Bell R.,ESC Inc.
Advances in Systems Safety - Proceedings of the 19th Safety-Critical Systems Symposium, SSS 2011 | Year: 2011
Over the past twenty-five years there have been a number of initiatives worldwide to develop guidelines and standards to enable the safe exploitation of programmable electronic systems used for safety applications. In the context of industrial applications (to distinguish from aerospace and military applications) a major initiative has been focused on IEC 61508, and other standards based on IEC 61508, which have emerged as key international standards. This paper considers some of the key features of IEC 61508 (IEC 2000), which has now been available for over ten years, and indicates the main changes that have been incorporated into the new Edition 2 (IEC 2010a), published in April 2010. © 2011 Springer-Verlag London Limited. Source
Thyagarajan S.,ESC Inc. |
Muhunthan B.,Washington State University |
Sivaneswaran N.,U.S. Federal Highway Administration |
Petros K.,U.S. Federal Highway Administration
Journal of Transportation Engineering | Year: 2011
Many sources of uncertainty are inherent in pavement design. These uncertainties must be incorporated systematically in a reliability analysis to compute their combined effects on the probability of failure of a given pavement structure. Monte Carlo simulation has been the technique of choice in the past to simulate the effects of uncertainties in input parameters on pavement distress and the resultant reliability analyses. The impractical computational time associated with a Monte Carlo scheme, however, has prompted the deferral of the implementation of similar techniques in the current Mechanistic-Empirical Pavement Design Guide (MEPDG). Instead, the reliability analysis implemented in the current MEPDG is performed on the basis of a simple assessment of the overall standard error of the predicted distress compared to the observed distress of the long-term pavement performance (LTPP) sections. It relies on a set of predetermined variability values obtained from a performance database instead of the site-specific input parameters that induce such uncertainty in distress predictions. Past efforts found that techniques (such as the Latin hypercube method) that require a substantially reduced number of simulations compared with Monte Carlo accuracy still suffered from the need for repeated execution of the MEDPG calculations. This study proposes to combine an efficient numerical scheme to conduct statistical simulations with the MEPDG calculations. It makes use of the concept of the representative linear elastic (LE) structure to minimize the number of repeated executions involved in simulations. The numerical scheme can be combined with any simulation technique of random variables to perform a reliability analysis of flexible pavements. The relative merits of Monte Carlo simulation, Latin hypercube simulation, and Rosenblueth's 2K+1 point-estimate method are compared. The simulations show that the Latin hypercube method is an efficient alternative to the computationally intensive Monte Carlo technique. On the other hand, although Rosenblueth's 2K+1 point-estimate method is much simpler, it is not capable of capturing the important attributes of the distribution of either input or output variables. © 2011 American Society of Civil Engineers. Source
Li X.-J.,ESC Inc. |
Marasteanu M.O.,University of Minnesota
Experimental Mechanics | Year: 2010
This work presents a repeatable semi circular bending (SCB) fracture test to evaluate the low temperature fracture resistance of asphalt mixture. The fracture resistance of six asphalt mixtures, which represent a combination of factors such as binder type, binder modifier, aggregate type, and air voids, and two testing conditions of loading rate and initial notch length, was evaluated by performing SCB fracture tests at three low temperatures. Fracture energy was calculated from the experimental data. Experimental results indicated strong dependence of the low temperature fracture resistance on the test temperature. Experimental plots and low coefficient of variation (COV) values from three replicates show a satisfactory repeatability from the test. The results of the analysis showed that fracture resistance of asphalt mixtures is significantly affected by type of aggregate and air void content. Experimental results also confirmed the significance of binder grade and modifier type with relation to cracking resistance of asphalt mixtures. Analysis of result also indicated that both the loading rate and initial notch length had significant effect on the fracture energy at the highest test temperature, whereas the effect was strongly diluted at the two lower temperatures. No clear trend was found with the fracture peak load from either the effect of loading rate or notch length. © 2009 Society for Experimental Mechanics. Source
Nicks J.E.,FHWA |
Adams M.T.,FHWA |
Runion M.,ESC Inc.
