Rupnik fortification line is a system of military fortifications on the Rapallo border, built just before the second world war. Its purpose was to defend the Kingdom of Yugoslavia from an attack by Italian forces. The fortification line consists of about 4000 military structures built between 1935 and 1940. Most of the structures were made with reinforced concrete and were split between the upper line (high positions) and the lower line (low positions) made of different concrete mixtures. Some of the structures are in a terrible state and in need of restoration, so we designed a concrete mix suitable for their restoration based on the research included in this thesis. The study compared the mechanical and physical properties of the replicated concrete samples to the samples of historic concrete gained from the structures on the fortification line. Samples of concrete were taken from selected fortifications and shaped into cylinders measuring 95 mm in diameter and 100 mm in height. Of all the samples 9 were taken from the bunkers of the upper line and 6 were taken from those of the lower line. Two different mixtures of concrete were prepared. For the restoration of the upper line, limestone aggregate was used, and for the restoration of bunkers of the lower line, dolomite aggregate was used. Each mixture was used to produce six 100 × 100 × 100 mm cubes and six 150 × 150 × 150 mm cubes, 24 in total. Using a hydraulic press, we set a base value for the concrete specimens' compressive strength and modulus of elasticity. The samples were then exposed to 30 freeze-thaw cycles, with the presence of chlorides, where compressive strength and dynamic modulus of elasticity were measured using ultrasound. Loss of mass was also measured at the same time. The results were used as the basis for determining the suitability of the concrete mixtures for the restoration of the structures on the Rupnik military line.
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