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24 ment that gyratory compaction for FDR samples is appropri- the value(s) of structural design coefficients was not col- ate; however, the use of this compaction device for CIR mixes lected in this survey; however, several values commonly that still have significant water content needs to be evaluated. used were found in the literature. A coefficient of 0.44 was recommended for HIR layers (In-Place Recycling Confer- The average of the agency and contractor responses was ence 2008). For CIR materials, coefficients of between 0.25 used to rank and summarize the types of mix designs cur- and 0.28 were recommended by Kansas, 0.26 by Nevada, rently used (Table 27). and 0.35 in a NCHRP Report 224 (Harrington 2008). Roma- noschi et al. (2004) recommended a coefficient of 0.18 for foamed asphalt-stabilized FDR. The Ontario MTO uses TABLE 27 0.20 to 0.28 (estimated from gravel equivalent) for foamed MIX DESIGN PRACTICES FDR (Thompson et al. 2009). Mix Design HIR CIR FDR Methods TABLE 28 No Formal Design Sometimes Sometimes** Often** STATE AND CONTRACTOR RESPONSES TO STRUCTURAL Superpave Rarely Rarely Rarely DESIGN CONSIDERATIONS Marshall Sometimes Often Often* Structural Design: During project development, I consider the structural capacity of the recycled layer using: Hveem Sometimes Often* Sometimes Structural Agency Responses Wirtgen Rarely Sometimes* Sometimes* Design Rarely = lower than 10% average of agency and contractor with experience. Considerations HIR CIR FDR Sometimes = between 10% and 25% average of agency and contractor with AZ, CO, IA, AZ, CO, IA, AL, CO, IA, experience. Often = between 25% and 50% average of agency and contractor with Established MT, NE, UT, MN, MT, NE, MN, NE, NV, experience. Structural WA NV, OR, RI, OR, SD, UT, WI Frequently = greater than 50% average of agency and contractor with Coefficients SD, UT, WA, experience. WI, WY *Contractor response was significantly higher than agency with experience. AR, AZ, ID, AZ, ID, ND, AK, AL, CA, **Agency response was significantly higher than contractor with experience. MD, NE, TX, NE, NV, OR, ID, MD, MT, FWD UT, VT, WA SD, UT, VA, ND, NE, NV, VT, WA OR, SD, TX, Superpave mix design methods need to be developed for UT, VA, VT designing recycled mixes. Curing times, temperatures, and AZ, FL, NC, AZ, CA, DE, CA, DE, MD, humidity need to be standardized for CIR and FDR regard- Pre-Determined WA NC, NV, SD, NC, NV, SC, SD Layer Thickness less of the type of compaction used to prepare the samples. WA Laboratory MD VA MD, NE, VA Resilient Modulus STRUCTURAL DESIGNS Wirtgen (2004) uses a nomograph to estimate the layer coefficient, other layer properties, and anticipated amount of foamed asphalt (Figure 21). The nomograph is used by iden- tifying a given property and then moving vertically up or down to obtain estimates for the other layer properties. For example, given a structural number coefficient of 0.16 after stabilization, the anticipated initial stiffness would be about 750 MPa, a steady stiffness of 450 MPa, and indirect tensile Structural design methods rely on the assessment of remain- strength of 150 kPa when using about 4% foamed asphalt for ing pavement life and the needed structural changes for future a range of AASHTO soil classifications. The advantage to traffic. Layer properties, thickness, and distress information the graph is that material properties are tied to the selection are needed to determine the appropriate changes during main- of the coefficients. This graph can also be used to estimate tenance and rehabilitation activities. In some cases, neither material properties for use with newer mechanisticempiri- the agency nor contractor assesses the structural capacity (i.e., cal design methods. Alternatively, FWD testing to deter- the "No" answer option to the question in Step 3). In this case, mine the existing pavement layer stiffness (i.e., modulus) the process moves directly to construction (chapter three). can be used to estimate structural coefficients. Established structural coefficients for the traditional A number of agencies simplify their design process by AASHTO design are the most commonly used design using predetermined thicknesses for each of the recycling approach by the agencies, followed by the use of FWD test- methods. Only a limited number of states use laboratory ing for layer properties (Table 28). Specific information on resilient moduli values for their designs.
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25 FIGURE 21 Relationships between layer properties and layer coefficients (based on original figure by Wirtgen 2004). Contractors are most likely to use structural coefficients written responses include the use of compressive strength, and FWD values for structural design considerations (Fig- distress level, and R-value. ure 22). Contractors are more likely than agencies to use either set recycled mix thickness or mix stiffness (resilient The average of the agency and contractor responses was modulus) information for their designs. Differences between used to rank and summarize the current use of structural contractors and agencies are more noticeable in their choice design approaches for in-place recycling (Table 29). of FDR designs. Other design considerations noted in the TABLE 29 STRUCTURAL DESIGN APPROACHES Design Information Used HIR CIR FDR Structural Coefficient Sometimes Frequently Frequently FWD Sometimes Often Often Set Thickness Sometimes Often Often* Lab Resilient Modulus Rarely Sometimes Often* Rarely = lower than 10% average of agency and contractor with experience. Sometimes = between 10% and 25% average of agency and contractor with experience. Often = between 25% and 50% average of agency and contractor with experience. Frequently = greater than 50% average of agency and contractor with experience. * Contractor response was significantly higher than agency with experience. Information regarding structural coefficients and layer stiffness is needed for structural design considerations. FIGURE 22 Information used for structural design approaches These design parameters need to be agreed on before con- by agencies and contractors. Percentages are based on the struction so that the final product meets or exceeds the number of agencies and contractors with experience using the desired performance. specific recycling process.