Benjamin Underwood

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. In recent years the NCDOT has conducted different research efforts to characterize the friction and texture characteristics of North Carolina mixes. Most recently, NCDOT RP 2020-11 quantified the impact of new asphalt overlays on friction and texture values and RP 2022-05 has been evaluating preliminary friction and texture performance models. Both projects have evaluated the effect of mixture compositional factors on the short- and long-term performance of both friction and texture. Friction and texture observations collected in both projects suggest that NCDOT dense-graded mixes have initial low texture values because of the fine gradations that are used in the current mix design specifications. Low macrotexture may contribute to reduced skid resistance values in the field. Revising the existing asphalt mixture categories to solve these problems may result in many practical issues due to contractor practices, familiarity with mixture designs, and unintended consequences to durability. On the other hand, a preliminary evaluation of the surface mixture guidelines in South Carolina and Virginia shows important differences with the NCDOT current practice. Both state DOTs use coarser gradations for dense-graded surface mixtures and have SMA mixes as an option to use in roads with high traffic volumes and high friction demand. Therefore, a research study is needed to identify alternative structural mixture designs that can be specified to ensure adequate friction and texture in North Carolina. The primary outcome of the proposed research will include new or improved asphalt mix design specifications updated to include new mixture categories that could be selected by NCDOT pavement designers in situations that warrant higher surface texture and/or friction. Given the existence of specifications for such mixes in other states, it is likely that the recommendations can have immediate impact. These outcomes can be used by the Traffic Safety and Materials and Test Units of the North Carolina DOT, the products of this research will provide immediate indications as to whether pavement mixture design specifications in adjacent states result in asphalt surface mixtures with improved macrotexture and friction.

Infrastructure resilience has become an important topic for North Carolina. Recent hurricanes and other extreme events have caused more than $450 million in direct damage to the State???s transportation infrastructure and innumerable indirect damage from losses in mobility, additional travel times needed while repairs were made, and other impacts. Though the North Carolina DOT (NCDOT) has been conducting studies to understand vulnerability and risk to its assets, most of these have focused on hydraulic structures and/or pavements that were located on top of, or adjacent to, hydraulic structures. In light of these issues, the proposed research plan will seek to achieve five objectives. 1) Provide a better understanding of the failure pathways and factors contributing to pavement failures during past events. 2) Identify the gaps and critical data linkages that hinder the use of existing NCDOT information to support resilience-based planning with respect to pavements. 3) Develop a framework for identifying and prioritizing road segments as part of resilience-based improvement plans/programs. 4) Develop a design feature selection and repair strategy decision tree that considers specific features, planned needs, sustainability considerations, and possible extreme event stressors at a given pavement site. 5) Identify data gaps and critical data linkages that hinder the use of existing NCDOT information to support this effort and provide recommendations to improve data collection and information to support resiliency efforts. The primary outcome of the proposed research will be a design feature selection and repair strategy decision tree that considers specific features, planned needs, sustainability considerations, and possible stressors at a given pavement site.??Another product will be identification of critical data linkages that hinder the use of existing NCDOT information to support this effort and recommendations to improve data collection and information to support resilience efforts. This research will provide NCDOT personnel with the tools necessary to take a proactive approach to inform pavement resilience project identification and prioritization based upon the as-built and current condition of roadway segments. It will also identify the design and repair options that could be used to improve the pavement performance during and after extreme events such as hurricanes. This research will also improve specifications that are used for design or repair so that pavements are better able to withstand extreme event damage and/or recover more quickly and/or the damage that does occur has a more limited impact on safety and mobility.

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.

The objective of this task order is the continued advancement of the performance specification continuum for BMD+ and mechanistic pavement design. Advancement of efforts will be accomplished by the following tasks: 1. Development of Level 1 BMD+ methods and threshold values for Level 2 BMD index parameters (Sapp and RSI) 2. Performance testing for transfer functions to support BMD+ and advanced pavement design 3. Comparison of BMD and BMD+ testing for mixture performance comparisons and analysis 4. Advance in a collaborative approach for FlexPAVE pavement design methods 5. BMD+ training and parallel field construction projects 6. Interim stakeholders status meeting 7. Create a system of tools including guidelines and sample specifications for agencies to transition from their traditional standard specifications to BMD+ specifications using performance tests and advanced mechanistic pavement design

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.

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.

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 objective of this research is to develop guidance to integrate the performance predictive capabilities of the PASSFlexTM software and its suite of tools (FlexPAVETM version 2.0, FlexMATTM, and FlexMIXTM) within a statistically sound QA system in a PRS framework. The research shall address: (a) the use of the cyclic fatigue Sapp and SSR allowable traffic for rutting (ATR) index test parameters, index thresholds, and acceptance limits in support of performance engineered mixture design (PEMD) approaches and to facilitate further implementation of the tests and performance predictions, (b) material selection and mixture design changes that can impact the test results (cyclic fatigue, SSR, and their index parameters) and trends associated with owner agency specified performance thresholds, and (c) the major elements of a QA system (per TRB E-Circular 235, Glossary of Transportation Construction Quality Assurance Terms (http://onlinepubs.trb.org/onlinepubs/circulars/ec235.pdf) and associated buyer/seller and payment risks.

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.