An experimental research is performed on the complete compressive stress–strain relationship for concrete after heating to temperatures of 100–800 °C. All concrete specimens are ϕ15 cm
×
30 cm ...standard cylinders, made with siliceous aggregate. The heated specimens are tested at 1 month after they are cooled to room temperature. From the results of 108 specimens with two original unheated strengths, a single equation for the complete stress–strain curves of heated concrete is developed to consider the shape varying with temperature. Through the regression analysis, the relationships of the mechanical properties with temperature are proposed to fit the test results, including the residual compressive strength, peak strain and elastic modulus. Compared with the experimental curves, the proposed equation is shown to be applicable to unheated and heated concrete for different temperatures. In addition, the split-cylinder tests of 54 specimens are also carried out to study the relationship of splitting tensile strength with temperature.
In this report, we assessed the importance of internal microstructure on TiO2-containing cement-based mortars during the degradation of NOx gases through a photocatalytic oxidation process. Six ...samples of mortar containing the same amount of cement (14%) and TiO2 (1%) were prepared by changing the amount and type of sand used in their formulation, as well as the water/cement ratio, thus yielding mortars with different overall porosities and pore size distributions. The particle size distribution of the raw materials had a strong influence on the final microstructure of the hardened products. The volume of macropores (in this study, pores with diameter larger than 80 nm), which appeared according to the size of particle used in the mortar preparation and the extent of the hydration reaction of the cementitious compounds, was related to the presence of active sites accessible to the photocatalytic oxidation (PCO) process. Samples with large macropore volume and lower amount of hydration products exhibited the highest PCO efficiency. Taking into account the low weight-percent of TiO2, the PCO performance was found to be outstanding, with NO conversion values generally larger than 20%. Moreover, the microstructure was related to the amount of TiO2 present in the mortar surface.
Display omitted
•NOx photocatalytic degradation is enhanced by regulating the microstructure of mortars.•The highest macropores population yielded outstanding values of NOx degradation.•Good pollutant degradation is obtained with very low amount of TiO2 in mortars.
Alkali-activated slag concretes stored for 7 years under atmospheric conditions are assessed, and the structural characteristics of naturally carbonated regions are determined. Concretes formulated ...with a 400 kg/m
3
and water/binder (w/b) ratio between 0.42 and 0.48 present similar natural carbonation depths, although these concretes report different permeabilities after 28 days of curing. The inclusion of increased contents of binder leads to a substantial reduction of the CO
2
penetration in these concretes, so that negligible carbonation depth values (2 mm) are identified in concretes formulated with 500 kg/m
3
of binder. Calcite, vaterite, and natron are identified as the main carbonation products formed in these concretes. These observations differ from the trends which would be expected in comparable ordinary Portland cement-based concretes, which is attributable to the physical (permeability) and chemical properties of alkali-activated slag concretes promoting high long-term stability and acceptably slow carbonation progress under natural atmospheric conditions.
Replacement of ordinary Portland cement (OPC) by blast-furnace slag (BFS) alters the durability behaviour of concrete. In this research, the influence of BFS on the concrete's acid or sulphate ...resistance is investigated by accelerated degradation tests and the results are related to microstructural and physico-chemical parameters.
A significant reduction of acid deterioration was recorded for BFS concrete, which is mainly attributed to the different chemical composition of the binder. General durability indicators like open porosity cannot solely explain the different performances of OPC and BFS concrete. In contrast, the resistance of concrete cyclically and partially submerged in sulphate solutions decreases when high amounts of cement are replaced by BFS. Crystallisation pressure causes more severe deterioration in these cases.
This paper provides an improved mathematical analysis of chloride penetration into concrete employing a time-dependent diffusion coefficient for the solution of Fick's second law of diffusion. In the ...paper the possible errors caused by the application of oversimplified mathematical expressions used in some models for the evaluation of service life of reinforced concrete structures are discussed. The results from this mathematical analysis demonstrate that some models based on the oversimplified error function complement (ERFC) solutions may easily overestimate the service life by orders of magnitude, especially when the age factor is high. Some chloride profiles after up to 10 years' field exposure were used to compare the oversimplified with the improved models. The results show that both the oversimplified and the improved models fairly well predict the 10 years' chloride ingress in Portland cement concrete, but the oversimplified ERFC model significantly underestimates the chloride ingress in concrete with fly ash.
Homeowners burn wood of a wide range of species and moisture content (MC) in residential cordwood and pellet stoves. An effective emission certification test protocol must account for and accurately ...measure the impact of those variables in order to ensure a reasonable correlation between laboratory results and in-use emissions and to promote the design and manufacture of cleaner burning appliances. This study explored the effect of wood species and MC on emissions and efficiency in four cordwood and four pellet stoves. PM emissions were consistently lower with pellets manufactured from softwood than for hardwood species and were highly correlated with ash content. Higher MC oak fuel substantially increased PM emissions in a non-catalytic cordwood stove; however, a hybrid cordwood stove was able to meet federal emissions limits even with the higher MC fuel. The results of this study, in combination with previous research, suggest that certification tests that use softwood fuel likely report lower emissions than tests that use hardwood. Requiring hardwood and higher MC cordwood fuel in certification tests would enable the assessment of an appliance's ability to operate well even when fuel conditions are not optimized.
Implications: The emission testing results reported in this paper call into question the adequacy of the fuel moisture content and fuel species specifications in testing protocols approved for certifying compliance with EPA's New Source Performance Standards for cordwood and pellet stoves. We recommend changes in those specifications, including the prohibition of testing with Douglas fir and other low ash softwood species, requiring the use of cordwood test fuel with a higher moisture content, and requiring pellet stoves to be tested using hardwood pellets. Adoption of these measures would increase the replicability of tests. allow for the identification of stoves that are unlikely to perform well in the field when fuel conditions are not ideal, and, ultimately, result in the design of cleaner burning stoves.
