Reclaimed Asphalt Pavement - Material Description

ORIGIN

Currently there are many thousands of miles of roads, many of which are near, at or past their design life (1). The need for roadway maintenance and roadway deconstruction has afforded a material that can be readily used for the repairs, recycled asphalt pavement (RAP). Reclaimed asphalt pavement (RAP) is the term given to removed and/or reprocessed pavement materials containing asphalt and aggregates. These materials are generated when asphalt pavements are removed for reconstruction, resurfacing, or to obtain access to buried utilities. When properly crushed and screened, RAP consists of high-quality, well-graded aggregates coated by asphalt cement. Both population growth and economic development have caused the need for the extensive network of roadways in the past 50-70yrs. Currently there are many thousands of miles of roads, many of which are near, at or past their design life (1) . The need for roadway maintenance and roadway deconstruction has afforded a material that can be readily used for the repairs, recycled asphalt pavement (RAP). Reclaimed asphalt pavement (RAP) is the term given to removed and/or reprocessed pavement materials containing asphalt and aggregates. These materials are generated when asphalt pavements are removed for reconstruction, resurfacing, or to obtain access to buried utilities. When properly crushed and screened, RAP consists of high-quality, well-graded aggregates coated by asphalt cement. Asphalt recycling is not a novel concept, cold recycling dates back to the early 1900's. The first hot in-place recycling was reported in the literature in the 1930's. Modern asphalt recycling technologies that are used today evolved in the 1970s (2,4) . Asphalt pavement is generally removed either by milling or full-depth removal. Milling is typically done in rehabilitation projects where the upper level of pavement is removed and then replaced to increase the pavement's service life. RAP that is produced from milling is ready to be recycled with little to no processing, depending on the amount being used in the mixture. If the percentages of the mixture exceed 15 to 25 percent then additional screening, crushing and fractioning may be necessary. RAP that is milled is frequently segregated in separate stockpiles at processing facilities. This is done because the RAP has come from a specific site where the pavement was consistent when placed, hence making the RAP consistent in quality. This allows for the potential of the RAP to be included in more mix types as it is well classified (10) . In full-depth removal bulldozers or front end loaders break the entire pavement structure into manageable slabs and then load them into trucks, which transport the pavement to a reprocessing site. RAP that was removed in the full-depth fashion may be segregated like millings, but not necessarily. Large quantities of uniformly consistent full-depth RAP may be segregated for crushing and sizing later. This is done because once the RAP is processed it will provide consistent stone gradation, quality, asphalt content and asphalt characteristics. However, it is more common that small quantities of full-depth RAP arrive at a reprocessing facility. Typically full-depth RAP from various sites is stored in common piles before crushing and blending. The RAP is then crushed down to the largest aggregate size. This allows for the creation of a consistent product from various sources. Experience has demonstrated that with thorough blending and crushing RAP with a consistent stone gradation and asphalt content can be manufactured (10) . Although the majority of old asphalt pavements are recycled at central processing plants, asphalt pavements may be pulverized in place and incorporated into granular or stabilized base courses using a self-propelled pulverizing machine. Hot in-place and cold in-place recycling processes have evolved into continuous train operations that include partial depth removal of the pavement surface, mixing the reclaimed material with beneficiating additives (such as virgin aggregate, binder, and/or softening or rejuvenating agents to improve binder properties), and placing and compacting the resultant mix in a single pass. Reliable figures for the generation of RAP are not readily available from all state highway agencies or local jurisdictions. Based on incomplete data, it is estimated that as much as 41 million metric tons (45 million tons) of RAP may be produced each year in the United States. (9) Additional information on recycling of asphalt pavement can be obtained from the following organizations:

National Asphalt Pavement Association 5100 Forbes Boulevard Lanham, Maryland 20706-4413 http://www.hotmix.org/

Asphalt Institute Research Park Drive Lexington , Kentucky 40512

Asphalt Recycling and Reclaiming Association # 3 Church Circle, Suite 250 Annapolis , Maryland 21401

Asphalt Pavement Alliance 5100 Forbes Blvd. Lanham , MD 20706 -44017 http://www.asphaltalliance.com/

CURRENT MANAGEMENT OPTIONS

Recycling The majority of the RAP that is produced is recycled and used, although not always in the same year that it is produced. Recycled RAP is almost always returned back into the roadway structure in some form, usually incorporated into asphalt paving by means of hot or cold recycling, but it is also sometimes used as an aggregate in base or subbase construction. It has been estimated that as much as approximately 33 million metric tons (36 million tons), or 80 to 85 percent of the excess asphalt concrete presently generated, is reportedly being used either as a portion of recycled hot mix asphalt, in cold mixes, or as aggregate in granular or stabilized base materials. (2) Some of the RAP that is not recycled or used during the same construction season that it is generated is stockpiled and is eventually reused. Disposal Excess asphalt concrete is disposed of in landfills or sometimes in the right of way. In most situations, this occurs where small quantities are involved, or where the material is commingled with other materials, or facilities are not readily available for collecting and processing the RAP. It is estimated that the amount of excess asphalt concrete that must be disposed is less than 20 percent of the annual amount of RAP that is generated.

MARKET SOURCES

In most cases, recycled hot mix asphalt can be obtained from central RAP processing facilities where asphalt pavements are crushed, screened, and stockpiled for use in asphalt concrete production, cold mix, or as a granular or stabilized base material. Most of these processing facilities are located at hot mix asphalt plant sites, where the RAP is either sold or used as feedstock for the production of recycled hot mix asphalt pavement or recycled cold mix. The properties of RAP are largely dependent on the properties of the constituent materials and asphalt concrete type used in the old pavement. Since RAP may be obtained from any number of old pavement sources, quality can vary. Excess granular material or soils, or even debris, can sometimes be introduced into old pavement stockpiles. The number of times the pavement has been resurfaced, the amount of patching and/or crack sealing, and the possible presence of prior seal coat applications will all have an influence on RAP composition. Quality control is needed to ensure that the processed RAP will be suitable for the prospective application. This is particularly the case with in-place pavement recycling.

HIGHWAY USES AND PROCESSING REQUIREMENTS

Milled or crushed RAP can be used in a number of highway construction applications. These include its use as an aggregate substitute and asphalt cement supplement in recycled asphalt paving (hot mix or cold mix), as a granular base or subbase, stabilized base aggregate, or as an embankment or fill material. Asphalt Concrete Aggregate and Asphalt Cement Supplement Recycled asphalt pavement can be used as an aggregate substitute material, but in this application it also provides additional asphalt cement binder, thereby reducing the demand for asphalt cement in new or recycled asphalt mixes containing RAP. When used in asphalt paving applications (hot mix or cold mix), RAP can be processed at either a central processing facility or on the job site (in-place processing). Introduction of RAP into asphalt paving mixtures is accomplished by either hot or cold recycling. Hot Mix Asphalt (Central Processing Facility) Recycled hot mix is normally produced at a central RAP processing facility, which usually contains crushers, screening units, conveyors, and stackers designed to produce and stockpile a finished granular RAP product processed to the desired gradation. This product is subsequently incorporated into hot mix asphalt paving mixtures as an aggregate substitute. Both batch plants and drum-mix plants can incorporate RAP into hot mix asphalt. Hot Mix Asphalt (In-Place Recycling) Hot in-place recycling is a process of repaving that is performed as either a single or multiple pass operation using specialized heating, scarifying, rejuvenating, laydown, and compaction equipment. There is no processing required prior to the actual recycling operation. Cold Mix Asphalt (Central Processing Facility) The RAP processing requirements for cold mix recycling are similar to those for recycled hot mix, except that the graded RAP product is incorporated into cold mix asphalt paving mixtures as an aggregate substitute. Cold Mix Asphalt (In-Place Recycling) The cold in-place recycling process involves specialized plants or processing trains, whereby the existing pavement surface is milled to a depth of up to 6 in, processed, mixed with asphalt emulsion (or foamed asphalt), and placed and compacted in a single pass. There is no processing required prior to the actual recycling operation. Granular Base Aggregate To produce a granular base or subbase aggregate, RAP must be crushed, screened, and blended with conventional granular aggregate, or sometimes reclaimed concrete material. Blending granular RAP with suitable materials is necessary to attain the bearing strengths needed for most load-bearing unbound granular applications. RAP by itself may exhibit a somewhat lower bearing capacity than conventional granular aggregate bases. Stabilized Base Aggregate To produce a stabilized base or subbase aggregate, RAP must also be crushed and screened, then blended with one or more stabilization reagents so that the blended material, when compacted, will gain strength. Embankment or Fill Stockpiled RAP material may also be used as a granular fill or base for embankment or backfill construction, although such an application is not widely used and does not represent the highest or most suitable use for the RAP. The use of RAP as an embankment base may be a practical alternative for material that has been stockpiled for a considerable time period, or may be commingled from several different project sources. Use as an embankment base or fill material within the same right of way may also be a suitable alternative to the disposal of excess asphalt concrete that is generated on a particular highway project.

MATERIAL PROPERTIES

Physical Properties The properties of RAP are largely dependent on the properties of the constituent materials and the type of asphalt concrete mix (wearing surface, binder course, etc.). There can be substantial differences between asphalt concrete mixes in aggregate quality, size, and consistency. Since the aggregates in surface course (wearing course) asphalt concrete must have high resistance to wear/abrasion (polishing) to contribute to acceptable friction resistance properties, these aggregates may be of higher quality than the aggregates in binder course applications, where polishing resistance is not of concern. Both milling and crushing can cause some aggregate degradation. The gradation of milled RAP is generally finer and denser than that of the virgin aggregates. Crushing does not cause as much degradation as milling; consequently, the gradation of crushed RAP is generally not as fine as milled RAP, but finer than virgin aggregates crushed with the same type of equipment. The particle size distribution of milled or crushed RAP may vary to some extent, depending on the type of equipment used to produce the RAP, the type of aggregate in the pavement, and whether any underlying base or subbase aggregate has been mixed in with the reclaimed asphalt pavement material during the pavement removal. During processing, virtually all RAP produced is milled or crushed down to1.5 in or less, with a maximum allowable top size of either 2 in or 2.5 in. Table 1 lists the typical range of particle size distribution that normally results from the milling or crushing of RAP. Milled RAP is generally finer than crushed RAP. Studies on pavements in California , North Carolina , Utah and Virginia have shown that before and after milling, the pavement fraction passing a 2.36 mm (No. 8) sieve can be expected to increase from a premilled range of 41 to 69 percent to a postmilled range of 52 to 72 percent. The fraction passing a 0.075 mm (No. 200) sieve can be expected to increase from approximately 6 to 10 percent to a range of 8 to 12 percent. (7)Most sources of RAP will be a well-graded coarse aggregate, comparable to, or perhaps slightly finer and more variable than, crushed natural aggregates. The unit weight of milled or processed RAP depends on the type of aggregate in the reclaimed pavement and the moisture content of the stockpiled material. Although available literature on RAP contains limited data pertaining to unit weight, the unit weight of milled or processed RAP has been found to range from 120 to 140 lb/ft 3 , which is slightly lower than that of natural aggregates. Information on the moisture content of RAP stockpiles is sparse, but indications are that the moisture content of the RAP will increase while in storage. Crushed or milled RAP can pick up a considerable amount of water if exposed to rain. Moisture contents up to 5 percent or higher have been measured for stored crushed RAP. (12) As noted earlier, during periods of extensive precipitation, the moisture content of some RAP stockpiles may be as high as 7 to 8 percent. (3) Lengthy stockpiling of crushed or milled RAP should, therefore, be kept to a minimum. The asphalt cement content of RAP typically ranges between 3 and 7 percent by weight. The asphalt cement adhering to the aggregate is somewhat harder than new asphalt cement. This is due primarily to exposure of the pavement to atmospheric oxygen (oxidation) during use and weathering. The degree of hardening depends on several factors, including the intrinsic properties of the asphalt cement, the mixing temperature/time (increases with increasing high temperature exposure), the degree of asphalt concrete compaction (increases if not well compacted), asphalt cement/air voids content (increases with lower asphalt/higher air voids content), and age in service (increases with age).

Table 1. Typical range of particle size distribution for reclaimed asphalt pavement (RAP) (percent by weight passing).

Screen Size (mesh)

Percent Finer After Processing or Milling

37.5 mm (1.5 in) 25 mm (1.0 in) 19 mm (3/4 in) 12.5 mm (1/2 in) 9.5 mm (3/8 in) 75 mm (No. 4) 2.36 mm (No. 8) 1.18 mm (No. 16) 0.60 mm (No. 30) 0.30 mm (No. 50) 0.15 mm (No. 100) 0.075 mm (No. 200)

100 95 - 100 84 - 100 70 - 100 58 - 95 38 - 75 25 - 60 17 - 40 10 - 35 a 5 - 25 b 3 - 20 c 2 - 15 d

a. Usually less than 30 percent b. Usually less than 20 percent c Usually less than 15 percent d. Usually less than 10 percent
The RAP obtained from most wearing surface mixes will usually have an asphalt content in the 4.5 to 6 percent range. The recovered asphalt from RAP usually exhibits low penetration and relatively high viscosity values, depending on the amount of time the original pavement has been in service. Penetration values at 25°C (77°F) are likely to range from 10 to 80 while the absolute viscosity values at 60°C (140°F) may range from as low as 2,000 poises (equivalent to AC-20) up to as high as 50,000 poises or greater, depending on the extent of aging. Viscosity ranges from 4,000 to 25,000 poises can normally be expected from the asphalt cement that is recovered from RAP material. (5) Table 2 provides a summary of the typical ranges of physical properties of RAP, other than gradation.

Table 2. Physical and mechanical properties of reclaimed asphalt pavement (RAP).

