The most important conclusion of the Federal Highway Administration (FHWA) report, Pavement Recycling Executive Summary and Report, is “pavement performance and detailed evaluations indicate that recycled HMA that is designed and controlled during production will perform comparably to conventional HMA and can improve material properties of the existing pavement layer,” according to the National Asphalt Pavement Association (NAPA).
Poor HMA performance is the same with both virgin and reclaimed asphalt pavement (RAP) mixes if proper mix design and construction control procedures are not followed.
A mix design incorporating RAP, just like any other mix design, should be carefully developed, taking into account the job specifications, available materials and economics. Proper testing starts with a representative sampling of the stockpiles of RAP. It can’t be emphasized strongly enough that proper sampling techniques must be used to obtain RAP samples for analysis.
The use of RAP in HMA
While plant technology, and the amount of fines in RAP present practical limitations, it is noted that overly conservative specifications may be the greatest obstacle limiting the use of RAP in HMA. The mix design process, production realities and performance history should be the limiting factors for the percentage of RAP, not arbitrary specification limits.
Two schools of thought exist for RAP mix design. The blending chart concept assumes that a 100% blend of the virgin binder and the binder from the RAP occurs. This concept implies that all of the binder in the RAP aggregate blends with the new binder—all in less than perhaps one minute of mixing time in the HMA facility. The second school of thought is that RAP goes into the mix as a “black rock” with the RAP binder providing no binder contribution to the mix. Obviously, neither concept represents actual binder blending which occurs when RAP is added to the HMA mixture. Using a 50-50 combination of the 100% blend/black rock concept is perhaps the most reasonable, albeit conservative, approach to represent the blending which occurs.
In 1979, a procedure for hot-mix recycling was described that would: utilize existing HMA plant equipment with minimum modifications; achieve acceptable productivity levels; produce stable HMA mixtures; and would meet environmental standards.
The evaluation process, representing the 100% blend concept, consisted of the following steps:
• Evaluate salvaged mix for gradation, asphalt binder content and binder properties;
• Select desired properties of final asphalt binder;
• Select grade of asphalt binder for the recycled mix;
• Determine quantity of new asphalt binder required;
• Determine mix proportions and evaluate mix properties;
• Determine combined aggregate properties of recycled mixture; and
• Establish job mix formula.
The procedure is essentially the same as presented in the Asphalt Institute’s MS-2 and MS-20 publications and has been the primary method used for RAP mix design since its development.
Viscosity blends in RAP
When the viscosity blending approach was first introduced, the graphical solution to percentage RAP was used for each RAP mix design. However, with years of experience, many mix designers have established “rules-of-thumb” to minimize the testing required. For example, in mixes with less than 15% RAP, many mix designers make no change in binder grade and for under 25% RAP, one viscosity grade softer is sometimes specified.
With the introduction of the Superpave performance grade binder specifications, changes are required in the mix design process in order to accommodate the use of RAP. The existing viscosity blending chart approach does not meet the specification requirements of Superpave.
The National Center for Asphalt Technology (NCAT) has completed a pilot project to define parameters for designing recycled hot-mix asphalt mixtures using Superpave technology. A more in-depth project has just begun under the sponsorship of NCHRP (Project 9-12) and recommendations from the NCHRP effort are a couple of years away. In the interim, the approach proposed by NCAT provides useful information for mix design.
NCAT’s study reviewed intermediate and high temperature binder properties for blending purposes. The study, following recommendations of the FHWA Mixture Expert Task Group, proposed a three-tier evaluation plan for RAP use.
The tiers are defined as follows:
Tier 1 - The amount of RAP in the HMA mix is equal to or less than 15%. The selected PG grade of the virgin binder should be the same as the Superpave specified PG grade.
Tier 2 - The amount of RAP in the HMA mix is more than 15% but equal to or less than 25%. The selected PG grade of the virgin asphalt binder should be one grade below, both high and low temperature, the Superpave specified PG grade.
