Technical Reports, Civil and Environmental Engineering
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Browsing Technical Reports, Civil and Environmental Engineering by Department "Virginia Tech Transportation Institute"
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- Composite Pavement Systems: Synthesis of Design and Construction PracticesFlintsch, Gerardo W.; Diefenderfer, Brian K.; Orlando Nunez (Virginia Center for Transportation Innovation and Research, 2008-11-01)Composite pavement systems have shown the potential for becoming a cost-effective pavement alternative for highways with high and heavy traffic volumes, especially in Europe. This study investigated the design and performance of composite pavement structures composed of a flexible layer (top-most layer) over a rigid base. The report compiles (1) a literature review of composite pavement systems in the U.S. and worldwide; (2) an evaluation of the state-of-the-practice in the U.S. obtained using a survey; (3) an investigation of technical aspects of various alternative composite pavement systems designed using available methodologies and mechanistic-empirical pavement distress models (fatigue, rutting, and reflective cracking); and (4) a preliminary life cycle cost analysis (LCCA) to study the feasibility of the most promising composite pavement systems. Composite pavements, when compared to traditional flexible or rigid pavements, have the potential to become a cost-effective alternative because they may provide better levels of performance, both structurally and functionally, than the traditional flexible and rigid pavement designs. Therefore, they can be viable options for high volume traffic corridors. Countries, such as the U.K. and Spain, which have used composite pavement systems in their main road networks, have reported positive experiences in terms of functional and structural performance. Composite pavement structures can provide long-life pavements that offer good serviceability levels and rapid, cost-effective maintenance operations, which are highly desired, especially for high-volume, high-priority corridors. Composite pavements mitigate various structural and functional problems that typical flexible or rigid pavements tend to present, such as hot-mix asphalt (HMA) fatigue cracking, subgrade rutting, portland cement concrete (PCC) erosion, and PCC loss of friction, among others. At the same time, though, composite systems are potentially more prone to other distresses, such as reflective cracking and rutting within the HMA layer. Premium HMA surfaces and/or reflective cracking mitigation techniques may be required to mitigate these potential problems. At the economic level, the results of the deterministic agency-cost LCCA suggest that the use of a composite pavement with a cement-treated base (CTB) results in a cost-effective alternative for a typical interstate traffic scenario. Alternatively, a composite pavement with a continuously reinforced concrete pavement (CRCP) base may become more cost-effective for very high volumes of traffic. Further, in addition to savings in agency cost, road user cost savings could also be important, especially for the HMA over CRCP composite pavement option because it would not require any lengthy rehabilitation actions, as is the case for the typical flexible and rigid pavements.
- Determination of the In-Place Hot-Mix Asphalt Layer Modulus for Rehabilitation Projects Using a Mechanistic-Empirical ProcedureLoulizi, Amara; Flintsch, Gerardo W.; McGhee, Kevin K. (Virginia Center for Transportation Innovation and Research, 2006-07-01)This project evaluated the procedures proposed by the Mechanistic-Empirical Pavement Design Guide (MEPDG) to characterize existing hot-mix asphalt (HMA) layers for rehabilitation purposes. Thirty-three cores were extracted from nine sites in Virginia to measure their dynamic moduli in the lab. Falling-weight deflectometer (FWD) testing was performed at the sites because the backcalculated moduli are needed for the Level 1 procedure. The resilient modulus was also measured in the lab because it is needed for the Level 2 procedure. A visual pavement rating was performed based on pavement condition because it is needed for the Level 3 procedure. The selected cores were tested for their bulk densities (Gmb) using the AASHTO T166 procedure and then for their dynamic modulus in accordance with the AASHTO TP62-03 standard test method. Then the cores were broken down and tested for their maximum theoretical specific gravity (Gmm) using the AASHTO T-209 procedure. Finally an ignition test was performed to find the percentage of binder and to reclaim the aggregate for gradation analysis. Volumetric properties were then calculated and used as input for the Witczak dynamic modulus prediction equations to find what the MEPDG calls the undamaged master curve of the HMA layer. The FWD data, resilient modulus data, and pavement rating were used to find the damaged master curve of the HMA layer as suggested for input Levels 1, 2, and 3, respectively. It was found that the resilient modulus data needed for a Level 2 type of analysis do not represent the entire HMA layer thickness, and therefore it was recommended that this analysis should not be performed by VDOT when implementing the design guide. The use of Level 1 data is recommended because FWD testing appears to be the only procedure investigated that can measure the overall condition of the entire HMA layer.
