Construction Guidelines for Cold Central Plant Recycling and Cold In-Place Recycling (2023)

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37 5 SIMILARITIES AND DIFFERENCES BETWEEN CIR/CCPR WITH FOAMED ASPHALT/EMULSIFIED ASPHALT 5.1 Project Selection Selecting between CIR and CCPR depends on the desired application by the agency and/or the location of distresses within the pavement section. CIR could be a preferred option if the distresses are located within the bound asphalt layers and minimizing the amount of materials hauled to and from the project site is desired. CCPR is usually used when it is desired to remove a substantial amount of existing asphalt pavement by milling to address deeper distresses and then to reuse the milled material, when it is not possible to perform CIR for other reasons, or for preparing a new pavement or new pavement layer using recycled materials. Additional details are provided in the following paragraphs. CIR could be a preferred recycling-based method of pavement rehabilitation when the distresses to be addressed are generally located within the top 3 to 6 inches of an existing asphalt pavement, or when, after milling, distresses are located at these same depths below the milled surface. The term “pre-mill” is often used to describe milling operations prior to CIR. CIR is beneficial in that large quantities of existing materials are not hauled to and from the project site to complete the pavement rehabilitation. Reducing the materials hauled to and from a project reduces costs, emissions, and wear on neighboring roadways. CCPR is a pavement recycling process that results in a material having similar behavior to that of CIR (NASEM 2017). Rather than being produced in situ, CCPR is produced either in a specialty plant or in the processing unit of a multi-unit CIR train. Recent published findings have shown that it is also possible to produce the CCPR material from a conventional HMA plant with modifications (Bowers and Powell 2021). CCPR could be a preferred recycling-based method of pavement rehabilitation when the asphalt layers of an existing pavement need to be removed to access and strengthen the underlying foundation materials. Such a need may arise if the agency desires to improve the properties of the existing foundation using FDR or a similar process. CCPR can be used with CIR to economically treat distress in thicker bound layers. CCPR can also be useful to prepare a new bound pavement layer using RAP from recent milling or from existing stockpiles of RAP. The Virginia DOT showed a recent example of this process when completing a lane widening project on Interstate 64. To complete the I-64 project, existing RAP from nearby stockpiles was hauled to a nearby CCPR plant where it was blended with the recycling agent and active filler and then hauled to the project and paved using conventional practices. CCPR can also be used to place multiple layers to complete a very thick pavement using recycled materials. Typical maximum lift thicknesses for CCPR are approximately 6 to 7 inches but multiple layers of this thickness could be combined to make a very thick pavement section if desired. Using CCPR allows the fractionating of RAP which may offer additional benefits by being able to more precisely control the gradation of the RAP used in the pavement layer, but the research team is not aware of any publications that quantify these potential benefits.

38 5.2 Material Sampling 5.2.1 Section Uniformity Pavement section uniformity (layer thickness, material composition, structural condition, etc.) is an important consideration for both CIR and CCPR. Agencies may rely on historical information about the pavement structure and past maintenance and rehabilitation efforts to determine the level of uniformity within a pavement section. Historical records of interest may include design thicknesses, material types (new and maintenance/rehabilitation), and maintenance and rehabilitation timing/efforts. Testing, both destructive and non-destructive, also needs to be conducted to provide additional information. Testing may include using falling weight deflectometer and ground penetrating radar to determine the uniformity of the sections with respect to both layer type and thickness, as well as in-place modulus. Coring and dynamic cone penetrometer (DCP) testing are additional methods of determining layer thicknesses and deterioration. Through these methods, if it is found that the pavement sections are not consistent then multiple mix designs are recommended. If the pavement is found to contain a paving fabric, the pavement can generally still be recycled as long as larger pieces are remover prior to compaction. 5.2.2 Pavement Deterioration Mechanism The pavement deterioration mechanism is an important consideration plays a role in selecting CIR, CCPR, a combination of the two, or other methods. Distresses that indicate base failure are not good candidates unless developed as a CCPR project including some rehabilitation of the base layer (e.g., using FDR). Distresses that indicate material quality issues or distresses within the bound layers are often good candidates for CIR or CCPR if the designer can ensure that the entire distressed location is recycled. 5.2.3 Sampling Method Material sampling is typically performed for CIR and CCPR by either (a) collecting cores or slabs from the roadway that is to be recycled or (b) milling a small section of the roadway and then using these materials to develop a mix design. Milling with small cold planers is not recommended as the gradations tend to be finer than that produced by larger CR equipment. If cores or slabs are collected, they must then be broken down by a laboratory jaw crusher or another method. However, CCPR can also be designed and constructed using existing RAP stockpiles, whereas CIR only uses material in the existing roadway. 5.3 Mix Design Process The design process for CIR and CCPR is mostly the same with the main differences being in how the RAP, used to produce the recycled mixture, is obtained. A mix design using either process includes the same equipment in the laboratory and the same tests and test requirements. 5.3.1 Gradation The gradation requirements for CIR or CCPR design are the same. For CCPR, if an existing stockpile is being used, then the gradation of that stockpile is used in the design of the mixture. Differences between the two processes arise in that it is typical practice to see both a course and

