Cassie Castorena

The vast majority of asphalt mixtures produced in North Carolina contain recycled materials, including Reclaimed Asphalt Pavement (RAP) and/or Recycled Asphalt Shingles (RAS). This research project seeks to identify how asphalt plant processing and stockpiling variables affect the consistency of RAP and RAS materials with time in a given stockpile and across different plants in North Carolina. Furthermore, the project will assess how changes in recycled material properties affects asphalt mixture performance is needed to understand the practical implications of variability. Collectively, the results of this research will inform improved measures within the NCDOT specifications to mitigate variability of recycled material sources and, in turn, improve the reliability of asphalt mixture performance.

Past and on-going NCDOT research projects have provided insights on critical aspects of the performance of asphalt mixtures containing recycled asphalt materials (RAM). NCDOT RP 2019-21 found that agglomerations of RAM particles exist in asphalt mixtures. These agglomerations act as ���black rocks��� and prohibit complete recycled binder availability. However, all experiments conducted in NCDOT RP 2019-21 employed a laboratory bucket mixer to prepare asphalt mixture samples, which may not reflect mixing in an asphalt plant. Furthermore, the NCDOT does not currently specify how to preheat virgin aggregate and RAM when producing asphalt mixtures in the laboratory, which may affect recycled binder contribution. NCDOT RP 2019-21 identified three adjustments to mixture design to account for recycled binder availability, termed availability adjusted mix design (AAMD). One of the proposed changes was to use only the available recycled binder to calculate the ���effective��� rather than total recycled binder replacement percentage (RBR%). The effective RBR% is lower than the total RBR% for a given mixture. Consequently, the reliance on the total RBR% may result in an effective binder system that is softer than what was expected. Shifting to specifications based on the effective RBR% would increase the amount of RAM that can be used in a mixture without adjusting to a PG 58-28 virgin binder and increase the maximum RAM that can be incorporated into a mixture without exceeding maximum limits.

Current procedures for asphalt mixture design in North Carolina require contractors to conform to volumetric requirements on the air void content, voids in mineral aggregate, and other parameters at a fixed, traffic- and layer-specific compaction effort. The presumption in this case is that the mixtures produced under the same guidelines will have similar properties. However, recent findings by NCSU suggests that this presumption may be inaccurate and may have substantial implications in the design, performance, and management of roadways. This research study will address this issue by: i) identifying the most appropriate durability related testing protocol for incorporation into mix design and quality assurance/control operations; ii) establishing initial threshold limits for the test identified; iii) developing a draft balanced mix design (BMD) procedure for North Carolina, and iv) developing a draft protocol for integrating the identified performance tests into quality assurance and quality control operations. The primary outcome of the proposed research will be a test method and procedure that the NCDOT can deploy in asphalt mixture design and production to ensure that the mixtures delivered in the state have an acceptable level of performance.

This project will explore an innovative method to determine Reclaimed Asphalt Pavement (RAP) binder availability. RAP binder availability refers to the percentage of total RAP binder that is released and available to blend with virgin asphalt during asphalt mixture production. Research has shown that the primary source of unavailable recycled binder is agglomerations of adhered RAP particles. The binder bound within the agglomerations is prohibited from coming contacting and therefore, blending with virgin asphalt. The proposed innovation determines the extent of RAP agglomeration and, in turn, RAP binder availability by comparing the gradation of recovered RAP aggregates to that of the RAP itself. Quantifying the inactive recycled binder content of RAP sources using the proposed innovation offers a means to discredit unavailable recycled binder within design practices. It is expected that discrediting unavailable recycled binder will improve the design and performance of high RAP content mixtures, consequently increasing pavement service life and reducing life cycle costs. The research products may also enable the design of satisfactory asphalt mixtures with higher RAP contents, resulting in environmental and cost benefits. Preliminary results of the proposed innovation are promising. This project will rigorously optimize the innovation, evaluate the performance implications of discrediting inactive recycled binder within design procedures, and develop a provisional AASHTO standard procedure.

