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57 0.0 2.0 4.0 6.0 8.0 10.0 12.0 14.0 16.0 2 4 8 16 32 Aging Time, hrs A sp ha lt fil m th ic kn es s, m ic ro n S1-9.5-mm S1-19-mm S2-9.5-mm S2-19-mm Figure 5-11- Asphalt film thickness in the four mixtures at various aging times CHAPTER 6- CONCLUSIONS AND RECOMMENDATIONS 6.1 General This study was conducted to prepare precision estimates for AASHTO and ASTM standards used to determine selected volumetric properties of HMA using absorptive aggregates. An interlaboratory study was planned to develop a precision statement applicable to (1) AASHTO T312, which is used to prepare and determine the density of HMA specimens using a Superpave Gyratory compactor, (2) AASHTO T166, which is used to determine bulk specific gravity of compacted asphalt mixtures, (3) ASTM D2041, which is used to determine maximum specific gravity of bituminous paving mixtures, and (4) ASTM D6752, which is used to determine bulk specific gravity and density of compacted bituminous mixtures using an automatic vacuum sealing method. The study was also aimed at evaluating the effect of aging time on the selective volumetric properties of HMA with absorptive aggregates. An appropriate laboratory aging time was intended to be proposed based on the observed effects of aging time. The study conclusions and recommendations are as follows.
58 6.2 Conclusions and Recommendations Related to Interlaboratory Study 6.2.1 ASTM D2041 6.2.1.1 Conclusions The Sr and SR estimates for D2041 of 9.5-mm and 19-mm mixtures were not significantly different. Therefore, the precision estimates from the two mixtures were combined and presented in a precision statement in Appendix C. The comparison of the precision estimates for D2041 computed using mixtures with absorptive aggregates versus mixtures with non-absorptive aggregates indicated that the precision estimates are significantly different. 6.2.1.2 Recommendations Based on the significant difference in Sr and SR precision estimates for D2041 of mixtures with absorptive aggregates versus mixtures with non-absorptive aggregates, the precision statement in Appendix C, which provides separate estimates for mixtures with absorptive aggregates and mixtures with non-absorptive aggregates, is recommended. 6.2.2 AASHTO T166 6.2.2.1 Conclusions The Sr and SR estimates for T166 of 9.5-mm and 19-mm mixtures with absorptive aggregates were not significantly different. Therefore, the precision estimates from the two mixtures were combined. The comparison of the precision estimates for T166 computed using mixtures with absorptive aggregates versus mixtures with non-absorptive aggregates indicated that the precision estimates are comparable and can be considered for combining. 6.2.2.2 Recommendations Based on no significant difference in Sr and SR precision estimates for T166 using mixtures with absorptive aggregates and mixtures with non-absorptive aggregates, the precision statement in Appendix D, which includes combined precision estimates of mixtures with absorptive aggregates and mixtures with non-absorptive aggregates, is recommended. 6.2.3 ASTM D6752 Results 6.2.3.1 Conclusions The Sr and SR estimates for D6752 of 9.5-mm and 19-mm mixtures with absorptive aggregates were not statistically significant. Therefore, the precision estimates from the two mixtures were combined.
59 The bulk specific gravity (Gmb) values obtained using D6752 were significantly lower than those obtained using T166. However, the Sr and SR estimates obtained using T166 and D6752 were not significantly different for either 9.5-mm or 19-mm mixtures. The comparison of the precision estimates for D6752 computed in this study with precision estimates for D6752 computed in Phase 1 [1] using mixtures with non- absorptive aggregates indicated that the precision estimates are comparable and can be considered for combining. 6.2.3.2 Recommendations Based on no significant difference in Sr and SR precision estimates for D6752 using mixtures with absorptive aggregates and mixtures with non-absorptive aggregates, the precision statement in Appendix E, which includes combined precision estimates of mixtures with absorptive aggregates and mixtures with non-absorptive aggregates, is recommended. The combined precision estimates can be included in AASHTO TP69, Bulk Specific Gravity and Density of Compacted Asphalt Mixtures Using Automatic Vacuum Sealing Method. 6.2.4 AASHTO T312 6.2.4.1 Conclusions At either Nin or Nd, the relative density computed using D6752 Gmb was significantly lower than the relative density obtained using T166 Gmb. However, the corresponding Sr and SR estimates for relative density using T166 Gmb and D6752 Gmb for either 9.5-mm or 19-mm mixtures with absorptive aggregates were not significantly different. Therefore, the precision estimates were combined. The comparison of the precision estimates for T 312 computed in this study with precision estimates computed in Phase 1 [1] using mixtures with non-absorptive aggregates indicated that the precision estimates are significantly different and should be presented separately. 6.2.4.2 Recommendations Based on the significant difference in Sr and SR precision estimates for T312 using mixtures with absorptive aggregates and mixtures with non-absorptive aggregates, the precision statement in Appendix F, which includes separate precision estimates for mixtures with absorptive aggregates and mixtures with non-absorptive aggregates, is recommended.
