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3  CHAPTER 1 Background Pavement recycling is a technology that can restore the depth of pulverization achieved with the reclaimer depends service life of pavement structures and stretch available on the thickness of the bound layers of the existing pave- funding for pavement rehabilitation (Asphalt Recycling and ment and is typically up to 12 in. (ARRA 2015). For existing Reclaiming Association [ARRA] 2015). In general, pavement pavements, FDR is performed on the bound layers and a recycling techniques remix the existing pavement material predetermined portion of the underlying unbound materials. (either in situ or through a mobile plant) and reuse it in the FDR may consist of simply pulverizing and remixing the final pavement in the form of a stabilized layer. Some of the roadway foundation (termed âmechanical stabilizationâ), but most commonly cited benefits of using pavement recycling it most often incorporates one or several stabilizing agents. techniques to rehabilitate and repair asphalt concrete pave- Chemical stabilization describes FDR performed using ments include reductions in costs, emissions, use of virgin cementitious products such as cement, lime, fly ash, and materials, fuel consumption, construction time, and dis- cement and lime kiln dust. Bituminous stabilization describes ruption to traffic (Nataatmadja 2001, Thenoux et al. 2007, FDR using asphalt-based stabilizer, namely asphalt emulsion Robinette and Epps 2010, Stroup-Gardiner 2011, Pakes et al. or foamed asphalt (ARRA 2015). Bituminous stabilization is 2018). Pavement recycling methods include the following most commonly performed using an asphalt-based stabilizer processes: cold planing (CP), hot in-place recycling (HIR), plus an active filler such as lime or cement (Wirtgen 2010). cold recycling (CR), and full-depth reclamation (FDR). An asphalt mixture overlay or surface treatment (e.g., chip CR includes the techniques cold in-place recycling (CIR) seal) is usually applied after the FDR layer has been allowed to and cold central-plant recycling (CCPR) (ARRA 2015). This cure. FDR has been used successfully by numerous highway NCHRP study focused on FDR, CIR, and CCPR using asphalt agencies in several states (Mallick et al. 2002, Bemanian et al. stabilizing/recycling agents with and without a cementitious 2006, Lewis et al. 2006, Guthrie et al. 2007, Jones et al. 2008, active filler. Hilbrich and Scullion 2008, Diefenderfer and Apeagyei 2011, Pavement recycling techniques, including FDR, CIR, Johanneck and Dai 2013, Diefenderfer et al. 2015, Howard and CCPR, are viable and economically and environmen- and Cox 2016) and countries (Saleh 2004, Berthelot et al. tally advantageous rehabilitation strategies for many asphalt 2007, Loizos 2007, Lane and Kazmierowski 2012). A photo- pavements. The benefits of pavement recycling are derived graph of a reclaimer performing FDR on imported material primarily from reusing the in-situ pavement materials or is shown in Figure 1.1. existing pavement millings (RAP) and from using stabilizing/ CIR rehabilitates the upper portions of the bound layers recycling agents to bind the RAP particles at ambient temper- of an asphalt pavement, typically extending to depths of atures rather than heating the materials to high temperatures. 4 to 6 in. (ARRA 2015). CIR has been shown to be an effec- Robinette and Epps (2010) reported both the life-cycle cost tive treatment process by many agencies, although its earliest analysis (LCCA) and life-cycle assessment (LCA) of multiple use in the United States was primarily in the central and in-place recycling methods, quantifying cost savings and western portions of the nation. One reason for this is that positive environmental impacts. CIR was originally developed as a process in which several FDR can be used to correct severe structural deficiencies large pieces of equipment were joined together to form a long and defects that are deep within an existing pavement struc- CIR train. These trains could consist of tanker trucks, milling ture or to prepare a stabilized foundation on a new roadway machines, sizing and grading machines, crushers, pavers, using imported material (termed âimported FDRâ). The and rollers. Because of the substantial length of these trains,
4 Figure 1.1.â FDR process used on I-64 in Virginia, 2017. Photo by Wirtgen. Figure 1.3.â CIR using a rear-discharge cold recycler. they were most effectively used on long stretches of open highway. However, it is becoming increasingly common to see shorter CIR trains where the equipment may consist of only 2000, Forsberg et al. 2002, Sebaaly et al. 2004, Morian et al. a water and bitumen tanker, cold recycler, paver (needed only 2004, Lane and Kazmierowski 2005, Bemanian et al. 2006, if a paving screed is not included as part of the cold recycler), Emery 2006, Loizos and Papavasiliou 2006, Cross and and rollers, as shown in Figure 1.2. Other recent advance- Jakatimath 2007, Jahren et al. 2007, Loizos et al. 2007, Loria ments include a rear-discharge cold recycler that discharges et al. 2008, Thompson et al. 2009, Schwartz and Khosravifar the recycled material into the hopper of an asphalt paver, 2013, Sanjeevan et al. 2014, Diefenderfer et al. 2015). as shown in Figure 1.3. CCPR is a process through which RAP, generated by taking Typical recycling agents for CIR include asphalt emulsion millings from the existing project, other projects, or existing and foamed asphalt. In many cases, an active filler such as stockpiles, is recycled and used to construct a roadway. The cement, lime, fly ash, and lime kiln dust is used in combi RAP is brought to a centrally located plant (an example is nation with asphalt recycling agents to improve dispersion of shown in Figure 1.4) that is used to mix recycling