Dr. Underwood’s research and education program focuses on materials and their interaction with society and the natural and built environments. This program pursues research, education, and service activities that evaluate, inform, and shape the science of these interactions, particularly how material decisions influence the use, function, and impact of civil infrastructure. As he and his students pursue these questions both broadly (by expanding the scope or significance of impact assessment) and deeply (by building the methods and models to better explain the physical behaviors that occur) new findings are integrated into his teaching. He and his students pursue this research in two ways. First, they use experimental mechanics and constitutive models to evaluate and understand the behavior of infrastructure materials, principally asphalt concrete. This knowledge is then exploited to better engineer the materials and/or structures to achieve sustainability and resiliency. Second, they use emerging perspectives and analytical tools to assess the role of social constructs in governing the impact of technological advances on infrastructure. Specific attention is given to pavements and with a focus on resilience due to a recognition that pavement engineering and indeed engineering of infrastructure in the built environment is facing great uncertainty.
- An investigation into the nonlinear rheological behavior of modified asphalt binders using large amplitude oscillatory shear rheology , INTERNATIONAL JOURNAL OF PAVEMENT ENGINEERING (2023)
- Asphalt mixture fatigue damage and failure predictions using the simplified viscoelastic continuum damage (S-VECD) model , INTERNATIONAL JOURNAL OF FATIGUE (2023)
- Balanced Mixture Design Framework for Asphalt Mixtures Based on Index- and Performance-Volumetrics Relationships , TRANSPORTATION RESEARCH RECORD (2023)
- Benchmarking Recycled Binder Blends Using Statistical Analysis: A Case Study of Virginia and North Carolina , TRANSPORTATION RESEARCH RECORD (2023)
- Case Studies of Asphalt Pavement Quality Assurance Specifications, Performance-Related Specifications, and Performance-Based Specifications , TRANSPORTATION RESEARCH RECORD (2023)
- Criterion to Select the Maximum Aggregate Size of Fine-Aggregate Asphalt Matrices , TRANSPORTATION RESEARCH RECORD (2023)
- Early Friction and Texture Evolution After an Asphalt Overlay , TRANSPORTATION RESEARCH RECORD (2023)
- Effect of soil amendments on the compaction characteristics, hydraulic conductivity, and tire sinkage potential of roadside soils , TRANSPORTATION GEOTECHNICS (2023)
- Effect of synthetic fibers on the mechanical performance of asphalt mixture: A review , JOURNAL OF TRAFFIC AND TRANSPORTATION ENGINEERING-ENGLISH EDITION (2023)
- Laboratory Performance Evaluation of Alternative Approaches to Incorporate Recycled Binder Availability into Mixture Design Procedures , TRANSPORTATION RESEARCH RECORD (2023)
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 Stateâ€™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.Â 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.
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.
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 ÃƒÂ¢Ã¢â€šÂ¬Ã…â€œtypical / everydayÃƒÂ¢Ã¢â€šÂ¬Ã‚Â 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: ÃƒÂ¢Ã¢â€šÂ¬Ã‚Â¢ 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. ÃƒÂ¢Ã¢â€šÂ¬Ã‚Â¢ Verify (or refine) the performance thresholds on the basis of mechanistic approach, rather than empirical approach. ÃƒÂ¢Ã¢â€šÂ¬Ã‚Â¢ 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 AASHTOWareÃƒâ€šÃ‚Â® 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.
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 ÃƒÂ¢Ã¢â€šÂ¬Ã¢â‚¬Å“ Develop final research plans and project schedule ÃƒÂ¢Ã¢â€šÂ¬Ã¢â‚¬Å“ The proposed plan will be revised based on feedback from the FHWA. Task 2 ÃƒÂ¢Ã¢â€šÂ¬Ã¢â‚¬Å“ Kickoff meeting ÃƒÂ¢Ã¢â€šÂ¬Ã¢â‚¬Å“ NCSU will meet with the project panel to review the statement of work and work plans. Task 3 ÃƒÂ¢Ã¢â€šÂ¬Ã¢â‚¬Å“ Perform work plan and document efforts ÃƒÂ¢Ã¢â€šÂ¬Ã¢â‚¬Å“ NCSU will carry out the approved work plan by following the appropriate ASTM standard test methods to develop a rugged test method. Task 4 ÃƒÂ¢Ã¢â€šÂ¬Ã¢â‚¬Å“ Ruggedness study presentations and webinar ÃƒÂ¢Ã¢â€šÂ¬Ã¢â‚¬Å“ NCSU will present the findings to targeted stakeholders. Task 5 ÃƒÂ¢Ã¢â€šÂ¬Ã¢â‚¬Å“ Publication ready final deliverables ÃƒÂ¢Ã¢â€šÂ¬Ã¢â‚¬Å“ the AASHTO standards will be revised into a final form. Task 6 ÃƒÂ¢Ã¢â€šÂ¬Ã¢â‚¬Å“ Revise work ILS work plan ÃƒÂ¢Ã¢â€šÂ¬Ã¢â‚¬Å“ NCSU will revise the original work plan from Task 1 based on findings from Tasks 3-5 for conducting the ILS study. Task 7 ÃƒÂ¢Ã¢â€šÂ¬Ã¢â‚¬Å“ Perform work plan and document efforts ÃƒÂ¢Ã¢â€šÂ¬Ã¢â‚¬Å“ 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 ÃƒÂ¢Ã¢â€šÂ¬Ã¢â‚¬Å“ ILS study presentation and webinar ÃƒÂ¢Ã¢â€šÂ¬Ã¢â‚¬Å“ NCSU will present their findings to targeted stakeholders. Task 9 ÃƒÂ¢Ã¢â€šÂ¬Ã¢â‚¬Å“ Publication ready final deliverables ÃƒÂ¢Ã¢â€šÂ¬Ã¢â‚¬Å“ the AASHTO standards revised in Task 5 will be modified to include repeatability and reproducibility statements.
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.
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 StatesÃƒÂ¢Ã¢â€šÂ¬Ã¢â€žÂ¢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 NCDOTÃƒÂ¢Ã¢â€šÂ¬Ã¢â€žÂ¢s procedures for the design of surface mixtures containing RAP and RAS to improve performance and (2) modify the NCDOTÃƒÂ¢Ã¢â€šÂ¬Ã¢â€žÂ¢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 NCDOTÃƒÂ¢Ã¢â€šÂ¬Ã¢â€žÂ¢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.