39 a fine gradation developed during the mix design for CIR. This is done to account for any potential/expected variability in the gradation of the RAP coming from the roadway (especially if the project is of sufficient length that multiple asphalt mixtures may be encountered). While it is not as common to see these two gradations developed for CCPR during mix design, as most designs are based on existing stockpile gradations, it is of potential value to account for any variation that could occur throughout the stockpile. The gradation does have some required differences based on if foamed asphalt or emulsified asphalt is being used as the recycling agent. In the case of foamed asphalt, the foamed asphalt binder is dispersed throughout the mixture by the fine material. Therefore, the range of passing the #200 sieve recommended for foamed asphalt mixtures is higher than mixtures using emulsified asphalt. Typical passing #200 requirements for foamed asphalt and emulsified asphalt mixtures are 4-12 percent and 2-10 percent, respectively, as recommended by Wirtgen (2012). Emulsified asphalt tends to coat the fine particles and some of the coarse particles, whereas the foamed asphalt binder forms interparticle “spot welds” during mixing which are pressed together during compaction. 5.3.2 Active Filler Use One method of increasing the fines content is the addition of an “active filler” such as lime or cement. This helps in the distribution of the foamed asphalt, decreases the moisture susceptibility of the mixture, and contributes positively to early and long-term stiffness of the mixture (NASEM 2017). The mechanical role of active fillers was explored by Fu et al. (2008), however the impact of active fillers on the cracking susceptibility of recycled mixtures has only recently been explored when applying the balanced mix design concept (Diefenderfer et al. 2019). Additional research is needed in this area to see if the stiffness and flexibility of recycled mixtures can be balanced to achieve optimum performance. Active fillers can be used with emulsified asphalt mixes as well, though care must be taken to ensure that the emulsified asphalt will not “break,” or set up, more rapidly due to the presence of the lime or cement. It is currently more common to see active fillers used with foamed asphalt mixtures. Many agencies recommend a maximum of 1% cement and/or a 2.5:1 ratio of residual asphalt to cement to help prevent brittle behavior. Lime is usually limited to a maximum of 1.5%. 5.3.3 Mix Design Performance Indicators The tests required for mix design of recycled mixtures using foamed asphalt and emulsified asphalt tend to be different. Foamed asphalt CR mixtures are most often required to meet a minimum indirect tensile strength (ITS) in both dry and conditioned states, along with a tensile strength ratio (TSR). Emulsified CR mixtures may require the ITS tests, but they more commonly require the mixture to meet a Marshall stability test in both a dry and conditioned state, along with a passing Marshall stability ratio (MSR). When mixing with emulsified asphalt, a raveling test can be required to ensure that raveling does not occur during construction or early trafficking. Typical test thresholds are provided in Table 1 (ARRA 2015)

