Material Properties of Cold In-Place Recycled and Full-Depth Reclamation Asphalt Concrete (2017)

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1 Pavement recycling techniques, including cold in-place recycling (CIR), cold central-plant recycling (CCPR), and full-depth reclamation (FDR), are effective techniques for rehabilitat- ing existing pavements or constructing new pavements while reducing construction costs, environmental impacts, and construction time. Emulsified asphalt or foamed asphalt can be used as a recycling agent for CIR and CCPR or as a stabilizing agent for FDR, but it is not clear if performance differences exist among the recycling techniques or recycling/stabilizing agents. In addition to using recycling/stabilizing agents, chemical additives such as hydraulic cement, lime, fly ash, or lime kiln dust may be added for some mixtures. The lack of quantitative values for the engineering properties of CIR/CCPR/FDR materi- als that can be used with confidence in pavement structural design is a major impediment to more widespread use of these fast, cost-effective, and sustainable rehabilitation strategies. The Mechanistic–Empirical Pavement Design Guide (MEPDG) methodology developed under NCHRP Project 1-37A and implemented in the AASHTOWare Pavement ME Design soft- ware provides little guidance for using these processes. Accordingly, NCHRP Project 09-51 was undertaken to determine the relevant material properties for CIR/CCPR/FDR materials using bituminous stabilizing agents for mechanistic-empirical pavement design. Specifically, the determination of typical values of dynamic modulus (|E*|) and repeated load permanent deformation (RLPD) structural properties was the primary objective of this study, as these are the inputs required by the Pavement ME Design software. Although CIR/CCPR layers could be candidates for bottom-up fatigue cracking, very little in the literature suggests this as an important distress mode for the types of pavements considered in this study; thus, no fatigue characterization was performed for the cold-recycled materials. An important complication is the effect of field curing on the properties of cold-recycled materials. Stiffness has been observed to increase substantially during field curing; it is assumed that permanent deformation resistance similarly increases during curing. Measurement of the structural properties of cold-recycled materials during design is problematic because of the difficulties of simulating field mixing, compaction, and curing conditions in the laboratory. Consequently, this study focused on the evaluation of typical structural properties (dynamic modulus, permanent deformation) of CIR/CCPR/FDR materials using bituminous stabilizing agents under field-mixed, field-compacted, and field-cured conditions via laboratory testing of field cores taken 12 or more months after placement. Cores were obtained from 27 projects located throughout the United States and Canada. Projects were selected for coring if the recycled layer was approximately 12–24 months old at the time of sampling and a mix design was available for the recycled layer. The original intention was to find projects spanning a matrix of environmental conditions, recycling techniques, and recycling agents, but it was difficult to obtain cores from a sufficient number S U M M A R Y Material Properties of Cold In-Place Recycled and Full-Depth Reclamation Asphalt Concrete

2 Material Properties of Cold In-Place Recycled and Full-Depth Reclamation Asphalt Concrete of projects to meet all the desired criteria of the matrix. Within the timeframe of the study, cores were therefore sought from as many projects as possible that met the requirements of time since construction and availability of mix design. The final project mix included 15 CIR projects (three foamed and 12 emulsified asphalt), three CCPR projects (all emulsi- fied asphalt), and six FDR projects (4 foamed and 2 emulsified asphalt). Project mixes also included a range of additives (cement, lime) and, in some cases, no additives. The thin layers for most CIR/CCPR/FDR construction preclude testing of conventional full-size 100 mm diameter by 150 mm tall cylindrical specimens. Instead, small-scale cylindri- cal specimens were fabricated for testing. The small-scale cylindrical specimens enabled the same boundary conditions and specimen geometry for both dynamic modulus and RLPD testing. Cylindrical 50 mm diameter test specimens were extracted by sub-coring perpendicu- lar to the long axis of a field core. Following the sub-coring procedure, the ends of the 50 mm diameter sub-cores were trimmed with a diamond wet saw to create a 110 mm tall specimen with flat ends. Tests conducted in this study and by others in previous studies found very good agreement between full-size (100 mm diameter, 150 mm length) and small-scale (50 mm diameter, 110 mm length) specimens for dynamic modulus testing. Unconfined dynamic modulus testing of the cold-recycled materials was conducted on the small-scale cylindrical specimens extracted from field cores. The testing was performed gener- ally in accordance with AASHTO TP 79. Modifications to the specification included a reduced set of temperatures (4.4°C, 21.1°C, and 37.8°C), the small-scale cylindrical specimens, and adjustments to the accepted test result variability. RLPD testing of the cold-recycled materials also was conducted on small-scale cylindrical specimens extracted from field cores. A repeated deviator stress of 482.6 kPa was applied at a constant confining stress of 68.9 kPa. The testing was performed generally in accordance with AASHTO TP 79. Modifications to the test included a lower test temperature (45°C), the small-scale cylindrical specimen geometry, and adjustments to the accepted test result variability. Predicted pavement performance was evaluated for all of the cold-recycled materials tested in this study. Two baseline pavement scenarios were considered: (1) a rehabilitated pavement having a cold-recycled inlay and (2) an asphalt surface wearing course and a rehabilitated pavement having a hot mix asphalt (HMA) recycled inlay and an asphalt sur- face wearing course. Three wearing course thicknesses with appropriate traffic levels and three climate scenarios were evaluated. All performance predictions were performed using Version 2.0 of the Pavement ME Design software with laboratory-measured (i.e., Level 1) dynamic modulus and RLPD property inputs for the cold-recycled inlay, the HMA inlay, and the asphalt surface wearing course. The investigation focused on rutting as the principal distress mode. A significant result of this study is the development of an initial catalog of measured typi- cal dynamic modulus and RLPD properties for bituminously stabilized CIR, CCPR, and FDR cold-recycled materials. Prior to this study, little was known regarding appropri- ate dynamic modulus and RLPD values for cold-recycled materials to use as inputs to mechanistic-empirical pavement design. Another significant result of this study is the evaluation of differences in the measured dynamic modulus and RLPD properties of FDR, CIR, and CCPR mixtures using different recycling/stabilizing agents and chemical additives. These evaluations included statistical analyses (with a systematic procedure for eliminating outliers) and data envelopes. Data envelopes, bounded by the maximum and minimum average material property values, were developed to compare project and material types by visual observation. The data envelopes