Vehicle collisions and increases in collisions rates during wet conditions are one of the major safety concerns for the NCDOT. Collision rates increase when the surface is wet because skid resistance reduces under these conditions. The precise amount of loss is dependent on many factors, but the consensus among experts is that pavement friction and macrotexture are important factors that affect the skid resistance and changes in this resistance under wet conditions. This research will achieve three objectives; 1) characterize friction and texture performance models, 2) develop friction and texture performance thresholds, and 3) identify asphalt mixture compositional factors (gradation, asphalt content, presence of modified versus non-modified asphalt, etc.) that affect the as-constructed macrotexture and friction. The primary outcome of the proposed research will be an initial set of performance models that can be used to assess immediate and potentially long-term friction/macrotexture issues. The research will also produce a set of threshold limits for friction/macrotexture where investigatory and intervention steps need to be taken to control for safety. Finally, the research will produce information on the mixture design factors that contribute to higher or lower friction/macrotexture. These outcomes can be used by the Traffic Safety and Materials and Test Units of the North Carolina DOT to predict and manage friction and texture performance on roadways and to understand when measurements represent a potential hazard exists. It will also be used to help identify asphalt mixtures with potential friction and macrotexture issues and develop better guidelines, specifications, and operational controls (if necessary) for recently overlaid pavements. This could lead to reduced collision rates on these pavements. Thus, this research will result in overall improved procedures for flexible pavement overlay operations.

In this research study, NCSU will design, conduct, and provide recommendations relating to a two-phase ruggedness and interlaboratory study on a test method that has been identified as critical to asphalt pavement performance and design practice. AASHTO TP 132 (2019) Standard Method of Test for Determining the Dynamic Modulus for Asphalt Mixtures Using Small Specimens in the Asphalt Mixture Performance Tester (AMPT) has been developed, refined, and recently published as an AASHTO provisional standard; a statistically sound refinement procedure is needed to facilitate widespread adoption and implementation. Not only can this standard be used to obtain inputs to the AASHTO PavementME pavement structural analysis software, the standard is being used in ongoing FHWA efforts as part of a performance-related specification framework which seeks to increase pavement life through fundamental testing and predictive relationships. AASHTO TP 132 is of interest because of its fundamental nature, determination via the AMPT standardized equipment, and its ability to model and predict material performance over a wide range of loading and climate conditions a pavement may experience; resulting in better performing, safe, quiet, durable, long lasting asphalt roadways. Additionally, a draft practice for preparing small-scale specimens has been developed and published as AASHTO PP 99 (2019) Standard Method of Practice for Preparation of Small Cylindrical Performance Test Specimens Using the Superpave Gyratory Compactor (SGC) and Field Cores. This draft practice is of significant interest to the asphalt materials community due to anticipated materials, time, and cost savings associated with preparing and evaluating smaller performance test. NCSU will carry out the following tasks. Task 1 ������������������ Develop final research plans and project schedule ������������������ The proposed plan will be revised based on feedback from the FHWA. Task 2 ������������������ Kickoff meeting ������������������ NCSU will meet with the project panel to review the statement of work and work plans. Task 3 ������������������ Perform work plan and document efforts ������������������ NCSU will carry out the approved work plan by following the appropriate ASTM standard test methods to develop a rugged test method. Task 4 ������������������ Ruggedness study presentations and webinar ������������������ NCSU will present the findings to targeted stakeholders. Task 5 ������������������ Publication ready final deliverables ������������������ the AASHTO standards will be revised into a final form. Task 6 ������������������ Revise work ILS work plan ������������������ NCSU will revise the original work plan from Task 1 based on findings from Tasks 3-5 for conducting the ILS study. Task 7 ������������������ Perform work plan and document efforts ������������������ NCSU will coordinate the ILS according to the approved work plan. This will include identifying participants, sending materials, and analyzing their results statistically. Task 8 ������������������ ILS study presentation and webinar ������������������ NCSU will present their findings to targeted stakeholders. Task 9 ������������������ Publication ready final deliverables ������������������ the AASHTO standards revised in Task 5 will be modified to include repeatability and reproducibility statements.

The use of high Recycled Binder Replacement Percentages (RBRs%) in asphalt surface mixtures is increasing. The asphalt binders in recycled materials are generally hardened and embrittled from oxidization and may not fully mobilize and blend with virgin materials. Consequently, high recycled content mixtures may be prone to cracking if appropriate measures to consider this effect are not taken during the mixture design process. The objectives of the proposed research project are to: (1) modify the NCDOT������������������s procedures for the design of surface mixtures containing RAP and RAS to improve performance and (2) modify the NCDOT������������������s specifications to improve the consistency within and across RAP and RAS stockpiles within North Carolina. 1. To achieve these objectives, an operational review will be conducted to identify how contractors process, stockpile, characterize and use RAP and RAS under the current NCDOT guidelines. In addition, relationships between asphalt content and performance will be developed for recycled mixtures sourced from North Carolina. These relationships will be used to identify the maximum virgin binder content allowable and to maximize cracking performance without having the rutting performance fall below a critical performance threshold for each mixture. The collective results will be used to identify appropriate revisions to the NCDOT������������������s current recycled mixture design procedure to ensure reliable performance. The research results will lead improved specifications that will facilitate the design of better-performing surface mixtures containing recycled materials. These specifications will improve the durability of NCDOT pavements and consequently decrease life-cycle costs.

