Lubinda F. Walubita

Human health effects of residual carbon nanotubes and traditional water treatment chemicals in drinking water

The volume of industrial and domestic wastewater is increasing significantly year by year with the change in the lifestyle based on mass consumption and mass disposal brought about by the dramatic development of economies and industries. Therefore, effective advanced wastewater treatment is required because wastewater contains a variety of constituents such as particles, organic materials, and emulsion depending on the resource. However, residual chemicals that remain during the treatment of wastewaters form a variety of known and unknown by-products through reactions between the chemicals and some pollutants. Chronic exposure to these by-products or residual chemicals through the ingestion of drinking water, inhalation and dermal contact during regular indoor activities (e.g., showering, bathing, cooking) may pose cancer and non-cancer risks to human health. For example, residual aluminium salts in treated water may cause Alzheimers disease (AD). As for carbon nanotubes (CNTs), despite their potential impacts on human health and the environment having been receiving more and more attention in the recent past, existing information on the toxicity of CNTs in drinking water is limited with many open questions. Furthermore, though general topics on the human health impacts of traditional water treatment chemicals have been studied, no comparative analysis has been done. Therefore, a qualitative comparison of the human health effects of both residual CNTs and traditional water treatment chemicals is given in this paper. In addition, it is also important to cover and compare the human health effects of CNTs to those of traditional water treatment chemicals together in one review because they are both used for water treatment and purification.

The production of carbon nanotubes from carbon dioxide: challenges and opportunities

Abstract Recent advances in the production of carbon nanotubes (CNTs) are reviewed with an emphasis on the use of carbon dioxide (CO2) as a sole source of carbon. Compared to the most widely used carbon precursors such as graphite, methane, acetylene, ethanol, ethylene, and coal-derived hydrocarbons, CO2 is competitively cheaper with relatively high carbon yield content. However, CNT synthesis from CO2 is a newly emerging technology, and hence it needs to be explored further. A theoretical and analytical comparison of the currently existing CNT-CO2 synthesis techniques is given including a review of some of the process parameters (i.e., temperature, pressure, catalyst, etc.) that affect the CO2 reduction rate. Such analysis indicates that there is still a fundamental need to further explore the following aspects so as to realize the full potential of CO2 based CNT technology: (1) the CNT-CO2 synthesis and formation mechanism, (2) catalytic effects of transitional metals and mechanisms, (3) utilization of metallocenes in the CNT-CO2 reactions, (4) applicability of ferrite-organometallic compounds in the CNT-CO2 synthesis reactions, and (5) the effects of process parameters such as temperature, etc.

Hot-Mix Asphalt Permanent Deformation Evaluated by Hamburg Wheel Tracking, Dynamic Modulus, and Repeated Load Tests

Permanent deformation (or rutting) is a common distress in hot-mix asphalt (HMA) pavements. As part of the HMA mix and structural design processes to optimize field performance, the Hamburg wheel-tracking, dynamic modulus, and uniaxial repeated load permanent deformation tests have been developed to characterize the HMA rutting resistance potential. The primary objective of this study was to compare the three laboratory rutting tests of HMA mixes and to relate their rutting predictive potential to actual field performance. The research methodology incorporated a two-phase approach: laboratory testing and field performance monitoring of selected mixes under both conventional traffic loading and accelerated pavement testing. For the HMA mixes evaluated, a good correlation was observed in the three laboratory tests and in comparison with actual in situ field performance. Overall, the findings indicated that the Hamburg wheel-tracking test was the most feasible test for daily routine HMA mix design and screening, while both the dynamic modulus and repeated load permanent deformation tests exhibited greater potential for comprehensive characterization of HMA material property (e.g., modulus) and applications for pavement structural design as research-level test tools.

