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13
MATERIAL SELECTION AND MIX DESIGN
The second step in the development of an in-place recycling
project includes assessing the in-situ layer and reclaimed
asphalt pavement (RAP) properties, selecting materials,
and providing mix designs for setting the job mix formulas.
The information varies on the basis of the recycling process
(Figure 12).
FIGURE 11 Types of surface courses used with in-place
recycling processes. Percentages are based on the number of
agencies and contractors with experience using the specific
recycling process.
The agency and contractor responses were ranked and
summarized to indicate currently used surface treatments
for in-place recycling projects (Table 15).
TABLE 15
SURFACE TREATMENT SELECTION
Type of Surface HR CIR FDR
Treatment
Overlays Structural Often Frequently Frequently
Non- Sometimes Sometimes Sometimes FIGURE 12 Work needed to select materials and establish job
Structural mix formulas.
Integral Sometimes Sometimes+ Sometimes*+
In-Situ Layer Properties
OGFC Rarely* Rarely Rarely
Fog/Chip Sometimes* Sometimes Often*
In-situ properties are needed to evaluate the need for different
Microsurfacing Sometimes* Sometimes* Sometimes designs for different segments of the project. The ability of the
Slurry Sometimes* Rarely Rarely underlying layers to support the construction equipment and
Fog Seal Rarely* Rarely Rarely the variability in layer thicknesses that can affect a reason-
Rarely = lower than 10% average of agency and contractor with experience. able selection of milling depths also need to be determined.
Sometimes = between 10% and 25% average of agency and contractor with A number of approaches can be used to define the thickness
experience. and stiffness of the underlying layers. The most common
Often = between 25% and 50% average of agency and contractor with
experience. methods of assessment include coring, boring logs for base
Frequently = greater than 50% average of agency and contractor with and soils classifications, dynamic cone penetrometer testing
experience.
*Contractor response was significantly higher than agency with experience. (DCP), California bearing ratio, or resistance value (R-value)
+By definition, "integral" refers to HIR processes, however some state from soils testing or historical records, FWD layer modulus,
agencies indicated use on CIR ad FDR.
ground-penetrating radar (GPR), or local experience (Jahren
et al. 1999; Loizos and Papavasiliou 2006; Loizos 2007;
The preference for using a structural overlay when struc- Malick et al. 2007). The initial use for this testing is to
tural capacity improvement is not needed requires further
research to define the criteria required to select this option. · Determine the ability of the subgrade to support the
The ability of other surface treatments to provide acceptable weight of the recycling equipment,
surface courses needs to be explored. · Evaluate needs for increased structural capacity,
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· Provide information for the structural design (FDR), and In summary, the availability or collection of in-place
· Identify sections in need of different treatments. material properties information needs to be considered
when developing the project design, specifications, and
States typically use the same preconstruction field test- agency estimates of project costs.
ing, regardless of the recycling process (Table 16). Agency
preferences for field-testing methods are borings and coring
investigations. Fewer than a third of the agencies use FWD
testing for determining the layer modulus and project vari-
ability. Fewer than 9% of the states use GPR for preconstruc-
tion project assessments. The use of GPR testing will likely
increase as the technology becomes more widely available in
the coming years because it can provide information quickly
on layer thickness, presence of moisture, and sections of the
project in need of different designs.
TABLE 16
AGENCY TESTING FOR LAYER PROPERTIES
Preconstruction Field Testing: Before construction, I typically use:
Preconstruc- States
tion Work HIR CIR FDR FIGURE 13 Methods of assessing in-place layer properties
by agencies and contractors. Percentages are based on the
Coring to AK, AL, CA, AR, AZ, CA, AK, AL, CA, CO,
number of agencies and contractors with experience using the
Determine CO, DE, GA, CO, FL, GA, CT, DE, GA, ID,
Thickness ID, IA, MD, ID, IA, KY, IA, MD, MN, specific recycling process.
MN, NC, ND, MD, MO, MT, MO, MT, NE,
NE, NV, OR, NE, NH, NV, NH, NV, OR, SC,
SC, SD, TX, UT, NY, OR, RI, SD, TX, UT, VA, TABLE 17
VA, VT, WY SD, UT, VA, WY SOURCES OF EXISTING IN-PLACE MATERIAL PROPERTIES
VT, WA, WY (average of agency and contractor percentages)
Boring to CO, ID, KS, CO, CT, DE, ID, AR, AZ, CA, CO, Layer Property Testing HIR CIR FDR
Check Depth MD, MO, MT, KS, MD, MO, FL, ID, IA, KS,
of Base and TX, WA MT, NH, NV, KY, MD, MO, Coring to Determine Thickness Often** Frequently* Frequently*
HMA OR, SD, UT, MT, NC, NE,
Boring to determine thickness Sometimes Often Frequently
VA, WA, WI NY, TX, VT, WA
FWD Sometimes Sometimes Often
FWD AR, AZ, CO, AZ, ID, MD, AK, AL, CA, ID,
Testing FL, ID, MD, MN, NC, NE, MD, MN, MT, GPR Rarely Rarely Rarely
NC, NE, TX, NV, OR, RI, NC, NE, NV, OR,
VT, WA SD, UT, VA, SD, TX, UT, VA, Rarely = lower than 10% average of agency and contractor with experience.
