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