- Development of a Framework for Evaluating Yellow Timing at Signalized IntersectionsRakha, Hesham A.; El-Shawarby, Ihab; Amer, Ahmed (Virginia Center for Transportation Innovation and Research, 2011-02-01)Studies show that the proper design of clearance intervals has significant implications for intersection safety. For example, in 2001, approximately 218,000 red-light-running crashes occurred at signalized intersections in the United States. These crashes resulted in nearly 181,000 injuries and 880 fatalities and an economic loss of $14 billion. Driver behavior while the driver is approaching high-speed signalized intersections at the onset of a yellow indication varies as a function of many parameters. Some of these parameters are related to the driver's attributes, e.g., age, gender, perception-reaction time, and acceptable deceleration levels. Other parameters that relate to the intersection geometry include the approach speed, distance, and time to the intersection at the onset of the yellow indication. This study developed a novice approach for computing the clearance interval duration that explicitly accounts for the reliability of the design (probability that drivers are not caught in a dilemma zone). Lookup tables based on the limited data available from this study are provided to illustrate how the framework could be used in the design of yellow timings. The approach was developed using data gathered along Virginia's Smart Road test facility for dry and clear weather conditions for two approach speeds: 72.4 km/h (45 mph) and 88.5 km/h (55 mph). Each dataset includes a complete tracking of the vehicle every deci-second within 150 m (500 ft) before and after the intersection. A total of 3,454 stop-run records were gathered. These include 1,727 records (687 running records and 1,040 stopping records) for an approach speed of 45 mph and 1,727 records (625 running records and 1,102 stopping records) for an approach speed of 55 mph. Using these data, models that characterize driver perception-reaction times and deceleration levels were developed. The application of the proposed approach demonstrates that the current design procedures are consistent with a reliability level of 98%.
- Evaluation of the Cargill Safelane Surface OverlaySprinkel, Michael M.; Roosevelt, Daniel S.; Flintsch, Gerardo W.; de León Izeppi, Edgar; Mokarem, David W. (Virginia Center for Transportation Innovation and Research, 2009-02-01)A recent development in polymer concrete overlays is the Cargill SafeLane surface overlay (SafeLane overlay). The 3/8-in-thick overlay is constructed with epoxy and broadcast aggregates, as are typical multiple-layer epoxy overlays that are used to provide a skid-resistant wearing surface for bridge decks that protects the decks again intrusion by chloride ions. Reportedly, the SafeLane overlay is unique in that Cargill indicates that the limestone aggregate used in the overlay can absorb and store liquid deicing chemicals that are applied to the surface of the roadway. The purpose of this research was to compare the SafeLane overlay and the Virginia Department of Transportation (VDOT) modified EP-5 epoxy concrete overlay (hereinafter called the VDOT modified EP-5 overlay) based on an evaluation of their construction, initial condition, and effectiveness in preventing frost, ice, and snow formation on the surface of the roadway. The comparison was limited to overlays placed on four bridges on I-81 in 2004 and 2005 (two SafeLane and two VDOT modified EP-5 overlays) and on four sections of continuously reinforced concrete pavement on the Virginia Smart Road in 2006. The evaluation with respect to the initial condition of the overlays on I-81 was based on a comparison of the as-constructed properties, including aggregate properties, bond strength, permeability, skid resistance, and chloride content. The evaluation with respect to the initial condition of the overlays on the Smart Road was limited to skid resistance. The evaluation of the overlays with respect to their effectiveness in preventing frost, ice, and snow formation was based on visual observations and skid measurements of overlay surfaces under typical interstate winter conditions at the I-81 sites and under artificial snow and ice conditions at the Smart Road. In addition, the effectiveness of the overlays at the Smart Road in preventing frost, ice, and snow formation was compared with that of a bare-tined concrete surface. The evaluation indicated that the SafeLane overlay can provide a skid-resistant wearing and protective surface for bridge decks. The study was not able to determine the performance of the overlay with respect to providing a surface with less accumulation of ice and snow. Further, there has not been sufficient time to evaluate chloride penetration into the decks overlaid with SafeLane overlays in Virginia.
