|FHWA > Engineering > Pavements > Pavement Recycling Guidelines for State and Local Governments > Chapter 11|
Pavement Recycling Guidelines for State and Local Governments
|Cost per square meter||$2.00||$3.21|
|Cost per Mg||$23.97||$25.67|
|Cost per kilometer (2-lane)||$14,600.00||$41,400.00|
Three sections of roadway were selected for hot in-place recycling in the City of Edmonton. These three sections included Victoria Park Road from the Goat Bridge Interchange I-115 Street, 137 Avenue eastbound from 113A Street to 97 Street and Learning Drive southbound from 167 Avenue to 144 Avenue and 50 Street to 137 Avenue. Each of the roadways had distress features consistent with surface disintegration (figure 11-13). On Learning Drive, some slight rutting was noted. Cores from each project were tested for density, asphalt content, and gradation.
Figure 11-13. Condition of pavement before recycling.
Penetration and viscosity tests on recovered asphalt binder were also conducted. Preliminary mix design consisted of the following steps:
The mix design results indicated several trends in the material properties. In-situ field air voids for the projects before recycling ranged from 2.1 percent to 3.2 percent. After the addition of "Cyclogen L," the void content ranged from 1.2 percent to 2.2 percent. The air voids after the addition of "Cyclogen L" was too low for two of the projects so the addition of blend sand was considered. The addition of 7 percent blend sand resulted in the voids for 137 Avenue and Victoria Park Road increasing to 2.0 to 3.2 percent, respectively. This level of air voids, although slightly lower than desirable, was considered acceptable.
The field control parameters were as follows:
Hot in-place recycling operations began on September 19, 1993, at the west end of Victoria Park Road and proceeded east. The initial admixture settings were as follows:
As recycling operations progressed, the recycled mix (after the addition of 0.2 percent cyclogen) appeared quite "wet." The mix also had a slight tendency to "flush" upon completion of the compaction operations. Based on these visual observations, a decision to reduce the amount of "Cyclogen" to a 0.15 percent addition rate was implemented. The mix was continually monitored on a visual basis. After compaction on the initial stretch of the road was completed, the finished pavement surface was inspected and some "stripping" of the larger aggregate particles was evident. Some uncoated particles (crushed during milling operations) were also observed to exist as the recycled mix was fed into the paver. At this time a decision was made to increase the amount of virgin asphalt cement content in the sand mix to aid, and ensure effective coating of the aggregates. The asphalt cement content was increased incrementally to 3.0 percent. After this, the recycled mix appeared satisfactory with respect to consistency, coating, and workability - both before and after placement. These settings/addition rates were maintained for the duration of the Victoria Park Road project.
Hot in-place recycling operations on Victoria Park Road were completed on September 20, 1993 and HIR operations on 137 Avenue began on September 21, 1993, at the west end of the project on the east bound lanes. The start-up admixture ("cyclogen" and virgin sand mix) addition rates for 137 Avenue Project were kept the same as that used on the Victoria Park Road Project.
With these settings in place, the recycling mix produced appeared to be relatively "drier" than that achieved on the previous project. Therefore, the "Cyclogen" addition rate was increased to 0.2 percent. After this, the recycled mixture exhibited a satisfactory appearance and paving operation progressed without any problems.
Field construction monitoring was conducted by the City for Manning Drive. Hot in-place recycling operations on Manning Drive began September 23, 1993, at the north end of the project in the west southbound lane.
The initial admixture settings for the Manning Drive were as follows:
Sand hot mix for Manning Drive was supplied by the City of Edmonton Asphalt Plant to attain additional flexibility in adjusting the asphalt content and gradation of the sand mix. As the recycling operation progressed, the recycled mix appeared "wet" and test results were indicating low voids. Based on this observation and test results, the asphalt cement content in the sand mix was reduced from 3.0 percent to 2.5 percent.
On September 24, 1993, existing asphalt material conditions changed and the recycled product appeared dry. In an attempt to increase the voids in the recycled product, "Cyclogen" and/or asphalt content in the sand mix were not increased but the gradation of the sand mix was altered by reducing the amount of sand and increasing the amount of 6 mm (¼ in) material. This alteration resulted in the mixture exhibiting a satisfactory appearance.
Hot in-place recycling was attempted on September 25, 1993, but was discontinued due to low ambient temperature (7°C, 45°F) and high winds. As a result of these conditions, temperatures behind the screed of higher than 85-90°C (185-194°F) could not be attained. The 90 linear meters (295 ft) of roadway that was completed under these conditions was redone the following day. Hot in-place recycling was completed on Manning Drive on September 27, 1993.
