Guidelines for the Design of Superpave Mixtures Containing Reclaimed Asphalt Pavement (RAP)
By: John R. Bukowski, Chair
FHWA Superpave Mixtures Expert Task Group
The concept of using reclaimed asphalt pavement (RAP) in asphalt mixtures has existed for some time in the hot mix asphalt (HMA) industry. The advent of cold-milling machines in the 1970's provided an increased supply of RAP which the industry began to recycle into new pavements. Currently, the use of RAP in new asphalt mixes is common practice. The recycling of old pavements into new pavements is the largest single recycling practice in the United States.
Performance of the HMA containing RAP has shown that this approach to resource management to be very effective. Significant quantities of both asphalt cement and aggregates have been utilized. These savings translate into significant economic advantage to both the public and private sectors.
The Superpave method of mix design, developed during the Strategic Highway Research Program (SHRP) does not contain specific recommendations for RAP. However, no reason exists for the exclusion of RAP in Superpave mixtures, even though specific methods for inclusion of RAP were not developed during SHRP.
This guideline is the result of an FHWA, Superpave Mixture Expert Task Group activity to make specific recommendations for inclusion of RAP into Superpave mixture design procedures. Provided are guidelines to proportion RAP, select grade and quantity of virgin asphalt binder, and preparation of a final mix design. Superpave mixture design procedures are fully described in FHWA publication SA-95-003, "Background of Superpave Asphalt Mixture Design and Analysis."
Procedures for the design of mixtures containing RAP are divided
into the following three categories:
Tier 1: 15% RAP by weight of total mixtureIt is recognized that experience with RAP varies among States. Thus, these tiers are given as guidelines which may need to be changed based on local experience. For mix designs containing RAP, the following requirements are suggested:
Tier 2: 16% to 25% RAP by weight of total mixture
Tier 3: 25% RAP by weight of total mixture
- General mix design requirements remain unchanged for Superpave mixtures containing RAP. Requirements for aggregate properties, gradation and volumetric properties should be met by the blend of virgin and reclaimed materials.
- The gradation of aggregate in the RAP should be used in calculation of the mix gradation. RAP is treated like a stockpile of aggregate during this analysis. Aggregate consensus properties may be run on the individual RAP aggregate stockpile at the agency's discretion. While fine aggregate angularity, sand equivalency, and flat and elongated particles might not be measured on the individual RAP aggregate stockpile, some amount of RAP aggregate will need to be extracted, combined with the total aggregate blend and tested for compliance with aggregate consensus properties.
- The percentage of asphalt binder in the RAP should be considered when determining the trial asphalt content. Asphalt binder content of the total mixture for mix batching includes virgin and reclaimed asphalt binder. The mixture trial asphalt content is calculated or estimated by experience during the trial blend analysis. Thus the amount of asphalt binder in the RAP is considered when determining how much virgin asphalt binder is required. The ability to obtain satisfactory mix volumetric properties is a requirement for all tiers.
- RAP should conform to the following requirements, as well as any other individual agency requirements.
- -maximum 2% deleterious materials, or as specified by the agency.
- -no particle in the mixture made with RAP should exceed the mixture maximum aggregate size at the time of production and discharge into the transport vehicle.
- For mixture design, the specific gravity of the virgin asphalt binder should be used as the specific gravity of the asphalt binder in the RAP.
- The effective specific gravity of the aggregate in the rap should be determined and used as the bulk specific gravity of the RAP aggregate for calculation purposes. When the RAP contains highly absorptive materials, the amount of absorbed asphalt should be estimated based on experience and used to back calculate the bulk specific gravity of the aggregate.
- During the laboratory mix design procedure, the RAP is handled as a combined material. Asphalt and aggregate are not extracted and handled individually, but are left together and handled as a separate material.
- Moisture content of the RAP should be initially determined to facilitate batching for mix design. A representative sample of RAP should be "pre-dried" to a constant mass prior to the batching of the mix specimens. This sample used for determination of the moisture content should not be used for other mix testing because it will be overheated.
- During batching of specimens, virgin aggregates and RAP should be heated to mixture temperature. RAP once heated to mixing temperature should not be reheated.
- Further hardening of the RAP asphalt binder during heating of the material to the mixing temperature should be avoided. Therefore, heating time for RAP should be kept at a minimum. The RAP should not be held at mixing temperature for more than one hour.
- After laboratory mixing and prior to compaction, the mix of virgin and reclaimed materials should be short term oven aged. The same short term oven aging procedure is used for the specimens containing RAP as would be used for a mix with all virgin materials.
- Use mixing and compaction temperatures for intended asphalt binder grade or as specified by the agency. Mixing and compaction temperature for virgin asphalt binders may be based upon equi-viscous temperatures measure with a rotational viscometer using AASHTO TP48 (ASTM D4402) or as specified by the agency. For combinations of virgin and reclaimed asphalt binder, actual measurement on a homogenous blend is not required. Mixing and compaction temperature can be obtained from a typical virgin binder or as specified by the agency.
- Tier 1 ( 15% RAP)
The asphalt binder grade for the mixture is selected for the environmental and traffic conditions the same as required for a mixture with all virgin materials. No grade adjustment is made to compensate for the stiffness of the asphalt binder in the RAP.
- Tier 2 (16% to 25% RAP)
The selected binder grade for the new asphalt binder is one grade lower for both the high and low temperature than the grade reqired for a virgin asphalt binder. Thus, if the specified binder grade for a virgin mixture is PG 64-22, the required grade for the binder in the recycled mixture would be a PG 58-28.
In moderate climate areas, the low temperature portion of the binder grade may not need to be adjusted at this RAP tier. Thus, based on local climate and experience, some areas may only need to adjust the high temperature portion of the binder grade to account for inclusion of RAP.
The asphalt binder grade can also be selected using an appropriate blending chart if the designer chooses to adjust the binder selection to compensate for the stiffness of the reclaimed asphalt binder.
- Tier 3 (> 25% RAP)
- The binder grade for the new asphalt binder is selected using an appropriate blending chart for high and low temperatures.
The design of Superpave mixtures with RAP requires only a few changes to the volumetric design procedures. For the most part aggregate in the RAP is handled as aggregate and asphalt binder in the RAP is considered considered as asphalt binder. All aggregate requirements for the aggregate blend (virgin and RAP) must be satisfied. The asphalt binder grade is adjusted, depending upon the amount of RAP included in the mixture. The following table summarizes the tests required on the RAP and selection of asphalt binder grade.
a. At the discretion of the agency|
b. Unless blending chart is used
c. Or use blending chart
Tier 1: < 15% RAP by weight of total mix
Tier 2: 16 % to 25% RAP by weight of total mix
Tier 3: >25% RAP by weight of total mix