Sometimes = between 10% and 25% average of agency and contractor with
VT, WA, WY VT
experience.
GPR Testing MT, TX MN, MT MN, MT, TX, AK 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.
**Agency response was significantly higher than contractor with experience.
Fewer field tests are conducted on HIR projects by con-
tractors and agencies compared with CIR and FDR projects
(Figure 13). Core thickness, boring logs, and samples are the
most commonly used preconstruction field tests. Contrac-
tors are significantly more likely than agencies to conduct
field tests for FDR projects.
The average of the percentage of agencies and contrac-
tors using a given method of assessing the in-place material
properties was used to rank method preferences (Table 17).
Wirtgen (2004) provides a suggested comprehensive
testing plan for conducting a detailed investigation. The
testing program includes cutting a test pit, coring, and DCP
testing (Figure 14). A combination of the distress surveys
and field tests provides the engineer with sufficient informa-
tion to evaluate any design adjustments needed for various FIGURE 14 Example of comprehensive preconstruction
sections of roadway. testing program (based on original figure by Wirtgen 2004).
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15
RAP Properties
RAP binder content, RAP binder properties, and RAP
aggregate gradation are needed for the appropriate selection
of grades of new aggregates, new binders, recycling agents,
and additives. The most common agency preconstruction
laboratory testing focuses on RAP gradations and binder
contents, material properties, and recovered binder proper-
ties (Table 18). The same testing program is used regardless
of the recycling process; however, agencies are more likely
to test the recovered binder properties for HIR projects than
for either CIR or FDR projects.
TABLE 18
FIGURE 15 Comparison of testing for RAP properties
AGENCY RESPONSES FOR PRECONSTRUCTION between agencies and contractors. Percentages are based on
LABORATORY TESTING FOR IN-PLACE RECYCLING the number of agencies and contractors with experience using
PROJECTS the specific recycling process.
Preconstruction Laboratory Testing: Before construction, I typically
determine:
Laboratory States TABLE 19
Testing HIR CIR FDR LABORATORY TESTING PROGRAMS
Aggregate AR, CA, FL, ID, CA, CT, DE, AK, AL, CA, Preconstruction HIR CIR FDR
Gradations of KS, KY, MF, ID, KS, MD, DE, GA, IA, Laboratory Testing
Cores or NC, NE, TX, MN, ND, NH, MD, MN, SD,
Binder Content Often Frequently* Often*
Millings WA NY, RI, SD, UT, VT
UT, VT, WY Recovered Binder Often Often* Sometimes*
Properties
Application CA, CO, IA, KS, CA, CO, CT, AL, CA, CO,
Rates of Bind- MD, NE, NY, IA, KS, MD, GA, ID, IA, Gradations Often Frequently* Often*
ers or Other TX, WA MN, NE, NH, MD, MN, NE,
Additives NY, SD, UT, SC, UT, VT, % Fines Often Frequently* Often*
VT, WY WY Application Rates Often Frequently* Often*
Binder Content AR, CA, FL, ID, CA, CO, CT, AL, CA, GA, Material Properties Often Frequently* Often*
of Cores or KS, KY, MD, ID, KS, MD, MD, MN, WA for Additives
Millings NC, NE, TX, MN, ND, NE,
WA NH, NY, WY Rarely = lower than 10% average of agency and contractor with experience.
Sometimes = between 10% and 25% average of agency and contractor with
Material CA, CO, FL, CA, CO, IA, AL, CA, CO, experience.
Properties of GA, IA, KS, KS, MD, NE, GA, IA, MD, Often = between 25% and 50% average of agency and contractor with
Any Liquids, MD, NC, NE, SD, UT, WY NE, TX, UT, experience.
Stabilizers, NY, TX, WA VT, WY Frequently = greater than 50% average of agency and contractor with
Rejuvenators, experience.
Additives, or *Contractor response was significantly higher than agency with experience.
Admixtures to
Be Added
Preconstruction testing is key to designing recycling
Percent Fines CA, FL, ID, KY, CA, ID, MD, AK, CA, ID, mixes. The time needed for this testing as well as the costs
of Millings MD, NC, NE, UT, VT, WY IA, MD, NE,
TX UT to the project need to be considered in developing cost esti-
Recovered CA, FL, GA, CA, MD, WY AK, CA, MD
mates and project timelines.