- Evaluation of Traffic Responsive Control on the Reston Parkway Arterial NetworkAbbas, Montasir M.; Abdelaziz, Sherif (Virginia Center for Transportation Innovation and Research, 2009-02-01)Traffic responsive plan selection (TRPS) control is considered an effective operational mode in traffic signal systems. Its efficiency stems from the fact that it can capture variations in traffic patterns and switch timing plans based on existing traffic conditions. Most of the research performed to date has focused on either small traffic networks-with up to five intersections-or theoretical networks. Past research has also focused on the threshold mechanism implemented in the National Electrical Manufacturers Association (NEMA) traffic controllers. There is very limited research on the pattern-matching mechanism implemented in the 170 controllers. This report documents a new approach to generating traffic scenarios for large networks, addressing issues such as the unequal traffic distribution and the large combination of traffic movements from multiple intersections. This approach is based on the selection of significant critical movements controlling the network using statistical correlation analysis of actual detector data and the use of synthetic origin-destination analysis of the entire network. The proposed approach was applied in the design of the traffic responsive control mode for the Reston Parkway arterial network, which has 14 intersections. Detector data were used to validate the results of the proposed approach. The validation process showed that the traffic system was correctly modeled and sufficiently represented by the proposed approach. Multi-objective optimization was used to generate the final timing plans and the TRPS pattern-matching parameters. Simulation analysis revealed that implementation of the traffic responsive control mode in the Reston Parkway network can achieve an average delay reduction of 27 percent and an average stops reduction of 14 percent on weekends and an average delay reduction of 18 percent and an average stops reduction of 21 percent on regular week days. The methodology documented in this report should be followed to implement TRPS control on large arterials in an optimal and stable manner. Optimal and stable operation of TRPS could significantly reduce congestion while capitalizing on existing traffic control infrastructure with a 46:1 benefit-cost ratio.
- Field Investigation of High Performance Pavements in VirginiaFlintsch, Gerardo W.; Al-Qadi, Imad L.; Loulizi, Amara; Lahouar, Samer; McGhee, Kevin K.; Trenton Clark (Virginia Center for Transportation Innovation and Research, 2005-01-01)This study evaluated 18 pavement sections located in high-traffic highways in Virginia to find a premium pavement design with a life span of 40 years or more using current and past field experience. The selected pavement sections were thought to perform well. Eight flexible pavements, six composite pavements, two continuously reinforced concrete pavements, and two jointed plain concrete pavements were investigated. Field testing consisted of (1) falling weight deflectometer (FWD) testing to assess the structural capacity of the different pavements and to backcalculate the pavement layer materials' moduli, (2) ground-penetrating radar (GPR) scanning to determine layer thicknesses and to locate any abnormalities inside the pavements, (3) digital imaging to determine condition indices, (4) longitudinal profile measurements to calculate International Roughness Index, and (5) coring and boring to perform material characterization of pavement layers. Hot mix asphalt tests included resilient modulus and creep compliance. Concrete was tested for compressive strength. The analysis of the collected data suggests that premium pavement designs can be obtained. The field investigations suggest that all the tested sites are performing satisfactorily and show very low structural distress. Limited material-related problems were found at some sites, which induced non-load related distresses. It was also confirmed that FWD, GPR, and digital imaging are very useful tools to assess the condition of existing pavements. Since the three categories of pavements (flexible, composite, and rigid) were found to perform well, the study recommends that evaluation of other pavement sections, which are thought to perform in a less than optimal state, be conducted to define the causes of the less than desired performance. The selection of the most appropriate premium pavement design should be based on a detailed life-cycle cost analysis; hence, such analysis should be performed. Mechanistic empirical modeling of the best performing section within each category would allow the prediction of future pavement performance for use in the life-cycle cost analysis.