60 6.3 Conclusions and Recommendations Related to Aging Time Study 6.3.1.1 Conclusions The effect of aging time on selective volumetric properties of four different mixtures with absorptive aggregates was investigated. Although data from this study indicated that absorption of asphalt into aggregates continued over the 32 hours of aging, the selection of proper aging time should not be based on the prolonged absorption period. The statistical analysis of Gmm and air void data indicated that volumetric properties of mixtures with absorptive aggregates could change significantly after 4 hours of aging. In addition, the change in air voids and asphalt absorption was shown to be practically significant after 4 hours of aging. Therefore, aging of a mixture with absorptive aggregates for a period longer than 4 hours might result in volumetrics that are significantly different from those at the original mixture design. Therefore, the laboratory aging of mixtures with absorptive aggregates should be limited to 4 hours to avoid deviation from the mixture design. 6.3.1.2 Recommendations Based on the findings of this study a revision to AASHTO R 30 standard practice is recommended. A revision should specify a laboratory aging time that does not exceed 4 hours for volumetric testing of mixtures with absorptive aggregates. It is important to note that the suggested aging time is specific to laboratory procedure and does not apply to silo time since the laboratory aging does not necessarily represent the aging that takes place in the field. However, if mix is intended to be stored for a long time, the increased amount of absorption needs to be considered. The aging of mixtures with absorptive aggregates has two phases. One phase is the absorption of asphalt into the pores of aggregates that affect the volumetrics of the mixture, and the other phase is the stiffening of the binder, which change the shear strength and stiffness of the material. This study aimed at determining the laboratory aging time that meets the volumetric requirements of the mixture design. It is of interest to determine the appropriate aging time for the performance testing of mixtures with absorptive aggregates. For example, the difference between 2 to 4 hours of aging might be significant for performance testing as shown by dynamic modulus or flow numbers of mixtures with absorptive aggregates. Therefore, it is recommended to examine the effect of aging time on performance testing of asphalt mixtures with absorptive aggregates. The simple performance tests (SPT) of dynamic modulus and flow number can be conducted and the change in performance with the change in aging time can be evaluated. The appropriate aging time for performance purpose can then be suggested based on the observed changes in dynamic modulus or flow number of the mixtures. As mentioned above the aging process in laboratory includes both absorption and stiffening of the binder. However, the aging in silo is assumed to include mainly absorption and not the stiffening of the binder. The reason is that the mixtures in laboratory are spread thin in flat pans. The air that is in contact with the mixture oxidizes the binder and in result changes the mixture stiffness. In silo, the asphalt mixture is
61 placed in a large pile. The exposure to the air does not necessarily result in age hardening of the binder and in result the mixture. Therefore, laboratory aging is assumed to cause more severe changes to the mixture than the aging in silo. For example, 32 hours of laboratory aging would destroy the binder and prevent the mixture to compact properly; however, 32 hours in silo might not cause any difficulty in the field compaction. In order to enable adjustment of the laboratory aging based on the expected field time aging, it is recommended that the correlation between the aging in a flat pan and aging in a silo to be investigated. This study could involve comparison of the performance of plant-aged and laboratory-aged specimens. In the current study two sources of aggregates with 4% to 5% levels of absorption were used. It is recommended to conduct a similar study using aggregates with 3% to 4% absorption. The recommended study would increase the validity of the current study since the whole range of absorptive aggregates would be covered. In addition, it provides further insight to the current findings of the aging time study.