Binder Proper- KY, MD, NC,
ties from Cores NY, TX, VT, New Materials and Additives
or Millings WA
A range of new materials and additives can be used to pro-
Contractors conduct more tests before construction than duce desired mix properties and early performance. New
the agencies (Figure 15). Contractors and agencies tend to aggregates may be added to adjust the final gradation.
agree more often on testing of HIR than on either CIR or New binders (paving-grade asphalts, emulsions) are used
FDR projects. to soften aged asphalt in the RAP and provide more flex-
ibility of the final asphalt concrete layer. Recycling agents
The agency and contractor responses were used to rank and rejuvenators can be used instead of, or in conjunction
and summarize current practices for laboratory testing with, the new binders to improve the binder performance
(Table 19). properties. Although each material can be added individu-
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ally, it is common practice to introduce new aggregates · Emulsions
and asphalt by adding new HMA to the recycled materials. CSS-1, CSS-1h, and CSS-1hP
Additives and stabilizers can be added to improve stiff- CMS-2S
ness, moisture resistance, and rut resistance; reduce ravel- HFMS-2, HFMS-2S
ing; help dry moist RAP and soils; and control the rate of HF-150, HF-300P
set of emulsions. Proprietary solventless emulsions
· Asphalt binders in fresh mix
New Aggregates Performance-graded asphalt, softer grades
Viscosity-graded asphalts (e.g., AC 10, AC 20)
When new aggregates are added, existing aggregate grades Foamed asphalt
are typically used such as 3/8 in. minus, no. 57 stone, ½ in.
minus sizes. Standard aggregate gradations locally available Emulsions are a combination of small asphalt globules
are typically used when gradations of the final mix need to suspended in water by the use of surfactants. A sample of
be adjusted. the emulsion grades used in recycling projects is shown
in Table 20. Regardless of the source of the emulsion
Asphalt Binders specification, three groups of material property tests are
typically needed to determine the properties of emul-
Asphalt binders used in recycling processes can be typical sions (water, asphalt, additives), distillation of emulsions
paving-grade asphalts or emulsions: (removal of water), and the recovered base asphalt (resi-
TABLE 20
REQUIREMENTS FOR CATIONIC EMULSIFIED ASPHALTS (based on ASTM D2387-05; D977-05; Oregon DOT 2010)
Type Medium Setting Slow Setting
HFMS-2 HFMS-2s HF-150 CMS-2S CSS-1 CSS-1h
Grade
Min Max Min. Max Min Max Min Max Min Max Min. Max
Tests on Emulsions
Viscosity, Saybolt Furol at 25ºC (77ºF) SFS 100 50 35 150 20 100 20 100
Viscosity, Saybolt Furol at 50ºC (122ºF) SFS 100 450
Storage Stability Test, 24-h, % 1 1 1.5 1 1 1
Demulsibility, 35 mlL,
40
0.8% Dioctyl Sodium Sulfonsuccinate, %
Coating Ability and Water Resistance
Coating , Dry Aggregate good good good
Coating, After Spraying fair fair fair
Coating, Wet Aggregate fair fair fair
Coating, After Spraying fair fair fair
Particle Charge Test positive positive positive
Sieve Test, % 0.1 0.1 0.1 0.1 0.1 0.1
Cement Mixing Test, % 2 2
Tests on Distillation
Oil Distillate, by Volume of Emulsions, % 0.5 4 12
Residue, % 65 65 62 60 57 57
Tests on residue from distillation test
Penetration, 25ºC (77ºF), 100g, 5 s 100 200 200 150 250 100 250 100 250 40 90
Ductility, 25ºC (77ºF),
40 40 40 40 40
5 cm/min, cm
Solubility in Trichloroethylene, % 97.5 97.5 97.5 97.5 97.5 97.5
Float Test, 60ºC
1,200 1,200 1,200
(140ºF), s
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due). Traditional emulsion specifications use one or more Asphalt--Foamed Asphalt
tests to define the asphalt residue properties: absolute and
kinematic viscosity, penetration, and ductility testing. The One method of adding fresh binder to cold recycling pro-
existing specifications rely on older methods of grading cesses (CIR and FDR) is to foam it during the mixing pro-
asphalts (e.g., penetration grades, viscosity grades); how- cess in the recycling train. Foamed asphalt is produced by
ever, most states now specify asphalt products using the injecting a small amount of water into hot asphalt as it is
performance grading (PG) specifications. mixed with the recycled materials (Figure 16). As the hot
liquid and water mix, the liquid expands as the water turns
Clyne et al. (2003) explored PG specification testing to steam, creating a thin film of asphalt with about 10 times
(AASHTO MP1) to classify the asphalt residue from three more coating potential. Foaming facilitates better dispersion
emulsions used in the same region of Minnesota (Table 21). of the asphalt into the materials to be recycled. The two key
Based on these results, the engineered emulsion (EE) and parameters that control the quality of the foam are the:
HFMS-2P would be expected to remain more flexible at
colder temperatures than CSS-1. Both CSS-1 and HFMS-2P
would be expected to be less sensitive to movement under
traffic at summer temperatures than the EE product. This is
also supported by research for Federal Lands (Johnston and
King 2008). Research by Epps et al. (2001) suggested that
emulsion specifications use the concept of PG binder prop-
erties so that recycling binders can be selected for project-
specific environmental conditions.
