Recycling and Reclamation of Asphalt Pavements Using In-Place Methods (2011)

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26 CHAPTER THREE CONSTRUCTION WEATHER CONDITIONS A number of recommended weather conditions for pav- ing were found in several presentations posted to the In- Place Recycling Conference (2009) and ARRA website (2010). Variations found in the presentations depend on the properties of the binders and additives used in the recycling. Typical ranges of weather conditions include • Ambient temperature above – 40°F to 50°F – 45°F to 65°F • Pavement temperature above – 50°F – 50°F to 70°F • No anticipated overnight freeze • Dry roadway • Construction dates limited to between – May 15 to Oct. 15 – May 1 to Sep. 30 • Weather conditions that allow for the proper placement • Curing conditions for CIR or FDR (depend on addi- tives and stabilizers) range from a – Minimum of 14 days to maximum of 30 days (CIR) – One to 7 days (foamed asphalt stabilized) – Once moisture content is below 1.0% (CIR) Better-defined guidance for weather conditions for each in-place recycling process is needed for successful in-place recycling projects. SURFACE PREPARATION AND COMPACTION Equipment used in front of recycling and for compaction after recycling is that typically used in conventional maintenance and HMA overlay placement projects. Surface preparation is not specific to a particular recycling process. Roller selection is typical of that used in standard roadway construction. Vari- ous recommendations for surface preparation are archived in ARRA (2009) presentations and Wirtgen (2004) project sum- maries. The most commonly cited practice is to remove any vegetation in cracks, scrub dirt deposits, and broom the surface before recycling. When lane widening is to be completed, the vegetation along the shoulder needs to be removed. Wirtgen (2004) provides some guidance on selecting the appropriate type of primary roller based on the general gradation of the mix (Figure 23). Sandy (i.e., coarse) mixes, such as FDR mixes, need a sheep’s foot roller first, whereas clayey (i.e., fines) mixes, such as CIR mixes, can use either a steel wheel or pneumatic roller. HIR Construction HIR is an on-site, in-place process that preserves or maintains deteriorated asphalt pavements while minimizing the use of new material. The HIR method is used to correct surface dis- tresses that are not caused by structural problems (i.e., stable and adequate base). One of three types of HIR processes is used, depending on the distresses present in the existing roadway: FIGURE 23 Suggested primary roller selection (based on original figure by Wirtgen 2004).

