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111Â Â The literature review documented information about pavement design, mix designs, perfor- mance testing, service life, cost savings, and environmental benefits. The agency survey, discus- sions with current and former agency staff, and a review of agency reports provided information about project selection, construction procedures and practices, and barriers to increased use of cold recycling. Literature Review 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. The newer AASHTO MEPDG pavement design methodology uses laboratory testing to establish material properties that provide inputs for the performance prediction models. Several sources of data for dynamic moduli values were found, but few sources were found that can be used for cracking and rutting models. A variety of specimen preparation methods and modifications to standard test method temperatures and conditioning processes are used to evaluate cold recycled mixes. Specific gravities are frequently determined using the 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 evaluates Marshall stability or indirect tensile strength. Moisture sensitivity of the mix is assessed by eval- uating 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 among 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 34 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 conventional mill and fill. Greenhouse gas emissions can be reduced by about 50% compared to conventional mill and fill. C H A P T E R 5 Conclusions
112 Practice and Performance of Cold In-Place Recycling and Cold Central Plant Recycling Agency Survey and Case Examples According to the survey responses and interviews, most cold recycling programs pave less than 50 lane-miles per year. Cold recycling is frequently used on roadways with annual average daily traffic 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 emul- sion 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 process or material over another. Experienced agencies select potential cold recycled projects based on pavement distresses and conventional pavement maintenance and rehabilitation triggers. Site investigations, coring, and assessments of adequate subgrade support and drainage are an essential part of the project selection process. With thorough site investigations and pavement designs, CIR can be used to recycle existing pavement down to the subgrade when the soil has adequate strength. Many agencies also use traffic levels as a project selection criterion. Less experienced agencies limit cold recycling to AADT levels generally under 5,000, while experienced agencies use cold recycled mixes on roadways with over 10,000 AADT. Successful CIR projects can be constructed in hot desert climates and on mountain roadways with logging truck traffic. Construction processes and material quantities are influenced by the existing pavement prop- erties and environmental conditions. The hardness of the existing pavement is a function of the oxidation of the asphalt and the temperature of the pavement. Changes in hardness and mat temperature influence milling operations and the resulting RAP gradations, needed field adjust- ments for liquid quantities, cold recycled mix workability, curing time, and strength gain. 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. Consistent grada- tions and moisture contents improve the consistency of material properties 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. Test strips can be used to establish the maximum density that can be achieved for the project- specific cold recycled mix and current environmental conditions. However, a few agencies enforce density requirements based on a percentage of the laboratory specific gravities. The in-place density is measured with a nuclear density gauge. The gauge can be calibrated with wet maximum specific gravity measurements, which are calculated using the mix design maximum specific gravity and the moisture content of the field mix. Cold recycled mixes with emulsions need to show some evidence that the emulsion is starting to break before rolling starts. A sufficient number of rollers is needed so the rolling can keep up with the paving operation without needing to increase the speed of the rollers. If the cold recycled mix is placed in a windrow, the paver elevator needs to be close enough to the pugmill so the emulsion does not break in the windrow and it needs sufficient horsepower to pick up the mix. The shear vane test can provide useful information for when rolling will be most effective and when traffic can be allowed on the new layer.