TABLE 21
PG GRADING FOR MINNESOTA EMULSIONS (based on Clyne
et al. 2003)
Emulsion Performance Grade
CSS-1 PG 52-28
FIGURE 16 Foamed asphalt process during construction
Engineered Emulsion (EE) PG 46-34
(based on original figure by Wirtgen, 2004).
HFMS-2P PG 52-34
· Expansion ratio (minimum of 10 times; Wirtgen 2004)
and
Emulsions historically used in the same environmental · Half-life of the foam (minimum of 8 s; Wirtgen 2004).
conditions may have base asphalts with a wide range of
performance-graded asphalt properties that will likely influ- The expansion ratio is defined as the ratio between the
ence the success or failure of recycling projects. Emulsion maximum achieved volumes of the foam to its original vol-
specifications need to be updated so that users can select ume. The half-life is defined as the time elapsed from the
binders on the basis of performance properties. time the foam was at the maximum volume to the time it
reaches half of the maximum volume. Larger expansions
Asphalts--Paving Grades and longer half-life are considered desirable properties for
foamed asphalt.
Paving-grade asphalt can be specified by the standard PG
specification by using the desired properties of the com- Marquis et al. (2002) noted that the quality of foamed
bined asphalt (i.e., combination of new and RAP asphalt). asphalt mix is strongly related to the quality of the foam as
A formal blending program can be conducted to select the measured by the expansion ratio and the half-life. Optimum
fresh binder PG specification, or a less formal "bumping" settings for the foaming process need to be set in the mix
down one grade to account for the stiffening of the fresh design phase on the basis of parameters needed to produce
binder because the aged RAP binder can be used. The Texas peaks in stiffness or modulus of the mix. It should be noted
Department of Transportation is conducting research into that there need to be between 8% and 20% fines in the FDR
the use of Superpave® PG specifications for asphalts used in to achieve the desired results for foamed asphalts, although
near-surface applications. 100% RAP mixes can be prepared with lower percentages of
fines (Matthews 2008). The Wirtgen (2004) manual recom-
Specific guidance is needed for using PG specifications mends a range of gradations suitable for foamed asphalt (Fig-
for recycling project asphalts. ure 17). One of the main advantages to using foamed asphalt
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instead of emulsions is that the roadway can be opened to cal hourly HMA plant production, and plant operators will
traffic immediately after compaction because no curing time be reluctant to change plant operations for small orders of
is needed (Lane and Kazmierowski 2005a). If the fines con- HMA. When they are willing to change mixes, the cost of
tent of the RAP is insufficient, then additional mineral filler the specialty mix will be substantially higher than typical
will be needed (see the section on additives and stabilizers). HMA, and the availability will depend on the plant's abil-
ity to interrupt its production schedule. The most economi-
cal and practical approach is to start with the properties of
the typical new HMA that is locally available and adjust the
RAP gradation if at all possible.
Recycling Agents and Rejuvenators
Recycling agents (RAs) are used to restore the aged asphalt to
the desired binder properties. ASTM D4552 (Table 22) classi-
fies petroleum product additives specifically for hot mix recy-
cling methods. The RA classifications are viscosity graded,
with the lower the number designation representing the low-
est viscosity. Products meeting the RA 1 through RA 75 des-
ignations are typically used for recycled mixes with more
than 70% RAP in the mixes. When more than 30% of new
FIGURE 17 Suggested gradation range for foamed asphalt aggregate is used, the RA 250 and RA 500 grades are more
(based on original figure by Wirtgen 2004). appropriate. The Pacific Coast Conference on Asphalt Speci-
fications defines RAs as hydrocarbon products with physical
New HMA characteristics selected to restore the aged asphalt binder to
the current asphalt binder specifications. By this definition, a
Fresh mix is commonly used in HIR, but it is used very softer grade of asphalt can be classified as an RA.