- Field Performance of High Friction Surfacesde León Izeppi, Edgar; Flintsch, Gerardo W.; McGhee, Kevin K. (Virginia Center for Transportation Innovation and Research, 2010-06-01)This report describes an evaluation of high friction surface (HFS) systems. The goal of this evaluation was to develop guidance for agencies when considering whether an HFS was an appropriate solution when addressing specific instances of low skid resistance and/or especially high friction demand. HFS systems are specially designed thin surface treatments that provide significant additional skid resistance of pavements and bridge decks without significantly affecting other qualities of the surface such as noise, ride quality, or durability. This report documents the location and climatic conditions where some of these systems are placed, recounts the experiences reported by the agencies that were responsible for their placement, and summarizes key HFS service-level indicators (friction and texture). The agency experiences include a sample benefit-cost analysis from an installation in Wisconsin that justified an HFS application through crash reductions that resulted following the measured increase in skid resistance. Analysis of the service-level indicators included development of the coefficients necessary to obtain the International Friction Index (IFI) values for each of the tested systems. Review of the IFI values suggested that more experiments with different types of wearing surfaces, to include HFS systems as well as more conventional surface treatments, are necessary in order to demonstrate the validity of the speed gradient and friction coefficients recommended by the ASTM standard for the IFI.
- Full-Scale Laboratory Evaluation of Hybrid Composite Beams for Implementation in a Virginia BridgeMoen, Cristopher D.; Roberts-Wollmann, Carin L.; Cousins, Thomas E. (Virginia Transportation Research Council, 2018-08)This research project studied a steel-reinforced concrete and fiber-reinforced polymer (FRP) structural element called the Hybrid-Composite Beam (HCB). The beam was used in a skewed simple span superstructure replacement project over the Tides Mill Stream in Colonial Beach, Virginia. For typical HCB construction, each beam is transported to site as a lightweight FRP beam shell. Self-consolidating concrete is pumped into the shell interior arch form, and when the concrete hardens, it stiffens and strengthens the beam so that it can act as falsework to carry the weight of a cast-in-place concrete bridge deck. Unstressed prestressing strands are embedded in the FRP shell bottom flange during the resin placement, and these strands equilibrate thrust in the arch and stiffen the beam to meet service deflection criteria. After the deck is placed, the HCB system performs as a longitudinal flexural member, with the bridge deck resisting compression and prestressing strands and the FRP bottom flange resisting tension. The primary research goal was to document the HCB as a structural component and as a bridge system, with the outcome being validation of key assumptions that can be applied to future designs such as, for example, strain compatibility between the FRP shell and steel strands. The research was conducted in five phases. In Phase 1, the HCB flexural rigidity and through-depth strain distributions were quantified considering just the FRP shell with unstressed strands. These tests confirmed flexural rigidity estimated by hand calculations and strain compatibility under uniform loads. Phase 2 evaluated flexural behavior of the HCB FRP shell and poured concrete arch. Phase 3 testing was performed after three HCBs were made integral with cast-in-place concrete end diaphragms and a reinforced concrete bridge deck. Point loads, to simulate an HL-93 design truck as specified in American Association of State Highway and Transportation Officials (AASHTO) LFRD Bridge Design Specifications, were applied to the