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1  Cold recycling processes provide agencies with cost-effective and environmentally friendly pavement maintenance and rehabilitation options for aged asphalt pavements. Cost savings are achieved by reducing the need for new materials; requiring fewer haul trucks; lowering fuel consumption; and eliminating the need for adjustments to curb, gutter, and overhead clearances. Environmental benefits include the reduced need for natural resources and lower generation of greenhouse gases by both the construction process and the reduced time vehicles spend in traffic queues. Cold in-place recycling (CIR) is a process in which 3 to 4 in. (75 to 100 mm) of the existing asphalt pavement layers are pulverized, mixed with a recycling agent, and repaved in place (Wagner 2018). Cold central plant recycling (CCPR) is similar to CIR, but it can recycle thicker pavement layers and the recycling operation occurs at a nearby, usually mobile, plant location. CCPR can use reclaimed asphalt pavement (RAP) obtained from the roadway under construction, a mixture of RAP from various nearby projects, or previously stockpiled RAP. Neither CIR nor CCPR is used to modify the underlying base and subgrade materials, so roadways need to be structurally sound with good drainage. The objective of this synthesis is to document how projects suitable for CIR and CCPR processes are selected, considered in the pavement design phase, constructed, and eval- uated. Three methods were used to accomplish this objective: (1) an online agency survey, (2) a literature review, and (3) interviews to collect agency experiences for case examples. The online state agency survey assessed the size, extent, and experience with CIR and CCPR as well as with newly published AASHTO and Asphalt Recycling and Reclaiming Association (ARRA) standards. A total of 40 agencies responded for a response rate of 80%, although not all respondents answered all questions. Most cold recycling programs pave less than 50 lane-miles per year. Cold recycling is frequently used on roadways with annual average daily traffic (AADT) under 10,000, but more experienced agencies use cold recycling on roadways with AADTs between 10,000 and 25,000. Agencies have the most experience using CIR with emulsion recycling agents. Additional communications with various current and former agency staff revealed that the cold recycling process and the type of recycling agent used by an agency are limited by what is locally available and not usually the result of a preference for one material or process over another. Agencies most frequently cited three barriers to increased use of cold recycling processes: the lack of agency experience, the lack of experienced contractors, and the lack of well- defined project selection criteria. Other barriers to increased use were the lack of quality assurance testing and construction specifications for CCPR with a foamed asphalt recycling agent and previous unsuccessful experiences with an emulsion CIR recycling agent. Most S U M M A R Y Practice and Performance of Cold In-Place Recycling and Cold Central Plant Recycling
2 Practice and Performance of Cold In-Place Recycling and Cold Central Plant Recycling agencies have not yet used the recently released AASHTO and ARRA standards; however, several agencies are planning to use the AASHTO standards for upcoming projects. The literature review evaluated published CIR and CCPR literature, research reports, and conference proceedings published on agency research websites, in peer-reviewed journals, and by regional user-producer organizations. The literature review documented information about pavement design, mix designs, performance testing, service life, cost savings, and environmental benefits. Agencies using the older AASHTO 1993 pavement design method employ a range of structural layer coefficients. AASHTOâs newer Mecha- nistic Empirical Pavement Design Guide (MEPDG) methodology uses laboratory testing to establish material properties that provide inputs for the performance prediction models. Several sources of data were found for dynamic moduli values, but few sources were found that can be used for cracking and rutting models. A variety of specimen preparation methods and modifications to the standard test method temperatures and conditioning processes are used to evaluate cold recycled mixes. Specific gravities are frequently determined using traditional hot asphalt mixture test methods, but several agencies use vacuum sealing methods because cold recycled mixture specimens have higher air voids and RAP particles that are not fully coated. Basic mix design testing evalu- ates Marshall stability or indirect tensile strength. Moisture sensitivity of the mix is assessed by evaluating the ratio of either stability or strength after saturation and soaking. Soaking and testing temperatures as well as saturation levels are usually, but not always, decreased, and conditioning may or may not include a freeze cycle. Agencies may use performance testing as part of balanced mix designs. Compaction levels, curing times, and curing temperatures vary widely between agencies and have a significant impact on performance testing results. Cold recycled mixtures can be easily damaged during sample preparation and can deform at platen and specimen clamp points. The reported service life of cold recycled pavements ranges from 20 to 34Â years when the cold recycled mix is used in conjunction with an overlay. The service life is somewhat shorter and more variable when chip seals are used as the wearing surface. Poor drainage can reduce the service life by 30% or more. Cold recycling with an overlay can reduce the cost of a project by 40% to 60% compared to a conventional mill and fill. Greenhouse gas emissions can be reduced by about 50% compared to a conventional mill and fill. Six agency case examples were developed to address individual key topics that were not specifically covered with the survey and the literature review. The topics include (1)Â project selection criteria based on pavement distresses (Indiana), (2) an agencyâs first experience with CCPR (Minnesota), (3) a CCPR plant owned and operated by an agency (Maine), (4)Â innovative mix design and construction control testing (Utah), (5)Â smooth- ness specification applied to cold recycled mixes (Caltrans), and (6) more than four decades of experience with cold recycling (Federal Lands Highway). The major advantage with CCPR is control over the consistency and quality of the cold recycled mix. Stockpiling RAP and then processing it immediately before use helps rework and blend RAP from various sources. It also permits specifying a smaller maximum size RAP because oversized particles can be reground to meet the finer gradation requirements. With the more prevalent use of finer cutting heads on the millers, smaller maximum size RAP is more easily obtained with little further processing. The smaller maximum particle size helps reduce segregation. Stockpiling RAP also helps stabilize the RAP moisture content. Con- sistent gradations and moisture contents improve the consistency of material properties
Summary 3Â Â and in-place densities. An added advantage to finer gradations is that the cold recycled mix can also be used as a leveling course for other paving and maintenance projects. The ride quality of cold recycled mix projects can meet existing ride quality specifica- tions. More experienced contractors profile the existing roadway to identify areas that can be pre-milled to improve the final ride quality. When an overlay is included, experi- enced contractors can meet ride quality incentives with a single lift overlay over the cold recycled layer. Gaps in the available information include documentation on how emulsion tempera- ture during construction influences the mix characteristics, documented procedures and benefits of supplemental compaction, and existing databases of cold recycled mix properties that can be used as inputs for pavement design performance prediction models. Recommendations for future research include the following: (1) modify and standardize conventional testing procedures for use with cold recycled mixes, (2) define the factors behind problems with emulsions that a number of agencies reported as a barrier to increased use, (3) define key factors for improved mix properties with supplemental compaction, (4)Â define time limits and procedures for field quality assurance testing, (5)Â update current definitions for CIR and CCPR to include a description of key material properties, and (6)Â evaluate where cold recycling belongs on the pavement deterioration curve.
