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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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14 · 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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16 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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17 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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18 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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19 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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20 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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21 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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22 · 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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23 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-