bridge deck to maximize shear, flexure, and torsion in the skewed bridge. Live load distribution between the three girders was approximately equal and the assumption of strain compatibility between the bridge deck, FRP shell, and steel strand was confirmed. Stresses in bottom flange FRP strands and the top of deck concrete were less than 30% of material limits under service level live loads. The concrete arch fell below the composite neutral axis, placing it in tension along the span. After the live load system tests, a more detailed investigation was performed in Phase 4 to explore transverse deck behavior. Transverse flexural demands were approximately 20% of the design capacity and standard truss bars, as specified by the Virginia Department of Transportation, are not necessary because of the small clear span of the slab between beams. In Phase 5, the bridge system was saw-cut longitudinally to separate it into three individual HCB composite beams. Two beams were load tested to failure at the Structures Laboratory at Virginia Tech. For one of the two beams tested at Virginia Tech, 14 out of a total 22 strands were cut at mid-span to simulate strand deterioration and for comparison the other beam remained undamaged prior to testing. The observed beam failure modes were mid-span concrete crushing for the undamaged beam and mid-span strand-FRP bond failure for the damaged beam. In support of Phase 5, a three-dimensional (3D) finite element model was developed to explore flexural and shear force distributions along the span, which led to a shear design procedure in which shear force is distributed based on the relative moments of inertia of the FRP shell and arch. Shear resistance is provided by the FRP shell webs and the concrete arch and fin.
- Ground-Penetrating Radar Calibration at the Virginia Smart Road and Signal Analysis to Improve Prediction of Flexible Pavement Layer ThicknessesAl-Qadi, Imad L.; Lahouar, Samer; Loulizi, Amara (Virginia Center for Transportation Innovation and Research, 2005-01-01)A ground-penetrating radar (GPR) system was used to collect data over the different pavement sections of the Virginia Smart Road from June 1999 until December 2002. Three antennae at different frequencies were used for this research. The collected data were successfully used to evaluate the physical GPR detection limitations, to evaluate the GPR accuracy for pavement layer thickness determination, to control the installation of three different types of reinforcing meshes installed within the pavement, and to estimate the in-situ complex dielectric constant of several types of hot-mix asphalt (HMA). The data analysis results were verified by the well-documented structure and composition of each section of the road, in addition to the embedment of 35 copper plates (perfect electromagnetic reflectors) at the different layer interfaces during construction of the pavement. It was found that GPR is a feasible nondestructive tool to estimate the layer thicknesses of bound and unbound aggregate layers, HMA layers, and concrete slabs. However, interface detection can be altered if the layers have comparable dielectric constants. A technique was developed to estimate the frequency-dependent in-situ complex dielectric constant of HMA materials. Results have shown that the effect of the variations of the dielectric properties within the GPR bandwidth is insignificant vis-a-vis the accuracy of thickness estimation. The use of GPR as a quality control tool to verify the success of steel reinforcing mesh installation was also found to be feasible. Given the success of using GPR for the aforementioned applications in the Virginia Smart Road, it is recommended that the Virginia Department of Transportation use GPR more frequently as a quality control tool during new pavement construction projects and as an assessment tool prior to project rehabilitation and as part of Virginia's pavement management system.