International standards have proven invaluable in the technology sector for developing functional and reliable products for the global marketplace. Standards provide performance criteria that ...technical engineers can use to design products to optimize the reliability and safety of new products. For example, standards have played a decisive role in the development of products associated with the senses of vision, audition, and touch. The design of products that perform automated "visual" tasks including unmanned vehicles, autonomous robots, optical tracking systems, and highway traffic monitoring devices has relied heavily on standards as well as technical regulations. Likewise, standards related to the sense of hearing have played a major role in the development of devices and systems that assist or mimic "audition" including cochlear implants, hearing aids, and voice and speech recognition systems. Standards related to the sense of touch have been seminal in the design of robotic arms and prosthetic hands. Unlike the senses of vision, audition, and touch, there are, however, no formal standards for electronic devices called e-noses and e-tongues that are designed to detect and evaluate odors and tastes. The purpose of this opinion piece is to give a brief background on the senses of smell and taste, to describe why standards for e-noses and e-tongues are needed, and to call for IEEE volunteers to participate and collaborate on technical standards development to ensure that machine olfaction and taste provide reliable and reproducible results that are comparable to human smell and taste.
This work explains the elastic properties of the reactive suspensions of metakaolin in sodium silicate solutions immediately after mixing. The flow properties of the interstitial fluid were obtained ...by mimicking it with synthetic aluminosilicate gels of different Si/Al molar ratios. By comparing these results with the rheological properties of fresh geopolymer pastes and with the ones of unreactive suspensions, we showed that the early age mechanical properties of geopolymer mixes cannot be explained by the colloidal interactions between metakaolin grains but rather by the formation of a gel with a molar ratio Si/Al<4.5. In addition, we measured the evolution of the total concentration of tetrahedral Al in the interstitial liquid by static NMR. It was thus evidenced that the afore-mentioned aluminosilicate gel is formed at a very early stage of the geopolymerisation reaction resulting in a heterogeneous suspension with an Al-rich gel formed at the grain boundaries.
This paper deals with the correlation between the time evolution of the degree of hydration and the compressive strength of Recycled Aggregate Concrete (RAC) for different water to cement ratios and ...initial moisture conditions of the Recycled Concrete Aggregates (RCAs). Particularly, the influence of such moisture conditions is investigated by monitoring the hydration process and determining the compressive strength development of fully dry or fully saturated recycled aggregates in four RAC mixtures. Hydration processes are monitored via temperature measurements in hardening concrete samples and the time evolution of the degree of hydration is determined through a 1D hydration and heat flow model. The effect of the initial moisture condition of RCAs employed in the considered concrete mixtures clearly emerges from this study. In fact, a novel conceptual method is proposed to predict the compressive strength of RAC-systems, from the initial mixture parameters and the hardening conditions.
•The concrete industry is more and more concerned with sustainability issues.•The use of recycled aggregates is a promising solution to enhance sustainability.•Recycled aggregates affect both hydration processes and compressive strength.•A fundamental approach is proposed to unveil the influence of recycled aggregates.•Some experimental comparisons are presented to validate the proposed approach.
Sleep deprivation is rampant within the military population, and insufficient sleep can lead to physical and mental health problems impacting soldier's readiness and deployability. Past research has ...shown the importance of leadership's role in subordinates' sleep health. Understanding the values, beliefs, and quality of military leader sleep is essential to the development of effective interventions to optimize occupational performance and overall sleep health. Therefore, the purpose of this study was to examine the military leaders' values, beliefs, and sleep quality and the impact on occupational performance. The authors aimed to (1) identify military leaders' sleep quality and beliefs; (2) explore the relationship between military leaders' sleep quality, beliefs and attitudes about sleep, and impact on occupational performance; and (3) examine the value leaders place on sleep for themselves and subordinates in relation to occupational performance.
This observational, mixed-methods study design recruited a convenience sample of 109 Army active duty medical service members currently serving in a leadership role. Participants completed an electronic survey to include general demographic information and three self-report measures: the Pittsburgh Quality of Sleep Index, the Dysfunctional Beliefs about Sleep, and the Functional Outcomes of Sleep Questionnaire. Eleven participants completed the semi-structured qualitative interview focusing on sleep values and the impacts on performance. Univariate and multivariate regressions were performed for statistical analysis of the quantitative survey data, whereas thematic analysis was used to analyze the qualitative interview data. This study was approved by the U.S. Army Medical Center of Excellence Institutional Review Board.
Multivariate regression analysis demonstrated small-to-medium effect sizes (R2 = 0.355-0.559) for relationships between sleep quality, sleep beliefs, functional performance, and demographic variables. More specifically, military grade, position, use of alcohol, time in service, and gender were all found to contribute significantly to scores on the Pittsburgh Sleep Quality Index, the Dysfunctional Beliefs About Sleep Scale-16, and the Functional Outcomes of Sleep Questionnaire-10 (P < .05). Qualitative data resulted in three primary themes: (1) Poor sleep degrades performance, (2) sleep is a top priority, and (3) leaders have a responsibility for subordinates' sleep health.
This sample of military leaders was found to perceive themselves as poor-quality sleepers despite demonstrating more functional attitudes and beliefs about sleep and reporting normal-to-mild impairments in daily functioning as a result of daytime sleepiness. Furthermore, findings suggest that leaders' sleep quality and beliefs stand to be further improved, whereas their sleep values need to be consistently demonstrated to subordinates. With a clearer understanding of military leaders' values, beliefs, and sleep quality, future research could focus on implementing and developing holistically based and individualized sleep interventions intended to optimize performance and sleep health.
PDF