Examination of the results of testing indicated the following:
|Victoria Park Road||5.4 - 6.2 percent|
|137 Avenue||5.2 - 6.4 percent|
|Manning Drive||5.2 - 6.0 percent|
The recycling train (figure 11-14) consisted of four pieces of equipment: a preheater (figure 11-15), a heater miller (figure 11-16), a second heater miller with conveyor (figure 11-17), and a paver. The preheater was used to raise the surface temperature. The first heater miller was also used to raise the surface temperature. However, it also milled the in-place material up to a depth of 25 mm (1 in). The milled material was left as windrow in the center of the roadway. Recycling agent was added during the milling operation. The second heater miller heated up the exposed surface, milled an additional 25 mm, and also reworked the windrow of material. New mix and sand blend were added at this point. This heater miller contained a pugmill which completely reblended all milled and new material along with the recycling agent. The final unit was the paver, which was equipped with a windrow elevator. It picked up the windrow of the remixed material and laid down as a conventional paver. All equipment used to heat the pavement were equipped with environmental controls to reduce the amount of emissions. Figure 11-18 shows the condition of the road before and after recycling.
Figure 11-14. Recycling train.
Figure 11-15. Preheater.
Figure 11-16. Heater miller (primary).
Figure 11-17. Heater miller (secondary) with paver.
Figure 11-18. Condition before and after recycling.
The total cost of the hot in-place recycling project was $262,701. This cost amounted to $5.01/m2 ($4.00/yd2). Compared to this, the cost of conventional remove and replacing was estimated to be $6.60 - $7.70/m2 ($5.28 - $6.16/yd2).
HIR has also been used in Canada for treating rutting at intersections. Figures 11-19 through 11-22 show recycling operations in the city of Edmonton. Figure 11-19 shows a rutted intersection. Figure 11-20 shows the recycling train. Figure 11-21 shows a closeup of the rutting intersection, and figure 11-22 shows a view of the pavement after recycling.
Figure 11-19. A rutted intersection.
Figure 11-20. Recycling train.
Figure 11-21. Closeup view of the rutting intersection.
Figure 11-22. A view of the pavement after recycling.
The two necessary steps required for ensuring satisfactory construction of a hot in-place recycling (HIR) project are the development of an adequate specification, and ensuring that the specification requirements are met during mix design and construction. As in the case of conventional asphalt overlay, a set of guidelines or specifications is required in hot in-place recycling to describe the materials, workmanship, and other general requirements for the project. Where appropriate, agencies should consider the hot in-place recycling process along with other alternatives.(5) This process can help the user agency to evaluate the two or more methods and determine the most cost effective approach.
A set of general specification guidelines for effective completion of hot in-place recycling process is presented in table 11-1.(6, modified by 5) The major steps involved are preliminary pavement evaluation to determine structural adequacy, determination of applicability of hot in-place recycling, a detailed pavement evaluation, and selection of the particular hot in-place recycling technique.
|1. Preliminary Pavement Evaluation||Table 11-2||Mainly to determine if pavement structure is adequate.|
|2. Applicability of HIR||Table 11-2||If HIR not applicable, develop alternative rehabilitation or reconstruction method(s).|
|3. Detailed Pavement Evaluation||Table 11-3||Mainly quality and properties of existing pavement surface course.|
|4. Selection of HIR Option||Table 11-4||Surface recycling, remix, repave or remix-repave.|
|5.||Method for determining optimum amount of recycling agent, from ASTM D 4887|
|6. Completion of HIR Project||Table 11-5||Quality control important.|
Before any rehabilitation process is specified, the most suitable rehabilitation process should be determined on the basis of the source and cause of any surface defects such as rutting, cracking and/or deficient surface frictional resistance. If the cause of the defect remains unknown, then the proper rehabilitation technique cannot be applied, and the defect would most likely reappear in future. Use of hot in-place recycling can remedy the existing problem if it is caused by mix problems, such as excess asphalt, inadequate aggregate interlock, or too hard/too soft asphalt.(5) Table 11-2 presents the information required for evaluation of the existing pavement so that a proper recycling process can be specified. The different information required include an inventory information, details about the pavement structure, knowledge of the prior treatments, geometry and profile of the pavement, and the presence of miscellaneous structures such as manholes or utility covers on the pavement. These information help the user agency to evaluate the applicability of the hot in-place recycling process, determine the need for any prior treatment (such as cold milling), and specify the particular hot in-place recycling technique applicable (such as remixing or repaving). Table 11-3(6, modified by 5) shows the importance of information from detailed evaluation of existing pavement surface. The most important surface features which must be evaluated, include cracks, wear, and rutting. The important existing HMA features include thickness, binder content, grading, density, and penetration and viscosity of the binder. Finally, the particular hot in-place recycling technique can be identified by the guidelines presented in table 11-4.