Geotechnical Special Publication | Year: 2015
During road realignment activities in 2012 on the I-495 Washington Capital Beltway, the Virginia Department of Transportation (VDOT) demolished a MSE wall originally built in 1976. The wall supported traffic on the interstate bridge ramp and was therefore salted each winter as part of VDOT's standard winter maintenance program; the wall did not have any observable performance issues. Demolition provided an opportunity for researchers at the Federal Highway Administration (FHWA) to investigate the condition of the reinforcement in the 36-year old wall and determine any key factors affecting corrosion related to the design, construction, and maintenance practices. Steel and soil samples were collected from various locations behind the concrete panels along the length of the wall to assess the thickness of the remaining zinc coating on the bar mats. This thickness was analyzed as a function of wall elevation and distance from the bridge abutment face. This paper will describe the corrosion sampling and testing program and present preliminary conclusions on the results. © ASCE 2015. Source
Gibson N.,U.S. Federal Highway Administration |
Li X.,ESC Inc.
Asphalt Paving Technology: Association of Asphalt Paving Technologists-Proceedings of the Technical Sessions | Year: 2012
This study quantified the effect of reduced gyration levels on mix design and performance. Laboratory tests captured changes in fatigue resistance and rutting resistance when design gyrations were decreased using different approaches. Three variations were studied using four separate mixtures, a standard 75 gyration design, a reduced 65 gyration design meeting standard volumetrics using additional binder, and a reduced 65 gyration design where standard volumetrics were met by adjusting the fine aggregate gradation rather than through the addition of binder. The properties of the various mix designs were evaluated using a suite of laboratory tests including: dynamic modulus, fatigue resistance and rutting resistance. Comparability was assessed using the shear response measured during gyratory compaction. Laboratory tests showed that reducing gyrations and adjusting the fine aggregate gradation increased the average modulus, and the average modulus decreased when binder was increased at lower gyrations. Flow number tests yielded the same trend as dynamic modulus where adjusted fine aggregate gradations increased average resistance to permanent deformation, and added binder decreased average resistance to permanent deformation relative to the standard, reference mix design. However, these trends in the average modulus and permanent deformation responses could not overcome the inherent variability of the lab tests and the differences were largely insignificant. The Compaction Force Index computed from the gyratory shear resistance indicated both types of reduced design gyrations improved the compactability relative to the reference mix with higher design gyration. Trends in the average fatigue resistance due to the reduced design gyration level were mixed. The fatigue resistance tended to be higher for lower gyration level mixes containing added binder. Some mixtures exhibited improved overall average fatigue performance when the aggregate blend was adjusted, however the average fatigue resistance of one mixture decreased for both types of design gyration reduction. The choice of fatigue failure criteria from the lab tests influenced the rank order of the fatigue resistance for each mix design variation. One of the mixes purported to exhibit difficult constructability exhibited different patterns in compaction shear resistance as well as erratic permanent deformations in the flow number repeated load test. This suggests that problematic mixtures could be screened with these tools though a more in-depth investigation would be necessary. Lastly, predicted performance using the variations in the job mix formulas were evaluated in light of the laboratory performance using a beta version of HMA Quality Related Specifications Software (QRSS). The analysis found the tool was able to reflect the permanent deformation trends from the small difference in measured dynamic modulus. A large variation in the predicted fatigue performance was observed that was also evident in the laboratory results. Thus, trends in the average fatigue response could only be compared, which showed the reduced gyration mix with added binder was always slightly better than the reference mix and the mix with adjusted aggregate blends. The implication of this study is that lowering design gyrations by a moderate level can, but not always, be used to achieve more comparable and more crack resistant mixtures without significantly jeopardizing rutting resistance. The crucial caveat is that performance tests need to be completed because a consistent or general rule does not apply. Some mixtures in the study were negatively affected by the reduced gyration level. laboratory performance tests which can be conducted with an Asphalt Mixture Performance Tester are recommended for guidance because of the particulars of local materials. Source