Type of Property

RAP Property

Typical Range of Values

Physical Properties

Unit Weight 1940 - 2300 kg/m 3 (120-140 lb/ft 3)
Moisture Content Normal : up to 5%Maximum: 7-8%
Asphalt Content Normal : 4.5-6% Maximum Range: 3-7%
Asphalt Penetration Normal : 10-80 at 25°C (77°F)
Absolute Viscosity or Recovered Asphalt Cement Normal : 4,000 - 25,000 poises at 60°C (140°F)

Mechanical Properties

Compacted Unit Weight 1600 - 2000 kg/m 3 (100-125 lb/ft 3 )
California Bearing Ratio (CBR) 100% RAP: 20-25% 40% RAP and 60% Natural Aggregate: 150% or higher
Chemical Properties Mineral aggregates constitute the overwhelming majority (93 to 97 percent by weight) of RAP. Only a minor percentage (3 to 7 percent) of RAP consists of hardened asphalt cement. Consequently, the overall chemical composition of RAP is essentially similar to that of the naturally occurring aggregate that is its principal constituent. Asphalt cement is made up of mainly high molecular weight aliphatic hydrocarbon compounds, but also small concentrations of other materials such as sulfur, nitrogen, and polycyclic hydrocarbons (aromatic and/or naphthenic) of very low chemical reactivity. Asphalt cement is a combination of asphaltenes and maltenes (resins and oils). Asphaltenes are more viscous than either resins or oils and play a major role in determining asphalt viscosity. Oxidation of aged asphalt causes the oils to convert to resins and the resins to convert to asphaltenes, resulting in age hardening and a higher viscosity binder. (8) Mechanical Properties The mechanical properties of RAP depend on the original asphalt pavement type, the method(s) utilized to recover the material, and the degree of processing necessary to prepare the RAP for a particular application. Since most RAP is recycled back into pavements, there is a general lack of data pertaining to the mechanical properties for RAP in other possible applications. The compacted unit weight of RAP will decrease with increasing unit weight, with maximum dry density values reported to range from 100 lb/ft 3 to 125 lb/ft 3 . (11) California Bearing Ratio (CBR) values for RAP material containing trap rock aggregate have been reported in the 20 to 25 percent range. However, when RAP is blended with natural aggregates for use in granular base, the asphalt cement in the RAP has a significant strengthening effect over time, such that specimens containing 40 percent RAP have produced CBR values exceeding 150 after 1 week. (6) Table 2 provides a summary of the mechanical properties of RAP discussed in the preceding paragraphs.

ENVIRONMENTAL CONSIDERATIONS

Asphalt pavement consists of aggregate and petroleum derived asphalt binder containing volatile and semi-volatile constituents (e.g., polycyclic aromatic hydrocarbons (PAHs)). Additionally the asphalt pavement roadway may contain surface treatments, rubberized materials or contaminants from vehicle or other emissions (e.g., historically lead). The environmental issues are different for RAP based upon various beneficial uses. For bound applications such as hot-in-place and cold-in-place recycling, research into the difference in emissions to that of virgin materials have not been conducted. For unbound applications, leachability from the RAP may also be a concern. This same leachability would be a concern if RAP was stockpiled or stored and exposed to precipitation. Testing has been conducted to determine the leaching characteristics of RAP in Florida. In all batch test measurements of VOCs, PAHs, and heavy metals (Ba, Ca, Cr, Cu, Pb, Ni and Zn) results were below the detection limit and below the applicable state regulatory groundwater guidance concentrations. This indicates that all RAP samples tested pose minimal risk under current waste policy in Florida. Lysimeter (column leaching) tests were also performed and columns were exposed to synthetic precipitation for 42 days. The VOC, PAH and heavy metal (Ba, Ca, Cr, Cu, Ni and Zn) measurements were all below detection limits, except for lead which exhibited a concentration of 24 µg/L and 23 µg/L on days 12 and 14 of the experiment, but then was otherwise below the applicable Florida Groundwater Guidance Concentration (15 µg/L) for the duration of the experiment. (13) Other batch and column leaching tests were completed on RAP which also found constituents leached were low and generally below European drinking water standards (The Drinking Water Directive (DWD), Council Directive 98/83/EC). (14) Additionally, the University of Minnesota completed a review of current literature on PAHs in asphalt pavement concluding that PAH concentrations depend on the type of pavement (coal-tar versus petroleum based). Petroleum based asphalt pavement contained PAHs at concentrations below Minnesota Pollution Control Agency human health risk clean-up levels (15). The only exceedance was when when PAHs were converted to benzo(a)pyrene equivalents, they could exceed the lowest limit (Tier I). The report further concluded that when RAP is used as subbase aggregate, it is mixed with soils or other aggregates that do not contain PAH, so this mixture would not likely exceed applicable limits. (15)

REFERENCES

  1. Basic Asphalt Recycling Manual, Asphalt Recycling and Reclaiming Association, PUB:NHI 01-022, Annapolis , Maryland , 2001.
  2. Engineering and Environmental Aspects of Recycling Materials for Highway Construction , Federal Highway Administration and U.S. Environmental Protection Agency, Report No. FHWA-RD-93-008, Washington , DC , May 1993.
  3. Decker, D. S. and T. J. Young, "Handling RAP in an HMA Facility." Proceedings of the Canadian Technical Asphalt Association , Edmonton , Alberta , 1996.
  4. Designing HMA Mixtures with High RAP Content, A Practical Guide, National Pavement Association, Quality Improvement Series 124, Lanham , Maryland , 2007.
  5. Epps, J. A., D. N. Little, R. J. O'Neal, and B. M. Gallaway. "Mixture Properties of Recycled Central Plant Materials." American Society for Testing and Materials, Special Technical Publication No. 662, Recycling of Bituminous Pavements , West Conshohocken, Pennsylvania , December, 1977.
  6. Hanks, A. J. and E. R. Magni. The Use of Bituminous and Concrete Material in Granular Base and Earth . Materials Information Report MI-137, Engineering Materials Office, Ontario Ministry of Transportation, Downsview , Ontario , 1989.
  7. Kallas, B. F. Flexible Pavement Mixture Design Using Reclaimed Asphalt Concrete , FHWA/RD-84/088, June, 1984.
  8. Noureldin, Ahmed Samy and Leonard E. Wood. "Variations in Molecular Size Distribution of Virgin and Recycled Asphalt Binders Associated with Aging." Transportation Research Board, Record No.1228, Washington , DC , 1989.
  9. Pavement Recycling Executive Summary and Report , Federal Highway Administration, Report No. FHWA-SA-95-060, Washington , DC , 1995.
  10. Recycling Hot-Mix Asphalt Pavements , National Pavement Association, Information Series 123, Lanham , Maryland , 2007.
  11. Senior, S. A., S. I. Szoke, and C. A. Rogers. "Ontario 's Experience with Reclaimed Materials for Use in Aggregates." Presented at the International Road Federation Conference, Calgary , Alberta , 1994.
  12. Smith, Richard W. "State-of-the-Art Hot Recycling." Transportation Research Board, Record No. 780, Proceedings of the National Seminar on Asphalt Pavement Recycling , Washington , DC , 1980.
  13. Brantley, A.S. and Townsend, T., Leaching of pollutants from reclaimed asphalt pavement, Environmental Engineering Science, Vol. 16, no. 2, pp. 105-116. Apr. 1999.
  14. Legret, M., Odie, L., Demare, D., Jullien, A., Leaching of heavy metals and plycyclic aromatic hydrocarbons from reclaimed asphalt pavement, Water Research, Vol. 39, 3675-3685, 2005.
  15. Grosenheider, K., Bloom, P., Halbach T., Johnson, M., A Review of the Current Literature Regarding Polycyclic Aromatic Hydrocarbons in Asphalt Pavement, Mn/DOT contract No. 81655, October, 2005.

Reclaimed Asphalt Pavement - Asphalt Concrete (Hot Recycling)

INTRODUCTION

The Federal Highway Administration estimates that of the 90 million tons of hot mix asphalt (HMA) removed every year, 90% is reused in highway applications. Approximately 30 million tons of HMA removed is recycled back into HMA pavements, this translates into a savings of about $300 million annually by reducing material and disposal costs (20,40) Reclaimed asphalt pavement (RAP) can be used as an aggregate in the hot recycling of asphalt paving mixtures in one of two ways. The most common method (conventional recycled hot mix) involves a process in which RAP is combined with virgin aggregate and new asphalt cement in a central mixing plant to produce new hot mix paving mixtures. (1) A second method (hot in-place recycling) involves a process in which asphalt pavement surface distress is corrected by softening the existing surface with heat, mechanically removing the pavement surface, mixing it with a recycling or rejuvenating agent, possibly adding virgin asphalt and/or aggregate, and replacing it on the pavement without removing the recycled material from the pavement site. (14)

PERFORMANCE RECORD

Although some form of pavement recycling had been practiced as early as 1915, (23) the first sustained efforts to recover and reuse old asphalt paving materials were conducted during 1974 in Nevada and Texas. (41) Bolstered by the sponsorship of the Federal Highway Administration (FHWA), more than 40 states performed and documented RAP demonstration projects between 1976 and 1982. RAP is now routinely accepted in asphalt paving mixtures as an aggregate substitute and as a portion of the binder in nearly all 50 states. Substitution rates of 10 to 50 percent or more, depending on state specifications, are normally introduced in pavements, and recently developed technology has even made it possible to recycle 90 to 100 percent RAP in hot mix. Recycled Hot Mix This recycling technology is currently the most widespread technology used in the world. Hot recycling involves combining RAP with virgin aggregates, new asphalt binder and/or recycling agents in a central processing plant to produce a recycled mix. This method employs the heat transfer method to soften the RAP. When heat transfer is used to soften RAP it is crucial that the moisture in the RAP be at a practical minimum. Extra moisture within the RAP will decrease the production rate, as the heat is first consumed to convert the moisture to steam before the RAP is softened (13) . The use of processed RAP to produce conventional recycled hot mix (RHM) is the most common type of asphalt recycling and is now considered standard asphalt paving practice. There are abundant technical data available indicating that properly specified and produced recycled hot mix asphalt is equivalent in quality and structural performance to conventional hot mix asphalt in terms of rutting, raveling, weathering, and fatigue cracking. Recycled hot mix asphalt mixtures also generally age more slowly and are more resistant to the action of water than conventional hot mix asphalt. (See references 2,13,16,18,22,26,28,29,30,36 and 37.) The maximum limit for RAP content in RHM produced in conventional hot mix asphalt batch plants is widely considered to be 50 percent, limited by both the heat capacity of the plants and gaseous hydrocarbon emissions. As much as 60 to 70 percent RAP may be processed in drum mix plants. Special plants based on microwave technology have been developed to limit gaseous emissions from hot mix asphalt production using very high RAP contents (up to 100 percent RAP), but the cost of heating is much higher than that of conventional systems. This process was developed in California and has only seen limited use. (24) Currently the ratio of RAP to virgin aggregates in batch plants is between 15-25 percent and 30-50 percent for drum mix plants (13) . Advantages to hot recycling include (13) :
  • Conservation of non-renewable resources
  • Energy conservation compared to other reconstruction methods
  • Disposal problems are eliminated
  • Problems with existing aggregate gradation and/or asphalt binder can be corrected with proper selection of virgin aggregates, asphalt binders and/or recycling agents
  • Provides the same, if not better, performance as pavements constructed with 100% virgin materials
  • Economic savings
Hot In-Place Recycling Hot in-place recycling (HIR) allows for 100% recycling of the existing asphalt pavement onsite. Depths of treatment typically range from ¾- 2 inches. In this process the existing asphalt is heated and softened, which allows it to be lifted or hot rotary milled to a specific depth. The pavement is then mixed, placed and compacted. Virgin aggregates, new asphalt binder, recycling agents and/or new HMA can be added as needed. Typically the virgin aggregate or HMA addition is limited by equipment constraints to about 30% by mass of the HIR mix. The rates at which additives may be added are determined through the analysis of the existing asphalt pavement properties and laboratory mix designs (13) . There are three HIR processes: 1. Surface Recycling, 2. Remixing and 3. Repaving. There are multiple pieces of equipment used in HIR and they include pre-heaters, heaters, heater/sacrificers, mixers, pavers and rollers. Surface recycling, the oldest HIR process is typically used to treat depths from ¾ in to 1 ½ in. This process requires the softening of the surface layer of pavement. The softened pavement is then sacrificed to the desired treatment depth. The loose recycled mix is then thoroughly mixed and placed with a standard paver screed. The remixing process is when the existing pavement is heated, softened and then sacrificed. The sacrificed pavement is then mixed with virgin aggregate, new asphalt binder, a recycling agent and/or new HMA may be added. This mixture is then placed in one layer. Remixing is used when the properties of the existing pavement need to be significantly altered. Repaving includes both surface recycling and remixing processes. The recycled mix is used as the leveling course and a new HMA mixture is used as the surface or wearing course. The thickness of the new HMA can range from ¾ in to 3 in thick (13) . Advantages to HIR (13) :
  • Conservation of non-renewable resources
  • Energy conservation compared to other reconstruction methods
  • Reduced truck hauling compared with other rehabilitation methods
  • Eliminates disposal problems
  • Treatment of complete roadway width or only the driving lanes
  • Surface irregularities and cracks are interrupted and filled
  • Rutting, potholes and raveling are eliminated
  • Curb reveal height and overhead clearance can be maintained
  • Oxidized asphalt binder can be rejuvenated
  • Problems with existing aggregate gradation and/or asphalt binder can be corrected
  • Friction numbers can be restored
  • Roadway opened to traffic at end of day with little or no edge drop off
  • Economic savings
Pavements that exhibit structural base failure, irregular patching or the need for major drainage or grade improvements are not suitable candidates for HIR. (14) HIR is an effective method for treating raveling, potholes, bleeding, friction number, rutting, corrugations, shoving, slippage, cracking (longitudinal, transverse and reflection) as well as poor ride quality, which may be caused by swells, bumps, sags or depressions (13) . Not all states have experience in HIR applications, although HIPR technology is a fairly well accepted practice. There are 32 states that report having some experience with HIPR, although 22 of these states consider their use of HIPR to be experimental. The 10 states that have the most experience with HIR are Arkansas, Colorado, Florida, Kansas, Maryland, New York, Ohio, Texas, Utah, and Virginia. None of these states has had more than five HIR projects per year. A survey of these states found that, in general, all have reported good or fair performance. (14) The survey of HIR experience at the state DOT level further indicated that the use of the three different HIR processes has been fairly evenly divided, with 13 states having had some experience with heater-scarification, and 16 states each having some experience with either the repaving or the remixing process. Of the 10 states with the most HIR experience, 5 have used heater-scarification, 4 have used the repaving process, and 6 have used the remixing process. (14)

MATERIAL PROCESSING REQUIREMENTS

Recycled Hot Mix Reclaimed asphalt pavement must be processed into a granular material prior to use in hot mix applications. A typical RAP processing plant consists of a crusher, screening units, conveyors, and stacker. It is desirable to produce either a coarse or a fine fraction of processed RAP to permit better control over input to the hot mix plant and better control of the mix design. The processed RAP used in recycled hot mix asphalt should be as coarse as possible and the fines (minus 0.075 mm (No. 200 sieve)) minimized. Gentle RAP crushing (controlled crusher speed and clearance adjustment on exit gate) is recommended to minimize the fracture of coarse aggregate and excess fines generation. Hot In-Place Recycling In the HIPR process, the surface of the pavement must be softened with heat prior to mechanical scarification. The HIPR process has evolved into a self-contained, continuous train operation that includes heating, scarifying, rejuvenator addition, mixing, and replacement.