Modeling Accident Duration and Its Mitigation Strategies on South Korean Freeway Systems

Understanding the relationship between the characteristics of an accident and its duration is crucial for efficient response to and timely clearance of an accident, which can minimize traffic delays and congestion on the freeway. Although about 3,000 traffic accidents occur annually on South Korean freeways, only limited studies have been conducted that comprehensively model traffic accidents, including characteristic attributes such as accident duration. The objective of this study is to analyze the critical factors that affect accident duration by means of an accelerated failure time (AFT) metric model and to develop strategic plans and mitigation measures for reducing accident duration on South Korean freeway systems. In total, 2,940 accident data sets, spanning one year (2004), are used. On the basis of the log logistic AFT metric model and survival analysis, the paper suggests some strategic plans and mitigation measures to reduce accident duration on freeway systems in South Korea. In particular, it is hypothesized that, for accidents that occur at night or in a work zone area or are taxi related, the accident duration can be reduced by, among other measures, adding more roadside facilities, employing better freeway management strategies, changing taxi-related policies, or using any combination of those measures. Overall, analytical modeling of traffic accidents and their characteristic attributes, as demonstrated in this paper, should be used routinely as an aid in the strategic planning and formulation of mitigation measures for reducing accident duration on Korean freeway systems.

Effects of Layer Interfacial Bonding Conditions on the Mechanistic Responses in Asphalt Pavements

The bonding condition between pavement layers plays an important role in the performance of pavement structures. In this paper, a three dimensional finite-element (3D-FE) program was used for modeling the mechanistic responses (stresses and strains) in the asphalt concrete (AC) layers by simulating two layer interfacial bonding conditions, namely fully bonded and debonded (i.e., the layer separated but still considering friction). The 3D-FE modeling incorporated actual measured vertical tire-pavement contact pressure (TPCP) and assumed horizontal TPCP, including investigating the effects of vehicle acceleration and deceleration. The results of these computational modeling are presented in this paper and indicated that the layer interfacial bonding condition has a significant effect on some pavement mechanistic responses such as the tensile, compressive, and shear stresses/strains in AC pavement structures. In general, layer interface debonding (or separation) was analytically found to indirectly exacerbate pavement distresses such as slippage cracking, fatigue cracking, shoving, shear deformation, and rutting, which is undesirable.

Proposed Loading Waveforms and Loading Time Equations for Mechanistic-Empirical Pavement Design and Analysis

Under the same applied traffic loading, time is traditionally assumed to be a function of only the vehicle speed and the depth beneath the pavement surface. Additionally, no definite loading waveform has been established and/or recommended for any one special loading situation. After numerous computations and analyses in this study, it was found that the loading time is not only a function of the vehicle speed and the depth beneath the pavement surface but is also a function of the moduli ratio between the layer of interest and the immediate succeeding layer below. Furthermore, the loading waveform changed with depth beneath the pavement surface and the moduli ratio. Based on the results of this study, new equations for more accurately determining the loading time were proposed. Additionally, the corresponding loading waveform, which is a function of the depth beneath the pavement surface and the moduli ratio, was recommended. Plausible results were also obtained when an iterated method was introduced to calculate the loading time using the equations proposed in this paper.

Modelling Tensile Strain Response in Asphalt Pavements: Bottom-up and/or Top-down Fatigue Crack Initiation

ABSTRACT Traditionally, HMA pavement designs and modeling methods have historically been based on the fundamental assumption that fatigue cracking generally initiates at the bottom of the HMA layer due to excessive tensile stresses/strains, and then propagates upwards to the surface. However, fatigue cracking can either be bottom-up or top-down initiated depending on the location of the maximum horizontal tensile stresses and strains in the HMA layer. Various factors such as the pavement structure and wheel/tire loading configurations influence both the location and magnitude of the fatigue crack-related tensile strains that are induced in the HMA layer. In this study, the influence of these factors on the tensile strains responses and potential location of fatigue crack initiation in HMA pavements was investigated. To address the non-uniformity distribution of the tire-pavement contact pressure (TPCP), including the vertical tire-pavement contact pressure and horizontal tire-pavement stress, a three-dimensional finite element program was utilized for the computational simulations and numerical analyses. Specifically, the actual TPCP of different tire types and tread patterns measured under variable load levels and tire inflation pressures were utilized to evaluate and analyze the potential locations of maximum tensile strains in terms of fatigue crack initiation. The results showed that the longitudinal and low latitudinal tire-pavement stress hardly influence the maximum tensile strains in the HMA layer; and that the maximum tensile strains can occur either at the top or bottom (or both) of the HMA layer, thus inducing top-down and/or bottom-up fatigue crack initiation. Just like heavy-truck tire loading, the study also demonstrated that light-truck tire loading can induce excessive fatigue crack-related tensile strains in the pavement.

