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Suggested Citation:"Chapter 7 - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2017. Material Properties of Cold In-Place Recycled and Full-Depth Reclamation Asphalt Concrete. Washington, DC: The National Academies Press. doi: 10.17226/24902.
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Page 69
Page 70
Suggested Citation:"Chapter 7 - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2017. Material Properties of Cold In-Place Recycled and Full-Depth Reclamation Asphalt Concrete. Washington, DC: The National Academies Press. doi: 10.17226/24902.
×
Page 70
Page 71
Suggested Citation:"Chapter 7 - Conclusions." National Academies of Sciences, Engineering, and Medicine. 2017. Material Properties of Cold In-Place Recycled and Full-Depth Reclamation Asphalt Concrete. Washington, DC: The National Academies Press. doi: 10.17226/24902.
×
Page 71

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69 This study measured and evaluated the dynamic modulus and RLPD characteristics from field- produced and field-cured bituminously stabilized cold-recycling mixtures. Dynamic modulus and RLPD properties are inputs to the mechanistic-empirical pavement design methodology embodied in the AASHTOWare Pavement ME Design software. Before this study, little was known regarding appropriate dynamic modulus and RLPD values for cold-recycled materials for use as inputs to mechanistic-empirical pavement design. This study has developed an initial catalog of these properties for bituminously stabilized FDR, CIR, and CCPR cold-recycled materials. In addition to providing representative values, the investigations examined whether signifi- cant differences existed in the dynamic modulus values of FDR, CIR, and CCPR mixtures using different recycling/stabilizing agents and chemical additives. The investigation included statistical analyses of dynamic modulus data at 10 Hz and temperatures of 4.4°C, 21.1°C, and 37.8°C, as well as an evaluation of data envelopes developed from the master curves. The principal conclu- sions regarding stiffness that were derived from these investigations include the following: • All three recycling processes had a similar range of dynamic modulus values at intermediate and high reduced frequencies. This conclusion was supported by the statistical testing and is similar to the trend observed by Diefenderfer, Bowers, and Diefenderfer (2015) based on FWD testing. Many highway agencies specify lower structural values (whether layer coefficients or moduli) for FDR than for CIR and CCPR; these lower values may be too conservative. • FDR showed less temperature dependency and higher stiffness at low reduced frequencies (or higher temperatures), as supported by the statistical testing. Given that CIR and CCPR are composed mostly or entirely of RAP, whereas FDR is composed of RAP and material from an unbound layer, this finding suggests that the existing RAP binder may play a role in the temperature-dependent stiffness properties of CIR and CCPR. • The master curve data envelopes exhibited much overlap between emulsified asphalt versus foamed asphalt as stabilizing/recycling agents, and no significant difference was shown by the statistical tests. Visual observation of the master curve data envelopes suggests that recycled mixtures using foamed asphalt as the stabilizing/recycling agent may be slightly stiffer at higher temperatures, whereas recycled mixtures using emulsified asphalt as the stabilizing/recycling agent may be slightly stiffer at lower temperatures. • Visual observation of the master curve data envelopes showed that the presence of a chemical additive generally increased the dynamic modulus values of the recycled mixtures as compared to mixtures with no chemical additive present. When separating the recycling process, the recycling/stabilizing agent, and presence and kind of chemical additive, the statistical tests showed significant differences. • Visual observation of the master curve data envelopes showed that the presence of a chemical additive generally reduced the recycled materials’ stiffness temperature dependency. When evaluating the master curve fitting parameters, no significant difference was shown in the C H A P T E R 7 Conclusions

