Kalibracija i verifikacija modela konačnih elemenata za dugotrajno ponašanje AB greda

This study presents the research carried out in finding an optimal finite element (FE) model for calculating the long-term behavior of reinforced concrete (RC) beams. A multi-purpose finite element software DIANA was used. A benchmark test in the form of a simply supported beam loaded in four point bending was selected for model calibration. The result was the choice of 3-node beam elements, a multi-directional fixed crack model with constant stress cut-off, nonlinear tension softening and constant shear retention and a creep and shrinkage model according to CEB-FIP Model Code 1990. The model was then validated on 14 simply supported beams and 6 continuous beams. Good agreement was found with experimental results (within ±15%).U ovom radu je predstavljeno istraživanje sprovedeno u svrhu pronalaženja optimalnog modela konačnih elemenata za predviđanje ponašanja armirano-betonskih greda pod dugotrajnim opterećenjem. U istraživanju je korišćen višenamenski komercijalni softver DIANA. Za kalibraciju modela je odabran benčmark test u vidu proste grede opterećene savijanjem u četiri tačke. Kao rezultat kalibracije, odabrani model se sastojao od linijskih konačnih elemenata sa tri čvora. Odabran je model fiksne prsline u više pravaca. Sadejstvo zategnutog betona izmenu prslina opisano je nelinearnom funkcijom. Odabrana je konstantna rezidualna nosivost pri smicanju. Skupljanje i tečenje je modelirano prema CEB-FIP Model Code 1990. Model je dodatno verifikovan na 14 prostih greda i 6 kontinualnih greda sa dva polja. Rezultati su pokazali dobro slaganje sa eksperimentalnim vrednostima (±15%)

Advanced Filtering Waveguide Components for Microwave Systems

Microwave filter design is a popular topic in the area of modern microwave engineering. Novel technologies, novel applications and more demanding component miniaturization are some of the key drivers for the development of novel microwave filters. For the systems operating with high power and low losses, waveguide filters represent sustainable solutions, in spite of their size. Herein, a method for the advanced bandpass and bandstop waveguide filter design is presented. Properly designed printed-circuit inserts, with simple resonators, are used as resonating elements inside a standard rectangular waveguide. In this manner, multi-band bandpass or bandstop waveguide filters are developed. Multiple resonant frequencies can be obtained using single insert, with properly positioned resonators. Filter design is exemplified by numerous three-dimensional electromagnetic models of the considered structures, equivalent microwave circuits and fabricated devices. Some of the advantages of the proposed design are simplicity, ease of implementation, possibility of miniaturization and experimental verification. The waveguide filters considered here are designed to operate in the X frequency band, so their application is recognized with the radar and satellite systems. Further improvement of the proposed method is possible, according to the future use of the presented devices

Discussion of the article 'Prediction of creep of recycled aggregate concrete using back-propagation neural network and support vector machine'

In a recent study, Rong et al.1 investigate the prediction of recycled aggregate concrete (RAC) creep using back-propagation neural network and support vector machine. For this purpose, the authors compiled a database of experimental results on the creep of RAC on which they first tested five analytical RAC creep prediction models2-6 and concluded that the performance of all five models is inadequate, thereby justifying the use of a back-propagation neural network and a support vector machine. The main argument for declaring the performance of the five analytical models inadequate is the analysis of “performance indices” of the correlation coefficient (R), mean absolute error (MAE), mean square error (MSE), and integral absolute error (IAE). The found ranges of values were 0.45–0.55 for R, 0.41–0.64 for MAE, 0.33–0.70 for MSE, and 0.33–0.53 for IAE. Nonetheless, there are errors and uncertainties regarding the study that are pointed out herein, some methodological and some formal.Postprint (published version

Model Code 2010 creep and shrinkage models extension to recycled aggregate concrete