Initial Validation of a New Surface Performance-Graded Binder Specification

Surface treatment design and material selection currently are based on traditional specifications and experience, which are not performance based and sometimes result in inadequate performance of the surface treatment. In 2000, the first phase of a Texas Department of Transportation research study developed a surface performance-graded (SPG) specification for the selection of surface treatment binders. The SPG specification is performance based and uses binder properties directly related to surface treatment performance and associated distresses. The specification takes into account environmental conditions, aging effects of the binder, viscoelastic behavior, and reliability. The objective of the second phase of the study was to (a) investigate and establish the validity and applicability of the proposed SPG specification by comparing laboratory-measured SPG binder grades to actual observed field performance and making modifications where necessary and (b) recommend the SPG specification for practical implementation. The research methodology involved highway section identification, including project data collection, laboratory testing including binder SPG grading, field performance monitoring, and data analysis. Factors included in the experimental design were binder type and suppliers, environment, aggregates, and traffic. Analyses of the results showed that there was generally a good correlation between the proposed SPG specification and actual field performance. Overall, the results indicate that the proposed SPG specification is functional; if properly applied, it promises to be a relatively cost-effective method for selecting binders to ensure adequate surface treatment performance.

Development, Calibration, and Verification of a New Mechanistic-Empirical Reflective Cracking Model for HMA Overlay Thickness Design and Analysis

The purpose of this paper is to present a new mechanistic-empirical (ME) reflective cracking model developed for hot-mix asphalt (HMA) overlay thickness design and analysis. After reviewing existing models, the reflective cracking model based on Paris’ law of fracture mechanics was considered as the state of the practice and was selected as the basis for the ME model development in this study. The model consists of stress intensity factor and fracture properties ( A and n ) as the fundamental input parameters for modeling reflective crack propagation caused by both traffic loadings (bending and shearing) and thermal effects (temperature variations). For practical application, 32 SIF regression equations were developed for HMA overlays with three levels of load transfer efficiencies (10, 50, and 90%) at joints/cracks under various traffic loading spectrums (bending and shearing) based on more than 1.6 million finite element simulations and computations. For the thermal induced reflective cracking, a “hybri...

Search for a Laboratory Test to Evaluate Crack Resistance of Hot-Mix Asphalt

Fatigue cracking is one of the predominant distresses that occur in hot-mix asphalt (HMA) pavements and often costs highway agencies millions of dollars in road maintenance and rehabilitation activities. Ways to minimize this distress include material screening and selecting appropriate mix designs that produce crack-resistant HMA mixes. However, no standardized laboratory tests for fatigue cracking have been universally adopted for routine mix design or screening purposes in the evaluation of HMA crack resistance. Four cracking test methods—overlay tester (OT), direct tension (DT), indirect tension (IDT), and semicircular bending (SCB)—were compared for their potential application as simple tests for the routine crack evaluation and screening of HMA mixes in the laboratory. The evaluation criteria, based on commonly used Texas mixes, included (a) rationality of the test concept and tie to field performance, (b) repeatability and variability, (c) simplicity and practicality, (d) specimen fabrication process, and (e) simplicity of data analysis. Compared with the repeated-loading OT test, preliminary findings indicated that although the monotonically increasing loading DT, IDT, and SCB tests were more repeatable, those tests could not adequately discriminate crack resistance among the mixes evaluated on the basis of the tensile strength and strain parameters. Thus none of these monotonic tests would be recommended as a simple fatigue cracking test. Additional development of the test protocols is strongly recommended, including exploration of the repeated-loading IDT and SCB tests, use of strain–fracture energy concepts, establishment of pass-or-fail criteria for screening mixes, and validation with field performance data.