ENGINEERING PROPERTIES

Some of the engineering properties of RAP that are of particular interest when RAP is incorporated into new asphalt pavements include its gradation, asphalt content, and the penetration and viscosity of the asphalt binder. Gradation : The aggregate gradation of processed RAP is somewhat finer than virgin aggregate. This is due to mechanical degradation during asphalt pavement removal and processing. RAP aggregates usually can satisfy the requirements of ASTM D692 "Coarse Aggregates for Bituminous Pavement Mixtures" and ASTM D1073 "Fine Aggregate for Bituminous Pavement Mixtures." (2,10) Asphalt Content and Properties : The asphalt content of most old pavements will comprise approximately 3 to 7 percent by weight and 10 to 20 percent by volume of the pavement. Due to oxidation aging, the asphalt cement has hardened and consequently is more viscous and has lower penetration values than the virgin asphalt cement. Depending on the amount of time the original pavement had been in service, recovered RAP binder may have penetration values from 10 to 80 and absolute viscosity values at 60°C (140°F) in a range from as low as 2,000 poises to as high 50,000 poises or greater. (19)

DESIGN CONSIDERATIONS

Recycled Hot Mix Mix Design The use of processed RAP in hot mix asphalt pavements is now standard practice in most jurisdictions and is referenced in ASTM D3515. (7) The primary steps in the design of mixes include the determination of material properties of RAP and new materials, the selection of an appropriate blend of RAP and virgin aggregate to meet gradation, the selection of an appropriate asphalt cement blend to satisfy specified viscosity and/or penetration requirements, the need to add a recycling or rejuvenating agent to soften the existing binder, and the need to comply with stability, flow, and air voids requirements. The first mix designs for the successful inclusion of RAP in HMA mixes were been designed over 15 years ago. Historically, either the Marshall (3) or the Hveem (4) mix design procedures were used by most state agencies for determining the asphalt cement and acceptable RAP content of recycled paving mixes. (1) Today there are additional mix designs which can be used to successfully incorporate RAP into HMA mixes. These mix designs range from low percentages of RAP to high percentages of RAP and Superpave mixes. (16,20,25,31) It is important to note that it has been demonstrated that RAP can successfully be incorporated in Superpave technology. (16,20,25) Recent research focuses on finding new, more efficient tests to determine various parameters of RAP and appropriate mix designs. (39) In order to determine how to design a mix, the recycled mixtures must be evaluated. Recycled mixtures should be evaluated on a 3 tier basis. Tier 1 is mixtures that include up to 15% Rap and do not require anything be changed in the mix design process. Tier 2 includes mixtures with 15%-25% RAP. Tier 2 mixtures require the asphalt grade be dropped by one grade on the high and low ends of the PG grade. Finally tier 3 mixtures include more than 25% RAP and these mixes require that asphalt be recovered from the RAP and be blended with virgin asphalt to produce a blended asphalt with the desired properties (25) . Recycling and rejuvenating agents can be divided into three main types: "super-soft" asphalt cements, napthenic (aromatic) oils, and paraffinic oils. These products consist of organic compounds derived from petroleum extracts during petroleum hydrocarbon processing. ASTM D4552 (8) provides a classification of recycling or rejuvenating agents. Procedures for selecting the quality of asphalt cement or recycling agent are outlined in ASTM D4887. (9) This specification includes a viscosity blending chart, which enables the designer to determine the percentage of recycling or rejuvenating agent (or soft asphalt cement) to add to the total binder in order to achieve a desired value of absolute viscosity for the recycled asphalt cement. The Asphalt Institute's manual on asphalt hot mix recycling also provides trial mix design examples that indicate how to use a viscosity blending chart to design a recycled hot mix. The Asphalt Institute's manual on mix design methods for asphalt concrete (32) provides a method to determine necessary mix design characteristics (such as stability, flow, and air voids content) for either the Marshall or the Hveem mix design methods. The final mix design proportions for the recycled hot mix paving mixture will be determined by completing mix design testing using standard procedures to satisfy applicable mix design criteria. Additional virgin aggregates may be required to satisfy gradation requirements to improve stability and to limit the RAP content in recycled hot mixes. In the production of hot mix, superheated virgin aggregate is needed to provide indirect heat transfer to the RAP while maintaining the proper mix temperature without the generation of "blue smoke." Structural Design Conventional AASHTO pavement structural design methods are appropriate for asphalt pavements incorporating reclaimed asphalt pavement in the mix. Hot In-Place Recycling Mix Design Mix design procedures for HIR have remained relatively unchanged for a long time. There is a potential that the Superpave system may be applied to HIR mix design. Traditional HIR mix design includes some or all of the following steps: 1. Evaluation of the existing HMA asphalt, 2. Determining whether the existing asphalt binder needs rejuvenation, 3. Selecting the type and amount of recycling agent, 4. Determining the need for and amount of admix including aggregate gradation, type and amount of soft, new asphalt binder. (13) Mix design procedures for HIR are not as well established as those for conventional recycled hot mix. Many states as a minimum require that cores be taken of the candidate pavement to determine in-place pavement properties, including binder content, viscosity, and aggregate grading. (14)Original HIR mix designs primarily focused on the viscosity of aged asphalt binder and the amount of rejuvenation required. HIR mix designs that are more comprehensive not only address the rheological properties of the asphalt binder, they also consider the recycled mix properties . (13) The material properties of the existing asphalt pavement (to at least the depth of scarification) should be determined prior to construction in order to permit any necessary adjustments to aggregate gradation to develop the required voids in mineral aggregate (VMA) and selection of the appropriate viscosity binder. This will require coring of the pavement to be recycled and laboratory testing of the recovered paving samples. Unlike conventional recycled hot mix where the RAP is combined with a significant amount of new aggregate material (making up typically between 60 to 80 percent of the RHM), HIPR may involve up to 100 percent recycling of the existing pavement. Consequently, the extent to which the existing pavement can be improved or modified is limited by the condition and characteristics of the old mix. The amount of rejuvenating agent that can be added through HIRP is limited by the air voids content of the existing asphalt. When the air voids content of the old asphalt mix is too low to accommodate sufficient recycling agent for proper rejuvenation or softening of the old asphalt binder without mix flushing, it may be necessary to add additional fine aggregate or to beneficiate with virgin hot mix to open up the mix or increase the air voids. The selection of the appropriate addition (either fine aggregate or virgin hot mix), and the amount to be added, are determined by Marshall or Hveem mix design methods. The type of recycling or rejuvenating agent and the percentage to be added to the binder can be estimated using procedures outlined in ASTM methods D4552 (8) and D4887. (9) The recycling or rejuvenating agent, if used, should be compatible with the recycled and new asphalt binder. Currently HIR recycling agents are hydrocarbon materials. They may include soft asphalt binders, specialty/proprietary products or some types of asphalt emulsions. The least expensive of these are soft asphalt binders, however they are also not as efficient at rejuvenation. (13) Structural Design HIR is generally considered a rehabilitation technique for addressing superficial pavement distress to a maximum depth of about 1 ½ in. The recycled layer is considered to be structurally equivalent to new hot mix asphalt.

CONSTRUCTION PROCEDURES

Recycled Hot Mix Material Handling and Storage RAP is produced by milling, ripping, breaking, crushing, or pulverizing types of equipment. To ensure that the final RAP product will perform as intended, inspection of incoming RAP with rejection of contaminated loads (excess granular material, surface treatment, joint sealant, etc.) should be undertaken. Some jurisdictions also require that RAP from a particular project not be blended or commingled with RAP from other projects. Once processed, RAP has traditionally been handled as a conventional aggregate material. Historically, because of the variability of RAP in comparison with virgin aggregates, many agencies do not permit the blending of RAP from different projects into combined stockpiles. The Asphalt Institute recommended that the height of RAP stockpiles be limited to a maximum of 3 meters (10 ft) to help prevent agglomeration or sticking together of the RAP particles. (1) Stockpiling time should also be minimized to keep the moisture content of RAP stockpiles from becoming excessive. Experience has proven that conical stockpiles are preferred to horizontal stockpiles and will not cause RAP to re-agglomerate in large piles. RAP has the tendency to form a crust (due to a solar/thermal effect from the sun) over the first 8 to 12 in of pile depth for both conical and horizontal stockpiles. This crust tends to help shed water, but is easily broken by a front-end loader, and may help keep the rest of the pile from agglomerating. RAP has a tendency to hold water and not to drain over time like an aggregate stockpile. Therefore, low, horizontal, flat stockpiles are subject to greater moisture accumulation than tall, conical stockpiles. It is not unusual to find RAP moisture content in the 7 to 8 percent range during the rainy season at facilities using low, horizontal stockpiling techniques. (13,16) RAP stockpiles are typically left uncovered because covering with tarps can cause condensation under the tarp and add moisture to the RAP stockpile. For this reason, RAP stockpiles are either left uncovered or RAP is stored in an open-sided building, but under a roof. (13,16) Some producers are beginning to pave underneath the RAP stockpiles. This practice may help to improve stockpile drainage (when graded properly) and it reduces potential moisture absorption from the ground. Possible contamination is also eliminated when a front-end loader collects materials from close to grade level. (16) When large quantities of RAP from different sources are available, it is advisable to keep stockpiles separated and identified by source. Consistent RAP from a "composite" or "blended" pile can be produced using a crushing and screening operation and reprocessing stockpiles that come to the yard from different sources. Material handling machinery, such as front-end loaders and bulldozers, should be kept from driving directly on the stockpile. Agglomerating RAP particles can make it very difficult for the loader to handle the RAP. Mixing, Placing and Compacting When RAP is added to hot mix asphalt, measures must be taken to avoid exposing the RAP to temperatures in excess of 427°C (800°:F). Exposure of the RAP to temperatures above this limit can result in excessive hydrocarbon emissions (blue smoke). To reduce this problem, hot mix asphalt plants have been modified to permit the recycling of RAP. (21) In a batch plant operation, the RAP is usually added to superheated new aggregate at the pugmill. In drum-mix plants, RAP is usually introduced with new aggregate into the drum using a dual feed system. The new aggregate is typically introduced at the hot end of the drum (normally the front end of the drum), while the RAP is introduced at the middle or rear of the drum to prevent overheating damage to the RAP. (16,33) In a batch plant, typical RAP substitution rates are limited by the heat capacity of the plant and the ability to superheat the aggregate to temperatures that will produce a suitable mix temperature. This normally limits batch plant blends to between 10 and 30 percent RAP. In a drum mix plant, from 30 to 70 percent RAP. As a general rule, gaseous emissions are typically undesirable with RAP percentages above 50 percent . (16) No special techniques or equipment is necessary for laydown and compaction of recycled mixtures. It is important to note that recycled mixtures are typically placed at slightly cooler temperatures than virgin mixtures in an effort to reduce the negative impact of superheated temperatures, the paving crew may find rolling and compacting times reduced. Conventional materials, techniques and indicators are all relevant. (16) Quality Control To produce consistently high-quality recycled hot mix asphalt, the need for systematic quality control of the RAP is essential. The process should be monitored for processed RAP moisture content, gradation, and asphalt cement content. (17) Controlled plant operations have been developed to produce a consistent (homogeneous) RAP. Extraction tests to monitor the RAP gradation and asphalt cement content, and penetration and viscosity tests on the recovered asphalt cement, should be performed regularly to monitor the RAP characteristics for comparison with the job mix formula and enable appropriate adjustments to the mix. The same field testing procedures used for conventional hot mix asphalt mixes should be used for mixes containing reclaimed asphalt pavement. Mixes should be sampled in accordance with AASHTO T168, (11) and tested for specific gravity in accordance with ASTM D2726 (5) and in-place density in accordance with ASTM D2950. (6) Hot In-Place Recycling Mixing, Placing and Compacting There are three basic HIR construction processes in use: surface recycling, repaving, and remixing. All involve a specialized plant in a continuous train operation. Surface recycling involves a plant that heats the pavement surface (typically using propane radiant heaters), scarifies or loosens the pavement surface using a bank of non-rotating teeth, such as a spring loaded leveling rake/scaring teeth/ tines. A liquid rejuvenating additive is added, and then mixed, traditionally in a set of standard augers. The recycled asphalt pavement is then compacted using conventional compaction equipment. (13)The process is limited in its ability to repair severely rutted pavements, which are often overlaid with conventional hot mix asphalt. Repaving is a more sophisticated process that includes removing (by heating and scarification and/or grinding) the top 1 to 2 in of the old asphalt pavement, adding and mixing in a rejuvenating agent to improve asphalt viscosity, placing the recycled material as a leveling course using a primary screed, and simultaneously placing a thin (usually less than 1 in but up to 2 in some systems) hot mix asphalt overlay. Conventional equipment and procedures are used immediately behind the train to compact both layers of material to ensure a monolithic bond between the new and recycled layer. (35) The remixing process is used when additional aggregates are required to improve the strength or stability of the recycled asphalt concrete. Scarified or milled RAP is blended with rejuvenator and new virgin aggregate or new hot mix asphalt, then placed by a compacting screed. Conventional equipment and procedures are used to place and compact the remixed material. Quality Control The initial step in the quality control of hot in-place recycled mixes is in the selection of the pavement to be recycled. Not all pavements are good candidates for this type of recycling. Cores of the pavement being considered for HIPR must be taken during the early planning for the project. The cores should first be visually examined for pavement problems such as delaminations, stripping, or stripping potential, or water in the voids or delaminations. Pavements with delaminations, especially saturated delaminations, in the top 5 cm (2 in) should not be considered for HIPR projects. Also, pavements that have been rutted, heavily patched, or chip-sealed are not good candidates for HIPR projects. Next, as noted in the Mix Design section, field core specimens should be analyzed in the laboratory to determine (based on the asphalt content, viscosity, and penetration of the recovered binder) the required amount of rejuvenating agent to be added to the mix in order to attain the desired viscosity of the recycled mix. If too much rejuvenating agent (1.0 percent or more by weight of mix) must be added in order to attain this viscosity, the mix should probably not be recycled in place. As a guideline, pavements being considered for HIPR should not be too severely aged. It is recommended that such pavements have an absolute viscosity lower that 200,000 poises (and preferably below 100,000 poises) in order to be considered for HIPR projects. (38) Field core specimens should also be evaluated for air voids content during the pavement selection process. An existing pavement being considered for HIPR should have an air voids content in excess of 6 percent, in order to accommodate the addition of a rejuvenating agent without the loss of stability in the recycled mix. If material properties are not completely satisfactory for 100 percent recycling, the addition of 20 to 30 percent by weight of virgin hot mix during recycling should be considered. (38) Field quality control measures during HIPR operations include monitoring the depth of scarification, the temperature of the recycled mix, the visual appearance and homogeneity of the scarified or milled RAP, the compaction procedure, and the visual appearance of the recycled pavement surface after compaction. Loose samples of the recycled mix should be obtained and extraction tests performed to monitor RAP gradation, asphalt cement and air voids contents, and penetration and viscosity of the recovered asphalt binder for comparison with the job mix formula. (27) The recycled mix should be monitored for in-place density in accordance with ASTM D2950. (6)

