National Cooperative Highway Research Program Research Articles

Crash performance evaluation of a new movable median guardrail on highways

In this study, a new movable median guardrail is proposed to overcome the structural defects of and to improve the low anti-collision performance of two conventional central/ median guardrails comprising of the concrete and W-beam. The design parameters of this movable median guardrail are optimized based on the uniform design method and a linear weighted combination method in accordance with the requirements given in the National Cooperative Highway Research Program (NCHRP) Report 350. This proposed guardrail is systematically evaluated in terms of its energy absorbing capacity, vehicular acceleration, post-impact trajectory of the impacting vehicle, and behavior of the guardrail upon impact through a three dimensional computer simulation using LS-DYNA code. In the evaluation process, performance comparisons of the moveable median guardrail are made with the concrete and W-beam guardrails. The results shows that the performance of the proposed movable median guardrail is superior as compared to the conventional concrete and W-beam guardrails as it offers: a higher level of crash performance is exhibited, the occupants of the impacting vehicle is better protected, and the impacting vehicle exhibits better post collision stability.

The effect of axle load spectra from AASHTO method on flexible pavement performance

Currently, methods used for pavement design are empirical and based on experiments conducted in the 1960s. Due to the number of variables that can influence an empirical analysis, the National Cooperative Highway Research Program (NCHRP) proposed a mechanistic-empirical method. Based on this method, the MEPDG (Mechanistic-Empirical Pavement Design Guide) software was developed and later, its improved version, the AASHTO Ware Pavement ME Design. In both software versions, default values were developed from traffic data of the United States. However, characteristics of the Brazilian traffic are different from characteristics in the United States. Therefore, this study aimed to verify the effect on national pavement performance due to the use of the default axle load spectra that are available on both software versions. Thus, computer simulations were performed using Brazilian axle load spectra, based on data collected at the Imigrantes Highway in Sao Paulo, and using the default load spectra of the software. The results show that the Brazilian axle load spectra studied damages more the national pavement than the default spectra of the software. Therefore, it is not recommended the use of default spectra for designing Brazilian highways similar to the one studied, because this would lead to under-designed structures.

Approaches for local calibration of mechanistic-empirical pavement design guide joint faulting model: a case study of Ontario

ABSTRACT The Mechanistic-Empirical Pavement Design Guide (MEPDG) has been employed by agencies as an innovative method for pavement design since the National Cooperative Highway Research Program (NCHRP) Project 1-37A was implemented in 2004. Over the years, the MEPDG has evolved into the AASHTOWare Pavement ME Design software (AASHTOWare®). Local calibration of the performance models in the AASHTOWare® is a crucial and challenging task to improve the effectiveness of its application. The accuracy of the calibration depends on efficient methods and validation processes. This paper aims at developing local calibration methods for joint faulting prediction model of Jointed Plain Concrete Pavement (JPCP). This study not only focuses on improving the prediction accuracy of the joint faulting model but also demonstrating the various optimisation procedures in detail. A total of 27 representative JPCP sections were used in the processes of calibration. Three optimisation approaches were used: (1) One-At-a-Time (OAT) through the trial-and-error procedure, (2) generalised reduced gradient (GRG) using MS Excel® Solver, and (3) Levenberg-Marquardt Algorithm (LMA) fitting the functions. The prediction accuracy of local models was improved as compared with the global ones. Average Bias (AB) reduced from 0.3083 to 0.0578, and Standard Error of the Estimate (SEE) reduced from 0.3345 to 0.1912. Among the three local calibration approaches, approach 2 and approach 3 had more significant improvement on results than approach 1. Finally, the integral procedures were provided for local calibration in Ontario.

