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5 1.1 Background Pavement recycling techniques, including cold in-place recycling (CIR), cold central-plant recycling (CCPR), and full-depth reclamation (FDR), are effective techniques for rehabilitating existing pavements or constructing new pavements while reducing the construction costs, envi- ronmental impacts, and construction time (Bemanian et al. 2006; Thenoux et al. 2007; Stroup- Gardiner 2011). The use of these techniques is not widespread, however, partially because of the lack of quantitative values for the engineering properties that can be used with confidence for pavement structural design. In addition, the measurement of relevant structural properties can be problematic given the lack of consensus and difficulties of simulating field mixing, compaction, and curing conditions in the laboratory. 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. Chemical additives often are included in mixtures to improve early strength and improve resistance to the detrimental effects of moisture, as well as for other potential uses (Asphalt Academy 2009; ARRA 2014); however, it is not well established whether the chemical additives contribute positively to the long-term performance of recycled mixtures. CIR often is used to rehabilitate existing asphalt pavements by recycling a portion of the exist- ing bound layers to a depth of 2 in. to 5 in. (ARRA 2014). CIR may be completed using a single- unit train, wherein the milling and recycling agent addition processes (and blending of chemical additives, if used) are incorporated into a single machine. CIR also may be completed using a multi-unit train that includes a cold planer, a screening and crushing unit, and a pug mill unit. For either process, the resulting material may be picked up from a windrow into a conventional paver or deposited directly into a paver hopper. CCPR is similar to CIR, but the recycling agent and secondary additives (if used) are added at a mobile plant located at or near the recycling project or RAP source stockpile. If the source material for the CCPR process comes directly from an existing pavement, the materials are milled, processed at the CCPR plant, and then placed using traditional asphalt mixture paving equipment. If the source material for the CCPR process is an existing RAP stockpile, the mobile plant can be centrally located, and processed material can be hauled to the construction project and placed using traditional asphalt mixture paving equipment. CCPR is advantageous because materials from an existing project can be removed and stockpiled, thus allowing access to stabi- lize or replace the underlying foundation, if needed. Additionally, the CCPR process can be used to (1) place multiple lifts for thicker applications of recycled materials and (2) produce a recycled base course for new construction projects, including lane widening, shoulder strengthening, C H A P T E R 1 Introduction
6 Material Properties of Cold In-Place Recycled and Full-Depth Reclamation Asphalt Concrete and other uses. Typical layer thicknesses for CCPR range from 2 in. to 6 in. (ARRA 2014), but multiple lifts may be used to increase the total thickness of the recycled material. If deeper structural deficiencies are encountered, FDR can be used to reconstruct a pavement section by recycling a portion of the existing bound and unbound layers. FDR is used to stabi- lize a layer between 4 in. and 12 in. thick (ARRA 2014). The FDR process is completed using a reclaimer, motor grader, and compaction equipment. FDR can be advantageous because it provides a foundation layer that can aid in reducing the strain in the overlying pavement under load but at a lower cost than complete replacement with conventional base materials (Diefenderfer et al. 2016). Despite the many advantages of incorporating pavement recycling techniques into pavement rehabilitation or new construction projects, for many reasons, highway agencies generally have not widely embraced these processes (Stroup-Gardiner 2011; Diefenderfer and Bowers 2015). These reasons include a lack of familiarity with the processes, hesitation to try processes still thought to be experimental, inconsistencies with specifications across agencies, limited long-term field performance data, and a lack of consensus about the fundamental engineering properties used for pavement design, among others. Much published work related to determining the fundamental engineering properties of recy- cled materials has focused on measuring either the stiffness of an asphalt-like material (Kim et al. 2009; Diefenderfer and Link 2014) or the shear properties of a stabilized aggregate-like material (Jenkins et al. 2007). Given that ongoing work has shown that recycled materials can success- fully be incorporated as a load-bearing layer within a heavily trafficked pavement (Diefenderfer, Bowers, and Diefenderfer 2015; Diefenderfer et al. 2017), a need exists to assess the permanent deformation properties using test procedures that can relate the two perspectives. 1.2 Study Objectives 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 MechanisticâEmpirical Pavement Design Guide (MEPDG) methodology developed under NCHRP Project 1-37A and now available as the AASHTOWare Pavement ME Design software provides little guidance for using these processes (AASHTO 2015). This study was undertaken to determine the relevant material properties for CIR/CCPR/FDR materials using bituminous stabilizing agents for pavement structure design. The determination of typical values of dynamic modulus (stiffness) and permanent defor- mation structural properties is the primary objective of this study, as these are the inputs required for mechanistic-empirical pavement structural design. 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. The excep- tions cited in the literature are primarily from South Africa, where the pavements have high stress-to-strength ratios because of the thin surfacing over the CIR coupled with high traffic/ load levels. In the United States, only very lightly trafficked roads are likely to have thin sur- facing over the CIR/CCPR layer. Most other pavementsâand specifically the types of higher traffic volume pavements that would be designed using the MEPDG, the focus of this studyâ will have moderately thick HMA surface/wearing courses that will suppress stress ratios below the threshold at which fatigue cracking develops. The stiffness of the recycled materials was quantified by conducting dynamic modulus tests. The rutting susceptibility was quantified by conducting RLPD tests.
Introduction 7 A final and 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 dif- ficulties of simulating field mixing, compaction, and curing conditions in the laboratory. Conse- quently, this study focuses on the evaluation of typical structural properties (dynamic modulus, permanent deformation) of CIR/CCPR/FDR materials using bituminous stabilizing agents under field-mixed, compacted, and cured conditions via laboratory testing of field cores taken 12 or more months after placement.