ENVIRONMENTAL CONSIDERATIONS

Asphalt pavement consists of aggregate and petroleum derived asphalt binder containing volatile and semi-volatile constituents (e.g., polycyclic aromatic hydrocarbons (PAHs)) Additionally the asphalt pavement roadway may contain surface treatments, rubberized materials or contaminants from vehicle or other emissions (e.g., historically lead). The environmental issues are different for RAP based upon various beneficial uses. For bound applications such as hot-in-place recycling, research into the difference in emissions to that of virgin materials have not been conducted. Leachability would be a concern if RAP was stockpiled or stored and exposed to precipitation. Testing has been conducted to determine the leaching characteristics of RAP in Florida. In all batch tests measurements of VOCs, PAHs, and heavy metals (Ba, Ca, Cr, Cu, Pb, Ni and Zn) were below the detection level and below the applicable state regulatory groundwater guidance concentrations. This indicates that all RAP samples tested pose minimal risk under current waste policy in Florida. Lysimeter (column leaching) tests were also performed and columns were exposed to synthetic precipitation for 42 days. The VOC, PAH and heavy metal (Ba, Ca, Cr, Cu, Ni and Zn) measurements were all below detection limits, except for lead which exhibited a concentration of 24 µg/L and 23 µg/L on days 12 and 14 of the experiment, but then was otherwise below the applicable Florida Groundwater Guidance Concentration (15 µg/L) for the duration of the experiment. (42) Other batch and column leaching tests were completed on RAP which also found constituents leached were low and generally below European drinking water standards (The Drinking Water Directive (DWD), Council Directive 98/83/EC). (43) Additionally, the University of Minnesota completed a review of current literature on PAHs in asphalt pavement concluding that PAH concentrations depend on the type of pavement (coal-tar versus petroleum based). Petroleum based asphalt pavement contained PAHs at concentrations below Minnesota Pollution Control Agency human health risk clean-up levels (44). The only exceedance was when when PAHs were converted to benzo(a)pyrene equivalents, they could exceed the lowest limit (Tier I). The report further concluded that when RAP is used as subbase aggregate, it is mixed with soils or other aggregates that do not contain PAH, so this mixture would not likely exceed applicable limits. (44)

UNRESOLVED ISSUES

While the asphalt pavement recycling technologies are well established, there is still need for additional performance information, particularly with regard to the maximum percentages of RAP that can be incorporated in relation to creep (rutting resistance), fatigue endurance and durability, and the use of reclaimed asphalt pavement in premium surface course mixes and Superpave. There is also a need for more correlation of field and laboratory measurements to refine guidelines for laboratory prediction of field performance (for instance, laboratory curing procedures that best simulate field conditions). Some additional issues that require resolution include:
  • further information on the variability of RAP, especially from blended stockpiles;
  • an environmental code of practice regarding gaseous emissions from hot mix plant recycling and HIPR (see above);
  • the suitability of HIPR for surface-treated and rubberized materials; and
  • evaluation methodologies for structural characterization of HIPR asphalt concrete and CIPR asphalt concrete.

REFERENCES

  1. Asphalt Institute. Asphalt Hot-Mix Recycling , Manual Series No.20, Second Edition, Lexington , Kentucky , 1986.
  2. ASTM D1073-94. "Standard Specification for Fine Aggregate for Bituminous Paving Mixtures." American Society for Testing and Materials,Annual Book of ASTM Standards , Volume 04.03, West Conshohocken , Pennsylvania .
  3. ASTM D1559-89. "Standard Test Method for Resistance to Plastic Flow of Bituminous Mixtures Using Marshall Apparatus." American Society for Testing and Materials, Annual Book of ASTM Standards , Volume 04.03, West Conshohocken , Pennsylvania .
  4. ASTM D1560-92. "Standard Test Methods for Resistance to Deformation and Cohesion of Bituminous Mixtures by Means of Hveem Apparatus." American Society for Testing and Materials, Annual Book of ASTM Standards, Volume 04.03, West Conshohocken , Pennsylvania .
  5. American Society for Testing and Materials, Standard Specification D2726-96, “Bulk Specific Gravity and Density of Non-Absorptive Compacted Bituminous Mixtures,” Annual Book of ASTM Standards , Volume 04.03, West Conshohocken , Pennsylvania , 1996.
  6. American Society for Testing and Materials, Standard Specification D2950-96, “Density of Bituminous Concrete in Place by Nuclear Methods,”Annual Book of ASTM Standards , Volume 04.03, West Conshohocken , Pennsylvania , 1996.
  7. ASTM D3515-89. “Standard Specification for Hot-Mixed, Hot-Laid Bituminous Paving Mixtures.” American Society for Testing and Materials,Annual Book of ASTM Standards , Volume 04.03, West Conshohocken , Pennsylvania
  8. ASTM D4552-92. “Standard Practice for Classifying Hot-Mix Recycling Agents.” American Society for Testing and Materials, Annual Book of ASTM Standards , Volume 04.03, West Conshohocken , Pennsylvania .
  9. ASTM D4887-93. “Standard Test Method for Preparation of Viscosity Blends for Hot-Recycled Bituminous Materials,” American Society for Testing and Materials, Annual Book of ASTM Standards , Volume 04.03, West Conshohocken , Pennsylvania .
  10. ASTM D692-94a. “Standard Specification for Coarse Aggregate for Bituminous Paving Mixtures.” American Society for Testing and Materials,Annual Book of ASTM Standards, Volume 04.03, West Conshohocken , Pennsylvania .
  11. American Association of State Highway and Transportation Officials, Standard Method of Test, “Sampling Bituminous Paving Mixtures,” AASHTO Designation: T168-82, Part II Tests, 16th Edition, 1993.
  12. Banasiak, David. “States Plane Off Excess in RAP Specs.” Roads and Bridges , Vol. 34, No. 10, October, 1996.
  13. Basic Asphalt Recycling Manual, Asphalt Recycling and Reclaiming Association, PUB:NHI 01-022, Annapolis , Maryland , 2001.
  14. Button, J.W., D.N. Little, and C.K. Estakhri. Hot In-Place Recycling of Aspha]t Concrete , National Cooperative Research Program Synthesis of Highway Practice 193, Transportation Research Board, Washington , DC , 1994.
  15. Decker, D. S. and T. J. Young, “Handling RAP in an HMA Facility.” Proceedings of the Canadian Technical Asphalt Association , Edmonton , Alberta , 1996.
  16. Designing HMA Mixtures with High RAP Content, A Practical Guide, National Pavement Association, Quality Improvement Series 124, Lanham , Maryland , 2007.
  17. Earl, F. J. and J. J. Emery, “Practical Experience With High Ratio Hot-Mix Recycling,” Proceedings of the Canadian Technical Asphalt Association 32nd Annual Conference, Toronto , Ontario , p. 326, November 1987.
  18. Eaton, M. “RAP Maintenance No Different Than Virgin, Engineers Say.” Roads and Bridges, Vol. 28, No. 10, October, 1990.
  19. Epps, J. A., D. N. Little, R. J. O'Neal, and B. M. Gallaway. “Mixture Properties of Recycled Central Plant Materials.” Recycling of Bituminous Pavements , American Society for Testing and Materials, Special Technical Publication No. 662, West Conshohocken, Pennsylvania , December, 1977.
  20. Federal Highway Administration, RAP and Superpave: An Excellent Blend, Focus, April, 2002.
  21. Harvey, Fitz, E. T. Larves, and R. G. Warburton. Hot Recycling. Wyoming Dryer Drum. Federal Highway Administration, Report No. FHWA TS-80-234, Washington , DC , April, 1980.
  22. Hossain, Mustaque, Dwight G. Metcalf, and Larry A. Scofield. Performance of Recycled Asphalt Concrete Overlays in South Western Arizona . Transportation Research Board, Record No. 1427, Washington , DC , 1993.
  23. “Hot Recycling of Yesterday.” Recycling Report , Volume 1, Number 2, National Asphalt Pavement Association, Lanham , Maryland , September, 1977.
  24. Howard, P. D. and D. A. Reed, “ L.A. Street Maintenance Recycles with 100% RAP,” Roads and Bridges , Vol. 28, No. 5, p. 63, May 1989.
  25. Kandhal, P., and Koo,K., Designing Recycled Hot Mix Asphalt Mixtures Using Superpave Technology, NCAT Technical Report # 96-5, National Center for Asphalt Technology, Auburn University, Alabama, 1997.
  26. Kandahl, Prithri S., Shridhar S. Rao, Donald E. Watson, and Brad Young. Performance of Recycled Hot-Mix Asphalt Mixtures in Georgia . Transportation Research Board, Record No. 1507, Washington , DC , 1996.
  27. Kazmierowski, Thomas J., Pamela Marks, and Alison Bradbury. “The Evolution of Hot In-Place Recycling in Ontario .” Presented at the 74th Annual Meeting of the Transportation Research Board, Washington , DC , January, 1995.
  28. Kiggundu, B. M. and J. K. Newman. Asphalt-Aggregate Interactions in Hot Recycling . New Mexico Engineering Research Institute, Report No. ESL-TR-87-07, Albuquerque , New Mexico , July, 1987.
  29. Little, Dallas N. and Jon A. Epps. “Evaluation of Certain Structural Characteristics of Recycled Pavement Material.” Proceedings of the Association of Asphalt Paving Technologists , Vol. 49, 1980, pp. 219-251.
  30. Little, D. N., R. J. Holmgreen, and J. A. Epps. “Effect of Recycling Agents on the Structural Performance of Recycled Asphalt Concrete Materials.” Proceedings of the Association of Asphalt Paving Technologists , Vol. 50, 1981, pp. 32-63.
  31. McDaniel, R. and Anderson, R.M Recommended Use of Reclaimed Apshalt Pavement in the Superpave Mix Design Method: Technicians Manual. NCHRP Report 452. Transportation Research Record, Washington , DC , 2001.
  32. Mix Design Methods for Asphalt Concrete and Other Hot-Mix Types . Asphalt Institute, Manual Series No. 2, Lexington , Kentucky , 1993.
  33. Olson, Roger C. and Ronald H. Cassellius. Hot Recycling of Bituminous Main Line and Shoulders. Minnesota-Modified Dryer Drum. Federal Highway Administration, Report No. FHWA-TS-80-233, Washington , DC , February, 1979.
  34. Pavement Recycling Executive Summary and Report . Federal Highway Administration, Report No. FHWA-SA-95-060, Washington , DC , October, 1995.
  35. Rathburn, J.R., “One Step Repaving Speeds County Work ,” Roads and Bridges, March 1990.
  36. Recycling Hot-Mix Asphalt Pavements , National Pavement Association, Information Series 123, Lanham , Maryland , 2007.
  37. Recycling Practices for HMA , Special Report, 187, National Pavement Association, Lanham , Maryland , 2000.
  38. Rogge, David F., Walter P. Hislop, and Dick Dominic. “Hot In-Place Recycling of Asphalt Pavements – The Oregon Experience.” Presented at the 75th Annual Meeting of the Transportation Research Board, Washington , DC , January, 1996.
  39. Stevens, J. E., Mahoney, J., and Dippold, C., Determination of the PG Binder Grade to Use In a RAP Mix, Final Report, JHR00-278 Project 99-1, April, 2001.
  40. Take the RAP: It Saves Money and the Environment , National Pavement Association, Lanham , Maryland , 1998.
  41. Transportation Research Board. Recycling Materials from Highways . National Cooperative Highway Research Program Synthesis of Highway Practice No. 54, Washington , DC , 1978.
  42. Brantley, A.S. and Townsend, T., Leaching of pollutants from reclaimed asphalt pavement, Environmental Engineering Science, Vol. 16, no. 2, pp. 105-116. Apr. 1999.
  43. Legret, M., Odie, L., Demare, D., Jullien, A., Leaching of heavy metals and plycyclic aromatic hydrocarbons from reclaimed asphalt pavement, Water Research, Vol. 39, 3675-3685, 2005.
  44. Grosenheider, K., Bloom, P., Halbach T., Johnson, M., A Review of the Current Literature Regarding Polycyclic Aromatic Hydrocarbons in Asphalt Pavement, Mn/DOT contract No. 81655, October, 2005.