In 2007, the Virginia Department of Transportation (VDOT) introduced specifications to allow higher percentages of reclaimed asphalt pavement (RAP) (i.e., up to 30%) in hot-mix asphalt (HMA) surface mixtures without adjustment of the virgin binder grade. The increased use of RAP was expected to result in a lower cost of produced asphalt mixtures given the continuous rising cost of oil and thus asphalt binders and fuel needed to produce asphalt pavements. By 2013, VDOT had begun to consider the feasibility of allowing the use of surface mixtures containing up to 45% of RAP material. Several trial sections were constructed containing mixtures with 20%, 30%, 40%, and 45% RAP for evaluation (Nair et al., 2019). In general, those trials found that mixtures containing up to 45% RAP could be designed, produced, and constructed if proper procedures are followed. In 2019, the research team at Virginia Transportation Research Council (VTRC) initiated another study to evaluate field trials of high RAP asphalt mixtures (i.e., more than 40%) designed following the Balanced Mix Design (BMD) special provision for VDOT������������������s surface mixtures. The primary concern with such mixtures has been that the use of high percentage of RAP will overly stiffen mixtures; making them more brittle and prone to premature cracking. The use of high percentage of RAP can lead to numerous construction and performance issues including, but not limited to, compactibility and workability in cool weather, low-temperature cracking with accumulation of thermally induced stresses, fatigue cracking and micro damage accumulation leading to crack initiation and propagation with repeated loading, reflection cracking with repeated loading and daily / seasonal thermal stresses, and raveling with subsequent aging or moisture damage. The challenges arising from the use of high RAP content mixtures can be addressed through the use of softer binders or additives such as recycling agents (RAs). These additives were utilized in HMA in the early period of widespread recycling in the 1970s and 1980s for the purpose of realizing three types of benefits: environmental, economic, and engineering. The use of RAs holds promise as long as there is a proper understanding of how effectively they restore binder rheology and how that effectiveness evolves with aging of mixtures in the laboratory, making them proper additives to be incorporated in mixtures to be placed in field. Hence, there is a need for an engineered framework to evaluate RAs in terms of their stiffness and cracking resistance when incorporated into the binder blends of corresponding mixtures. Currently, there are no unique and / or detailed handy guides or specifications that outline a framework to evaluate acceptability of RAs in the state of Virginia. Therefore, this study aims to identify and / or develop a testing protocol to evaluate the effectiveness of RAs in alleviating the brittleness of high RAP asphalt mixtures. In addition, a performance-based parameter(s) with its threshold limits / criteria will be identified or developed to accept or reject a certain product (i.e., recycling agents). Both objectives will facilitate responsible use of innovative materials as part of Virginia������������������s Balanced Mix Design (BMD) initiative. In this study, NCSU researchers will conduct experiments on asphalt binder, asphalt from reclaimed asphalt pavement (RAP), rejuvenator agents, mortars, and asphalt mixture.

The use of asphalt mixtures containing high Recycled Binder Replacements (RBRs) is increasing. Recycled binders are oxidized and thus, harder and more susceptible to cracking than virgin binders. Consequently, the use of higher recycled content mixtures has prompted heightened interest in recycling agents and necessitated the use of asphalt extenders to produce softer virgin binder grades. Recycling agents include a wide-range of both softening agents and rejuvenators that are intended to restore the physical and chemical properties of aged asphalt binders. Petroleum-based extender products have been in existence for a long time (e.g., Re-refined Engine Oil Bottom (REOB)). Non-petroleum based products have been more recently introduced (e.g., bio-oils).

Asphalt mixtures used in pavement construction are required to meet the North Carolina Department of Transportation (NCDOT) moisture sensitivity specifications. To improve resistance to moisture damage various antistrip additives are used by the asphalt plants producing asphalt mixtures. These antistrip additives help improve the adhesion between asphalt and aggregate and thus improve the resistance of the asphalt mixtures to moisture damage. The antistrip additives are added to the asphalt mixture at the plant by various mechanisms based on the type of antistrip additive being used or they come premixed with the asphalt liquid. The additive is usually added to the asphalt liquid. A problem in the mechanism used to add the antistrip additive to the asphalt might lead to a lesser amount, or no antistrip additive being added into the asphalt mixture. This might lead to the asphalt mixture not meeting the NCDOT moisture sensitivity specifications. Since the additive is added to the asphalt liquid, any problem in the mechanism will not be noticed until the asphalt mixture is tested for its moisture sensitivity.