Enhanced MASH TL-3 Short-Radius System: Promising Design to Accommodate Flat Terrain and Ditches

A challenging guardrail installation situation presents itself when two roadways intersect. Combining the guardrails from intersecting roadway results in what is commonly known as a short radius or T-intersection. It is difficult if not physically impossible to provide the required tensile capacity to the geometrical constraints of the curved section. Researchers and practitioners in the roadside safety area have been investigating the short-radius issue for many years. Investigators conducted numerous crash tests for different short-radius guardrail designs, yet none of those designs passed the National Cooperative Highway Research Program (NCHRP) Report 350 Test Level 3 (TL-3) criteria. In 2009, the crash testing guidelines were updated in the Manual for Assessing Safety Hardware (MASH). MASH guidelines increased the impact severity for TL-3 tests over those in NCHRP 350. This paper presents a MASH TL-3 short-radius design that was successfully crash tested for both a flat terrain and a 3H:1V sloped terrain behind the installation. The impact conditions adopted from the MASH terminal/crash cushion matrix were MASH 3-33, 3-32, 3-31, and 3-35 for the flat terrain. Additionally, a slightly modified design that was installed in front of a 3H:1V slope was successfully evaluated using MASH 3-33 and 3-32 test conditions. These tests used a 25° impact angle since it was shown to be more critical for installation during simulation of the system.

Performance measures on two-lane highways: Survey of practice

Two-lane highways constitute a significant component of the highway system and serve an essential function for the movement of people and goods. The Highway Capacity Manual is primarily used for operational analysis on two lane highways in the U.S. and Canada. As part of a project sponsored by the National Cooperative Highway Research Program for improving two-lane highway operational analysis methodology, a practice survey was conducted to gather information on agency experiences in regards to performance measurement on two-lane highways. The survey was sent to all state Departments of Transportation in the United States and Canada. A total of 35 usable responses were received, representing 25 states and 4 Canadian provinces. This paper summarizes the results of the agency survey and presents the most important findings. Results suggest that the top three criteria for good performance measures are: sensitivity to traffic conditions, sensitivity to road conditions, and relevance to road user perception. Further, agencies identified average travel speed as the most relevant traffic flow aspect to two-lane highway operations. Other performance measures that were found meritorious were volume-to-capacity ratio and flow rate, for class I and class II highways, respectively, versus the latter measures and percent-time-spent-following for class III highways.

USING ANALYTICAL MODELS TO CALIBRATE A DUAL-ROUNDABOUT INTERSECTION IN MICROSIMULATION

Calibration of roundabout microsimulation models is a challenging task that often relies on various field-related data that might not be always available. The calibration process of dual roundabouts might be even more demanding due to their rare presence in the field and literature. This paper proposes a straightforward but novel approach toward the calibration of dual roundabouts. The proposed method heavily relies on suggestions provided in the National Cooperative Highway Research Program (NCHRP) report 572 and easily-available information from the field (such as geometry and volume data). NCHRP is utilized to assess the key calibration parameters and corresponding capacity of a dual-roundabout model. The model validation is based on matching the capacity produced by the simulation model with the estimated capacity as it is proposed in NCHRP report. The numerical results of a single lane dual roundabout case study reveal the close match between capacities obtained from microsimulation tool (VISSIM) and NCHPR report. Moreover, the proposed approach can benefit traffic practitioners and planners dealing with the microsimulation models of roundabouts and limited data sets.

Organizational Best Practices for Transportation Projects Delivered Using Alternative Contracting Methods

The use of Alternative Contracting Methods (ACM) to deliver US transportation projects has reached a point where a definitive set of best practices can be identified to leverage the lessons learned by early ACM adopters. The most pressing need is for guidance on how public agencies organize to implement ACMs in a budget-constrained environment where the possibility of increasing the number of public agency engineers is nil. This paper is based on mining the survey response data from 6 National Cooperative Highway Research Program (NCHRP) synthesis reports on ACM topics and proposes a framework for analyzing ACM practices deemed effective by peer-reviewed research to determine if each practice can be classified as a best practice. Importance index theory provides the analytical foundation for the framework and provides a ranking of candidate best practices in order of each practice’s importance and effectiveness. Nine effective ACM practices were identified and evaluated with only one, “appointing an agency ACM champion,” meeting the objective criteria for a best practice. The paper’s major contribution is to provide the suite of 1 best and 8 effective practices that can be employed when developing the organization for an agency that has decided to implement ACM project delivery.Keywords: Alternative contracting methods, best practices, organizational structure, index number theory.