(6, modified from 5) Surface recycling is employed to improve the profile of a surface course deformed by rutting or wearing, but in comparatively unaged condition with minor cracking. Repaving is used to improve the profile of a surface course severely deformed by rutting or wearing, with new hot-mix overlay placed in one pass. This method improves frictional characteristics, and provides some pavement strengthening. Remixing can be used to improve the quality of old, cracked, aged surface course by the addition of recycling agent and/or new hot mix asphalt. The existing HMA mixture should be evaluated to help specify the particular hot in-place recycling technique required and to determine the type and amount of any recycling agent required. The process basically consists of extraction and recovery of the asphalt binder from the mix, up to the scarification depth. The type and amount of recycling agent chosen should be such as to restore the binder to the as-placed rheological condition. Adjustments of amount of recycling agent may be required in the field, since laboratory evaluations often indicate more recycling agent than is actually needed.(5)
|Pavement Inventory Information|
|Prior Treatments (See Pavement Inventory Also)|
|Geometry and Profile|
|Pavement Evaluation Item||Pavement Evaluation Parameter||Surface Defect|
|Existing Asphalt Concreteb (usually surface course, but must be at least to proposed scarification depth)||M||M||M||M|
|To improve the profile of a surface course deformed by rutting or wearing, but in comparatively unaged condition with minor cracking.b||Surface recycle||Heating, Scarification, Rejuvenator (if needed), Leveling, Reprofilingc, Compactiond|
|To improve the profile of surface course severely deformed by rutting or wearing, with new hot-mix overlay placed in one pass. To improve frictional characteristics. To provide some pavement strengthening.||Repave||Heating, Scarification, Rejuvenation, Leveling, Laying New Hot Mixf, Reprofiling, Compactiond|
|To improve the quality of old, cracked aged surface course by the addition of rejuvenator and/or new hot mixe.||Remix||Heating, Scarification, Rejuvenator, Mixing and/or New Hot Mix, Mixing, Leveling, Reprofiling, Compactiond|
For construction guidelines in specifications for recycling, the average recycled mixture temperature required for satisfactory compaction is in the 105°C to 115°C (221°F to 239°F) range at the breakdown rolling,(6) depending on specific site and ambient condition, and for scarification of the existing old HMA to be effective and efficient, the minimum temperature at the depth of the scarification should be the softening point for the project's recovered asphalt cement before rejuvenation.(5)
The two common types of specifications used for HMA construction are the method and the end-result specifications. The method specification describes in detail the equipment and procedures used to obtain the desired quality of asphalt mixture.This type specification requires sufficient detail to describe all the variables required to obtain a satisfactory asphalt mixture. On the other hand, an end-result specification sets the limits on certain properties of the HMA, and the method of construction is left at the discretion of the contractor. There are advantages and disadvantages for both types of specifications. For example, if a user agency has sufficient experience with the construction method, a method specifications may be easier to write. However, method specification requires that an inspector be on the job at all times to ensure proper compliance with the procedure. The end-result specification may be easier to write, since it is shorter and less detailed than the method specification. However, it may be very difficult to determine what properties to specify and what limits to set for these properties. This becomes more of a problem when dealing with material with high amount of variability, such as recycled asphalt pavement (RAP) material. Hence, in the case of recycling it is best to adopt a specification which is a combination of method and end-result specification. In this way, the experience of the user agency, contractors, material suppliers and equipment manufacturers can be utilized to obtain a recycled HMA with desirable quality.(6) Generally, the specifications for recycling projects are more of end-result nature than of method specification. This allows, and also encourages, the contractor to develop and use new construction methods and equipment for HMA recycling. However, in some cases it may be required to specify the type of equipment that will produce an acceptable pavement.(5) It is advisable to specify that the application rate of the recycling agent and any virgin HMA must be related directly to the forward movement of the equipment to minimize variation during speed changes and stoppages.(5)
Quality control (QC) refers to those tests necessary to control a product and to determine the quality of the product being produced. These QC tests are usually performed by the contractor. Because of greater variation expected in the case of recycled HMA, the frequency of testing should be more, even though the same tests that are used in the case of conventional HMA may be used. Before starting construction, it is very important to know the quality of the existing pavement, in terms of aggregate gradation and asphalt content of the mix. The pavement should first be delineated into subprojects, on the basis of differences in design, maintenance and rehabilitation actions. Once the subprojects are identified, samples should be taken from each of the subprojects to obtain representative materials. In this way the variation in the existing material can be identified and evaluated.(7)
In general, the quality control and quality assurance (QC/QA) measures for hot in-place recycled mixes should be similar to those for the hot mix recycled mixtures. The reader is referred to chapter 8 for this. However, unlike recycled mixtures produced in a HMA plant, the hot in-place recycled mixes are produced and modified in the field. Therefore, some features involving sampling and testing of HIR mixes need to be discussed.