additive(s), the foamed asphalt, improve resistance to moisture damage, similar to those used with CIR, consistently with the RAP. help achieve early strength, and expedite opening to traffic The plants are portable in that they can be temporarily set up (ARRA 2015). On higher volume routes, a single or multi- on or near a project or kept at a fixed location. course asphalt mixture overlay is typically applied, but other Recent studies have also shown mechanical properties of treatments (such as chip seals) may be used on lower volume CCPR and CIR to be similar (Apeagyei and Diefenderfer facilities (Bemanian et al. 2006, Maurer et al. 2007). CIR has 2013, Diefenderfer et al. 2016a, Schwartz et al. 2017). CCPR been successfully used for many projects in the United States, also offers the opportunity to process the RAP through a Canada, and other countries (Crovetti 2000, Thomas et al. mobile crusher on site before adding it to the CCPR plant Figure 1.2.â CIR using a single-unit cold recycling train Figure 1.4.â CCPR plant; the recycled product is on I-81 in Virginia, 2011. discharged from elevator at right of image.
5  1.2âObjectives The objectives of this study were to develop (1) time- critical tests for asphalt-treated CIR, FDR, and CCPR materials; and (2) guide specifications for using these tests for process control and product acceptance that provide agencies with a basis for determining when the pavement can be opened to traffic or when it can be surfaced. 1.3âCurrent Recycled Material Quality Tests Currently, agencies use several proxy tests to assess the Figure 1.5.â Paving CCPR on I-64 in Virginia, 2017. quality of a recycled pavement during construction. These proxy tests are most often related to level of compaction and moisture content. Density measurement is one of the most for improved gradation control compared to CIR, although common tests used by agency and contractor personnel to this has not been shown conclusively to benefit the final assess the quality of the recycled material during construc- product. The primary benefits of using the CCPR process are tion. Density measurements have been shown to be some- twofold. First, material can be removed from the roadway and what correlated with stiffness properties of recycled materials then returned as a recycled layer after the underlying foun- (Schwartz et al. 2017), and the experience of the recycling dation is either stabilized (using FDR) or replaced if needed. community has suggested that poor compaction density Second, existing stockpiles of RAP can be treated and used in leads to poor material quality (ARRA 2015, Asphalt Academy the construction of new pavements or in the rehabilitation 2009). However, density measurements do not fully indi- of different existing pavements. Although the CCPR process cate whether the recycled material is of sufficient quality or has not been used as widely as CIR, it has been successfully stability for trafficking or surfacing. Further, density measure- implemented recently on high-traffic sections of roadway ments do not account for the curing process that is known (Diefenderfer et al. 2016b, Ma et al. 2017, Timm et al. 2018). to occur with asphalt-stabilized recycled materials. Previous Figure 1.5 is a photograph of CCPR paving. studies have shown that asphalt-stabilized recycled materials gain stiffness and strength with time (Lane and Kazmierowski 2005, Loizos et al. 2007, Diefenderfer and Apeagyei 2011, 1.1â Problem Statement Diefenderfer et al. 2016b) while density remains constant. Despite the significant benefits, several impediments have Aside from density testing, quality tests that are often hampered the widespread use of pavement recycling tech- performed by the contractor may include using a proof- niques by agencies. Important among these is the lack of rolling process or compacting molded specimens in the field technical standards for rapid process control and product as part of a process control or quality assurance program. acceptance during construction. From the contractor perspec Proof rolling can effectively identify deficient structural tive, a lack of valid and rapid process control procedures issues; however, there are few standard methods to apply makes it difficult to deliver and document consistent place- the test, and thus results are rarely transferable from one ment that meets the design requirements. Current tests include project to another. Molded specimens fabricated in the field the seldom-used proof rolling and the more popular nuclear are exposed to accelerated curing procedures in the labora- density gauge (NDG) density and moisture measurements, tory to simulate the long-term curing process that occurs neither of which has been shown individually to correlate in the field prior to testing for strength properties. Again, well with performance. From an agency perspective, it is there are no AASHTO or ASTM standards for this curing difficult to make a time-critical assessment of material quality process, only loosely agreed-upon temperatures depending when there are no rapid product acceptance procedures. on the recycling/stabilizing agent, none of which had been Rapid product acceptance procedures are critical to help an proven to simulate field conditions. In addition, the curing agency decide when the recycled layer is ready to be opened simulation may be a multiday process and thus does not to traffic or when it is ready to be surfaced. The procedures provide time-critical information. are also vital for predicting whether the current engineering Although moisture content measurements are some- properties of the recycled material meet the design intent times used at early ages as an indicator of the curing process, in the fully field-cured state. the measurement methods employed have inherent issues.