Selection and preliminary evaluation of laboratory cracking tests for routine asphalt mix designs

Cracking has become a primary mode of distress in recent years that frequently drives the need for rehabilitation of asphalt pavements. Meanwhile, asphalt mix designs are becoming more and more complex with the increasing uses of recycled materials, recycling agents, binder additives/modifiers, and multiple warm mix asphalt technologies. Thus, there is an urgent need to identify reliable cracking tests that can be used for routine mix design to eliminate brittle mixes. This paper critically reviewed cracking mechanisms and laboratory tests. A total of 12 cracking tests were discussed at a cracking test workshop held as part of the National Cooperative Highway Research Program Project 9-57. Seven cracking tests were selected for further laboratory evaluation and field validation. Four of the simpler cracking tests from the seven were evaluated in this paper, these being the Texas Overlay Test (OT), Disk-shaped Compact Tension (DCT) test, Semi-Circular Bend test from the Louisiana Transportation Research Center (SCB-LTRC), and SCB test at room temperature from Illinois (SCB-IL). A laboratory sensitivity study was performed, and the results showed that all four cracking tests were generally sensitive to asphalt mix components. However, there were some concerns with the DCT, SCB-LTRC, and SCB-IL. Both the DCT and SCB-IL were found to be not sensitive to asphalt binder content; and both the DCT and SCB-LTRC showed an unexpected increase in cracking resistance when adding RAS to the mix. Additionally, two sets of field test sections were used for preliminary validation of these four cracking tests. It was found that the OT, DCT, and SCB-IL provided rankings which matched the measured field performance for the two sections on US62, Texas; and the OT and SCB-LTRC were valid for six APT test sections. Further validation with different mixes, traffic, and climate is needed.

The implications of increasing temperature due to climate change for asphalt concrete performance and pavement design

The dynamic moduli have been recently introduced and effectively used to express the intrinsic behaviour of the viscoelasticity of an asphalt concrete material across a given range of temperatures and frequencies. Those can generally be represented in a form of the master curve. This study investigated the dynamic moduli of Western Australian (WA) asphalt mixes. In addition, the study modified the dynamic modulus predictive equation of the national cooperative highway research program (NCHRP) project 1-37A to suit WA conditions. Furthermore, in this study, the effect of increased temperatures due to climate change upon flexible road pavement was investigated through the pavement design exercise using WA as the study area. To determine the effective dynamic modulus (|E*|eff), temperature and historical climate data were sourced from a Perth Airport Station to conduct a pavement design exercise. This data covered a 20-year period, a typical asphalt pavement design life. The allowable number of cycles (of traffic) to failure (Nf) under four different pavement design scenarios was then calculated based on this |E*|eff to examine of an increase in temperature potentially resulting from climate change effects. The results of the examination showed that once an increase in temperature due to climate change was taken into account, Nf could be decreased by approximately six million repetitions. This equals a shorter pavement life or design period of around four years compared to a result of the original design, which does not consider the increase in temperature resulting from climate change.

Crashworthiness analysis of a bridge rail-to-guardrail transition

ABSTRACTIn this study, a bridge rail-to-guardrail transition is analysed to fully evaluate its crashworthiness. A non-linear, large-deformation finite-element simulation programme LS-DYNA is used for the analysis. Previous studies have predicted that the transition was able to successfully contain and redirect a 2000 kg pickup truck under National Cooperative Highway Research Program (NCHRP) Report 350 test level 3 conditions. In this study, the transition was subjected to an 8000 kg single-unit truck impact according to NCHRP Report 350 test level 4 conditions. Both the model of the 8000 kg truck and the transition structure used in this study were acquired from previous successful studies. Results of the simulation study showed that the transition structure was found to be adequate in containing and redirecting an 8000 kg truck. It was also determined that the post-impact trajectory of the truck, occupant risk values, article deflections and exit conditions were very similar to those obtained from a similar transition structure previously crash-tested under test level 4 conditions. It was concluded that the finite-element model of the vertical flared-back bridge rail-to-guardrail transition is fairly accurate and can be used in further studies, such as the MASH test level 4 crash simulation with confidence.