Recycled aggregate concrete (RAC) produced with recycled concrete aggregate (RCA) is one of the most promising ways of eliminating concrete waste and saving natural resources. However, shrinkage and creep behaviour of RAC, important for serviceability design of reinforced and prestressed concrete structures, are still insufficiently studied and guidelines for RAC serviceability design are still not incorporated into design codes and standards. This study aims to systematize the knowledge gained on RAC shrinkage and creep behaviour thus far and to offer analytic expressions for predicting RAC shrinkage strain and creep coefficient. For this purpose, databases of previously published results on RAC shrinkage and creep were compiled and the results on RAC were analysed relative to companion natural aggregate concrete (NAC). The results showed a systematically higher shrinkage and creep of RAC relative to NAC. Finally, analytic expressions for correction coefficients, dependent on RAC compressive strength and RCA replacement ratio, were formulated for predicting RAC shrinkage strain and creep coefficient using the fib Model Code 2010.This work was supported by the Ministry for Education, Science and Technology, Republic of Serbia under grant number TR 36017 and Spansh Ministerio de Economía, Industra y Competividad under the SAES project BIA2016-78742-C2-1-R. This support is gratefully acknowledgedPostprint (published version

Sustainability assessment of recycled aggregate concrete structures: a critical view on the current state-of-knowledge and practice

The environmental impacts of activities such as raw material extraction, construction of infrastructure, and demolition, place construction as one of the sectors that exert the highest pressures on the environment, society, and economy. Some of the major environmental impacts for which the construction industry is responsible are mineral resource depletion, greenhouse gas emissions, and waste generation. Among the different strategies that exist to decrease such impacts, recycling demolition waste into recycled concrete aggregates has been considered a promising alternative. As such, at present, the literature dealing with the impact assessment of recycled aggregate concrete structures is very extensive. Therefore, the objective of this article is to present a critical view of the state-of-the-art in terms of sustainability assessment of recycled aggregate concrete structures, taking a holistic perspective by considering environmental, social, and economic impacts.Peer ReviewedPostprint (published version

Advanced Teaching Methods Application and its Benefits in Descriptive Geometry at the Faculty of Civil Engineering and Architecture in Niš

Teaching tools, such as computer drawings in lecture templates, "step by step" presentations, Android application, got introduced so as to improve teaching process quality in Descriptive geometry subject, at the Faculty of Civil Engineering and Architecture in Niš. Advanced teaching methods in practice classes, and on-the-screen video beam projected "step by step" presentations were available only to half of the enrolled students (experimental group –the EG), while the rest attended the practice classes deploying classical teaching methods, and manual drawings on the blackboard (control group –the CG). Deliberation of data on previous education and intellectual capabilities of students involved proved that the groups do not differ. Comparative analysis of the final results in academic year of 2016/17 indicated that in the EG students there was a higher knowledge of the subject matter, and higher exam pass rate. The conclusion is that the application of the advanced teaching methods makes learning and teaching process more successful, thus their usage in the future is recommended

Sustainability of the concrete industry: Current trends and future outlook

Achieving sustainability of all human actions has been recognized as an urgent and top priority since the warnings of anthropogenic climate change are overwhelming. However, the precise goal, aim and method of shifting the global paradigm towards sustainability are still contested. Among all human activities, the concrete industry has one of the largest environmental footprints, not only because concrete is the second most used material in the world, but also because the production of cement for concrete is highly energy-intensive and inevitably releases large amounts of CO2. In this paper, a historic and theoretical background to the environmental problems, arising from the production and use of concrete, is presented. The specific problems it poses are recognized as natural resource consumption, CO2 emissions, and waste generation. A technical discussion based on Life Cycle Assessment analyses is presented alongside a societal interpretation within the framework of common resource and externality management. Possible technical solutions in the form of recycling waste concrete and replacing cement with industrial by-products are presented and finally, a necessity for a shift towards a holistic and environmental paradigm is highlighted

