Influence of crosslinking parameters on the quality of rubber product

Osnova pri proizvodnji elastomernog materijala ili gume je kaučuk koji postaje umrežen tijekom procesa umreživanja. Umreživanje je kemijsko-fizikalna promjena pri kojoj pretežno plastični kaučuk prelazi u gumasto-elastično stanje. Za umreživanje je potrebno umreživalo, najčešće sumpor, a odvija se tijekom više sati pri povišenim temperaturama. Vrsta upotrebljenog kaučuka određuje temeljna svojstva gotovog gumenog proizvoda, prije svega postojanost, starenje, i savitljivost na niske temperature i dr., ali i mehanička svojstva, kao što su čvrstoća i elastičnost. U radu su primjenom centralno kompozitnog plana pokusa ispitana mehanička svojstva gumenih tvorevina od smjese prirodnog (NR) i stiren-butadienskog (SBR) kaučuka s obzirom na različite parametre umreživanja (vrijeme i temperatura umreživanja).The basis in the production of elastomeric materials is rubber which becomes cured during crosslinking process. Curing is a chemical-physical modification in which mostly plastic rubber transfers to rubber-elastic state. The curing process requires a curing agent, most frequently sulphur, and the process is performed at higher temperatures for several hours. The type of the used rubber defines the basic properties of product, for example: stability, ageing and flexibility at lower temperatures, as well as strength and elasticity. The paper presents the study of influence of crosslinking temperature and crosslinking time on some mechanical properties of natural rubber and styrene/butadiene rubber

Seismic Retrofitting of Dual Structural Systems—A Case Study of an Educational Building in Croatia

On 29 December 2020, a devastating Mw6.4 earthquake struck near the town of Petrinja, Croatia. The main earthquake was preceded by a Mw4.9 foreshock the day before. The earthquakes caused extensive damage to buildings, especially historic buildings made of unreinforced masonry but also to buildings of other typologies and to critical infrastructure. Today, recovery efforts in Croatia focus primarily on reconstruction and seismic retrofitting. Family homes and public, cultural, educational, and other facilities are top priorities. In this paper, a comprehensive study of existing building in the educational sector is presented as a case study. The seismic performance of the building is evaluated using numerical methods, first for the as-built condition and then for the retrofitted building. For each condition, the collapse mechanisms of the building were determined and critical structural elements were identified. The presented retrofit strategy of the dual structural system consisting of RC frame system and masonry walls aims to reduce the displacements of the RC frame system to a level sufficient to prevent the early brittle failure of the concrete. Additionally, the discrepancies when using different modelling approaches are discussed

Damage Evaluation and Seismic Assessment of a Typical Historical Unreinforced Masonry Building in the Zagreb 2020 Earthquake: A Case Study—Part I

The city of Zagreb, the national capital and economic hub of Croatia, is situated in a seismically active region and hosts a significant array of historical buildings, from the medieval to Austro-Hungarian periods. These buildings possess varying but generally high degrees of vulnerability to seismic loading. This was highlighted in the Zagreb earthquake of 22 March 2020, emphasizing the need for seismic retrofitting in order to preserve this architectural heritage. In this paper, the seismic capacity of one such unreinforced masonry building is considered through a number of analysis methods, including response spectrum, pushover, and out-of-plane wall failure analyses. Given the advantages and disadvantages of the individual methods, their applicability and value in a seismic analysis is considered. Ambient vibration measurements before and after the Zagreb 2020 earthquake, used for model calibration, are also presented. Conclusions are drawn from each individual analysis and later compared. In conclusion, no single analysis method considers all relevant failure modes, and a combination of nonlinear static or dynamic analysis and out-of-plane analysis is recommended. Due to the large volume of the material, it is published in two parts, with ground motion record selection, dynamic analysis, and a comparison of the results published in part two