40 Table 5.1. Typical Test Requirements for Cold Recycled Materials in Mix Design (ARRA 2015) Test and Recycling Agent Specification Requirement Mixture Type Indirect Tensile Strength AASHTO T 283 ≥ 45 psi Foamed asphalt and Emulsified asphalt* Tensile Strength Ratio AASHTO T 283 ≥ 0.70 Foamed asphalt and Emulsified asphalt* Marshall Stability Test AASHTO T 245 ≥ 1,250 lbf (4-inch diameter) Emulsified asphalt Marshall Strength Ratio AASHTO T 283 ≥ 0.70 Emulsified asphalt Raveling Test ASTM D 7196 AASHTO PP 86 ≤ 7% loss Emulsified asphalt *Deviation from ARRA (2015) 5.4 Equipment Differences The primary differences in equipment between CIR and CCPR occur during the production of the recycled material. CIR is produced using a cold recycler which can be either a single unit or a multi-unit train. In a single unit train, the cutting head removes the pavement to the required depth and cross-slope, and blends the recycling agent with the RAP. Single unit trains achieve the desired gradation by operating the cutting head in a down cutting mode, controlling the forward speed of the train, and with the use of pressure and breaker bars. The recycling agent is added based on the treatment volume which is determined by treatment width, depth, and anticipated forward speed of the unit. When the material is produced, it is either placed by a screed attached to the unit, left in a windrow as the machine advances, or it is discharged by a conveyor into a paver hopper as shown in Figure 5.1. Figure 5.1. Single Unit CIR Train.

41 Multi-unit CIR trains typically consist of a large full lane width milling machine, a trailer mounted screening/crushing unit, and a trailer mounted pugmill mixer. In most multi-unit trains the screening/crushing unit and the pugmill are combined into one large unit. The milling machine removes the materials from the pavement to the desired depth or cross-slope. The maximum RAP size is controlled by the screen sizes used in the screening/crushing unit. Any oversize material is sent to the crusher, typically a horizontal impactor, and then returned to the screen. The RAP is moved from the screening/crushing unit to the pugmill and a belt scale on the conveyor carrying RAP to the pugmill determines the mass of RAP entering the pugmill. The amount of liquid recycling agent introduced is controlled by a metering system using the mass of material on the belt scale. The material leaving the pugmill is deposited in a windrow or directly into the paver hopper. An example of a multi-unit CIR train is provided in Figure 5.2. Figure 5.2. Multi-Unit CIR Train (courtesy ARRA). A mobile plant specifically designed for cold central plant recycling is most often used to produce the CCPR material (Figure 5.3), but the processing unit of a multi-unit CIR train can also be utilized (Figure 5.4). Rather than a milling and mixing chamber, a CCPR plant often includes a dual-shaft pugmill where the RAP, recycling agent, and active filler (if used) are blended. Following mixing, the CCPR is often discharged into waiting dump trucks using an attached conveyor.

42 Figure 5.3. CCPR Plant. Figure 5.4. CCPR Produced with Processing Unit of Multi-Unit CIR Train (ARRA 2015). Several options also exist for placing the produced CIR or CCPR material. Most often, the material produced in the CIR process is deposited into a conventional asphalt paver either by a pickup machine delivering the material from a windrow or by a conveyor on board the cold

