Ivan Gabrijel

Analysis of Thermal Properties of Cement Paste During Setting and Hardening

An experimental investigation is made in order to study changes of thermal properties of hardening cement paste. Using a hot disc principle for determination of thermal properties, specific heat capacity, effusivity and thermal conductivity are analyzed during first 4 days of hydration. Specific heat capacity value changed only slightly during hydration. Thermal conductivity decreased with the progress of hydration 20% compared to initial value. Hydration process was monitored using ultrasonic pulse velocity (UPV) test. A discussion of results is made taking into account porous nature of cement paste and transformation of liquid and hardened phases i.e. water and clinker minerals, into hardened structure.The motivation for conducted research is improvement of numerical models for calculating temperature changes in mass concrete structures using hydration dependant thermal properties. Also, another field of application could be the quality control of cement based materials.

Mechanical Properties of Self-Compacting Concrete with Different Mineral Aditives After High Temperature Exposure

This paper presents an experimental research on the performance of high-strength selfcompacting concrete (SCC) with different mineral additives after exposure to high temperatures of up to 600°C. For this purpose, four SCC mixtures were studied: one reference and three mixtures where the Portland cement was replaced with mineral additive (fly ash, metakaolin and limestone) in certain proportions. After natural cooling in the furnace, compressive strength and static modulus of elasticity were determined and compared to results obtained from other studies and those provided in EN 1992-1-2 and EN 1994-1-2 for normal-vibrated concrete. Additionally, acoustic emission (AE) parameters during compression test of heated and unheated specimens were also obtained which showed good non-destructive tool for identifying exposure temperature of the concrete needed for the assessment of concrete structures after fire.

Modifications of dolomite-based self-compacting concrete properties using mineral additives

This paper addresses fresh and hardening properties of self compacting concretes (SCC) made with dolomite filer and pozzolanic materials, fly ash and matekaolin as cement replacement in different amounts. For this study, seven mixtures were prepared with a constant water-to-powder ratio and powder content. Fresh SCC properties were assessed by means of slump flow, L-box and sieve segregation test, while hardened properties were evaluated by means of compressive strength and modulus of elasticity at ages of 2, 7, 14, 28 and 365 days. In addition, the activity of mineral additives was assessed by heat of hydration measurement. Obtained results showed that mixtures containing fly ash exhibited better performance concerning workability properties needed for SCC, while cement replacement with metakaolin enhanced the mechanical properties. Slower pozzolanic activity of fly ash reflected in slower strength development, but ultimate strength obtained at the age of 365 days was not reduced by the presence of fly ash.

ODREĐIVANJE TIJEKA OSLOBAĐANJA TOPLINE HIDRATACIJE U BETONU

Nepovoljna distribucija temperature u betonskim elementima može dovesti do nastanka pukotina. Sprjecavanje nastanka pukotina provodi se planiranjem tehnologije izvođenja. Velika pomoc pritom su proracuni raspodjele temperature. Proracun temperaturnih promjena u betonu mora uzeti u obzir međudjelovanje betonskih elemenata s okolisem, uz istovremeno generiranje topline u betonu zbog hidratacije cementa. Buduci da su uvjeti u okolisu i toplina koja se oslobađa promjenjivi u vremenu, predviđanje distribucije temperature postaje složen problem. Tijek oslobađanja topline hidratacije u betonu predstavlja vrlo bitan parametar, jer služi za proracun temperaturnih promjena, a također je i pokazatelj napretka hidratacije cementa. U ovom radu prikazan je matematicki model kojim se opisuje razvoj topline hidratacije tijekom vremena. Provedena su laboratorijska ispitivanja oslobađanja topline hidratacije u betonu diferencijskim kalorimetrom. Analize su provedene na sest mjesavina betona izrađenih s razlicitim vodocementnim (v/c) omjerima i dvije vrste cementa.