was used to control the testing temperature (Figure 4). The test chamber was mounted inside a walk-in environmental chamber. The temperature inside the test chamber was controlled by connecting a water bath to a heat exchanger mounted in a plenum adjacent to the test chamber. Air was exchanged between the plenum and the test chamber resulting in variations of less than ± 0.015 °C in the test chamber. Two 100-W light bulbs were used as a heat source to make small adjustments in the temperature of the air passing between the plenum and the test chamber.
Figure 4. Schematic of temperature chamber and controller.
A schematic of the test fixture including a test specimen and the transducers is given in Figure 5. A jig was used to align the mounting heads and test specimen during the gluing process. The jig ensures that the ends of the mounting heads as they are attached to the testing frame are parallel and that the axes of the test specimen and mounting heads are concentric.
Figure 5. Schematic of test fixture.
In order to prepare a specimen for testing, the test specimen is placed in the jig along with the epoxy-coated upper and lower mounting heads. After the epoxy has cured, the specimens are maintained in the temperature chamber for 24 hours at the test temperature for conditioning and achieving temperature equilibrium. The lower mounting head and the mounting flange are screwed together, and then the upper mounting head is screwed into the load cell. The hydraulic ram is lowered until the mounting flange seats on the lower platen extension. The three bolts connecting the mounting flange and the lower platen extension are then tightened. With this configuration, the ends of the test specimen are held rigidly in the testing machine. By following this procedure damage to the test specimen caused by traction, compression or torque is minimized.
The original experiment design included four different asphalt mixtures with three replicates for each. However, because a number of problems were incurred during the course of the project, changes were made to the original experiment design. Table 2 presents the mixtures and specimens prepared for this study.
|Mix ID||PG Grade||Target AV%||Specimen #||Actual AV%||%AC|
With the exception of the I-80 specimen the specimens were compacted using a Pine™ Superpave Gyratory Compactor. The compacted specimens were 150 mm in diameter and 150 mm in height. Attempts were made to achieve air voids in the range of 7.0 ± 0.5 percent. The I-80 specimens were compacted as part of a previous fatigue study using the French rolling wheel compactor as described elsewhere( 4 ).
The gradation and properties for the mixtures prepared for this study are presented in Appendix A. Three of the mixes were obtained from a PennDOT-sponsored project, Superpave In-Situ Stress-Strain Investigation (SISSI).
The I-80 mix was used in the previous fatigue study( 4 ). Unfortunately, due to time constraints no tests were conducted on the last mix (M5201) and limited testing was conducted on the remaining mixes.
During a previous study( 6 ), a new fatigue testing protocol was proposed in which three stages of continuous loading without any rest period were considered, as shown in Figure 6. Schematic of loading in Stages I, II, and III. The same strain level, not exceeding the endurance limit of the HMA and consequently at a level that does not produce fatigue damage, is applied during Stages I and III. During Stage II, a strain with a magnitude exceeding the endurance limit and consequently causing fatigue damage is applied. The difference between the moduli at the end of Stages I and III (represented by two asymptotes) should directly indicate the level of "true" fatigue damage imposed during Stage II, as shown in Figure 6. Schematic of loading in Stages I, II, and III.
Damage should occur during Stage II only if the strain level during Stage II is greater than the endurance limit. In the study presented here, strain levels were expected to be considerably less than if the endurance limit were applied. For both Stages I and III, strain magnitude is maintained at the same level (σ1). For Stage II, a larger strain level σ2 is applied. At each stage, loading continues until the curve of modulus versus number of cycles reaches a zero slope (i.e., SAB = SCD = SFG = 0, where S indicates the slope of line presenting modulus as a function of loading cycles). The expectation is that the modulus at the end of Stage III will return to the same level as was observed at the end of Stage I if the applied strains are below the endurance limit. Therefore, the following goals will be achieved from such an experiment:
Figure 6. Schematic of loading in Stages I, II, and III.
Figure 7. Testing at specific strain levels to validate existence of the endurance limit.
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