6 Current procedures usually include using an NDG (or similar to become rewetted, the properties of the recycled material device), where the measured moisture content is affected by would not automatically revert to those it had in the uncured the presence of hydrogen in the asphalt binder and recycled condition; its properties would depend on the degree of pavement, or destructively testing a sample of the recycled bond quality that was established prior to the rewetting. material for moisture loss using forced-draft oven or micro- Even given the aforementioned deficiencies, the test wave oven drying. Although the microwave oven drying methods currently used to determine when a recycled pave- process is faster, it is difficult to remove all of the moisture, ment can be opened to traffic or surfaced have been used and some binder may be lost; thus, some error is inherent extensively in the past, and many pavement recycling process in the measurement. The forced-draft oven method may practitioners are comfortable with their use based on experi- remove nearly all free moisture, but it is completed at elevated ence. However, these methods sometimes fail to discriminate temperatures and typically requires a day or more to provide successfully between sufficient and deficient material quality, results. However, the main limitations of these approaches can often result in significant delays to project completion, include that the moisture content of a material does not and may lead to inappropriate âemergencyâ corrective actions always correlate well with its structural properties or when a such as adding more active filler. The development of recycled layer can be opened to traffic or surfaced. appropriate rapid quality tests will significantly improve When an agency does attempt to quantify the appropriate the ability of agencies to accept well-performing materials time to open a recycled layer to traffic or surfacing, most while minimizing the risks of accepting deficient materials. highway agencies currently follow one of two approaches. In addition, the pavement recycling industry will have the The first approach is to wait a predetermined time for the process control tools to demonstrate material quality rapidly. material to gain enough stability to carry traffic via a material curing process. Wait times from agency specifications range 1.4â Scope of Report from a few days to 2 weeks. This process is highly inefficient in that under certain conditions, the material may have This report summarizes the work completed under NCHRP reached the ability to carry traffic without deterioration much Project 9-62 to identify and develop time-critical quality tests sooner than allowed by the specification. This increases and guide specifications for using these tests with asphalt- agency costs by delaying the project and increases costs to the treated CIR, FDR, and CCPR. The report is divided into five traveling public through increased travel time and reduced chapters, including this background Chapter 1. Chapter 2 utility of the roadway. The second approach includes perform- discusses the research approach, and Chapter 3 presents the ing a moisture content test and allowing surfacing or traffic findings and applications based on responses to the online once a predetermined moisture content is reached, usually stakeholder survey, specification review, laboratory testing, around 50% of the optimum value obtained during the mix and field testing. Chapter 4 presents the conclusions and design process. Although this criterion is perceived to be more offers suggestions for continued research. Chapter 5 discusses scientific by virtue of a quantifiable measurement, it too ideas for training and implementation. The references used can be problematic since the moisture content is only loosely in the preparation of this report follow Chapter 5. Detailed correlated with material structural properties. Studies have responses from the stakeholder survey, field testing data suggested that as a recycled material loses moisture, the par- sheets, preliminary draft standard practice documents, and ticle bonds are enhanced and strength properties are increased preliminary draft revisions to existing test methods are (Fu et al. 2010a, Fu et al. 2010b). However, if the material were included in the appendices.
