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    Analysis of concrete taken from a loaded column by determining the modulus of elasticity
    (Czech Technical University in Prague, 2024-05-16) Hrabová, Kristýna; Cikrle, Petr; Olejková, Sabina; Kocáb, Dalibor
    Sustainability places an emphasis on preserving and using existing structures, particularly those that are historically or artistically valuable. The preservation of buildings is closely related to a good assessment of their condition. When making a diagnosis, it is suitable to use non-destructive methods to the greatest extent possible. However, these must also be complemented by core samples, which can be used to determine not only the compressive strength but also other important properties. The paper focuses on the evaluation of a reinforced concrete column that was gradually loaded until failure occurred. During loading, the quality of the concrete throughout the column was monitored using the ultrasonic pulse velocity method and, after loading was finished, the cores were taken from the column. The specimens, obtained from the core samples, were then carefully analysed using both non-destructive and destructive tests. The conclusion describes a possible approach to the assessment of concrete using a core examination.
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    MONITORING THE INTERNAL STRUCTURE BEHAVIOUR OF ALKALI-ACTIVATED SLAG PASTE: EFFECT OF THE CURING MODE
    (CZECH TECHNICAL UNIV PRAGUE, 2023-12-31) Nápravník, Petr; Kocáb, Dalibor; Bílek, Vlastimil; Lisztwan, Dominik; Kucharczyková, Barbara
    This paper deals with the monitoring of the internal structure behaviour of an alkali -activated slag (AAS) paste. The slag was activated with a 4M solution of sodium hydroxide. The behaviour of the internal structure of the paste was regularly monitored through the changes in the resonant frequency and the mechanical properties, until the paste reached the age of 90 days. The main aim of the article is to show the long-term maturation and degradation process of an AAS paste under different curing modes. The results obtained suggest that the curing mode of the specimens has a significant effect on the behaviour of the internal structure of the paste based on the AAS. The development of both the dynamic properties and the flexural strength indicates the occurrence of a higher porosity in the internal structure of the paste, especially when the free drying process is started earlier. Insufficient hydration of the binder system is also a likely cause of cracks. The reduction in the relative dynamic moduli values ranging from 50 % to 80 % was observed for drying specimens at the age of 90 days. What is very interesting is that the occurrence of cracks was not prevented even by intensive moist curing of the paste as, between the 21st and the 28th day of maturing, there was a significant decrease of about 20 % in the relative dynamic modulus of elasticity and also a 50 % reduction in the flexural strength.
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    Effect of siliceous sand volume fraction on the properties of alkali-activated slag mortars
    (IOP Science, 2023-08-17) Hajzler, Jan; Bílek, Vlastimil; Kejík, Marek; Kucharczyková, Barbara
    One of the critical factors affecting the performance of alkali-activated slag (AAS) is the nature and dose of alkali activator. The activator type can play a significant role during the transition from pastes to mortars or concretes. Therefore, three basic sodium activators (water glass, carbonate, and hydroxide) of the same molarity of 4M Na+ were used to prepare AAS-based mortars with different volume fractions of siliceous sand. These were compared by means of workability, mechanical strength, and long-term shrinkage under autogenous conditions. The results were compared to those obtained on pastes with similar workability. Increasing the content of the sand tended rather to decrease the mechanical properties, while greatly decreased autogenous shrinkage. Nevertheless, the most remarkable differences for different activators were observed when comparing the mortars with pastes. The transition from pastes to mortars resulted in the highest reduction in both compressive and flexural strength for sodium hydroxide. The flexural strength of the mortars with sodium water glass and sodium carbonate even increased considerably in presence of sand.
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    The influence of steel fibre amount on the consistency, volume changes and compressive strength of concrete
    (IOP Publishing, 2023-08-25) Vymazal, Tomáš; Misák, Petr; Hrabová, Kristýna; Kocáb, Dalibor
    The paper focuses on the consistency of fresh steel fibre reinforced concrete in relation to the amount of steel fibre used. Consistency was determined by the slump test and the flow table test. Furthermore, the paper deals with monitoring the compressive strength and volume changes during the maturing process of steel fibre reinforced concrete. Volume changes were monitored using a shrinkage drain. The results of the steel fibre reinforced concrete properties are compared with the values of the reference concrete without fibres and with each other. The result is an evaluation of how the amount of fibre affects the properties of fresh and hardened concrete. The dependence between consistency, compressive strength and shrinkage of steel fibre reinforced concrete is also established.
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    Development of the internal structure of an alkali-activated paste related to the length of curing
    (IOP Publishing, 2023-08-25) Kocáb, Dalibor; Bílek, Vlastimil; Nápravník, Petr; Kucharczyková, Barbara
    This paper deals with the influence of the length of the moist curing period on the development of the internal structure of a paste based on alkali-activated slag. Waterglass was used as the activator. For the purpose of experiment, the test specimens subjected to the experiment were divided into five sets with different curing regimes combining moist and air curing. The reference set was kept in an environment with relative air humidity 95% during the entire maturing period. The development of the internal structure of the alkali-activated paste was mainly monitored using the resonance method. The obtained results show that the curing regime applied to the test specimens significantly influences the development of the internal structure of the material. The development of the observed dynamic properties (dynamic moduli and Poisson’s ratio) shows the formation of damage to the internal structure of the paste after its exposure to the air. Interestingly, a massive decrease in dynamic modulus of elasticity immediately after exposure to air drying was not prevented, even by intensive moist curing of the paste for 28 days.