Benjamin Underwood

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.

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.

In 2018, an initial effort was undertaken by Virginia Transportation Research Council (VTRC) to provide benchmark indications of performance for a number of “typical / everyday” asphalt surface mixtures produced and sampled in 2015 in anticipation of this new approach. Three fast, simple, practical, but empirical performance tests addressing different modes of distresses were selected for use as part of the BMD method. The selected tests were Cantabro test, the Indirect Tensile cracking test (IDT-CT), and the Asphalt Pavement Analyzer (APA) rut test for assessing durability, cracking and rutting potentials of asphalt mixtures, respectively. VDOT has been so far extensively building upon its BMD initiative based on Approach I, the empirical tests (for rutting and cracking) and associated thresholds have never been verified through the use of Approach II. This study would provide an opportunity to: • Establish links between laboratory performance-related asphalt mixtures empirical and fundamental properties (on one hand) and M-E structural pavement design (on the other hand). This is a vital step to a practical integration of mixture design and structural design. • Verify (or refine) the performance thresholds on the basis of mechanistic approach, rather than empirical approach. • Establish and verify initial traffic-based performance thresholds for the empirical tests tied /correlated to the fundamental tests and mechanistic analyses. In order to fulfill the objectives of this research study, the following six primary tasks are proposed. First, the existing literature on similar efforts by other state agencies will be summarized and reported. Then, the research team will conduct a laboratory experimental program with three major parts: material selection (18 different mixtures), performance testing on reheated mixtures, and performance testing on extracted and recovered binder. NCSU researchers will assist in all three tasks, but take the primary lead in the performance testing on asphalt mixtures and binders. Third, the laboratory measured engineering and performance properties will be coupled in a full mechanistic analysis framework. This is a vital step to quantify and effectively evaluate the impact of using BMD asphalt surface mixture on the overall performance of pavements. This will include the use of AASHTOWare® Pavement ME and FlexPaveTM. The mechanistic-based simulations will be executed using real existing and most commonly encountered pavement structures in Virginia (referred to herein as pseud-hypothetical pavement structures). NCSU researchers will lead the analysis of this task and carry out the requisite simulations. Fourth, links and correlations between BMD and mechanistic-based fundamental tests. In Task 5, NCSU researchers will VDOT personnel at the VTRC (or other approved site in Virginia) to perform AASHTO TP 132 (dynamic modulus), AASHTO TP 133 (cyclic fatigue) and AASHTO TP 134 (stress sweep rutting). In Task 6, a final report will be developed. VTRC personnel will lead this effort, but NCSU researchers will support the task.

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.

Infrastructure resilience has become an important topic for North Carolina. Recent hurricanes and other extreme events have caused more than $450 million in damage to the States’s transportation infrastructure. In addition to the cost of the infrastructure, the NCDOT spent considerable resources to redesign and repair many elements after each event. A review of the NCDOT records following Hurricane Florence indicates that more than 3,000 disruptions resulted from that event alone. Some of these locations were identical to those damaged during Hurricane Matthew but, the amount of damage was different between the two events, suggesting that DOT strategies were effective. However, detailed quantification of the performance differences have not been completed and thus NCDOT engineers must rely on qualitative and anecdotal evidence as to the effectiveness of various strategies. Though many agencies have studied the topic of infrastructure resilience to extreme events, the literature suggests that the generalizability of their findings is limited because of the contextual sensitivity of the available strategies. In this case, data on the effectiveness of design and repair strategies within the context of North Carolina is required. Thus, research is needed to identify and evaluate the specific elements of the new infrastructure that positively contributed to the improved performance during Hurricane Florence and those that did not positively contribute. With respect to this need, the proposed research plan will achieve four objectives; 1) evaluate the design process for roadway infrastructure that was repaired following Hurricanes Matthew and Florence, 2) identify the specific elements of the new infrastructure that positively contributed to improved performance during Hurricane Florence, and 3) develop recommendations on design elements that improve the resilience of NCDOT roadways. These objectives will be met with five tasks. 1. The relevant literature on resilient infrastructure and practices for ensuring transportation infrastructure resilience to extreme events will be reviewed and documented. 2. Locations where roadway infrastructure failed during Hurricanes Matthew and Florence will be identified, mapped, and compared. 3. The performance of different maintenance, repair, and reconstruction strategies deployed in the aftermath of Hurricane Matthew will be evaluated and quantitatively assessed. 4. A series of detailed case studies will be performed to identify the design factors and repair/maintenance decisions that led to better performance during Hurricane Florence. 5. A final report summarizing the methodology, results, and recommendations will be prepared The primary outcome of the proposed research will be data on the effectiveness of design strategies used to repair infrastructure following hurricanes specifically and extreme events in general. This knowledge can be helpful to improve the design and repair methodologies to be more robust and resilient against future extreme events. The research will also produce a set of guidelines and recommendations for hydraulic design, repair, and reconstruction that may improve the resiliency of roadway design in North Carolina. The guidelines that results from this research will allow NCDOT engineers to deploy design strategies that are proven to be cost effective in the long run. For example, the primary focus of engineers after the event is restoring mobility. For some cases, once this mobility is restored it may be cost effective to redesign or reconstruct a more robust design so that future events do also cause disruptions. This work will provide evidence as to when and how such major repairs can be effective. The proposed work is significant because it will provide quantified evidence as to the efficacy of existing strategies to provide this long-term effectiveness. Ultimately, the deployment of these strategies can reduce agency costs while also improving roadway resilience to extreme events.

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.

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.

The Federal Highway Administration (FHWA) has developed mechanistically based performance comparison models to evaluate the cracking and rutting performance of asphalt pavement mixtures. These models form the basis of an asphalt performance comparison development effort and are being implemented into a FlexPAVETM software program for analyzing pavements and predicting distress. In this research study, NCSU will assess current asphalt pavement cracking models that can be applied to reflective cracking and further research, develop, calibrate, train, and validate a mechanistically based asphalt pavement reflective cracking model that is consistent with existing FlexPAVETM methodology and performance tests; incorporate it into the FlexPAVETM software and the FlexMATTM and FlexMIXTM data analysis tools, and assess and incorporate run time improvements to the model, software, and analysis tools.

In this research study, NCSU will conduct experiments and analysis to improve the AASHTO TP 133 protocol by incorporating more scientifically based temperature selection guide and providing guidance on the maximum air void content for specimens that are subjected to this standard test method. In addition, NCSU will update the FlexPAVETM software to incorporate seasonal effects into the base layer and the user guides for improved usability. This research supports ongoing FHWA efforts as part of a performance-related specification framework which seeks to increase pavement life through fundamental testing and predictive relationships. Recent developments in these performance tests, adoption of standards, FlexMATTM, FlexMIXTM, and FlexPAVETM provide highway agencies and asphalt paving community with a unique opportunity to use performance tests and mechanistic models for asphalt PEMD, asphalt pavement design, and performance related specifications to integrate these different phases in pavement construction using the same test methods and mechanistic principles. These tools help link material characteristics from testing with mechanistic models to predict performance; and ultimately identify how to best design, construct, and accept a pavement. 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 – Draft Final Revised FlexPAVETM Software, Installation, and User Guides – NCSU will update the user interface, installation guide, and user guide for the FlexPAVETM software. Task 5 –Presentation and webinar – NCSU will present their findings to targeted stakeholders. Task 6 – Publication ready final deliverables – the reports will be revised and finalized and AASHTO standards will be revised into a final form.

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.