Primena hemijskog dodatka za sprečavanje zaleđivanja vode u svežem betonu pri betoniranju u zimskim uslovima

During the winter time construction of the concrete structures, it is not uncommon to expect occasional short periods of the cold weather with temperatures below 0°C, which can last for few days, when freezing of the water in the fresh concrete might permanently destruct the newly formed cement matrix. This paper presents the results of an investigation of new type of chemical admixture which protect fresh concrete from the frost during very low temperatures (up to - 50°C) and enables continuation of the concrete hardening process when outside temperature becomes favorable. The testing included the admixture's influence on fresh concrete mixture and on the hardened concrete, with and without low temperatures influence. The concrete specimens were cured in the water at the 20°C temperature, and the results showed that the tested admixture caused the increase of the compression strength of more than 10% comparing to one of the reference specimen. In order to test the admixture influence, three cases of the potential low temperatures were examined. In thesis simulations, in order to establish the limits of the admiture's influence, fresh concrete mixture was exposed to the temperatures, where the lowest was -25°C. All three cases showed better behavior of the fresh concrete mixture, and also displayed much higher compression strength (for more than 100% higher, and for some regimes, for 200% higher). The specimens exposed to the extreme regime (the third one) were also tested for the water absorption and release, in order to establish the degree of destruction caused by frost and low temperatures during concrete hardening. These tests also showed that the specimens with the admixtures had lower water absorption, i.e. preserved structure of the concrete.Prilikom izvođenja betonskih konstrukcija na licu mesta u zimskom periodu nisu retki slučajevi kada se može očekivati kratkotrajno zahlađenje od nekoliko dana sa temperaturama ispod nule koje mogu dovesti do zaleđivanja vode u svežem betonu i stvaranja leda koji trajno oštećuje strukturu tek formiranog cementnog kamena. U radu su prikazani rezultati ispitivanja uticaja novog hemijskog dodatka koji sprečava formiranja leda pri izuzetno niskim temperaturama (do -25 °C) u svežem betonu i omogućuje da se proces očvršćavanja nastavi čim to porast spoljne temperature dozvoli. Ispitivano je dejstvo dodatka na svež i očvrsli beton, bez i sa uticajem niskih temperatura. Prikazani rezultati na uzorcima koji su negovani u vodi temperature 20 °C pokazuju da ispitivani dodatak čak povećava čvrstoću na pritisak očvrslog betona u odnosu na etalon i za više od 10%. Sa ciljem da se ispita dejstvo dodatka u slučaju iznenadnih niskih temperatura simulirane su tri potencijalno realne situacije niskih temperatura. Radi utvrđivanja granica dejstva dodatka svež beton je, kroz ove simulacije, izlagan temperaturama i do -25 °C. U sva tri slučaja beton sa dodacima pokazao je znatno bolje ponašanje i postigao je znatno veće čvrstoće (za preko 100%, a za pojedine režime i za preko 200%). Opitna tela koja su prošla najteži termički režim (3. režim) ispitana su i na upijanje i otpuštanje vode, da bi se utvrdila mera oštećenja strukture usled dejstva mraza i niskih temperatura u periodu očvršćavanja svežeg betona. I ovde su uzorci sa dodatkom pokazali znatno manje upijanje vode, tj. očuvanu strukturu očvrslog betona

Building envelope influence on the annual energy performance in office buildings

The objective of the research is to determine the quantitative influence of building envelope on the annual heating and cooling energy demand in office buildings demonstrated on a reference office-tower building located in Novi Sad, Serbia. The investigation intended to find preferable and applicable solutions for energy performance improvement in currently inefficient office buildings. A comparative and evaluative analysis was performed among the heating energy expenses and simulated values from the multi-zone model designed in EnergyPlus engine. The research determines an improved window to wall ratio using dynamic daylight simulation and presents the influence of glazing parameters (U-value, Solar heat gain coefficient - SHGC) on the annual energy performance. Findings presented window to wall ratio reduction down to 30% and point out the significance of the SHGC parameter on the overall energy performance of buildings with high internal loads. The calculation of the air-ventilation energy demand according to EN 15251 is included respectively. Results offer effective methods for energy performance improvement in temperate climate conditions

Energy performance modelling and heat recovery unit efficiency assessment of an office building

This paper investigates and analyzes a typical multi-zone office building’s annual energy performance for the location and climate data of central Belgrade. The aim is to evaluate the HVAC system’s and HR unit’s performance in order to conduct the most preferable heating and cooling solution for the typical climate of Belgrade city. The energy performance of four HVAC system types (heat pump - air to air, gas-electricity, electrical and fan coil system) was analyzed, compared and evaluated on a virtual office building model in order to assess the total annual energy performance and to determine the efficiency of the HR unit’s application. Further, the parameters of an energy efficient building envelope, HVAC system, internal loads, building operation schedules and occupancy intervals were implemented into the multi-zone analysis model. The investigation was conducted in EnergyPlus simulation engine using system thermodynamic algorithms and surface/air heat balance modules. The comparison and evaluation of the obtained results was achieved through the conversion of the calculated total energy demand into primary energy. The goal is conduct the most preferable heating and cooling solution (Best Case Scenario) for the climate of Belgrade city and outline major criteria in qualitative enhancement

Nonlinear seismic analysis of continuous RC bridge

Nonlinear static analysis, known as a pushover method (NSPA) is oftenly used to study the behaviour of a bridge structure under the seismic action. It is shown that the Equivalent Linearization Method - ELM, recommended in FEMA 440, is appropriate for the response analysis of the bridge columns, with different geometric characteristics, quantity and distribution of steel reinforcement. The subject of analysis is a bridge structure with a carriageway plate - a continuous beam with three spans, with the 24 + 40 + 24 m range. Main girder is made of prestressed concrete and it has a box cross section of a constant height. It is important to study the behaviour, not only in the transverse, but also in the longitudinal direction of the bridge axis, when analysing the bridge columns exposed to horizontal seismic actions. The columns were designed according to EN1992, parts 1 and 2. Seismic action analysis is conducted according to EN 1998: 2004 standard. Response spectrum type 1, for the ground type B, was applied and the analysis also includes 20% of traffic load. The analysis includes the values of columns displacement and ductility. To describe the behaviour of elements under the earthquake action in both - longitudinal and transverse direction, pushover curves were formed