LFRD Calibration of Pullout Resistance Factors Using Survival Analysis for Soil Nails in North Texas

AbstractReliability-based analysis of the calibration of pullout resistance factors (φPO) for soil nails are presented in this paper. To conduct the calibration, a total of 47 verification tests conducted in cohesive soil within the Lyndon B. Johnson (LBJ) Express construction project in North Dallas Texas were collected. Three failure criteria, i.e., creep test data, test curve evaluation, and load-elongation curve were selected to estimate the nominal bond resistance between soil and grout. As a result, 25 tests met at least one of the failure criteria. In addition, 45 existing test results from the National Cooperative Highway Research Program (NCHRP) Report 701 in cohesive soil were incorporated to the test database and resulted in a total of 92 soil nail tests including 22 nonfailed tests. These 92 tests were separated into three databases for analysis and calibration purposes. Statistical analysis was conducted on these databases to evaluate the predicted (Rp) to measured (Rm) nominal bond resistanc...

Investigation causes of pavement structure destruction using new AASHTO mechanistic-empirical procedures for improving roads function in different climatic condition of Iran

In recent years, procedure of AASHTO (American Association State Highway and Transportation Officials) Guide for Design of Pavement Structures distanced from first empirical procedure and advanced toward mechanistic-empirical procedures. “Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures” in 2004 and its attached software M-EPDG is the result of this new procedure that AASHTO presented it through projects NCHRP 1-37 A and NCHRP 1-40 B with cooperation of NCHRP (National Cooperative Highway Research Program) and FHWA (Federal Highway Administration) institutions. In this paper, required data for software analyzing of three real pavement structures pieces collected from three different climatic areas and pavement structures modeled in software by entering data into software. Modeled sections by this software were analyzed destructions, and, regarding to obtained results, common designing pavement structures procedures compared with the new way of AASHTO, and efficiency rate of related software investigated in three different climatic areas of Iran. Toward this process, Save, Hamadan and Qazvin, Boin Zahra cities selected as a case study and then studied. Results of software analyzing showed that the designs of old AASHTO method in tempered climate of country met all criteria of designing but in both of cold and warm areas, some destructions at designed pavement structures via this method exceeded from allowed rate and according to presented destructions, will be excessed more in future. This destruction in case project of cold area was longitude crack and was furrowing of pavement structures bed in warm area. Then, probable causes of mentioned destructions studied in pavement structures projects and procedures designed for rehabilitating pavement structures for met all of the designated criteria. Also, in Iran some suggestions indicated about required conductions for application of new method of AASHTO.

Investigation causes of pavement structure failure using new AASHTO mechanistic-empirical procedures for optimization roads performance in different climatic condition of Iran

Abstract
 In recent years, procedure of AASHTO (American Association State Highway and Transportation Officials) Guide for Design of Pavement Structures distanced from first empirical procedure and advanced toward mechanistic-empirical procedures. “Guide for Mechanistic-Empirical Design of New and Rehabilitated Pavement Structures” in 2004 and its attached software M-EPDG is the result of this new procedure that AASHTO presented it through projects NCHRP 1-37 A and NCHRP 1-40 B with cooperation of NCHRP (National Cooperative Highway Research Program) and FHWA (Federal Highway Administration) institutions. In this paper, requireddata for software analyzing of three real pavement structures pieces collected from three different climatic areas and pavement structures modeled in software by entering data into software. Modeled sections by this software were analyzed failure, and, regarding to obtained results, common designing pavement structures procedures compared with the new way of AASHTO, and efficiency rate of related software investigated in two different climatic zone of Iran. Toward this process, Save- Hamadan and Qazvin- Boin Zahra cities selected as a case study and then studied. Results of software analyzing showed that the designs of old AASHTO method in tempered climate of country met all criteria of designing but in both of cold and warm areas, some failure at designed pavement structures via this method exceeded from allowed rate and according to presented failure, will be excessed more in future. This destruction in case project of cold area was longitude crack and was rutting of pavement structures subgrade in warm area. Then, probable causes of mentioned failures studied in pavement structures projects and procedures designed for rehabilitation pavement structures for met all of the designated criteria. Also, in Iran some suggestions indicated about required conductions for application of new method of AASHTO.