- Implementation of a Precast Inverted T-Beam System in Virginia: Part II: Analytic and Field InvestigationsMenkulasi, Fatmir; Cousins, Thomas E.; Roberts-Wollmann, Carin L. (Virginia Transportation Research Council, 2018-08)The inverted T-beam superstructure is a bridge system that provides an accelerated construction alternative for short-to-medium-span bridges. The system consists of adjacent precast inverted T-beams with a cast-in-place concrete topping. This bridge system is expected to not experience the reflective cracking problems manifested in short-to-medium-span bridges constructed with traditional adjacent voided slab or adjacent box beams. This report presents the results of three phases of a comprehensive research project to develop and implement an inverted T-beam system for Virginia. The three phases are: investigation of time-dependent and temperature effects, investigation of end zone stresses, and live load testing. The first investigation is of time-dependent effects in composite bridges with precast inverted T-beams. The analysis was performed for a two-span continuous bridge. An analytical study was performed to quantify the stresses generated as a result of differential shrinkage, creep and temperature gradient at various sections in both directions. At the cross-sectional level, an elastic sectional analysis approach using the age-adjusted effective modulus method was used to perform the investigation. At the structure level, the effects of uniform temperature changes, thermal gradients and differential shrinkage and creep were investigated and quantified in terms of axial restraint forces and restraint moments. It is shown that, by paying attention to detailing and by selecting a mix design for the cast-in-place topping that has relatively low shrinkage and high creep, the potential for excessive cracking can be reduced. The second investigation is of the stresses in the end zones of such a uniquely shaped precast element. The transfer of prestressing force creates vertical and horizontal tensile stresses in the end zones of the beam. A series of three-dimensional (3D) finite element analyses were performed to investigate the magnitude of these tensile stresses. Various methods of modeling the prestressing force, including the modeling of the transfer length, were examined and the effect of notches at the ends of the precast beams was explored. Existing design methods were evaluated; strut-and-tie models, calibrated to match the results of 3D finite element analyses, are proposed as alternatives to existing methods to aid designers in sizing reinforcing in the end zones. The final section reports the results of live load testing performed on the first inverted T-beam bridge in Virginia on U.S. 360 over the Chickahominy River. A finite element model of Phase I of the U.S. 360 Bridge was created and the live load distribution factors were analytically determined. Live load tests using a stationary truck were performed on Phase I of the U.S. 360 Bridge with the purpose of quantifying live load distribution factors and validating the results from the finite element analyses. It is concluded that it is appropriate to estimate live load distribution factors using AASHTO provisions for cast-in-place slab span bridges.
- Methods for Assessing the Polishing Characteristics of Coarse Aggregates for Use in Pavement Surface LayersWang, Linbing; Druta, Cristian; Lane, D. Stephen (Virginia Center for Transportation Innovation and Research, 2010-06-01)The predominant aggregate resources located in the western parts of Virginia are carbonate rocks. The mineral components of these rocks tend to be relatively soft and subject to abrasive wear under traffic that leads to a fairly rapid smoothing of the aggregate surface and the surface of pavements containing them. This smoothing or polishing leads to a loss of surface friction and thus skid resistance of the pavement, creating potential safety issues. As a consequence, surface courses of pavements are constructed with nonpolishing aggregates, which in the western districts can mean lengthy transport of materials. With rising economic and environmental costs associated with transportation, increasing the use of locally available materials through improved methods to discern subtle differences in polishing tendencies or blending of polishing with nonpolishing aggregates could translate into significant cost savings. In this study, a suite of carbonate aggregates suspected to show a range of polishing characteristics based on previous studies was selected for evaluation along with several sources currently used as nonpolishing aggregates. The aggregates were examined petrographically, and the carbonate sources were tested for insoluble residue and subjected to a dilute acid etch to form the basis for a tentative classification of polishing susceptibility. The aggregates were tested in the Micro-Deval apparatus using both the standard procedure for coarse aggregates and a cyclical A/P procedure with silica sand devised for this study to examine how the various aggregate types responded when subjected to various degrees of wearing under similar conditions. Following abrasion cycles, 2-D digital aggregate images were obtained and analyzed using specialized software to assess their morphological properties with emphasis on angularity and surface texture. These properties were assessed before and after abrasion to determine the degree and rate of change in surface texture as well as the texture distribution within an aggregate sample. In addition, pictures were taken of the exposed surfaces of three experimental pavement sections in the Virginia Department of Transportation's Bristol District containing carbonate rock as the fine aggregate blended with nonpolishing coarse aggregate to visually assess the effect of traffic in the last 3 years. The carbonate rocks evaluated could be grouped according to polishing and wear tendency based on petrographic characteristics and mass loss in the Micro-Deval tests. Although visual distinctions could be discerned, these differences were not identified statistically by the image analysis program, but factors affecting the program were identified. A second phase of work is proposed that focuses on developing an accelerated pavement wearing protocol for assessing polishing tendencies and evaluating various mixtures of carbonate and nonpolishing aggregates.
