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Asphalt Binder Cracking Device to Reduce Low-Temperature Asphalt Pavement Cracking: Final Report
The main objectives of this project were to refine the ABCD test method and to conduct the ABCD interlaboratory study (ILS). During Phase 1, ABCD test equipment and test procedures were refined. The ABCD ruggedness test was also completed and reported.(5) One of the major refinements in the ABCD test procedure made during Phase 1 was the cooling rate. The typical pavement cooling rate is considered to be about 5 °C/hr (9 °F/hr) or slower.(6) During the initial development of the ABCD, the rate of 10 °C/hr (18 °F/hr) was used as for other similar test methods. To study the effects of cooling rate during Phase 1, six binder types and five cooling rates (1 °C/hr, 3 °C/hr, 10 °C/hr, 20 °C/hr, and 40 °C/hr [1.8 °F/hr, 5.4 °F/hr, 18 °F/hr, 36 °F/hr, and 72 °F/hr]) were used for ABCD tests. For the cooling rate of 3 °C/hr (5.4 °F/hr) or faster, the effects of the cooling rate on ABCD cracking temperatures were relatively small and the rank of binder type in the cracking temperature did not change. On average, the increase of the cooling rate by 1 °C/hr (1.8 °F/hr) increased the cracking temperature by about 0.07 °C (0.13 °F). When the cooling rate was lowered to 1 °C/hr (1.8 °F/hr), the changes in the cracking temperature were asphalt dependent; some binders showed significantly decreased cracking temperature and some changed little. At this slow cooling rate, the effects of physical hardening seemed to have effects on the cracking temperature. The standard deviation of the cracking temperature remained small up to the rate of 20 °C/hr (36 °F/hr). At 40 °C/hr (72 °F/hr), the standard deviation increased significantly. Based on this finding, 20 °C/hr was the rate chosen for the ABCD testing, and it was used for all Phase 2 activities.
Two main tasks planned for Phase 2 of the program were manufacturing five units of ABCD and conducting an ABCD ILS. For the Superpave low-temperature grade of asphalt binder, the BBR test has been used in practice. The BBR critical temperature for thermal cracking is the warmer temperature between temperatures where the BBR creep stiffness reaches a critical stiffness of 300 MPa (43.51 ksi) or where the slope of log stiffness versus log time plot, known as m-value, reaches 0.300. The current standard test method (AASHTO T 313) describes the BBR test procedures and also provides the precision estimates for the BBR creep stiffness and m-value, but not for the BBR critical temperature. Before Phase 2 began, the program review panel recommended comparing the precision estimates of ABCD cracking temperature with those of the BBR critical temperature. In addition to the ABCD ILS, another ILS was conducted for BBR critical temperature. Both studies followed the guideline outlined in ASTM C802, "Standard Practice for Conducting an Interlaboratory Test Program to Determine the Precision of Test Methods for Construction Materials." Three binders with different levels of stiffness were chosen (low, medium, and high stiffness; labeled as Lxx, Mxx, and Hxx, respectively). The low-stiffness binder was an unaged PG 70-28 SBS modified. The medium-stiffness binder was an unaged PG 76-22 SBS modified. The high-stiffness binder was an unmodified mixing stock with penetration 0-10 (equivalent of PG 88 + 6).
Table 2 lists the 31 laboratories that volunteered for the ABCD ILS, including 18 State and Federal Government laboratories, 1 Superpave Center, 1 Canadian Province Ministry of Transportation, 5 universities, and 6 private laboratories. Ten laboratories also participated in the BBR critical temperature ILS. Because there were more ILS participants than expected, the binder samples ran out after the first 23 ABCD ILS laboratories had participated, and replacement binder samples with the same PG grade were used for the remaining laboratories. For determination of ABCD precision estimates, however, only data from the first 23 laboratories were used.
This report summarizes both the ABCD ILS and the BBR Critical Temperature ILS. In an effort to improve the precision of ABCD test results, the No-Trim ABCD test procedure was developed. This procedure and results obtained from No-Trim ABCD tests are discussed in this report. The appendix presents the results of the No-Trim ABCD tests.
* Laboratory also participated in the BBR critical temperature ILS.
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