Variability of Composition, Volumetric, and Mechanic Properties of Hot Mix Asphalt for Quality Assurance

AbstractAs hot mix asphalt (HMA) is the major paving material worldwide, how the quality of this material is assured is a critical issue. The current quality assurance (QA) practices include testing programs that yield inevitable variability as a result of, e.g., different operators, equipment, and methods. So far, very little research has focused on the variance and testing variability of HMA properties with respect to the material types and climatic conditions that are typical of Alaska. This study presents research to evaluate the variance in composition, volumetric, and mechanical properties between plant-produced and lab-designed mixtures of Alaska HMA, which was produced with various production/compaction scenarios and tested by different operating parties. The variability level of each tested property was quantified, compared with nationwide levels, and tested for influencing factors. This study further evaluated the method proposed in the National Cooperative Highway Research Program (NCHRP) 9-22 ...

Road side safety hardware: Current status and future outlook

As an integral part of engineering safer roads, road side safety devices passively interact with errant vehicles to redirect them safely back to the road or bring them to a safe and controlled stop. These devices take the form of crash cushions, cable barriers, concrete barriers, steel barriers, guard rails, guardrail terminals and others. Placement criteria and warrants are established in the AASHTO Road Side Design Guide (1). However, before those devices are placed on the roadways, they have to be evaluated under objective test conditions.. Given that possible combinations of impact speeds, impact angles, vehicle characteristics and roadway characteristics are infinite, it is impossible to design roadside safety hardware for all those combination. Thus a “Practical Worse Case” philosophy derived from crash data analyses is followed to determine such impact conditions. In the USA, the evaluation methodologies are established in the National Cooperative Highway Research Program (NCHRP) Report 350 (2) which is now superseded by Manual for Assessing Safety Hardware (MASH) guidelines (3). In Europe a comparable set of guideline (EN 1317) is used to evaluate road safety devices under vehicular impact. Many roadway authorities around the world adopt either EN 1317 or NCHRP Report 350 with some modifications to incorporate local requirements. In the early parts of this presentation demonstrate testing conditions and criteria for quantifying the device as crashworthy under NCHRP Report 350 and MASH for some of the common road safety devices. The most common test conditions are explained in details. The key criteria of acceptance shall be presented for the most common test conditions. State of the art nonlinear finite element methodology has been gaining tractions in designing and enhancing the safety of vehicles and roadside devices. This presentation will highlight roadside safety hardware (4, 5, and 6) that was designed through extensive simulation process and had subsequent successful crash test per the latest MASH guidelines. REFERENCES 1. America Association of State Highway and Transportation Officials , Roadside Design Guide, 4th Edition, AASHTO, 2011. 2. Ross, H.E., D.L. Sicking, R.A. Zimmer, and J.D. Michie, Recommended Procedures for the Evaluation of Highway Features, NCHRP Report No. 350, 1993, Transportation Research Board: Washington, D.C. 3. AASHTO, Manual for Assessing Safety Hardware, 2009, American Association of State Highway and Transportation Officials: Washington, D.C. 4. Abu-Odeh, K.M. McCaskey, R.P. Bligh, W.L. Menges, D.L. Kuhn. Crash Test and MASH TL-3 Evaluation of the TxDOT Short Radius Guardrail. Test Report. 0-6711-1. Project No. 0-6711. Texas A&M Transportation Institute, College Station, TX. March 2015 5. A. Abu-Odeh, R.P. Bligh, M.L. Mason, W.L. Menges. Development and Evaluation of a MASH TL-3 31-Inch W-Beam Median Barrier. 9-1002-12-8. Texas A&M Transportation Institute, College Station, TX. January 2014. 6. A. Abu-Odeh, W.F. Williams, M. Ferdous, M.T. Spencer, R.P. Bligh, W.L. Menges. Safety and Integrity of Median Barrier-Mounted Hardware. 0-6646-S. Texas A&M Transportation Institute, College Station, TX. 2013.