27 • Surface recycling—heater scarification, • Repaving, or • Remixing. These processes are single-stage and multistage HIR trains. The single-stage train processes the complete depth in one pass, whereas the multistage unit processes half of the depth per pass. Surface recycling is a preservation/maintenance process that restores cracked, brittle, and irregular pavement in prep- aration for a final thin wearing course. A single-pass method recycles and places the HIR material in one pass of the equipment. Repaving combines surface recycling with the simultaneous overlay of the new HMA overlay. Remixing heats, scarifies, collects material, and places it in a windrow that is picked up and mixed in a pugmill with new aggre- gates, rejuvenators, and new HMA (as needed). HIR processes may not be applicable to recycling pave- ments with multiple seal coats (FHWA 1997), crumb rubber surface treatments, or porous HMA mixes (Stroup-Gardiner, 2008). In these cases, the properties of the upper layers act as an insulator against the heat transfer to the underlying pave- ment. In some cases, the excess binder or additives, such as crumb rubber, may also create smoke and potential fire haz- ards (Caltrans-METS 2005). Excessive crack sealant can pose similar problems. Each category of the HIR process has its own sequence of standard and specialized heavy equipment. All of the HIR processes use one or more preheaters to soften the existing pavement so that it can be scarified or milled. The older style of preheater uses a simple set of burners fueled by propane. The disadvantage of this type of preheater is that open flames can be seen during use. The open flames may pose a fire hazard when there is dry brush near the roadway or in neighborhoods with landscaping near the project edge, and they can readily burn the pavement surface. Newer styles of preheaters include infrared, skirted recirculating heating systems, and most recently, a combina- tion of heating systems. The size and weight of HIR equipment are highly variable: • Preheaters – Weights range from 4,990 to 44,906 kg (11,000 to 99,000 lb). – Heights range from 2.3 to 3.6 m (7.6 to 11.8 ft). – Lengths range from 2.1 to 18.9 m (7 to 62 ft). • Scarifiers and miller units – Weights range from 7,711 to 39,463 kg (17,000 to 87,000 lb). – Heights range from 3.1 to 4.9 m (11 to 16.5 ft). – Lengths range from 9.1 to 16.8 m (30 to 55 ft). • Recycling combination units – Weights range from 15,876 to 83,008 kg (35,000 to 183,000 lb). – Heights range around 4.3 m (14 ft). – Lengths range from 8.3 to 22.3 m (28 to 73 ft). Any type of preheater may be in use, according to agency and contractor responses (Figure 24). FIGURE 24 Types of preheaters currently in use. Percentages are based on the number of agencies and contractors with experience using HIR processes. HIR surfacing equipment consists of a preheater followed by a combination preheater–scarifier (Figure 25, top). The heated, scarified HIR material is compacted using conven- tional compaction equipment. HIR repaving uses a standard haul truck to transport new HMA and load it into the front hopper of the recycling unit (Figure 25, middle). The mix is moved through the equip- ment to the paving screed at the back. A series of heaters is used to soften the existing pavement, and a scarifier is used to loosen the RAP. This is followed by the addition and mixing of binders or rejuvenators, and the mixture is spread across the lane. Mixing augers blend the materials and place the mix with the recycling screed. For an integral overlay, the new HMA is placed on top of the hot recycled mix (paving screed), and compaction is achieved using standard pneu- matic and steel wheel rollers.

28 be placed because sufficient time may be needed for water evaporation after placement. Standard haul trucks are used to provide new aggregates (typical East Coast practice) or new HMA (typical West Coast practice; Figure 26). Unlike the HIR recycling trains, one or two nurse trucks are usually in front of the recycling profiler and mixer unit to provide a continuous supply of liq- uids for the mix (e.g., recycling agents, water). The recycling unit mills, processes, and mixes the recycled materials and then transfers them to a paver. Standard compaction prac- tices are used to place and compact. FDR Construction The FDR method pulverizes the existing HMA pavement along with underlying granular materials. Stabilizers are added to the pulverizers or through separate passes of other units. The steps involved in the FDR construction process are shown in Figure 27. The first construction activity is to deposit fresh materials or additives and spread them evenly over the old roadway surface (Figure 28). Nurse trucks provide liquids to the pulverizing and mixing unit. FIGURE 25 Typical sequence of construction equipment for HIR processes (based on FHWA 1997; ARRA, 2001). HIR remixing mills provide the deepest milling depths of the HIR processes. Typically, two preheating units precede the miller-mixer (Figure 25, bottom). The miller-mixer is usually equipped with a front hopper for the new HMA mix, and the bottom front of the miller is fitted with another heater pushed in front of or pulled behind the recycling. The milled material is mixed with fresh materials and additives and placed using a heated, vibratory, or tamping screed. Compaction is accom- plished with standard compaction units. CIR Construction CIR mills only the existing HMA pavement surface. Screen- ing decks and onboard crushers size the reclaimed asphalt pavement. The sized material is then transferred to a twin- shaft pugmill and mixed with the emulsion or foamed asphalt. Wirtgen (2004) provides a comparison of the con- struction parameters for the typical asphalt binders used in either CIR or FDR processes (see Table 30). Note that “cold” refers to the ambient temperature of the milling and aggre- gate temperature. CIR can still use hot paving-grade binders with the foamed asphalt process. Breaking time (i.e., rate of set) for the emulsion will limit when the surface course can