- Multivariate Volumetric Specifications and Dynamic Modulus as a Quality Measure for Asphalt Concrete MaterialsKaticha, Samer W.; de León Izeppi, Edgar; Flintsch, Gerardo W. (Virginia Center for Transportation Innovation and Research, 2010-06-01)The Virginia Department of Transportation (VDOT) has worked toward end-result specifications (ERSs) in asphalt concrete since the mid-1960s. As stated by Hughes et al. (2007), true ERSs can lead to a reduction in VDOT's overall inspection force resulting in considerable savings and allow for the reallocation of inspection resources to key construction and placement processes that cannot be measured upon delivery (e.g., joint tacking and construction platform preparation). The latest efforts toward this end were conducted by Hughes et al. (2007) who suggested expanding the quality measures for asphalt concrete acceptance to include the asphalt concrete volumetric properties of voids in total mix (VTM) and voids in mineral aggregates (VMA), along with the already used asphalt content (AC) and gradation. This report builds on that and further investigates, through the use of the asphalt concrete dynamic modulus, how performance-related ERSs can be introduced into a quality assurance (QA) plan. Specifically, the report 1) documents the current variability of VTM, VMA, and AC; 2) explores different QA specification plans; and 3) develops and applies a method to predict asphalt concrete rutting performance from asphalt concrete dynamic modulus test results using the mechanistic-empirical pavement design guide (MEPDG). Contractor volumetric test results (for the years 2006 through 2008) for VTM, VMA, and AC were obtained from VDOT's central database for production asphalt concrete. Statistical measures of mean, variance and covariance were calculated. The experimental distribution of test results for each of the three volumetric measures was obtained and compared to the normal (Gaussian) distribution. This research used these data and exploratory analysis to present alternative QA plans, which ranged from a simple univariate plan to a multivariate percent within limits (PWL) plan. The choice of a specific plan to implement depends, among other criteria, on the variable-more specifically on the correlation between these variables-that are included as part of this plan. The PWL method for uncorrelated variables (in this case VTM and AC) is recommended as it presents a sound statistical approach that avoids the complexities that result from incorporating correlated variables. With advances in mechanistic-empirical pavement design methods (specifically the new MEPDG), a framework for performance-related ERSs is now available. The dynamic modulus as a function of temperature and frequency is the main asphalt concrete material input property in the MEPDG. It has significant influence on distress prediction, which makes it a quality candidate test for performance-related ERSs. A principal technical barrier to using the dynamic modulus test is the time required to perform the test temperature sweep. To address this obstacle, this report presents a method to reduce the required number of tests to characterize asphalt concrete rutting characteristics. It demonstrates that a single dynamic modulus test is sufficient to estimate asphalt concrete rutting potential as calculated by the MEPDG. This is an initial step towards using the dynamic modulus in performance-related ERSs. However, actual implementation still depends on broader acceptance and use of the dynamic modulus testing equipment and procedures, as well as the proper calibration of the MEPDG distress models to reflect observed field performance. If and when this is accomplished, the method can be extended to fatigue cracking.
