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24 4 STAKEHOLDER SURVEY RESULTS 4.1 Agency Survey A web-based stakeholder survey was conducted from September to December 2020. The objectives of this survey were: (1) to understand agency experience with CR in North America, (2) collect unsearchable state specifications and local specifications to be included and accounted for in the overall specification analysis, and (3) to identify participants to be used in the Targeted Interviews. The survey questions included the following (paraphrased): ï· First and last name; Agency name; Agency type; Location; Role in the agency; ï· Number of years of experience agency has with CIR and CCPR along with foamed asphalt and emulsified asphalt; ï· Approximate number of total projects the agency has had with CIR and CCPR each; ï· How many years of experience the respondent has with CIR and CCPR each; ï· Has the agency completed a CIR or CCPR project in the last five years (separately); ï· Opportunity to link or upload copies of CIR and/or CCPR specification(s); ï· Offer opportunity to be interviewed for the follow-up interviews. The survey was distributed to points of contact at the state and local agency levels with help from the AASHTO Committees on Maintenance, Materials and Pavements, and Construction, as well as the National Association of County Engineers (NACE), though some city or township agencies may not have been reached or may not have responded. Ninety-four individual responses representing 44 state/provincial agencies (43 U.S. DOTs and 1 Canadian province) and 33 local agencies (31 U.S. counties and 2 U.S. cities) were received. Incomplete or blank responses were not counted. Analysis of the survey responses showed that most of the experience with cold recycling methods relates to the use of cold in-place recycling using emulsified asphalt (CIR-E in Figures 4-1 and 4-2). 80% of state agency respondents and 70% of local agency respondents indicated having some experience with CIR-E, as seen in Figures 4.1 and 4.2. It was also found that in general, there is more experience with the use of CIR than CCPR, both at the state and local levels. Six of the 44 state agencies that responded to the survey do not have any experience with CR, while six of the 33 local agencies also indicated having no experience with these methods. This means that 86% of state agency respondents and 82% of local agency respondents have experience with at least one CR method.
25 Figure 4.1. Years of Experience with Cold Recycling â State/Provincial Agencies. CIR-F = cold in-place recycling using foamed asphalted asphalt, CIR-E = cold in-place recycling using emulsified asphalt, CCPR-F = cold central plant recycling using foamed asphalted asphalt, CCPR-E = cold central plant recycling using emulsified asphalt. Figure 4.2. Years of Experience with Cold Recycling â Local Agencies. CIR-F = cold in-place recycling using foamed asphalted asphalt, CIR-E = cold in-place recycling using emulsified asphalt, CCPR-F = cold central plant recycling using foamed asphalted asphalt, CCPR-E = cold central plant recycling using emulsified asphalt. The majority of state/provincial agencies report having completed CR projects (either CIR or CCPR), with most having done 1 to 5 projects (Figure 4.3). At the local level, however, the projects reported have been predominantly CIR projects and only 24% of respondents indicated having at least one CCPR project, as seen in Figure 4.4. It is interesting to note that a significant portion of state agencies (more than 30%) also indicated having performed more than 20 CIR projects, which emphasizes how this method has been more widely used for cold recycling.
26 Figure 4.3. Number of Cold Recycling Projects â State/Provincial Agencies. CIR = cold in-place recycling, CCPR = cold central plant recycling. Figure 4.4. Number of Cold Recycling Projects â Local Agencies. CIR = cold in-place recycling, CCPR = cold central plant recycling. To capture the most recent experience, participants were asked if their agency had completed a CR project in the last five years. 66% of state/provincial agencies and 73% of local agencies reported completing a CIR project in that timeframe, while only 30% of state/provincial agencies and 13% of local agencies indicated completing a CCPR project in that timeframe as shown in Figures 4.5 and 4.6, respectively.
27 Figure 4.5. Completion of Cold Recycling Projects in the Last Five Years â State/Provincial Agencies. CIR = cold in-place recycling, CCPR = cold central plant recycling. Figure 4.6. Completion of Cold Recycling Projects in the Last Five Years â Local Agencies. CIR = cold in-place recycling, CCPR = cold central plant recycling. For state/provincial agencies, Figure 4.7 shows the agencies that have completed a CR project in the last five years. It can be observed that those respondents with recent experience had either completed CIR or both CIR and CCPR projects during that time. The Ontario Ministry of Transportation (not shown in Figure 4.7) also reported completing both CIR and CCPR projects. There were no state/provincial agencies that completed only CCPR projects. Similar results were observed for local agencies, with 20 of them having completed a CIR project and only four of them having completed both types of projects. None of the local agencies reported completing only CCPR projects in the last five years.