In hot in-place recycling, the softened material from the existing pavement is scarified and recompacted after mixing with recycling agent. Samples of in-place mix can be obtained from the laydown machine for checking the mix components such as gradation and asphalt content. The scarified existing material can also be sampled behind the scarifiers. The National Center for Asphalt Technology (NCAT) ignition oven can be used to determine the amount of asphalt binder (or recycling agent) being incorporated in the recycled mix by checking the asphalt content of the existing, scarified mix and the asphalt content of the recycled mix.
Items of interest in hot in-place recycling to be checked or observed are recycled mix temperature, compacted mat density, surface smoothness, cross slope, handwork, mat depth, and general mat appearance. Proper care should be taken to avoid excessive smoke or flames on the pavement. Some recommended guidelines for quality control are presented in table 11-5.(6, modified by 5) Some of the tests mentioned in table 11-5 can also be used for quality assurance (QA) by the user agency. However, the frequency of QA tests is usually significantly lower than the frequency of QC tests. Many of the features are same as those of conventional hot mix construction. The important features are the applicability of ASTM D 4887 for recycling application, the new mixture (if any) addition rate, the thickness of the new hot mix overlay (if any), and the temperature at breakdown rolling. Some states require construction of a test strip to evaluate the recycling process, before starting actual operations. The New York DOT remixing specification requires that the contractor sample, test, and supply test results of the recycled loose mixture.(8)
|Depth of Scarification||Measure depth from existing surface adjacent to second mixer or Use circular ring methodc|
|Rejuvenator Application Rate (if any)||ASTM D 4887|
Calculate from quantity used
|Rejuvenator Quality (if any)||As usual (specifications and ASTM D 4552)|
|New Mixture Addition Rate (if any)||Calculate from quantity used (tons) and in-place density|
|Thickness of New Hot-Mix Overlay (if any)||Calculate from quantity used (tons) and in-place density|
|Temperature at Breakdown Rolling||Monitor at mid-point of re-profiled depth|
|Temperature of New Hot-Mix (if any)||As usual|
|Asphalt Cement Content, Gradation, and Stability Requirements||As usual|
|Compaction||As usual. Important to compare to relevant re-compacted density|
|Surface Tolerance||As usual|
|Penetration/Viscosity and Softening Point of Asphalt Extracted from Recycled Mixture||As usual|
In hot in-place recycling, the existing pavement is heated and the softened material is scarified and mixed with virgin aggregate and/or recycling agent and/or virgin asphalt mix. Different types of hot in-place recycling methods can be selected, based on the type of distress in the existing pavement. The three primary types of hot in-place recycling processes are surface recycling, repaving, and remixing. Case histories for these three kinds of recycling show that hot in-place recycling can effectively treat pavement distress at a much lower cost, compared to conventional rehabilitation treatments. However, to ensure proper performance, hot in-place recycling operation must be accompanied by strict quality control and quality assurance measures. The cause of distress in the existing pavement must be ascertained and the applicability of hot in-place recycling must be evaluated. Based on the type of distress and objective of rehabilitation the proper hot in-place recycling procedure should be selected. Testing of the existing asphalt binder is required to evaluate the effectiveness of the chosen recycling agent. For construction guidelines, recommendations regarding mat temperature during rolling should be followed. In general, QC/QA measures for hot in-place recycling are similar to those for hot mix recycling. However, softened and scarified material should be evaluated for gradation and asphalt binder properties in hot in-place recycling. Items of interest to be checked include depth of scarification, recycled mix temperature, compacted mat density, surface smoothness, cross slope, handwork, mat depth, and mat appearance.
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