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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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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.