rarely in CIR and not at all in FDR. The new HMA is used
to easily adjust the gradation of the HIR. The gradation of ASTM D5505 provides specifications for emulsified
the new HMA is selected to achieve the desired final grada- recycling (ER) agents (Table 23). The base asphalt in these
tion, and the binder grade of the new HMA is selected to products increases in stiffness (viscosity) with increases in
help soften and rejuvenate the RAP binder. Mixes reported the grade number. The ER-1 is a petroleum derivative com-
as used include minus 1.9 mm (¾ in.) dense-graded HMA, patible with asphalts. Its main function is to rejuvenate aged
fine-graded HMA, minus 12.5 mm (1/2 in.) open-graded asphalt. The ER-1 material is viscosity graded, and there are
HMA, and stone matrix asphalts. no requirements for viscosity measurements on the residue
after rolling thin film oven (RTFO) testing. The ER-2 and
The simplest means of obtaining economical and timely ER-3 grades are a combination of rejuvenators and asphalt
new HMA supplies is to use HMA mixes typically produced components. These ER agents are typically used when the
by the plant supplying the mix. The amount of new HMA recycled HMA needs additional asphalt (e.g., when adding
needed for a recycling job is small compared with the typi- new aggregate). They are considered a penetration-graded
TABLE 22
ASTM D4552 CLASSIFICATIONS FOR HOT MIX RECYCLING AGENTS (ASTM D4552 2009)
ASTM Test RA 1 RA 5 RA 25 RA 75 RA 250 RA 500
Test
Method Min Max Min Max Min Max Min Max Min Max Min Max
Viscosity at D 2170 or D
50 175 176 900 901 4,500 4,501 12,500 12,501 37,500 37,501 60,000
140°F, cSt 2171
Flash Point,
D92 425 -- 425 -- 425 -- 425 -- 425 -- 425 --
COC, °F
Saturates, wt% D 2007 -- 30 -- 30 -- 30 -- 30 -- 30 -- 30
Tests on residue from RTFO or TFO oven 325°F (D 2872 or D 1754)
Viscosity
-- -- 3 -- 3 -- 3 -- 3 -- 3 -- 3
Ratio
Wt Change ± % -- -- 4 -- 4 -- 3 -- 3 -- 3 -- 3
Specific D 70 or
Report Report Report Report Report Report
Gravity D 1298
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TABLE 23
ASTM D5505 SPECIFICATIONS FOR EMULSIFYING RECYCLING AGENTS (ASTM 2009)
ER-1 ER-2 ER-3
Tests Test Method
Min Max Min Max Min Max
Testing on emulsion
Viscosity, 50°C, SSF D224 100 20 450 20 450
Sieve, % D6933 0.1 0.1 0.1
Storage Stability, 24 h, % D6930 1.5 1.5 1.5
Residue, by Distillation, % D6997 65 65 65
Dilution -- ReportA
Specific Gravity D70 Report Report Report
CompactibilityB varies Report Report Report
Testing on residue from distillation
Viscosity, 60°C, cSt D2170 50 200 30 30
Saturates, % D2007 30
Solubility in Trichloroethylene D2042 97.5 97.5 97.5
On residue from distillation after RTFOC
Penetration, 4°C, 50 g 5 s D5 75 200 5 75
RTFO, Weight Change, % D2872 4 4 4
Notes:
AER-1 shall be certified for dilution with potable water.
BThis specification allows a variety of emulsions, including high-float and cationic emulsions. The engineer should take the steps necessary to keep incompatible
materials from co-mingling in tanks or other vessels. It would be prudent to have the chemical nature (flat test for high-float emulsions, particle charge test for
cationic emulsions, or other tests as necessary) certified by the supplier.
CRTFO shall be the standard. When approved by the engineer the Thin Film Oven Test (Test Method D 1754) may be substituted for compliance testing.
material because the penetration is used to set limits on the · Increasing base durability and strength,
residue after RTFO conditioning. · Reducing dust during construction,
· Waterproofing the in-situ soils,
In addition to the ASTM recycling agents and ASTM ER · Drying wet in-situ soils,
agents, some states may include a state-developed specifi- · Conserving natural resources (aggregates),
cation such as the one from Kansas (Table 24). There are · Reducing construction costs, and
also proprietary recycling products on the market, such as · Providing a temporary wearing surface.
engineered emulsions specifically designed to address dis-
advantages of conventional recycling agents, in particular TABLE 24
in-place recycling methods. Proprietary products specifi- ASPHALT REJUVENATING AGENT (based on Kansas
cally designed for in-place recycling that have been used by specification 1205)
agencies and contractors are Property Requirement
Viscosity, SayboltFurol at 25ºC, s 15100
· CIR-EE,
Residue, % min. 60
· Reflex,
· Fortress, Sieve Test, % max. 0.1
· Pass-R, Oil Distillate, % max. 2
· ERA-25, Storage Stability, 24 h, % max. 1
· ARA-1P, and Tests on Residue from Distillation
· Reclamite.
