Although some form of pavement recycling was practiced as early as 1915, the first sustained efforts to recover and reuse old asphalt paving materials were conducted in the mid 1970s. With financial support of the Federal Highway Administration (FHWA) and technical assistance from trade associations such as the National Asphalt Pavement Association and the Asphalt Institute, more than 40 states placed demonstration reclaimed asphalt pavement (RAP projects by 1982. RAP, is now routinely used in nearly all 50 states.
Though reliable figures for the generation of RAP are not readily available from all state highway agencies or local jurisdictions, the FHWA estimates that nearly 30 million tons are recycled into hot-mix asphalt (HMA) pavements each year. Saving taxpayers more than $300 million annually by reducing material and disposal costs, RAP has become an increasingly valuable asset.
This will work
Equally important is the well-documented fact that pavements incorporating RAP can provide the same safe, smooth, durable ride as conventional HMA. Agency and industry personnel, as well as researchers, seem to agree that as long as RAP is treated as an engineered construction material, it will work in Superpave. A product of the Strategic Highway Research Program, Superpave (SUperior PERforming Asphalt PAVEments) is a performance-based system for designing pavements to accommodate project-specific conditions including climate, materials and traffic. As originally configured, however, Superpave did not provide guidelines for the inclusion of RAP. Still, experience to date indicates that RAP can be used in Superpave mixes predictably and reliably, as long as the unique properties of the RAP are known. Recently completed research, conducted under the auspices of the National Cooperative Highway Research Program (NCHRP) and led by Rebecca McDaniel of the North Central Superpave Center and Mike Anderson of the Asphalt Institute, confirms that Superpave can easily and effectively accommodate the use of RAP.
As highway agencies transition from viscosity- or penetration-graded asphalt cements to Superpave Performance Grade (PG) binders, a key question is, "When the aged binder from RAP is combined with the new binder, what effect does it have on the resultant binder grade?"
Similarly, as highway agencies move from traditional Marshall and Hveem methods of mix design they ask, "What effect does the RAP have on mix volumetric properties and performance?"
Intuitively, and correctly, they reasoned that at low RAP percentages the changes would be negligible, but at higher percentages the effect would be significant. With RAP routinely used in HMA at substitution rates of 10- 50%, these questions were addressed in NCHRP Project 9-12, "Incorporation of Reclaimed Asphalt Pavement in the Superpave System." Briefly, the research objectives were two-fold: to determine the extent of binder blending (Does RAP act like a black rock, in which no blending of the old and new binder occurs, or does some blending occur between the old, hardened RAP binder and the added, virgin binder?) and the effect of RAP on the blended binder and resulting mix properties.
Being partial
Based on a comprehensive laboratory testing program using RAP materials from field projects in Arizona, Connecticut and Florida, the researchers concluded that RAP does not act like a black rock, nor does total blending of the RAP and virgin binders ever occur.
Instead, partial blending occurs to a significant extent. This partial blending means that at high RAP contents the hardened RAP binder must be accounted for in the selection of the virgin binder.
The research findings largely confirm current practice. Low amounts of RAP, typically 10-20%, can be used without testing the recovered binder. With higher RAP contents, Superpave binder tests can be used to determine how much RAP may be added or which virgin PG binder is needed. The recommended percentage of RAP within each tier is dependent upon the grade of the extracted RAP binder. The softer the extracted RAP binder, the greater the percentage of RAP allowed.
For low RAP contents of 10-20%, no testing is required because there is not enough of the old, hardened RAP binder present to change the combined binder properties.
For intermediate ranges of RAP, the recommended virgin binder is simply one grade softer than that used for conventional HMA.
At higher RAP contents, the RAP binder will have a noticeable effect and must be accounted for by using a softer grade of virgin binder. The RAP binder must be extracted, recovered and tested to develop blending charts. To construct a blending chart, the desired final binder grade and critical temperatures of the recovered RAP binder are needed. Once the physical properties and critical temperatures of the recovered RAP binder are known, the materials engineer has two approaches to consider. In the first approach, the percentage of RAP is specified and the blending chart is used to determine the appropriate virgin binder grade. In the second approach, the virgin binder grade is specified and the blending chart is used to determine the maximum percentage of RAP that may be used. The step-by-step procedures for constructing blending charts are included in NCHRP Report 452.
The overall Superpave mix design process with RAP is virtually identical to that described in AASHTO MP2 with only minor differences.
Although RAP is treated like any other stockpile for blending and weighing, it must be heated gently to avoid changing the RAP binder properties. Also, the RAP aggregate specific gravity must be estimated. The weight of the binder in the RAP must be accounted for when batching aggregates, and the total asphalt content reduced to compensate for the RAP binder. The virgin binder grade may need to be changed depending upon the RAP percentage and binder grade and the desired blend.
Though not Superpave-specific, other factors to consider when using RAP are those related to production and quality control testing. Higher plant temperatures are necessary if the ambient temperature is low or the moisture content of the materials is high. Greater energy consumption may affect plant production. Stricter stockpile management and more frequent sampling and testing of the RAP may be necessary to ensure consistency and quality.
Findings of the recently completed NCHRP 9-12 project confirm that Superpave can effectively accommodate the use of RAP. Specifically, low amounts of RAP, typically 10-20%, can be used without testing the recovered binder. With higher RAP contents, Superpave binder tests can be used to determine how much RAP may be added or which virgin PG binder is needed.