Summary 3 are useful because the statistical analyses indicate only that a difference exists between proj- ect or material types, not the direction of the difference. The material properties for similar project or material types were grouped together. The groupings included recycling process, stabilizing/recycling agent, and presence or absence of chemical additives. For dynamic modulus, the investigations included statistical analyses of |E*| data at 10 Hz and temperatures of 4.4°C, 21.1°C, and 37.8°C, as well as an evaluation of the data envelopes developed from the mixture master curves. The principal conclusions regarding stiffness derived from these investigations include the following: • All three recycling processes had a similar range of dynamic modulus values at intermedi- ate and high reduced frequencies. 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). The likely cause is that CIR and CCPR are composed mostly or entirely of RAP (reclaimed asphalt pavement) and the temperature dependency of stiff- ness is controlled by the existing RAP binder. • 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 observations of the master curve data enve- lopes suggest 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. • The master curve data envelopes showed that the presence of a chemical additive gen- erally increased the dynamic modulus values of the recycled mixtures as compared to mixtures with no chemical additive. When separating the recycling process, the recycling/ stabilizing agent and, the use of chemical additives, significant differences were shown. • 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 master curve data envelopes showed that the presence of a chemical additive gen- erally reduced the temperature dependency of stiffness for the cold-recycled materials. Although an increased temperature dependency was found for those mixtures having no chemical additive, no clear trend was shown by the statistical tests. • 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. • The acceptable coefficient of variation (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. No strong correlations were found between mixture characteristics (e.g., volumetrics, gradation, density) and stiffness. This result likely is a consequence of the small number of mixtures given the large range of processing types, stabilizing agents, and chemical additives. The slope and intercept properties of the RLPD data (in log-log space) were evaluated using the same techniques as for dynamic modulus. The principal conclusions regarding RLPD properties include the following: • All three recycling processes had similar RLPD characteristics as defined by their data envelopes. CIR and CCPR were found to behave very similarly. FDR was found to exhibit

4 Material Properties of Cold In-Place Recycled and Full-Depth Reclamation Asphalt Concrete lower permanent deformations than CCPR and CIR in some cases. This finding is con- sistent with the trends in the dynamic modulus envelopes. • Emulsified asphalt and foamed asphalt stabilizers performed similarly in terms of RLPD. This result is consistent with the trends in the dynamic modulus envelopes. • The data envelopes showed that the presence of chemical additives generally increased the 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 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. • The acceptable COV from AASHTO TP 79 does not adequately reflect the typical varia- tion seen in recycled materials. The allowable variation needs further study for cold- recycled materials. No strong correlations were found between slope and intercept values 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. Predicted rutting performance was evaluated for all of the cold-recycled materials con- sidered in this study and compared against predicted rutting for an equivalent conventional HMA rehabilitation scenario. 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 acceptable ranges. Thirty percent of the analysis cases exhibited poor rutting perfor- mance; most of these cases were sections with a thin HMA surface wearing course. • Predicted rutting for the cold-recycled inlay scenarios decreased as HMA surface wearing course thickness increased. As the cold-recycled layer is pushed deeper into the pavement structure, rutting approaches that of the HMA inlay reference scenario. • Cold-recycled materials that exhibited poor laboratory RLPD behavior (e.g., high traffic exponent k3, high temperature susceptibility exponent k2) also exhibited poor predicted rutting performance. • The asphalt mixture performance tester (AMPT) used for laboratory testing in this study did not report the resilient strains during RLPD testing, so these were estimated based on the measured unconfined dynamic modulus modified to correct for the influence of con- fining stresses in the RLPD test. It is recommended 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 mod- erately thick HMA surface wearing course (e.g., 2 in. thick or greater) exhibited predicted pavement performance comparable to that of conventional HMA rehabilitated sections.