VAA Virginia Asphalt Fall/Winter 2020

16 FALL/WINTER 2020 and a constant load is applied to the specimen at a rate of 50 ± 2.0 mm/min, with no seating load applied before the test. The test is stopped when the load drops below 0.1 kN after reaching the peak load. Some key details are essential when evaluating data quality: • No seating load is applied, • The load is applied at the specified constant load rate of 50 ± 2 mm/min, and • The test ends only when the applied load drops to 0.1 kN or less after reaching the peak load. An acceptable load-displacement curve will look like the example in Figure 1(a), and an acceptable displacement-time curve will look like that shown in Figure 1(b). These curves should be checked for each specimen as part of the test quality control. Non-Compliant Data There are many reasons that the load-displacement and displace- ment-time curves may not meet the test specification. Some load frames have a safety function limiting the ram travel that does not provide enough travel for the post-peak load to drop to 0.1kN or less during testing. Machine compliance may cause the rate of loading to change with increasing specimen resistance to loading. Improper LVDT setup, including misalignment, improper zeroing, or incorrect calibration, will affect displacement measurements. △ continued from page 15 Performing data quality checks will permit the identification of issues and allow them to be addressed. The time, load, and displacement measurements must be obtained from the testing software to identify non-compliant data. The operator manual or equipment manufacturer may have to be consulted to determine the best way to extract the raw data file. The time, load, and displacement measurements can then be plotted using a spreadsheet, and data quality can be assessed. If non-compliant data is found, the causes should be determined so they can be addressed and resolved. A single occurrence of non-compliance may indicate an isolated or random event; however, recurring events may be a sign of equipment issues, repetitive operator error, or other testing problems. Some examples of non-compliant data are shown in Figure 2, along with possible causes. Figure 2(a) shows data from a test wherein the data acquisition has been incorrectly set up, as the measured load should reach approximately 10kN at its peak. The difference suggests that the load data shown are in US units (lbf) instead of kN as labeled, while displacements are shown correctly in SI units (mm); the mixed units will result in incorrect calculations of the CTindex. In addition, the test terminated before the load dropped to 0.1kN. A test with an error in displacement measurements is shown in Figure 2(b); the LVDT may have slipped out of position, or the range may need to be checked. Figure 2(c) displays a seating load applied at the beginning of the test and an LVDT error towards the test’s end. The test software should be (a) Load magnitude is not as expected; likely caused by an issue with incorrect units. The test terminates prematurely before the load drops to 0.1kN . (b) Issue with displacement measurement; the LVDT may have slipped out of position. FIGURE 2: Examples of non-compliance data curves. Dashed black lines in displacement-time graphs indicate 50 mm/min loading rate.