BACKGROUND
Reclaimed asphalt pavement (RAP) has been used for several decades in hot mix asphalt (HMA) to make use of a valuable resource and reduce materials costs. Based on varying state practices, HMA can contain 10 to 25% RAP which, in most parts of the United States, does not consume the entire amount of available RAP. As a result, RAP is accumulating and is being used in low-value, non-bituminous applications or wasted.
RAP processing equipment and procedures have advanced since the recommendations on incorporation of RAP in the Superpave mix design method were made in NCHRP Report 452 (http://gulliver.trb.org/publications/nchrp/nchrp_rpt_452.pdf) and NCHRP Research Results Digest 253 (http://gulliver.trb.org/publications/nchrp/nchrp_rrd_253.pdf). Current crushing and screening technology allows processing of RAP in a more consistent and uniform manner. In addition, hot mix plants are routinely able to handle higher amounts of RAP. As a result, it is now possible to consistently produce HMA containing 25% to greater than 50% RAP. Such high-RAP content mixes have the potential to significantly reduce the cost of HMA paving while conserving natural resources.
RAP fractionating, a relatively new processing technique, refers to the crushing and screening of RAP into several predetermined sizes that can be fed into the hot mix plant according to the HMA mix requirements. Fractionating RAP allows for more uniform materials, which can be fed back with virgin material producing a uniform mix.
Current AASHTO recommendations make it difficult to design HMA mixes with high-RAP contents. Modifications to the current specifications may be needed to assure agencies that satisfactory performance will result from the use of high-RAP content HMA mixes.
OBJECTIVE
The objectives of this research are to (1) develop a mix design and analysis procedure for HMA containing high-RAP contents that provide satisfactory long-term performance and (2) propose changes to existing specifications to account for HMA containing high-RAP contents. High-RAP content is defined as greater than 25% and may exceed 50%.
The mix design procedure is expected to be an adaptation of AASHTO R 35, Superpave Volumetric Design for Hot-Mix Asphalt. The mix analysis procedure is expected to (1) include performance-related tests and key criteria to address permanent deformation, fatigue cracking, low-temperature cracking, and moisture susceptibility and (2) identify any promising method or procedure developed to assess the durability of HMA.
A mix design and analysis procedure will then be prepared in the form of an AASHTO Standard Practice for use by practicing engineers and technicians. The specification will be an adaptation of AASHTO M 323, Superpave Volumetric Mix Design. Practical guidelines for proper material management and processing of RAP (such as contained in NAPA QIS 124, Designing HMA Mixtures with High RAP Content: A Practical Guide) will also be developed.
Accomplishment of the project objectives will require at least the following tasks.
TASKS
Phase I (1.) Review the available literature on and existing mix design procedures for high-RAP content HMA, as well as the mix design procedures developed for RAP-HMA mixes in NCHRP Projects 9-12 and 9-33, where high-RAP content is defined as greater than 25% and possibly exceeding 50%. In addition, conduct a continuing review of (1) HMA specifications for the use of high-RAP content HMA; (2) guidelines for materials management, production, and placement of high RAP content HMA mixes; and (3) best practices for maintaining emissions within regulatory limits when high-RAP content HMA is produced. (2.) Based on the results of Task 1, propose mix design and mix analysis procedures for HMA containing high-RAP contents that provide satisfactory long-term performance. The mix design procedure should (1) address the measurement of binder, aggregate, and mix properties, directly, indirectly, or both, and (2) consider, at a minimum: (a) the use of modified binders and other admixtures, whether as virgin materials or in the RAP; (b) conditioning protocols for asphalt binder, aggregate, and mix; (c) laboratory compaction effort, and (d) modifications to existing methods, procedures, and criteria due to the use of high-RAP contents. The mix analysis procedure should address (1) a process to estimate the amount of blending of RAP binder with virgin binder, (2) the measurement of performance properties of the combined asphalt binder, (3) the measurement of performance properties of the high-RAP content HMA mix, and (4) the validity of direct and indirect measurements for these purposes. (3.) Prepare a laboratory work plan to evaluate the mix design and analysis procedures proposed in Task 2. In the plan, include a range of (1) RAP materials, processes, and contents; (2) modified and unmodified asphalt binders; (3) aggregate types; and (4) mix types for surface, intermediate, and base courses. (4.) Within 4 months of the effective date of the contract, submit an interim report of the findings of the Tasks 1 and 2 and the work plan developed in Task 3. The research agency will be required to meet with the project panel approximately 1 month later to obtain NCHRP approval before beginning Phase II.
Phase II (5.) Conduct the laboratory work plan approved in Task 4. Based on the results of this laboratory work, prepare a practical mix design and analysis procedure that yields satisfactory HMA mix designs with high-RAP contents. (6.) Compare the properties and performance predictions of high-RAP content HMA mixes with mixes prepared with all virgin materials for the same field conditions. Assess the causes of any substantial differences between the properties and performance of the high-RAP content and virgin HMA mixes. If needed, revise the mix design and analysis procedure, the material property criteria, or both, to minimize these differences. (7.) Evaluate the mix design and analysis procedure from Task 6 using materials from a minimum of three field projects utilizing high-RAP contents. If possible, the field projects should represent different climates and traffic volumes. Compare the properties and performance of these designs with those of agency designs developed for the specific projects. (8.) Identify proposed changes to HMA specifications, including AASHTO M 323, necessary to account for the use of high-RAP content HMA, and prepare guidelines for materials management, production, and placement of these mixes. Prepare best practices for maintaining emissions within regulatory limits when high-RAP content HMA is produced. (9.) Prepare a final report that documents results, summarizes findings, draws conclusions, and presents the proposed mix design and analysis procedure, guidelines, best practices, and suggested changes to specifications in the appropriate AASHTO format.
Status: Work is in progress on Phase II.
Product Availability: The Phase I interim report is available for loan on request to NCHRP.