28 It can be seen that these projects have been constructed across the country, in different climatic regions. From the map shown in Figure 4.7, it appears that the use of CIR alone is more predominant among state DOTs in the western half of the United States, while those in the eastern half tend to include both CIR and CCPR. Figure 4.7 also shows that recycling is more predominant in the northern half of the US versus the southern half (especially in the eastern and central US) which could be considered counterintuitive based on warmer climatic conditions favoring longer construction seasons. Figure 4.7. State agencies that completed a project in the last five years. CIR = cold in-place recycling, CCPR = cold central plant recycling. Most of the local agencies that indicated completing a CR project in the last five years are located in states where the DOT also has this type of experience (Figure 4.8). The state with the most local agencies that reported having projects in the last five years was Illinois (7), followed by Michigan (4), California (3), Iowa, Minnesota and New York (2 each), and Indiana, Kansas and Wisconsin (1 each). The only states where there were local agencies with recent projects but no reported DOT projects were Alabama, Florida, Kansas, and Michigan. Yes â CIR Yes â CIR and CCPR No Projects
29 Figure 4.8. State with local agencies that completed a project in the last five years. CIR = cold in-place recycling, CCPR = cold central plant recycling. A total of 55 agencies shared a copy of their specifications (34 state/provincial agencies and 21 local agencies). It was found that some local agencies rely on their state/provincial agency CR specification. 4.2 Focused Survey / Interview Members of the project team conducted extended interviews with 16 agency and industry recycling practitioners (10 agency, 6 industry). The project team selected highly experienced agency interviewees from volunteers who self-identified during the online survey and selected industry interviewees based on geographic location and level of experience known to the research team based on direct work or ARRA affiliation. Each extended interview was conducted using online conferencing software and ranged from approximately 20 to 60 minutes, depending on the discussion generated during the interview. All interviewees were asked the same set of questions. These questions are shown below: 1) What are the 3-4 parts of your specification that can help achieve good performance? 2) Are any parts of your specification unnecessary? 3) Are there any specification components that are missing? 4) When should the test strip be performed, as part of initial production or prior to production? Yes â CIR Yes â CIR and CCPR No Projects
30 5) How do you measure moisture in the field? 6) How is the production density target established? 7) Do you perform any mix verification tests? 8) Should agencies consider adjusting tests or test frequency depending on traffic level? 9) What have you found to be some of the best practices to achieve good smoothness? 10) Should smoothness be measured directly on the recycled layer or the final surface? Agency interviewees were asked to comment on their specific agency specification(s) while industry interviewees were asked to comment on those specifications with which they have the most experience. 4.2.1 Specification Components to Achieving Good Performance In response to the question about which 3-4 parts of a specification can help achieve good performance, most responses included standard specification sections such as: mix design, quality assurance, compaction, materials, construction methods, density requirements, weather limitations, and measurement and payment. Also included in the responses were several components not often found in recycling specifications. These included a pre-construction quality control plan, a pre-construction just-in-time training session, and availability of a technical consultant. A pre-construction quality control plan (QCP) is used to identify key personnel from both the agency and contractor who are responsible for the quality of the constructed recycled layer. The QCP also often lists typical areas of concern (such as insufficient density, precipitation during production, etc.) where the contractor is required to document their intended steps to remedy the concern and the personnel in charge. Identifying these actions prior to the occurrence of any issues has been found to aid in resolving many issues more quickly during construction. Similarly, a just-in-time training (JITT) session is often conducted no more than one to two weeks prior to construction and gives an opportunity for quality managers and technicians from the agency and industry to meet each other, review the project scope and the QCP, and other items. The JITT session was noted as being very helpful to those interviewees who use it, even those with many years of experience. General Takeaway: Interviewees mentioned many of the major portions of a standard construction specification as being important, but the value of a pre-construction QCP and JITT was notable. Interviewees noted the pre-construction QCP and JITT need to be considered in the AASHTO Construction Guide Specification. 4.2.2 Specification Components That May Be Unnecessary While it may be difficult to agencies to admit that certain specification components may be unnecessary, respondents with a nuanced understanding of pavement recycling noted that certain specification components may be included because they are easily measured rather than highly influential in the performance of the material. Examples include specifications that require gradation measurements for many sieves, low temperature cracking requirements (rather than relying on local PG binder grades), and laboratory testing of field produced material where the test specimen is fabricated in the laboratory rather than taken from the field.