Reclaimed Asphalt Pavement - Asphalt Concrete (Cold Recycling)

INTRODUCTION

Cold recycling (CR) is a way to recycled RAP without using heat during the recycling process. Reclaimed asphalt pavement (RAP) can be used as an aggregate in the cold recycling of asphalt paving mixtures in one of two ways. The first method (cold mix plant recycling) involves a process in which RAP is combined with new emulsified or foamed asphalt and a recycling or rejuvenating agent, possibly also with virgin aggregate, and mixed at a central plant or a mobile plant to produce cold mix base mixtures. (3) The second, more common, method involves a process in which the asphalt pavement is recycled in-place (cold central plant recycling (CCPR) process), where the RAP is combined without heat and with new emulsified or foamed asphalt and/or a recycling or rejuvenating agent, possibly also with virgin aggregate, and mixed at the pavement site, at either partial depth or full depth, to produce a new cold mix end product. (16) Most states have used cold in-place recycling in conjunction with a hot mix overlay or chip seal.

PERFORMANCE RECORD

Documented performance of cold plant mix recycling projects is not widely available. According to a 1994 survey of all state transportation agencies, at least 32 states have used or are using RAP in cold recycling of asphalt pavements. (13) Although cold recycling has been reportedly practiced in these states, data are unavailable to differentiate whether cold plant mix recycling, CCPR, or both, are being used. In all likelihood, CCPR is probably being utilized more frequently, especially on low-volume roads where transport costs to plant sites are likely to be higher. The states that appear to have had the most experience with CCPR techniques include California , Indiana , Kansas , New Mexico , Oregon , and Pennsylvania . The performance of CCPR projects in Indiana has been described as structurally comparable to those of cold mixes in which conventional aggregates and asphalt emulsions have been used. (25) Over 800 lane-km (500 lane-miles) of roadways in New Mexico have been successfully recycled using CCPR, and the extensive recycling experiences in California and Pennsylvania have also been very promising. (27) There have been approximately 672 km (420 mi) of low-volume roads in Oregon that were cold in-place recycled between 1984 and 1989, and over 75 percent of these projects were rated fair or better. (24) The performance of eight CIPR projects located throughout Pennsylvania were considered good to satisfactory, as long as a double seal coat was placed over the recycled cold mix. (19) CIR has been successfully used in California and New Mexico projects since for over 20yrs. (26) the interactive map below shows the use of CIR by states that responded to a survey distributed by The University of Rhode Island for the report "Development of Performace Based Mix Design for Cold In-Place Recycling (CIR) of Bituminous Pavements Based on Fundamental Properties":
Table 1. CIR Use for State Maintained Highways (1998) (26)
Various methods for CIR have been developed by the following: 1. Oregon, 2. California, 3. The Asphalt Institute, 4. US Army Corps of Engineers, 5. Kansas, 6. Pennsylvania, and New Mexico. (16) Performance studies indicate that CIR retards or eliminates the occurrence of reflective cracking from environmental distress, depending on the depth of treatment and crack depth. (5) Improper emulsion application can result in high residual asphalt content (leading to flushing) and excessive processing can result in high fines content (leading to rutting due to low stability). CR is best suited for the following: 1. raveling, 2. bleeding, 3. shoulder drop off, 4. rutting, 5.corrugations, 6.shoving, 7.removal of deteriorated, stripped or aged asphalt, 8. poor ride quality caused by swells, bumps, sags and depressions, 9. potential bonding problems between existing pavement and new HMA overlay, and finally 10. diminished curb reveal heights. (11) Advantages to Cold Recycling include: (11)
  • Conservation of non-renewable resources
  • Disposal problems are eliminated
  • Surface irregularities and cracks can be interrupted and filled
  • Rutting, potholes and raveling are eliminated
  • Base and subgrade materials are not disturbed
  • Pavement cross-slop and profile can be improved
  • Problems with existing aggregate gradation and/or asphalt binder can be corrected
  • Ride quality can be improved
  • Economic savings

MATERIAL PROCESSING REQUIREMENTS

Cold Central Plant Recycling Processing requirements for CCPR are similar to those for recycled hot mix. Recycled asphalt pavement must be processed into a granular material prior to use in cold mix applications. A typical RAP plant consists of a crusher, screening units, conveyors, and stackers. CCPR mix can be used immediately or can be stockpiled for later use. It is typically used in maintenance applications such as blade patching or pothole repair. (11) Cold In-Place Recycling CIPR (like hot in-place recycling (HIPR)), requires a self-contained, continuous train operation that includes ripping or scarifying, processing (screening and sizing/crushing unit), mixing of the milled RAP, and the addition of liquid rejuvenators. Special asphalt-derived products such as cationic, anionic, and polymer modified emulsions, rejuvenators and recycling agents have been developed especially for CIPR processes. These hydrocarbon materials are sometimes, but not always, used to soften or lower the viscosity of the residual asphalt binder in the RAP material so that it is compatible with the newly added binder.

ENGINEERING PROPERTIES

Some of the engineering properties of RAP that are of particular interest when RAP is used in cold recycled applications include its gradation, asphalt content, and the penetration and viscosity of the asphalt binder. Gradation : The aggregate gradation of processed RAP is somewhat finer than virgin aggregate. This is due to mechanical degradation during asphalt pavement removal and processing. RAP aggregates usually can satisfy the requirements of ASTM D692 for coarse aggregate and ASTM D1073 for fine aggregate. (6,7) Asphalt Content : The asphalt content of most old pavements will comprise approximately 3 to 7 percent by weight and 10 to 20 percent by volume of the pavement. Due to oxidation aging, the asphalt cement has hardened and consequently is more viscous and has lower penetration values than the virgin asphalt cement. Penetration and Viscosity : Depending on the amount of time the original pavement had been in service, recovered RAP binder may have penetration values from 10 to 80 and absolute viscosity values at 60°C (140°F) in a range from as low as 2,000 poises to as high 50,000 poises or greater. (18)

DESIGN CONSIDERATIONS

To satisfy the engineering requirements for use in cold recycled asphalt concrete pavements, it is usually necessary to rejuvenate or augment the asphalt binder in RAP to lower the viscosity and/or increase penetration. This is done by the addition of one or more recycling agents, consisting of either an emulsified or foamed asphalt and/or a rejuvenating agent. Some additional aggregate may also be added to adjust the mix gradation or air voids content. Cold Plant Mix Recycling Mix Design The specifications and design of cold plant mix recycling of asphalt pavements are referred to in ASTM D4215. (10) Cold plant mixtures can be dense-graded or open-graded. Cold-laid asphalt mixes may be used for surface, base, or subbase courses. Although there are no universally accepted mix design methods for cold mix recycling, the Asphalt Institute recommends and most agencies use a variation of the Marshall mix design method. (8) General procedures include a determination of the aggregate gradation and asphalt content of the processed RAP, determination of the percentage (if any) of new aggregate to be added, calculation of combined aggregate in recycled mix, selection of the type and grade of new asphalt, determination of the asphalt demand of the combined aggregate, estimation of the percent of new asphalt required in the mix, and adjustment of asphalt content by field mix trials. (23) The percent asphalt demand of combined aggregates can be determined by means of a formula that takes into account the various sieve size fractions of the combined RAP and virgin aggregate. These size fractions include the percentage retained on the 2.36 mm (No. 8) sieve, the percentage between the 2.36 mm (No. 8) and 0.075 mm (No. 200) sieves, and the percentage passing the 0.075 mm (No. 200) sieve. The percent of new asphalt is the difference between the percent asphalt demand and the percent of asphalt contained in the RAP. (3) Using the determined asphalt content, Marshall specimens can be prepared at various emulsion percentages to determine an optimum asphalt content on the basis of applicable stability and air voids criteria. Structural Design The AASHTO Design Guide (1) is applicable to recycled cold mix paving mixtures. While there are no universally accepted structural layer coefficient values for asphalt cold mix, it is generally recognized that cold mix asphalt is not the structural equivalent of hot mix asphalt, but is superior to gravel or crushed stone base courses. Asphalt cold mix is generally not recommended for use as a wearing surface, but only in base course layers because of both structural and durability considerations. The structural capacity of recycled cold mix can be considered equal to that of conventional cold mix paving materials. (22) Although most agencies have not published structural layer coefficient values for conventional or recycled cold mixes, a layer coefficient value of 0.25 to 0.35 for an asphalt stabilized base is considered within a reasonable range. Pennsylvania DOT has assigned a structural layer coefficient of 0.30 for a bituminous-aggregate stabilized base, (19) which is a conventional cold mix. Cold In-Place Recycling Mix Design In the past, The Asphalt Institute recommended a modified Marshall mix type procedure for the design of CIR mixes. (8) Such a design initially involves obtaining samples of the candidate pavement to determine the gradation of the aggregate, the asphalt content, and the penetration and viscosity of the asphalt binder. Marshall specimens are prepared at various emulsion percentages, as initially determined by calculating the asphalt demand on the basis of aggregate gradation and deducting the percentage of asphalt in the RAP. (22) The optimum asphalt content can be determined by a stability and air voids analysis, with target air voids in the 8 to 10 percent range, or the specimens may be evaluated using indirect tensile strength or resilient modulus testing. (20) This method, however upon evaluation has proven to be ill-suited for CIR mix design. Resent research has suggested a mix design using a volumetric mix design using the Superpave Gyratory Compactor (SGC) for CIR materials. This mix design was developed primarily for partial depth CIR using emulsion as the recycling additive. (26) It has recently been shown that the addition of virgin aggregates (20 to 25 percent) in the CIR process results in less voids and, consequently, less flushing, and improved stability. (23) The amount of recycling agent (either new asphalt or modifying oil) also has a significant effect on the behavior of the mix, with the ideal range of recycling agent being somewhere between 2 and 3 percent by weight of dry RAP. (12) Structural Design CIR is generally considered for rehabilitation of pavements showing distress to depths about 4 to 6 in. It can handle a pavement section in poorer condition and with more cracking than HIPR, provided that the pavement section (when recycled) is structurally sound and adequately drained. The AASHTO Design Guide (1) is recommended for the thickness design of cold in-place recycled asphalt mixes. Since there is essentially little or no difference in the composition and structural properties of recycled cold mix and cold in-place recycled paving materials, the range of structural layer coefficients recommended for recycled cold mixes (0.25 to 0.35) are also applicable for cold in-place recycled mixes. CIR mixes are not recommended for use as a wearing surface.

CONSTRUCTION PROCEDURES

Cold Plant Mix Recycling Material Handling and Storage RAP is produced by milling, ripping, breaking, crushing, or pulverizing types of equipment. To ensure that the final RAP product will perform as intended, inspection of incoming RAP and rejection of contaminated loads (excess granular material, surface treatment, joint sealant, etc.) should be undertaken. Some jurisdictions also require that RAP from a particular project not be blended or commingled with RAP from other projects. Once processed, RAP can be handled and stored as a conventional aggregate material. However, because of the variability of RAP in comparison with virgin aggregates, many agencies do not permit the blending of RAP from different projects into combined stockpiles. The Asphalt Institute recommends that the height of RAP stockpiles be limited to a maximum of10 ft to help prevent agglomeration of the RAP particles. (4) Experience has proven that conical stockpiles are preferred to horizontal stockpiles and will not cause RAP to re-agglomerate or congeal in large piles. RAP has the tendency to form a crust (due to a solar/thermal effect from the sun) over the first 8 to 12 in of pile depth for both conical and horizontal stockpiles. This crust tends to help shed water, but is easily broken by a front-end loader and may help keep the rest of the pile from agglomerating. RAP has a tendency to hold water and not to drain over time like an aggregate stockpile. Therefore, low, horizontal, flat stockpiles are subject to greater moisture accumulation than tall, conical stockpiles. It is not unusual to find RAP moisture content in the 7 to 8 percent range during the rainy season at facilities using low, horizontal stockpiling techniques. (14,15) RAP stockpiles are typically left uncovered because covering with tarps can cause condensation under the tarp and add moisture to the RAP stockpile. For this reason, RAP stockpiles are either left uncovered or RAP is stored in an open-sided building, but under a roof. (14,15) Some producers are beginning to pave underneath the RAP stockpiles. This practice may help to improve stockpile drainage (when graded properly) and it reduces potential moisture absorption from the ground. Possible contamination is also eliminated when a front-end loader collects materials from close to grade level. (15) When large quantities of RAP from different sources are available, it is advisable to keep stockpiles separated and identified by source. Consistent RAP from a "composite" or "blended" pile can be produced using a crushing and screening operation and reprocessing stockpiles from different sources. Material handling machinery, such as front-end loaders and bulldozers, should be kept from driving directly on the stockpile. Agglomeration can result, making it very difficult for the loader to handle the RAP. Mixing, Placing, and Compacting The RAP processing requirements for cold mix recycling are similar to those for recycled hot mix, except that the graded RAP product is incorporated into cold mix asphalt paving mixtures as an aggregate substitute. RAP is mixed with new aggregate and emulsified or foamed asphalt in either a central plant or a mobile plant. The blend is then placed as conventional cold mix asphalt. The pavement removal or milling is performed with a self-propelled rotary drum cold planing machine with RAP transferred to trucks for removal from the job site. Cold mix asphalt is usually placed on low-volume roadways with traffic volumes less than 3,000 vehicles per day and covered with either a double surface treatment or a hot mix wearing surface. (27) Cold plant mix recycling can be accomplished either by hauling the RAP to a central processing location, where it is crushed, screened, and blended with a recycling agent in a central mixing plant, or the RAP can be processed at the project site and prepared in a mobile mixing plant that has been transported to the job site. In either case, a pugmill mixing plant is commonly used. (17) Recycled cold mix material can be normally placed with a conventional paver, provided the mixing moisture can be adequately controlled to a level not requiring aeration. Cold mix pavement construction requires several warm days and nights for adequate curing. (24) Successful placement using conventional pavers requires that the mix be sufficiently fluid to avoid tearing. Alternatively, a Jersey or towed spreader can be used. Using a Jersey or towed spreader (which is essentially a front-wheeled hopper fastened to the front of tractor or the rear of a haul truck), the cold mix is dumped into a hopper and falls directly on the road where it is spread and struck off to the required thickness. The same equipment and techniques used to compact and cure conventional cold mix asphalt pavements are applicable to recycled cold mix. Quality Control To ensure the consistency and quality of a recycled cold plant mix, quality control of the RAP is essential. Random samples of the RAP or recycled material should be analyzed for aggregate gradation, asphalt cement content, and moisture content. The recycled material must be closely inspected to make sure that the RAP is consistent in size and appearance and that subgrade soil (or other possible contaminants) have not been included in the RAP. Plant operations should be monitored to ensure that the proper amount of emulsified or foamed asphalt is being added and that the moisture content of the recycled mix is in the proper range for maximum compaction at the project site. The amount of any additional aggregate being mixed with the RAP should also be monitored. Loose samples of the recycled mix should be obtained and extraction tests performed to monitor mix gradation and asphalt content, as well as moisture content. Mixes should be sampled in accordance with AASHTO T168. (2) Achieving the proper compaction or densification of the paving material is essential to proper performance. A test strip should be used at the start of the project to establish a target density and number of roller passes needed to achieve that density. The in-place density of the cold mix paving material can then be monitored by using a nuclear density gauge in accordance with ASTM D2950. (9) Cold In-Place Recycling Mixing, Placing and Compacting A typical CIR train consists of a cold milling machine (with water added as necessary for cooling and dust control) that is capable of reclaiming the old asphalt pavement to depths from about 4 in to 6 in. CIR plants consist of a screening and sizing or crushing unit, as well as a mixing unit for the addition of polymer-modified high-float emulsion, as determined by the mix design, and also water, if required. Mixing may be accomplished using a motor grader blade, a rotary pulvimixer, a windrow type mixer, or a traveling plant pugmill, which offers the highest degree of grading control. (17) A reclaim/paver unit is also part of the system to place the recycled cold mix. The mixing and placement units are combined in some trains in what are referred to as mixer-pavers. Care must be taken during the CIR operation to avoid the incorporation of the granular base material into the mixer. The CIR mix is usually compacted until the mixture begins to "break" (the mixture turns from brown to black) with a large sized pneumatic-tired roller or vibrating steel drum rollers. When asphalt emulsions or emulsified recycling additives are used this could take from 30 minutes to 2 hours. (11) Curing The compacted mix must then cure before a wearing surface can be placed. The rate of curing is quite variable and depends on a various factors. These factors include environmental conditions, drainage and moisture characteristics of the mix. Typical curing times can vary from days to 2 weeks, depending on the factors mentioned above, recycling additives and modifiers that may have been used. (11) Quality Control As with HIR, the crucial step in the quality control of CIR mixes is in the initial process of project selection. If an existing pavement exhibits distress resulting from a subgrade or base failure, it cannot be remedied simply by recycling the surface layer. Pavements that have been rutted, heavily patched, or chip-sealed are not good candidates for CIR projects. Also, core specimens of the pavement being considered for CIR should be taken and examined for variations in pavement layers, delaminations, and saturated material adjacent to voids or delaminations. To ensure the success of a CIR mix, quality control of the RAP is essential. Random samples of the RAP or recycled material should be analyzed for aggregate gradation, asphalt content, and moisture content. The recycled material must be closely inspected to make sure that the RAP is consistent in size and appearance and that subgrade soil (or other possible contaminants) have not been included in the RAP. Field quality control measures during CIPR operations include monitoring the depth of scarification, the coating of the aggregate by the emulsion, the proper curing of the emulsion, the visual appearance and possible segregation of the recycled material, the compaction procedure, and appearance of the recycled pavement surface after compaction. Loose samples of the recycled mix should be obtained and extraction tests performed to monitor mix gradation and emulsion content, as well as moisture content. The moisture content of recycled pavement should be less than 1 percent of the existing pavement prior to recycling. (21) Achieving the proper compaction or densification of the recycled paving material is essential to proper performance. The in-place density of the recycled mix should be monitored by using a nuclear density gauge in accordance with ASTM D2950. (9)