- Transit Signal Priority Project Phase II Field and Simulation Evaluation ResultsRakha, Hesham A.; Ahn, Kyoungho (Virginia Center for Transportation Innovation and Research, 2006-03-01)Transit Signal Priority (TSP) is recognized as an emerging technology that is capable of enhancing traditional transit services. Basic green-extension TSP was implemented on U.S. Route 1 in the Northern Virginia Area (or Washington, DC metropolitan area). This study quantifies the impact of TSP technology on transit-vehicle performance using field-collected Global Positioning System (GPS) data and evaluates the system-wide benefits of TSP operations using computer simulations to expand on the field evaluation study. The field study demonstrated that overall travel-time improvements in the order of 3% to 6% were observed for TSP-operated buses. However, the results also demonstrated that green-extension TSP can increase transit-vehicle travel times by approximately 2.5% during congested morning peak periods. In addition, the study demonstrated that TSP strategies reduce transit-vehicle intersection delay by as much as 23%. The field study demonstrated that the benefits associated with TSP were highly dependent on the roadway level of congestion and were maximized under moderate to low levels of congestion. However, the simulation results indicated that TSP did not result in statistically significant changes in auto or system-wide travel times (differences less than 1%). Furthermore, a paired t-test concluded that basic green-extension TSP did not increase side-street queue lengths. An increase in the traffic demand along Route 1 resulted in increased system-wide detriments; however, these detriments were minimal (less than 1.37%). The study demonstrated that an increase in side-street demand did not result in any statistically significant system-wide detriments. Increasing the frequency of transit vehicles resulted in additional benefits to transit vehicles (savings in transit vehicle travel times by up to 3.42%), but no system-wide benefits were observed. Finally, TSP operations at near-side bus stops (within the detection zone) resulted in increased delays in the range of 2.85%, while TSP operations at mid-block and far-side bus stops resulted in network-wide savings in delay in the range of 1.62%. Consequently, we recommend not implementing TSP in the vicinity of near-side stops that are located within the TSP detection zone. The simulation results indicated that a TSP system generally benefits transit vehicles, but does not guarantee system-wide benefits. In this study, a maximum transit vehicle travel-time savings of 3% to 6% was observed with the provision of green-extension TSP from both the field and simulation evaluation studies. However, the green-extension TSP operation did not benefit nor damage the non-transit vehicles in most cases. Also, it should be noted that the results of the study may be specific to Route 1 corridor because of the unique characteristics of the study corridor, the specific traffic demand, and TSP logic implemented. Finally, the study recommends the calibration of current TSP settings to improve the effectiveness of TSP operation. Also, different transit priority strategies or a combination of other TSP strategies should be investigated to increase the benefits of TSP operations. A conditional TSP system that only provides priority to transit vehicles behind schedule and an intelligent transit monitoring system are also recommended to improve the TSP system on the Route 1 corridor.
- Using high-speed texture measurements to improve the uniformity of hot-mix asphaltMcGhee, Kevin K.; Flintsch, Gerardo W.; de León Izeppi, Edgar (Virginia Center for Transportation Innovation and Research, 2003-05-01)This study introduces Virginia's efforts to apply high-speed texture measurement as a tool to improve the uniformity of hot-mix asphalt (HMA) pavements. Three approaches for detecting and quantifying HMA segregation through measuring pavement surface macrotexture were evaluated: (1) applying the methods proposed in NCHRP Report 441, which build on the ability to predict the expected "non-segregated" macrotexture; (2) using acceptance bands for texture similar to those used for HMA density; and (3) considering the standard deviation of the macrotexture as a measure of construction uniformity. Based on the findings from a series of field tests, the researchers concluded that macrotexture measurement holds great promise as a tool to detect and quantify segregation for quality assurance purposes. None of the available equations for predicting non-segregated macrotexture (the approach in NCHRP Report 441) was found to work for all the construction projects evaluated. Additional information is necessary to establish target macrotexture levels. The acceptance bands approach produced reasonable results in most of the field-verification experiments, but it was significantly influenced by the actual variability within the section. An approach that used target levels of standard deviations was selected for further testing and implementation on a pilot basis.