31 Another commonly cited specification component that was identified as unnecessary was the assumed certainty that a measured percent moisture content could define cured versus un-cured material. It is generally understood that the CIR and CCPR materials undergo a curing process in which the moisture content is reduced from production levels and that surfacing a recycled layer having higher moisture contents could lead to a lack of or a reduced strength gain. However, the threshold value for moisture that determines good versus poor performance is not well understood. Many specifications cite maximum allowable moisture values of 2% to 2.5% or half of the optimum value before surfacing. Often, specifications allow the discretion of the engineer to be used to override these maximum values and surface the layer earlier. The implications of this action are not well understood and have generally not had significant negative impacts across a wide number of projects. As more recycling work is undertaken on pavements carrying higher traffic volumes, the potential for negative impacts could be increasing. In particular, interviewees mentioned that long wait times were often unnecessary and that specific moisture content values should be replaced with the identification of a minimum plateau or trend. General Takeaway: Some testing requirements or curing regimes may not be necessary and are not yet well understood. Flexibility and appropriate commentary on items such as gradation with many sieves, cracking tests, laboratory testing of field materials, and curing could be considered in the AASHTO Construction Guide Specification. In addition, rather than identifying a particular moisture content, future research should investigate the use of a moisture plateau. 4.2.3 Specifications Components that Are Missing By far, the most commonly identified missing component of recycling specifications was the lack of a field test to determine strength properties of the produced material in situ. This topic was addressed in the recent NCHRP Project 09-62 study (Diefenderfer et al. 2021), but the identified tests do not measure the fundamental engineering properties of the materials such that the results could be used in current pavement structure design methods. Either this means that further tests should be investigated, or the current design methods do not focus on the properties that are significant in describing performance. Currently specified strength tests rely mainly on laboratory prepared test specimens created from loose materials sampled at a project. Unfortunately, if the test specimen preparation or curing regime does not well represent field conditions, the results of the test may not be as useful. The current criteria for these strength tests are based more on a history of acceptable performance rather than mechanistically derived properties. Several interviewees also noted three additional missing topics: best practices guidance, acceptance of the constructed layer using properties other than density and moisture content, and inclusion of balanced mix design (BMD) criteria during mix design. While a best practices guideline is not likely to be included in a specification, it was included in this listing since several interviewees mentioned it. In addition, the project team members are not aware of a significant body of research or existing literature related to this topic. Despite recent research efforts on testing of recycled materials, the most often used tests for material acceptance continue to be density and moisture content. A survey of specifications (Chapter 3) shows that the most often used equipment for both of these measurements is a nuclear density gauge despite noted concerns with measuring moisture of asphalt-based materials using this device. Recently, there has been an increased interest in using BMD-style tests to characterize recycled materials
32 during mix design. The primary benefit of using BMD tests for recycled materials is that designers can work to maximize the performance of the recycled material by studying its behavior using several performance tests. General Takeaway: Guidance documents, a product of this research project, are an identified gap by interviewees. Field acceptance and BMD are also topics of interest. The AASHTO Construction Guide Specification could include an option for the NCHRP 09-62 tests in addition to some additional commentary on performance tests that may be related to BMD. 4.2.4 Test Strip Timing Interviewees were asked if a test strip, which is used as a proof of constructability and often as a basis for target density, should be performed prior to mainline production or as part of the initial production of a project. The potential implications for agencies are that conducting the test strip before production can allow an agency to analyze the results of laboratory strength tests that are included in some specifications. From an industry perspective, however, conducting a test strip prior to production requires the mobilization of crews and equipment to a job site for the test strip, and then remobilization to another location or have the crew and equipment remain idle until the test results are obtained. Most agency and industry respondents stated that they felt the test strip should be included as part of the initial production. The reasons for this included reduced idle time for the contractor, the potential for other more rapid tests to determine sufficiency of the material (such as a modified proctor test conducted in the field on field produced material), and the need for a contractor to make small adjustments to the moisture or binder content as environmental and in-situ pavement conditions change during the project (and thus negating the applicability of the initial test strip conditions). General Takeaway: Consideration needs to be given to reducing mobilization efforts as performance verification options are developed within the test strip portion of the AAHSTO Construction Guide Specification. 4.2.5 Measuring Moisture in the Field While oven, hot plate, Speedy, and microwave methods were all moisture content measurement methods mentioned by the interviewees, use of the nuclear density gauge was most often cited. The reasons for this are that the nuclear density gauge is almost always used as the method for measuring in-place density of the material and thus the gauge is already present on most projects. However, it is understood (though sometimes ignored) that the nuclear density gauge moisture measurement of asphalt-based materials is influenced by the hydrogen present in the asphalt binder as well as any water. Some agencies use a correction procedure where another method is used to calibrate the moisture reading from the nuclear density gauge, but the correction process is not always employed. General Takeaway: If moisture content is considered an important element, guidance needs to be provided on the most accepted methods to measure it. Commentary could also be provided on nuclear density correction procedures if used for moisture content in the AASHTO Construction Guide Specifications.