Asphaltenes, % max. 15
Penetration @ 4°C, 100g, 5 sec. 150250
Additives and Stabilizers
Geiger et al. (2007) summarized the reasons for using sta- The types of additive(s) used with CIR processes are based
bilization to improve the characteristics of base materials as on the desired mix property improvements, such as improved
stripping resistance, rut resistance, layer stiffness for higher
· Reducing plasticity index, traffic levels, controlled rate of set of emulsions, minimized
· Reducing swelling potential of the in-situ soils, raveling until the wear course is placed, and additional fines
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needed to meet the desired gradation. FDR materials can be Lime (calcium hydroxide) works best when there is reac-
stabilized with most of the additives used for HIR and CIR tive clay in base materials, as lime reduces the plasticity of
improvements. Stabilization improves the load-bearing quali- the clay materials. Lime is typically used when the plasticity
ties of the mostly unbound pulverized materials and is classi- index (PI) is greater than 8 (Matthews 2008) and fines con-
fied by how it improves base properties (ARRA 2001): tents are greater than 10% (Franco et al. 2009). Thompson et
al. (2009) recommend using 1% hydrated lime when the PI
· Mechanical, is between 10 and 16 and 2% when the PI is greater than 16.
· Chemical, The reduction in plasticity helps minimize swelling, reduce
· Bituminous, and moisture damage, and improve the base strength. Like port-
· Combinations. land cement, lime can help reduce initial excess moisture in
the pulverized base materials. Too much lime can result in
Mechanical stabilization is developed by using par- shrinkage cracking.
ticle interlock typically achieved by pulverizing RAP and
base materials and then compacting to the desired density. Quicklime (calcium oxide) reacts with water to form cal-
Because all of the recycling methods include compaction, cium hydroxide, a reaction that generates heat, and the solid
mechanical stabilization can be considered a secondary sta- nearly doubles in volume. Because of the fast reaction of the
bilizing mechanism for all of the methods. quicklime, it is used for set control or early strength gains.
The benefits are the same as using hydrated lime.
Chemical stabilization mixes the pulverized RAP and
base or subgrade materials with cementitious materials such Fly ash, a pozzolanic material, also provides improved base
as calcium chloride, magnesium chloride, lime (hydrated or strength through a cementitious bonding of the particles when
quicklime), fly ash (Class F or C), kiln dust (cement or lime), in the presence of water. Moisture resistance is improved by a
portland cement, or other chemicals (ARRA 2001). Some reduction in the permeability of the base materials.
of these chemical stabilizers can be added either dry or in
slurry form. Asphalt emulsions, a mixture of asphalt cement, water,
and an emulsifying agent, improve the strength and mois-
Bituminous stabilization uses an asphalt emulsion, ER ture resistance of the base material, soften the aged asphalt
agent, or foamed (expanded) asphalt. It is not unusual to binder in the RAP, and reduce shrinkage cracking seen with
see combinations of stabilizers such as fly ash and asphalt cement and lime stabilizers. When the emulsion breaks,
emulsion or fly ash and portland cement. Combinations of the asphalt droplets join, and the water separates from the
stabilizing methods and additives are commonly used to asphalt. Compaction helps force the water out of the stabi-
improve properties. lized base, but sufficient time for the moisture content to
drop below about 1.0% is still needed for all of the moisture
Liquid calcium chloride is used to improve freeze/thaw to evaporate before the placement of the next lift.
resistance by lowering the freezing point of reclaimed base
material. The stiffness of the base is improved by the bond- Combinations of additives and stabilizers have been used
ing of the soil and RAP particles. The first application of the with asphalt binders to improve properties of the final prod-
liquid is blended with the pulverized material; the stabilized uct. For example, Naizi and Jalili (2009) found that using
base is shaped and graded and then sealed with a second emulsions with lime slurry or portland cement improved
application of calcium chloride. moisture resistance and increased both the final mix stiffness
and indirect tensile strength. Thomas et al. (2000) evaluated
Portland cement is used to increase compressive strengths the combination of fly ash and lime, which showed improved
of bases by providing a cementitious bonding of the soil and mix stiffness but promoted shrinkage cracking. A combina-
RAP particles. Portland cement works best with a plasticity tion of EE and lime slurry provided improved flexibility at
index of less than about 10 (Matthews 2008; Thompson et al. cold temperatures and minimized shrinkage cracking. The
2009) and fewer than 10% fines (Franco et al. 2009). Higher Wirtgen (2004) manual notes that cement is routinely used
percentages of fines can be tolerated while still improving with bitumen emulsions to improve moisture resistance,
the load-carrying capability of the soil. Cement-stabilized tensile strength, fatigue resistance, and retained strengths.
bases continue to slowly gain strength over time and work Cement and emulsion combinations need less curing time
best with granular materials with low plasticity. Another before traffic can be permitted on the recycled surface.
advantage to using cement as a stabilizer is that excess mois-
ture can be quickly removed from the pulverized material. Information on the use of typical additives and stabilizers
One disadvantage is when used as a stabilizer, the recycled compiled from the survey responses and from the literature
material has a tendency to show shrinkage cracking. is summarized in Table 25.