Injury Outcome in Crashes with Guardrail End Terminals

Objective: The goal of this study is to evaluate the crash performance of guardrail end terminals in real-world crashes. Guardrail end terminals are installed at the ends of guardrail systems to prevent the rail from spearing through the car in an end-on collision. Recently, there has been a great deal of controversy as to the safety of certain widely used end terminal designs, partly because there is surprisingly little real-world crash data for end terminals. Most existing studies of end terminal crashes used data from prior to the mid-1990s. Since then, there have been large improvements to vehicle crashworthiness and seat belt usage rates, as well as new roadside safety hardware compliant with National Cooperative Highway Research Program (NCHRP) Report 350, “Recommended Procedures for the Safety Performance Evaluation of Highway Features.” Additionally, most existing studies of injury in end terminal crashes do not account for factors such as the occurrence of rollover. This analysis uses more recent crash data that represent post-1990s vehicle fleet changes and account for a number of factors that may affect driver injury outcome and rollover occurrence.Methods: Passenger vehicle crashes coded as involving guardrail end terminals were identified in the set of police-reported crashes in Michigan in 2011 and 2012. End terminal performance was expected to be a function of end terminal system design. State crash databases generally do not identify specific end terminal systems. In this study, the coded crash location was used to obtain photographs of the crash site prior to the crash from Google Street View. These site photographs were manually inspected to identify the particular end terminal system involved in the crash. Multiple logistic regression was used to test for significant differences in the odds of driver injury and rollover between different terminal types while accounting for other factors.Results: A total of 1,001 end terminal crashes from the 2011–2012 Michigan State crash data were manually inspected to identify the terminal that had been struck. Four hundred fifty-one crashes were found to be suitable for analysis. Serious to fatal driver injury occurred in 3.8% of end terminal crashes, moderate to fatal driver injury occurred in 11.8%, and 72.3% involved property damage only. No significant difference in moderate to fatal driver injury odds was observed between NCHRP 350 compliant end terminals and noncompliant terminals. Car drivers showed odds of moderate to fatal injury 3.6 times greater than LTV drivers in end terminal crashes. Rollover occurrence was not significantly associated with end terminal type.Conclusions: Car drivers have greater potential for injury in end terminal crashes than light truck/van/sport utility vehicle drivers. End terminal designs compliant with NCHRP 350 did not appear to carry different odds of moderate driver injury than noncompliant end terminals. The findings account for driver seat belt use, rollover occurrence, terminal orientation (leading/trailing), control loss, and the number of impact events. Rollover and nonuse of seat belts carried much larger increases in injury potential than end terminal type. Rollover did not appear to be associated with NCHRP 350 compliance.

Improving the Selection of Virgin Binders for Recycled Hot Mixtures in Ontario

The design of Recycled Hot Mixtures (RHM) in the United States is based on the guidelines provided by the National Cooperative Highway Research Program (NCHRP) Report 452. However, the extreme climatic characteristics in Canada require a validation on the selection of the virgin binders especially for Superpave Surface layer mixtures. This paper shows the results of a laboratory characterization of twelve conventional Superpave 12.5 mm mixtures, four of them with high Reclaimed Asphalt Pavement (RAP) content. Thermal Stress Restrained Specimen Test (TSRST) was conducted on triplicate samples for each mixture, and also the continuous grades for the binders were obtained. The findings were compared with the recent guidelines provided by the NCHRP Report 752. Dynamic modulus was performed and used to determine the critical temperatures of the blended binder without extraction. The analyses suggest that the virgin binder could be selected based on the critical temperature of the climate zone and the RAP content, and that the use of softer binders could not guaranty that the mixtures have the desired performance to low temperature cracking.