ENVIRONMENTAL CONSIDERATIONS

Asphalt pavement consists of aggregate and petroleum derived asphalt binder containing volatile and semi-volatile constituents (e.g., polycyclic aromatic hydrocarbons (PAHs)) Additionally the asphalt pavement roadway may contain surface treatments, rubberized materials or contaminants from vehicle or other emissions (e.g., historically lead). The environmental issues are different for RAP based upon various beneficial uses. For bound applications such as cold-in-place recycling, research into the difference in emissions to that of virgin materials have not been conducted. Leachability would be a concern if RAP was stockpiled or stored and exposed to precipitation. Testing has been conducted to determine the leaching characteristics of RAP in Florida. In all batch tests measurements of VOCs, PAHs, and heavy metals (Ba, Ca, Cr, Cu, Pb, Ni and Zn) were below the detection level and below the applicable state regulatory groundwater guidance concentrations. This indicates that all RAP samples tested pose minimal risk under current waste policy in Florida. Lysimeter (column leaching) tests were also performed and columns were exposed to synthetic precipitation for 42 days. The VOC, PAH and heavy metal (Ba, Ca, Cr, Cu, Ni and Zn) measurements were all below detection limits, except for lead which exhibited a concentration of 24 µg/L and 23 µg/L on days 12 and 14 of the experiment, but then was otherwise below the applicable Florida Groundwater Guidance Concentration (15 µg/L) for the duration of the experiment. (28) Other batch and column leaching tests were completed on RAP which also found constituents leached were low and generally below European drinking water standards (The Drinking Water Directive (DWD), Council Directive 98/83/EC). (29) Additionally, the University of Minnesota completed a review of current literature on PAHs in asphalt pavement concluding that PAH concentrations depend on the type of pavement (coal-tar versus petroleum based). Petroleum based asphalt pavement contained PAHs at concentrations below Minnesota Pollution Control Agency human health risk clean-up levels (30). The only exceedance was when when PAHs were converted to benzo(a)pyrene equivalents, they could exceed the lowest limit (Tier I). The report further concluded that when RAP is used as subbase aggregate, it is mixed with soils or other aggregates that do not contain PAH, so this mixture would not likely exceed applicable limits. (30)

UNRESOLVED ISSUES

While cold asphalt pavement recycling technologies are well established, there is still a need for additional performance information, particularly with regard to the percentage of RAP that can be incorporated and the relationship to creep (rutting resistance), fatigue endurance, and durability. In addition, there is a need to assess whether RAP can be used in wearing surface cold mixes. Further investigation is also needed to evaluate the ability of cold recycled plant mixes to perform on higher traffic volume roadways. There is also a need for more correlation of field and laboratory measurements to refine guidelines for laboratory prediction of field performance, including, for instance, laboratory curing procedures that best simulate field conditions. Some specific issues that require resolution include:
  • further information on the variability of RAP, especially from blended stockpiles;
  • a consensus regarding mix design and testing procedures for plant recycled cold mix and CIR asphalt mixtures;
  • the suitability of CIR for use with surface treatments and/or rubberized paving materials;
  • a more accurate determination of the structural layer coefficient for plant recycled cold mix and CIR asphalt mixtures; and
  • an environmental evaluation of any potentially harmful impacts from cold mix plant recycling and/or cold in-place recycling.

REFERENCES

  1. AASHTO Guide for Design of Pavement Structures. American Association of State Highway and Transportation Officials, Washington , DC , 1993.
  2. American Association of State Highway and Transportation Officials. Standard Method of Test, "Sampling Bituminous Paving Mixtures," AASHTO Designation T168-82, Part II Tests, 16th Edition, 1993.
  3. Asphalt Institute. Asphalt Cold-Mix Recycling , Manual Series No. 21, Lexington , Kentucky , March, 1983.
  4. Asphalt Hot-Mix Recycling . Asphalt Institute. Manual Series No. 20, Second Edition, Lexington , Kentucky , 1986.
  5. "A Study of the Use of Recycled Paving Materials - Report to Congress," Federal Highway Administration and Environmental Protection Agency, Report No. FHWA-RD-93-147, EPA/600/R-93/095, Washington , DC , June, 1993.
  6. ASTM D692-94a. "Standard Specification for Coarse Aggregate for Bituminous Paving Mixtures." American Society for Testing and Materials,Annual Book of ASTM Standards , Volume 04.03, West Conshohocken , Pennsylvania .
  7. ASTM D1073-94. "Standard Specification for Fine Aggregate for Bituminous Paving Mixtures." American Society for Testing and Materials,Annual Book of ASTM Standards , Volume 04.03, West Conshohocken , Pennsylvania .
  8. ASTM D1559-89. "Standard Test Method for Resistance to Plastic Flow of Bituminous Mixtures Using Marshall Apparatus." American Society for Testing and Materials, Annual Book of ASTM Standards , Volume 04.03, West Conshohocken , Pennsylvania .
  9. ASTM D2950-96, "Standard Specification for Density of Bituminous Concrete in Place by Nuclear Methods." American Society for Testing and Materials, Annual Book of ASTM Standards , Volume 04.03, West Conshohocken , Pennsylvania .
  10. ASTM D4215. "Standard Specification for Cold-Mixed, Cold-Laid Bituminous Paving Mixtures." American Society for Testing and Materials,Annual Book of ASTM Standards , Volume 04.03, West Conshohocken , Pennsylvania .
  11. Basic Asphalt Recycling Manual, Asphalt Recycling and Reclaiming Association, PUB:NHI 01-022, Annapolis , Maryland , 2001.
  12. Castedo, Humberto. "Significance of Various Factors in the Recycling of Asphalt Pavements on Secondary Roads." Transportation Research Record No.1115 , Washington , DC , 1987.
  13. Collins, Robert J. and Stanley K. Ciesielski. Recycling and Use of Waste Materials and By-Products in Highway Construction . National Cooperative Highway Research Program, Synthesis of Highway Practice No. 199, Transportation Research Board, Washington , DC , 1994.
  14. Decker, D. S. and T. J. Young, "Handling RAP in an HMA Facility"AT Proceedings of the Canadian Technical Asphalt Association , Edmonton , Alberta , 1996.
  15. Designing HMA Mixtures with High RAP Content, A Practical Guide, National Pavement Association, Quality Improvement Series 124, Lanham , Maryland , 2007.
  16. Epps, Jon A. Cold-Recycled Bituminous Concrete Using Bituminous Materials . National Cooperative Highway Research Program, Synthesis of Highway Practice 160, July, 1990.
  17. Epps, J. A., D. N. Little, R. J. Holmgreen, and R. L. Terrel. Guidelines for Recycling Pavement Materials . National Cooperative Highway Research Program Report No. 224, Washington , DC , September, 1980.
  18. Epps, J. A., D. N. Little, R. J. O'Neal, and B. M. Gallaway. Mixture Properties of Recycled Central Plant Materials . American Society for Testing and Materials, Special Technical Publication No. 662, Recycling of Bituminous Pavements, West Conshohocken, Pennsylvania , December, 1977.
  19. Kandahl, Prithvi S. and William C. Koehler. "Cold Recycling of Asphalt-Pavements on Low-Volume Roads." Transportation Research Record No. 1106 , Washington , DC , 1987.
  20. Kennedy, T. W. and Ignacio Perez, "Preliminary Mixture Design Procedure for Recycled Asphalt Materials." Recycling of Bituminous Pavements , American Society for Testing and Materials Special Technical Publication No. 662, West Conshohocken , Pennsylvania , December, 1977.
  21. McKeen, R.G., D.I. Hanson, and J.H. Stokes. " New Mexico 's Experience with Cold In-Situ Recycling." Presented at the 76th Annual Meeting of the Transportation Research Board, Washington , DC , January, 1997.
  22. Mix Design Methods for Asphalt Concrete and Other Hot-Mix Types . Asphalt Institute. Manual Series No. 2, Lexington , Kentucky , 1993.
  23. Murphy, D. T. and J. J. Emery. "Modified Cold In-Place Asphalt Recycling." Presented at the 1995 Annual Conference of the Transportation Association of Canada , Victoria , British Columbia .
  24. Scholz, Todd V., R. Gary Hicks, David F. Rogge, and Dale Allen. "Use of Cold In-Place Recycling on Low-Volume Roads." Transportation Research Record No. 1291 , Washington , DC , 1991.
  25. Tia, Mang and Leonard E. Wood. "Use of Asphalt Emulsion and Foamed Asphalt in Cold-Recycled Asphalt Paving Mixtures." Transportation Research Record No. 898 , Washington , DC , 1983.
  26. Wayne Lee, K., Brayton, T.E., Huston, M. Development of Performance Based Mix-Design for Cold In-Place (CIR) of Bituminous Pavements Based on Fundamental Properties, FHWA-CIR-02-01, Federal Highway Administration, September, 2002.
  27. Wood, Leonard E., Thomas D. White, and Thomas B. Nelson. "Current Practice of Cold In-Place Recycling of Asphalt Pavements."Transportation Research Record No. 1178 , Washington , DC , 1988.
  28. Brantley, A.S. and Townsend, T., Leaching of pollutants from reclaimed asphalt pavement, Environmental Engineering Science, Vol. 16, no. 2, pp. 105-116. Apr. 1999.
  29. Legret, M., Odie, L., Demare, D., Jullien, A., Leaching of heavy metals and plycyclic aromatic hydrocarbons from reclaimed asphalt pavement, Water Research, Vol. 39, 3675-3685, 2005.
  30. Grosenheider, K., Bloom, P., Halbach T., Johnson, M., A Review of the Current Literature Regarding Polycyclic Aromatic Hydrocarbons in Asphalt Pavement, Mn/DOT contract No. 81655, October, 2005.

Reclaimed Asphalt Pavement - Granular Base

INTRODUCTION

Reclaimed asphalt pavement (RAP) can be used as granular base or subbase material in virtually all pavement types, including paved and unpaved roadways, parking areas, bicycle paths, gravel road rehabilitation, shoulders, residential driveways, trench backfill, engineered fill, pipe bedding, and culvert backfill. (13,17) Although the use of RAP in granular base applications does not recover the asphalt cement potential in the old pavement, it does provide an alternate application where no other markets (asphalt paving) are available or where unsuitable material (such as soil or mud) may have been combined with the RAP so that it cannot be used as part of a recycled pavement.