Conclusions 113Â Â Adequate curing of the cold recycled mix needs to occur before placement of the final wearing surface. This is commonly done by defining maximum allowable moisture content or by setting a limit on the time between recycling and overlay placement. The reduction of in-place moisture can be difficult to confirm during rainy periods. The ride quality of cold recycled mix projects can meet existing ride quality specifications. More experienced contractors profile the existing roadway to identify areas that can be pre- milled to improve the final ride quality. Experienced contractors can meet incentive limits for ride quality with the cold recycled layer and a single lift overlay. The most frequently cited barriers to increased use of cold recycling processes are the lack of agency experience, the lack of experienced contractors, and the lack of well-defined project selection criteria. The lack of quality assurance testing and construction specifications for CCPR with foamed asphalt recycling agent and previous unsuccessful experiences with emul- sion CIR recycling agent were also cited as barriers for increased use. Most agencies have not yet used the recently released AASHTO and ARRA standards, however, several agencies are planning on using the AASHTO standards for upcoming projects. Recommendations for Future Research Suggestions for future research include the following: â¢ Modify and standardize conventional testing procedures for use with cold recycled mixes. â¢ Develop a cold recycled mix design procedure that includes an assessment of specimens compacted at different temperatures. â¢ Define the factors behind problems with emulsions that a number of agencies reported were a barrier to increased use. â¢ Establish key temperatures (air and pavement) needed for producing acceptable cold recycled mixes with emulsions. â¢ Define key factors for improved mix properties with supplemental compaction. â¢ Define time limits and procedures for quality assurance testing in the field. â¢ Evaluate performance characteristics and service life of CIR and CCPR projects. â¢ Update current definitions of CIR and CCPR to include a description of key material properties. â¢ Evaluate where cold recycling belongs on the pavement deterioration curve. While determining the density and air voids of asphalt mixes is routinely done during mix design and for QA testing, the partially, or thinly, coated RAP particles and the porous nature of the cold recycled mixes makes the selection of the most appropriate test method problem- atic. Also, routine testsâsuch as Marshall stability, indirect tensile strength, and moisture conditioningâare used to evaluate cold recycled mixes. However, the specimen compaction levels, conditioning temperatures, and saturation levels are routinely adjusted for cold recycled mixes. These adjustments need to be standardized. The Utah DOT research showed that the densities that can be achieved at key temperatures likely to be encountered during construction need to be evaluated. This evaluation would pro- vide guidance on when additional materials are needed and would bracket a range of densities that can be expected during construction. A comprehensive study is needed to evaluate how temperature differences can be incorporated into cold recycled mix designs. Various agencies reported poor experiences with emulsions in cold recycled mixes as a barrier to increased use. However, ensuing discussions revealed the reasons for âpoor experiencesâ are
114 Practice and Performance of Cold In-Place Recycling and Cold Central Plant Recycling varied and may be a function of unexpected environmental conditions. Guidance is needed for selecting an appropriate emulsion that can be locally sourced for use in cold recycling projects. Emulsions are used to produce cold recycled mixes, but the emulsions are usually held at warm temperatures slightly above pavement temperatures. When the existing pavement temperatures and/or the emulsion temperatures are too low or too high, the quality of the field produced mix can suffer. The impact of the individual material component temperatures on the in-place mix properties needs to be investigated so that appropriate temperature limits can be defined. Supplemental compaction can significantly improve the in-place density. But at this time, the use and timing of supplemental compaction are generally left to the discretion of the contractor. Research is needed to identify key factors for using supplemental compaction. The time between sampling and testing significantly influences test results. Well-defined time requirements for sampling, specimen preparation, and testing are needed so that QA testing will provide useful and accurate results. While various agencies and researchers have documented the long-term performance of cold recycled mix properties, they each used their own methods for calculating the pavement condition rating and for statistical analyses. This variation limits the applicability of the find- ings because the underlying factors that influence service life are not consistently included in the analysis and resulting performance models. A comprehensive study is needed to develop material inputs and verify performance prediction models that include cold recycled mix layers. The current definitions of cold in-place and cold central plant recycling describe the processes but not the material characteristics. Cold recycled mixes are viscoelastic materials that are distinctly different from conventional hot asphalt dense-graded mixes. Cold recycled mixture pavement layers significantly reduce reflective cracking, have adequate indirect tensile strengths, can resist rutting from heavy traffic loads, and show a gradual increase in strength over the first few years. Including the material property characteristics in the definition will help users under- stand the most appropriate uses for these materials. CIR and CCPR have two uses: as an alternative to conventional mill and fill, and to rehabili- tate the entire asphalt layer. These two uses are applied at different locations on the pavement deterioration curve. Currently, the mill and fill option is shown on the pavement deterioration curve at about 60 PCI, but cold recycling is only shown as an option at about 40 PCI. Adding cold recycling as an alternative to mill and fill at the higher PCI level needs to be considered. The usefulness of cold recycling as an alternative to mill and fill may help increase the use of these processes earlier in the life of the pavement.