33 4.2.6 Establishing the Production Density Target The production density target identifies the amount of compaction that the contractor must achieve during placement of the recycled material to produce an acceptable material, usually as a percentage of some reference density. The achievable density of a particular material will vary with material temperature, particle shape, and moisture content of the mixture. During the specification review, the project team found several options that are used as the reference density. These options included the maximum density from the mix design, the density obtained by applying a standard compaction effort in a field-based test, or the peak density from a growth curve developed during the test strip. The project team found that few agency specifications require the production density target to be a percentage of the maximum density found during the mix design and this was echoed during the extended interviews. While used by some agencies, this practice may not be a best practice as it is known that for the same density level, the field density is usually more easily obtained than the same density level in the lab. Thus, for some projects additional density could be gained in the field rather than stopping when a laboratory-based reference is reached. A preferred practice is to base the production density target on the ability to achieve density given a standard compaction level. This is beneficial for both the agency and the contractor in that the differences between field and lab compaction are eliminated and that the density target reflects the current field and material conditions. This can most easily be accomplished using a modified Proctor test in the field and is common practice on many full depth reclamation projects but relatively few CR projects. The use of Proctor testing in the field for density assessment is a current research topic by others. The most commonly cited method of establishing the production density target currently was by measuring the peak density achieved during the test strip in what is known as a break-over process. With subsequent passes of the roller, the density will peak and then begin to decrease. The number of passes to achieve the peak density is then identified as the roller pattern. One agency interviewee identified a similar procedure using a lightweight deflectometer where the number of passes to achieve a minimum deflection was taken as the roller pattern. Regardless of the method, the number of passes to achieve the target density (or deflection) needs to be consistently monitored during production to determine whether a change in the material or environmental conditions necessitates the need for establishing a new roller pattern. Most specifications are silent on how to determine this need other than generically stating that conditions will dictate. General Takeaway: There are many ways that the target density is identified within current specifications. While many have some merit, commentary needs to be provided in the AASHTO Construction Guide Specification that addresses the challenges related to each method or emphasis on the need for clarity regarding when to re-establish the target density (i.e., when using the break-over method)
34 4.2.7 Mixture Verification Tests About half of the extended interview participants stated that they collect field produced materials for the purpose of mixture verification tests. These tests are most often the same strength tests used during mixture design with specimens fabricated in the laboratory from field produced materials. These tests are most often used by the contractor or agency for informational purposes and are rarely used to determine pay. The main drawback is that the test results are usually not available for several days and so it is difficult to make any meaningful corrective actions if necessary. A few of the extended interview participants stated that materials were collected from the field were used for materials testing such as binder content and gradation. General Takeaway: Mixture verification tests, especially those that require curing for strength testing, provide some challenges (e.g., time to results). While they may provide value, they are typically not used for acceptance or pay. This context needs to be considered in the AASHTO Construction Guide Specification. 4.2.8 Adjusting Test Requirements and Test Frequency Depending on Traffic Level About half the interviewees stated that they believed that testing should be adjusted depending on traffic level or level of acceptable project risk (i.e., higher risk projects should have different testing or testing requirements than lower risk projects). Most of these interviewees agreed that the type of test or test requirements should be adjusted but most did not think the test frequency should change. Most of the interviewees who agree that testing should be adjusted with traffic level were those agency or industry interviewees who have experience with using recycling on higher volume facilities. Most of the interviewees who only use recycling on lower volume roadways did not think that testing should be adjusted based on traffic level. One industry interviewee with a high level of experience noted that testing should not be adjusted with respect to traffic level since agency and industry practitioners did not have a high degree of understanding about how well the test results represented long-term performance. General Takeaway: The project team suggests that perhaps both the test requirements and test frequency should be adjusted with respect to traffic level and that this same thinking could be extended to higher risk versus lower risk projects and even projects having thinner versus thicker surfacings. It is likely that risk and traffic level may be somewhat proportional to surfacing thickness for most projects, but this may not always be the case. Commentary on this topic needs to be provided in the AASHTO Construction Guide Specification. 