PERFORMANCE RECORD

RAP that has been properly processed and in most cases blended with conventional aggregates has demonstrated satisfactory performance as granular road base for more than 20 years and is now considered standard practice in many areas. At least 13 state agencies ( Arizona , Illinois , Louisiana , Maine , Nebraska , New Hampshire , North Dakota , Oregon , Rhode Island , South Dakota , Texas , Virginia , and Wisconsin ) have used RAP as aggregate in base course. At least four state agencies (Alaska, New York, Ohio, and Utah) have used RAP as unbound aggregate in subbase, and at least two states (California and Vermont) have experience with RAP use in stabilized base course. (9) In addition to the states listed above, it has also been reported that RAP has been used as a base course additive in Idaho and New Mexico, and as a subbase additive in at least 10 other states, including Connecticut, Georgia, Iowa, Kansas, Massachusetts, Minnesota, Montana, Oklahoma, Tennessee, and Wyoming. (5) It has further been reported that Kentucky has had some limited experience with the use of RAP in roadbase, although no information is available concerning its performance. (16) Overall, the performance of RAP as a granular base or subbase aggregate, or as an additive to granular base or subbase, has been described as satisfactory, good, very good, or excellent. (5.9) Some of the positive features of RAP aggregates that have been properly incorporated into granular base applications include adequate bearing capacity, good drainage characteristics, and very good durability. However, RAP that is not properly processed or blended to design specification requirements may result in poor pavement performance. Increasing the RAP content results in a decrease in the bearing capacity of the granular base. In addition, where conventional granular material has been placed over processed RAP (and not homogeneously blended), the coarse granular material (sometimes referred to as float material) tends to ravel under traffic. (18) RAP by itself is not a qualified subbase by itself, however recent research in Louisiana focused on the use of foamed RAP as a subbase material. Foamed RAP is RAP that has been pretreated with a foamed asphalt design method, designed by Wirtgen America, Inc. This process combines hot asphalt and a small amount of water in a mixing chamber to produce an asphalt foam that can be incorporated into the base material. Due to the fact this process uses less water than an emulsion-based mix, compaction can be started earlier. A number of strength tests were preformed on the section with the RAP base and an adjacent stone base. In the tests the foamed RAP equaled or exceeded those of the stone base. In order to maintain quality construction the stockpile RAP needs to be closely monitored. The materials moisture content was also monitored throughout the construction process. (11)

MATERIAL PROCESSING REQUIREMENTS

Crushing and Screening Stockpiled RAP must be processed to the desired aggregate gradation using conventional equipment consisting of a primary crusher, screening units, secondary crusher (optional), conveyors, and a stacker. Blending To avoid agglomeration of crushed RAP, it should be blended as soon as possible with conventional aggregate (using a cold feed system) to a homogeneous mixture. However, blended material that is stockpiled for a considerable period of time, particularly in warm weather, may harden and require recrushing and rescreening before it can be incorporated into granular base applications. Stockpiling Blended RAP-aggregate stockpiles should not be allowed to remain in place for extended time periods in most climates because the stockpiled material is likely to become overly wet, possibly requiring some drying prior to use. Placement by In-Place Processing In-place processing consists of self-propelled pulverizing units that break up and crush the existing asphalt concrete, (typically up to a depth of about 100 mm (4 in)) and underlying granular material to a total maximum depth of 200 mm (8 in) and thoroughly mix the materials in place. The depth of processing must be closely monitored since cutting too deep can incorporate subbase material while cutting too shallow increases the percentage of RAP in the blend.

ENGINEERING PROPERTIES

Some of the engineering properties of RAP that are of particular interest when RAP is used in granular base applications include gradation, bearing strength, compacted density, moisture content, permeability, and durability. Gradation : The gradation for milled RAP is governed by the spacing of the teeth and speed of the pulverizing unit. Wider tooth spacing and higher speed result in larger particle sizes and coarser gradation. RAP can be readily processed to satisfy gradation requirements for granular base and subbase specifications, such as AASHTO M147. (2) Bearing Strength : The bearing capacity of blended RAP is strongly dependent on the proportion of RAP to conventional aggregate. The bearing capacity decreases with increasing RAP content. The California Bearing Ratio (CBR) is reduced below that expected for conventional granular base when the amount of RAP exceeds 20 to 25 percent. (12) CBR values have been shown to decrease almost directly with increasing RAP contents. (18) Compacted Density : Due to the coating of asphalt cement on RAP aggregate, which inhibits compaction, the compacted density of blended granular material tends to decrease with increasing RAP content. (18) Moisture Content : The optimum moisture content for RAP blended aggregates is reported to be higher than for conventional granular material, particularly for RAP from pulverizing operations, due to higher fines content and the absorptive capacity of these fines. (12) Permeability : The permeability of blended granular material containing RAP is similar to conventional granular base course material. (12) Durability : Since the quality of virgin aggregates used in asphalt concrete usually exceeds the requirements for granular aggregates, there are generally no durability concerns regarding the use of RAP in granular base, especially if the RAP is less than 20 to 25 percent of the base.

DESIGN CONSIDERATIONS

The key design parameter for incorporating processed RAP into granular base material is the blending ratio of RAP to conventional aggregate that is needed to provide adequate bearing capacity. The ratio can be determined from laboratory testing of RAP aggregate blends using the CBR test method (6) or previous experience. It has been reported that blends of up to 30 percent asphalt-coated particles from RAP have been incorporated into blended granular base material. (14) The presence of asphalt cement in the RAP, however, does have a significant strengthening effect with time. It has been reported that specimens with 40 percent RAP blended in granular base material have produced CBR values exceeding 150 after 1 week. (12) RAP produced by grinding or pulverizing has a lower bearing capacity than crushed RAP, due to the higher generation of fines. (10) As a result, for use in load-bearing applications, granular RAP is usually blended with conventional aggregates. Conventional AASHTO pavement structural design procedures can be employed for granular base containing reclaimed asphalt pavement. The AASHTO Design Guide (1) is recommended for the thickness design of base course or subbase layers that contain RAP as a percentage of, or possibly even all of, the base or subbase. If the RAP is only a portion of the base or subbase material (less than 30 percent), the structural layer coefficient normally recommended for granular base materials (0.11 to 0.14) can be used. If the RAP constitutes a greater percentage, or even all, of the base or subbase material, some adjustment of the structural layer coefficient may be considered.

CONSTRUCTION PROCEDURES

Material Handling and Storage Essentially the same equipment and procedures used to stockpile, handle, and place conventional aggregates in granular base are applicable to blended granular material containing RAP. For major projects where control of engineering properties is critical, controlled blending of the RAP with conventional granular material at a central plant provides better consistency than the product of in-place, full-depth processing. Since each source of RAP will be different, random sampling and testing of the RAP stockpile must be performed to quantify and qualify the RAP. Samples of the stockpiled RAP should be used to determine the optimum blend of materials. (15) Additional care is required during stockpiling and handling to avoid segregation or re-agglomerating. Placing and Compacting Recycled asphalt pavement, which is recovered, crushed, screened, and blended with conventional aggregates, is placed as conventional granular material. Alternatively, in-place processing, which involves pulverizing the existing pavement and thoroughly mixing the individual surface and granular base course layers into a relatively homogeneous mixture and re-compacting it as granular base, can also be used. Conventional granular aggregates do not bond well with RAP or blended granular material containing RAP. Consequently, raveling can occur if thin layers of conventional aggregates are placed over material containing RAP. During placement, finish grading can be difficult because of the adhesion of asphalt in the RAP. Particular attention should be paid to obtaining adequate compaction to avoid post-construction densification of granular base materials containing RAP. Both blended granular material and pulverized material can be similarly compacted using conventional compaction equipment. It has been reported that compaction is improved if little or no water is used. (18) Quality Control The same test procedures used for conventional aggregate are appropriate for granular base/subbase containing RAP. The same field test procedures used for conventional aggregate are recommended for granular base applications when using RAP. Standard laboratory and field test methods for compacted density are given by AASHTO T191, (3) T205, (4) T238, (7) and T239. (8) Testing of moisture content and compaction using nuclear gauges is affected by the presence of RAP. Both parameters tend to be overestimated because of the presence of hydrogen ions in the asphalt cement contributing to the total count. To avoid this problem, compaction of granular base containing RAP may be carried out using a control strip. (18) Laboratory moisture checks should be completed to calibrate nuclear density gauge moisture content readings. In Florida , testing has been done to determine the leaching characteristics of RAP. In all batch tests measurements of VOCs, PAHs, and heavy metals (Ba, Ca, Cr, Cu, Pb, Ni and Zn) were below the detection level and below the applicable regulatory groundwater guidance concentrations. This indicates that all RAP samples tested pose minimal risk under current waste policy in Florida . Lysimeter tests were also preformed and columns were leached for 42 days. The VOC, PAH and heavy metal (Ba, Ca, Cr, Cu, Ni and Zn) measurements were all below detection limits, except for lead. (19)

ENVIRONMENTAL CONSIDERATIONS

Asphalt pavement consists of aggregate and petroleum derived asphalt binder containing volatile and semi-volatile constituents (e.g., polycyclic aromatic hydrocarbons (PAHs)) Additionally the asphalt pavement roadway may contain surface treatments, rubberized materials or contaminants from vehicle or other emissions (e.g., historically lead). The environmental issues are different for RAP based upon various beneficial uses. For unbound applications, such as granular base, leachability from the RAP may be a concern. This same leachability would be a concern if RAP was stockpiled or stored and exposed to precipitation. Testing has been conducted to determine the leaching characteristics of RAP in Florida. In all batch tests measurements of VOCs, PAHs, and heavy metals (Ba, Ca, Cr, Cu, Pb, Ni and Zn) were below the detection level and below the applicable state regulatory groundwater guidance concentrations. This indicates that all RAP samples tested pose minimal risk under current waste policy in Florida. Lysimeter (column leaching) tests were also performed and columns were exposed to synthetic precipitation for 42 days. The VOC, PAH and heavy metal (Ba, Ca, Cr, Cu, Ni and Zn) measurements were all below detection limits, except for lead which exhibited a concentration of 24 µg/L and 23 µg/L on days 12 and 14 of the experiment, but then was otherwise below the applicable Florida Groundwater Guidance Concentration (15 µg/L) for the duration of the experiment. (19) Other batch and column leaching tests were completed on RAP which also found constituents leached were low and generally below European drinking water standards (The Drinking Water Directive (DWD), Council Directive 98/83/EC). (20) Additionally, the University of Minnesota completed a review of current literature on PAHs in asphalt pavement concluding that PAH concentrations depend on the type of pavement (coal-tar versus petroleum based). Petroleum based asphalt pavement contained PAHs at concentrations below Minnesota Pollution Control Agency human health risk clean-up levels (21). The only exceedance was when when PAHs were converted to benzo(a)pyrene equivalents, they could exceed the lowest limit (Tier I). The report further concluded that when RAP is used as subbase aggregate, it is mixed with soils or other aggregates that do not contain PAH, so this mixture would not likely exceed applicable limits. (21)

UNRESOLVED ISSUES

There are various methods for incorporating RAP into a granular base however there are no specifications. There is a need to establish standard specifications for the incorporation of RAP into granular base and standard methods for determining in-place compacted density.

REFERENCES

  1. AASHTO Guide for Design of Pavement Structures . American Association of State Highway and Transportation Officials, Washington , DC , 1993.
  2. American Association of State Highway and Transportation Officials. "Standard Specification for Aggregate and Soil-Aggregate Subbase, Base and Surface Courses," AASHTO Designation M147-70, Part I Specifications, 16th Edition, 1993.
  3. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil In-Place by the Sand Cone Method," AASHTO Designation: T191-86, Part II Tests, 14th Edition, 1986.
  4. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil In-Place by the Rubber-Balloon Method," AASHTO Designation: T205-86, Part II Tests, 14th Edition, 1986.
  5. Ahmed, Imtiaz. Use of Waste Materials in Highway Construction . Federal Highway Administration, Report No. FHWA/IN/JHRP-91/3, Washington , DC , January, 1991.
  6. American Society for Testing and Materials. Standard Test Method D1883-87, "Standard Test Method for CBR ( California Bearing Ratio) of Laboratory-Compacted Soils." Annual Book of ASTM Standards , Volume 04.08, West Conshohocken , Pennsylvania .
  7. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil and Soil-Aggregate in Place by Nuclear Methods (Shallow Depth)," AASHTO Designation: T238-86, Part II Tests, 14th Edition, 1986.
  8. American Association of State Highway and Transportation Officials. Standard Method of Test, "Moisture Content of Soil and Soil Aggregate in Place by Nuclear Methods (Shallow Depth)," AASHTO Designation: T239-86, Part II Tests, 14th Edition, 1986.
  9. Collins, R. J. and S. K. Ciesielski. Recycling and Use of Waste Materials and By-Products in Highway Construction . National Cooperative Highway Research Program Synthesis of Highway Practice 199, Transportation Research Board, Washington , DC , 1994.
  10. Engineering and Environmental Aspects of Recycling Materials for Highway Construction , Federal Highway Administration, Report No. FHWA-RD-93-008, Washington , DC , May 1993.
  11. Federal Highway Administration, Foamed RAP Makes the Grade in Louisianna , Focus, October, 2002.
  12. Hanks, A. J. and E. R. Magni. The Use of Bituminous and Concrete Material in Granular and Earth , Materials Information Report MI-137, Engineering Materials Office, Ontario Ministry of Transportation, Downsview, Ontario, 1989.
  13. Miller, R. H. and R. J. Collins. Waste Materials as Potential Replacements for Highway Aggregates , National Cooperative Highway Research Program Synthesis of Highway Practice No. 166, Transportation Research Board, Washington, DC, 1976.
  14. Mineral Aggregate Conservation Reuse and Recycling , Report Prepared by John Emery Geotechnical Engineering Limited for Aggregate and Petroleum Resources Section, Ontario Ministry of Natural Resources, Ontario, Canada, 1992.
  15. Pavement Recycling Executive Summary and Report , Federal Highway Administration, FHWA-SA-95-060, Washington , DC , 1995.
  16. Saeed, A., W. R. Hudson, and P. Anaejionu. Location and Availability of Waste and Recycled Materials in Texas and Evaluation of their Utilization Potential in Roadbase . University of Texas , Center for Transportation Research, Report No. 1348-1, Austin , Texas , October, 1995.
  17. Schroeder, R. L. "Current Research on the Utilization of Recycled Materials in Highway Construction," Federal Highway Administration, Washington, DC, Presented at the International Road Federation Conference, Calgary, Alberta, Canada, 1994.
  18. Senior, S. A., S. I. Szoke, and C. A. Rogers, " Ontario 's Experience With Reclaimed Materials for Use in Aggregates." Presented at the International Road Federation Conference, Calgary , Alberta , 1994.
  19. Brantley, A.S. and Townsend, T., Leaching of pollutants from reclaimed asphalt pavement, Environmental Engineering Science , Vol. 16, no. 2, pp. 105-116. Apr. 1999.
  20. Legret, M., Odie, L., Demare, D., Jullien, A., Leaching of heavy metals and plycyclic aromatic hydrocarbons from reclaimed asphalt pavement, Water Research, Vol. 39, 3675-3685, 2005.
  21. Grosenheider, K., Bloom, P., Halbach T., Johnson, M., A Review of the Current Literature Regarding Polycyclic Aromatic Hydrocarbons in Asphalt Pavement, Mn/DOT contract No. 81655, October, 2005.