43 recycler. Some manufacturers have developed cold recyclers with on board high-density paving screeds. Ancillary materials used in the production of CIR (foamed asphalt or emulsified asphalt, and active filler [if used]) will also need to be delivered to the job site to continue the recycling process. The material produced in the CCPR process is most often delivered to the project site by dump trucks where the CCPR material is directly dumped into the paver hopper. Recent published findings have shown that it is also possible to produce the CCPR material from a conventional HMA plant with modifications (Bowers and Powell 2021). Benefits include the ability for an existing HMA plant owner to produce a high-recycled content mixture, however, the CCPR production rate will not be as high as for a dedicated CCPR plant (which often average about 200 tons per hour). One final difference in CCPR is the need for equipment to manage, manipulate, and deliver the RAP to the CCPR plant. These pieces of equipment and practices are the same as those used to manage and deliver RAP and/or aggregates to a HMA plant so different expertise and training are not needed. 5.5 Surface Preparation Preparation of the existing pavement surface can be different if using CIR or CCPR. CIR is performed on an existing asphalt pavement and depending on the depth of recycling desired (among other considerations), the pavement surface may be milled prior to performing the CIR. Since the density of the recycled material is less than for the existing HMA, the CIR material will occupy a slightly larger volume than the original material. This increase in volume is usually termed fluff and the increase in volume is typically about 9 or 10%. The fluff may be accounted for by premilling in which the existing pavement is milled by the amount of expected surface elevation increase. It can also be accounted for by trench widening and using the excess volume of material to fill the trenched area. Combining recycling and trench widening can be an effective means to widen existing pavements having narrow lanes. CCPR does not often involve any special surface preparation other than having a layer of compacted material that forms a sufficient base suitable for any other paving material. 5.6 Production CIR and CCPR production is different with respect to the equipment used to produce the materials, as mentioned in the Equipment Differences section. Considerations for foamed asphalt mixtures are generally the same for CIR and CCPR. The emulsified asphalt for emulsified CCPR mixes can be formulated to allow for increased haul times (ARRA 2015). Production rates for CIR are typically about 18-20 ft per minute and will vary given the material properties and depth of the recycling process. At this forward speed, a production rate of a little more than a lane mile per day is achievable. ARRA (2015) states that newer CIR equipment has a production rate as high as 3 lane miles per day. Production rates for CCPR are about 200 tons per hour. However, placement rates (especially for thick applications) may outrun the production rate. In these cases, the CCPR can be produced a few hours ahead of the expected start of paving so that the production and desired placement quantities match by the end of the work period. However, work by Kazmi et al. (2021) showed

44 that stockpiling material for too long, 24 hours in the case of the studied CCPR mixture, can be detrimental to the strength properties, so monitoring the strength of stockpiled and processed CCPR is needed. 5.7 Production Tests Production tests for CIR and CCPR are most often the same, as noted in the literature review and focused survey/interview portion of this project and usually only differ depending on if foamed asphalt or emulsified asphalt is used (as shown in Table 1). The most common tests during production are percent compaction, depth checks, and yield checks. Additional tests for foamed asphalt mixtures include checking the expansion ratio and half-life properties of the binder. Additional tests for emulsified asphalt mixtures most often include a screen test to assess agglomeration of the asphalt in the emulsion, which may lead to handling challenges (ASTM D6933). Evaluating mix properties of field produced mix, ITS or Marshall stability, are occasionally required; however, the test specimens must be compacted and tested in a timely manner. One of the biggest hurdles with testing CIR and CCPR materials soon after construction include knowing when the material is ready to be trafficked or surfaced. The NCHRP Project 09-62 developed a series of tests that can measure the shear and raveling properties of newly placed recycled materials (Diefenderfer et al. 2021). Field validation activities for these tests are currently underway and will include relating the material response at early ages to the response at typical surfacing and opening to traffic times. 5.8 Laydown, Compaction, and Curing Laydown, compaction, and curing processes are generally the same for CIR and CCPR, regardless of recycling agent. However, secondary compaction can be useful for CR mixtures that use emulsified asphalt as the recycling agent. Secondary compaction is a beneficial way to remove any tire marks or minor rutting due to trafficking prior to the application of the overlay (ARRA 2015). Secondary compaction is not typically performed for foamed asphalt CR mixtures. According to ARRA’s recommended construction guidelines for CIR (ARRA 2017) any transverse joints, namely due to stopping and starting in successive days work, must overlap by 2 feet. For CCPR, typical HMA processes are used for placing in successive days and therefore no overlap is needed. Other differences are more likely observed when comparing the use of foamed asphalt versus emulsified asphalt rather than CIR versus CCPR. For example, the curing rate of a foamed asphalt projects is sometimes faster than one in which emulsified asphalt is used. Curing rates vary greatly depending on weather conditions, added moisture content, and presence of active filler. The NCHRP Project 09-62 developed raveling and shear tests are examples of this in that the test criteria are the same for CIR and CCPR whether foamed asphalted asphalt or asphalt emulsified asphalt is used. However, it is expected that the time after compaction until the test criteria are achieved will differ (Diefenderfer et al. 2021).