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TABLE 25
SUMMARY OF USES FOR ADDITIVES AND STABILIZERS IN RECYCLING PROCESSES
CIR FDR
Additive or Freeze/ Thaw Rut Chemical or
Stabilizer Moisture Layer Rate of Set Minimize Mechanical
Resistance Resis- Bituminous
Resistance Stiffness Control Raveling Stabilization
tance Stabilization
Calcium Chloride X X X
Portland Cement X X X X X X
Lime X X X X X
Quicklime X X X X X
Fly Ash X X X
Limestone Fines X X X X
Fibers X X X
Asphalt X X X X X X
Recycling Agents X X X X X
In summary, additives and stabilizers need to be selected
on the basis of their ability to improve key material and
mix properties.
Mix Designs
Regardless of which recycling process is used on a project,
the steps in the mix design process are similar (Figure 18).
New and RAP Binder, RA Selection
Once the gradation blend of RAP and new aggregate is
determined, the binder grade, quantity, and any recycling or
rejuvenating agent need to be identified. For HIR, this can
be done by the use of blending charts, which can be adapted
for viscosity or Superpave PG binder tests (Figure 19). The
viscosity or G*/sin for the RAP binder is plotted on the left
y-axis and the properties of the new asphalt, or RA for CIR,
are plotted on the right. A line is drawn horizontally across
the graph, from left to right, until it intersects the diagonal
viscosity line. The percentage of new asphalt or RA needed
is read off the bottom horizontal axis. More comprehensive
selection methods will blend the anticipated percentages
of RAP, and new binder will use the full Superpave binder
property to select the new binder grade.
Mix Design Methods
The most commonly used mix design methods vary by the
in-place recycling process. HIR mix designs are usually
based on standard HMA mix design methods. CIR and
FDR are based on emulsion or foamed asphalt methods, FIGURE 18 Basic steps in recycled mix designs
(based on FHWA 1997).
which include
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· EEs
Caltrans: Marshall stability at 104ºF
· 1,250 lb minimum dry stability
· 70% minimum retained strength ratio
Iowa DOT: Marshall stability at 100ºF
· 1,000 lb minimum stability
SemMaterials: Indirect tensile strength, resilient
modulus, and modified cohesiometer
· 35 to 40 psi minimum (dry)
· 20 to 25 psi minimum (wet)
· 70% minimum ratio
· 120 to 150 ksi minimum for resilient modulus
· Emulsions and foamed
FIGURE 19 Blending chart used to select the percent of Ontario MTO: Indirect tensile strength (dry and
new asphalt or additive needed to provide the desired binder wet), retained strength ratio
properties (based on FHWA 1997).
· 50 psi minimum (dry)
· 25 psi minimum (wet)
· EEs · 50% minimum for ratio
Caltrans: 75 blow Marshall
Iowa DOT: 4-in. gyratory with 30 revolutions The most common approach by state agencies in designing
SemMaterials: 6-in. gyratory with 30 gyrations recycled mixes is to do nothing (Table 26). When agencies do
· Emulsions mix designs, either the Superpave or Marshall methods are
Wirtgen: 75 blow Marshall commonly used. None of the agency respondents indicated
Ontario Ministry of Transportation (MTO): 75 that they use the standard Hveem method, and only four states
blow Marshall use the Wirtgen (2004) method for CIR or FDR. For states
· Foamed asphalt indicating "other," the design methods mentioned were the
Iowa DOT: 4-in. gyratory with 25 revolutions Portland Cement Association (PCA) soilcement mix design
Wirtgen: 75 blow Marshall (PCA 2005), Proctor method (optimum dry density and mois-
Ontario MTO: 75 blow Marshall ture content), modified Proctor (Kim and Labuz 2007), and
unconfined compressive strengths (geotechnical testing).
Because mix designs are intended to represent field condi-
tions, curing periods before testing are included in emulsion
(engineered or traditional) and foamed asphalt mix designs. TABLE 26
As with the compaction methods, each mix design method AGENCY AND CONTRACTOR RESPONSES TO MIX DESIGN
varies in its curing procedures (Thompson et al. 2009): METHODS
Mix Design Testing: Before construction, I or my contractor design our
recycled mixes based on the following method:
· EEs
Caltrans: Cure at 140ºF to constant weight Mix Design Agency Responses
Iowa DOT: 48 h at 140ºF Methods HIR CIR FDR
SemMaterials: 72 h at 140ºF CA, ID, IA, CA, DE, ID, CT, DE, ID, MN,
· Emulsions Do Not Do MO, VT, WA IA, NC, NH, MT, NC, NH, NV,
Wirtgen: 72 h at 104ºF. For high traffic (i.e., greater Mix Designs NV, RI, SD, NY, SD, VT, WI
VT, WA, WI
than 5 million ESALs), the specimens are com-
Hveem -- -- --
pacted at the anticipated final field moisture content
and cured in sealed containers for 40 h at 104ºF. AZ, KY, NE AZ, MN, NE, VA
Marshall
OR, VA, WY
Ontario MTO : 48 h at 140ºF, soaked for 24 h at 77ºF,
or vacuum saturated for 60 min at mmHg pressure. CO, KS, MO, CO, KS, MO, MD, MO, UT, VA
Superpave
ND, UT, VT ND, UT, VA
· Foamed asphalt
Iowa DOT: 72 h at 105ºF Wirtgen -- VA AK, CA, IA, VA
Wirtgen: Same as for emulsions Other
NY, TX CT, MT, NY AL, CO, GA, NE,
NY, SD, WY
Ontario MTO: Same as for emulsions
Once the specimens have cured, various properties of the Between 20% and 42% of the states do not develop
specimens are determined: mix designs for recycling projects (Figure 20). Comments
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by the states indicated that they require the contractor to peratures, and determination of specific gravities are the focus
supply the designs. Contractors typically design using the of a number of current agency and academic research projects.