70 Material Properties of Cold In-Place Recycled and Full-Depth Reclamation Asphalt Concrete alpha (a) parameter over the total reduced frequency range; however, a significant difference was observed in the gamma (g) parameter with respect to the effect of lime on CIR specimens using emulsion. This statistical evaluation was based on a limited data set due to an insuf- ficient number of specimens. • No significant difference was found when comparing the use of hydraulic cement versus lime as a chemical additive at 21.1°C and 37.8°C; however, only the CIR process had projects that used both hydraulic cement and lime as a chemical additive. • The presence of chemical additives was found to be beneficial with respect to stiffness even though the materials used for testing were 12–24 months old. This finding suggests that the benefits of chemical additives last beyond the initial performance period. • No strong correlations were found between mixture characteristics (e.g., volumetrics, grada- tion, density) and stiffness. This finding may be a consequence of the small number of mixtures given the large range of processing types, stabilizing agents, and chemical additives. The investigations also examined whether significant differences existed in the RLPD proper- ties of FDR, CIR, and CCPR mixtures using different recycling/stabilizing agents and chemical additives. The investigations evaluated data envelopes representing accumulated permanent strain versus load cycle. The principal conclusions regarding RLPD properties derived from these investigations include the following: • All three recycling processes had similar RLPD characteristics as defined by visual observation of their data envelopes. CIR and CCPR specimens were found to behave very similarly. FDR specimens were found to exhibit lower permanent deformations than CCPR and CIR specimens in some cases. This finding is consistent with the trends in the dynamic modulus envelopes. • Emulsified asphalt and foamed asphalt stabilizers performed similarly based on visual obser- vation of RLPD data envelopes. This finding is consistent with the trends in the dynamic modulus envelopes. • Visual observation of the RLPD data envelopes showed that the presence of chemical additives generally increased the mixture’s resistance to permanent deformation. In particular, cement reduced the amount of permanent deformation exhibited by the recycled materials. • The presence of chemical additives was found to be beneficial with respect to rutting suscepti- bility even though the materials used for testing were 12–24 months old. This finding suggests that the benefits of chemical additives last beyond the initial performance period. • No clear correlation was found between the slope and intercept values of the power-law RLPD curve and density. This result is most likely a consequence of the small number of mixtures given the large range of processing types, stabilizing agents, and chemical additives. • The findings suggest that the acceptable COV from AASHTO TP 79 does not adequately reflect the typical variation seen in recycled materials. The allowable variation needs further study for cold-recycled materials. Predicted performance was evaluated for all of the cold-recycled materials considered in this study. Two baseline pavement scenarios were considered: (1) a rehabilitated pavement having a cold-recycled inlay and an asphalt surface wearing course and (2) a rehabilitated pavement having a HMA recycled inlay and an asphalt surface wearing course. Three wearing course thick- nesses with appropriate traffic levels and three climate scenarios were evaluated. All performance predictions were performed using AASHTOWare Pavement ME Design software (Version 2.0) with Level 1 dynamic modulus and RLPD property inputs for the cold-recycled inlay, the HMA inlay, and the asphalt surface wearing course. The investigation examined rutting as the principal distress mode. Conclusions drawn from the rutting predictions for the cold-recycled mixtures considered in this study include the following: • The predicted rutting performance of the cold-recycled sections generally fell within accept- able ranges. Thirty percent of the analysis cases exhibited poor rutting performance, and these

Conclusions 71 were mostly sections with only a thin HMA surface wearing course. Predicted rutting decreased significantly as the wearing course thickness increased to 3 in. and 4 in. • Predicted rutting for the cold-recycled inlay scenarios decreased as the HMA surface wearing course thickness increased. As the cold-recycled layer is pushed deeper into the pavement structure, the behavior approached that of the HMA inlay reference scenario. • The temperature susceptibility (k2) and traffic (k3) exponents in the MEPDG rutting model have direct and major impact on the predicted rutting. These exponents are based on the laboratory-measured RLPD plastic strain response and on the resilient strains. Cold-recycled materials that exhibited poor laboratory RLPD behavior (e.g., high k3, high k2) also exhibited poor predicted rutting performance. • The AMPT used for laboratory testing in this study did not report the resilient strains during RLPD testing, so these strains were estimated based on the measured unconfined dynamic modulus modified to correct for the influence of confining stresses in the RLPD test. It is recom- mended that future testing use an AMPT that directly reports the resilient strains in the RLPD test. Alternatively, the dynamic modulus tests could be performed under confined rather than unconfined conditions so that the appropriate resilient strains can be estimated more accurately. Rehabilitated pavement sections having good quality cold-recycled materials and a moderately thick HMA surface wearing course (e.g., 2 in. thick or thicker) exhibited predicted pavement performance comparable to that of conventional HMA rehabilitated sections.

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TRB's National Cooperative Highway Research Program (NCHRP) Research Report 863: Material Properties of Cold In-Place Recycled and Full-Depth Reclamation Asphalt Concrete presents procedures for determining material properties of cold-recycled asphalt mixtures for input to pavement structural design programs. Highway agencies are placing increasing emphasis on sustainability, recycling, and making maximum use of existing pavement assets in rehabilitation strategies. Such emphasis has led agencies to explore the advantages of producing asphalt mixtures using cold-recycling technology, particularly cold in-place recycling (CIR), cold central-plant recycling (CCPR), and full-depth reclamation (FDR).

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