29 of the old asphalt surface. Care is needed when the surface is milled before pulverizing because the remaining pave- ment layer has to support the pulverizing equipment (Jahren et al. 1999). The second step prepulverized the remaining asphalt concrete layers to a depth of 12 in. Four percent lime was applied, repulverized, and allowed to mellow for 24 h. The second admixture, which was 6% fly ash, was blended into the lime-treated material. Water was added to the mix, which was compacted immediately. The stabilized base was then completed using 4 in. of new HMA. QUALITY MANAGEMENT PROGRAM Inspection FHWA (2005a,b) has developed pocket-sized guides for HIR and CIR processes. These guides provide a useful checklist for agency inspection staff. The checklist for HIR project inspection includes infor- mation for • Preheaters, • Milling/scarifying units, • Additive or admixture system, • Mixing unit/spreader, • Paver (repaving), and • Rolling. The checklist for CIR project inspection includes infor- mation for • Milling, crushing, and mixing; • Additives and mixtures; • Pick-machine and paver; and • Rolling procedure. Quality Control and Quality Assurance Quality management programs are used to ensure that contrac- tors meet or exceed the project requirements (specifications) and include systematic management throughout the process from TABLE 30 COMPARISON BETWEEN DIFFERENT TYPES OF BITUMEN APPLICATIONS (based on Wirtgen 2004) Factor Bitumen Emulsion Foamed Bitumen Aggregate Types Applicable – Crushed rock – Natural gravel – RAP, cold-mix – RAP, stabilized – Crushed rock – Natural gravel – RAP, stabilized – Marginal (sands) Bitumen Mixing Temperature 68°F to 15°C 320°C to 356°F (before foaming) Aggregate Tempera- ture During Mixing Ambient (cold) Ambient (cold) Moisture Content During Mixing 90% of OMC minus 50% of emulsion content 65% to 95% of opti- mum moisture content Type of Coating of Aggregate Partial coating of coarse particles and cohesion of mix Coating of fine Construction and Compaction Temperature Ambient Ambient Rate of Initial Strength Gain Slow Medium Modification of Binder Yes Unsuitable Important Parame- ters of Binder – Emulsion type (anionic, cationic) – Residual bitumen – Breaking time – Curing – Half-life – Expansion Ratio Initial compaction can be accomplished with a sheep’s foot roller. An additional water truck may be needed (additive/ stabilizer dependent) to provide sufficient moisture con- tent for optimum density. Once the pulverized materials are mixed and the initial compaction completed, the profile is restored using a standard motor grader. The FDR surface can be broomed to remove any loose particles before open- ing to traffic. When more than one admixture is used in the FDR activi- ties, admixes are typically added sequentially (Harris 2007). For example, one project in Delaware County, Ohio, placed a lime–fly ash FDR section by first milling and removing 4 in. FIGURE 26 Typical equipment in CIR recycling train (based on FHWA 1997; ARRA 2001).

30 FIGURE 27 Flow chart of activities for FDR projects (based on FHWA 1997). FIGURE 28 Typical equipment used for FDR reclamation trains (based on FHWA 1997; ARRA 2001; Wirtgen 2004).