Reclaimed Asphalt Pavement - Embankment or Fill

INTRODUCTION

In addition to recycling into asphalt paving or incorporation into bases or subbases, some reclaimed asphalt pavement (RAP) has been used for embankment construction. It can also be used as a fill material. When used as an embankment or fill material, the undersize portion of crushed and screened RAP, typically less than 2 in, may be blended with soil and/or finely graded aggregate. Uncrushed or more coarsely graded RAP may be used as the embankment base. Although the use of RAP in embankment construction does not take any advantage of the asphalt cement component, it does, nevertheless, provide an alternate application where no other markets for reuse are readily available, or where the RAP may be unsuitable for use in asphalt concrete pavement. The properties of RAP are largely dependent on the properties of the constituent materials and asphalt concrete type used in the old pavement. (10,14)

PERFORMANCE RECORD

Although use of RAP as an embankment construction material does not appear to be extensive, it has been reported that at least nine states have made some use of RAP for this purpose. States that have made use of RAP as an additive in embankment construction include Connecticut , Indiana , Kansas , Montana , New York , and Tennessee . States that have used RAP directly as an embankment base material include California, Connecticut, Illinois, Louisiana, and Tennessee. (4) The performance of RAP in these applications was generally considered to be satisfactory to good.

MATERIAL PROCESSING REQUIREMENTS

Crushing Processing requirements for embankment or fill applications are minimal. Primary crushing may be necessary to satisfy gradation requirements. However, some jurisdictions permit the use of broken pieces of old asphalt pavement, provided the specified maximum size (similar to boulders) is not exceeded. Blending Crushed RAP is sometimes mixed with conventional earth fill materials or crushed aggregates and used in embankment construction.

ENGINEERING PROPERTIES

Some of the engineering properties of RAP that are of particular interest when RAP is used in embankment applications include gradation, compacted density, moisture content, shear strength, consolidation characteristics, permeability, durability, drainage characteristics, bearing strength, and corrosivity. Gradation : The gradation of RAP is controlled by crushing and screening. The gradation and physical requirements of AASHTO M145 (1) are usually readily satisfied by RAP or blends of RAP and soil or crushed aggregate. If used as an embankment base material, the maximum particle size of RAP will probably be less than 24 in. Compacted Density : Due to its asphalt cement content, the compacted unit weight of RAP100 to 125 lbs/ft 3 is likely to be somewhat lower than that of earth or rock. (15) The finer the RAP is crushed and sized, the higher its compacted density. Moisture Content : The optimum moisture content for RAP-aggregate blends is reported to be higher than for conventional embankment material, particularly for RAP from pulverizing operations, due to higher fines generation. (11) Shear Strength : The shear strength of RAP that has been crushed and sized will be based on internal friction, with little or no cohesion, and should be comparable to that of a similarly graded natural aggregate. RAP-aggregate blends should also have an internal friction angle in the same range as the natural aggregate. The shear strength of RAP-soil blends will likely be based mainly on internal friction, with little or no cohesion, and will be dependent on the relative proportions of the RAP and the soil. Consolidation Characteristics : The compressibility or consolidation characteristics of RAP-soil blends will probably be within the range of a granular soil, depending on the gradation, moisture content, and proportion of soil added to the RAP. For coarsely graded RAP, or RAP-aggregate blends, the potential for compressibility should, for all practical purposes, be negligible. Permeability : The permeability of blended RAP is similar to that of conventional granular material or soil-aggregate blends having similar gradation.(11) Durability : Since the quality of virgin aggregates used in asphalt concrete usually exceeds the requirements for embankment/fill material, there are generally no durability concerns regarding the use of RAP in this application. Drainage Characteristics : RAP is nonplastic, free draining, is not frost susceptible, and can be blended and compacted with other suitable fill materials. Bearing Strength : The bearing strength of an embankment is mainly of importance only in the top 3 ft, which is the portion of the embankment that provides the subgrade support for the pavement structure. The bearing strength of subgrade materials is usually determined by the California Bearing Ratio (CBR) test. The CBR value for RAP should be comparable to that of crushed stone of a similar gradation. The CBR of RAP-soil blends should be comparable to that of a well-graded granular soil. The top portion of an embankment will normally consist of soil like materials, with the coarser materials (crushed stone or rock) in the lower portions of the embankment. Corrosivity : On the basis of limited testing results, RAP is considered noncorrosive. (9,12)

DESIGN CONSIDERATIONS

The design requirements for RAP in embankment construction are the same as for similar sized soil-aggregate blends, conventional aggregates, or shot rock fill. Where pieces of broken asphalt pavement are used as embankment base, size and placement restrictions should apply in the same manner as for boulders and cobbles. It is recommended that such uncrushed materials not be placed where they may have an impact on future construction activities. Some jurisdictions require that a minimum separation be maintained between watercourses and fill materials containing RAP to avoid submersion of RAP in water, which may or may not be a potential environmental concern. (13) Design procedures for embankments or fill containing RAP are the same as design procedures for conventional embankment materials. The design should take into consideration slope stability, settlement or consolidation, and bearing capacity concerns. If the embankment is to be constructed using a blend of RAP with soil and/or crushed aggregate, a representative sample of the blended material should be tested, if possible, for triaxial compression (5) and California Bearing Ratio (CBR). (6) The maximum particle size for the triaxial test is 5 mm (No. 4 sieve). The maximum particle size for the CBR test is 19 mm (3/4 in sieve).

CONSTRUCTION PROCEDURES

Material Handling and Storage The same methods and equipment used to store or stockpile conventional aggregates are applicable for reclaimed asphalt pavement. Since each source of RAP will be different, random sampling and testing of the RAP stockpile must be performed to quantify and qualify the RAP. Representative samples of the stockpiled RAP should be used in the optimum blend design. (2) Additional care is required during stockpiling and handling to avoid segregation or re-agglomeration. Placing and Compacting The same methods and equipment for compacting conventional fill can be used for compacting crushed RAP or blends of soil and RAP. It is reported that granular materials containing RAP appear to compact better if little or no water is used. (15) Where large, broken pieces of old asphalt pavement are incorporated in embankment construction, additional attention is needed during compaction to ensure that no large voids are formed within the fill that could contribute to subsequent long-term differential settlement. Standard laboratory and field test methods for compacted density are given by AASHTO T191, (2) T205, (3) T238, (7) and T239. (8) Quality Control The same field test procedures used for conventional soils or crushed aggregate materials are also appropriate for RAP, or blends of RAP and soils or crushed aggregates. When RAP is used for construction of an embankment base or foundation material, compaction operations must be visually inspected on a continuous basis to ensure that the specified degree of compaction can be achieved, or that there is no movement under the action of compaction equipment. The construction of embankment bases or foundations containing rock or oversize materials usually requires a method specification, in which the procedures and type of equipment for placement and compaction are stipulated, but no testing methods or acceptance criteria are indicated. In Florida , testing has been done to determine the leaching characteristics of RAP. In all batch tests measurements of VOCs, PAHs, and heavy metals (Ba, Ca, Cr, Cu, Pb, Ni and Zn) were below the detection level and below the applicable regulatory groundwater guidance concentrations. This indicates that all RAP samples tested pose minimal risk under current waste policy in Florida . Lysimeter tests were also preformed and columns were leached for 42 days. The VOC, PAH and heavy metal (Ba, Ca, Cr, Cu, Ni and Zn) measurements were all below detection limits, except for lead. (19)

ENVIRONMENTAL CONSIDERATIONS

Asphalt pavement consists of aggregate and petroleum derived asphalt binder containing volatile and semi-volatile constituents (e.g., polycyclic aromatic hydrocarbons (PAHs)) Additionally the asphalt pavement roadway may contain surface treatments, rubberized materials or contaminants from vehicle or other emissions (e.g., historically lead). The environmental issues are different for RAP based upon various beneficial uses. For unbound applications, such as embankment fill, leachability from the RAP may be a concern. This same leachability would be a concern if RAP was stockpiled or stored and exposed to precipitation. Testing has been conducted to determine the leaching characteristics of RAP in Florida. In all batch tests measurements of VOCs, PAHs, and heavy metals (Ba, Ca, Cr, Cu, Pb, Ni and Zn) were below the detection level and below the applicable state regulatory groundwater guidance concentrations. This indicates that all RAP samples tested pose minimal risk under current waste policy in Florida. Lysimeter (column leaching) tests were also performed and columns were exposed to synthetic precipitation for 42 days. The VOC, PAH and heavy metal (Ba, Ca, Cr, Cu, Ni and Zn) measurements were all below detection limits, except for lead which exhibited a concentration of 24 µg/L and 23 µg/L on days 12 and 14 of the experiment, but then was otherwise below the applicable Florida Groundwater Guidance Concentration (15 µg/L) for the duration of the experiment. (16) Other batch and column leaching tests were completed on RAP which also found constituents leached were low and generally below European drinking water standards (The Drinking Water Directive (DWD), Council Directive 98/83/EC). (17) Additionally, the University of Minnesota completed a review of current literature on PAHs in asphalt pavement concluding that PAH concentrations depend on the type of pavement (coal-tar versus petroleum based). Petroleum based asphalt pavement contained PAHs at concentrations below Minnesota Pollution Control Agency human health risk clean-up levels (18). The only exceedance was when when PAHs were converted to benzo(a)pyrene equivalents, they could exceed the lowest limit (Tier I). The report further concluded that when RAP is used as subbase aggregate, it is mixed with soils or other aggregates that do not contain PAH, so this mixture would not likely exceed applicable limits. (18)

UNRESOLVED ISSUES

Although RAP is not frequently incorporated into embankments, there is a need to establish standard specifications for the use of RAP in embankment construction, either by itself as an embankment base material, or blended with soil and/or crushed aggregate.

REFERENCES

  1. AASHTO Designation: M145-82. "Standard Method of Test for the Classification of Soils and Soil-Aggregate Mixtures for Highway Construction Purposes," American Association of State Highway and Transportation Officials, Part I Specifications, 16th Edition, 1993.
  2. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil In-Place by the Sand Cone Method," AASHTO Designation: T191-86, Part II Tests, 14th Edition, 1986.
  3. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil In-Place by the Rubber-Balloon Method," AASHTO Designation: T205-86, Part II Tests, 14th Edition, 1986.
  4. Ahmed, Imtiaz. Use of Waste Materials in Highway Construction . Federal Highway Administration, Report No. FHWA/IN/JHRP-91/3, Washington , DC , January, 1991.
  5. ASTM D2850-87. "Standard Test Method for Unconsolidated, Undrained Compressive Strength of Cohesive Soils in Triaxial Compression." American Society for Testing and Materials, Annual Book of ASTM Standards , Volume 04.08, West Conshohocken , Pennsylvania .
  6. ASTM D1883-87. "Standard Test Method for CBR ( California Bearing Ratio) of Laboratory-Compacted Soils." American Society for Testing and Materials, Annual Book of ASTM Standards , Volume 04.08, West Conshohocken , Pennsylvania .
  7. American Association of State Highway and Transportation Officials. Standard Method of Test, "Density of Soil and Soil-Aggregate in Place by Nuclear Methods (Shallow Depth)," AASHTO Designation: T238-86, Part II Tests, 14th Edition, 1986.
  8. American Association of State Highway and Transportation Officials. Standard Method of Test, "Moisture Content of Soil and Soil Aggregate in Place by Nuclear Methods (Shallow Depth)," AASHTO Designation: T239-86, Part II Tests, 14th Edition, 1986.
  9. Bansci, J. J., A. Benedek, J. J. Emery, and J. Lawrence, "The Leaching of Toxic Organic Compounds from Solid Wastes," Presented at U.S. EPA National Conference on Management of Uncontrolled Waste Sites, Washington , DC , 1980.
  10. Engineering and Environmental Aspects of Recycling Materials for Highway Construction , Federal Highway Administration, Report No. FHWA-RD-93-008, Washington , DC , May 1993.
  11. Hanks, A. J. and E. R. Magni. The Use of Bituminous and Concrete Material in Granular Base and Earth . Materials Information Report MI-137, Engineering Materials Office, Ontario Ministry of Transportation, Downsview , Ontario , 1989.
  12. Krietch, A.J. "Evaluation of RAP as Clean Fill," Asphalt , Vol.5, No.1, p.8, The Asphalt Institute, Lexington , Kentucky , Summer 1991.
  13. Krietch, A.J. Leachability of Asphalt and Concrete Pavements , Heritage Research Group Report, Indianapolis , Indiana , March, 1992.
  14. Pavement Recycling Executive Summary and Report , Federal Highway Administration, Report No. FHWA-SA-95-060, Washington , DC , 1995.
  15. Senior, S. A., S. I. Szoke, and C. A. Rogers. " Ontario 's Experience with Reclaimed Materials for Use in Aggregates." Presented at the International Road Federation Conference, Calgary , Alberta , 1994.
  16. Brantley, A.S. and Townsend, T., Leaching of pollutants from reclaimed asphalt pavement, Environmental Engineering Science , Vol. 16, no. 2, pp. 105-116. Apr. 1999.
  17. Legret, M., Odie, L., Demare, D., Jullien, A., Leaching of heavy metals and plycyclic aromatic hydrocarbons from reclaimed asphalt pavement, Water Research, Vol. 39, 3675-3685, 2005.
  18. Grosenheider, K., Bloom, P., Halbach T., Johnson, M., A Review of the Current Literature Regarding Polycyclic Aromatic Hydrocarbons in Asphalt Pavement, Mn/DOT contract No. 81655, October, 2005.