Marshall or Superpave mix designs. Contractors are more
likely to use either Marshall or Wirtgen or no mix design Recent published results indicate that gyratory compac-
at all for FDR designs. tion is useful for preparing samples for all of the recycling
processes. In particular, using gyratory compaction for FDR
seems to provide a compacted density closer to actual in-situ
densities than the other methods. Mallick et al. (2002) and
Kim and Labuz (2007) found 50 gyrations produced labora-
tory-compacted samples with densities similar to those found
in the field projects. Other researchers investigated using
30 gyrations for preparing FDR samples (Cross 2002; Lee
and Kim 2007b; Thompson et al. 2009). Some concern was
expressed about the need to provide drainage for the water
pressed out of the CIR and FDR mixes (Mallick et al. 2007),
and a slotted gyratory mold was used when compacting these
mixes. Gyratory compaction can also be used for foamed
asphalt samples (Kim and Lee 2006; Kim et al. 2007b).
The load-carrying capability of the recycled mix is evalu-
ated with Marshall or Hveem stabilities. The indirect ten-
FIGURE 20 Comparison of mix design methods used by
agencies and contractors. Percentages are based on the sile strength test (IDT) is also used either in place of, or in
number of agencies and contractors with experience using the addition to, the stabilities. Both dry and wet IDTs are used
specific recycling process. by a number of agencies and contractors to determine the
moisture sensitivity by evaluating the retained strengths
AASHTO, the Associated General Contractors of Amer- (i.e., tensile strength ratio, TSR). States that evaluate rutting
ica, and the American Road and Transportation Builders potential with loaded wheel testers (i.e., asphalt pavement
Association Joint Committee Task Force 38 adapted Mar- analyzer, Hamburg) for their HMA mixes also use these
shall (50 blow) and Hveem mix designs (ARRA 2001) tests for the recycled mixes.
for use with recycled mixes. Since the Task Force report
came out, several researchers have evaluated the suggested The PCA (2005) and general unconfined compressive
designs, primarily the 50 blow Marshall method, which con- strength approaches to design recommend a range of com-
sists of two parts: determination of optimum water content pressive strengths at various times after curing. For example,
and determination of optimum binder content. Two stud- the PCA method uses limits for strengths between 2.07 to
ies (Salomon and Newcomb 2000; Lee et al. 2002) evalu- 2.76 MPa (300 and 400 psi) at 7 days. Franco et al. (2009)
ated this CIR mix design method and noted the following recommend including the determination of the Atterberg
disadvantages: limits in the mix design methods for FDR.
· Time needed to complete mix design is 8 days. The FDR mix design method used depends on the type of
· Information on when new aggregate should be added stabilizer. Because FDR is essentially a method of produc-
to the mix (i.e., no suggested gradation bands) was ing a stabilized base material, typical geotechnical tests are
missing. commonly used by agencies. These include using a Proc-
· Time needed for emulsion to break was not considered tor or modified Proctor determination of optimum moisture
in sample preparation. content and maximum dry density. Strength testing is con-
· Heating time for emulsion was not specified. ducted using unconfined compressive strength, California
· Temperature differences for different emulsions were bearing ratio, or R-value tests. When cement is used, the
not addressed. PCA method for soilcement stabilization may be used
· Applicability of using standard HMA testing for bulk (PCA 2005). Other stabilized base mix designs for fly ash
specific gravity (i.e., direct immersion of high air void and lime stabilization can be used for CIR and FDR mixes.
mixes in water) was not addressed, but should be con- Combinations of additives and stabilizers such as emulsions
sidered because of the high air voids in recycling mixes. and cement can be designed with CIR mix designs or by
· Specific procedures for determining optimum values using geotechnical tests.
of water and emulsions were not clearly defined.
Regardless of the mix design method used, there was gen-
Preparation, mixing (order of addition), mixing tempera- eral agreement that there is a lack of established curing times,
tures, curing times (before and after compaction), curing tem- temperatures, or humidity conditions. There is some agree-