4.2.9 Best Practices to Achieving Good Smoothness The majority of interviewees stated that maintaining consistent paving and production as well as good materials management were keys to achieving good smoothness. While initially these may sound like unrelated concepts, being able to keep the paving operation moving in a steady and consistent manner is also known to be a good paving practice for HMA projects. Care must be taken for CIR projects to balance the forward speed of the paver with the speed of the cold recycler (when two separate devices are used) to maintain a constant head of material at either the external paver or paving screed on board the cold recycler (when the CR process and paving happen in one device). In addition, the forward speed must be such that the rollers on the project are able to continue to compact the material in a reasonable time after production. The goal of
35 consistency for CCPR is even more similar to HMA paving projects at both the paver and the materials production plant. The CCPR plant production rate, storage of produced materials, trucking, paving, and compaction processes must all be balanced so that the paver can place the CCPR material in a continuous process. As with HMA paving, changes in forward speed and the head of material at the auger chamber can vary the downward pressure applied by the paving screed and cause long wavelength roughness in the paved layer. For most recycling specifications, the contractor is allowed to operate at any forward speed such that the produced material meets the required specifications. However, at least one interviewee stated that agencies should consider a maximum forward speed of the cold recycler. In addition to the above practices, contractors may elect or be required to mill or pave using a reference ski or ultrasonic reference sensors. These devices help ensure that the paving operation is following a constant elevation by constantly measuring the distance from the sensor to the ground reference and adjusting the paving or milling operation. Many agency specifications also utilize either a surface tolerance or a smoothness requirement. A surface tolerance is usually given as allowing no more than approximately 1/8-to-1/4-inch change over a standard length such as 10 feet. A smoothness requirement states that the pavement must have a smoothness, usually in terms of the international roughness index (IRI), of so many inches per mile; maximum values of approximately 70-90 inches per mile are common. Some interviewees stated that their specification required the contractor to achieve smoothness with no corrective action or allowed the contractor to adjust the surface smoothness by micro milling. While there is no standard practice with respect to allowing micro milling or not, some interviewees dislike the process as they feel the best quality material (the material at the top of the layer and thus likely to have the highest density) is removed. Other interviewees shared the results of research by others that shows user costs and maintenance actions are reduced for roadways having better smoothness. Some agencies go so far as to provide a financial incentive to the contractor to achieve superior smoothness values. General Takeaway: Smoothness is an important topic to be considered. The various methods and their benefits/drawbacks can be discussed in the Best Practices document. 4.2.10 Should Smoothness be Measured on the Recycled Layer or Final Surface Since nearly all pavement recycling projects have some type of surfacing placed on the recycled layer. The interviewees were asked if they felt the smoothness measurement should be taken on the recycled layer or the final surface. The responses to this question were varied with some interviewees stating a measurement should be taken at the recycled surface. However, most interviewees stated that testing the final surface was preferred since this is where the traveling public will interact with the road. Interestingly, California recycling specifications have a maximum smoothness value on both the recycled layer (90 inches per mile) and on the final surface (60 inches per mile). One drawback to conducting the smoothness measurement on the recycled layer is timing to conduct the measurement. On some projects, there may be only a day to a few days between recycling and paving the final surface. If the agency conducts the smoothness testing, it may be difficult to schedule the testing. If the contractor is conducting the testing, scheduling is likely to be less of a concern. A benefit to conducting the smoothness measurement on the recycled surface is that if any irregularities are identified, they can be corrected prior to placing the final surface and this can help give the completed project a more
36 uniform appearance. For contractors, measuring smoothness at the final surface could lead to potential issues when the recycled layer and surface layer are placed by two different entities and the decision must be made on who is responsible for fixing any potential rough areas. General Takeaway: It is debatable which layer (CR or HMA) should be used to measure smoothness. Some guidance needs to be provided in the commentary of the AASHTO Construction Guide Specification regarding which layer to use to measure smoothness.