Održivost industrije betona - savremeni trendovi i izgledi za budućnost

Achieving sustainability of all human actions has been recognized as an urgent and top priority since the warnings of anthropogenic climate change are overwhelming. However, the precise goal, aim and method of shifting the global paradigm towards sustainability are still contested. Among all human activities, the concrete industry has one of the largest environmental footprints, not only because concrete is the second most used material in the world, but also because the production of cement for concrete is highly energy-intensive and inevitably releases large amounts of CO2. In this paper, a historic and theoretical background to the environmental problems, arising from the production and use of concrete, is presented. The specific problems it poses are recognized as natural resource consumption, CO2 emissions, and waste generation. A technical discussion based on Life Cycle Assessment analyses is presented alongside a societal interpretation within the framework of common resource and externality management. Possible technical solutions in the form of recycling waste concrete and replacing cement with industrial by-products are presented and finally, a necessity for a shift towards a holistic and environmental paradigm is highlighted.Postizanje održivosti svih ljudskih aktivnosti je prepoznato kao hitan i prioritetan zadatak s obzirom da su upozorenja o ljudskim uzrocima klimatskih promena brojna. Međutim, tačan cilj i način promene globalne paradigme ka održivosti su i dalje nejasni i neusaglašeni. Od svih ljudskih aktivnosti, industrija betona ima jedan od najvećih uticaja na životnu sredinu, ne samo usled toga što je beton drugi najkorišćeniji materijal na svetu, već i usled toga što je proizvodnja cementa energetski veoma zahtevna i neizbežno dovodi do emisije velikih količina CO2. U ovom radu su predstavljeni istorijski podaci i teorijske osnove ekoloških problema koji nastaju usled proizvodnje betona. Konkretni problemi do kojih dolazi su potrošnja prirodnih resursa, emisije CO2 i proizvodnja otpada. Tema rada je predstavljena sa tehničkog aspekta na osnovu analiza životnog ciklusa, a društveno-ekonomska interpretacija je data u teorijskom okviru upravljanja zajedničkim dobrima i eksternalijama. Predstavljena su moguća tehnička rešenja u vidu recikliranja otpadnog betona i zamene cementa industrijskim nusproizvodima. Na kraju je ukazano na potrebu za zaokretom globalne paradigme ka holističkom i ekološkom pogledu na svet

Multielement determination using inductively coupled plasma optical emission spectrometry for metal characterization of water from artesian wells in Semberija region: Multivariate analysis of data

The concentrations of fifteen metals (Mg, Na, Ca, K, Se, Zn, Mn, Fe, Pb, Cr, Cu, Cd, Sb, Ni, Co) were determined in water from ten artesian wells (AW) in Semberija to obtain a general metal profile of water in this region. Principal components analysis (PCA) was used in this classification. Using principal component analysis two factors controlling the metal variability were obtained, which accounted for nearly 71.5% of the total variance. Natural (lithogenic) factor is represented by PC1, while anthropogenic factor is represented by PC2. PC1 with high contribution of Mn, Mg, Na, K, Ca, Zn and Se accounting for 41.84% of the total variance, while PC2 exhibits high loading for Cd, Ni, Sb, Cr and Pb (29.66%). Three general areas (clusters) with different metal characteristics were detected. Water from artesian wells in first cluster (AW1, AW2, AW3, AW4, AW5 and AW6) had much higher metal concentration compared with those in the second (AW7, AW8 and AW9) and third cluster (AW10). That is as a result of anthropogenic inputs. Also, the analysis of water demonstrated slightly elevated values for Mn (concentrations up to 0.176 mg/L), while concentrations of the other investigated elements are below the values recommended by the World Health Organization (WHO) and the United States Environmental Protection Agency (US EPA). [Projekat Ministarstva nauke Republike Srbije, br. 172047

Environmental assessment of green concretes for structural use

This paper presents a comparative environmental assessment of several different green concrete mixes for structural use. Four green concrete mixes were compared with a conventional concrete mix: recycled aggregate concrete with a cement binder, high-volume fly ash concrete with natural and recycled aggregates, and alkali activated fly ash concrete with natural aggregates. All five concrete mixes were designed and experimentally verified to have equal compressive strength and workability. An attributional life cycle assessment, based on the scenario which included construction practice, transport distances, and materials available in Serbia, was performed. When treating fly ash impacts, three allocation procedures were compared: ‘no allocation’, economic, and mass allocation, with mass allocation giving unreasonably high impacts of fly ash. Normalization and aggregation of indicators was performed and the impact of each concrete mix was expressed through a global sustainability indicator. A sensitivity analysis was also performed to evaluate the influence of possibly different carbonation resistance and long-term deformational behavior on the functional unit. In this specific case study, regardless of the choice of the functional unit, the best overall environmental performance was shown by the alkali activated fly ash concrete mix with natural aggregates and the high-volume fly ash recycled aggregate concrete mix. The worst performance was shown by the recycled aggregate concrete mix with a cement binder