When understanding the effect of the morphology of coarse aggregate on the properties of a fresh concrete mixture, the strength and deformability of self-compacting high-performance fibre-reinforced concrete (SCHPFRC) can be seen to be critical for its performance. In this research, regular and irregular grains were separated from granite coarse aggregate. The morphology of these grains was described while using digital image analysis. As a result, the aspect ratio, roundness and area ratio were determined in order to better understand this phenomenon. Then, the principal rheological, physical, and mechanical properties of SCHPFRC were determined. The obtained results indicated that the morphology of the grains of coarse aggregate has an impact on the strength and stiffness properties of SCHPFRC. Moreover, significant differences in the transverse strain of concretes were observed. The morphology of the coarse aggregate also has an impact on the rheological parameters of a fresh concrete mixture. To better understand this phenomenon, the hypothesized mechanism of the formation of SCHPFRC caused by different morphology of coarse aggregate was proposed at the end of the article.

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The purpose of the article is to present the results of the evaluation of the structure of air voids in low-strength air-entrained concrete (AEC) floor screeds modified with mineral powders sourced from industrial wastes. Quartz-feldspar and basalt powders were selected for aggregate quarry waste to replace 10%, 20%, and 30% of the mass of the binder. The principal aim of this study is to investigate the effect of adding selected powders on the structure of the air voids in AEC floor screeds. The structure of the air voids was measured using X-ray micro computed tomography (micro-CT). In the analysis, two methodologically different approaches were used, i.e., the volume-equivalent sphere diameter of the air voids and the local structure thickness of the air voids. It was seen that the structure of the air voids in the modified AEC floor screeds is different when compared to the reference AEC floor screeds without powders and non-AEC screeds. It was also shown that the microstructure differs for different amounts and types of used mineral powder.

Original scientific paper The paper reviews the process of surface layer development with the use of carbonitriding and carbonitriding with oxidation. Employed for that purpose was a modern and ecological method called ZeroFlow. Research was carried out using HS 6-5-2 steel. Single-component nitrifying atmosphere of ammonia NH3 was applied. The temperature of the processes was 570 °C. Diffusion layers were obtained on the steel. The thickness of the layers achieved during the microstructural research equalled from about 125 µm after carbonitriding with oxidation to about 187 µm after carbonitriding. Surface hardness HV1 was the highest for the carbonitrided layer -1027 HV1, whereas in the case of carbonitriding with oxidation it amounted to 1018 HV1. As far as surface hardness HV30 is concerned, it was at the level of 813 HV30 for the carbonitrided layer and 795 HV30 for carbonitriding with oxidation. Surface hardness test results indicate surface layer hardening. Keywords: carbonitriding; microhardness; nitriding; oxidation; surface layer Dobivanje površinskog sloja HS 6-5-2 čelika karbonitriranjem uz primjenu metode ZeroFlowIzvorni znanstveni članak U radu se opisuje postupak dobivanja površinskog sloja primjenom karbonitriranja i karbonitriranja s oksidacijom uz pomoć suvremene i ekološke metode poznate kao ZeroFlow. Istraživanje je provedeno uporabom čelika HS 6-5-2. Primijenjena je jednokomponentna nitrificirajuća atmosfera amonijaka NH3. Postupak se vodio na temperaturi 570 °C. Na čeliku su nastali difuzijski slojevi. Debljina slojeva dobivenih tijekom mikrostrukturnog istraživanja bila je od oko 125 µm nakon karbonitriranja s oksidacijom do oko 187 µm nakon karbonitriranja. Površinska hrapavost HV1 bila je najviša za karbonitrirani sloj -1027 HV1, dok je u slučaju karbonitriranja s oksidacijom iznosila 1018 HV1. Kad se radi o površinskoj tvrdoći HV30, ona je za karbonitrirani sloj bila na nivou od 813 HV30, a kod karbonitriranja s oksidacijom iznosila je 795 HV30. Rezultati ispitivanja površinske tvrdoće ukazuju na otvrdnjavanje površinskog sloja.