31 quality planning, quality control (QC), quality assurance (QA), and quality improvement (Miron et al. 2008). Areas of concern to agencies, as noted in the ARRA Basic Asphalt Recycling Manual (2001), are summarized in Table 31. Aspects associated with QC (process control) and QA were evaluated in the survey. TABLE 31 SUMMARY OF POINTS OF CONCERN FOR MONITORING PROCESS CONTROL AND ACCEPTANCE OF IN-PLACE RECYCLED MIXES Project Topics of Concern to Agencies HIR CIR FDR Heating of the existing pavement X Blue or black smoke emissions from heating unit X Scorched or charred pavement surface X Excessive temperature variation across the mat X Existing surface sufficiently softened for scarification X Treatment depth X X X Addition of additives (type and quantity) X X X Additives thoroughly mixed X X X Placement of recycled mix X X X Adequate temperature for compaction X Excess moisture removed X X X Compaction of recycled mix X X X Surface appearance (need consistency) X X X Grade and cross slope X Recycled Mix Properties of Concern to Agencies Gradation of final mix X X X Asphalt content of final mix X X X Recovered binder properties X X X In-place density X X X Recycled mix strength properties X X X Uniformity of compaction X X X Process control is a contractor program that is intended to provide a uniform final product and consists of evaluations during preconstruction and construction. Survey responses indicate that contractors use a range of process control options, including • QC technician on the job (about one-third of contractors), • Real-time mix design adjustments for variable existing pavement materials, • Verification of density (compaction), • Verification of gradations, • Mix design property verifications, • Monitoring and verification of applications rate, • Verification of moisture content, • Verification of indirect tensile strengths, and • Short-term performance before surface course is placed. Recommendations for QC approaches found in the litera- ture include emulsion testing to determine the percentage of residue from distillation (Thompson et al. 2009). Field- testing recommendations include depth measurements, compaction monitored with nuclear density gauge (direct transmission), and moisture content verified before overlay (CIR and FDR). Contractor QC programs include field technician train- ing; validation of mix design properties, material proper- ties, and density; and documentation of application rates. Acceptance The acceptance of the project by the agencies includes a number of approaches: • Construction of acceptable test strip (establishing rolling patterns, optimum moisture and dry density requirements, acceptable equipment); • Measurements of one or more of the following: den- sity, thickness, profile deviations, material properties, material quantities, and mix properties. Materials testing noted in the written responses include emulsion testing, residual binder testing (penetration), and aggregate gradations. Mix properties evaluated by agencies cover a wide range of tests such as binder content, indirect tensile strength, Hamburg rut testing, moisture sensitivity (boiling water, TSR), and other standard HMA tests (e.g., volumetrics). SemMaterials recommends that QC/QA testing for FDR projects include the following: • Asphalt emulsion: residue from distillation, oil distil- late by distillation, sieve test, and penetration;. • Added rock or dry additive: – Confirmation of quantity and type used in mix design, – Minimum material size, – Moisture content before emulsion. Suggestions for QA testing and parameters were found in the literature and included 97% of the laboratory density or 92% to 98% of the theoretical maximum specific grav- ity. Thompson et al. (2009) summarized a range of require- ments, including • Density – 97% of laboratory density (laboratory-compacted samples, nuclear density, and moisture),

32 – 92% to 98% of theoretical maximum density (In-Place Recycling Conference 2008), – Maximum dry density and optimum moisture con- tent (Franco et al. 2009); • Dry Marshall stability, minimum, of – Greater than 5.56 kN (1,250 lb) (Caltrans; for EE), – Greater than 4.44 kN (1,000 lb) (IowaDOT, at 100°F); • Indirect tensile strengths, minimum, of – 241 to 276 kPa (35 to 40 psi; dry) and 138 to 173 kPa (20 to 25 psi; wet) (SemMaterials); – Dry indirect tensile strengths of 300 to 345 kPa (43 to 50 psi; dry) and 150 to 175 kPa (22 to 25 psi; wet) (Ontario MTO); • Retained TSRs of – Greater than 70% (SemMaterials; Caltrans), – Greater than 50% (Ontario MTO; foamed asphalt); • Greater minimum resilient modulus of 837 to 1,034 MPa (120 to 150 ksi) (SemMaterials). Lane and Kazmierowski (2005a) noted that the Ontario MTO uses the following for both QC and QA testing: • Indirect tensile strengths, minimum, of 300 to 350 kPa (43 to 50 psi; dry) and 150 to 175 kPa (22 to 25 psi; wet) for CIR with expanded asphalt; • Target density of 96.0% of job mix formulas, with no result below 95.0%; • Moisture content of less than 2.0%, with no sublot value above 3.0%. QC/QA programs include measurements of density, moisture content, recycling layer depth, verification of mate- rial properties, and performance-related mix testing.