Tier 3 - The amount of RAP in the HMA mix is more than 25%. Use the blending chart to select the high temperature grade of the virgin asphalt binder. The low temperature grade should be at least one grade lower than the binder grade specified by Superpave.
These recommendations are based on ETG recommendations from broad field experience. It is important to note that local mix design and performance experience should prevail in the decision making process. The tiers are recommended as general guidelines for materials for which no experience exists.
In an unpublished study, it was found that up to 25% RAP can be used with no effect on the binder grade. Industry experience indicates that most RAP applications are in the “less than 30%” range for practical and environmental reasons.
The percentages noted in the Tier concept may be appropriate if the 50-50 approach between 100% blend and “black rock” is taken. Tier 1 would then be appropriate for RAP use up to 30% which is supported by current practice. Tier 2 then becomes RAP use from 30 to 50% and Tier 3 greater than 50%.
The NCAT study has proposed a “specific grade” blending chart for determining the percentage of selected PG grade virgin asphalt binder or amount of RAP. This work also represents the 100% blend concept. The Y-axis of this chart is in log-log scale, similar to viscosity or penetration blending charts. The information needed to construct a “specific grade” blending chart is the G*/sind, rutting factor, of the aged asphalt binder from the RAP and selected virgin asphalt binder only.
Both G*/sind values must be measured at the high pavement service temperature indicated by the specified PG grade. The maximum and minimum percentages of virgin binder are selected corresponding to G*/sind values of 1.0 and 2.0 kPa, respectively, of the recycled asphalt binder.
NCAT recommends that the actual high temperature grade blending chart be used if modified asphalts are used.
While this proposed approach by NCAT may not prove to be the final approach selected by the NCHRP Project, the concept appears to be sound for designing mixes using RAP within the Superpave guidelines until the NCHRP project is completed if the 50-50 blend idea is overlaid onto the process.
RAP in Superpave
The FHWA Mixture Expert Task Group has developed a draft list of recommendations for use of RAP in Superpave. The recommendations are based on discussions of a group of industry professionals and, while the ideas do not represent results of a valid experiment, the concepts in the recommendations appear sound. Use of the recommendations by the industry will enable validation. The proposed recommendations are:
• Generally, 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 with some specific deviations as noted;
• The gradation of the aggregate in the RAP should be used in calculation of the mix gradation and fractured faces. RAP is treated like a stockpile of aggregate during analysis. Fine aggregate angularity, sand equivalent and flat and elongated particles are not measured on the RAP aggregate;
• The percentage of asphalt in the RAP should be considered when determining the trial asphalt content. The trial asphalt content is calculated or selected by experience during the trial blend analysis. The amount of asphalt contained in the RAP should be considered when determining how much virgin asphalt is required. The ability to obtain mix volumetric properties will control for all Tiers;
• Asphalt content of the total mixture for mix batching includes virgin and reclaimed asphalt binder. The asphalt binder in the RAP should be considered as part of the trial mix binder content;
• RAP should conform to requirements listed below plus other agency requirements. Requirements for RAP include:
- 100% passing 50 mm sieve;
- Maximum 2% deleterious materials or as specified by the DOT;
- No particle in the mixture made with RAP should exceed the maximum aggregate size at the time of discharge in the transport vehicle; and
- No aggregate in the RAP should be greater than half the layer thickness.
• The specific gravity of the virgin binder should be used as the specific gravity of the binder in the RAP for mixture design.
• 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.
• Use mixing and compaction temperatures for intended asphalt binder grade or as specified by the DOT. Mixing and compaction temperatures for virgin asphalt binders may be based upon equiviscous temperature measured with a rotational viscometer using the American Association of State Highway & Transportation Officials TP48 (ASTM D4402) or as specified by the DOT. For combinations of virgin and reclaimed asphalt, 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 DOT.
These comments have shown that all the answers are not yet available for mix design of RAP within the Superpave system. While the concepts appear to be sound, verification of the ideas need to occur. Verification activities are under way and should be completed in one to two years.