The effects of recycled clay brick content on the engineering properties, weathering durability, and resilient modulus of recycled concrete aggregate

This paper presents a laboratory investigation of recycled clay bricks (RCB) from demolished building when mixed with recycled concrete aggregates (RCA) as an unbound base course in road construction. The hydraulic properties (saturated and unsaturated) of 100% RCB, 30% RCB, 15% RCB, 5% RCB and 100% RCA were determined at the optimum water content and 95% of dry density using rigid-wall hydraulic conductivity test and multi-step outflow, respectively. The drying path of soil–water characteristic curves (SWCCs) was measured using hanging column tests. They are used for the estimation of unsaturated hydraulic conductivities and pores size distribution (pores diameter occupied by free water). Results show that the drainage of water is faster when the percentage of RCB increases due to larger pores. The effects of weathering (wet and dry cycles W–D, freeze and thaw cycles F–T) and abrasion (Micro-Deval and Los Angeles tests) were performed to see the stiffness changes with the number of cycles and the effects of abrasion on particle degradation of the specimens. The Summary Resilient Moduli (SRM) were calculated using the Power function model developed by Moosazedh and Witczak and the National Cooperative Highway Research Program (NCHRP) model under 1-28A procedure. The trend shows that the SRM and the constrained modulus decrease when the percentage of brick and the number of F–T cycles increase. Micro-Deval (MDE) and Los Angeles (LA) coefficients increase with the percentage of RCB on the specimens because they have low particle density and great porosity compared to natural aggregates. The W–D cycle results show that the durability of the material is affected by the number of W–D cycles and an increase in the percentage of fine particle is observed. The optimum moisture content of the specimens combined with the variation of temperature may influence the characteristics of the unbound road base pavement made out of recycled aggregates by the action of wet–dry or freeze–thaw cycling. The durability of a pavement can be influenced by repeated freeze–thaw cycles, wet–dry cycles, or a combination of both. During the thawing period, the melting of ice on the specimens lead to an unsaturated material, inducing an important bearing capacity loss.

A Study on Lane Operation Restriction (LOR) Policy at On-ramp Junctions

On-ramp junctions on freeways are subject to vehicle merging interactions between main travel lanes and ramp flows. The turbulence created by these interactions often causes congestion, and can lead to breakdown when the combined volume from the ramp and the mainline exceed the capacity of the downstream freeway segment. This study discusses a potential application for freeway control policy at the ramp junction – the Lane Operation Restriction (LOR) policy.This policy would provide particular attention to traffic operation improvements at on-ramp junctions with relatively high flow rates from the ramp. Principles of LOR policy would be based on empirical data and analysis findings. The empirical data are analyzed upstream and downstream of on-ramp junctions on six-lane freeways using a database for the National Cooperative Highway Research Program (NCHRP) 3-37. It was observed that lanes 2 (middle lane) and 3 (median side lane) show a higher density-speed relationship than the shoulder lane upstream of the junctions. In addition, the breakdown condition and congestion spread across all freeway travel lanes when the flow of lane 1 (shoulder side lane) is suddenly increased to near capacity at the ramp influence areas. A simulation analysis employing the LOR policy is introduced evaluating the ramp junction influence areas, its downstream and upstream areas. It concludes that traffic operations in the vicinity of on-ramp junctions can be significantly improved when upstream vehicles are guided to shift from lane 1 to lanes 2 and 3 before arriving at on-ramp junctions and restraint lane changes at the ramp influence area.

Stochastic Analysis of Thermal Cracking Distress of Asphalt Pavements Using the MEPDG Thermal Cracking Model

The objective of the National Cooperative Highway Research Program (NCHRP) 9-22 project was to develop a Quality Assurance (QA) system for asphalt pavements in conjunction with Performance Related Specifications (PRS). The project was originally intended to use the Mechanistic-Empirical Pavement Design Guide (MEPDG) as the performance prediction simulator. However, since the probabilistic assessment of the MEPDG required hundreds, if not thousands, of solution simulations; the computational time was found to be excessive as a quicker solution was critically necessary. The study presented in this paper has been conducted as part of the effort associated with the development of a rapid thermal cracking (TC) prediction methodology and the estimation of the variance of TC. The Rosenblueth method and the Beta frequency distribution were used to estimate the first and second moments (average and standard deviation) of TC and its service life. The proposed TC stochastic framework showed a successful performance and was accordingly incorporated into the PRS QA system developed under NCHRP 9-22. The ultimate purpose of the developed QA system is to stochastically evaluate the incentive/disincentive for the contractors.